Recent advances in ultraviolet photodetectors

Materials Science in Semiconductor Processing ] (]]]]) ]]]–]]]

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Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp

Review

Recent advances in ultraviolet photodetectors Z. Alaie a,n, S. Mohammad Nejad a, M.H. Yousefi b a b

Nanoptronics Research Center, Iran University of Science and Technology, Iran Nanolab, Malke Ashtar University of Technology, Iran

a r t i c l e i n f o

abstract

PACS: 85.60.Gz 85.60.Dw 85.30.De 85.30.Kk 85.35.Be 73.40.Sx 73.61.Ga 73.61.Ph

In recent years, ultraviolet (UV) photodetectors (PDs) have received much attention in the various field of research due to wide range of industrial, military, biological and environmental applications. In this paper, a special focus is given to the unique advantages of different UV PDs, current device schemes and demonstrations, novel structures and new material compounds which are used to fabrication of PDs. Additionally, we investigate numerous technical design challenges and compare characteristics of the various PD structures developed to date. Finally, we conclude this review paper with some future research directions in this field. & 2014 Elsevier Ltd. All rights reserved.

Keywords: Ultraviolet photodetection Semiconductors Nanoparticles Thin film

Contents 1. 2.

n

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 III-nitride-based UV PDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.1. GaN UV PDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.1.1. GaN MSM UV PDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.1.2. Schottky barrier GaN UV PDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.1.3. p–n, p–i–n and avalanche GaN UV PDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.1.4. Polarization sensitive GaN UV PDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.1.5. High speed GaN UV PDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.1.6. PDs based on GaN nanowires . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.1.7. Application aspects of GaN UV PDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.2. AlN UV PDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 2.3. AlGaN UV PDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 2.3.1. MSM AlGaN UV PDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 2.3.2. Multiple-quantum-well (MQW) AlGaN UV PDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 2.3.3. Schottky barrier AlGaN UV PDs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.3.4. p–i–n and avalanche AlGaN UV PDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Corresponding author at: Iran University of Science and Technology, Tehran, Iran. E-mail address: [email protected] (Z. Alaie).

http://dx.doi.org/10.1016/j.mssp.2014.02.054 1369-8001/& 2014 Elsevier Ltd. All rights reserved.

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2

3.

4. 5. 6. 7. 8. 9. 10. 11. 12.

2.3.5. Application aspects of AlGaN UV PDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. AlInGaN UV PDs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ZnO based UV PDs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. ZnO based MSM UV PDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Schottky barrier ZnO UV PDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. p–n and p–i–n and avalanche ZnO UV PDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Effect of annealing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5. Doping in ZnO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6. Piezoelectric property and surface acoustic wave UV PDs based on ZnO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7. Nanostructure ZnO UV PDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7.1. UV PDs based on ZnO nanowires . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7.2. ZnO nanorod UV PDs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7.3. ZnO nanoparticles UV PDs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7.4. Other types of nanostructure ZnO UV PDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MgZnO UV PDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Other types of oxide UV PDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Organic UV PDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SiC UV PDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Si UV PDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Diamond UV PDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ZnSe UV PDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Other types of UV PDs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

14 15 16 16 19 19 20 20 20 21 21 23 23 24 24 26 29 31 33 34 35 35 36 36

1. Introduction Recent development in technology of wide band gap semiconductors have stimulated up major interest in UV potential application. UV PDs have many applications in various areas such as engine control, solar UV monitoring, astronomy, lithography aligners, secure space-to-space communications, or detection of missiles [1]. In particular UV PDs have attracted significant attention in the recent years, due to the rise of new requirements of civil and military industries to improve UV instrumentation that is capable of operating at high temperature and in harsh environments. Therefore, many attempts have been made to fabricate PD devices with these features for operation in the UV region of the spectrum whilst remaining blind to visible wavelengths. Some PDs are commercially available such as PDs that are used in photodetection system of ultrafast imaging, measuring speckle motion to monitor ultrasonic vibrations, remote optical diagnostics of nonstationary aerosol media in a wide range of particle sizes, atmospheric clock transfer based on femtosecond frequency combs, measurement of the polarization state of a weak signal field by homodyne detection and so on. The UV region of the electromagnetic spectrum covers the wavelength range between λ
10 nm and λ
400 nm. It is often divided into the three spectral bands: UVA for λ ¼400–320 nm; UVB for λ ¼320–280 nm; and UVC for λ o280 nm. The fundamental operating principle of all solid-state photosensitive devices is the same. A photon with sufficient energy interacts with a semiconductor crystal, temporarily changing the distribution of electron energies within the crystal (Fig. 1). One electron gains enough energy to attain an energetic conductive state, where it is free to move about within the crystal. The promoted electron leaves behind a vacancy, called a hole, which can also move about within the crystal. Together, these are

referred to as an electron–hole pair, and in an ideal PD, one such pair is created for every absorbed photon. Eventually, if left in the crystal long enough, the electron–hole pair will recombine, giving up the extra energy in the form of heat. This happens on a characteristic time scale called the recombination lifetime, τr. While excited, the electron and hole will drift in the presence of an electric field, creating an electric current. This current can be detected by connecting the active area of the semiconductor into an electronic circuit. How the electric field is applied and the nature of the electrical contacts defines the class of detector [2]. For photodetection, there are many characteristics that describe the performance of the PD. These parameters indicate how a detector responds and they are as follows: Quantum efficiency (η) is defined as the ratio of countable events produced per number of incident photons. It is also equal to the current responsivity times

Fig. 1. Operating physics principle of PDs.

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the photon energy of the incident radiation. Pinc is the incident optical power in watts, Iph is the photocurrent, h is the Plank constant and ν is the radiation frequency. η¼

I ph =q I ph hν ¼ P inc =hν q P inc

ð1Þ

Responsivity (R) is the ratio of number of incident photons per incident optical power. It depends on wavelength, bias voltage, and temperature. The plot of responsivity as a function of wavelength gives the spectral response of the detector. λ is the radiation wavelength, c is the speed of light and q is the electron charge. R¼

I ph ηq ηλq ¼ ¼ P inc hν hc

ð2Þ

Noise equivalent power (NEP) is defined as the minimum optical power required for producing a photocurrent and is equal to the total noise (i.e. the root mean square of the sum of the Johnson, Shot and Excess noises) per responsivity. Detectivity is defined as the reciprocal of noise equivalent power. Therefore, the higher value of detectivity, the better performance of the detector. In general, detectivity varies with the detector sensitive area and the electrical bandwidth. Therefore, specific detectivity (D*) is introduced, which is the product of the detectivity and the square root of the product of the detector area and bandwidth. A is area of PD (cm2); B is bandwidth of measuring system (Hz). NEP ¼

Dn ¼

noise ðWattsÞ responsivity

pffiffiffiffiffiffi   AB cm ðHzÞ1=2 W  1 NEP

ð3Þ

ð4Þ

The response time τ, for any detector is of great importance as this will often determine the suitability of a device for a specific application. This parameter is characterized by the speed of response to a sudden change in the input signal, i.e., it is a measure of the time required for a PD to response to a light impulse. The response time is specified as the time taken for the photosignal to rise or to decay to specific value. In most cases, the response speed was limited by the fall time, rather than the rise time, i.e., it defined as the time in which photocurrent drops from 90% to 10% of its maximum value, when the device is excited with light pulses [3]. In fact, the response time of a photodiode is dependent on three fundamental factors: (1) drift time of the carriers to cross the depletion region, (2) diffusion time of carriers generated outside the depletion region and (3) RC time constant where R is the load resistance and C is the capacitance of the photodiode. The drift of carriers through the depletion region is usually quite rapid due to the built in electric field of the junction. Drift velocity can be further increased with an applied reverse bias until it reaches its saturation value. The diffusion process, on the other hand, is influenced by the recombination time and is thus relatively slow. The diffusion time can be improved by ensuring that all the electron–hole pairs are produced within the depletion region or within one diffusion length from the depletion region. In practice the most serious limitation arises from

3

the RC time constant. There is a tradeoff for both fast response time and high selectivity on selection of the load resistance. Response time requires the load resistance to be less whereas high selectivity requires high value load resistance. But, the capacitance associated with this load resistance should be low for both because capacitance removes any high frequency information in the signal from being measured. Also, actual device photocurrent may be greater or smaller than the primary photocurrent. It depends on what happens to the photo-generated e-h pairs before they reach the PD contacts. If all the carriers are swept out before recombining and if there is no replenishment by re-injection, the photocurrent is equal to the primary current: I ph ¼ I ph0 ¼ qGWLD ¼ qðη P opt =hVÞ

ð5Þ

W, L and D are the width, length and thickness of sample. G is steady state generation rate of carriers per unit volume: G ¼ n=τ ¼

ηðP opt =hνÞ WLD

ð6Þ

where η is the quantum efficiency and n is the carrier per volume. The photocurrent between the electrodes is I ph ¼ ðsEÞWD ¼ ðqmn nEÞWD ¼ ðqnV d ÞWD

ð7Þ

Substituting n in (6) and (7) gives I ph ¼ qðη P opt =hVÞðmn τE=LÞ ¼ I ph0 gain

ð8Þ

To account for various mechanisms increasing or decreasing the actual photocurrent there is an addition factor called gain. The photocurrent gain is: Gain ¼ I ph =I ph0 ¼ me τE=L ¼ τ=t r

ð9Þ

where tr ¼ L/Vd is the carrier transit time. Hence, the gain depends upon the ratio of carrier lifetime to the transit time [4]. The response time of the photoconductor is determined by lifetime. Therefore, the gain and response time of photoconductor are related together. These parameters frequently are used in article for comparing the quality of fabricated PDs. UV detection has usually been applied to semiconductor photodiodes, thermal detectors, photomultiplier tubes (PMT), or charge-coupled devices (CCD) because they exhibit high gain and low noise and they can be rather visible-blind. However, PMT is a fragile and bulky device which requires high power supplies. On the other hand, CCD detectors are slow and their response does not depend on the wavelength. Semiconductor PDs require only a mild bias, and can be benefited for their small size and light weight, and being insensitive to magnetic fields. Their low cost, good linearity and sensibility, and capability for high-speed operation make them excellent devices for UV detection [1]. Therefore, if UV PD's exceed the performance of PMT's and CCD's, they will not be suitable for deployment. In this paper, we review the recent progress in UV semiconductor/PDs and it focused on Various Advances that have been discussed in the related articles. The organization of this review paper is as follows: First, III-nitride PDs including GaN, AlN and AlGaN PDs are presented. Then, ZnO (bulk, nanorods, nanowires and

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4

Table 1 Several parameters of important UV PDs. Parameters

Si

Diamond 4H-SiC

6H-SiC

GaN

AlN

Eg (eV) Thermal conductivity (W cm  1 K  1) Melting point (K) Electron saturation velocity (107 cm S  1) Mobility (cm2 V  1 S  1) electrons Mobility (cm2 V  1 S  1) holes Dielectric constant Break down field (105 V cm  1)

1.12 1.5 1683 1 1400 600 11.8 3

5.5 20 3773 2.7 2200 1600 5.5 100

2.86 4.9 4 2800 2 400 75 9.7 24

3.4 1.3 42500 2.5 1000 30 8.9 26

6.2 3.4–6.2 3.37 3.19 5.4 42400 2242 1.4 135 170 14 8.1 9.1 20

3.2 3.7 4 2800 2 950 120 9.7 20

quantum dots), MgZnO and other types of oxide photodiodes are discussed in continue. This is followed by Organic, SiC, Si, diamond, ZnSe and other types UV PDs in continue. Finally, we conclude this review with some future research directions in this field. For comparison, Table 1 provides some features of important semiconductors which are reviewed in this paper. Comparing these properties has been shown that many materials have better critical parameters than Si which make them suitable for high temperature and high power industrial applications. Therefore, by obtaining fabrication knowledge of high quality PDs based on these materials is meaning of replacing Si in industry.

AlGaN

ZnO

MgO MgZnO TiO2

ZnSe

7.83 3.3–7.8 3.2 Anatase 2.82 4.82 1.17 0.18 3073 1570 1100 10 2 9.8

10 Anatase 3.2 Anatase

response of the PDs. GaN detectors typically have a higher leakage current, which is expected to decrease as the material quality improves. The main advantage of the GaN detectors is the sharp absorption edge and the ability to adjust the wavelength at which it occurs by using AlGaN [8]. In the past few years, various structures of GaN-based PDs such as photoconductive, Schottky barrier, (metal–insulator–semicondutor) MIS, (metal–semicondutor–metal) MSM, p–n, p–i–n junction PDs and heterojunction have been fabricated [9–12]. In this section, recent progress in various types of GaNbased PDs and the influence of dislocations, thermal annealing and using different substrates and electrodes on UV PD properties have been studied.

2. III-nitride-based UV PDs III-nitride semiconductor materials binary (GaN, AlN, and InN), ternary (AlGaN, InGaN, and InAlN) and quaternary (InAlGaN) systems have been extensively studied from the viewpoint of their extensive electronic and optoelectronic applications in the visible and UV spectral range. Band gap of III-nitride alloys vary from 0.7 eV for InN to 6.2 eV for AlN. The most attractive devices in the group of III-nitride semiconductors are UV detectors. Nitride-based UV PDs are potentially useful in civil, commercial and military applications such as UV astronomy, flame detection and engine monitoring [5]. Moreover, the PD cutoff frequency can be engineered by changing the mole fraction in their ternary and quaternary alloys. 2.1. GaN UV PDs Gallium nitride (GaN) is one of the most promising materials for the realization of visible–blind UV PDs because of its large direct band gap (3.4 eV at room temperature in wurtzitic symmetry), monocrystalline character, high saturation-electron drift velocity (310 cm/ s), excellent surface properties, good thermal and chemical stability, superior radiation hardness, high temperature resistance, their compactness, low consume and remarkable tolerability of aggressive environments [6]. These properties make them promising candidates for working under extreme conditions such as high temperature, high power, and high frequency applications [7]. Compared to Si, GaN has wide band gap, small dielectric constant, excellent thermal stability, chemical inertness and radiation hardness. The saturation velocity in GaN is a few times higher than in GaAs or Si, enhancing the transient

2.1.1. GaN MSM UV PDs A MSM PD is made by forming two Schottky contacts connected back-to-back on an undoped semiconductor layer. When a bias is applied within the MSM; this will put one Schottky barrier in forward direction (anode) and the other is reverse direction (cathode). The MSM PD operates when the incident light is directed on the semiconductor material between the electrodes, electrons will be generated in the conduction band, and thus creating holes in the valance band of the undoped region of the semiconductor. The reversed biased interface prevents the current from flowing through the device when there is no optical signal. The depleted regions between the electrodes are the PD active regions. A photon with energy greater than the band gap of the semiconductor will be absorbed by an electron; this electron will get excited to the conduction band. The photogenerated electron and hole are swept by the high applied fields to the positive and negative electrodes resulting in an electronic output signal. Among various types of GaN-based PDs, MSM PD exhibits sharp cut-off in the visible wavelength and a linear photo response characteristic that makes it an attractive choice for fabricating military counter measures, aerospace, automotive, petroleum, engine monitoring, flame detection and solar UV detection [7]. The MSM-PD is made by forming back-to-back two Schottky contacts on a nonintentionally doped or compensated semiconductor material. The Schottky barrier height is considered to affect the dark current of the MSM-PDs. In addition, parameters in the active layer also affect the device performance.

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MSM photodiodes have advantages such as very low dark currents due to the rectifying nature of the contacts, the high resistivity of the material, the need for only a single dopant active layer, high speed, the linearity with the optical power, an extremely low parasitic capacitance and low noise. These features make them ideal candidates for military and space applications and optimum structures for high-speed photodetection [7,9,13–14]. For example, GaN photodiodes for space imaging applications must be in the near-UV and vacuum UV ranges and must be potential for high-energy photodetection. Darkcurrent requirements for space applications are satisfied even in the high bias operation regime, VB 4 10 V [15]. Also, MSM UV PDs exhibit superior performance because of its fabrication simplicity and suitability for Highbandwidth and monolithic integration, which would enable their use without any preamplifier stage. The response of GaN-based MSM PDs that operate in the UV increases linearly with the applied constant current. The optical characterization shows the evidence of some internal gain. The gain was considered to be a result of trapping of minority carriers (holes) at acceptor impurities or defects. Achievement of very low dark current is critical to producing UV PDs with a high signal-to-noise ratio. Simulation of the dark current and photocurrent for the MSM-PD has been shown that the influence of carrier concentration of the active layer on the dark current is different depending on the doping of the active layer [16]. Also, the current transport is dependent on the thermionic and thermionic-field emission. In the p-type GaN MSM structure, as the applied voltage is increased, thermionicfield emissions dominate the dark current. In the n-GaN MSM structure, because of the small Schottky barrier height and high tunneling probability, the dark current is determined by thermionic and thermionic-field emission. The increase in layer thickness and finger spacing increase the photocurrent response but do not show a strong influence on the transient photocurrent. Transparency of the contact material determines the number of photons entering the semiconductor. Therefore, responsivity of normal MSM PDs is limited by the presence of the opaque metal contact electrodes in general. To achieve high performance MSM PDs, it is thus important to choose metal contacts with large Schottky barrier heights and high transparencies. When substrate is transparent in the wavelength range of interest, back illumination can be readily used to improve the sensitivity of MSM PDs by avoiding electrode shadowing. This structure due to using intrinsic GaN film, results in low dark current, stable contacts, and high quantum efficiency. Also, the membrane makes possible the backside illumination of the structure and reduces the interface defects and thus also improves the noise performances of the detector [17]. GaN membrane MSM PD structures with fingers and interdigits of 0.5 mm width have been manufactured using nanolithographic techniques. Very high values for the responsivity (50–150 A/W for a bias in the range 6–15 V) have been obtained. Although back-side illumination can be used to obtain high responsivity, it will also create critical problems in optical lithography alignment, device

5

processing and chip packaging. Previously, it has been shown that one can use transparent contact electrodes to solve this problem. GaN-based UV MSM PDs with transparent titanium nitride (TiN) and indium, tin oxide (ITO) Schottky contact electrodes have also been demonstrated [18,19]. Achieving a high responsivity has been one of the most important challenges for the application of UV detectors. Recently, GaN UV detectors have been fabricated by exploiting Ag nanoparticles' plasmon enhancement. By annealing Ag nanoparticles on the GaN surface, responsivity of these detectors was increased about 30 times. The high responsivity (4000 mA/W) was attributed to the plasmonic scattering effect of these Ag nanoparticles. The results of this work are significant for advancing the application of GaN-based UV devices [20]. 2.1.2. Schottky barrier GaN UV PDs Schottky diodes in their simplest form consist of a metal layer that contacts a semiconductor. The metal/ semiconductor junctions exhibit rectifying behavior. The rectifying property of the metal-semiconductor contact arises from the presence of an electrostatic barrier between the metal and the semiconductor which is due to the difference in work functions of the metal and semiconductor, respectively. Comparing with photoconductor and MSM photodiode, a Schottky photodiode has many advantages in the aspects of high quantum efficiency, high response speed, low dark current, high UV/visible contrast, and possible zero-bias operation. Schottky barrier GaN PDs were first fabricated by Khan et al. [21]. It was a Ti Schottky diode on p-type GaN. It is the first commercial PD based on III-nitrides, as announced by APA Optics in January 1998. The manufacturer reported a typical dark current of 2 nA at -1 V bias, a responsivity of 130 mA/W, an UV (365 nm)/visible (400 nm) contrast of 103 and a bandwidth of 10 MHz for GaN grown on c-sapphire with a diameter of 250 mm and vertical Pd Schottky barriers [22]. The advantage of Schottky barrier photodiodes compared with other structures is efficient

Fig. 2. Normalized responsivity of different types of GaN PDs (with permission from the publisher and authors) [23].

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absorption of UV light, their fabrication simplicity and high response speed. These devices are especially suitable for broadband photodetection, because their photoresponse are flat (Fig. 2). But, their maximum responsivity is limited by light reflection in the semitransparent top contact, while it is not necessary in p–n junction photodiodes [23]. However, the large dislocation density in GaN epitaxial layers will result in large leakage current for nitride-based Schottky barrier PDs [24]. Reducing the leakage current is an important issue for GaN-based Schottky barrier PDs. The leakage current of Schottky contacts strongly depends on the barrier height. A Schottky barrier of a metal/ semiconductor interface must be high in order to reduce the leakage current in the Schottky contacts. It was found that post-deposition annealing of the contact is also effective in reducing the leakage current of Schottky diodes with certain Schottky metals [25]. By annealing, the Schottky barrier height increased from 0.91 eV to 1.03 eV and the ideality factor decreased from 1.58 to 1.16 [24]. It is known that dislocation density in GaN epitaxial layers was high due to the large differences in lattice constant and thermal expansion coefficient between GaN and substrate. Therefore, using bulk GaN substrates with low dislocation density result in reducing the leakage current and therefore improving the performance of GaN PD. In another work, it is found that employing undoped GaN instead of Si-doped GaN as the n  -GaN layer reducing the dislocation density in the n  -GaN layer and brings about a higher responsivity due to a lower Ga vacancy concentration [26]. Furthermore, it is found that the responsivity in the region near the edge of the electrode is closely related to the frequency of incident light [27]. It is suggested that the surface states near the edge region capture photogenerated carriers and lead to an enhancement of electron injection, being an important mechanism responsible for the photocurrent gain of the GaN Schottky barrier UV PDs. Studies have demonstrated that the dark current in Schottky based GaN photoconductors can be reduced significantly by applying a low-temperature-grown nucleation layer prior to epitaxial growth [28]. Simultaneously, transparent Schottky contacts, such as ITO have been used as electrodes to enhance the responsivity of PDs. Also, it was shown that leakage current was much smaller and much less bias dependent for the PD with SiN/GaN nucleation layer, as compared to the PD with conventional low-temperature GaN nucleation layer and the effective Schottky barrier height increased from 1.27 to 1.53 eV with the insertion of the SiN layer [29]. Recently, a dual-operation-mode UV Schottky-barrier PD has been fabricated on high resistivity GaN homoepitaxial layer. The operation mode of the PD was controlled by bias polarity. Under reverse and zero bias, the PD worked in depletion mode with low dark current and high UV/visible rejection ratio. Under forward bias, the PD worked alternatively in photoconductive mode and it exhibited high photo-responsivity and a narrow detection band around 365 nm [30].

2.1.3. p–n, p–i–n and avalanche GaN UV PDs In a simple junction photodiode, (p–n junction), incident light with energy greater than the bandgap creates electrons in the p region and holes in the n region. Those that are within the diffusion length of the junction are swept across by the field. The light also creates electron– hole pairs in the junction region, and these are separated by the field. In both cases, an electron charge is contributed to the external circuit. In the case of no bias, the carrier movement creates a voltage with p region being positive. The maximum voltage is equal to the difference in the Fermi levels in the p and n regions and approaches the bandgap energy Eg. The first fabrication and characterization of intrinsic visible-blind photovoltaic detectors based on GaN p–n junctions has been reported in 1995 [31]. This PD has been shown zero-bias responsivity as high as 0.09 A/W at 360 nm which is comparable to that of an UV enhanced Si detector. In design of p–n junction devices, carrier mobility is an important key. Measurement has been shown that the minority electron mobility was smaller by about 3 orders of magnitude than the majority electron mobility owing to the higher concentration of Mg dopants in p-GaN [32]. Also, the simulation calculation have been shown that the spectral response of p–n þ structure GaN UV PD changes remarkably with the reversed bias and the carrier concentration of p-type GaN based on the value of reversedbias voltage can be derived [33]. P–i–n PD is a simple diode structure with heavily doped p- and n- regions separated by an intrinsic layer. Photons with energy greater than the energy gap, incident on the front surface of the device, create electron–hole pairs in the material on both sides of the junction. The PIN diode has an extra intrinsic high field layer between the p and n regions, designed to absorb the light. The carriers that are generated in the junction region experience the highest field and so, being separated rapidly. This minimizes the generation of slow carriers and results in a fast response detector [3]. Although, the reverse bias leakage current across a p–n junction is lower than that across a Schottky barrier due to larger barrier height. GaN based p–i–n PDs have better sensitivity and faster response [10]. These PDs typically exhibit excellent UV-to-visible rejection ratios, sharp spectral responsivity cutoffs, and fast response times. The thickness of depletion region (the intrinsic layer) can be tailored to optimize the quantum efficiency and frequency response of PD. An optimal thickness of n-layer for visible blind GaN/ AlGaN p–i–n UV PD based on a balance between high rejection ratio and low fabrication costs have been numerically obtained [34]. It is found that rejection ratio of short-wavelength side monotonically rises with the increased thickness of the n-layer, and then saturates. By increasing doping concentration of the absorption layer, radiative and Auger recombinations result in the decrease of photoresponse. Recently, GaN deep UV p–i–n PDs with a 40 nm thin p-GaN contact layer have been fabricated on sapphire substrates. The PDs have been shown good rectification

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Device structure

Fabrication method

Electrode

Light of detection

Dark current

GaN MSM GaN MSM

MOCVDa Hydride vapor phase epitaxy MOCVD MOCVD MOCVD

Ni/Au Ni/Au

300–400 nm 365 nm

10 pA at 1 V o 2 pA at 10 V

Ni/Au Au Ni/Au

365 nm 362 nm 365 nm

o 5 pA at 15 V 200 pA at 3 V 80 pA at 2 V

ZnO:Ga

365, 250 nm

2 nA at 10 V

Ni/Au

360 nm

Ni

365 nm

2.4  10  11 A at  9 V 6.13 mA

Ti/Al/Ti/Au

300 nm

0.52 pA at  1 V

Ti/Al

360 nm

3.26 PA at  1 V

Ni/Au Ni/Au

360 nm 365 nm,
5.5 μW/mm2

0.5 pA at  5 V

GaN MSM GaN MSM GaN MSM

GaN MSM

GaN Schottky

Hydride Vapor phase epitaxy Hydride Vapor phase epitaxy Thermal vapor deposition Plasma assisted MBEb MOCVD

GaN Schottky GaN Schottky (dual mode operation) GaN Schottky

MOCVD Hydride Vapor phase epitaxy MOCVD

GaN MSM GaN on Si MSM GaN/AlN QDisc MSM

AlGaN/GaN Schottky

Polyaniline Pt/Au

250–400 nm, 2 mW/cm2

0.94 fA at  2.5 V

GaN p–i–n GaN p–i–n

MOCVD MOCVD

Ti/Al Ni/Au

365 nm 360 nm

20 pA at 45 V 1 pA at  10 V

GaN p–i–n on sapphire AlGaN/GaN p–i–n

MOVPEc

p-GaN Ti/Al/Ti/Au

360 nm 365 nm

5.1 nA/cm2 at  5 V 50 nA at 4100 V

AlGaN/GaN p–i–n on SiO2

MOVPE

Cr/Au

280 nm, 3.9 mW

GaN (nanowire) n þ –n–n þ

Ti/Al/Ti/Au

GaN avalanche gain: 105 AlGaN avalanche gain: 1570

Plasma assisted MBE MOCVD MOCVD

Ti/Al/Ti/Au

350 nm, 0.1 mW/cm2 280 nm, 360 nm 272 nm

GaN M–I–S

MOCVD

Ti/Al

360 nm

GaN M–I–S

MOCVD

SiN/Al

By increasing the bias, longer wavelength

10  7 A/cm2 8 fA at 420 V 3.3  10  10 A/cm2 at  5 V 2.1  10  8 A at  0.5 V

Photo to dark current ratio

UV to vis: 41  105

Responsivity

Response time

50–150 A/W at 6–15 V 0.05 A/W at 5 V

[17] [36]

UV to vis: 45 Front to backside illumination responsivity ratio : 4.3

Ref.

[14] [37] [7]

0.19 A/W at 5 V Front side: 1.45 A/W, back side: 0.37 W at 2.5 V

[9] UV to vis: 1479

0.26 A/W at 5 V

Contrast ratio: 233 at 5 V Photo to dark current: 5  102 UV to vis: 1.05  103 at  6 V

0.285 A/W at 5 V

UV to vis: 6000 at 5 V Rectification ratio:
1.8  106 at 2 V UV to vis: 43

UV to vis: 6.7  103 UV to vis:
2.16  104 at 0 V and
2.87  103 at  8 V UV to vis: 4104 Photo to dark current: 5 Photo to dark current: 1  105 UV to vis: 43

UV to vis: 2  104 at 350 nm, 0 V UV to vis: 2.5  103 at  3 V Forward to reverse current: 5.5  102

[13] Rise time: 7.1 ms, decay time: 10.4 ms

[38]

2000 A/W

[39]

0.19 A/W at  6 V

[24]

0.29 A/W at  14 V 4.6 A/W at  5 V

Fall time: 46 ms at  5 V

[27] [30] [40]

0.18 A/W at 280 nm and 0.012 A/W at 340 nm 0.23 A/W at 5 V
0.17 A/W to
0.18 A/W from 0 V to  8 V


0.19 A/W at 0 V 70 A/mW at 100 V

[41]

14 fs (rise time)

6 ms (decay time)

180 mA/W at 0 V, 250 nm

[42] [10]

[5] [43]

Z. Alaie et al. / Materials Science in Semiconductor Processing ] (]]]]) ]]]–]]]

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Table 2 Recent III-nitride based UV PDs with different configurations.

[44] Time response o 10/100 ms

[45]

0.13 A/W at  20 V

[46] [47]

0.018 A/W at  3 V

[48] [11]

7

[39]

[12]

[50]

[49]

[49]

Ref.

behavior and low dark current in pA level for reverse bias up to  10 V. Under zero bias, UV/visible rejection ratio was more than 4 orders of magnitude. Quantum efficiency of the PD was around 47.5% at short wavelength of 280 nm [10]. In another work, GaN-based p–i–n UV PDs have been fabricated on patterned sapphire substrate for the first time. It was found that the structure on patterned sapphire substrate has been shown considerably lower defect density than that of a similar structure grown on standard sapphire substrate. The PD on patterned sapphire substrate has been exhibited a low dark current density of
5.1 nA/cm2 under 5 V, a high UV/visible rejection ratio of more than 104, and a zero-bias peak responsivity of
0.19 A/W at 360 nm, which corresponded to a maximum quantum efficiency of 65% [5]. In avalanche photodiodes, when the reverse bias of a photodiode is increased to near the breakdown voltage, carriers in the depletion region can be accelerated to the point where they will excite electrons by impact ionization from the valence band into the conduction band, creating more carriers. This current multiplication is called avalanche gain, and typical gains of 50 are available. Avalanche diodes are specially designed to have uniform junction regions to handle the high applied fields [3]. Recently, a visible–blind UV GaN back-illuminated avalanche photodiode with separate absorption and multiplication regions has been simulated based on driftdiffusion equation. Peak responsivity was 106.5 mA/W at the cutoff wavelength of GaN (364 nm). It was found that the thickness of the multiplication layer is important to improve the electrical field profiles and spectral response characteristics [35]. Due to the tremendous research efforts and the space limitations, this article is unable to cover all the recent exciting works reported in this field. Table 2 is the up-to-date summaries of most important reports on PDs made of GaN with different configurations. It summarizes the data on responsivity, dark current, photo to dark current and response time of GaN based PDs. From the data exit in this table, we can conclude that the p–i–n and (metal–insulator–semiconductor–insulator– metal) MISIM type PDs have the potential to achieve the lowest dark current densities, the avalanche type PDs have the potential to achieve the highest photo to dark currents and the Quantum discs type PDs have the potential to achieve the highest responsivities.

Photo to dark current: 500

Detectivity: 7.9  1012 Hz0.5 W  1 2  103 A/W

Metal Organic Chemical Vapor Deposition. Molecular Beam Epitaxy. Metal-organic vapor-phase epitaxy. c

b

a

Ti/Al/Ti/Au Plasma-assistedMBE

300 nm, 5 mW/cm2

Ti/Al MOCVD

246 nm

1.49  10  7 A at  5 V

UV to vis: 90

0.19 A/W 320 nm, 365 nm Ni MOCVD

246 nm

AlGaN/GaN multi quantum well (p–i–n) AlGaN/GaN multi quantum well (M–S–M) GaN/AlN quantum discs n–i–n

365 nm MOCVD

AlGaN/GaN MISIM (insulating layer:Al2O3) AlGaN/GaN MISIM (insulating layer:Al2O3, SiO2)

Ni

UV to vis: 20 at 10 V UV to vis: 5.7  103 at 10 V UV to vis: l.2  l02 and 2.3  103 for sensors with SiO2 and Al2O3, respectively 6.2  10  7 A/cm2 at 20 V 5  10  9 A/cm2 at 20 V 6.2  10  8 A/cm2 and 5.0  10  9 A/cm2 at 20 V for sensors with SiO2 and Al2O3, respectively 365 nm MOCVD AlGaN/GaN MSM

Ni

Photo to dark current ratio Dark current Light of detection Electrode Fabrication method Device structure

Table 2 (continued )

Responsivity

Response time

[51]

Z. Alaie et al. / Materials Science in Semiconductor Processing ] (]]]]) ]]]–]]]

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2.1.4. Polarization sensitive GaN UV PDs Recently, control of the polarization state for the UV spectral range has obtained great importance in a wide range of scientific and technological application [52]. M-plane group-III nitrides exhibit highly anisotropic optical properties that allow for the polarization-sensitive detection in the UV spectral ranges. The anisotropy can be further enhanced by the presence of in-plane anisotropic strain generated by the lattice mismatch between GaN and substrate. It enhances maximum contrast that can be used for relaxed thick films under weak signal detection conditions. It has been shown that by utilizing a polarizationsensitive photodectector in combination with a polarization filter, a limited wavelength range was obtained. In this

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work, M-plane GaN films on γ-LiAlO2 (100) substrate was used to achieve a high responsivity, very narrow-band detection and a high polarization contrast [53]. This detection system was sensitive to the state of polarization of the incident light. This added functionality may be helpful for a further reduction in spurious background signals. 2.1.5. High speed GaN UV PDs The fastest GaN-based PDs are p–i–n devices with decay time constants on the order of nana-second. MSM PDs fabricated on GaN have been shown extremely slow time-constants (typically no better than
10–100 ms) presumably due to a high density of traps. High speed GaN and GaN:Mg MSM PDs with sharp wavelength cutoff have been reported [54]. Minimum response times were o10 ns and about 200 ns for the GaN and GaN:Mg MSM devices, respectively. The noise power spectral density remained below the background level of the system (10  24 A2/Hz) up to 5 V for the GaN MSM detector. For comparison, both MSM and p–i–n UV PDs have been fabricated on single crystal GaN [55]. The best MSM devices have shown a fast 10–90% rise-time of
28 ps under comparatively low UV excitation of
0.1 W/cm2 average irradiance. Frequency response measurement was shown value of
3.5 GHz at a reverse bias of 25 V. For the p–i–n devices, rise-time has been of
43 ps at 15 V reverse bias for a 60 mm diameter mesa with 1 mm thick intrinsic region. 2.1.6. PDs based on GaN nanowires Recently, semiconductor nanowires (or nanorods) have considerable attention since they are building blocks that enable diverse applications in nanoscale electronics and photonics [45]. The small nanowire diameter and the high photoconductive gain of nanowire PDs are promising properties for diffraction limited UV PDs with high responsivity. Nanowire surface has a strong effect on the wire photoconduction due to formation of sub-band-gap photocurrent. Demonstration of an UV PD based on a single GaN nanowire containing GaN/AlN quantum discs was reported. The responsivity at λ ¼300 nm and at  1 V applied bias was measured 2  103 A/W [56]. It was shown that the insertion of an axial heterostructure drastically reduces the dark current with respect to the binary nanowires and for an incoming light intensity of 5 mW/cm2, the ratio between the photocurrent and the dark current enhances up to 5  102. 2.1.7. Application aspects of GaN UV PDs 2.1.7.1. Application of buffer layer for improved GaN film quality. The surface states are known to cause enhanced leakage current, non-radiative interface recombination and other undesirable effects. By passivating them, the photoresponse of Schottky detector was enhanced and the UV to visible contrast was improved [57]. Suppression of dark-current-related shot noise, which mainly originated from dislocations and carrier trapping by surface states is important. Because of this, recently many researches were

9

focused on the fabrication of GaN-based metal–insulator– semiconductor with different gate dielectrics. The purpose of buffer layer is to optimize the transition between substrate and active layers. One possible way to reduce leakage current is to employ an insulating gate layer by adopting a metal–insulator–semiconductor structure. Si3N4, Al2O3, Ga2O3, Gd2O3, SiO2, Ta2O5 and polymer poly (PBT) gate dielectrics have been attempted in nitridebased devices [48]. But, they were all ex situ deposited that the contamination probably might occur at insulator/ semiconductor interface. Recently, in situ insulators were deposited. An in situ grown, unactivated, Mg-doped GaN into a nitride-based PD was used to reduce the leakage current [48]. With a 3 V reverse bias, it was found that the UV to visible rejection ratio was 2.5  103. A dual wavelength detecting AlGaN/GaN MISIM UV sensor with a tunneling insulator layer has been fabricated. Al2O3 and SiO2 have been used as effective layer materials to reduce the dark current density of the UV sensor. The device has been exhibited two cutoff wavelengths of 320 nm and 365 nm corresponding to the bandgaps of AIGaN and GaN. The PD with a thin Al2O3 layer showed the lower leakage current and high UV–visible rejection ratio than that with an Al2O3 layer at the high bias conditions. The results of this work show that the AIGaN/GaN MISIM structure with thin Al2O3 is useful for detecting dual wavelengths in a single device [50]. Also, an MISIM-type UV sensor has been fabricated using an AlGaN/GaN hetero-structure epitaxially grown on a sapphire substrate that a thin Al2O3 layer inserted between GaN and Ni Schottky electrodes. This sensor has been responded to the two UV wavelengths. At a 20 V bias, the photo-responsive current density has been reported 2.2  10  5 under 330 nm UV irradiation and that at 365 nm was 1.1  10  5 A/cm2. The UV/visible rejection ratio for the MISIM UV sensor was 2.3  103 at the cut-off wavelength of 320 nm and 2 V bias, and that at the cut-off wavelength of 365 nm and 20 V bias was 5.7  103 [49]. 2.1.7.2. Defect free GaN based materials: different substrates. Using different substrates has recently gained increasing attention due to its potential in obtaining lower defect densities. Conventional GaN-based epitaxial layers were grown either on sapphire or on SiC substrates [58,59]. For overcoming the large density of defects and cracks of GaN film grown on Si substrates, buffer layers can be used. For porous β-SiC buffer layer, it was found a very high photo/dark current ratio of 2427.23, which is about 60 times the value for cubic β-SiC, thus evidenced the porous β-SiC buffer layer indeed can help low leakage current and high-quality GaN thin films grown on Si substrates [60]. Homoepitaxial GaN layer on bulk GaN substrate was used to fabricate MSM UV PDs [36]. The photoresponsivity was dependent on the incident optical power density and illumination conditions. The PD has been shown a high UV-to-visible rejection ratio of up to 1  105 and exhibited a low dark current of o2 pA at room temperature under 10 V bias. High density threading dislocations exit in GaN epilayers grown on other substrate result in poor properties

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of PDs [61]. The screw dislocations show a strong influence on the dark current of the PDs because they lower Schottky barrier height, while edge dislocations show the predominant effects on their responsivities because dangling bonds along those edge dislocation lines enhance the recombination of photogenerated electron–hole pairs. Membrane increases the responsivity of the GaN UV PDs and makes possible the backside illumination of the structure and then reduces the interface defects and thus also improves the noise performances of the detector. It was reported that manufacturing GaN UV PDs on 0.5 mm-thick GaN membranes result in high values for the responsivity (50–150 A/W for a bias in the range 6–15 V) [62]. 2.1.7.3. Transparent contact electrodes. The responsivity of normal MSM optical sensors was significantly limited by the presence of their opaque metal contact electrodes due to the blocking of incoming light by the metal electrodes. To solve this problem back illumination can be used but such back illumination will also create critical problems in optical lithography alignment, device processing and chip packaging. It has been reported that one can use transparent contact electrodes to solve this problem. Transparent metals such as sputter ITO, TiN and E-gun ITO films can be deposited onto GaN as the transparent electrodes. The effective barrier height of sputter ITO, TiN and E-gun ITO films to GaN was 0.46, 0.59 and 0.95 eV, respectively. The photo/dark contrast of sputter ITO, TiN and E-gun ITO MSM PDs was 0.36, 3 and 4.25 orders at 5 V bias, respectively [19]. Therefore, the E-gun ITO was choiced as the best candidate for application in MSM PDs due to its high transmittance and high barrier height to n-GaN. Also, transmittance of ITO, TiN, RuO and IrO electrodes decreases rapidly in the UV-B region with the wavelength between 280 and 320 nm. Therefore, responsivity of detectors with these transparent contact electrodes decreases rapidly in the UV-B region. Maximum responsivities of TiW and Ni/Au contact electrodes on n  -GaN MSM PDs were reported 0.187 and 0.0792 A/W, corresponding to quantum efficiencies of 64.7% and 27.4%, respectively [6]. The values of detectivity (D*) for devices with TiW and Ni/Au electrodes were calculated to be 1.313  1012 and 3.914  1011 cm Hz0.5 W  1, respectively. Also, lateral Schottky UV detectors were fabricated in GaN using indium–tin–oxynitride (ITON) as a contact metal [63]. Spectral responsivity has been determined more than 30 A/W at 240 nm and a photo-to-dark current ratio was 1000. The high spectral responsivity is explained by a high internal gain caused by generation–recombination centers at the ITON/GaN interface. The antimony-doped tin oxide (ATO) film possesses good optical and electrical properties, which make it good candidate for optoelectronic applications. The undoped GaN-based MSM UV PDs with ATO transparent electrodes were reported [64]. The barrier height between ATO and GaN was determined to be 1.12 eV and the responsivity of PDs with ATO electrodes measured at 325 nm under the bias voltage of 1 and 5 V was 0.051 and 0.095AW  1, respectively. Semitransparent metals with high work functions have low Schottky barrier heights and high reflectance

to UV light which yield limited quantum efficiency and responsivity of the MSM PDs. Conducting metal oxides have advantages such as a low resistivity, a high work function, excellent thermal stability, high transmittance to UV light, and high Schottky barrier height. Also, the two-dimensional electron gas (2DEG) sample has the higher mobility and the better thermal stability than the bulk GaN so the 2DEG PD has the faster response speed, the better thermal stability, the larger responsivity, the lower noise level and the larger detectivity. It was reported that a Ni/Au semi-transparent contact layer was deposited onto unintentionally doped GaN and AlGaN/GaN 2DEG structure [65]. By photo-chemical annealing of these PDs in O2, a larger Ni/Au transmittance, higher Schottky barrier heights and larger photocurrent to dark current contrast ratios can be achieved. The maximum quantum efficiencies were reported 13% and 57% for the photo-chemical annealing u-GaN and 2DEG PDs, respectively. Recently, fabrication of graphene contact to GaN nanowire ensemble in order to the demonstration of PDs has been reported. The detector presented a responsivity of 25 A/W at 1 V bias at 357 nm at low excitation power. The device has been shown a strong response up to 4.15 eV confirming a good transparency of the top graphene contact in the spectral region where ITO already presents a strong absorption [66]. 2.1.7.4. Effect of annealing. Thermal annealing of UV PDs based on GaN, which are mainly used due to the high thermal stability of GaN; results in enhancement of the electrical and morphological properties of the metal contacts of the photodiodes. As previously mentioned, transparency of the contact material determines the number of photons entering the semiconductor. To achieve high performance MSM PDs, it is thus important to have smooth surface morphology of the photodiodes–metal contact. It was shown that thermal treatment reduces the leakage current in Schottky diodes and the dark current in a Schottky contact based MSM PDs [67]. By thermal annealing of GaN PDs, dark current decreases down to 200 pA at 3 V bias, which is attributed to the interstitial Au atoms obtaining enough energy to fill N and Ga vacancies [37]. A responsivity of 0.19 A/W under 3 V bias was also achieved at 362 nm; the corresponding detectivity was 1.2  1011 cm Hz1/2/W. With proper annealing in oxygen by the photo-CVD systems, it was found that the transmittance of the deposited Ni/Au onto unintentionally doped nitride epitaxial layers in MSM UV PDs increased from 67% to 85% in the region between 350 and 450 nm. A photo to dark current contrast ratio of 2.54  103 achieved with a 1 V applied bias [68]. Application of thermal annealing treatment to Ni/GaN MSM photodiodes at high annealing temperature (600 and 700 1C) with the assistance of cryogenic treatment was investigated [69]. It was shown that cryogenic treatment after annealing helps in the Smoother surface morphology of the metal contacts and the enhancement of the electrical of the photodiodes especially at high temperature thermal treatment.

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2.2. AlN UV PDs AlN possesses the widest direct energy band gap (
6.2 eV) among the III-nitride semiconductors, therefore, it is a very important material for the development of deep UV (DUV), vacuum UV (VUV), and extreme UV (EUV) detectors. Solar-blind MSM PDs based on AlN/AlGaN heterostructures offer a low dark current and a maximal sensitivity at a wavelength of 240 nm [70]. These devices have been fabricated on sapphire substrates which allow backside irradiation of the detector, preventing light losses due to reflection from the contacts. Stacked AlN/AlGaN buffer layers in between GaN and the underneath Si (111) substrate can reduce dislocation density in the GaN epitaxial layer. Nitride-based MSM UV PDs with this layer have been prepared on Si (111) substrate [71]. With 5 V applied bias, dark current of device was only 7.95  10  12 A. Peak responsivity was 0.06 A/W at 10 V bias, corresponding to quantum efficiency of 21.2% with UV/visible rejection ratio of 244. With 5 V applied bias, noise equivalent power and D* of fabricated device were 1.70  10  13 W and 1.18  1013 cm Hz0.5 W  1, respectively. Effect of AlN thickness of 500, 1500, and 2500 nm on the photo/dark currents of the MSM PD has been investigated [72]. XRD and SEM have been shown that the crystallization of AlN film increases with increase of the AlN thickness from 500 to 2500 nm; this results in increasing of the photo current. This device has been exhibited the ratio of photo-current/dark-current of 5, 4, and 3 order of magnitude for AlN thickness of 500, 1500, and 2500 nm, respectively. Also, AlN layer has excellent lattice match to SiC (1%) unlike pure indirect band gap of SiC, AlN/n-SiC hybrid detectors have higher detectivity with adjustable direct band gap. AlN/n-SiC hybrid Schottky barrier PDs have been exhibited peak responsivity at 200 nm with very sharp cutoff wavelength at 210 nm [73]. Their dark currents were as low as 10 fA at a reverse bias of 50 V. the detectivity at zero bias was reported about 1.0  1015 cm Hz1/2 W  1. Also, cubic boron nitride (cBN) is one of the largest band gap semiconductors, suggesting that cBN is an ideal material for PDs working in DUV (wavelength o300 nm) range. DUV MSM PDs based on this material have been shown a sharp cutoff wavelength at around 193 nm [74]. The rejection ratio between 180 and 250 nm has been four orders of magnitude [75]. 2.3. AlGaN UV PDs The ternary AlxGa1  xN material with a direct band gap and relatively high mobilities is a promising material for developing UV PDs. By changing the Al composition, x, from 0 to 1, the energy band gap of these material varies from 3.4 to 6.2 eV. This corresponds to the band edge wavelength range from 365 nm to 200 nm. The first AlGaN PD was reported in 1996 by Walker et al. and Lim et al. AlGaN-based detectors have important applications in aerospace, automotive, petroleum and military industries. In addition, because of a small lattice mismatch between

Fig. 3. The spectral response of the AlxGa1  xN photoconductors with different mole ratios (x). (with permission from the publisher and authors) [76].

AlN and GaN, these materials are used to developing heterostructures for improving the performance. Responsivity of AlGaN PDs increases with wavelength, showing high quantum efficiency (Fig. 3). Near the bandgap, it exhibits a sharply decreasing responsivity. The cutoff wavelength of responsivity can be tuned by the Al composition, as one can see in Fig. 3 it varies from about 360 nm for GaN to 260 nm for Al0.50Ga0.50N [76]. For more knowledge of their advantages, Different types of AlxGa1  xN UV PDs present in this section. 2.3.1. MSM AlGaN UV PDs The main efforts so far have been concentrated to obtain a PD with low dark currents, solar-blind responsivity and gigahertz-level frequency response. AlxGa1–xN MSM UV PDs with low dark current, high speed operation and high sensitivity, are attractive devices. They do not require p-type doping that simplifies fabrication technology. Also, they can illuminate from back side therefore semitransparent finger metallization is not necessary. It was shown that resonant cavity enhances responsivity of PDs [77]. Resonant-cavity-enhanced UV MSM PDs have been fabricated. In this structure, responsivity enhanced by the resonant cavity effect i.e. by incorporating a multiple pass detection scheme. It was reported that dual-color PD structures have been fabricated with an epitaxial filter layer that incorporated between two detector active layers and device was designed for back illumination [78]. Two UV MSM PDs have been fabricated on the same chip. The detector that was fabricated on the as-grown surface have been shown 0.12 A/W of responsivity peak at 310 nm with a 10 V bias and 11 nm FWHM, whereas the detector that was fabricated on the recess-etched surface have been shown 0.1 A/W of responsivity peak at 254 nm with a 25 V bias and 22 nm FWHM. In another work, two-color Al0.75Ga0.25N and Al0.38Ga0.62N MSM PDs with excellent dark current characteristics have been demonstrated [79]. It was reported

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that a 229 nm cut-off wavelength, a peak responsivity of 0.53 A/W at 222 nm, and seven orders of magnitude visible rejection have obtained from Al0.75Ga0.25N MSM PD. Al0.38Ga0.62N MSM PD have been shown a 3-dB bandwidth of 5.4 GHz. Also, AlGaN-based UV MSM PDs with Schottky contact electrodes were fabricated by MOCVD on sapphire substrate [80]. Device has been shown a very low dark current about 20 pA under 1 V bias. The photocurrent/dark current rejection was more than five orders of magnitude. A peak responsivity of 0.19 A/W at 308 nm was measured at 1 V bias and the corresponding detectivity was about 4.43  1011 cm Hz1/2/W. The 10–90% rise and fall time were 10 ns and 190 ns, respectively. 2.3.2. Multiple-quantum-well (MQW) AlGaN UV PDs MQW-based detectors have different advantages over bulk devices. Strong piezoelectric fields existing in GaN/ AlGaN quantum wells improve the carrier transport and the absorption coefficient of MQWs enhances the quantum efficiency of the active region of devices. By adjusting different well widths and barrier heights, the cutoff wavelength of PDs can be tuned. Recently, it was found that using of InGaN/GaN multi quantum-wells in the active region of PDs can further reduce the leakage current and increase the rejection ratio. InGaN/GaN multi quantum-well MSM PDs have been fabricated with a beta-Ga2O3 cap layer formed by the furnace oxidation of a GaN epitaxial layer. The betaGa2O3 cap layer was found to suppress the reverse leakage current by at least about two orders of magnitude with a 5-V applied bias by increasing the barrier height [12]. With back-illuminated vertical Schottky type MQW UV detector, flat spectral responsivity between 325 and 350 nm and peak responsivity of 0.054 A/W were achieved [81]. By variation the quantum well thickness and increasing of the Al mole fraction, the spectral responsivity range

Fig. 4. Comparison of the unbiased spectral responsivity for all three MQW PDs. Inset: The transmission ratio between the single and doubleside polished sapphire (with permission from the publisher and authors) [81].

can be adjusted to shorter wavelengths. The cutoff wavelength of the MQW PD can be tuned by variation the well width, well composition and barrier height of PD. Fig. 4 shows spectral responsivity of three samples of svt1171, svt1172 and svt1174 that their thicknesses of barriers were 7, 5 and 3 nm, respectively. As you see, due to the polarization-induced internal electric field in the MQWs, the cutoff wavelengths have a redshift. Also, inset of Fig. 4 shows the transmittance ratio between single-side and double-side polished sapphire. It is shown that the double-side polished sapphire transmits the light more than two times compared to singleside polished sapphire in the spectral region. The p–InGaN/GaN(MQW)–n dual function devices have been reported [82]. These devices have exhibited the PD properties under reverse bias voltage and LED performance under forward bias voltage. The turn on voltage in forward bias and the break down voltage in reverse bias were about 3.2 V and  30 V, respectively. The larger size of devices has been exhibited higher photo- and darkcurrent densities under reverse bias voltage. High-speed MSM AlGaN/GaN solar-blind photodiodes with Al mole fraction up to 0.55 have been demonstrated [75]. These PDs have been shown low dark current density and true solar-blind detectivity. Schottky barrier height was 1.1 eV for Ni and 1.4 eV for Mo contacts on AlGaN. Detectors have exhibited low dark currents and high sensitivity within the range of 250–290 nm. It is mentioned that the detector speed of response is limited by parasitic capacitance of interdigitated diode structure, transit time of carriers and the space-charge of the holes at low and high excitation level respectively. Also, modeling of AlGaN/GaN MQW-UV detectors based on p–i–n heterostructures in the presence of external electric field has been reported [83]. This device includes 20 AlGaN/GaN MQW structures in i-region with different Al mole fraction. The MQW structures have 2 nm GaN quantum well width and 15 nm AlxGa1  xN barrier width. The cutoff wavelength of the PDs can be tuned by adjusting the well width and barrier height. It was shown that by affecting the polarization field effects, on increasing Al mole fraction, the transition energy decreases, the total noise increases, and the responsivity has a red shift, and so the detectivity decreases and has a red shift. 2.3.3. Schottky barrier AlGaN UV PDs Applications of AlxGa1  xN Schottky PDs span wide domains: control of the environment, flame detection, missile launching detection, UV-A and UV-B dosimeters, etc. AlxGa1  xN-based PDs which have an Al mole fraction of about 0.3 are very promising devices for UV-B range, due to their superior radiation hardness, high temperature resistance, and visible-blind characteristics. In Schottky barrier AlGaN detectors, responsivities decrease with Al mole fraction, from about 180 mA/W for Al 0% (GaN) down to 10 mA/W for Al 35% photodiodes [84]. Modeling of the spectral response of the UV Schottky detectors has shown that the spectral response increases when both the doping density and the thickness of the n-type layer decrease [85]. Therefore, optimization of the basic parameters must be

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used to get a detector with the expected performances (bandwidth, spectral response magnitude, etc). Schottky-type AlGaN layer which was grown on the μpillar GaN template by MOCVD, has been exhibited three steps of responses at about 326, 346, and 356 nm with responsivity of 1.1  10  2, 5.9  10  3, and 4.04  10  3 A/ W, respectively [86]. The diameter, spacing, and height for each pillar are 3, 3, and 1.2 μm, respectively. The main cutoff of the Schottky PDs at 356 nm should be contributed by the GaN layer under the AlGaN layer. The cutoff wavelength of the Schottky PDs at 326 and 346 nm should be contributed by the AlGaN layer on the sidewall of cone shaped pillars and the rest of the area of the AlGaN, respectively. Recently, the inverted Schottky structure in submicronthin AlGaN layers was designed especially for optimized backside sensitivity illuminated two-dimensional arrays with a very small pixel-to-pixel pitch [87]. When the device is illuminated from the backside detrimental absorption and recombination in the doped layer occur so that the conventional Schottky structure is not suitable for EUV detection. Cut-off wavelength was at 280 nm and intrinsic rejection ratio was three orders of magnitude of the visible radiation. The interface in these devices causes high dark currents which limits the minimum detectable optical power and the suitability for UV detection. Also, it is shown that AlGaN/GaN-based PDs with Schottky contacts have low dark currents in picoampere-range up to a bias voltage of 10 V and a low responsivity in the range of 0.1 A/W [88]. An Al0.25Ga0.75N/GaN heterostructure with Ohmic contacts was reported which a mesa design with meander geometry was used to disconnect the conductive channel at the heterostructure interface to reduce the dark current [43]. By employing a mesa structure design with meander geometry, very low dark currents below 50 nA up to a bias voltage of 100 V were achieved. The response time was determined to be 6 ms. The PD has been shown a cut-off wavelength of 365 nm according to the band gap energy of the GaN absorption layer. A high responsivity with a maximum of 70 A/mW at 312 nm and 100 V bias voltage was obtained. 2.3.4. p–i–n and avalanche AlGaN UV PDs Among exiting structures, p–i–n PD structures consume less power that can easily be integrated with other electronic circuitry. However, design and optimization of the PD structure is important key in obtaining high performance devices; there is a trade-off between speed and responsivity of the p–i–n photodiodes. The device with thicker i-layer exhibits higher quantum efficiency but its time response is slow. Therefore for the applications that speed is not important, thicker depletion region is more efficient. Misorientation angles of a-plane 11 off axis substrate results in defect reduction and therefore lower dark current, higher response sensitivity, and higher rejection ratio compared to that grown on 01 on-axis sapphire substrate. It was reported that GaN p–i–n PD fabricated on 11 off-axis c-plane sapphire substrate [89]. the dark current density decreased from 2.4  10  9 A/cm2 to

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1.82  10  11 A/cm2 at  3 V, the responsivity increased from 0.06 A/W to 0.105 A/W at 360 nm with no applied bias; the UV/visible (UV/vis) rejection ratio increased from 2.32  103 to 2.48  104 (comparing wavelength 360– 450 nm). For comparison, AlxGa1  xN heterostructures Schottky, p–i–n, and MSM photodiodes have been fabricated [90]. The p–i–n samples exhibited extremely low dark currents. Leakage current was below 3 fA, at 6 V reverse bias. Detectivity was 4.9  1014 cm Hz1/2 W  1. The MSM devices exhibited photoconductive gain, while Schottky and p–i–n samples have shown 0.09 and 0.11 A/W maximum responsivity values at 267 and 261 nm, respectively. Schottky samples have been shown visible rejection of 2  104. The fastest devices were MSM photodiodes with a maximum 3-dB bandwidth of 5.4 GHz that measured at 267 nm. AlxGa1  xN with x¼0.4 are intrinsically solar-blind, in which no additional UV filters are needed. In spite of low noise and fast response times, they have lack of high internal gain that is the major limitation for the usage of AlGaN PDs as high sensitivity detectors. However, AlGaNbased avalanche PDs (APDs) with high avalanche gain was reported [47]. Cutoff wavelength was around 276 nm. The dark current was lower than 8 fA for bias voltages up to 20 V. The responsivity was 0.13 A/W at 272 nm under 20 V reverse bias. Detectivity for a 40 μm diameter device was achieved D* ¼1.4  1014 cm Hz1/2 W  1. Also, it was shown that by designing different epitaxial avalanche structure with thinner active multiplication layer, the rejection ratio improves. By decreasing the active layer thickness, device exhibited lower defect density [91]. Recently, it was shown that the formation of charge at the hetero-interface arising from spontaneous and piezoelectric polarization can dramatically affect APD performance. A thin AlN layer inserted at the AlGaN/SiC heterointerface has been shown to be effective in reducing the net interface charge [92]. Researchers have been shown that the performances of the avalanche photodiodes can be also improved further by polarization doping effect in the p-type AlGaN layer. To improve the performances of separate absorption and multiplication AlGaN avalanche photodiodes, a polarization field and a polarization doping effect have been introduced into the APDs by adjusting the Al composition of the p-AlGaN layer. It was shown that the polarization induced electric field, which has the same direction as the reverse bias in the multiplication layer, can significantly lower the avalanche breakdown voltage, whilst the multiplication gain increases pronouncedly [52]. By refining the low-pressure MOCVD growth of AlGaN as well as the UV PD p–i–n structure, Razeghi's group has successfully fabricated the world's highest quantum efficiency (external quantum efficiency of 89%) solar-blind UV PDs grown on sapphire substrate, an advance in technology that could aid in the detection of missiles and chemical and biological threats [93]. Although sapphire is the most common choice for back-illuminated devices, researchers also developed alternative low-cost UV PDs grown on silicon substrate. This group used a novel maskless Lateral Epitaxial Overgrowth technique for the

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growth of a high-quality aluminum nitride template layer on silicon substrate [94]. 2.3.5. Application aspects of AlGaN UV PDs 2.3.5.1. Application of buffer layer for improved AlGaN film quality. The growth of thick and uniform Al content AlGaN layer with high crystal quality is difficult. AlGaN layers have large leakage current that is attributed to traps and dislocations. It has been reported that growing a buffer layer improves the crystal quality of the AlGaN layer and results in lower leakage current and higher detectivity. It must be care that for metal–insulator–semiconductor PDs, quality of the insulator layer has critical role. For this reason, optimization of buffer layer is important. MSM UV PDs based on Al0.16Ga0.84N/GaN heterostructures have been grown on sapphire (0001) by MOCVD [95]. There are optimized HT AlN intermediate layers by thicknesses ranged from 30 to 70 which result in high responsivity, fast response time, and very low dark current. Their responsivity was achieved 0.2 A/W under –10 V bias at a cut-off wavelength of 330 nm. Response time was 15.4 ns, and the dark current was as low as 3.1 pA/cm3 under zero bias. It was found that the insertion of Al0.2Ga0.8N/GaN superlattices improved the crystal quality of AlxGa1  x N/GaN heterostructures [96]. This detector exhibited dark current of 0.1 nA at  5 V, a responsivity of 0.097 A/W at zero bias, and a specific detectivity of D* ¼8  1013 cm Hz1/2 W  1 at λ ¼290 nm. AlGaN/GaN MIS UV phtodetectors using photo-CVD SiO2 as a current suppressing layer was fabricated and an AR-coating layer was used to obtain a high UV/visible rejection contrast PD [97]. Photocurrent to dark current contrast ratio was 1.27  104. It was also found that UV-tovisible rejection ratio was more than 3 orders of magnitude and the responsivity in 350 nm incident light wavelength was 0.144 A/W with a 5 V applied bias. AlN interlayer have important role in improving photocurrent and leakage current of AlGaN layer due to it reduces optical defects. UV detectors with AlxGa1–xN/GaN heterostructures have shown a low dark current of 0.1 nA at  1 V, a responsivity at zero bias of 0.150 (0.140) A/W at 300 (250) nm, and a value of specific detectivity of 1.28  1014 cm Hz1/2 W  1 at 300 nm [98]. 2.3.5.2. Defect free AlGaN based materials: different substrates. Crystal quality of the GaN epitaxial layers grown on various substrates is different. Also orientation of substrate is important key in determining crystal quality. Therefore, AlGaN with different substrates is investigated. AlxGa1  xN (0 oxo0.35) Schottky photodiodes on Si (1 1 1) have been fabricated and characterized which responsivity cutoffs were from 365 to 295 nm [99]. Photoconductors have shown that high responsivities (
10 A/W) for low Al contents (o10%), zero-bias responsivities from 12 to 5 mA/W (x¼0–0.35), a UV/visible contrast higher than 103 and a time response of
20 ns. AlGaN MSM PDs have been grown on Si (1 1 1) substrates by low-pressure MOCVD and low temperature (LT) AlN or LT GaN cap layers have deposited on top of the

undoped AlGaN layers [100]. The PD with LT GaN cap layer wasn’t shown reduction in dark current which observed from PDs prepared on sapphire substrates. With an incident wavelength of 305 nm and an applied bias of 5 V, it was reported that peak responsivities were 0.02, 0.005 and 0.007 A/W for the PDs with LT GaN cap layer and without cap layer, respectively. The corresponding detectivities were 2.2  1010, 1.36  1010 and 1.55  1010 cm Hz0.5 W  1, respectively. As before mentioned, heterostructure PDs with a 2DEG provide large optical gain. The high electron saturation velocity of such structure (
107 cm/s) results in a considerable reduction in the photocarrier transit time relative to its lifetime that it shows a high photoconductive gain. AlGaN/GaN heterostructure based on 2DEG PDs have been shown responsivity of 8.7  106 A/W With an incident light wavelength of 270 nm [101]. The measurement of absorption in a sub-band gap energy range is the most direct way to obtain information about defect structure. Studies have shown two resonance absorption peaks for detectors grown on sapphire and silicon substrates that centered at 2.25 eV and 3.6 eV, respectively. The resonance absorption at 3.6 eV attributed to the carbon-related deep acceptor–shallow donor transition [102]. 2.3.5.3. Time response of AlGaN UV PDs. Time response of AlGaN UV photoconductive detectors is found to be strongly related to the grain boundary density in the epilayers because of effects on hole trap and electron barrier. Improvement of the V/III ratio of the crystal– nucleus coalescence can be shortened response time of the detector about an order of magnitude. The carrier lifetime can be determined by simulation of the voltage dependent responsivity [103]. This study has been done for solar blind photoconductors using AlGaN epitaxial layers with aluminum fraction of 49.6% and 54.1%. The cutoff wavelength was as low as 275 and 271 nm, respectively. The carrier lifetime of these detectors were 0.15 ms for x ¼0.49 and 0.13 ms for x ¼0.54. Also, carrier lifetime of AlxGa1  xN (0 o x o 1) PDs experimentally measured [104]. In this letter, the effective majority carrier lifetime derived from frequencydependent photoconductivity measurements. A thin undoped AlN layer was deposited at high temperatures prior to the growth of AlxGa1  xN layers as buffer layer. The maximum detectivity has been reported 5.5  108 cm Hz1/2/W at a modulating frequency of 14 Hz. The carrier lifetime has been estimated to be from 6 to 35 ms for AlxGa1  xN (0 o x o 1) PDs. 2.4. AlInGaN UV PDs Binary GaN materials can be used for fabrication UV PDs due to its absorption edge of 360 nm, however, ternary AlGaN materials is used to shorten their cut-off wavelengths below 280 nm. Unfortunately, the lattice mismatch between AlGaN and GaN in the AlGaN/GaN heterostructure results in creation cracks and dislocations in the surface of AlGaN layers and therefore low quantum efficiency achieves. The band gap energy of AlInGaN varies

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Fig. 5. Responsivity comparison of In0.02Al0.15Ga0.83N with Al0.14Ga0.86N (with permission from the publisher and authors) [106].

from 1.95 to 6.2 eV by varying the composition of the alloys, therefore using the near-matched quaternary Inx AlyGa1 x yN materials results in crack-free structures in the short wavelength region [105,106]. Hence the responsivity of InxAlyGa1 x yN is greater than that of AlGaN material. Fig. 5 shows the normalized photoresponse versus wavelength for these two samples. It was shown that the response of the In0.02Al0.15Ga0.83N quaternary alloy was about five times greater than that of Al0.14Ga0.86N of comparable cutoff wavelength. Recently, the Ni/Ir/Au multilayer contact was used to decrease leakage current and better performance of AlInGaN PDs [105]. Deposition of high work function metal (Ir) that results in the formation IrO2, improves performance of PDs. it was found that the dark current densities of PDs with a Ni/Au conventional contact and a Ni/Ir/Au multilayer contact were 3.7  10  8 A/cm2 and 8.3  10  9 A/cm2, respectively. Also, InGaN/GaN MQW structure was used to shift cutoff wavelength and enhance responsivity of p–n junction PDs [18]. These PDs have exhibited a 20 V breakdown voltage. The photocurrent to dark current contrast ratio was higher than 105 by applying 0.4 V reverse bias. The maximum responsivity was 1.28 and 1.76 A/W with a 0.1 and 3 V applied reverse bias, respectively. Recently, MSM UV PD has been fabricated based on the Al0.40 In0.02Ga0.58N film on GaN buffer layer. At 10 V bias, the PD has been shown a peak responsivity of 0.065 A/W at 295 nm with a cutoff wavelength at 310 nm. The UV–visible rejection ratio (R295 nm/R450 nm) was more than two orders of magnitude at 10 V bias. Another response peak at 360 nm corresponding to the band gap of GaN has been detected which proved dual band detection of PD in UV region [107].

3. ZnO based UV PDs ZnO is one of the most important II–VI group semiconductors with a wide direct band gap (3.37 eV) and high exciton binding energy (60 meV) at room temperature, which enable it very promising material for the UV detection. Wide band gap semiconductors such as ZnO, GaN, ZnSe, ZnS, and 4H-SiC have shown similar properties with their crystal structures and band gaps. Initially, ZnSe and GaN based technologies made significant progress in the blue and UV optoelectronic devices such as light emitting diode and injection laser. No doubt, GaN is considered to be the best candidate for the fabrication of optoelectronic devices. However, ZnO has great advantages for light emitting diodes and laser diodes over the currently used semiconductors. Recently, it has been suggested that ZnO is promising for various technological applications, especially for optoelectronic short wavelength devices due to its wide and direct band gap. Furthermore, ZnO has very excellent radiation hardness which becomes it very suitable for using in harsh environments. Its radiation-resistive is more than Si, GaAs, SiC and GaN [108]. Also, it is used in surface acoustic wave devices, pyroelectric devices, gas sensors, combustion monitoring, varistors and transparent electrodes. ZnO UV PDs with various structures has been fabricated. In the following sections, a review of recent progress in these structures has been exhibited. ZnO single crystal microtubes were used to fabrication UV PDs [109]. Responsivity of this PD was about 6.2 A/W under 5 V bias in air. Also, photoresponsivity of the ZnO microtubes strongly depended on the ambient atmosphere. Higher peak current with slow response time obtained in pure nitrogen or in pure argon atmosphere

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and lower peak current with fast response time obtained in pure oxygen or in air. Treading surface of ZnO thin film significantly changes the photoconductivity, photocurrent and the photoresponse time constants [110]. Investigation has been shown that adsorption/desorption of oxygen with surface states on ZnO is crucial, many attempts have been made to modify the surface states and improve ZnO thin film properties. For this purpose, the influence of trap levels on the slow and fast rising components of the photoresponse has been investigated. The magnitude of the photocurrent and the rise time are found to decrease considerably with increasing number of trap levels. A slow rise in the photocurrent directly relates to the adsorption and desorption of oxygen on the film surface, and the fast rise is due to a bulk-related phenomena involving embedded oxygen. ZnO bilayer PDs with ZnO film and different metals (Cu, Al, Sn, Au, Cr, and Te) on platinum interdigital electrodes have been designed and tested [111,112]. Photoresponse of these configurations were reported four to seven times greater than the pure ZnO thin films. Sn (30 nm)/ZnO sample has had highest responsivity of
8.57 kV/W and Te (20 nm)/ZnO structure have shown highest sensitivity of
31.3  103 compared to ZnO thin film. Many applications require a low-cost and large-scale mode of flexible electronics with reasonably high photoresponse. Recently, ZnO UV sensor has been reported that it made on common pencil drawn circuit over a paper. The core of a pencil is made of graphite, where graphene sheets are connected together by weak van der Waals attractions. In this work, pencil drawn interdigitated electrodes and screen printed ZnO nanocrystals have been used to achieve a UV sensor on paper. ZnO nanocrystal paste was screen printed onto the pencil drawn interdigitated circuit. The authors claimed that these sensors demonstrate characteristics comparable to those made with complex and expensive procedures [113]. 3.1. ZnO based MSM UV PDs Several challenges exist for obtaining reliable p-type ZnO that limit the realization of p–n junction-based devices. For this reason, MSM structured UV detectors with either Schottky or Ohmic contacts have been interested. MSM detectors exhibit high gain which attributes to the trapping of hole carriers at the semiconductor–metal interface. Despite high gain, MSM structured UV detectors based on ohmic junction have very long decay time and large dark current. As a result, Schottky type detectors are more attractive due to their high gain, high speed and low noise performance. Firstly reported ZnO MSM structure is based on sol–gel synthesized ZnO: Al thin film that it has been shown that at illumination wavelength of 350 nm, photocurrent has been reported 58.05 mA at a bias of 6 V [114]. Optimum device geometric parameters and properties of the ZnO film determine performance of the ZnO MSM PD. It has been shown that geometry parameters variations including the finger spacing, finger width and finger length directly affect dark current, carrier's transit time

and device capacitance [115]. Moreover, there is a tradeoff between device geometry parameters according to the application of the device. Responsivity has a great significance in determining the performance of a PD. Therefore, many research have been focused to improve the responsivity of a PD. It was reported that by trapping many carriers at the metal semiconductor interface in MSM structure, the responsivity of the PD is greatly improved [116]. In fact, when the PD receives one photon, many carriers generates. This route is used to obtain high responsivity and the authors have reported that they obtained the highest responsivity of 26,000 A/W at 8 V bias of Au/ZnO/Au MSM structure. Different substrates were used to fabricate MSM UV PDs. For example, quartz substrate was used to fabrication of photoconductive UV detector with MSM structure based on c-preferred oriented ZnO film by RF magnetron sputting [117]. With a 3 V bias, responsivity of detector has been 30 A/W at 360 nm. Rise time of PD was 20 ns and PD has been shown slow decay time of
10 ms as a result of excess lifetime of trapped holes. In another work, ZnO films were deposited on sapphire substrates by MOCVD at three different temperatures and UV PDs with a MSM structure have been fabricated [118]. It was reported that application of the diamond film as substrate improves the performance of ZnO UV detectors for high-temperature and high-power electronic applications [119]. Therefore, highly c-axis-oriented ZnO thin films have been used to fabricate MSM structural UV PDs on the freestanding diamond substrate by RF reactive magnetron sputtering. Dark current was as low as 6.6 nA and photocurrent was as high as 196.8 nA under 10 V bias voltage. This research has shown that the low dark currents and high photocurrents of PDs were related to the formation of bigger grain size of ZnO films. In MSM UV PDs, large Schottky barrier height at metal– semiconductor interface results in improved responsivity and photocurrent to dark current contrast ratio that are critical keys to achieve high performance. Therefore, metals with high work functions should be used to obtain large Schottky barrier height on ZnO [120]. Different electrode contacts were used to fabrication MSM UV PDs. Here, some of them are discussed. ZnO-based MSM PDs with Pd contacts have been grown on sapphire (0001) substrates which have exhibited 0.701 eV barrier heights for electrons [121]. For a given bandwidth of 100 Hz and an applied bias of 1 V, noise equivalent power and corresponding detectivity D* have been reported 1.13  10  12 W and 6.25  1011 cm Hz0.5 W  1, respectively. At 370 nm incident wavelength and with applied bias of 1 V, responsivity for the PDs was 0.051 A W  1 corresponding to a quantum efficiency of 11.4%. The maximum responsivity of the Ni/ZnO/Ni MSM PD on poly propylene carbonate (PPC) plastic substrate has been found to be 1.59 A/W [122]. The measurement of electrical characteristics of the detector has been shown that the dark- and photo-current were 1.04 and 93.80 mA at 5 V respectively. Photo to dark current contrast ratio of this PD was 90.19 times. Also, Pt/ZnO/Pt MSM UV PD has been fabricated by DC sputtering deposition of ZnO thin films on PPC plastic substrates [123]. Platinum (Pt) is a stable Schottky contact which has the

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highest metal work function of 5.65 eV [124]. Dark current and photocurrent of ZnO MSM PD have been reported 0.7957, 126.5163 A, respectively and the photocurrent to dark current contrast ratio has been measured of 159. For comparison, in another work, fabrication of ZnO-based MSM sensors with different (Ag, Pd and Ni) contact electrodes has been investigated [125]. Barrier height at Ag/ZnO, Pd/ZnO and Ni/ZnO interfaces were found to be 0.736, 0.701 and 0.613 eV, respectively. For a given bandwidth of 100 Hz and 1 V applied bias, the corresponding normalized detectivity were (D*) of 1.04  1012, 6.25  1011 and 1.1  1011 cm Hz 0.5 W  1 and the maximum responsivity,in an incident wavelength of 370 nm and 1 V applied bias, were 0.066, 0.051 and 0.09 A/W (Fig. 6), which correspond to quantum efficiencies 17.3%, 11.4% and 23.8%. As you can see, the PD responsivities were nearly constant in the below band gap UV region (300–370 nm) while sharp cutoffs with a drop of 2–3 orders of magnitude occurred at approximately 370 nm.

17

Au-electrodes have been used to fabrication of Schottky type ZnO MSM UV detector on SiO2 substrate by the RF magnetron sputtering method [126]. With the incident wavelength of 360 nm at 3 V operating bias, the responsivity has been 0.337 A/W and the dark current was about 1 nA under 3 V bias. The fast rise time and fall time were 20 ns and 250 ns, respectively. It is important that Au and Ag Schottky contacts have serious problems at elevated temperatures higher than 330 K [120]. Because of severe inter-diffusion at metal–ZnO interface, an insulating layer between metal and the underneath ZnO is used which can effectively suppress leakage current of the PDs. Comparing ZnO-based MIS and MSM PDs has been shown that smaller dark current, larger photocurrent to dark current contrast ratio and larger UV to visible rejection ratio from the ZnO MIS UV PD can be obtained [127]. Photocurrent to dark current contrast ratios have been reported 2.9  102 and 3.2  104 for MSM and MIS, respectively. It was mentioned that responsivity of ZnO MSM and MIS PDs have been 0.089 and 0.0083 A/W respectively at 370 nm incident light wavelength and with 5 V applied bias (Fig. 7). The smaller responsivity observed from ZnO MIS PD were attributed to the insertion of highly resistive SiO2 layer. 3.2. Schottky barrier ZnO UV PDs

Fig. 6. Responsivity of ZnO MSM UV PDs with different contact electrodes (with permission from the publisher and authors) [125].

Several groups have reported the different Schottky contacts on ZnO thin films by using Pt, Pd, Au, Ag and so on. Among them, studies on Pt as Schottky metals for ZnO films has been shown a relative rare properties that a sublayer would allow ZnO films re-growth [128]. Schottky UV photodiode has been reported which consisted of a hydrothermally grown (0001) ZnO single crystal, a semitransparent Pt film for the Schottky electrode and an Al thin film for the Ohmic electrode [129]. The photodiode had polarity dependences on current–voltage characteristics and on responsivity. At the wavelength of 365 nm and Schottky electrode on the zinc surface, responsivity was 0.185 A/W (higher responsivity) and for oxygen surface,

Fig. 7. Photoresponsivities of the (a) ZnO MSM and (b) MIS PDs (with permission from the publisher and authors) [127].

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the responsivity was 0.09 A/W. The authors attributed these results to the polarity dependences of surface chemical reactivity and the surface state density on ZnO surfaces. Also, PEDOT: PSS thin film has a resistivity of as low as 10  3 Ω cm and a large work function of 5.0 eV. This film is transmittance in a wavelength range from 250 to 800 nm that covers UV range [130]. Thus PEDOT: PSS can be used as a transparent Schottky contact on ZnO. ZnObased Schottky UV photodiode using PEDOT: PSS as a transparent metal electrode has been fabricated with a simple spin-coating process at room temperature in air [131]. Visible rejection ratio was about 103 under zero-bias condition. Photodetectivity was 3.6  1014 cm Hz1/2/W at 370 nm. Iridium (Ir) as a stable Schottky contact has been used for GaN based UV PDs. The thermal reliability of Ir contacts on ZnO and temperature-dependent characteristics of ZnO-based Schottky diodes with Ir contact electrodes have been investigated [132]. Norde model has been shown that the effectively Schottky barrier heights of Ir on n-ZnO have been around 0.837, 0.829, 0.801, 0.750 and 0.719 eV when measured at 25, 30, 50, 100 and 150 1C, respectively and Cheung's method has been shown that Schottky barrier heights between Ir and the n-ZnO were 0.824, 0.823, 0.789, 0.743 and 0.740 eV when measured at 25, 30, 50, 100 and 150 1C, respectively. However, both Norde model and Cheung's method have been reported that the Schottky barrier heights have been larger than 0.8 eV at room temperature. In another work, a back-illuminated vertical-structure ZnO UV detector has been fabricated [133]. This structure was produced by using an ITO electrode on a quartz substrate. Responsivity of PD was 1616 A/W at 5 V bias, with a rise time of 71.2 ns and a decay time of 377 μs. Also at this bias, the photocurrent was 16.8 mA under UV illumination (365 nm, 10 μW) and the dark current was as high as 640 μA. Recently, a Schottky junction UV PD has been fabricated by coating a free-standing ZnO nanorod array with a layer of transparent monolayer graphene film. PD exhibited quick response of millisecond rise time/fall times with excellent reproducibility. Also, it was capable of monitoring a fast switching light with a frequency as high as 2250 Hz [134]. 3.3. p–n and p–i–n and avalanche ZnO UV PDs As a result of native defects, as-grown ZnO is always ntype and fabrication of p–n heterojunctions of ZnO is under development because of difficulties attributed to growing of reproducible p type ZnO films. Hence, a lot of efforts on the fabrication of p–n heterojunctions have been reported by combining n-type ZnO with various other ptype materials. However, a major technical issue for these devices is a high leakage current due to imperfection of the heterojunction interface. Many efforts have been concentrated to find a p type material with small lattice mismatch that results in very low imperfections. Some of these works review in this section. Within last few years, many research papers published solely related to ZnO PDs. Due to space limitation, we tabulate the representative results on PD properties of

ZnO p–n heterojunction photodiodes recently reported, along with a brief description of the corresponding growth method, detection wavelength, and PD performance (Table 3). All these results indicate that RF sputtering, PLD, CVD, chemical routes, MBE and MOCVD are very suitable methods to fabricate ZnO-based UV PDs. However, the technology of ZnO-based materials and PDs is not yet mature. This should inspire more research efforts to address the challenges that remain. From the data exit in this table, we can still conclude that the ZnO QDs/PVK nanocomposites type PDs have the potential to achieve the lowest dark current densities and the highest photo to dark currents, the ZnO-ZnS nanocomposites type PDs have the potential to achieve the highest responsivities and multiwalled carbon nanotubes/ZnO nanowires/GaN nanocomposites have the potential to achieve the lowest response time. In addition, ZnO based p–i–n PDs have been reported [135]. They have demonstrated maximum responsivity of 8 A W  1 when the reverse bias reached 12 V that was considerable compared to other structured ones, such as MSM, Schottky and photoconductive type. In another work, nanocrystalline ZnO films have been used for fabrication An ITO/ZnO/NiO n–i–p heterostructure at room temperature by using oxygen ion assisted e-beam evaporation [136]. Average grain size of nanocrystalline ZnO films was 13 71 nm. Author has mentioned that due to using this nanocrystaline ZnO film, diode has exhibited lower leakage current level (
100 nA/cm2 at  5 V) and higher current rectification ratio (104–105 at 72 V) than those reported for bulk ZnO-based diodes. Furthermore, this PD has been shown spectral response that was suitable for low level of UVA (320–400 nm) detection. Recently, an impact ionization process, in which additional carriers can be generated in an insulating layer at a relatively large electric field, has been employed to increase the responsivity of a semiconductor PD. It is a general route to enhance the responsivity of a PD, thus may represent a step towards high-performance APDs [137]. For first time, ZnO based UV APDs have been fabricated from Au/MgO/ZnO/MgO/Au structures. An impact ionization process occurring in MgO insulation layer caused the carrier avalanche multiplication. The responsivity of PD was reported 1.7  104 A W  1, and the avalanche gain was about 294 at 73 V. The results reported in this paper may promise a route to high-performance avalanche ZnO UV PDs [138]. 3.4. Effect of annealing Studies have been shown that post-annealing affect crystalline quality, uniformity of ZnO films, modification of native defects and the electrical properties of ZnO films. For example, effect of thermal treatment on the performance of Au/Cr/SiO2/ZnO/Al Schottky barrier MIS PD has been investigated [163]. It has been shown that electrical and optical performance of PD improves for annealing temperature that is restricted up to 250 1C and performance of device degrades dramatically for annealing temperature beyond 250 1C. Post-annealing effects on the electrical properties of Pt/ZnO thin films contacts lead to exploring Pt contact in

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Device structure

Fabrication method

Electrode Light of detection Dark current

ZnO–Si n–p ZnO–NiO n–p

MBE RF sputtering

Ti/Au Al

378 nm 365 nm

1 nA at  2 V 6.64  10  8 A/cm2 at  2 V

ZnO–CuO n–p ITO-ZnO–NiO p–i–n

Hydrothermal e-Beam evaporation

ITO

365 nm

ZnO–SnO2 n–p

Electrospinning

ITO

100  10  9 A/cm2 at  5 V 1.7 pA at 10 V

ZnO (NW)-GaN n–p

MOCVD

ITO

300 nm, 0.45 mW/cm2 365 nm

ZnO:Mn–GaN p–n ZnO NR–GaN: Mg n–p

RF sputtering MOCVD

Au ITO

365 nm 360 nm

n-GaN/i-ZnO NR/n-ZnO:In p–i–n ZnO–ZnS nanocomposite

Vapor cooling condensation Thermal evaporation

ZnO (NW)–Si n–p

5

0.58  10 A at 5 V 2.95 μA at 3 V

5 pA at 5 V 0.67 mA at 5 V

MOCVD

320 nm, 0.91 mW/cm2 365 nm

ZnO (NR)–Si n–p

Aqueous method

365 nm

ZnO (NR)–Si n–p ZnO:Li–Si p–n

CVD Hydrothermal

375 nm 385 nm, 30 W

ZnO (NR)-polyfluorene n–p

Electrochemical

Au

350 nm

ZnO NW-polyaniline n–p

Au

150 mW cm 2

ZnO tetrapod-PEDOT:PSS n–p

Chemical solution deposition CVD

Ag

325 nm, 0.16 mW 1 pA at 0 V

ZnO NRs-PET polymer

Aqueous solution

ZnO QD/PVK-PEDOT:PSS p–n

CVD

Al

365 nm, 70 mW/cm2 365 nm, 6 W

MgZO–ZnO p–n

PE-MBE

In, Ni/Au

330 nm

MgZO–ZnO p–n

Sputtering

Au

297 nm

ZnO QD/graphene nanocomposite

Wet spin coating

ZnO–SiO2 nanocomposite

Sputtering

ZnO (NW)/graphene Schottky

CVD

Multiwalled carbon nanotubes/ZnO (NW)/ p-GaN composite ZnO–CdS core/shell

Hydrothermal Hydrothermal

365 nm, 6 W MSM

0.1084 mA at 5 V

7.98  10  11 A at 0.4 V 30  10  9 A at  5 V

8.32  10  1 mA 3.5  10  13 A 0.05 mA at 2 V 0.33 nA at  5 V 12.9 nA

Response time

Ref. [139] [140]

UV to vis: 4100 Current rectification ratio 4104 at 7 2 V Photo to dark current: 4600 at 10 V Photo to dark current: 15 at  5 V

[141] [136]

UV to vis: 66

Rectification ratio: 1.6  102 at 4 V Rectification ratio: 361 at 73 V UV to vis ratio: 1000 Photo to dark current: 364% Photo to dark current: 104 at 0 V Minimum rectification ratio at
93 mW cm  2 On to off ratio: 1100

Photo to dark current: 108 at 0.4 V UV to vis: 42

Rise time: 32.2 s decay time: 7.8 s

[142] [143]

276 A/W at 2 V 4.6  102 A/W at 5 V 1461 A/W at 5 V 5.0  105 A/W Rise time: o0.3 s decay at 5 V time: 1.7 s

[144] [145] [146] [147] [148]

0.3 A/W

[149]

1.4 A/W at 1 V

[150] [151]

0.18 A/W at -2 V

[152] [153] Rise time: 3.5 s decay time: 4.5 s Response time: 100 s decay time: 120 s

2.16 mA/W at -5 V Photo to dark current: 0.45 A/W at -5 V Rise time: 4 s recovery 20 at  5 V time: 4 s Photo to dark current: Rise time: 2 s decay 1.1  104 time: 1 s UV to vis ratio: 1.75  105 3.86 A/W at -10 V Photocurrent on/off ratio: 800 Rise time: 0.7 s recovery time: 0.5 s UV to vis: 511 at 7 2 V, Photo 6 mA/W Rise time: 5 ms, fall to dark current: 300 at  15 V time: 6 ms Photo to dark current: 12 Response time: 0.5 s

[154] [155] [156] [157] [158] [159] [108] [160] [161] [162]

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240 nm, 186.53 mW/cm2 graphene 325 nm, 100 mW/mm2 Au 335 nm, 31.65 mW/cm2 372 nm, 6.36  10  5 W/cm2

1.17  10  7 A at  4 V

Responsivity

UV to vis: 45 Photo to dark current: 11.56

UV to vis: 1.7  103 at  5 V

360 nm Cr/Au

Rejection ratio

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Table 3 Recent ZnO-based heterojunction UV PDs.

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zero-biased PD [128]. ZnO films oriented along c-axis have been deposited by pulsed laser deposition on Pt/Ti buffer layer and SiO2/Si substrate. The largest barrier height of this PD was 0.8 eV and the reverse leakage current was 1.5  10  5 A/cm2. At zero biased, PD has been shown responsivity of 0.265 A/W at 378 nm and fast photoresponse with rise time of 10 ns and fall time of 17 ns. Recently, it was reported that the degraded performance of annealed ZnO-based PD can be recovered by embedding Ag2O nanoparticles resulted from the transformation of as-deposited Ag layer. The dark current of ZnO PD could be suppressed by silver oxide nanoparticles after thermal treatment. The rejection ratio of thermaldegraded ZnO-based PD can be recovered by inserting silver oxide nanoparticles. ZnO PD with the appropriate Ag2O nanoparticles possesses the best rejection ratio [164]. 3.5. Doping in ZnO It has been shown that doping in ZnO can change its properties which are useful in some applications. Recently, several findings have been reported in this field. Here, some of them are investigated. Sb doping in ZnO nanobelts result in depressing of recombination rate of the photogenerated electron–hole pairs in the Schottky barrier and reducing electrical resistivity of UV PD [165]. Response time of these PDs has been found to be less than 100 ms and without external bias voltages, sensitivity of photoresponse has been reported 2200%. Photoconductor devices with a high-quality Ga-doped ZnO film grown on an R-plane sapphire substrate and Al/Ti Ohmic contacts have been reported [166]. Peak responsivity was 1.68 A/W at 20 V bias for 374 nm light. Because of presence of deep levels, slow transient responses, i.e. rise time of 95 s and fall time of 2068 s have been observed. C-axis oriented Ga-doped ZnO (ZnO: Ga) thin film has been deposited on quartz by radio-frequency magnetron sputtering and a MSM photoconductive detector has been fabricated on it [167]. With a 10 V bias, responsivity has been reported about 2.6 A/W at 370 nm. Measurement of time photoresponse has been shown a rise time of 10 ns and a fall time of 960 ns. In another work, Ga doped ZnO photoconductive UV detector has been grown by spray pyrolysis on glass substrates [168]. Responsivity was about 1125 A/W at 5 V bias at 365 nm peak wavelength. Indium-doped zinc oxide field effect transistor has been fabricated by employing inkjet printing [169]. The authors suggested that with this method a low cost highly sensitive UV PD has been obtained. Current on/off ratio of this device was 104–105. By illuminating with 363 nm, 1.7 mW cm  2 UV light, photocurrent was 2 mA. Dark current was
20 nA at gate voltage of 2 V. Rise time and fall time of this device were about 5 ms and 10 ms, respectively. Researches on the Al-doped ZnO thin films previously have been concentrated on the applications of the transparent and conductive thin films. Recently, it has been shown that the Al-doped ZnO is a promising material for fabrication fast photoresponse UV detectors. Researchers have deposited the Al-doped ZnO thin films on quartz substrates by radio frequency magnetron sputtering

technique and then annealed them in oxygen to improve the performances of the devices [170]. At 6 V bias, responsivity of the devices was higher than 4 A/W in the wavelength shorter than 350 nm. Device has been shown fast photoresponse with a rise time of 9 ns and a fall time of 1.2 μs.

3.6. Piezoelectric property and surface acoustic wave UV PDs based on ZnO ZnO thin films have piezoelectric property that can be used for fabrication surface acoustic wave (SAW) PDs. A PD can be realized by a semiconductor and SAW device. Semiconductor absorbs the incident light and generates electron–hole pairs. When free carriers propagate in SAW, their velocity reduces due to interacting with the electric field and piezoelectric stiffening, resulting in a phase shift and time delay across the SAW device, therefore detecting UV light. SAW devices are passive that is important for low power sensors in particular UV PDs. Therefore, the ZnO SAW UV detector can be used as a passive zero-power remote wireless UV sensor. Acoustoelectric interaction between the photogenerated charge carriers and the potential associated with the acoustic waves lead to a change in insertion loss and frequency of operation [171]. By UV illumination, the SAW UV sensor has been exhibited a downshift in frequency of
45 kHz, and a change in insertion loss
1.1 dB which have confirmed that this device is promising for low cost wireless SAW UV sensors. Recently, dynamic responses of SAW array UV PDs under illumination of different optical wavelengths have been demonstrated. Response time of the SAW UV sensors are all extremely fast under various UV light illumination while it takes longer to recovery at 350 nm. However, regardless recovery time, the differences between on and off were all larger than 10 dB during each on-off cycle, which means the value of insertion loss difference between UV on and off was significant [172]. The piezo-phototronic effect is about the use of the piezoelectric potential created inside some materials for enhancing the charge carrier generation or separation at the metal–semiconductor contact or pn junction. Recently, a core–shell ZnO–CdS micro/nanowire with the CdS NW array as the shell and a ZnO micro/nanowire as the core have been used to fabrication of UV/vis PDs. The authors mentioned that upon illumination of visible and UV light, performance of PD can be both further enhanced for more than 10 times with the participation of the piezophototronic effect when the device was subjected to a  0.31% compressive strain. It was due to modulation of the Schottky barrier heights at the source and drain contacts [173]. Also, a core–shell ZnO–CdS micro/nanowire with the CdS NW array as the shell and a ZnO micro/ nanowire as the core have been used to fabrication of UV/vis PDs. The authors mentioned that upon illumination of visible and UV light, performance of PD can be both further enhanced for more than 10 times with the participation of the piezo-phototronic effect when the device is subjected to a 0.31% compressive strain that it was due

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to modulation of the Schottky barrier heights at the source and drain contacts [162]. 3.7. Nanostructure ZnO UV PDs ZnO-based nanostructures are one of the most attractive materials for UV PDs. Recently, much attention has been paid to ZnO nanostructures, such as nanowires, nanobelts, nanorods, nanotubes, nano-tripods, nanoribbon and nano-tetrapods that enhance performance of UV PDs based on nanostructure materials including high responsivity and high gain which are attributed to the large surface-to-volume ratio. Here, some of them and their advantages have been investigated. 3.7.1. UV PDs based on ZnO nanowires Nanowires have increased surface to volume which enable it a promising material for UV detection. Because of large surface area, the impact caused by lattice and thermal mismatches can be efficiently reduced in nanowires, thereby leading to the much improved device performance. Furthermore, the small footprint of nanowires renders themselves as promising candidates for the device-level integration with Si CMOS technology. Moreover, nanowires have many other advantages such as enhanced light absorption and trapping/confinement so that photosensitivity can be enhanced significantly. Additionally, nanowire PDs offer the possibility to separate optical absorption and carrier transport paths [174,175]. But, compared with the mature planar structure PDs, the nanowire PDs are still at a fundamental research stage. The currently investigated nanowires PDs typically possess a low responsivity (A/W). Developing nanowire PDs that can be integrated to Si CMOS technology, especially in the telecommunication wavelength range is urgent. ZnO nanowires and their heterostructures have been widely investigated as optoelectronic components for the emerging nanoscale devices in the past decade, such as nanowire PDs. Many aspects exit for fabrication ZnO nanowire based UV PDs and Different routes have been reported for synthesizing ZnO nanowires that each of them has an advantage. In this section, some of them are reviewed. Laterally-grown ZnO nanowire PDs on glass substrates have been reported [176]. Cutoff of the fabricated PD was at
360 nm. At wavelength of 350 nm and with a bias of 0.1 V, photoresponsivity of the PD was 6.04  10  3 A/W and UV-to-visible rejection ratio was larger than 600. To make sensitive PD with improved performance, it is important to use a growing technique lead to closely packed ZnO films. Radio frequency cosputtering method was used to fabricate closely packed ZnO nanowire arrays on a n-type Si (100) as a substrate [148]. This photodiode has been shown a good rectification ratio above 1.6  102 at 4 V in the dark which was attributed to that this technique resulting highly closed packed ZnO thin film. It is necessary to solidly connect electrical contact pads to both ends of the nanowires because tips of the vertical ZnO nanowires were contacted softly with the underneath ITO/glass and vibration can disconnect them. It has been shown that by dispersing the nanowires onto precoated

21

electrodes or using electron beam lithography, it can be possible to fabricate electrodes on top of the nanowires. Recently, a vertical UV PD based on single ZnO NW sandwiched between two graphene sheets has been fabricated. Due to the existence of Schottky barrier between graphene and ZnO NW, the photoresponse performance was much higher than that of the traditional ZnO NW photoconductivity based UV detectors. Besides that, due to the excellent transparent property of graphene, the graphene/ZnO NW/graphene based UV detectors could have larger light absorption efficiency and effectual interface area than other ZnO NWs based Schottky barrier UV detectors utilizing metal electrodes [160]. Interlaced ZnO nanowires have been fabricated by bridging the gap of interdigitated gallium-doped ZnO pattern deposited on a silicon oxide (SiO2) thin film [177]. At wavelength of 375 nm, responsivity was 0.055 A/W with a 1 V applied bias. The noise equivalent power and normalized detectivity (D*) of PD were 2.32  10  9 W and 7.43  109 cm Hz0.5 W  1, respectively. Rise time and fall time of PD were 12.72 ms and 447.66 ms, respectively. It has been shown that dielectrophoresis (DEP)-fabricated ZnO nanowire UV photosensors could detect UV light down to 10 nW cm  2 intensity [178]. DEP is the electrokinetic motion of dielectrically polarized materials in non-uniform electric fields. ZnO nanowires can be trapped in the microelectrode gap where the electric field is higher and thus they can be aligned along the electric field line and bridged the electrode gap. Under UV irradiation, the conductance of the DEP-trapped ZnO nanowires exponentially increases as at higher UV intensity, the conductance response becomes larger. Therefore, they have higher UV sensitivity than ZnO or ZnO nanowires based UV PDs. Recently, researchers suggested that rational integration of ZnO wire and metal nanoparticles is a viable approach to improving the performance of ZnO nanowire PDs. It was shown that after covering with Au nanoparticles, the dark current of the ZnO NW PDs decreases by 2 orders of magnitude and the ratio of photo current to dark current increases. Also, decorated Au nanoparticles can drastically improve the response speed of ZnO nanowire PDs [179]. In another work, it was reported that Ag nanoparticles can enhance the performance of a ZnO nanowire-based UV detector. This enhancement was attributed to a Schottky barrier at the interface of Ag and ZnO because of the formation of a thin AgOx layer [180]. TiO2 nanoparticle effects on the surface of an aluminum-doped zinc oxide nanowire network have been investigated [181]. The aluminum-doped zinc oxide nanowires modified with TiO2 nanoparticles have shown approximately 2-fold faster response time and an approximately 3-fold higher UV photosensitivity than the bare nanowires. ZnO nanowires can be used as n type ZnO material in p–n structures. Different reports exit for p–n heterojunction of ZnO nanowires and another p type material. For example, GaN and ZnO have low lattice mismatch (
1.8%), therefore p–n heterojunction of ZnO and GaN is a promising structure for use in UV PDs. Furthermore, by using ZnO nanowires and thus increasing the surface to volume ratio,

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the quality of the junction between ZnO and GaN improves and the sensitivity of the devices enhances. A p–n heterojunction PD was reported that it fabricated by ZnO nanowires/p-GaN [143]. Dark current and photocurrent of this structure at a reverse voltage of  5 V were 0.58  10  5 and 7.49  10  5 A, respectively, and the ratio of illuminated current to dark current was almost 15. In another work, ZnO nanowire/n-Si heterojunction has been fabricated by deposition closely packed ZnO nanowire on n-type Si (100) substrate by a magnetron cosputtering method [148]. This device has been shown rectification ratio of above 1.6  102 at 4 V in the dark. Recently, multiwalled carbon nanotubes/zinc-oxide nanowires/p-GaN composite has been used to fabricate heterjunction UV PDs. Compared with ZnO/p-GaN photodiodes, this PD demonstrated significant rectifying characteristics and relative fast transient response with response time on the order of milliseconds. Also, a narrow bandpass photoresponse spectrum with FWHM of 13 nm has been realized by taking advantage of the GaN substrate as a filter. The enhanced device performance was attributed to improved carrier transport and collection efficiency through multiwalled carbon nanotubes network deposited on top of the ZnO NWs [161]. Also, p–n homojunction of ZnO nanowires has been reported [182]. In this letter, ZnO p–n homojunction composed of hydrothermally grown n-type ZnO nanowires array covered with p-type Al, N co-doped ZnO film has been deposited on transparent indium tin oxide (ITO) coated on glass substrates. The reverse leakage current and photo to dark current ratio have been reported
5 μA and 150 at bias of 3 V, respectively. The homojunction can act as a UV PD when light is shined from the glass side of the device. At illumination wavelength of 384 nm, UV to visible responsivity ratio (R384 nm/R550 nm) was
70 at a bias of 3 V. A smaller response at red light (λ at 600– 700 nm) was attributed to the presence of deep-level defects in the material. Recently, self powered ZnO NW UV metal-semiconductor PD integrated with a GaInP/GaAs/Ge solar cell has been reported. The solar cell transforms solar light to electrical power and provides a bias of 2.5 V to enhance the response of the detector. This PD has been shown rejection ratio of
218 and the measured responsivity is 3.39  10  4 A/W [154]. ZnO nanowire array UV PD with Pt Schottky contacts has been fabricated on a glass substrate. This PD has been shown a high sensitivity of 475 without external bias. This phenomenon has been explained by the asymmetric Schottky barrier height at the two ends causing different separation efficiency of photogenerated electron–hole pairs, which resulted in the formation of photocurrent under zero bias [182]. Recently, a novel conception of decreasing the concentration of oxygen vacancy for decreasing photoresponse time has been proposed that it achieved by a hydrothermal treatment. Performance instability originates from the interaction of oxygen vacancies near ZnO surface with the atmosphere during service. It was reported that by controlling the concentration of oxygen vacancies on the surface of ZnO-nanowire PD through a hydrothermal treatment in pure water can tune recovery time and

photosensitivity [183]. High-density La-doped ZnO nanowires have been grown hydrothermally on flexible polyimide substrate. It was reported that the dark current of the p-ZnO:La NWs decreased with increased relative humidity, while the photocurrent of the p-ZnO:La nanowires increased with increased relative humidity. The higher relative humidity environment has been improved UV response performance. In a water environment, the photo to dark current ratio of p-ZnO:La NWs has been reported 212.1 which was the maximum UV response [184].

3.7.2. ZnO nanorod UV PDs ZnO nanorods, compared with ZnO bulk materials, have been exhibited additional advantages for fabrication optoelectronic devices because they have increased junction area, the enhanced polarization dependence, and the improved carrier confinement in one dimension. It has been shown that nanorod UV PDs have been higher photoresponse and UV-to-visible rejection ratio than the traditional ZnO MSM PDs [185]. It has been attributed to the very large surface-to-volume ratios of ZnO nanorod arrays enhancing oxygen adsorption and desorption at the NR surfaces and thus suggesting superior materials for UV detection. Despite many advantages, ohmic contacts easily form at the electrode/ZnO nanorods interface which result in slow response and recover behaviors of PD [186]. For comparision, spectral responsivities of the ZnO nanorod and ZnO film MSM PDs with 5 V applied bias have been compared in Fig. 8 [185]. The ZnO nanorod MSM PDs show much higher photoresponse than the traditional ZnO MSM PDs (41.22 A/W, 0.13 A/W). It can be attributed to the defects of nanorods surfaces, which is the origin of internal gain exists in the nanorod device [187]. ITO/ZnO, poly-nvinylcarbazole (PVK) hybrid layer/ PEDOT: PSS/Au UV PD has been reported. In which ITO glass and PVK acted as filters to make a cut-off in the short-wavelength region (shorter than 350 nm) [188]. Therefore, PD has been shown a narrow band centered at 364 nm with a FWHM of only 26 nm. At  5 V bias, current density was 132 μA/cm2 and corresponding photoresponse has been reported 110 mA/W. Single-crystalline ZnO nanorods have been grown on glass substrates by a hydrothermal method and then by

Fig. 8. Spectral reponsivities of the ZnO (nanorod and film) MSM PDs at 5 V applied bias (with permission from the publisher and authors) [185].

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the in-situ lift-out technique in a focused ion beam (FIB/ SEM) instrument, a single ZnO nanorod-based PD has been fabricated [189]. At illumination wavelength of 370 nm and with an applied bias of 1 V, the responsivity of the single ZnO nanorod was 30 mA/W which was promising for fabrication nanoscale PDs and nano-optics applications. Also, the holes on a polymeric template that is fabricated by nanoimprint lithography can used to growing single crystalline ZnO nanorod in each hole as a one hole–one rod configuration. Therefore, equal number of ZnO nanorods in each pixel which is suitable for an integrated photodectector array circuit. With this method, (ITO)/ZnO nanorod/platinum PD array has been fabricated [190]. Responsivity was 4381.4 A W  1 at 365 nm and a UV-tovisible rejection ratio of 83 has been achieved from a 2 μm  2 μm PD pixel areas. Because of formation of ohmic contacts at the electrode/ZnO nanorods interface, many researchers have investigated the Schottky contact between ZnO nanorods and metal. A simple route to gain Schottky barrier is deposition metal electrodes on the top of different oxide material-coated n-ZnO nanorods on different seed layers. For example, the Schottky barrier PD based on vertically oriented hexagonal ZnO nanorod arrays were hydrothermally grown on F-doped SnO2 (FTO) substrates and the Au electrodes [191]. Photocurrent was around 6.71 mA at the applied voltage of 0.4 V. Also, effect of annealing on Schottky contacts at the electrode/ZnO nanorods interface has been shown that with increasing annealing temperature up to 300 1C, responsivity increases [186]. At 254 nm the biggest responsivity of
4.5 A/W has been obtained with the forward bias of 2 V. Also, the ratio of D*254 to D*546 has been calculated as high as 103. P–n junction PD based on ZnO nanorods and another material was reported [152]. A near-UV detector based on a pn-heterojunction consisting of p-type polyfluorene polymer and n-type ZnO nanorods have been shown responsivity of 0.18 A/W at 300 nm by applying a bias of 2 V. Recently, self powered ZnO-nanorod/CuSCN photodiodes have been fabricated. These devices exhibited a photocurrent at zero bias, creating a self-powered PD. A photocurrent response of 30 μA (at 6 mW cm  2 irradiance) was measured, with a rise time of
25 ns, and sensitivity to both UV and visible light (475–525 nm) [192].

3.7.3. ZnO nanoparticles UV PDs Increasing in surface-to-volume ratio leads to improvement in the photoresponse behavior so that it is interesting to investigate photoconductivity in a system where in which the nanostructures have been used. Also, quantum confinement effects lead to continuous tuning of the emission and detection wavelengths and improved device performance. Nanoparticles have excellent absorption efficiency, therefore, the number of photogenerated charge carriers is increased and the number of charge carriers which jump over the barrier will increase, resulting in an enhancement of the photocurrent and photo to dark current ratio. Also, because of numerous junction barriers formed between nanoparticles, the dark current dramatically reduces.

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UV PDs based on ZnO nanoparticles have been shown the photo to dark current ratio of 106 and the responsivity of 0.1 mA/W at 1 V bias [193]. The rise and decay time constants were 48 s and 0.9 s, respectively. It is confirmed that the adsorption and readsorption of oxidizing gas molecules result in increasing and decreasing of the photocurrent, respectively. A variety of high-quality ZnO nanostructures namely well-aligned nanorods, tetrapod-like nanorods, and hairlike nanowires were synthesized on Si (100), porous silicon (PS/Si), and quartz substrates, respectively and Pd/ZnO/Pd MSM PDs have been successfully fabricated. This study demonstrated that ZnO nanowires/PS exhibited a relatively fast photoresponse, with a rise time of 0.089 s and a fall time of 0.085 s. The ZnO nanorods/Si and ZnO nanotetrapods/quartz exhibited a slow response with rise times of 0.128 and 0.194 s and fall times of 0.362 and 0.4 s, respectively. The study suggests that the response time of the ZnO nanostructures to UV exposure is dependent on the type of substrate used [194]. As previously mentioned, like ZnO nanowires, dielectrophoresis has proven to be a versatile method for assembling of ZnO nanoparticles to form functional submicron ZnO-nanoparticle cluster arrays for UV sensing [195]. Also, it was reported that ratio of photo to dark current of the polyvinyl-alcohol surface passivated zinc oxide nanoparticle PD was 5 times higher than that of the devices fabricated using uncoated ZnO nanoparticles [196]. The hybrid PVK–ZnO quantum dot composites with incorporating a graphene layer have been used to fabrication UV PDs that combined advantages of the high sensitivity of the ZnO quantum dots and the high conductivity of the grapheme [156]. These UV PDs have the photo to the dark current ratio of approximately 108 under illumination with 365 nm light and with applied bias of 0.4 V. Also, fabrication of UV PDs has been reported utilizing colloidal ZnO QDs on graphene/PET sheets. These PDs under illumination with a 365 nm wavelength at 3 V have been shown ratio of the photo to the dark current of approximately 1.1  104. The rise and the decay times of the PD were 2 and 1 s, respectively [159]. 3.7.4. Other types of nanostructure ZnO UV PDs ZnO nanowall network MSM PDs have been fabricated by plasma-assisted MBE on Si (111) substrates [197]. The UV (360 nm) to visible (450 nm) rejection ratio was around two orders. The dark current was below 6 μA with 5 V bias and PD has been exhibited peak responsivity of 15 A/W at 360 nm. The self-assembled nanowire-nanoribbon junction arrays have been reported [198]. They were grown by thermal evaporation of the mixture of ZnO and SnO2 powders at 1300 1C through a vapor–liquid–solid process. The junction arrays of these ZnO nanostructures have ultrahigh surface sensitivity and they are candidates for building sensors with ultrahigh sensitivity. In future, these structures certainly receive much attention and newly UV PDs based on these nanostructures will be reported. Recently, self powered UV PD based on a single ZnO tetrapod and PEDOT:PSS heterostructure has been constructed. At zero bias, the detector has been shown an

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on/off ratio of
1100, a rise time of
3.5 s and a decay time of
4.5 s when the 325 nm UV (0.16 mW) illuminated the p–n heterojunction. The self-powered properties were driven by the photovoltaic effect. Furthermore, selfpowered properties have been found when the UV irradiated the ZnO tetrapod only. The short-circuit current and the open-circuit voltage decreased with the increasing distance between the illuminated spot at the ZnO tetrapod and the heterojunction [199]. 4. MgZnO UV PDs The MgxZn1  xO material has unique properties such as intrinsic visible blindness, availability of lattice-matched single-crystal substrates, relatively low film growth temperatures [200]. Also, by designing different Mg mole fractions, their tunable direct band gap energy extends from that of wurtzite ZnO (
3.37 eV) to that of cubic MgO (
7.8 eV). Therefore, MgxZn1  xO are extremely useful for UV light sensing. In following, fabrication difficulties and photodetector properties of some reported MgxZn1  xO based UV PDs are investigated. Because of difficulties in obtaining high quality and reliable p-type MgZnO layer, there are few reports on MgZnO p–n or p–i–n photodiodes for UV detection and in some works another p type material has been used to fabrication UV PDs. p-MgZnO/n-ZnO based p–n heterojunction UV PD has been fabricated on c-plane sapphire substrate [157]. The device showed a rectifying behavior with a turn-on voltage of 4.5 V. PD has been shown a peak response at the wavelength of 330 nm under backillumination. Several UV PDs based on MgZnO have been reported that in them different growth methods and various substrates were used. Each of them has its advantages. Here, some of them are reviewed. For example, MSM structured PD fabricated on a single phased cubic Mg0.48Zn0.52O thin film on sapphire substrates by MOCVD, which has been shown a low dark current of 6.5 pA at 10 V bias, maximum responsivity of 16 mA/W at 268 nm, cutoff wavelength at 283 nm and external quantum efficiency of 7% [201]. Time photoresponse measurement of PD has been shown that the rise and

decay time of the device were 10 ns and 150 ns, respectively. In another work, MOCVD grown MgZnO thin films with different Mg content (0.06–0.25) has been used to fabrication MSM structured PDs on sapphire substrates [202]. Among them, Mg0.06Zn0.94O film has been exhibited peak responsivity of about 14.62 A/W at 340 nm. Time photoresponse measurement has been shown a rise time of 20 ns and a fall time of 400 ns. Also, it was reported for Mg content from 0.5 to 0.7 of MgxZn1  xO thin films on sapphire substrates, cutoff wavelengths varied from 225 to 287 nm [203]. Dark current of PDs were 15 pA under 10 V bias and MgxZn1  xO PD with Mg content of 52% has been shown a rejection ratio of four orders of magnitude. RF magnetron cosputtering has been used to growing Mg0.47Zn0.53O thin film on a quartz substrate for fabrication MSM PD [204]. Peak responsivity was 10.5 mA/W at 290 nm. Dark current was about 3 pA at 5 V bias and the UV–visible rejection ratio (R 290 nm/R400 nm) has been more than 4 orders of magnitude. Time response measurement of PD has been exhibited the rise time of 10 ns and the fall time of 30 ns. It was reported that deep UV detectors can be fabricated from high Mg component MgZnO epitaxial films on Si substrate. But, many difficulties exit for growing highquality MgZnO films on Si substrate due to the formation of amorphous SiOx layer. By using a buffer layer, clean Si surface is protected from oxidation and single-crystalline MgZnO thin film achieved. Also, buffer layer provides a suitable epitaxial template for high-Mg-content MgZnO. There are many reports of insertion of monoxide buffer layers such as TiN, AlN, CaF2, deposition of Zn, Mg and Be metals for adopting ZnO or MgZnO epitaxy. In some works, oxide buffer layers have been used to segregation semiconductor from electrode, carrier multiplication and further gain in this layer. For example, it has been reported that Au/MgO/MgZnO metal–oxide–semiconductor-structured PD has been shown a responsivity of about 2 orders of magnitude larger than that of the Au/MgZnO PD fabricated under the same procedure except that no MgO layer was introduced (Fig. 9(a)) [205]. The enhanced responsivity has been attributed to internal gain that was a result of carrier multiplication occurring in the MgO

Fig. 9. (a) Spectral responsivity of the MgZnO-based MS and MOS PDs at 5 V bias. The inset is a plot of spectral responsivity of the MOS PD at 0 V bias. (b) Spectra response of p–n and MSM MgZnO detectors (with permission from the publisher and authors) [205,206].

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layer via impact ionization. According to the inset plot, at 0 V bias, there is obvious photoresponse for the MOS PD while the response of the MS PD is undetectable. Also, for comparison, Fig. 9(b) shows that responsivity of n-MgZnO/p-Si p–n structure is one order of magnitude larger than that MgZnO MSM PDs with same Ti/Au electrodes. This indicates p–n junction produces a lower extent of electron scattering and high efficiency in the separation of photo-generated carriers by the large built-in electric field [206]. MgxNi1 xO alloys with different mole fractions x (0oxo1) have been shown high quality because both NiO and MgO have very similar lattice constants (0.4209 nm for MgO and 0.4177 nm for NiO. A MSM structured PD has been fabricated from the Mg0.2Ni0.8O film on quartz substrates by electron beam evaporation [207]. By increasing in the Mg content from 0.2 to 0.8, the absorption edge has been shown a blue shift from 340 nm to 260 nm. The dark current of the PD has been about 70 nA with 5 V bias. Photoresponsivity has been shown a peak about 147.3 μA W  1 at 320 nm. Also, UV (320 nm) to visible (400 nm) rejection ratio was nearly two orders of magnitude. Using Nanostructures in UV PDs have significant importance. Recently, Mg0.05Zn0.95O nanowall networks UV PDs have been shown more than four orders of magnitude visible rejection (R352 nm/R400 nm) and at illumination wavelength of 352 nm, maximum responsibility was 24.65 A/W with 5 V bias [208]. MgZnO is a promising candidate for UV dual- or even multi-wavelength detector. Recently, attention is devoted to the fabrication of multichannel UV PDs based on MgZnO. In design of wavelength-selective PDs, the spectral bandwidth of the device is given by the bandgap difference ΔEg of both layers. However, the chemical interdiffusion of Mg at the interface between filter and active layer and the diffusion of carriers generated within the filter layer to the active layer can affect the spectral response of such PDs. in order to avoid these effects, the filter layer blocking high energy irradiation was separated from the active layer and allows the tuning of cutoff energies and bandwidth of the photodiode. The change in bandgap can be easily adjusted by the chemical composition of precursor. In this work, photodiode arrays has been shown the cutoff energies between 3.4 and 3.9 eV and bandwidths from 190 meV down to 50 meV [209]. In another work, a type of cutoff wavelength-selectable UV PD has been fabricated by growing high-quality wurzite MgZnO layers with different Mg contents on Si substrate. By controlling the polarity of the applied bias, this PD responded to either solarblind waveband or visible-blind waveband with sharp cutoff wavelengths. In this work, three response peak were located at 289 nm (H-MgZnO), 299 nm (L-MgZnO), and 362 nm (B-MgZnO), corresponding to the band edge emissions from three different epitaxial layers [210]. Also, dual-color UV PD based on mixed-phase-MgZnO/ i-MgO/p-Si double heterojunction has been reported. Both responses at UVC and UVA ranges, which corresponding to solar blind and visible blind bands, have been reported. Two photoresponse peaks, located at UVC (250 nm) and UVA (around 330 nm) range, were observed at different

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reverse bias. It was attributed to the change of the interface barrier under the increasing reverse bias [200]. For the first time, MgZnO APDs have been fabricated from Au/MgO/MgZnO/MgO/Au Schottky structures. The carrier avalanche multiplication occurred in the MgO layer under relatively large electric field via an impact ionization process. The avalanche gain can reach 587 at 31 V bias and the response speed was in the order of ms. The results reported in this paper may promise high-performance UV or even deep-UV MgZnO-based APDs [211]. We tabulate the up to date representative results on PD properties of MgZnO structures reported so far, along with a brief description of the corresponding growth method, detection wavelength, and PD performance (Table 4). All these results indicate that sputtering, MBE and MOCVD are very suitable methods to fabricate ZnMgO solar-blind PDs. From the data exit in this table, we can conclude that the avalanche type MgZnO PDs have the potential to achieve the highest dark current and the highest responsivities. 5. Other types of oxide UV PDs Ternary oxide nanowires have larger bandgap and hence higher wavelength selectivity. Also, they are chemically and thermally stable. But very limited studies have been devoted on these type PDs. Recently, it was shown that wide-bandgap Zn2GeO4 nanowires can be used as efficient UV PDs [222]. Unlike ZnO (Eg ¼3.4 eV) that responses to the whole UV band (
200–400 nm), ternary Zn2GeO4 (Eg ¼4.68 eV) is blind to UV-A/B (
290–400 nm) and only responsive to UV-C band (
200–290 nm). The visible-blind deep-UV (DUV) PDs of Zn2GeO4 nanowires have been reported [223]. At an 8 V bias voltage, dark current was as low as (o0.1 pA), responsivity was 38.3 A/W (corresponding gain
200). Device has been shown a high DUV-to-visible ratio of
104. The band gap of NiO is approximately 3.6 eV (direct, λ¼ 344 nm) and the ternary NixMg1  xO exhibits band gap tunability across the 3.6–7.8 eV spectral region [207,224]. It was shown that this ternary compound has been shown high DUV responsivity of 12 mA/W at wavelength peak of 250 nm with DUV to visible rejection of 800:1 [225]. Tin dioxide (SnO2) is an n-type functional oxide with a wide bandgap (Eg) around 3.6 eV at 300 K. The dark current and UV-exposed current of the SnO2 nanowires were around 7.7  10  5 A and 1.3  10  4 A, respectively [226]. The resistance variation of SnO2 nanowires has been shown that the resistance for the UV-off state was 9 KΩ higher than UV-on state. This resistance variation is promising for using SnO2 nanowires to trigger nanosensor devices. Titanium dioxide (TiO2) with its wide bandgap (anatase 3.2 eV and rutile 3.0 eV) is a strong candidate in the application of UV photo-detection. Sol–gel method has great importance in deposition semiconductor thin film particularly TiO2 with advantages like ease of processing, ability of coating low cost, large and complex areas. MSM UV PDs based on sol–gel-derived TiO2 films have been reported [227]. The peak responsivity of this device was 17.5 A/W at 5 V bias. The current density was 3.84 nA/cm2

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Metal oxide

Devices

Light of detection

Dark current

Rejection ratio

Responsivity

Response time

Ref. [212]

Mg0.52Zn0.48O

Au

MOCVD

MSM

238 nm

16 pA at 15 V

UV to vis: 4

129 mA/W at 15 V

Rise time: 0.6 ms Fall time: 1.67 ms

MgZnO/i-MgO/n-Si n–Mg0.5Zn0.5O–p-Si Mott-type MgZnO

Au Ti/Au In

MOCVD RF-MBE MOCVD

p–i–n p–n MSM

250 nm, 330 nm 280 nm 325 nm

2 nA at  3 V

UV to vis: 2

6 mA/W at 1 V 431 mA/W at 30 V

Response time o 100 ms Rise time: 0.6 ms Decay time: 1.5 ms

Mg0.27Zn0.73O MgZnO nanorods MgZnO-p-Si MgZnO/i-SiO2/n-Si MgO/MgZnO Mg0.58Zn0.42O/MgO Mg0.44Zn0.56O/MgO

Au

PA-MBEa

Ti/Au Ni/Au Au, Al Au Au

PA-MBE RF sputtering MOCVD MOCVD PA-MBE

Schottky p–n MSM MSM MIS MSM Avalanche (gain: 587 at 31 V)

330 nm 260 nm 300–400 nm 340 nm 366 nm 240 nm 315 nm

1.9 nA at  3 V 10 nA at 5 V

UV UV UV UV

MgZO/ZnO

In

PA-MBE

p–n

MgZnO/BeO Mg0.40Zn0.60O Mg0.47Zn0.53O

Ti/Au Au Au

PA-MBE RF-sputtering RF-sputtering

Mg0.47Zn0.53O

MgZnO:Al

Magnetron sputtering

a

Plasma assisted-MBE.

to to to to

vis: vis: vis: vis:

4 at 5 V 6.24  102 2 4

2.01 A/W 1 A/W 0.2 A/W 0.11 A/W at 0 V 15.8 mA/W at 15 V 0.018 A/W at 2 V, 1000 A/W at 31 V

1.6 pA at 15 V 2.3  10  5 A at 12 V

UV to vis: 4

330 nm

30 nA at  5 V

UV to vis: 2

MSM MSM MSM

260 nm 276 nm 290 nm

100 pA at 2 V 3 pA at 5 V

UV to vis: 2 UV to vis: 4 UV to vis 44

For back-illumination: 2.16 mA/W and frontillumination: 0.53 mA/W 20 mA/W at 0 V 0.0012 A/W at 276 nm 10.5 mA/W at 5 V

Schottky

340 nm

70 pA at 15 V

UV to vis: 3

13.31 mA/W at 10 V

Decay time:100 s

Rise time: 63.8 ns Fall time: 1.2 ms

[200] [210] [213] [214] [215] [216] [217] [205] [218] [211] [157]

Rise time: 10 ns Fall time: 30 ns

[219] [220] [204] [221]

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Table 4 Recent MgZnO-based UV PDs published in literatures.

Z. Alaie et al. / Materials Science in Semiconductor Processing ] (]]]]) ]]]–]]]

and the attributed dark current was 5.38 pA at 5 V bias. This low dark current was attributed to the high effective Schottky barrier between Au and TiO2 films which was a result of low concentration of oxygen vacancies in the surface of the TiO2 films. Also, a MISIM structured TiO2 UV PD has been shown significantly larger responsivity than that of the MSM structured PD [228]. The existence of an insulation layer causes improvement of the PD responsivity because the carrier multiplication occurs in the insulation layer under a high electric field. For comparison, response spectra of the MISIM and MSM structured PDs under 30 V bias are shown in Fig. 14. Responsivity peaks of two PDs are at around 310 nm. Maximum responsivity of the MISIM PD is about 23 times larger than that of the MSM one. The dependence of the peak responsivity of the MISIM and the MSM structured TiO2 PDs on the applied bias is shown in the inset of Fig. 10. Both PDs increases almost linearly in the range from 5 to 30 V. Due to carrier multiplication, discrepancy between the responsivity of the two PDs becomes larger at large bias. A new way to improve high dark resistance of TiO2 PDs based on double-walled carbon nanotube/TiO2 nanotube heterojunctions has been reported. TiO2 nanotube arrays were coated with a double-walled carbon nanotube film, which functioned as a semitransparent electrode and a photoactive layer. Via pre-electroforming, the device was switched from a high resistance state to a low resistance state, as a result of the formation of oxygen vacancy related conducting filaments. It was reported that the photoresponse was higher than it was obtained from the high resistance device under the same conditions [229]. Silver microgrid/TiO2/ITO sandwich structures PDs with ohmic contacts have been fabricated. The high UV transmittance of electrode resulted from an enhanced bulk Plasmon effect of the silver microgrid electrode. The sandwich structure revealed higher responsivity and SNR than those of the MSM structure. The short transport distance of carriers and the low blocking of incoming UV

27

light could account for the results. The silver microgrid transparent conductive electrode based on the bulk plasmon effect opens the way to UV-wavelength application [230]. Recently, Schottky junctions made from a T nanotube array in contact with a monolayer graphene film were fabricated and utilized for UV light detection. Photoconductive analysis showed that the fabricated Schottky junction PD was sensitive to UV light illumination with good stability and reproducibility. The corresponding responsivity, photoconductive gain and detectivity were calculated to be 15 A W  1, 51, and 1.5  1012 cm Hz1/ 2 W  1, respectively. Also, it was reported that the fabricated PD exhibits spectral sensitivity and a simple powerlaw dependence on light intensity [231]. In another work, an UV PD was fabricated from WO3 nanodiscs/reduced graphene oxide composite material. The UV PD showed a fast transient response and high responsivity, which were attributed to the improved carrier transport and collection efficiency through graphene [232]. The UV-sensitive TiO2/glycerol is embedded inside a cavity on silicon chip and then encapsulated by CVD parylene C film. The performances of presented sensor with various electrode gap designs (1581–335 μm) under 20 wt% TiO2 have been shown the photo to dark current ratio ranging 11.4–5.85, response time ranging 19–23 s, and recovery time: ranging 1.5–6 s [233]. Also, the application of a photoelectrochemical cell (PECC) as a selfpowered UV-PD for detecting the UV light has been shown. This device has been based on the photovoltaic effect of PECCs. By connecting a PECC to an ammeter, the intensity of UV light have been quantified using the output short-circuit photocurrent of the PECC without a power source. This self-powered UV-PD has been exhibited a high photoresponse sensitivity of 269,850%, a rise time of 0.08 s and a decay time of 0.03 s for output short circuit current signal [234]. Here, we tabulate the representative results on PD properties of other types of oxide based PDs reported so far, along with a brief description of the corresponding growth method, detection wavelength, and PD performance (Table 5). From the data exit in this table, we can conclude that among PDs with MSM structure, the B-Ga2O3 PDs have the potential to achieve the lowest dark current densities, the Zn2GeO4 PDs have the potential to achieve the highest UV to vis ratio, the B-Ga2O3 PDs have the potential to achieve the highest responsivities and the SrTiO3 PDs have the potential to achieve the lowest response time. Also, we can conclude that among PDs with p–n structure, the SnO2/ZnO or SnO2/TiO2 composite PDs have the potential to achieve the highest photo to dark current ratio and the In2Ge2O7 PDs have the potential to achieve the highest responsivities. 6. Organic UV PDs

Fig. 10. Photoresponse of the MISIM and MSM TiO2 PDs measured at 30 V bias; dependence of the peak responsivity of the two PDs on applied bias is shown in the inset (with permission from the publisher and authors) [228].

Inorganic semiconductor based PDs require expensive substrates and costly complicated manufacturing processes. Organic UV PDs as a complementary alternative for the inorganic ones have advantages of simpler fabrication processes, lighter weight and lower costs. The

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28

Metal oxide

Growth method

Devices

Light of detection

Dark current

Zn2GeO4 NW

VLS

MSM

254 nm, 0.2 mW/cm2

0.02 nA

Zn2GeO4 NW

CVD

MSM

254 nm, 50.2 mW/cm2


0.1 pA at 710 V

DUV to vis: 104

NixMg1  xO NW

MBE

MSM

250 nm, 400 nW

o 25 nA

DUV to vis: 800:1

SnO2 SnO2 nanowires Ni doped-SnO2

MSM MSM MSM

365 nm 365 nm 354 nm

0.77 mA 2.3 pA

Photo to dark current: 1.67

Β-Ga2O3 Β-Ga2O3 nanowires

CVD ALD Sol–gel based electrospining MOCVD CVD

MSM MSM

260 nm 250 nm, 49 mW/cm2

4492 A/W at RT, 3000 A/W at 553 K

Β-Ga2O3 nanowires

CVD

MSM

254 nm, 470 mW/cm2

1.6  10  13 A at 5 V 0.1 PA at 10 V, Photo to dark current 43 323 K 0.01 nA UV to dark current: 42

TiO2 TiO2 (Plasmon effect) TiO2

Sol–gel RF sputtering Sol–gel

MSM MSM MSM

340 nm, 1.38 mW/cm2 365 nm, 40 mW/cm2 260 nm, 6.1 mW/cm2

5.38 pA at 5 V 0.81 mA at 3 V 1.9 nA at 5 V

17.5 A/W at 5 V 3.6 A/W at 3 V 199 A/W at 5 V

TiO2 TiO2 Zr0.5Ti0.5O2

ALD MSM ALD MISM Chemical MSM solution process

310 nm 310 nm 250 nm

17 pA

Photo to dark current: 3

0.02 mA/W at 30 V 0.48 mA/W at 30 V 620 mA/W

SrTiO3

Sputtering

MISM

310 nm

0.4 nA at 50 V

UV to vis: 2

105 mA/W

Rise time: 330 ps, Fall time: 480 ps at 355 nm

[242]

SrZn0.1Ti0.9O3

Sol–gel

MSM

260 nm

41 pA at 5 V

UV to vis 43

94 mA/W

Rise time: 3.8 ms Fall time: 565 ms

[243]

NaTaO3

Hydrothermal

MSM

260 nm

1.5 nA at  5 V

UV to dark current: 13

Rise time: 4.6 s Decay time: 15.4 s

[244]

TiO2 TiO2 SnO2–ZnO

Solvothermal Hydrothermal Electrospinning method

p–n p–n p–n

365 nm 380 nm, 3.6 mW/cm2 300 nm, 0.45 mW/cm2

0.02 mA/cm2

UV to noise: 3

1.7 pA

Photo to dark current: 4600

Response time: 200 ms Response time: 300 ms Rise time: 32.2 s; decay time: 7.8 s

[245] [246] [142]

p–n p–n p–n

330 nm 373 nm, 70 mW 395 nm

p–n p–n p–n

365 nm, 2 mW/cm2 200–250 nm 335 nm, 31.65 mW/cm2

TiO2/SnO2 self powered PD TiO2/ZnO core–shell TiO2-polymer

Sol–gel Low temperature solution Hydrothermal TiO2–NiO In2Ge2O7 Sol–gel WO3/reduced graphene oxide Homogeneous precipitation

Rejection ratio

Responsivity

Response time

Ref.

Response time: 0.3 Recovery time: 0.2

[222]

38.3 A/W at 8 V

Rise time: 12 s Fall time: 0.6 s

[223]

12 mA/W at 10 V

Rise time: 0.59 s Fall time: 7.10 s

[225]

Photosensivity: 10

10

 11

A

UV to vis: 3 Photo to dark current: 3

3

Response time: 0.3 s Recovery speed: 1 s

[237] [238] Response time: 0.32 s Recovery time: 0.08 s

On/off ratio: 4550 Swith current ratio: 140 Photo to dark current: 3

0.6 A/W 250 A/W at  5 V 33.2 mA/W at 0 V

Photo to dark current: 600 at 20 V

3.9  105 A/W 6.4 A/W at 347 nm and under 20 V

[226] [235] [236]

Rise time: 6 s Decay time: 15 s

Rise time: 424.1 s Fall time: 154 ms

[239] [240] [230] [241] [228] [228] [234]

Rise time: 0.03 s; decay time: 0.01 s [247] [248] Response time o 200 ms [249]

Rise time: 0.1 s, Decay time: 0.1 s Decay time: 3 ms Response time
1 ms

[233] [250] [232]

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Table 5 Recent other types of oxide-based UV PDs.

[231]

[252]

15 A/W at  5 V

Rise time: 0.5 s; decay time: 0.7 s Schottky TiO2 nanotube/graphene

Polyvinyl-alcohol coated In2O3 Anodic TiO2 nanotube

Electrochemical anodization Anodization and CVD

Schottky 312 nm,1.06 mW/cm

2

Schottky 335 nm, 31.65 mW/cm2

1 nA at 0.5 V

UV to dark current: 44

13 A/W at 2.5 V

Rise time: 500 s; fall time: 1600 s

[251]

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29

fundamental difference between organic and inorganic semiconductors lies beneath their chemical structures. Nevertheless, this reduction in crystallinity in organic solids trades in for an attractive advantage: flexibility. Flexibility allows organic layers to be grown or deposited on vast selection of substrates and deposition conditions. Lattice matching problems found in heterogeneous inorganic devices do not apply to organic semiconductors. Excitons (photo-generated electron–hole pairs) in inorganic photodiodes are weakly bonded, while excitons in organic photodiodes are tightly bonded. This explains the fact that exciton diffusion lengths in inorganic semiconductors are a few orders of magnitude higher than that of organic semiconductors (few hundreds of angstroms). Since the carrier movement in organic molecules is somewhat restricted, the carrier mobility is expected to be low. Also, organic UV PDs still exhibit lower responsivity. But these PDs have undergone rapid development in recent years. It has been shown that the light absorption efficiency, amount of carrier mobilities and the energy level alignment of donor and acceptor materials have critical roles in determining performance of organic PDs. Important parameters in the performance of organic PD rely on the light absorptions, carrier mobilities and the energy level alignment of donor and acceptor materials. Important material which is frequently used in organic PDs is 1,3,5-tris(3-methylphenyl-phenylamino)-triphenyamine (m-MTDATA). Different types of organic UV PDs based on this material and other UV sensitive organic materials have been reported. For example, the planar heterojunction (PHJ) and bulk-heterojunction (BHJ) devices have been fabricated with structures of ITO/mMTDATA (350 Å)/1,3,5-tris(N-phenyl-benzimidazol-2-yl)benzene (TPBi) (500 Å)/bathocuproine (BCP) (120 Å)/LiF (10 Å)/Al and ITO/m-MTDATA(300 Å)/m-MTDATA:TPBi (1:1, 100 Å)/TPBi(450 Å)/BCP(120 Å)/LiF(10 Å)/Al, respectively. Peak responsivity was 100 mA/W at zero bias [253]. Also, η is higher for PHJ than BHJ structure devices. This is opposite of Yang's work who reported that blenddevice (BHJ) generally provides a higher photovoltaic property than that of a bilayer (PHJ) device [254]. Also, organic UV PD has been fabricated by using a blend of m-MTDATA and tris-(8-Hydroxyquinoline) gallium (Gaq3) as the electron donor and acceptor, respectively [255]. This device has been shown a photocurrent of 405 mA/cm  2 at  8 V which was corresponding to a photoresponse of 338 mA/W under an illumination of 365 nm UV light with an intensity of 1.2 mW/cm2. In another work, a blend layer of donor and acceptor polymers of m-MTDATA and 4,7-diphenyl-1,10-phenanthroline-(bathophenanthroline) (Bphen) has been used to fabrication of deep UV PD [256]. In addition to having role of donor material in blend layer, m-MTDATA can be used to suppression of radiative decay. For example, a PD has been reported which has had the structure of ITO/m-MTDATA/m-MTDATA: [Cu(1, 2-bis (diphenylphosphino) benzene) (bathocuproine)] BF4 (CuBB)/CuBB/LiF/Al and different blend layer thicknesses [257]. Photocurrent was about 173 mA/cm2 at  10 V bias. At 365 nm UV light illumination with intensity of 0.691 m W/cm2, photoresponse was 251 mA/W.

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By using organic materials with different absorption wavelengths, PDs sensitive to various UV wavelengths can be obtained. In attention to that m-MTDATA has a maximum absorption at about 350 nm and TPBi has maximum absorption around 315 nm, double UV wavelength sensitive PD has been fabricated that has had bilayer of these two materials [258]. It has been shown that illuminating the photodiode at 365 and 330 nm UV wavelengths via the anode and cathode sides, led to photogenerated carriers which were collected by anode and cathode, respectively. The device has been shown a photoresponse of 75.2 and 22.5 mA/W under illuminations of 365 and 330 nm UV light which were corresponding to the respective absorption of the donor and acceptor components. Different donors (PFH, m-MTDATA, PFP and PVK) have been used to fabrication organic UV PDs with wide bandgap acceptor of triazine derivative (NSN). Structure of device was ITO/PEDOT: PSS/donor/NSN/LiF/Al [259]. It has been shown that varied response cutoff wavelengths can be obtained by using different energy gaps of the donors. Fig. 11 shows by selecting donors with different bandgaps to combine with NSN, the response cutoff wavelength can be tuned. Heterojunction of organic polymers and inorganic wide band gap materials can be used to fabrication UV PDs. For example, p–n heterojunction of polyaniline–silicon has been fabricated as a photo detector for detection of UV radiation [260]. Micro and nano-films ( o300 nm thick) of polyaniline deposited on the top of a silicon wafer through the spin-coating technique, aluminum and gold were used as electric contacts in the backside of silicon and on top of polyaniline, respectively. This PD has been shown good sensitivity (
200%) especially in the UV region
2.0– 3.5 eV compared to commercial all-silicon diodes. Active layers for UV PD can be obtained by incorporating ZnO nanoparticles into the organic hosts, such as incorporating ZnO nanoparticles in 2,7-di(9-(2-ethyl)hexylcarbazole-3-yl)-9,9-bis(4-diphenylaminophenyl)fluorine (CFC) and 3,6-di(9,9-bis(4-diphenylaminophenyl)fluorene-2-yl)-9-(2ethyl)hexylcarbazole (FCF), and then spin-coating uniform hybrid films [261]. Photo-to-dark current ratio was about

Fig. 11. Normalized responsivity spectra of ITO/PEDOT: PSS/donor (PFH, m-MTDATA, PFP and PVK)/NSN/LiF/Al for illumination through ITO side (with permission from the publisher and authors) [259].

three orders of magnitude. The decay time of photo current was less than 200 ms. FCF and CFC based PDs have been shown different spectral response that covered 300– 385 nm and 300–402 nm for FCF and CFC, respectively. Degradation of the organic/electrode interfaces in UV organic PDs is essentially induced by UV-generated excitons in their vicinity and may be responsible for nearly 100% of the photo current loss of UV organic PDs. Recently, it was shown that a thin (
0.5 nm) interfacial layers such as lithium acetylacetonate at organic/metal interfaces and an appropriate hole transport materials such as N,N0 bis (naphthalen-1-yl)-N,N0 -bis(phenyl) benzidine at ITO/ organic interfaces can improve the photo-stability of organic/electrode interfaces, and thus the lifetime of UV organic PDs [262]. Recently, a new organic (PEDOT: PSS)–inorganic (ZnSSe) hybrid structure avalanche photodiodes has been reported. An energy barrier height of 1.0 eV was revealed in the interface of i-ZnSSe and PEDOT: PSS layer by photoresponse method. this device exhibited external quantum efficiency of 90% at 325 nm, an intrinsic avalanche breakdown at about 28 V and multiplication factor of 650 at a reverse bias of 34.2 V [263]. Structural spectral response tuning employ different structures that exhibit different response peaks. It was shown that bulk heterojunctions of TAPC: BAlq reveal the response peak at
280 nm and mixed planar-bulk heterojunctions of TAPC/TAPC: BAlq/BAlq reveal the response peak at
260 nm. Peak responsivities are 100.8 mA/W at
280 nm and 33.6 mA/W at
260 nm, respectively [264]. Also, it was reported a device that was capable of NUV and DUV-selective response for radiation, respectively from ITO and Al sides. These spectral response tuning was available in planar heterojunction device: ITO/PEDOT: PSS/donor/NSN/LiF/Al by light illumination from ITO and Al sides [259]. Most of the inorganic PDs have high cost and low responsivity (o0.2 A/W), organic materials and/or nanomaterials could be significantly cheaper to manufacture, but their performance so far has been limited. A nanocomposite active layer composed of ZnO nanoparticles blended with semiconducting polymers has been reported that it can significantly outperform inorganic PDs. The nanocomposite layer as an electron blocking/hole-conducting layer combined the hole-injection and hole-transport capabilities of TPD-Si2 (4,40 -bis((p-trichlorosilylpropylphenyl)phenylamino)-biphenyl) with the electron-blocking capability of PVK. As a result of interfacial trap-controlled charge injection, the PD transited from a photodiode with a rectifying Schottky contact in the dark, to a photoconductor with an ohmic contact under illumination, therefore it combines the low dark current of a photodiode and the high responsivity of a photoconductor (
721–1001 A/W). Detectivity was 3.4  1015 Jones at 360 nm and under a bias of o10 V that it is two to three orders of magnitude higher than that of existing inorganic semiconductor UV PDs. A quantitative comparison of the organic based UV PDs proposed in the scientific literature based on light of detection, dark current and responsivity have been reviewed and summarized in Table 6. From the data exit in this table, we can conclude that the organic PDs with

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31

Table 6 Recent organic-based UV PDs published in literatures. Device structure

ITO/m-MTDATA(350 Å)/TPBi(500 Å)/ BCP(120 Å)/LiF(10 Å)/Al ITO/m-MTDATA(300 Å)/m-MTDATA:TPBi (1:1, 100 Å)/TPBi(450 Å)/BCP(120 Å)/LiF (10 Å)/Al, ITO/m-MTDATA:TPBi(1:1)/A1, Ag ITO/m-MTDATA/m-MTDATA:Bphen(1:1)/ Bphen/CsCO3:Bphen/Al/TPD ITO/m-MTDATA/m-MTDATA:CuBB/CuBB/LiF/Al ITO/PEDOT:PSS/PFP/NSN/LiF/Al ITO/PEDOT:PSS/PFP/NSN/PVK/NSN/LiF/Al ITO/m-MTDATA/m-MTDATA:BAlq (1:1)/ BAlq/LiF/Al ITO/NPB/PBD/LiF/Al ITO/PEDOT:PSS/PFH:NDI/Al ITO/PVK:PBD/Al ITO/n-ZnO:PFP/PEDOt:PSS/Au ITO/n-ZnO:CFC:FCF/PEDOt:PSS/Al Glass/ITO/PEDOT:PSS/P3HT:PCBM/Ca/graphene

Active area

Dark current

Responsivity

Ref.

365 nm, 0.426 mW/cm2 365 nm, 0.426 mW/cm2

57 mA/cm2 at  4 V

100 mA/W at 0 V

[253]

56 mA/cm2 at  4 V

85 mA/W at 0 V

[253]

0.02 mA/cm2 at  6.5 V 0.25 mA/cm2 at  8 V

75.2 mA/W at 0 V 309 mA/W at 0 V

[258] [256]

251 mA/W at  10 V

[257]

0.696 mA/W at  12 V 0.245 mA/W at  12 V 514 mA/W at  7 V

[259] [259] [265]

4.5 A/W at 3 V 224 mA/W at  4 V 18.6 A/W at  1 V 0.18 A/W at  2 V, 300 nm 72 mA/W at  2 V 0.083 A/W

[266] [267] [268] [152]

410 mA/W at  4 V 0.074 A/W 560 mA/W at  12 V

[270] [271] [272]

872 mA/W at  12 V

[273]

0.12 A/W 4600 A/W (APD, gain:650) 721 A/W

[274] [263] [275]

33.6 mA/W at  10 V

[264]

100.8 mA/W at  10 V

[264]

2  3 mm2

365 nm, 1 mW/cm2 280 nm, 0.428 mW/cm2 2 mm  2 mm 365 nm, 0.691 mW/cm2 365 nm, 1 mW/cm2 340 nm, 0.5 mW/cm2 365 nm, 1.2 mW/cm2

3.6 mm2

1.4 mA/cm2 at  12 V 0.32 mA/cm2 at-7 V 0.011 mA/cm2 at 3 V

350 nm, 60 mW/cm2 365 nm, 1 mW/cm2 340 nm, 213 mW/cm2 2.2  10-3 mA/cm2 at 0 V 350 nm, 0.69 mW/cm2

0.09 mm2

365 nm, 1 mW/cm2 320–380 nm, 10 mW/cm2 368 nm, 1 mW/cm2 12 mm2 250 nm 2 mm  3 mm 365 nm, 1.05 mW/cm2

ITO/PEDOT:PSS/PFE:BNDI (3: 1)/Al Glass/ZnO:Ga/ZnMgO/PEDOT:PSS ITO/m-MTDATA/m-MTDATA:Cu(I) complexes/Cu(I) complexes/LiF/Al ITO/m-MTDATA/m-MTDATA:BPhen/BPhen)/TPBi/ 2 mm  4 mm LiF/Al Glass/ITO/PEDOT:PSS/TAPC:PBD/LiF Ag/PEDOT:PSS/i-ZnSSe/n-ZnSSe/n-ZnSe/ n-GaAs/In ITO/PEDOT:PSS/TPD-Si2:PVK/P3HT: ZnO NPs/BCP/Al PEDOT:PSS/HJs/TAPC/BAlq PEDOT:PSS/BHHJs/TAPC/BAlq

Light of detection, power

6.2 nA/cm2

100 nA up to  5 V

365 nm, 2.82 mW/cm2 330 nm, 0.95 mW/cm 325 nm

2

360 nm, 125 mW/cm2

6.8 nA at  9 V

260 nm, 0.91 mW/cm

2

260 nm, 0.91 mW/cm

2

PVK composite layers have the potential to achieve the lowest dark current densities, the organic/ZnSe composite APDs and the organic/ZnO NPs composite PDs have the potential to achieve the highest responsivities even higher than that of existing inorganic semiconductor UV PDs. 7. SiC UV PDs Silicon carbide (SiC) is a very promising semiconductor material for UV photodetection. Its band gap energy matches the UV photon energy region while it can withstand in harsh conditional environments such as high temperatures. Also, SiC exhibits inherent favorable properties of wide band-gap energy, higher thermal conductivity and higher breakdown field compared to the conventional semiconductor materials such as silicon. These properties are very attractive for other applications such as high power electronic devices, high temperature operating gas sensors, pressure sensors and accelerometers for automotive and space industry applications [276]. For example, silicon Carbide has a high thermal conductivity and temperature has a little influence on its switching and

8

2

3.35  10 A/cm at  10 V 2.94  10  8 A/cm2 at -10 V

[261] [269]

thermal characteristics. With special packaging, it is possible to have operating temperatures of over 500 K, which allows passive radiation cooling in aerospace applications. The first UV-detector was made of 6H-SiC which utilized diffusion of Al (at 2000 1C) into n-type substrate [277]. Diffusion let to structural decomposition of surface layers and therefore resulting high leakage current with low efficiency. Glassow et al. improved this structure by offering the conventional structure of PDs i.e. the n þ -p photodiode [278]. They utilized n-implantation to form n þ -p junction in 5 μm p-type epitaxial layer grown on a p-type substrate. The diode exhibited 75% quantum efficiency at a peak wavelength of 280 nm at room temperature. Leakage current was as high as 10  5 A/cm2 at 10 V. In comparison to p–n junction PDs, the p–i–n SiC PDs exhibit a low-noise, high-speed, visible blind response and high photosensitivity at low reverse bias due to a low terminal capacitance and a large shunt resistance. It was reported that planar p–i–n PDs has higher photo-response compared with MSM structure detectors. Also, based on the application, intrinsic thickness (spacing) of p–i–n structure must be choiced. For example, small spacing is

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desirable for the high speed devices and reasonable large spacing is good for high-sensitive applications. For the first time, design and fabrication of 4H-SiC p–i–n UV PDs have been presented in 2006 [279]. Peak wavelength and cutoff wavelength of responsivity were about 275 nm and 375 nm, respectively. The ratio of responsivity of these wavelengths have been reported nearly 100. These SiC p–i–n PDs have been shown a high dark current of
35 pA/mm2 at  0.5 V. Even by improvement in the performance of these PDs, a significant dark current of 2.5 pA/mm2 at a low reverse bias of 5 V has been obtained [280]. However, vertical p–i–n UV PDs based on 4H-SiC homoepilayers have been shown low dark current [281]. The spectral response of PD was from 260 to 360 nm. The calculated peak value of spectral detectivity D* was 5.9  1013 cm Hz1/2/W at 310 nm. Dark current was about 5.4 pA and ratio of the illuminated current to the dark current has been reported at least two orders of magnitude at a bias of below 12 V which demonstrating good visible blind performance and ability to detect radiation. The fabricated PDs in annealing condition show lower dark current, higher light current and higher responsivity than that of as-deposition condition. Responsivity of 4HSiC MSM PDs with annealing was 0.094 A/W at 20 V and their peak response wavelength was at 290 nm [282]. The authors mentioned that their low leakage current (0.45 pA at 5 V bias voltage) and high breakdown voltage (4150 V) is a pointer of high-quality contacts between metal and epitaxial layer. In another work, by considering the UV transmittance of a metal electrode, dependency of responsivity and performance of MSM UV detector to various electrode heights (H), spacings (S) and widths (W) has been investigated [283]. Results have been shown that responsivity is inversely proportional to electrode height i.e. the lower the height of the electrode results in the higher the spectral response. Also, responsivity is enhanced by increasing of electrode spacing and width. The maximum responsivity, peak quantum efficiency and outstanding UV-to-visible contrast have been 180.056 mA/W, 77.93% and 1875, respectively. These results have been obtained from a detector with 50 nm electrode height, 3 μm width and 9 μm spacing which has best structure for achieving excellent performance for UV detection. Theoretical demonstration of the performance enhancement of MSM detector has been shown that triangle electrode deposited on PD results in better performance of MSM detector as compared with conventional electrode MSM device. Recently, simulation has been shown that 4H-SiC triangular electrode MSM UV PDs have a 113% photocurrent increase of 25.4 nA and similar low dark current of 3.16 pA at 30 V bias over the conventional electrode MSM device [284]. Also, by considering triangular electrode and changing electrode angle α, width W and spacing S, MSM PDs have been optimized to obtain the enhanced UV-to-visible rejection ratio and responsivity. 4H-SiC triangular electrode MSM PD with parameters of α ¼ 603 , W ¼ 3 μm and S ¼ 4 μm have been shown maximum UV-to-visible rejection ratio, responsivity and external quantum efficiency of 13049, 0.1712 A/W and 68.48%, at 310 nm wavelength and under 30 V bias, respectively. In

Fig. 12. Simulation and experimental dark and illuminated I–V characteristics of TEMSM and CEMSM (with permission from the publisher and authors) [284].

Fig. 12, it has been shown that the triangular electrode MSM (TEMSM) PD exhibits larger photocurrent than that of the conventional electrode MSM (CEMSM) structure. While, the TEMSM and CEMSM detectors show a similar dark current around 3 pA at a bias of 30 V. Therefore, TEMSM PDs have high contrast of photocurrent to dark current. Al2O3 and SiO2 are the most important oxide film materials with the least absorption in the UV spectral region. The refractive index of Al2O3 films is most suitable for 4H-SiC substrates. It was demonstrated that Al2O3/ SiO2/4H-SiC MSM PDs have been shown much better responsivity, higher quantum efficiency and larger photocurrent than conventional 4H-SiC MSM devices [285]. These devices have been shown a peak responsivity of 0.12 A/W at 290 nm and maximum external quantum efficiency of 50% at 280 nm under 20 V electrical applied bias. As previously mentioned, polymers are promising material for future development of PDs. It was shown that polyaniline (PANI)/SiC heterojunctions can be used to fabricate radiation sensors. Recently, by spin coating of PANI films onto n-type 6H-SiC and 4H-SiC substrates, PANI–SiC heterojunctions have been fabricated and interface trap density has been evaluated [286]. The authors showed that interface trap density for 4H-SiC/PANI heterojunctions is higher than that for 6H-SiC/PANI heterojunctions. Like silicon carbide, ternary silicon carbon nitride (SiCN) is a wide band gap semiconductor that has promising properties such as hardness, oxidation resistance, high thermal stability, and corrosion resistance which enable it for UV detection in harsh environment. The n-SiCN/p-SiCN homojunction on Si substrate has been fabricated [287]. At  5 V bias, the photo/dark current ratio, with and without irradiation of 254 nm UV light have been 1940 and 96.3, at room temperature and 175 1C, respectively. These results show that this PD is suitable for high temperature UV detecting applications. Also, in many applications, the temperature is always raised due to absorption of high-

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energy UV light; therefore, detector’s material with higher thermal stability must be used. The temperature dependence of current–voltage and spectral response characteristics of a 4H-SiC MSM UV PD has been investigated. It was shown that, as the temperature rises, a remarkable red-shift of 12 nm occurs and overall responsivity is enhanced for longer wavelength. While the short-wavelength response remains relatively independent of temperature. It was reported that the mechanism of indirect and direct band absorption transition is responsible for temperature-dependent spectrum distribution [288]. A way to improve the UV PD performance is to increase the current density at reverse bias. Researchers demonstrated the improvement in electrical characteristics achievable by either optical or electrical means. This study has shown that increase in photon harvesting and wavelength selectivity is possible by using a specific patterned surface, a surface grating which behaves as a photonic crystal. To ensure that the photon absorption is carried out inside the space-charge region, this simulation study proposed a UV-PD comprising a “flat-top pyramid” surface [20]. 8. Si UV PDs Silicon (Si) PDs usually are used to detection for the visible spectral range. UV penetration into Si PDs because of their narrow band gap (indirect Eg
1.1 eV), is very limited in depth. Also, these PDs have low responsivity and they are very expensive. As a result of narrow band gap, Si UV detectors are less sensitive than other wide band gap semiconductors. Many wide band gap materials can be used to enhance detectivity of Si detectors in UV region. For example, NiO is a wide band gap p-type semiconductors (direct band gap, Eg
3.7 eV) that can be used as p-type material in p–n junction of Si PDs. A heterojunction p-NiO/n-Si diode using the thermal evaporation of NiO film and n-Si has been reported [289]. This device can detect both UV and visible photons. Responsivity of this PD has been
0.36 A/W at 0 V and
0.40 A/W at 30 V for visible light (red, 633 nm) while it was 0.15 A/W at 0 V and 0.17 A/W at 30 V for UV (290 nm). Solution processed organic semiconductors enable manufactures to easily produce hybrid organic–inorganic semiconductor devices. conjugated polymer thin film blends provide a simple and convenient method to greatly enhance responsivity of UV PDs, easily choice of materials and alter blend ratios, choice different layer thicknesses by varying spin speed, solvent and solution concentration. By using fluorine copolymers, hybrid organic semiconductor/ silicon PDs have been demonstrated [290]. These PDs exhibited a quantum efficiency of 60% at 200 nm and their quantum efficiencies have been greater than 34% over the entire 200–620 nm range. Because of small penetration depth of the VUV radiation in silicon, the photo-generated charge is very close to the device surface and the depletion zone of the photodiode is very close to the device surface. Doping causes ultra-shallow junction depth and therefore enhances

Fig. 13. Variations of resposivity as a function of wavelength at reverse voltage of 2 V for different structures (with permission from the publisher and authors) [292].

radiation detection. The CVD doping technique has been used to deposition ultrashallow boron-doped layers on the active area of Si photodiode to fabrication of high-quality p þ –n junctions [291]. The authors reported that this device has a good performance in terms of low dark current, high responsivity and irradiation stability. Introducing an oxide layer between metal and semiconductor increases the total absorption depth. This type of structure is used in solar cells and can be used in UV PDs to enhance degree of trapping UV radiation. As insulating buffer layer, different types of oxides (SiO2, SnO2 and SiO) were deposited on silicon substrate [292]. Fig. 13 shows a comparison between the responsivity of different structures as a function of wavelength. Except SnO2 oxide that does not enhance the device operation, responsivity of other structures increases as the wavelength increases. This behavior was attributed to increase total absorption depth in the wasted portion of the solar spectrum. Multilayer coating on the surface of the photodiode during the photodiode fabrication process, provide wavelength bandpass filter in the extreme UV and soft x-ray spectral regions while providing low noise photodetection. Silicon photodiode detectors with thin metallic coatings have been developed for establishing wavelength bandpasses [293]. It was reported that multilayer coatings of Fe/ Al, Mn/Al, V/Al, Ti/C, PdITi, Ti/Zr/Al, Ag/CaF2/Al, and Ti/Mo/ C on Si photodiodes have been shown bandpass wavelengths of 17–30 A, 19–30 A, 24–35 A, 27–40 A, 27–50 A, 27–50 A, 36–50 A and 50–150 A, respectively. Recently, silicon-based ultrashallow junction p þ -n photodiodes has been fabricated by pure boron CVD technology. It was evaluated for detection in the extreme

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UV spectral range spanning from 3 nm to 15 nm. Responsivity of this PD was 0.265 A/W at a wavelength of 13.5 nm, which is the operating wavelength of the next-generation lithography systems. Results of this work have been shown that the increasing extreme UV induced dark current is due to the radiation-caused damage along the Si–SiO2 interface. It was recommended that this damage can be minimized by introducing a silicon nitride layer to the surface- passivation layer stack [294]. Also, silicon photodiodes with an integrated Zr/Si filter for extreme UV spectral range has been developed. The optical properties of silicon photodiodes in the extreme UV and visible spectral ranges were investigated. a narrow region beyond the operating aperture was found to be sensitive to the visible light. the detector sensitivity to visible light was limited by the value of 10  2. This effect has been shown the necessity of high-quality collimators and apertures when using silicon detectors with filters in the EUV spectral region [295]. 9. Diamond UV PDs Diamond is a very promising material with UV absorption range from 400 nm to 1 nm. Its unique properties are wide bandgap, high carrier mobility, a short carrier lifetime, a high dark resistance, a high electric strength, enhanced temperature, radiation tolerance of its crystals and low self-noise. Diamond is an attractive material in the fabrication of UV and X-ray radiation detectors, highfrequency microwaves, high-power, high temperature, and optoelectronic devices [296]. It was shown that polycrystalline and synthetic diamond MSM detectors have a significantly lower responsivity than natural diamond MSM detector [297]. Low quality of polycrystalline diamond crystals with a number of grain boundaries causes small μτ Simulation and experimental dark and ill product and thus low responsivity of PD. In order to obtain high-quality diamond films, growth rate must be decreased. High-quality single crystalline diamond film has been grown in CVD system by a slow growth rate of
0.05 μm/h [298]. This PD has been shown small noise equivalent power (NEP) o1 pW, for 220-nm UV light. Its responsivity ratio for 220-nm UV light to 400nm visible light was at least four. In another work, single crystal CVD diamond based UV detector has been shown more than five orders of magnitude of visible/UV rejection ratio with a drop of about 104 corresponding to the diamond energy gap [299]. Also, various efforts (design and material treatments) have been made to reduce the photoresponsivity of diamond UV PDs in the visible region that is a result of existence impurities and structural defects in grown diamond film. For example, it was shown that plasma post deposition of oxygen and carbon tetrafluoride at room temperature can remove the visible component of the photoresponse and therefore improves the performance of diamond UV PDs [300]. It was reported that the UV photoresponse at 225 nm respects to the visible wavelength has been enhanced nearly four orders of magnitude. Diamond can be used to detect deep UV radiation. The Ib (100) diamond substrate has been used to fabricate DUV

Fig. 14. (a) Specific contact resistance of the Ti, Mo, Cr, Pd, and Co contacts before and after annealing vs. NA at room temperature [296] and (b) photoresponsivity of CVD diamond PDs before and after roomtemperature plasma treatment (with permission from the publisher and authors) [300].

MSM diamond photoconductors [301]. This work has been shown that by using a microwave plasma-enhanced chemical vapor deposition reactor, diamond thin film can detect DUV radiation. DUV/visible ratio was up to 108 between 210 nm and visible light and photoconductivity gain was 33 at 220 nm light for a bias of 3 V. Also, the dark current of the photoconductor was 10  12 A. In addition, it was reported that Diamond PDs are suitable for detection of deep UV pulses from excimer lasers. The detector exhibited a linear response over the laser fluence range. Also, excimer vacuum UV detection in different atmospheres (air, oxygen or nitrogen) by using the thin film CVD diamond device has been shown that these three atmospheres exhibited a low similar dark current around 0.3 nA [302]. But a significantly prolonged “turn-off” time has been seen when working in air that was attributed to moisture penetrating into the surface layer and following increased surface conductivity of the device. It has been found that thermally stable Schottky contact materials for p-diamond are metal compounds which

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did not react with diamond at elevated temperatures. For example, it has been shown that metal carbide and nitride contacts are useful for fabrication thermally stable diamond DUV PD [296]. Fig. 14(a) shows the dependence of the specific contact resistance values on boron (B) acceptor concentration (NA) for Ti, Mo, Cr, Pd, and Co contacts (open symbols for before annealing and closed symbols for after annealing) [296]. This figure shows reaction between the diamond and the contact metals results in reduction of the specific contact resistance. Also, Fig. 14(b) shows plasma post treatment of detector results in a sharp increased response (nearly four orders of magnitude higher response) in the UV region peaking at 225 nm, which corresponds to the diamond band-to-band transition [300]. In another work, a semi-transparent tungsten carbide (WC) or hafnium nitride (HfN) Schottky contact and an annealed Ti/WC Ohmic contact on a boron-doped homoepitaxial p-type diamond layer has been used to fabricate deep-UV PDs [296]. Ratio of deep UV to visible was as large as 106. Also, thermal annealing resulted in dramatic enhancement of responsivity by a factor of 103 at 220 nm. B doping concentration significantly has reduced the response time. Recently, realization of compact beam-profiling system for UV and X-ray imaging, based on polycrystalline CVD diamond detectors has been reported. Diamond properties, such as radiation hardness and solar blindness, have enabled real-time monitoring with no need for attenuators or wavelength converters. This work proves that diamond-based UV and X-ray beam-profiling systems, coupled to fast multichannel read-out electronics, can definitely compete with commercial silicon-based devices [303]. 10. ZnSe UV PDs ZnSe is an important II–VI semiconductor material with wide direct bandgap energy of 2.67 eV that enable it to UV photodetection. Compared to other compound wide band gap semiconductors, such as gallium nitride (GaN), which is also sensitive in UV region, ZnSe has a broader spectral sensitivity. It shows the relatively broad spectral characteristics of ZnSe-based photodiode in the UV region of 300–450 nm, which is needed for fabricating optoelectronic devices in the UV A and B ranges. Also, it was also found that the ZnSe based detectors have low leakage current compared to GaN counterparts. ZnSe is closely lattice matched with GaAs substrates and GaAs substrates are much available than ZnSe substrates. Therefore, many structures of UV PDs based on ZnSe have been grown on GaAs substrates. However, slight lattice mismatch (0.27% at room temperature) between ZnSe and GaAs substrates can produce a huge amount of defects on ZnSe thin film. Therefore, growing lattice matched ternary or quaternary compounds of ZnSe on top of GaAs substrates can effectively suppress this problem. For example, ZnSSe n þ –i–p structure on p-type GaAs has been used [304]. With this structure, sensitivities have been reported of 0.30 A/W (external quantum efficiency: η¼83.6%) in the blue (450 nm), 0.24 A/W (η ¼75.8%) in the blue-violet (400 nm: for blue ray) and

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Fig. 15. Spectral responses of ZnSe UV PDs with a 1 V applied bias (with permission from the publisher and authors) [308].

0.12 A/W (η¼ 50.9%) in the UV (300 nm) regions. noise equivalent power value was as low as 10  14 W Hz  1/2 cm  1 in the blue-violet region (450 nm). Compared to ternary compounds, the quaternary structures have an extra degree of freedom to control the bandgap energy and lattice constant that adjusting bandgap energy becomes easier. ZnMgBeSe quaternary alloys are promising direct wide-band gap materials with the gap ranging from 2.75 to more than 3.8 eV. Because of difficulties to achieve p-type doping in these materials, fabrication of p–n junction is difficult. But, Schottky barrier photodiodes based on ZnMgBeSe alloys have been fabricated [305]. These PDs have been shown quantum efficiencies above 50% at 380–315 nm spectral range and their detectivity were 2  1010 mHz1/2 W  1. Also, Single-crystalline ZnSe nanobelts have been fabricated via the ethylenediamine assisted ternary solution technique and subsequent thermal treatment. The photoconductive devices of ZnSe nanobelts have been revealed an ultralow dark current (below the detection limit, 10  14 A, of the current meter), a high photocurrent immediate decay ratio ( 499%), and a fast time response (o0.3 s) that it was the fastest among all conventional ZnSe PDs. These properties were attributed to the large surface to volume ratios, the pure chemistry and singlecrystalline character of the fabricated ZnSe nanobelts, and the designed short channel distances between the electrodes within the prepared photodevices [306]. Recently, p-type ZnSe nanowires with tunable electrical conductivity have been fabricated by evaporating a mixed powder composed of ZnSe and Sb in different ratios. A crossed p–n nano-heterojunction PD has been made from the Sb-doped ZnSe nanowires displays pronounced rectification behavior, with a rectification ratio as high as 103 at 75 V and a response time of less than 1 s [307]. Homoepitaxial ZnSe MSM PDs with different contact electrodes of ITO, TiW and Ni/Au have been fabricated [308]. These structures have shown maximum responsivities of 120, 50.6 and 28.1 mA/W and noise equivalent power of 8.14  10  13, 1.73  10  12 and 9.25  10  13 W, respectively (see Fig. 15). Their barrier heights for electrons

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were 0.66, 0.695 and 0.715 eV for ITO, TiW and Ni/Au, respectively. The authors concluded semi-transparent metals with high work functions, such as Ni/Au and TiW are better because low Schottky barrier result in a large dark leakage current. 11. Other types of UV PDs Fluorides are relatively wide band gap materials. By using wide gap fluorides, the development of VUV photoconductive detector, which has no sensitivity in deep UV region without any filters, will become possible. The growth of neodymium fluoride (NdF3) thin films on quarts glass substrates by pulsed laser deposition has been reported. It exhibited the maximum photocurrent and the response below 180 nm in sensitivity spectrum. The increase in current achieved 4-digit growth before and after VUV illumination. These observed characteristics open up the possibility of high sensitive VUV photoconductive detectors using fluorides [309]. Also, researchers have been shown that CeF3 thin films could be an attractive alternative to wide band gap materials for UV photodetection. They deposited CeF3 thin films on quartz glass substrates by pulsed laser deposition. At an applied bias of 600 V, the dark current was 0.32 nA and the photocurrent was 0.54 nA for a mercury lamp at a power density of 12 mW cm  2. Changing the substrate temperature to either 470 K or 870 K caused a decrease in photoconductivity due to crystalline degradation [310]. Visible-blind microscale ZnS nanobelt-based UV sensors have been reported. The devices exhibited low dark and immediate currents after turning off the UV-light and high stability in air or under vacuum conditions even at a temperature of 150 1C photoresponsivity ratios about three orders of magnitude under UV-light illumination compared with visible light [311]. In another work, enhanced stability and sensitivity of the UV sensors composed of multiple ZnS-nanobelt devices compared to the individual nanobelt-based sensors has been demonstrated. The photoresponsivity of ZnS-nanobelt-based UV-light sensors exhibited over three orders of magnitude gain under UVlight illumination as compared to visible light. The high spectral selectivity combined with high photosensitivity and fast response time (o0.3 s) exhibit the ZnS nanobelts valuable for visible blind UV light PDs, especially in the UV-A region [312]. MBE grown MgS on GaAs (111) substrate was resulted in wurtzite phase. It has a direct bandgap at around 5.1 eV. The response peak was at 245 nm with quantum efficiency of 9.9% and rejection ratio was more than three orders at 320 nm. The sharp cutoff and large visible rejection power of this photodiode system have proven that it is a strong candidate in solar-blind UV detection applications [313]. Recently, Ge/Zn2SiO4 thin films were fabricated on the Si substrates through a thermal evaporation method. Nickel (Ni) contacts were deposited on the Ge/Zn2SiO4 films to fabricate MSM structure. The photoelectric properties of the MSM in the deep UV demonstrate that the film contributes to photosensitivity. The spectral response curves of the films integrated as MSM PDs has shown that the films have respectable responses to UV light

irradiation and a significant response in deep UV-C regions. Therefore, Ge/Zn2SiO4 films can be potential PDs in short wavelength applications [314]. Pd nanoparticles with a size around 2 nm have been deposited on top of the LaAlO3 surface, and the LaAlO3/ SrTiO3 interfacial two-dimensional electron gas presented optical switching to UV light with a wavelength shorter than 400 nm. This giant optical switching behavior was attributed to Pd NPs' catalytic effect and surface/interface charge coupling. These results are interesting for sensor applications such as UV light sensing and gas sensing [315].

12. Conclusion The importance of semiconductor UV PD has expanded the semiconductor industry. Even though the responsivity of Si-based optical PDs in the UV region is low, because of accessible fabrication technology, they are still being used for light detection. This has promoted some researchers to use wide direct band gap materials to fabricate UV optoelectronic devices. Wide band gap materials have superior physical properties compared Si such as higher thermal conductivity, better radiation hardness and durability and greater resistance to dielectric breakdown. These properties permit the use of wide band gap PDs in environmentally demanding application that require resistance for example to high pressure, high temperature, high voltage or high incident power. Hence, SiC, Diamond, ZnSe, AlGaN and GaN-based UV PDs have already become commercially available. ZnO is another semiconductor of wide direct bandgap that is also sensitive in the UV region and is of low cost and easy to manufacture. Initially, ZnSe and GaN based technologies made significant progress in optoelectronic devices. No doubt, GaN is considered to be the best candidate for the fabrication of optoelectronic devices. Also, ZnO is promising wide band gap material for optoelectronic short wavelength devices. Recent efforts in UV PDs have been focused to advance reliable, high responsivity, wide band gap, low cost and various functionality devices. However, they can still not replace silicon detectors in the short term due to the poor reproducibility and reliability of the devices. There is still plenty of room for the development of wide bandgap semiconductors and their PD applications. The development of high-quality substrates for homoepitaxial growth is the key point to improve the reproducibility and reduce costs. The development of nanostructures multiplies these advances. Recently, ternary and quaternary compounds are used in UV PD structures obtaining high selectivity in wavelengths and lattice matching to substrate. Research on II–IV compounds and organic materials have attracted interest in the past few years, as in the future they open up the new stages in PD development. In addition, the ability of wide bandgap PDs to function under extreme conditions offers new markets for new technologies and in the future, it is clear that these PDs are likely to be very promising areas for many commercial applications.

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References [1] E. Monroy, F. Omnès, F. Calle, Semicond. Sci. Technol. 18 (2003) R33. [2] S. Gateva, in: T. Smiljanic (Ed.), Photodetectors, In Tech, 2012. [3] P.N.J. Dennis, Photodetectors: An Introduction to Current Technology. Uptodates in Applied Physics and Electrical Technology, Springer, 1986. [4] S.M. Sze, Physics of Semiconductor Devices, second ed. Wiley, 1981. [5] W. Guosheng, L. Hai, C. Dunjun, R. Fangfang, Z. Rong, Z. Youdou, Photonics Technol. Lett., IEEE 25 (2013) 652–654. [6] R.W. Chuang, S.P. Chang, S.J. Chang, Y.Z. Chiou, C.Y. Lu, T.K. Lin, et al., J. Appl. Phys. 102 (2007) 073110-4. [7] A. Müller, G. Konstantinidis, M. Androulidaki, A. Dinescu, A. Stefanescu, A. Cismaru, et al., Thin Solid Films 520 (2012) 2158–2161. [8] J.T. Torvik, J.I. Pankove, B.J. Van Zeghbroeck, IEEE Trans. Electron Devices 46 (1999) 1326–1331. [9] R.X. Wang, L.C. Yang, Y.M. Zhang, S.J. Xu, K. Fu, B.S. Zhang, et al., Appl. Phys. Lett. (2013) 102:. [10] L. Hai-Feng, W. Guo-Sheng, L. Hai, R. Fang-Fang, C. Dun-Jun, Z. Rong, et al., Chin. Phys. Lett. 30 (2013) 017302. [11] K. You, H. Jiang, D. Li, X. Sun, H. Song, Y. Chen, et al., Appl. Phys. Lett. 100 (2012) 121109–4. [12] H. Zheng-Da, W. Wen-Yin, C. Shoou-Jinn, H. Yuan-Fu, C. Chiu-Jung, H. Ting-Jen, et al., Sens. J., IEEE 13 (2013) 1187–1191. [13] H. Chao-Wei, C. Yung-Feng, S. Yan-Kuin, IEEE J. Quantum Electron. 50 (2014) 35–41. [14] F. Xie, H. Lu, D. Chen, R. Zhang, Y. Zheng, M.S.M. GaN, Phys. Status Solidi C 8 (2011) 2473–2475. [15] E. Monroy, T. Palacios, O. Hainaut, F. Omnes, F. Calle, J.F. Hochedez, Appl. Phys. Lett. 80 (2002) 3198–3200. [16] Y. Tian, S.-J. Chua, H. Wang, Solid-State Electron. 47 (2003) 1863–1867. [17] A. Müller, G. Konstantinidis, M. Dragoman, D. Neculoiu, A. Dinescu, M. Androulidaki, et al., Microelectron. J. 40 (2009) 319–321. [18] Y.-Z. Chiou, Y.-K. Su, S.-J. Chang, Y.-C. Lin, C.-S. Chang, C.-H. Chen, Solid-State Electron. 46 (2002) 2227–2229. [19] Y.-Z. Chiou, J.-R. Chiou, Y.-K. Su, S.-J. Chang, B.-R. Huang, C.-S. Chang, et al., Mater. Chem. Phys. 80 (2003) 201–204. [20] S. Biondo, W. Vervisch, L. Ottaviani, J. Appl. Phys. (2012) 111. [21] M. Asif Khan, J.N. Kuznia, D.T. Olson, M. Blasingame, A.R. Bhattarai, Appl. Phys. Lett. 63 (1993) 2455–2456. [22] E. Monroy, F. Calle, E. Muñoz, B. Beaumont, F. Omnès, P. Gibart, Phys. Status Solidi A 176 (1999) 141–145. [23] E. Monroy, F. Calle, J.L. Pau, E. Muñoz, F. Omnès, B. Beaumont, et al., J. Cryst. Growth 230 (2001) 537–543. [24] P.C. Chang, Y.K. Su, K.J. Lee, C.L. Yu, S.J. Chang, C.H. Liu, J. Alloys Compd. 504 (Supplement 1) (2010) S429–S431. [25] Y. Zhou, C. Ahyi, C.-C. Tin, J. Williams, M. Park, D.-J. Kim, et al., Appl. Phys. Lett. 90 (2007) 121118. [26] D.G. Zhao, D.S. Jiang, J.J. Zhu, Z.S. Liu, S.M. Zhang, J.W. Liang, et al., Appl. Phys. Lett. 90 (2007) 062106. [27] Z. De-Gang, J. De-Sheng, L. Zong-Shun, Z. Jian-Jun, W. Hui, Z. ShuMing, et al., Chin. Phys. Lett. 26 (2009) 058501. [28] S.J. Chang, M.L. Lee, J.K. Sheu, W.C. Lai, Y.K. Su, C.S. Chang, et al., IEEE Electron Device Lett. 24 (2003) 212–214. [29] Y.D. Jhou, S.J. Chang, Y.K. Su, Y.Y. Lee, C.H. Liu, H.C. Lee, Appl. Phys. Lett. 91 (2007) 103506-3. [30] F. Xie, H. Lu, D. Chen, F. Ren, R. Zhang, Y. Zheng, Photonics Technol. Lett., IEEE 24 (2012) 2203–2205. [31] Q. Chen, M.A. Khan, C.J. Sun, J.W. Yang, Electron. Lett. 31 (1995) 1781–1782. [32] L. Pomortseva, Semiconductors 45 (2011) 436–444. [33] D.G. Zhao, D.S. Jiang, J.J. Zhu, H. Wang, Z.S. Liu, S.M. Zhang, et al., J. Alloys Compd. 492 (2010) 300–302. [34] X.D. Wang, W.D. Hu, X.S. Chen, L. Wang, X.Y. Li, W. Lu, Photoresponse simulation of visible blind GaN/AlGaN p–i–n photodiode, in: Proceedings of the 10th International Conference on Numerical Simulation of Optoelectronic Devices (NUSOD), 2010, pp. 31–32. [35] W. Jingjing, S. Zhaoqi, S. Shiwei, W. Mingzai, H. Gang, L. Guang Dependence of electrical field and photoresponse on multiplication region thickness for GaN APDs. Numerical Simulation of Optoelectronic Devices (NUSOD), 2013 13th International Conference on2013. p. 29-30. [36] F. Xie, H. Lu, X. Xiu, D. Chen, P. Han, R. Zhang, et al., Solid-State Electron. 57 (2011) 39–42. [37] M. Zhao, J. Bao, X. Fan, F. Gu, Y. Guo, Y. Zhang, et al., Phys. B: Condens. Matter 404 (2009) 275–277.

37

[38] K.M.A. Saron, M.R. Hashim, M.A. Farrukh, Superlattices Microstruct. 64 (2013) 88–97. [39] L. Rigutti, M. Tchernycheva, A. De Luna Bugallo, G. Jacopin, F.H. Julien, L.F. Zagonel, et al., Nano Lett. 10 (2010) 2939–2943. [40] Y. Nakano, N. Matsuki, Y. Irokawa, M. Sumiya, Phys. Status Solidi C 7 (2010) 2007–2009. [41] M. Kumar, C.Y. Lee, H. Sekiguchi, H. Okada, A. Wakahara, Semicond. Sci. Technol. 28 (2013) 094005. [42] B. Butun, T. Tut, E. Ulker, T. Yelboga, E. Ozbay, Appl. Phys. Lett. 92 (2008) 033507-3. [43] M. Martens, J. Schlegel, P. Vogt, F. Brunner, R. Lossy, J. Wurfl, et al., Appl. Phys. Lett. 98 (2011) 211114. [44] L. Ming-Lun, Y. Yu-Hsiang, T. Shang-Ju, IEEE J. Quantum Electron. 48 (2012) 1305–1309. [45] F. González-Posada, R. Songmuang, M. Den Hertog, E. Monroy, Phys. Status Solidi C 9 (2012) 642–645. [46] Y. Zhang, D. Yoo, J.-B. Limb, J.-H. Ryou, R.D. Dupuis, S.-C. Shen, et al., Performance evaluation of III-nitride avalanche photodiodes grown on SiC and GaN substrates, Meeting Abstracts, 2007, MA200702:1263. [47] T. Tut, M. Gokkavas, E. Ozbay, Phys. Status Solidi C 5 (2008) 2316–2319. [48] K.T. Lam, P.C. Chang, S.J. Chang, C.L. Yu, Y.C. Lin, Y.X. Sun, et al., Sens. Actuators A: Phys. 143 (2008) 191–195. [49] C.-J. Lee, Y.-J. Kwon, C.-H. Won, J.-H. Lee, S.-H. Hahm, Appl. Phys. Lett. (2013) 103:. [50] L. Chang-Ju, C. Hyeon-Gu, W. Chul-Ho, L. Jung-Hee, H. Sung-Ho, High UV–visible rejection ratio of dual-wavelength detecting MISIM UV sensor with a thin Al2O3 layer, in: Proceedings of the IEEE International Conference of 2013 on Electron Devices and Solid-State Circuits (EDSSC), 2013, pp. 1–2. [51] A. Rostami, N. Ravanbaksh, S. Golmohammadi, K. Abedi, Int. J. Numer. Model.: Electron. Netw. Devices Fields 27 (2014) 309–317. [52] D. Ke Xiu, C. Dun Jun, L. Hai, L. Bin, H. Ping, Z. Rong, et al., Photonics Technol. Lett., IEEE 25 (2013) 1510–1513. [53] S. Ghosh, C. Rivera, J.L. Pau, E. Munoz, O. Brandt, H.T. Grahn, Appl. Phys. Lett. 90 (2007) 091110-3. [54] D. Walker, E. Monroy, P. Kung, J. Wu, M. Hamilton, F.J. Sanchez, et al., Appl. Phys. Lett. 74 (1999) 762–764. [55] J. Carrano, T. Li, C. Eiting, R. Dupuis, J. Campbell, J. Electron. Mater. 28 (1999) 325–333. [56] L. Rigutti, M. Tchernycheva, A.D.L. Bugallo, G. Jacopin, F.H. Julien, R. Songmuang, et al., Phys. Status Solidi A 207 (2010) 1323–1327. [57] H. Hung, S.J. Chang, Y.C. Lin, H. Kuan, R.M. Lin, M.S.M. AlGaN, Optoelectronics, IET 1 (2007) 147–149. [58] M. Razeghi, P. Sandvik, P. Kung, D. Walker, K. Mi, X. Zhang, et al., Mater. Sci. Eng.: B 74 (2000) 107–112. [59] Z.M. Zhao, R.L. Jiang, P. Chen, D.J. Xi, Z.Y. Luo, R. Zhang, et al., Appl. Phys. Lett. 77 (2000) 444–446. [60] S.-H. Chang, Y.-K. Fang, K.-C. Hsu, T.-C. Wei, U.V. GaN, Sens. Actuators A: Phys. 147 (2008) 1–5. [61] D. Li, X. Sun, H. Song, Z. Li, Y. Chen, G. Miao, et al., Appl. Phys. Lett. 98 (2011) 011108. [62] H.Z. Xu, Z.G. Wang, M. Kawabe, I. Harrison, B.J. Ansell, C.T. Foxon, J. Cryst. Growth 218 (2000) 1–6. [63] N. Vanhove, J. John, A. Lorenz, K. Cheng, G. Borghs, Appl. Surf. Sci. 253 (2006) 2930–2932. [64] M.-L. Tu, Y.-K. Su, S.-J. Chang, R.W. Chuang, U.V. GaN, J. Cryst. Growth 298 (2007) 744–747. [65] Y.K. Su, P.C. Chang, C.H. Chen, S.J. Chang, C.L. Yu, C.T. Lee, et al., Solid-State Electron. 49 (2005) 459–463. [66] A.V. Babichev, H. Zhang, P. Lavenus, F.H. Julien, A.Y. Egorov, Y.T. Lin, et al., Appl. Phys. Lett. (2013) 103:. [67] T. Sawada, Y. Ito, K. Imai, K. Suzuki, H. Tomozawa, S. Sakai, Appl. Surf. Sci. 159–160 (2000) 449–455. [68] Y.D. Jhou, C.H. Chen, S.J. Chang, Y.K. Su, P.C. Chang, P.C. Chen, et al., Microelectron. J. 37 (2006) 328–331. [69] Y.C. Lee, Z. Hassan, M.J. Abdullah, M.R. Hashim, K. Ibrahim, Microelectron. Eng. 81 (2005) 262–267. [70] S. Averin, P. Kuznetsov, V. Zhitov, N. Alkeev, V. Kotov, A. Dorofeev, et al., Tech. Phys. 56 (2011) 295–297. [71] S.-P. Chang, S.-J. Chang, C.-Y. Lu, Y.-Z. Chiou, R.W. Chuang, H.-C. Lin, J. Cryst. Growth 311 (2009) 3003–3006. [72] R.-Y. Yang, C.-M. Hsiung, H.-H. Chen, H.-W. Wu, M.-C. Shih, Microw. Opt. Technol. Lett. 50 (2008) 2863–2866. [73] R. Dahal, T.M. Al Tahtamouni, Z.Y. Fan, J.Y. Lin, H.X. Jiang, Appl. Phys. Lett. 90 (2007). (263505-3). [74] A. Soltani, H.A. Barkad, M. Mattalah, B. Benbakhti, J.C. De Jaeger, Y. M. Chong, et al., Appl. Phys. Lett. 92 (2008) 053501–3.

Please cite this article as: Z. Alaie, et al., Materials Science in Semiconductor Processing (2014), http://dx.doi.org/10.1016/ j.mssp.2014.02.054i

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[75] S.V. Averine, P.I. Kuznetzov, V.A. Zhitov, N.V. Alkeev, Solid-State Electron. 52 (2008) 618–624. [76] D. Walker, X. Zhang, P. Kung, A. Saxler, S. Javadpour, J. Xu, et al., Appl. Phys. Lett. 68 (1996) 2100–2101. [77] K. Kishino, M. Yonemaru, A. Kikuchi, Y. Toyoura, Phys. Status Solidi A 188 (2001) 321–324. [78] M. Gokkavas, S. Butun, H. Yu, T. Tut, B. Butun, E. Ozbay, Appl. Phys. Lett. 89 (2006). (143503-3). [79] M. Gökkavas, S. Butun, T. Tut, N. Biyikli, E. Ozbay, Photonics Nanostructures – Fundam. Appl. 5 (2007) 53–62. [80] J. Li, M. Zhao, X.F. Wang, Phys. B: Condens. Matter 405 (2010) 996–998. [81] A. Teke, S. Dogan, F. Yun, M.A. Reshchikov, H. Le, X.Q. Liu, et al., Solid-State Electron. 47 (2003) 1401–1408. [82] Y.D. Jhou, C.H. Chen, R.W. Chuang, S.J. Chang, Y.K. Su, P.C. Chang, et al., Solid-State Electron. 49 (2005) 1347–1351. [83] A. Asgari, E. Ahmadi, M. Kalafi, Microelectron. J. 40 (2009) 104–107. [84] Q. Chen, J.W. Yang, A. Osinsky, S. Gangopadhyay, B. Lim, M. Z. Anwar, et al., Appl. Phys. Lett. 70 (1997) 2277–2279. [85] A. Bouhdada, M. Hanzaz, P. Gibart, F. Omnes, E. Monroy, E. Munoz, J. Appl. Phys. 87 (2000) 8286–8290. [86] W.-C. Lai, L.-C. Peng, C.-C. Chen, J.-K. Sheu, S.-C. Shei, Appl. Phys. Lett. 96 (2010) 102104. [87] P.E. Malinowski, J.-Y. Duboz, P. De Moor, K. Minoglou, J. John, S. M. Horcajo, et al., Appl. Phys. Lett. 98 (2011). (141104-3). [88] H. Jiang, T. Egawa, H. Ishikawa, C. Shao, T. Jimbo, Jpn. J. Appl. Phys. (2011) 43. [89] S.-S. Liu, P.-W. Li, W.H. Lan, W.-j. Lin, Mater. Sci. Eng.: B 121 (2005) 86–91. [90] E. Ozbay, N. Biyikli, I. Kimukin, T. Kartaloglu, T. Tut, O. Aytur, Sel. Top. IEEE J. Quantum Electron. 10 (2004) 742–751. [91] T. Tut, B. Butun, M. Gokkavas, E. Ozbay, Photonics Nanostructures – Fundam. Appl. 5 (2007) 140–144. [92] L.E. Rodak, A.V. Sampath, C.S. Gallinat, R.W. Enck, J. Smith, H. Shen, et al., Aluminum gallium nitride/silicon carbide separate absorption and multiplication avalanche photodiodes, in: Proceedings of the Lester Eastman Conference on High Performance Devices (LEC), 2012, pp. 1–4. [93] E. Cicek, R. McClintock, C.Y. Cho, B. Rahnema, M. Razeghi, Appl. Phys. Lett. 103 (2013) 191108. [94] E. Cicek, R. McClintock, C.Y. Cho, B. Rahnema, M. Razeghi, Appl. Phys. Lett. 103 (2013). (181113-4). [95] D.-W. Kim, K.-S. Chea, Y.-J. Park, I.-H. Lee, C.-R. Lee, Phys. Status Solidi A 201 (2004) 2686–2690. [96] K.Y. Park, B.J. Kwon, Y.-H. Cho, S.A. Lee, J.H. Son, J. Appl. Phys. 98 (2005) 124505. [97] P.-C. Chang, C.-H. Chen, S.-J. Chang, Y.-K. Su, C.-L. Yu, B.-R. Huang, et al., Thin Solid Films 498 (2006) 133–136. [98] K.Y. Park, B.J. Kwon, J.H. Son, Y.-H. Cho, Phys. Status Solidi C 3 (2006) 2299–2302. [99] J.L. Pau, E. Monroy, E. Muñoz, F.B. Naranjo, F. Calle, A. Sáncheź et al., J. Cryst. Growth 230 (2001). (544-8). Garcıa, [100] S.J. Chang, T.K. Ko, J.K. Sheu, S.C. Shei, W.C. Lai, Y.Z. Chiou, et al., Sens. Actuators A: Phys. 135 (2007) 502–506. [101] S.J. Chang, T.M. Kuan, C.H. Ko, Y.K. Su, J.B. Webb, J.A. Bardwell, et al., Solid-State Electron. 47 (2003) 2023–2026. [102] V. Lebedev, I. Cimalla, V. Cimalla, R. Wagner, U. Kaiser, O. Ambacher, Phys. Status Solidi C 2 (2005) 1360–1365. [103] C. Touzi, F. Omnès, T. Boufaden, P. Gibart, B. El Jani, Microelectron. J. 37 (2006) 336–339. [104] D. Walker, X. Zhang, A. Saxler, P. Kung, J. Xu, M. Razeghi, Appl. Phys. Lett. 70 (1997) 949–951. [105] H.C. Lee, Y.K. Su, J.C. Lin, Y.C. Cheng, T.C. Li, K.J. Chang, Solid-State Electron. 54 (2010) 488–491. [106] T.N. Oder, J. Li, J.Y. Lin, H.X. Jiang, Appl. Phys. Lett. 77 (2000) 791–793. [107] W. Dongbo, J. Shujie, S. Sujuan, Z. Liancheng, Al 0.40 in 0.02 Ga 0.58 N based metal–semiconductor–metal photodiodes for ultraviolet detection, in: Proceedings of the International Conference on 2012 of Optoelectronics and Microelectronics (ICOM), 2012, pp. 412–416. [108] W.-C. Lai, J.-T. Chen, Y.-Y. Yang, Appl. Phys. Lett. 102 (2013). (1911154). [109] J. Cheng, Y. Zhang, R. Guo, J. Cryst. Growth 310 (2008) 57–61. [110] H.K. Yadav, K. Sreenivas, V. Gupta, Appl. Phys. Lett. 90 (2007) 172113. [111] H.K. Yadav, K. Sreenivas, V. Gupta, Appl. Phys. Lett. 90 (2007). (172113-3). [112] H.K. Yadav, K. Sreenivas, V. Gupta, J. Appl. Phys. 107 (2010) 044507. [113] K. ul Hasan, O. Nur, M. Willander, Appl. Phys. Lett. (2012) 100.

[114] Z.-Q. Xu, H. Deng, J. Xie, Y. Li, X.-T. Zu, Appl. Surf. Sci. 253 (2006) 476–479. [115] A.M. Nahhas, ELECTRICA 10 (2008) 1–7. [116] J.S. Liu, C.X. Shan, B.H. Li, Z.Z. Zhang, C.L. Yang, D.Z. Shen, et al., Appl. Phys. Lett. 97 (2010) 251102. [117] K.W. Liu, J.G. Ma, J.Y. Zhang, Y.M. Lu, D.Y. Jiang, B.H. Li, et al., SolidState Electron. 51 (2007) 757–761. [118] H.F. Lui, W.K. Fong, C. Surya, Fabrication and characterization of Ni/ GaN Schottky junction erythemal UV detectors, in: Proceedings of the 2009 OptoElectronics and Communications Conference, OECC, 2009, pp. 1–2. [119] J.M. Liu, Y.B. Xia, L.J. Wang, Q.F. Su, W.M. Shi, J. Cryst. Growth 300 (2007) 353–357. [120] F.D. Auret, S.A. Goodman, M. Hayes, M.J. Legodi, H.A. van Laarhoven, D.C. Look, Appl. Phys. Lett. 79 (2001) 3074–3076. [121] S.J. Young, L.W. Ji, T.H. Fang, S.J. Chang, Y.K. Su, X.L. Du, Acta Mater. 55 (2007) 329–333. [122] N.N. Jandow, F.K. Yam, S.M. Thahab, H. Abu Hassan, K. Ibrahim, Curr. Appl. Phys. 10 (2010) 1452–1455. [123] N.N. Jandowa, K.A. Ibrahim, H.A. Hassana, S.M. Thahab, O.S. Hamad, J. Electron Devices 7 (2010) 225–229. [124] X.G. Zheng, Q.S. Li, J.P. Zhao, D. Chen, B. Zhao, Y.J. Yang, et al., Appl. Surf. Sci. 253 (2006) 2264–2267. [125] S.J. Young, L.W. Ji, S.J. Chang, Y.K. Su, J. Cryst. Growth 293 (2006) 43–47. [126] D. Jiang, J. Zhang, Y. Lu, K. Liu, D. Zhao, Z. Zhang, et al., Solid-State Electron. 52 (2008) 679–682. [127] S.J. Young, L.W. Ji, S.J. Chang, S.H. Liang, K.T. Lam, T.H. Fang, et al., Sens. Actuators A: Phys. 135 (2007) 529–533. [128] Y.Z. Li, X.M. Li, X.D. Gao, J. Alloys Compd. 509 (2011) 7193–7197. [129] H. Endo, M. Sugibuchi, K. Takahashi, S. Goto, S. Sugimura, K. Hane, et al., Appl. Phys. Lett. 90 (2007) 121906-3. [130] M. Nakano, A. Tsukazaki, R.Y. Gunji, K. Ueno, A. Ohtomo, T. Fukumura, et al., Appl. Phys. Lett. 91 (2007). (142113-142113). [131] M. Nakano, T. Makino, A. Tsukazaki, K. Ueno, A. Ohtomo, T. Fukumura, et al., Appl. Phys. Lett. 93 (2008) 123309-3. [132] S.J. Young, S.J. Chang, L.W. Ji, T.H. Meen, C.H. Hsiao, K.W. Liu, et al., Microelectron. Eng. 88 (2011) 113–116. [133] Z. Bi, X. Yang, J. Zhang, X. Bian, D. Wang, X. Zhang, et al., J. Electron. Mater. 38 (2009) 609–612. [134] B. Nie, J.-G. Hu, L.-B. Luo, C. Xie, L.-H. Zeng, P. Lv, et al., Small 9 (2013) 2872–2879. [135] F. Sun, C.-X. Shan, S.-P. Wang, B.-H. Li, Z.-Z. Zhang, C.-L. Yang, et al., Mater. Chem. Phys. 129 (2011) 27–29. [136] K. Wang, Y. Vygranenko, A. Nathan, Thin Solid Films 515 (2007) 6981–6985. [137] J. Yu, C.-X. Shan, Q. Qiao, X.-H. Xie, S.-P. Wang, Z.-Z. Zhang, et al., Sensors 12 (2012) 1280–1287. [138] J. Yu, C.X. Shan, X.M. Huang, X.W. Zhang, S.P. Wang, D.Z. Shen, J. Phys. D: Appl. Phys. 46 (2013) 305105. [139] T.C. Zhang, Y. Guo, Z.X. Mei, C.Z. Gu, X.L. Du, Appl. Phys. Lett. 94 (2009) 113508. [140] S.-Y. Tsai, M.-H. Hon, Y.-M. Lu, Solid-State Electron. 63 (2011) 37–41 . [141] W. Sheng-Bo, H. Chih-Hung, C. Shoou-Jinn, Z.Y. Jiao, Y. Sheng-Joue, H. Shang-Chao, et al., IEEE Trans. Nanotechnol. 12 (2013) 263–269. [142] W. Tian, T. Zhai, C. Zhang, S.-L. Li, X. Wang, F. Liu, et al., Adv. Mater. 25 (2013) 4625–4630. [143] C.-H. Chen, S.-J. Chang, S.-P. Chang, M.-J. Li, I.C. Chen, T.-J. Hsueh, et al., Chem. Phys. Lett. 476 (2009) 69–72. [144] H. Zhou, G. Fang, Y. Zhu, N. Liu, H. Wang, H. Huang, et al., Europhys. Lett. 97 (2012) 68001. [145] C. Chia-Hsun, L. Ching-Ting, Photonics Technol. Lett., IEEE 25 (2013) 348–351. [146] C.-H. Chen, J.-T. Yan, C.-T. Lee, ECS Trans. 28 (2010) 27–32. [147] L. Hu, J. Yan, M. Liao, H. Xiang, X. Gong, L. Zhang, et al., Adv. Mater. 24 (2012) 2305–2309. [148] Z. Guo, D. Zhao, Y. Liu, D. Shen, J. Zhang, B. Li, Appl. Phys. Lett. 93 (2008) 163501-3. [149] H. Huang, G. Fang, X. Mo, L. Yuan, H. Zhou, M. Wang, et al., Appl. Phys. Lett. 94 (2009) 063512-3. [150] Y. Xie, M. Madel, Y. Li, W. Jie, B. Neuschl, M. Feneberg, et al., J. Appl. Phys. (2012) 112. [151] C.Y. Kung, S.L. Young, M.C. Kao, H.Z. Chen, J.H. Lin, H.H. Lin, et al., Thickness effect of sputtered ZnO seed layer on the electrical properties of Li-doped ZnO nanorods and application on the UV photodetector, in: Proceedings of the 2013 IEEE 5th International Nanoelectronics Conference (INEC), 2013, pp. 417–20.

Please cite this article as: Z. Alaie, et al., Materials Science in Semiconductor Processing (2014), http://dx.doi.org/10.1016/ j.mssp.2014.02.054i

Z. Alaie et al. / Materials Science in Semiconductor Processing ] (]]]]) ]]]–]]] [152] Y.-Y. Lin, C.-W. Chen, W.-C. Yen, W.-F. Su, C.-H. Ku, J.-J. Wu, Appl. Phys. Lett. 92 (2008) 233301. [153] Q. Tang, L. Lin, X. Zhao, K. Huang, J. Wu, Langmuir 28 (2012) 3972–3978. [154] L.i.H. Jei, C. Shoou Jin, W. Chih Hung, H. Ting Jen, IEEE Electron Device Lett. 34 (2013) 1023–1025. [155] I.C. Yao, T.-Y. Tseng, P. Lin, Sens. Actuators A: Phys. 178 (2012) 26–31 . [156] H.Y. Yang, D.I. Son, T.W. Kim, J.M. Lee, W.I. Park, Org. Electron. 11 (2010) 1313–1317. [157] W.W. Liu, B. Yao, B.H. Li, Y.F. Li, J. Zheng, Z.Z. Zhang, et al., Solid State Sci. 12 (2010) 1567–1569. [158] Y. Zhu, H. Zhou, G. Fang, M. Li, Semicond. Sci. Technol. 27 (2012) 065003. [159] D. Ick Son, H. Yeon Yang, T. Whan Kim, W. Il Park, Appl. Phys. Lett. (2013) 102. [160] X.-W. Fu, Z.-M. Liao, Y.-B. Zhou, H.-C. Wu, Y.-Q. Bie, J. Xu, et al., Appl. Phys. Lett. (2012) 100. [161] S. Dali, Y. Mingpeng, L. Jie, IEEE Electron Device Lett. 34 (2013) 1169–1171. [162] F. Zhang, S. Niu, W. Guo, G. Zhu, Y. Liu, X. Zhang, et al., ACS Nano 7 (2013) 4537–4544. [163] G.M. Ali, P. Chakrabarti, Appl. Phys. Lett. 97 (2010) 031116-3. [164] Z.-S. Hu, F.-Y. Hung, K.-J. Chen, S.-J. Chang, W.-K. Hsieh, T.-Y. Liao, et al., Appl. Surf. Sci. 279 (2013) 31–35. [165] Y. Yang, W. Guo, J. Qi, J. Zhao, Y. Zhang, Appl. Phys. Lett. 97 (2010) 223113. [166] L.J. Mandalapu, F.X. Xiu, Z. Yang, J.L. Liu, Solid-State Electron. 51 (2007) 1014–1017. [167] J. Sun, F.-J. Liu, H.-Q. Huang, J.-W. Zhao, Z.-F. Hu, X.-Q. Zhang, et al., Appl. Surf. Sci. 257 (2010) 921–924. [168] S.S. Shinde, K.Y. Rajpure, U.V. High-performance, Appl. Surf. Sci. 257 (2011) 9595–9599. [169] Y. Wu, E. Girgis, V. Ström, W. Voit, L. Belova, K.V. Rao, Phys. Status Solidi A 208 (2011) 206–209. [170] J. Sun, Q. Dai, F. Liu, H. Huang, Z. Li, X. Zhang, et al., Sci. China Phys., Mech. Astron. 54 (2011) 102–105. [171] S. Kumar, G.-H. Kim, K. Sreenivas, R. Tandon, J. Electroceram. 22 (2009) 198–202. [172] W.-S. Wang, C.-J. Tsai, C.-C. Ma, J. Appl. Phys. (2013) 114. [173] Q. Yang, X. Guo, W. Wang, Y. Zhang, S. Xu, D.H. Lien, et al., ACS Nano 4 (2010) 6285–6291. [174] C. Soci, A. Zhang, X.-Y. Bao, H. Kim, Y. Lo, D. Wang, J. Nanosci. Nanotechnol. 10 (2010) 1430–1449. [175] L. Vj, J. Oh, A.P. Nayak, A.M. Katzenmeyer, K.H. Gilchrist, S. Grego, et al., Sel. Top. IEEE J. Quantum Electron. 17 (2011) 1002–1032. [176] W.Y. Weng, T.J. Hsueh, S.J. Chang, S.P. Chang, C.L. Hsu, Superlattices Microstruct. 46 (2009) 797–802. [177] C. Sheng-Po, L. Chien-Yuan, C. Shoou-Jinn, C. Yu-Zung, H. Ting-Jen, H. Cheng-Liang, Sel. Top. IEEE J. Quantum Electron. 17 (2011) 990–995. [178] S. Junya, N. Nobutaka, H. Shin-ichiro, U. Makoto, I. Kiminobu, H. Mitsuhiro, et al., Nanotechnology 17 (2006) 2567. [179] K. Liu, M. Sakurai, M. Liao, M. Aono, J. Phys. Chem. C 114 (2010) 19835–19839. [180] S.-K. Tzeng, M.-H. Hon, I.-C. Leu, J. Electrochem. Soc. 159 (2012) H440–H443. [181] K.-P. Kim, D. Chang, S.K. Lim, S.-K. Lee, H.-K. Lyu, D.-K. Hwang, Jpn. J. Appl. Phys. 50 (2011) 1–3. [182] Y.H. Leung, Z.B. He, L.B. Luo, C.H.A. Tsang, N.B. Wong, W.J. Zhang, et al., Appl. Phys. Lett. 96 (2010) 053102. [183] S.-K. Tzeng, M.-H. Hon, I.-C. Leu, Mater. Chem. Phys. 139 (2013) 963–967. [184] C.-L. Hsu, H.-H. Li, T.-J. Hsueh, ACS Appl. Mater. Interfaces 5 (2013) 11142–11151. [185] L.W. Ji, S.M. Peng, Y.K. Su, S.J. Young, C.Z. Wu, W.B. Cheng, Appl. Phys. Lett. 94 (2009) 203106. [186] H. Zhou, G.-J. Fang, N. Liu, X.-Z. Zhao, Mater. Sci. Eng.: B 176 (2011) 740–744. [187] C. Soci, A. Zhang, B. Xiang, S.A. Dayeh, D.P.R. Aplin, J. Park, et al., Nano Lett. 7 (2007) 1003–1009. [188] L. Wang, D. Zhao, Z. Su, F. Fang, B. Li, Z. Zhang, et al., Org. Electron. 11 (2010) 1318–1322. [189] O. Lupan, L. Chow, G. Chai, L. Chernyak, O. Lopatiuk-Tirpak, H. Heinrich, Phys. Status Solidi A 205 (2008) 2673–2678. [190] J. Huisu, K. Ki Seok, K. Yong Hwan, J. Hyun, S. Hui, H.o.L. Kwang, et al., Nanotechnology 22 (2011) 275310. [191] Y. Li, X. Dong, C. Cheng, X. Zhou, P. Zhang, J. Gao, et al., Phys. B: Condens. Matter 404 (2009) 4282–4285.

39

[192] S.M. Hatch, J. Briscoe, A. Sapelkin, W.P. Gillin, J.B. Gilchrist, M. P. Ryan, et al., J. Appl. Phys. 113 (2013) 204501–9. [193] J.H. Jun, H. Seong, K. Cho, B.-M. Moon, S. Kim, Ceram. Int. 35 (2009) 2797–2801. [194] H.I. Abdulgafour, Z. Hassan, N.M. Ahmed, F.K. Yam, J. Appl. Phys. (2012) 112:. [195] Y. Wenjing, M. Norman, H. Horst, K. Ralph, Nanotechnology 21 (2010) 115501. [196] L. Qin, C. Shing, S. Sawyer, P.S. Dutta, Opt. Mater. 33 (2011) 359–362 . [197] S. Su, X. Yang, C. Hu, J. Semicond. 32 (2011) 074008. [198] P. Gao, Z.L. Wang, J. Phys. Chem. B 106 (2002) 12653–12658. [199] F. Yi, Q. Liao, Y. Huang, Y. Gu, Y. Zhang, Semicond. Sci. Technol. 28 (2013) 105023. [200] X.H. Xie, Z.Z. Zhang, C.X. Shan, H.Y. Chen, D.Z. Shen, Appl. Phys. Lett. (2012) 101:. [201] L.K. Wang, Z.G. Ju, C.X. Shan, J. Zheng, D.Z. Shen, B. Yao, et al., Solid State Commun. 149 (2009) 2021–2023. [202] C.Y. Zhao, X.H. Wang, J.Y. Zhang, Z.G. Ju, C.X. Shan, B. Yao, et al., Thin Solid Films 519 (2011) 1976–1979. [203] Z.G. Ju, C.X. Shan, D.Y. Jiang, J.Y. Zhang, B. Yao, D.X. Zhao, et al., Appl. Phys. Lett. 93 (2008). (173505-3). [204] Y. Zhao, J. Zhang, D. Jiang, C. Shan, Z. Zhang, B. Yao, et al., ACS Appl. Mater. Interfaces 1 (2009) 2428–2430. [205] H. Zhu, C.X. Shan, L.K. Wang, J. Zheng, J.Y. Zhang, B. Yao, et al., J. Phys. Chem. C 114 (2010) 7169–7172. [206] Y.N. Hou, Z.X. Mei, H.L. Liang, D.Q. Ye, S. Liang, C.Z. Gu, et al., Appl. Phys. Lett. 98 (2011) 263501-3. [207] Z. Yanmin, Z. Jiying, J. Dayong, S. Chongxin, Z. Zhenzhong, Y. Bin, et al., J. Phys. D: Appl. Phys. 42 (2009) 092007. [208] D. Jiang, J. Qin, X. Zhang, Z. Bai, D. Shen, Mater. Sci. Eng.: B 176 (2011) 736–739. [209] Z. Zhang, H. von Wenckstern, M. Grundmann, Appl. Phys. Lett. 103 (2013) 171111-4. [210] Y.N. Hou, Z.X. Mei, H.L. Liang, D.Q. Ye, C.Z. Gu, X.L. Du, Appl. Phys. Lett. (2013) 102:. [211] J. Yu, C.X. Shan, J.S. Liu, X.W. Zhang, B.H. Li, D.Z. Shen, Phys. Status Solidi (RRL) – Rapid Res. Lett. 7 (2013) 425–428. [212] S. Han, Z. Zhang, J. Zhang, L. Wang, J. Zheng, H. Zhao, et al., Appl. Phys. Lett. (2011) 99:. [213] X.H. Xie, Z.Z. Zhang, B.H. Li, S.P. Wang, M.M. Jiang, C.X. Shan, et al., Appl. Phys. Lett. 102 (2013) 231122-4. [214] S. Han, J.Y. Zhang, Z.Z. Zhang, L.K. Wang, Y.M. Zhao, J. Zheng, et al., J. Phys. Chem. C 114 (2010) 21757–21761. [215] C.-Z. Wu, L.-W. Ji, S.-M. Peng, Y.-L. Chen, S.-J. Young, Electrochem. Solid-State Lett. 14 (2011) J55–J57. [216] H. Yaonan, M. Zengxia, L. Huili, Y. Daqian, G. Changzhi, D. Xiaolong, et al., IEEE Trans. Electron Devices 60 (2013) 3474–3477. [217] D.-Y. Jiang, X.-Y. Zhang, Q.-S. Liu, Z.-H. Bai, L.-P. Lu, X.-C. Wang, et al., Appl. Surf. Sci. 256 (2010) 6153–6156. [218] S. Han, J. Zhang, Z. Zhang, Y. Zhao, L. Wang, J. Zheng, et al., ACS Appl. Mater. Interfaces 2 (2010) 1918–1921. [219] H.L. Liang, Z.X. Mei, Y.N. Hou, S. Liang, Z.L. Liu, Y.P. Liu, et al., J. Cryst. Growth 381 (2013) 6–9. [220] D. Jiang, C. Shan, J. Zhang, Y. Lu, B. Yao, D. Zhao, et al., Cryst. Growth Des. 9 (2008) 454–456. [221] C. Li, Z. Zhang, H. Chen, X. Xie, D. Shen, Thin Solid Films 548 (2013) 456–459. [222] C. Yan, N. Singh, P.S. Lee, Appl. Phys. Lett. 96 (2010) 053108. [223] C. Li, Y. Bando, M. Liao, Y. Koide, D. Golberg, Appl. Phys. Lett. 97 (2010) 161102–3. [224] Z. Ji, Z. He, K. Liu, S. Zhao, Z. He, J. Cryst. Growth 273 (2005) 446–450. [225] J.W. Mares, R.C. Boutwell, M. Wei, A. Scheurer, W.V. Schoenfeld, Appl. Phys. Lett. 97 (2010) 161113-3. [226] J.-M. Wu, C.-H. Kuo, Thin Solid Films 517 (2009) 3870–3873. [227] Y. Xie, H. Huang, W. Yang, Z. Wu, J. Appl. Phys. 109 (2011) 023114. [228] W.J. Wang, C.X. Shan, H. Zhu, F.Y. Ma, D.Z. Shen, X.W. Fan, et al., J. Phys. D: Appl. Phys. 43 (2010) 045102. [229] G. Zhang, J.-L. Sun, J. Wei, H. Sun, J.-L. Zhu, Nanotechnology 24 (2013) 465203. [230] J. Shang, H. Qi, H. Feng, L. Wang, J. Zhang, Y. Wang, et al., Phys. Status Solidi (RRL) – Rapid Res. Lett. 7 (2013) 1071–1075. [231] M.-Z. Wang, F.-X. Liang, B. Nie, L.-H. Zeng, L.-X. Zheng, P. Lv, et al., Part. Part. Syst. Charact. 30 (2013) 630–636. [232] D. Shao, M. Yu, J. Lian, S. Sawyer, Nanotechnology 24 (2013) 295701. [233] K. Khun, Z.H. Ibupoto, M. Willander, Phys. Status Solidi A 210 (2013) 2720–2724.

Please cite this article as: Z. Alaie, et al., Materials Science in Semiconductor Processing (2014), http://dx.doi.org/10.1016/ j.mssp.2014.02.054i

40

Z. Alaie et al. / Materials Science in Semiconductor Processing ] (]]]]) ]]]–]]]

[234] M. Zhang, X. Gu, K. Lv, W. Dong, S. Ruan, Y. Chen, et al., Appl. Surf. Sci. 268 (2013) 312–316. [235] K. Deng, H. Lu, Z. Shi, Q. Liu, L. Li, ACS Appl. Mater. Interfaces 5 (2013) 7845–7851. [236] S. Huang, K. Matsubara, J. Cheng, H. Li, W. Pan, Appl. Phys. Lett. (2013) 103:. [237] C.-Y. Huang, R.-H. Horng, D.-S. Wuu, L.-W. Tu, H.-S. Kao, Appl. Phys. Lett. (2013) 102:. [238] R. Zou, Z. Zhang, J. Hu, L. Sang, Y. Koide, M. Liao, Nanotechnology 24 (2013) 495701. [239] H. Feng, W. Hao, C. Zhao, X. Xin, J. Cheng, Y. Cui, et al., Phys. Status Solidi A 210 (2013) 1861–1865. [240] H. Huolin, X. Yannan, Y. Weifeng, Z. Feng, C. Jiafa, W. Zhengyun, IEEE Electron Device Lett. 32 (2011) 530–532. [241] H. Xue, X. Kong, Z. Liu, C. Liu, J. Zhou, W. Chen, et al., Appl. Phys. Lett. 90 (2007) 201118. [242] J. Xing, Z. Changchun, G. Erjia, F. Yang, Sens. J., IEEE 12 (2012) 2561–2564. [243] Z. Min, C. Yu, Z. Haifeng, C. Weiyou, L. Kaibo, R. Shengping, IEEE Electron Device Lett. 34 (2013) 420–422. [244] Z. Min, L. Guohua, Z. Jingran, M. Xiaohui, L. Zhiqi, C. Weiyou, et al., IEEE Electron Device Lett. 34 (2013) 1539–1541. [245] Y. Han, G. Wu, M. Wang, H. Chen, Polymer 51 (2010) 3736–3743. [246] Y. Han, G. Wu, M. Wang, H. Chen, Appl. Surf. Sci. 256 (2009) 1530–1533. [247] X. Li, C. Gao, H. Duan, B. Lu, Y. Wang, L. Chen, et al., Small 9 (2013) 2005–2011. [248] T.D. Dao, C.T.T. Dang, G. Han, C.V. Hoang, W. Yi, V. Narayanamurti, et al., Appl. Phys. Lett. (2013) 103:-. [249] Y. Han, C. Fan, G. Wu, H.-Z. Chen, M. Wang, J. Phys. Chem. C 115 (2011) 13438–13445. [250] L. Li, P.S. Lee, C. Yan, T. Zhai, X. Fang, M. Liao, et al., Adv. Mater. 22 (2010) 5145–5149. [251] D. Shao, L. Qin, S. Sawyer, Appl. Surf. Sci. 261 (2012) 123–127. [252] J. Zou, Q. Zhang, K. Huang, N. Marzari, J. Phys. Chem. C 114 (2010) 10725–10729. [253] G. Zhang, W. Li, B. Chu, Z. Su, D. Yang, F. Yan, et al., Org. Electron. 10 (2009) 352–356. [254] F. Yang, M. Shtein, S.R. Forrest, Nat. Mater. 4 (2005) 37–41. [255] Z. Su, W. Li, B. Chu, T. Li, J. Zhu, G. Zhang, et al., Appl. Phys. Lett. 93 (2008) 103309. [256] Li W-l Wu S-h, B. Chu, C.S. Lee, Z.-s. Su, J.-b. Wang, et al., Appl. Phys. Lett. 96 (2010) 093302–3. [257] Z. Xu, C. Xu, Q. Wang, W. Jiang, C. Liu, L. Liu, et al., Synth. Met. 160 (2010) 2260–2264. [258] F. Yan, H. Liu, W. Li, B. Chu, Z. Su, G. Zhang, et al., Appl. Phys. Lett. 95 (2009) 253308. [259] W.u.G. Li H-g, H.-Z. Chen, M. Wang, Org. Electron. 12 (2011) 70–77. [260] J.R. Cárdenas, E.A. de Vasconcelos, W.M. de Azevedo, E.F. da Silva, I. Pepe, A.F. da Silva, et al., Appl. Surf. Sci. 255 (2008) 688–690. [261] W.u.G. Li H-g, M.-m. Shi, H.-Z. Chen, M. Wang, Synth. Met. 160 (2010) 1648–1653. [262] Q. Wang, H. Aziz, Org. Electron. 14 (2013) 3030–3036. [263] Y. Inagaki, M. Ebisu, M. Otsuki, N. Ayuni, T. Shimizu, T. Abe, et al., Phys. Status Solidi C 9 (2012) 1852–1855. [264] L. Zhu, W.-S. Wang, Z.-G. Yao, X.-Q. Zhang, Y.-S. Wang, Solid-State Electron. 80 (2013) 14–18. [265] Li W-l Wu S-h, B. Chu, C.S. Lee, Z.-s. Su, J.-b. Wang, et al., Appl. Phys. Lett. 97 (2010) 023306. [266] Q. Dai, X.Q. Zhang, Opt. Exp. 18 (2010) 11821–11826. [267] H.-G. Li, G. Wu, H.-Z. Chen, M. Wang, Curr. Appl. Phys. 11 (2011) 750–754. [268] J. Sun, K. Peng, L. Zhu, Z. Hu, Q. Dai, X. Zhang, et al., Chin. Opt. Lett. 9 (2011) 052501. [269] M. Burkhardt, W. Liu, C.G. Shuttle, K. Banerjee, M.L. Chabinyc, Appl. Phys. Lett. (2012) 101:-. [270] G. Memisoglu, C. Varlikli, Highly efficient organic U.V. photodetectors based on polyfluorene and naphthalenediimide blends: effect of thermal annealing, Int. J. Photoenergy 2012 (2012) 11. [271] T. Ikenoue, N. Kameyama, S. Fujita, Phys. Status Solidi C 8 (2011) 613–615. [272] C.-B. Liu, M. Liu, G.-B. Che, B. Su, L. Wang, X.-X. Zhang, et al., SolidState Electron. 89 (2013) 68–71. [273] C. Liu, L. Liu, G. Che, Y. Cui, Q. Wang, W. Li, et al., Solar Energy Mater. Solar Cells 96 (2012) 29–32. [274] L. Zhu, W.S. Wang, Z.G. Yao, X.Q. Zhang, Y.S. Wang, IEEE Trans. Electron Devices 59 (2012) 3583–3586. [275] F. Guo, B. Yang, Y. Yuan, Z. Xiao, Q. Dong, Y. Bi, et al., Nat. Nano 7 (2012) 798–802.

[276] J. Wei, Cryst. Res. Technol. 41 (2006) 186–191. [277] R.B. Campbell, C. Hung-Chi, Detection of ultraviolet radiation using silicon carbide P–N junctions, in: Proceedings of the International 1966 Electron Devices Meeting, 1966, pp. 26. [278] M. Razeghi, A. Rogalski, J. Appl. Phys. 79 (1996) 7433–7473. [279] X. Chen, W. Yang, Z. Wu, Microelectron. Eng. 83 (2006) 104–106. [280] X. Chen, H. Zhu, J. Cai, Z. Wu, J. Appl. Phys. 102 (2007) 024505-4. [281] X.F. Liu, G.S. Sun, J.M. Li, J. Ning, Y.M. Zhao, M.C. Luo, et al., Phys. Status Solidi C 4 (2007) 1609–1612. [282] W. Yang, F. Zhang, Z. Liu, Z. Wu, Mater. Sci. Semicond. Process. 11 (2008) 59–62. [283] B. Chen, Y. Yang, C. Chai, K. Song, Z. Ma, J. Semicond. 32 (2011) 084001. [284] B. Chen, Y.-T. Yang, C.-C. Chai, X.-J. Zhang, Chin. Phys. Lett. 28 (2011) 068501. [285] F. Zhang, W. Yang, H. Huang, X. Chen, Z. Wu, H. Zhu, et al., Appl. Phys. Lett. 92 (2008) 251102. [286] J.F. Felix, E.A. deVasconcelos, E.F. da Silva, W.M. de Azevedo, J. Phys. D: Appl. Phys. 44 (2011) 205101. [287] T.-H. Chou, Y.-K. Fang, Y.-T. Chiang, C.-I. Lin, C.-Y. Yang, Sens. Actuators A: Phys. 147 (2008) 60–63. [288] B. Chen, Y. Yang, X. Xie, N. Wang, Z. Ma, K. Song, et al., Chin. Sci. Bull. 57 (2012) 4427–4433. [289] J.-M. Choi, S. Im, Appl. Surf. Sci. 244 (2005) 435–438. [290] J.W. Levell, M.E. Giardini, Opt. Exp. 18 (2010) 3219–3225. [291] L. Shi, F. Sarubbi, L.K. Nanver, U. Kroth, A. Gottwald, S. Nihtianov, Proc. Eng. 5 (2010) 633–636. [292] W.F. Mohamad, A. Abou Hajar, A.N. Saleh, Renew. Energy 31 (2006) 1493–1503. [293] J.F. Seely, R.S. Korde, F.A. Hanser, J. Wise, G.E. Holland, J.L. Weaver, et al., in: S. Fineschi, B.E. Woodgate, R.A. Kimble (Eds.), Characterization of Silicon Photodiode Detectors With Multilayer Filter Coatings for 17 to 150 A, 1st ed. SPIE, Denver, CO, USA, 1999, pp. 103–109. [294] L. Shi, S.N. Nihtianov, F. Scholze, L.K. Nanver, Electrical performance stability characterization of high-sensitivity Si-based EUV photodiodes in a harsh industrial application, in: Proceedings of the 38th Annual Conference on 2012 IEEE Industrial Electronics Society, IECON, 2012, pp. 3952–3957. [295] P.N. Aruev, M.M. Barysheva, B.Y. Ber, N.V. Zabrodskaya, V.V. Zabrodskii, A.Y. Lopatin, et al., Quantum Electron. 42 (2012) 943. [296] K. Yasuo, Appl. Surf. Sci. 254 (2008) 6268–6272. [297] S.C. Binari, M. Marchywka, D.A. Koolbeck, H.B. Dietrich, D. Moses, Diam. Rel. Mater. 2 (1993) 1020–1023. [298] T. Teraji, S. Yoshizaki, H. Wada, M. Hamada, T. Ito, Diam. Rel. Mater. 13 (2004) 858–862. [299] A. Balducci, A. De Sio, M. Marinelli, E. Milani, M.E. Morgada, E. Pace, et al., Diam. Rel. Mater. 14 (1980-3). [300] S.G. Wang, P.J. Sellin, A. Lohstroh, Q. Zhang, Appl. Phys. Lett. 86 (2005) 093503. [301] M. Liao, Y. Koide, Appl. Phys. Lett. 89 (2006) 113509–3. [302] J.J. Yu, I.W. Boyd, Diam. Rel. Mater. 16 (2007) 494–497. [303] M. Girolami, P. Allegrini, G. Conte, D.M. Trucchi, V.G. Ralchenko, S. Salvatori, IEEE Electron Device Lett. 33 (2012) 224–226. [304] K. Miki, T. Abe, J. Naruse, K. Ikumi, T. Yamaguchi, H. Kasada, et al., Physica Status Solidi B 243 (2006) 950–954. [305] F. Vigue, E. Tournie, J.P. Faurie, E. Monroy, F. Calle, E. Munoz, Appl. Phys. Lett. 78 (2001) 4190–4192. [306] X. Fang, S. Xiong, T. Zhai, Y. Bando, M. Liao, U.K. Gautam, et al., Adv. Mater. 21 (2009) 5016–5021. [307] B. Nie, L.-B. Luo, J.-J. Chen, J.-G. Hu, C.-Y. Wu, L. Wang, et al., Nanotechnology 24 (2013) 095603. [308] S.J. Chang, T.K. Lin, Y.K. Su, Y.Z. Chiou, C.K. Wang, S.P. Chang, et al., Mater. Sci. Eng.: B 127 (2006) 164–168. [309] M. Ieda, T. Ishimaru, S. Ono, Y. Yokota, T. Yanagida, A. Yoshikawa, Filterless vacuum ultraviolet photoconductive detector fabricated on NdF3 thin film, in: Proceedings of the IEEE 2012 Photonics Conference (IPC), 2012, pp. 382–383. [310] N. Horiuchi, Nat Photonics 6 (2012) 501. [311] X. Fang, Y. Bando, M. Liao, T. Zhai, U.K. Gautam, L. Li, et al., Adv. Funct. Mater. 20 (2010) 500–508. [312] X. Fang, Y. Bando, M. Liao, U.K. Gautam, C. Zhi, B. Dierre, et al., Adv. Mater. 21 (2009) 2034–2039. [313] Y.H. Lai, Q.L. He, W.Y. Cheung, S.K. Lok, K.S. Wong, S.K. Ho, et al., Appl. Phys. Lett. (2013) 102:. [314] M. Muzafa Jumidali, M.R. Hashim, K. Al-Heuseen, Mater. Sci. Semicond. Process. 16 (2013) 1360–1364. [315] N.Y. Chan, M. Zhao, N. Wang, K. Au, J. Wang, L.W.H. Chan, et al., ACS Nano 7 (2013) 8673–8679.

Please cite this article as: Z. Alaie, et al., Materials Science in Semiconductor Processing (2014), http://dx.doi.org/10.1016/ j.mssp.2014.02.054i