GaN superlattices structure

Materials Science and Engineering B 126 (2006) 33–36

The improvement of GaN p-i-n UV sensor by 8-pair AlGaN/GaN superlattices structure Su-Sir Liu a,∗ , Pei-Wen Li a , W.H. Lan b , Yi-Cheng Cheng c a

b

Department of Electrical Engineering, National Central University Taoyuan, Taiwan, ROC Department of Electrical Engineering, National University of Kaohsiung Taoyuan, Taiwan, ROC c Chung-Shan Institute of Science and Technology, Lung-Tan, Taoyuan 325, Taiwan, ROC Received 1 April 2005; accepted 20 July 2005

Abstract GaN with pairs of AlGaN/GaN superlattices (SLs) structure for p-i-n UV photo detector are fabricated on sapphire by metal organic chemical vapor deposition (MOCVD). For 8-pair AlGaN/GaN SLs not only eliminates cracking through this strain management, but it also significantly decreases the threading dislocation density by acting itself as an effective dislocation filter. The related structure has exhibited excellent film qualities such as enhanced crystallinity, lower specific contact resistance, lower etching pit density or mean roughness in the film. GaN p-i-n diode fabricated with 8-pair SLs, the dark current of device is reduced by two orders of magnitude than that without SLs structure at reverse bias of −3 V. Moreover, the peak UV responsivity is 0.12 A/W, which is higher than that without SLs is 0.07 A/W at 360 nm. The rejecting ratio is also by two orders of magnitude higher than that without SLs structure. © 2005 Elsevier B.V. All rights reserved. Keywords: Superlattice; Responsivity; EPD; AFM

1. Introduction Ultraviolet (UV) photo detection has drawn a great deal of attention in the recent years, due to the rise of new requirements [1,2]. Both civil and military industries demand better UV instrumentation, for applications such as engine control, solar UV monitoring, source calibration, UV astronomy, flame sensors, detection of missile plumes, and secure space-to-space communications [3–8]. Due to their compactness, low consume, and high stability, semiconductor devices are the best choice for UV photo detectors. The well-established silicon technology offers cheap and efficient solutions for UV detection, although these devices are sensitive to visible and infrared photons, and suffer from aging effect when exposed to high-energy radiation. Photo detectors based on wide-band gap semiconductors (diamond [9], SiC [10], III-nitrides [11] and wide-band gap II–VI materials [12,13]) can achieve UV selectivity without optical filters. Moreover, wide-band gap materials are chemically, mechanically and thermally stable, which is an advantage

∗

Corresponding author. E-mail address: [email protected] (S.-S. Liu).

0921-5107/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2005.07.027

for operation in harsh environments. III-nitrides (AlN, GaN, AlGaN, and related alloys) present some advantages over other wide-band gap semiconductors, such as the possibility of selecting the cut-off wavelength by changing the mole fraction of their ternary compounds, and their capability for hetero junction devices. High threading dislocation density induce crack is currently a main issue in the growth of GaN/sapphire epitaxy, which results in low performance of devices, such as current-scattering centers for light propagation as well as poor crystal quality. The crack is generally assumed caused by tensile stress due to a mismatch of thermal expansion coefficient between GaN and sapphire during cooling after growth. Recently, several groups have attempted to overcome the cracking problem through the insertion of strainrelief low temperature GaN or AlN interlayers between the AlGaN layers and the underlying GaN [14–17]. However, the LT-interlayer approach does not eliminate the cracking problem completely, In this work, the improvements of AlGaN/GaN ˚ ˚ superlattices (SLs) structure were carried out for (50 A/50 A) GaN p-i-n sensor for UV detection by metal organic chemical vapor deposition (MOCVD), through defect reducing by 8-pair ˚ ˚ superlattices (SLs) structure, not of AlGaN/GaN (50 A/50 A) only a better film quality can be achieved; but also the dark cur-

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S.-S. Liu et al. / Materials Science and Engineering B 126 (2006) 33–36

rent and spectrum responsivity of the device can be significantly improved on our sensor. 2. Experiments Samples used in this study were prepared by metal organic chemical vapor deposition (MOCVD) on sapphire substrates. The GaN p-i-n photodiode with/without different pairs ˚ ˚ superlattices (SLs) structure are shown AlGaN/GaN (50 A/50 A) in Fig. 1(a) using trimethylgallium (TMG) and ammonia (NH3 ) as gallium and nitrogen sources and H2 is used as the carrier gas, all structures consists of a LT–GaN buffer layer 1 ␮m GaN layer, 2 ␮m lightly doped Si–GaN layer (n = 2.5 × 1017 cm−3 ) con˚ ˚ tinued with/without different pairs of AlGaN/GaN (50 A/50 A) superlattices (SLs) structure, a 1 ␮m heavy doped N–GaN layer (n = 5 × 1018 cm−3 ) followed by 1 ␮m undoped GaN layer, and terminated by a 200 nm Mg-doped p-GaN layer. Mesa etching is then performed by ICP-RIE etching technique, using Cl2 and Ar as etching sources for device isolation. The device consists of circular contact electrodes, Ti/Al/Ti/Au (20 nm/100 nm/20 nm/150 nm) and Ni/Au (20 nm/150 nm) as n and p contact electrodes, respectively, followed by a 600 ◦ C tube annealing process in N2 ambient for 10 min. Samples are rinsed in H3 PO4 at 280 ◦ C for 5 min, then the etching pits density is examined by scanning electron microscope (SEM). The contact resistance was measured by circular transfer length method (CTLM) and the interface state capacitance with frequency is analyzed by using HP 4284 parameter analyzer which company with light shield model Hg-402L mercury probe. Atomic force microscopic (AFM) and a surface profiler (Dektak3) are used to

Fig. 1. (a) GaN p-i-n UV sensor structure with/without AlGaN/GaN SLs structure; (b) Top view of device.

characterize the surface morphology. The dark current of the pi-n photo detectors were characterized by an HP-4156 parameter analyzer and the studies of spectral responsivity are performed using a 75 W Xenon lamp companied with a monochromator illuminated at the front side on device effective area (detector size area Φ = 500 ␮m minus the P-contact ring area). Moreover, the light source (5.06 × 10−4 W/cm2 at 360 nm) which is focused by lens to ensure all the light source energy illuminated on device effective area shown in Fig. 1(b). A standard Si-based UV enhanced photo detector is also used for purpose of calibration on our p-i-n UV sensor. 3. Results and discussion Specific contact resistance are first characterized as GaN groups of samples grown to a thickness 2 ␮m on different (6-, 8-, and 10-pair) SLs structure, respectively, shown as Fig. 2. It is observed that the specific contact resistance is improved by 8-pair SLs structure. However, the specific contact resistance of GaN on 6-pair or 10-pair of SLs structure are not competed with 8-pair SLs structure. The reason is the piezoelectric stress effect of 6-pair SLs structure could not defend all the defects stretch into GaN film. On the other hand, 10-pair SLs structures with too much pairs of the films, which exceed the critical thickness and worsen the GaN films quality. Fig. 3 shows the average dark current density of p-i-n UV sensors grown on 8-pair SLs structures (sample 1) and without SLs (sample 2), respectively. It is known that defect, such as dislocations originated from the lattice mismatch between epitaxial layers and sapphire substrate, can reveal themselves in a high dark current of p-i-n device. It is also known that dry etching induced crystal damages could result in a high dark current. Experimental results show that sample 2 has resulted in high density V-shape defects and cracking in GaN layer due to the severe lattice mismatch between epilayers and sapphire substrate. The relatively high dark current density of 3.05 × 10−9 A/cm2 at −3 V measured in sample 2 can be attributed to the severe hopping of charge carriers occurred via localized defects in the epilayers. However, the dark current of sample 1 is only about

Fig. 2. Specific contact resistance of GaN grown to a thickness of 2 ␮m with 6-, 8-, and 10-pair of AlGaN/GaN SLs structure, respectively.

S.-S. Liu et al. / Materials Science and Engineering B 126 (2006) 33–36

Fig. 3. Dark current analysis of the GaN p-i-n UV sensor with/without 8-pair AlGaN/GaN SLs structure.

2.25 × 10−11 A/cm2 which is reduced by two orders of magnitude than that of sample 2 at reverse bias of −3 V. Moreover, in the case of the turn-on voltage on samples 1 and 2 are 0.98 and 1.5 V, with the corresponding ideality factors of 1.3 and 2.9, respectively. We observed that with 8-pair SLs structures, the lower turn-on voltage and lower ideality factor could be achieved on sample 1 represents a lower defect density of the film. The corresponding electrical characteristics such as lower dark current in sample 1 can be attributed to the improvement of GaN film quality. The film quality of GaN film grown to 2 ␮m on sapphire substrate with 8-pair SLs structures (sample 1) and without SLs (sample 2) was also characterized by capacitance frequency as shown in Fig. 4. It was observed that the related interfacial states capacitance CP , produced by defect extension into the surface of GaN film were measured to be 1.1 × 10−11 and 2.4 × 10−10 F for samples 1 and 2, at (100 Hz/30 mV) respectively. The result of interfacial state capacitance spectrum agrees well with local defects or interface states hopping with that of dark current analysis.

Fig. 4. Interface state capacitance vs. frequency as GaN grown to a thickness of 2 ␮m with/without 8-pair of AlGaN/GaN SLs structure.

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Fig. 5. Spectrum responsivity of the GaN p-i-n UV sensor with/without 8-pair of AlGaN/GaN SLs structure at zero bias.

Fig. 5 shows the spectrum response of visible–blind UV photodiodes at zero bias. The average responsivity exhibit significantly larger among UV spectral response ranges 250–360 nm visible–blind window on samples 1 and 2, respectively. Two devices roll off abruptly at wavelengths greater than 360 nm since they are transparent in this region. A slight decrease in responsivity to higher energy photon incidence is given by the photon energy. The response tail for photon energies below the band gap can be explained by the Mg or carbon impurity level. These defect centers generate energy levels in the semiconductor band gap and that can be ionized by photons with energies below the band gap. Moreover, high-energy photons could ionize these centers and provoke band-to-band transition in the bulk, which with a reduction of the UV–vis contrast ratio is observed. It is observed that the maximum responsivity of sample 1 is 0.12 A/W at 360 nm, which corresponds to an internal quantum efficiency of 40%. The responsivity of sample 2 is much lower (about 0.07 A/W) at the same wavelength. The rejection ratio of sample 1 is about 1.67 × 105 (measured among wavelength from 360 to 410 nm); however, in sample 2, the rejection ratio is only about 2.72 × 103 , which is two orders of magnitude lower than that in sample 1. Fig. 6(a) and (b) shows the SEM photograph of etching pit density (EPD) on top p-GaN surface of samples 1 and 2, respectively. The EPD of p-GaN film on sample 1 is about 2.5 × 10−6 cm−2 , which is two orders of magnitude lower than that of sample 2 (∼1.3 × 108 cm−2 ) due to sample 1 with 8-pair SLs structures, which defends most of all the defects stretch into GaN film. It also decreases the incorporation of unwanted impurities (possibly silicon, carbon, or oxygen) and reducing the density of nitrogen vacancies. The improved grain alignment and lower EPD of sample 1 may be responsible for the superior electrical properties than sample 2. Fig. 6(c) and (d) shows the AFM roughness analysis on top p-GaN surface of samples 1 and 2, respectively. The mean roughness of sample 1 is (Ra = 0.25 nm), which is much lower than sample 2 (Ra = 0.73 nm). Through defect reduction or lower mean roughness, a good film quality can be achieved. Moreover, the efficiency of carrier collection

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S.-S. Liu et al. / Materials Science and Engineering B 126 (2006) 33–36

Fig. 6. (a) and (b) shows the etching pit density analysis on top p-layer as devices with/without 8-pair AlGaN/GaN SLs structure; (c) and (d) shows the AFM surface roughness morphology on top p-layer as devices with/without 8-pair AlGaN/GaN SLs structure, respectively.

can be increased and a higher response or rejection ratio on sample 1 is thus formed. 4. Conclusions The growth by low-pressure metal organic chemical vapor deposition, fabrication and characterization of GaN p-i-n UV sensor with/without AlGaN/GaN SLs structure is reported. GaN p-i-n UV sensor fabricated with 8-pair SLs, the peak UV responsivity is 0.12 A/W at 360 nm. The rejecting ratio measured is two orders larger than that without SLs structure. On the other hand, GaN p-i-n UV sensor with 8-pair AlGaN/GaN SLs structure, the dark current of device is reduced by two orders of magnitude than that without SLs structure at reverse bias of −3 V. Meanwhile, a lower etching pit density (EPD) or mean roughness (Ra) can be achieved with 8-pair AlGaN/GaN SLs structure. The proposed structures not only filter the threading dislocations or defects level, but also improve the optical properties on our devices. References [1] M. Razeghi, A. Rogalski, Appl. Phys. Lett. 79 (1996) 7433. [2] Y.A. Goldberg, Semicond. Sci. Technol. 14 (1999) R41. [3] M. Razeghi, A. Rogalski, Appl. Phys. Lett. 79 (1996) 7433.

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