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Barrier enhancement of Ge MSM IR photodetector with Ge layer optimization
Superlattices and Microstructures 88 (2015) 685e694
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Barrier enhancement of Ge MSM IR photodetector with Ge layer optimization a, b € Tarık Asar a, b, *, Süleyman Ozçelik a b
Department of Physics, Faculty of Science, Gazi University, 06500 Ankara, Turkey Photonics Application and Research Center, Gazi University, 06500 Ankara, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 August 2015 Received in revised form 19 October 2015 Accepted 20 October 2015 Available online 28 October 2015
Germanium thin films were deposited on n-type Silicon substrates with three different sputter power by using DC magnetron sputtering system at room temperature. The structural and morphological properties of the samples have been obtained by means of Xray diffraction and atomic force microscopy measurements. Then, Germanium metalsemiconductor-metal infrared photodetectors were fabricated on these structures. The carrier recombination lifetime and the diffusion length of the devices were also calculated by using the carrier density and mobility data was obtained from the room temperature Hall Effect measurements. The dark currentevoltage measurements of devices were achieved at room temperature. The electrical parameters such as ideality factor, Schottky barrier height, saturation current and series resistance were extracted from dark current evoltage characteristics. Finally, it has been shown that the barrier enhancement of Ge MSM IR photodetector can be achieved by Ge layer optimization. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Ge MSM IR photodetector Barrier height enhancement XRD AFM HALL IeV
1. Introduction Silicon (Si), Germanium and their heterostructures are excellent materials for the photonic and optoelectronic devices on the infrared (IR) based technologies. Si is the bare material for these type devices. However, Si based devices can be negatively affected from the insignificant responsivity and the self-absorption, whereas Germanium (Ge) with its low band gap is a key material for overcoming these troubles. The absorption edge can be changed toward the infrared in low band gap material [1,2]. So, Ge is preferred for a wide variety of applications because of its properties which are superior to silicon. One of the important steps to usage of Ge was the first bipolar transistor which was invented in 1949 [3]. First transistor was built using Ge. However, Ge lost its popularity respect to silicon due to better thermal noise and abundance in the nature. After successful demonstration of high purity germanium for detector applications at 1970's, this material had regained its popularity [4]. In addition to the proving this electronic functionality of Ge, it also has been frequently used in optical and optoelectronic devices because of its small energy band gap which is about 0.66 eV. It is also an indirect bandgap material like Si, its direct bandgap of 0.8 eV is only 140 meV above the dominant indirect bandgap. So, Ge presents much higher optical absorption than Si in 1.3e1.55 mm wavelength range. Thus Ge IR photodetector can be an applicant for Si photonics integration [5]. Ge based photodetectors [6e14], solar cells [15,16], waveguides [17], and gamma-radiation detectors [18e20] are some of the optical and optoelectronic devices that use Ge. Among the miscellaneous types of Ge
* Corresponding author. Department of Physics, Faculty of Science, Gazi University, 06500 Ankara, Turkey. E-mail address: [email protected] (T. Asar). http://dx.doi.org/10.1016/j.spmi.2015.10.034 0749-6036/© 2015 Elsevier Ltd. All rights reserved.
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based photodetectors, Germanium Metal-Semiconductor-Metal (MSM) photodetectors have been widely studied [21,22]. Ge MSM photodetectors can be examined in two categories as its response and carrier collection. The response and carrier collection depend on short carrier lifetime and diffusion length. For fast response, the semiconductor is heavily doped to obtain a high density of recombination centers. However, this type of heavy doping limits the carrier mobility and reduces the quantum efficiency [21]. Despite lower quantum efficiency, Ge MSM photodetectors have two individual advantages over p-i-n type photodetectors with respect to process integration. One of these advantages is the simple fabrication. It does not require impurity implantation and subsequent activation annealing. The other advantage is the simple metal layout, which enables the photodetector to be obtained more flexible than that of a p-i-n type [23,24]. The layout flexibility is also important for integrated optical circuits. Easy and flexible fabrication of Ge MSM photodetectors allow to use of the standard silicon process lines for signal processing. For this purpose, a Ge infrared photodetector structure based on the thermal evaporation at 300 C was first demonstrated in the pioneering work in 2000 [25]. The scientists found that the polycrystalline Ge can be deposited at 300 C. It allows simple and low cost integration with Si processes. After that, monolithic integration of an array of eight polycrystalline Ge pixels with Complementary Metal Oxide Semiconductor (CMOS) readout electronics and the realization of a digital camera were obtained on the basis of this method [7,26]. Actually, the significant challenge behind these technological development of Ge/Si structure is the huge lattice mismatch between Ge and Si. The formation of lattice mismatch has shown the high dislocation densities and the rough surface morphology due to Stranski-Krastanov island formation [21]. Both of these defects involved in the generation of high leakage current which decreases the efficiency of the photodetector. The scientists overcome these challenges through various applications such as the direct hetero-epitaxy growth of Ge on Si through the use of a low temperature thin SiGe buffer layer [27], the growth of SiGe layer [22], and the optimization of two thin SiGe buffer layers with varying Ge concentration [28]. In addition to these, this paper shows that mentioned troubles can be overcome by optimizing Ge layer at room temperature. In this paper, Ge/n-Si structures for Ge MSM IR photodetector applications were reported. Three Ge thin films were deposited on n-type Si substrates by using DC magnetron sputtering system at room temperature. The structural parameters such as the crystal quality, grain size (D), dislocation density (d) and strain (ε) were determined by X-Ray Diffraction (XRD) measurements. Surface properties of the deposited thin films were analyzed by atomic force microscopy (AFM) measurements. The fabrication of Ge MSM IR photodetectors were held with lithographic processes at room temperature. In addition, carrier lifetime (t) and diffusion length (L) in fabricated MSM devices were calculated using measured mobility (m) and carrier density (N) at room temperature by Hall Effect measurements. Moreover, the dark currentevoltage (IeV) measurements of devices were achieved at room temperature. The electrical parameters such as ideality factor (n), Schottky barrier height (ФB), saturation current (I0) and series resistance (Rs) were extracted from dark IeV characteristics. 2. Experimental Ge MSM photodetectors were fabricated on the phosphorus doped (n-type) Silicon (Si) substrates with 380 ± 25 mm thickness, (100) orientation, and 1e10 Ucm resistivity. Prior to the fabrication process, n-Si wafers were decreased in organic solvent of CHClCClL2, CH3COOH and CH3OH, etched in a sequence of H2SO4 and H2O2, 25% HF, a solution of 7HNO3:1HF:40H2O, 25% HF and finally quenched in deionized water (DI-H2O) with resistivity of 18 MUcm for a prolonged time [29]. Preceding each cleaning step, the substrates were rinsed thoroughly in DI-H2O. Then, the back side of substrates were separately clamped to the stainless steel sputtering holders and loaded into the DC magnetron sputtering system. After all, Ge thin films with 140 nm thickness were deposited on the upper surface of the substrates by using high purity (99.999%) Ge target at the constant pressure (4.2 103 mbar) and temperature (22 C). The film thickness were confirmed by stylus type profilometer. In order to investigate the effect of the sputter power on the properties of the films, the sputter power was varied from 50 W to 100 W with an increment of 25 W. The other deposition parameters such as temperature, time, pressure, argon flow, and target distance are kept constant at whole depositions. Thus, they were called as N340, N341, and N342 and given in Table 1. After the deposition, the structures were divided into square pieces (12 12 mm2). In this way, the Ge/n-Si structures were ready for their characterizations and fabrications. After the deposition processes, structural and morphological properties of the samples were analyzed by XRD, and AFM measurements. XRD measurements were carried out on a D-8 Bruker diffractometer by using CuKa1 (1.540 Å) radiation, a prodded mirror, and a 4-bounce Ge (220) symmetric monochromator. The angular resolution of the diffractometer was 0.004 with the Si calibration sample. AFM measurements were performed by using a room temperature high performance atomic force microscope (NanoMagnetics Instruments Ltd., Oxford, UK) by using dynamic mode scanning. A single crystal
Table 1 The identification of the Ge MSM IR photodetector structures.
Sample
Deposition
Temperature ( C)
Pressure (mbar)
Thickness (Å)
Sputter power (W)
N340 N341 N342
Ge/n-Si Ge/n-Si Ge/n-Si
22 22 22
4.2 103 4.2 103 4.2 103
1400 1400 1400
100 75 50
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silicon tip was attached to the end of a cantilever oscillating at or near its resonant frequency (146e236 kHz). The scan area was set at 5 5 mm2. All measurements were made at room temperature. The root mean square (RMS) and grain size values of the samples were calculated from AFM images. For the electrical characterizations of the Ge MSM IR photodetectors, all three 12 12 mm2 samples were fabricated with the lithographic processes and special designed masks. The fabrication included a native oxide removal, and Schottky electrodes forming steps. First, the native oxide on the Ge surfaces were removed by 1H2SO4:1H2O2:80DI-H2O wet chemical etch solution in 1 min. The samples were rinsed in DI-H2O and dried with nitrogen gas. Then, the electrodes with 100 mm line width were obtained by a co-sputter system (NanoVak Ltd., Ankara, TR) with 1.4 106 mbar base pressure. These were also formed by the deposition of high purity (99.999%) Cr and Au on Ge layer deposited sides at room temperature with 30 nm, and 200 nm thicknesses, respectively. Schematic representation of fabricated MSM device is given in Fig. 1. These Ge MSM IR photodetectors were called MSM1 (Au/Cr/N340), MSM2 (Au/Cr/N341) and MSM3 (Au/Cr/N342) and given in Table 2. In a MSM IR photodetector, the infrared radiation depends on that the Schottky barrier height (fB) is absorbed within the material by interaction with electrons. The detection of the infrared light is achieved by measuring electrical output signal produced by the change of the electronic energy distribution in the material [30]. After the fabrication of devices, in order to perform the electrical analyses, the wire-bonder system was used to bond wires from electrode pads of devices to measuring pads. Then, IeV characteristics were performed using a semiconductor parameter analyzing system (Keithley 4200). In addition, to calculate the carrier lifetime and diffusion length in the Ge MSM devices, mobility and carrier density of the devices were measured by Hall Effect system (Lake Shore) at room temperature.
3. Results and discussion The performance of an optoelectronic device such as MSM IR photodetector is critically dependent on some significant parameters such as structural property, surface morphology, mobility, carrier density, recombination lifetime and diffusion length [30e32]. The recorded u-2q XRD spectra of the three Ge layers (N340, N341, and N342) are shown in Fig. 2. As seen from the Fig. 2, the high intense peak comes from n-Si substrate while the other peaks that belong Ge, GeO2, and SiO2. It can be seen from the Fig. 2 that, there are Si (400), Si (211), Si (220), Ge (201), Ge (401), Ge (410), Ge (413), GeO2 (102), GeO2 (114), GeO2 (311), GeO2 (204), and SiO2 (203) peaks. That means the deposited Ge has polycrystalline structure. Additionally, the Si (400) peak position is well known at 69 [33]. However, the Si (211) peak and Ge (201) peak are situated in nearly the same position. For clearly specification of that, the XRD spectra is plotted between 32.93 and 33.00 and given in the Fig. 3. If the XRD characteristics of Ge/n-Si structures given in the Fig. 3 are examined, it is clearly seen that this broadening behavior of the peaks cannot be arisen from the Si substrate. Generally, Si substrate peaks appear very sharply. So, this broadening behavior can be attributed to the Ge (201). In addition, the full width at half maximum (FWHM) values of Ge (201) peaks for the N340, N341, and N342 structures were determined as 0.018 , 0.025 and 0.010 , respectively. Although, these lower FWHM values indicate that there is no sharpness on the peaks. So, it can be said that the rough changes on the peaks can represent the amorphous morphology. The structural parameters of the three samples such as the grain size (D), dislocation density (d) and strain (ε) were calculated by using the equations D ¼ (0,9l)/(bcosq), d ¼ 1/(D2) and ε ¼ ((l/Dcosq)b)$(1/tanq), where l is the X-Ray wavelength (1.540 Å), q is the peak angle value in the u-2q scans, hkl are the Miller indices, and b is the FWHM value of the Ge (201) peaks [33e35]. The calculated structural parameters are given in Table 3. It is seen from Table 3 that the grain size of the N341 is lower than the others. However, N341 has higher dislocation density and strain values. Additionally, the electrical results of devices which was fabricated onto N341 may be affected by the higher dislocation density and strain. The surface morphology is also very important for the optoelectronic devices such as MSM IR photodetectors. For example, a non-uniform surface can cause breakdown or shortening with upper layers. So, this can result in a reduction
Fig. 1. Schematic representation of the MSM device.
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Table 2 The identification of the Ge MSM IR photodetectors. Device
Metallization
Description
MSM1 MSM2 MSM3
Au/Cr/N340 Au/Cr/N341 Au/Cr/N342
N340 is Ge on n-Si with 100 W Sputter Power N341 is Ge on n-Si with 75 W Sputter Power N342 is Ge on n-Si with 50 W Sputter Power
of the performance of optoelectronic devices. Thus, some measurement methods have been developed to analyze the surface morphology. AFM is one of these methods which can be used to characterize the surface and also to estimate grain size and surface roughness. Fig. 4 shows that three-dimensional (3D) and two-dimensional (2D) AFM images with 5 5 mm2 scan area of the samples. It can be seen in the Fig. 4 that the crystallinity of the films is changed by the sputter power. The grain sizes were evaluated as 410.23 nm, 371.42 nm, and 449.35 nm from AFM images for N340, N341, and N342, respectively. If the grain sizes are compared with each other, it is clearly seen that the grain size of N341 is smaller than the others. This trend of grain size was nearly matched with calculated ones from the XRD measurements as seen in Table 4. Also, the root-mean square (RMS) values which were obtained from the samples' surfaces are listed in Table 4. The RMS value decreases from 21.03 nm to 18.91 nm as the sputter power is increased from 50 W to 100 W. So, it is thought that the changes on roughness and grain size are associated with the performance of the Ge MSM devices. The electro-optic performance of Ge MSM IR photodetectors were also determined. Recombination lifetime (t) and diffusion length (L) for Ge MSM devices were calculated by using the carrier density (N) and mobility (m) obtained from the room temperature Hall Effect measurements. The total recombination lifetime is given as [36].
1 1 1 1 ¼ þ þ : t tSRH tR tA
(1)
In terms of doping concentrations N, the Eq. (1) can be rewritten as
i1 h 2 t ¼ t1 : SRH þ BN þ CN
(2)
As seen from the equations, the total recombination lifetime depends on the N separate into the three parts as ShockleyeReadeHall (SRH), Auger and radiative recombination [30]. However, Auger recombination dominates the others at the doping level that is higher than 1018 cm3, and the lifetime varies with the inverse square of the doping concentration [37,38]. Then, the total lifetime can be given as
i1 h t ¼ CN 2 :
(3)
Here, the Auger recombination coefficients (C) of the Ge and Si are taken as nearly equal (2 1031 cm6 s1) [37,39,40]. The diffusion length (L) leads the distribution of carrier in the devices depending on the minority carrier injection and diffusion. The diffusion length can be derived from the carrier lifetime t as
Ln;p ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffi t$Dn;p ;
(4)
where the diffusion coefficient Dn,p is given by the Einstein relation as
Dn;p ¼ mn;p
kT : q
(5)
Here, mn,p is the carrier mobility, k is the Boltzmann's constant and q is the electrical charge on electron [41]. Finally, the carrier lifetime and the diffusion length calculated from the theoretical modeling are given in Table 5. Important recombination mechanisms in indirect bulk semiconductors are processes in which intermediate trap levels are evoked [42]. In bulk Si, for instance, the impurity dominated Auger process gives rise to low temperature carrier lifetimes in the range of 1 nse1 ms depending on the binding energy of the impurity [43,44]. The recombination statistics of these processes in bulk indirect semiconductors were comprehensively studied in the literature [42,44]. A common feature to all these processes is the saturation of the recombination as the photoinduced carrier density exceeds the trap density [45]. The forward bias dark IeV characteristics of Ge MSM devices were measured to investigate the electrical properties of structures such as the ideality factor (n), Schottky barrier height (ФB), saturation current (I0) and series resistance (Rs). Fig. 5
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Fig. 2. The u-2q XRD spectra of Ge/n-Si structures.
shows the semi-logarithmic IeV characteristics of the Ge MSM devices at room temperature. According to the thermionic emission theory, the relationship between the applied forward voltage (V kT/q) and the current can be expressed as shown below [46,47]:
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Fig. 3. The Si (211) and Ge (201) XRD characteristics of Ge/n-Si structures.
Table 3 The structural parameters of the samples. Structure
2q (deg)
b (deg)
D (nm)
d (109 cm2)
ε (103)
N340 N341 N342
32.9655 32.9636 32.9649
0.0183 0.0253 0.0095
452.7179 327.4584 872.0764
0.4879 0.9326 0.1315
1.1990 1.6580 0.6230
I ¼ Io exp
qV nkT
qV 1 exp ; kT
(6)
where, V is the forward bias voltage, q is the electronic charge, n is the ideality factor, k is the Boltzmann constant, T is the temperature in K and Io is the saturation current. The series resistance (Rs) of devices were determined from the resistance ðRi ¼ dVi =dIi Þ vs. applied bias voltage (Vi) plots obtained from the IeV characteristics. It was observed that at sufficiently high forward bias voltage region the series resistance of devices approaches to a constant value. The Rs values of the MSM1, MSM2 and MSM3 were found as 9.45 kU, 35.32 kU and 11.47 kU and given the Table 6. In Fig. 5, the saturation current I0 is derived from the straight-line intercept of lnI at zero bias. On the other hand, the saturation current I0 is defined as
qF I0 ¼ AA* T 2 exp B ; kT
(7)
where A, A*, q and ФB are the diode area, the effective Richardson constant (for Ge 143 A/cm2 K2) [61], the electronic charge, and the zero-bias Schottky barrier height, respectively. From Eq. (6), the ideality factor can be written as
n¼
q dV : kT dlnðIÞ
(8)
n is used to calculate the deviation from the ideal thermionic model. ФB was calculated using the theoretical value of A* and extrapolated I0 at room temperature according to Eq. (9),
FB ¼
* 2 kT AA T ln : q I0
(9)
The calculated values of n and ФB obtained from Eqs. (8) and (9) are shown in Table 6. As seen from the Table 6, the ФB values of MSM1 and MSM3 devices were calculated as 0.61 eV. Additionally, the fB value of MSM2 device were calculated as 0.73 eV. These values correspond the infrared wavelengths of 2.032 mm, and 1.699 mm. Thus, it can be said that the Ge MSM IR photodetectors can work at these wavelengths. Actually, the ФB is as low as 0.09 eV for
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Fig. 4. 3D and 2D AFM images of the structures. The scan area is 5 5 mm2 in the all images.
p-Ge and 0.54 eV for n-Ge in conventional Ge MSM photodiodes [48]. This is due to the small band-gap energy of Ge and Fermi level pinning between 0.54 eV and 0.61 eV below the conduction band edge, independent of the contacting metallization [22]. However, in this study, the fB was increased to 0.73 eV. This increment in the Schottky barrier height resulting from Ge and Cr/Au contacts led to a reduction of the dark current. It can be seen in Table 6 that MSM2 produced lower dark
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Table 4 The RMS and grain size values of the samples. Sample
Grain size (nm)
N340 N341 N342
XRD results
AFM results
452.72 327.46 872.08
410.23 371.42 449.35
RMS (nm)
Sputter power (W)
18.91 17.96 21.03
100 75 50
Table 5 Carrier density, mobility, diffusion coefficient, carrier lifetime and diffusion length of the MSM devices. Structure
T (K)
N (1/cm3)
m (cm2/V.s)
Rs (ohm.cm)
D (cm2/s)
t (ns)
L (mm)
MSM1 MSM2 MSM3
302.925 302.744 302.703
1.875 1018 2.805 1018 1.558 1018
1239.200 780.610 1298.200
0.0027 0.0029 0.0031
32.348 20.365 33.863
1422.526 635.348 2059.055
67.835 35.971 83.503
Fig. 5. Measured forward bias dark currentevoltage characteristics of Ge MSM devices.
Table 6 Electrical parameters for the Ge MSM devices. Device
n
ФB (eV)
Io (A)
Rs (kU)
MSM1 MSM2 MSM3
4.200 2.110 3.580
0.610 0.730 0.610
8.590 106 8.460 108 9.660 106
9.450 35.320 11.470
current than the others. The relatively high dark current of the MSM1 and MSM3 is due primarily to the high surface field in the samples. It is the fact that the Ge surface cannot be effectively passivated with Ge oxides. In fact, the unsteady Ge oxides can induce point defects at the surface [22]. These surface states can act as recombination-generation centers, which increase the surface leakage current. The effectiveness of Ge at reducing the dark current is believed to be due to the enhanced fB by increasing the separation between the mobility edges in Ge, which plays a role equivalent to the band-gap energy in a crystalline semiconductor [22]. The separation between the mobility edges in a-Si:H was reported to be about 1.8 eV, which is higher than the band gap energy of crystalline Si [49,50]. In addition, it can be seen from Table 6 that the MSM2 Ge device has a lower saturation current compared with that of MSM1 and MSM3. Also in agreement with XRD, AFM, and IeV results, the MSM2 with lower Ge grain size shows a lower saturation current than that of both MSMs with larger Ge grain size. It can be related that the size of Ge grain plays a major role on operating the MSM. Therefore, the optimizing of Ge layer may cause a decrease in the saturation current. The literature result for dark currents of Ge MSM suggests that the presence of Ge islands beneath the metal electrode has suppressed the flow of carriers since there are two interfaces to traverse across, Metal/Ge interface and Ge/Si interface [51]. This causes lower the dark currents than Si MSM. This reason can be used to explain why MSM2 with small grain has lower saturation current than that of MSM1 and MSM3 with large grains. For same electrode design, lots of clusters with small Ge grains provide more
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rooms for carriers to traverse across metal/semiconductor interface which acts as the easiest path. On the other hand, MSM with large Ge grains has less room for carriers to traverse across. The carriers in these type MSM can endure much more collisions with each other. This may be the cause of the series resistance, and can increase the saturation current. It may be resulted with reduction of output current. So, it can be said that MSM2 with small Ge grains has more rooms for carriers hence resulting in the suppression of saturation current. 4. Conclusions It has been shown that the barrier enhancement of Ge MSM IR photodetector has been investigated by the optimization of Ge layer. The grain sizes of the structures were evaluated as 452.72 nm, 327.46 nm, and 872.08 nm from XRD analyses, and 410.23 nm, 371.42 nm, and 449.35 nm from AFM images for N340, N341, and N342, respectively. It was seen that the grain size firstly decrease and then increase with the increasing sputter power. The grain size of N342 which had lowest sputter power was the biggest one. However, MSM3 which was fabricated on N342 did not reveal the best electrical results. So, it can be said that the optimized grain size is so important for such MSM devices. This behavior can affect the MSM IR photodetector properties. Also, the RMS values which were extracted from AFM results decreases from 21.03 nm to 18.91 nm when the sputter power increase from 50 W to 100 W. In addition, the carrier lifetime and diffusion length in the Ge MSM IR photodetectors were calculated by using the mobility and carrier density data obtained from Hall Effect measurements. These values were 1422.526 ns, 635.348 ns, 2059.055 ns, and 67.835 mm, 35.971 mm, 83.503 mm for MSM1, MSM2, and MSM3, respectively. Important device parameters such as ideality factor, Schottky barrier height and saturation current of fabricated Ge MSM IR photodetectors were obtained by IeV measurements. The values of fB were calculated as 0.61 eV for MSM1 and MSM3, and 0.73 eV for MSM2 from room temperature forward bias dark IeV measurements. It was seen that the Schottky barrier height fB was increased from the conventional Schottky barrier height to 0.73 eV. These results showed that an improvement of the Schottky barrier height was obtained by the optimizing of the Ge layer deposition with sputter power adjustment. This improvement can be attributed to obtain less carrier lifetime and diffusion length. Besides these parameters, the electrical properties of the devices can be improved with the good quality of fabrication. After all, it can be said that N341 type structure which was deposited at 75 W sputter power at room temperature can be more convenient for MSM IR photodetector applications especially due to easy deposition and fabrication processes at room temperature. Thereby, this type MSM IR photodetectors can be produced with low production cost.
Acknowledgments This work is supported by the Ministry of Development of Turkey under project number: 2011K120290.
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Barrier enhancement of Ge MSM IR photodetector with Ge layer optimization a, b € Tarık Asar a, b, *, Süleyman Ozçelik a b
Department of Physics, Faculty of Science, Gazi University, 06500 Ankara, Turkey Photonics Application and Research Center, Gazi University, 06500 Ankara, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 August 2015 Received in revised form 19 October 2015 Accepted 20 October 2015 Available online 28 October 2015
Germanium thin films were deposited on n-type Silicon substrates with three different sputter power by using DC magnetron sputtering system at room temperature. The structural and morphological properties of the samples have been obtained by means of Xray diffraction and atomic force microscopy measurements. Then, Germanium metalsemiconductor-metal infrared photodetectors were fabricated on these structures. The carrier recombination lifetime and the diffusion length of the devices were also calculated by using the carrier density and mobility data was obtained from the room temperature Hall Effect measurements. The dark currentevoltage measurements of devices were achieved at room temperature. The electrical parameters such as ideality factor, Schottky barrier height, saturation current and series resistance were extracted from dark current evoltage characteristics. Finally, it has been shown that the barrier enhancement of Ge MSM IR photodetector can be achieved by Ge layer optimization. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Ge MSM IR photodetector Barrier height enhancement XRD AFM HALL IeV
1. Introduction Silicon (Si), Germanium and their heterostructures are excellent materials for the photonic and optoelectronic devices on the infrared (IR) based technologies. Si is the bare material for these type devices. However, Si based devices can be negatively affected from the insignificant responsivity and the self-absorption, whereas Germanium (Ge) with its low band gap is a key material for overcoming these troubles. The absorption edge can be changed toward the infrared in low band gap material [1,2]. So, Ge is preferred for a wide variety of applications because of its properties which are superior to silicon. One of the important steps to usage of Ge was the first bipolar transistor which was invented in 1949 [3]. First transistor was built using Ge. However, Ge lost its popularity respect to silicon due to better thermal noise and abundance in the nature. After successful demonstration of high purity germanium for detector applications at 1970's, this material had regained its popularity [4]. In addition to the proving this electronic functionality of Ge, it also has been frequently used in optical and optoelectronic devices because of its small energy band gap which is about 0.66 eV. It is also an indirect bandgap material like Si, its direct bandgap of 0.8 eV is only 140 meV above the dominant indirect bandgap. So, Ge presents much higher optical absorption than Si in 1.3e1.55 mm wavelength range. Thus Ge IR photodetector can be an applicant for Si photonics integration [5]. Ge based photodetectors [6e14], solar cells [15,16], waveguides [17], and gamma-radiation detectors [18e20] are some of the optical and optoelectronic devices that use Ge. Among the miscellaneous types of Ge
* Corresponding author. Department of Physics, Faculty of Science, Gazi University, 06500 Ankara, Turkey. E-mail address: [email protected] (T. Asar). http://dx.doi.org/10.1016/j.spmi.2015.10.034 0749-6036/© 2015 Elsevier Ltd. All rights reserved.
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based photodetectors, Germanium Metal-Semiconductor-Metal (MSM) photodetectors have been widely studied [21,22]. Ge MSM photodetectors can be examined in two categories as its response and carrier collection. The response and carrier collection depend on short carrier lifetime and diffusion length. For fast response, the semiconductor is heavily doped to obtain a high density of recombination centers. However, this type of heavy doping limits the carrier mobility and reduces the quantum efficiency [21]. Despite lower quantum efficiency, Ge MSM photodetectors have two individual advantages over p-i-n type photodetectors with respect to process integration. One of these advantages is the simple fabrication. It does not require impurity implantation and subsequent activation annealing. The other advantage is the simple metal layout, which enables the photodetector to be obtained more flexible than that of a p-i-n type [23,24]. The layout flexibility is also important for integrated optical circuits. Easy and flexible fabrication of Ge MSM photodetectors allow to use of the standard silicon process lines for signal processing. For this purpose, a Ge infrared photodetector structure based on the thermal evaporation at 300 C was first demonstrated in the pioneering work in 2000 [25]. The scientists found that the polycrystalline Ge can be deposited at 300 C. It allows simple and low cost integration with Si processes. After that, monolithic integration of an array of eight polycrystalline Ge pixels with Complementary Metal Oxide Semiconductor (CMOS) readout electronics and the realization of a digital camera were obtained on the basis of this method [7,26]. Actually, the significant challenge behind these technological development of Ge/Si structure is the huge lattice mismatch between Ge and Si. The formation of lattice mismatch has shown the high dislocation densities and the rough surface morphology due to Stranski-Krastanov island formation [21]. Both of these defects involved in the generation of high leakage current which decreases the efficiency of the photodetector. The scientists overcome these challenges through various applications such as the direct hetero-epitaxy growth of Ge on Si through the use of a low temperature thin SiGe buffer layer [27], the growth of SiGe layer [22], and the optimization of two thin SiGe buffer layers with varying Ge concentration [28]. In addition to these, this paper shows that mentioned troubles can be overcome by optimizing Ge layer at room temperature. In this paper, Ge/n-Si structures for Ge MSM IR photodetector applications were reported. Three Ge thin films were deposited on n-type Si substrates by using DC magnetron sputtering system at room temperature. The structural parameters such as the crystal quality, grain size (D), dislocation density (d) and strain (ε) were determined by X-Ray Diffraction (XRD) measurements. Surface properties of the deposited thin films were analyzed by atomic force microscopy (AFM) measurements. The fabrication of Ge MSM IR photodetectors were held with lithographic processes at room temperature. In addition, carrier lifetime (t) and diffusion length (L) in fabricated MSM devices were calculated using measured mobility (m) and carrier density (N) at room temperature by Hall Effect measurements. Moreover, the dark currentevoltage (IeV) measurements of devices were achieved at room temperature. The electrical parameters such as ideality factor (n), Schottky barrier height (ФB), saturation current (I0) and series resistance (Rs) were extracted from dark IeV characteristics. 2. Experimental Ge MSM photodetectors were fabricated on the phosphorus doped (n-type) Silicon (Si) substrates with 380 ± 25 mm thickness, (100) orientation, and 1e10 Ucm resistivity. Prior to the fabrication process, n-Si wafers were decreased in organic solvent of CHClCClL2, CH3COOH and CH3OH, etched in a sequence of H2SO4 and H2O2, 25% HF, a solution of 7HNO3:1HF:40H2O, 25% HF and finally quenched in deionized water (DI-H2O) with resistivity of 18 MUcm for a prolonged time [29]. Preceding each cleaning step, the substrates were rinsed thoroughly in DI-H2O. Then, the back side of substrates were separately clamped to the stainless steel sputtering holders and loaded into the DC magnetron sputtering system. After all, Ge thin films with 140 nm thickness were deposited on the upper surface of the substrates by using high purity (99.999%) Ge target at the constant pressure (4.2 103 mbar) and temperature (22 C). The film thickness were confirmed by stylus type profilometer. In order to investigate the effect of the sputter power on the properties of the films, the sputter power was varied from 50 W to 100 W with an increment of 25 W. The other deposition parameters such as temperature, time, pressure, argon flow, and target distance are kept constant at whole depositions. Thus, they were called as N340, N341, and N342 and given in Table 1. After the deposition, the structures were divided into square pieces (12 12 mm2). In this way, the Ge/n-Si structures were ready for their characterizations and fabrications. After the deposition processes, structural and morphological properties of the samples were analyzed by XRD, and AFM measurements. XRD measurements were carried out on a D-8 Bruker diffractometer by using CuKa1 (1.540 Å) radiation, a prodded mirror, and a 4-bounce Ge (220) symmetric monochromator. The angular resolution of the diffractometer was 0.004 with the Si calibration sample. AFM measurements were performed by using a room temperature high performance atomic force microscope (NanoMagnetics Instruments Ltd., Oxford, UK) by using dynamic mode scanning. A single crystal
Table 1 The identification of the Ge MSM IR photodetector structures.
Sample
Deposition
Temperature ( C)
Pressure (mbar)
Thickness (Å)
Sputter power (W)
N340 N341 N342
Ge/n-Si Ge/n-Si Ge/n-Si
22 22 22
4.2 103 4.2 103 4.2 103
1400 1400 1400
100 75 50
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silicon tip was attached to the end of a cantilever oscillating at or near its resonant frequency (146e236 kHz). The scan area was set at 5 5 mm2. All measurements were made at room temperature. The root mean square (RMS) and grain size values of the samples were calculated from AFM images. For the electrical characterizations of the Ge MSM IR photodetectors, all three 12 12 mm2 samples were fabricated with the lithographic processes and special designed masks. The fabrication included a native oxide removal, and Schottky electrodes forming steps. First, the native oxide on the Ge surfaces were removed by 1H2SO4:1H2O2:80DI-H2O wet chemical etch solution in 1 min. The samples were rinsed in DI-H2O and dried with nitrogen gas. Then, the electrodes with 100 mm line width were obtained by a co-sputter system (NanoVak Ltd., Ankara, TR) with 1.4 106 mbar base pressure. These were also formed by the deposition of high purity (99.999%) Cr and Au on Ge layer deposited sides at room temperature with 30 nm, and 200 nm thicknesses, respectively. Schematic representation of fabricated MSM device is given in Fig. 1. These Ge MSM IR photodetectors were called MSM1 (Au/Cr/N340), MSM2 (Au/Cr/N341) and MSM3 (Au/Cr/N342) and given in Table 2. In a MSM IR photodetector, the infrared radiation depends on that the Schottky barrier height (fB) is absorbed within the material by interaction with electrons. The detection of the infrared light is achieved by measuring electrical output signal produced by the change of the electronic energy distribution in the material [30]. After the fabrication of devices, in order to perform the electrical analyses, the wire-bonder system was used to bond wires from electrode pads of devices to measuring pads. Then, IeV characteristics were performed using a semiconductor parameter analyzing system (Keithley 4200). In addition, to calculate the carrier lifetime and diffusion length in the Ge MSM devices, mobility and carrier density of the devices were measured by Hall Effect system (Lake Shore) at room temperature.
3. Results and discussion The performance of an optoelectronic device such as MSM IR photodetector is critically dependent on some significant parameters such as structural property, surface morphology, mobility, carrier density, recombination lifetime and diffusion length [30e32]. The recorded u-2q XRD spectra of the three Ge layers (N340, N341, and N342) are shown in Fig. 2. As seen from the Fig. 2, the high intense peak comes from n-Si substrate while the other peaks that belong Ge, GeO2, and SiO2. It can be seen from the Fig. 2 that, there are Si (400), Si (211), Si (220), Ge (201), Ge (401), Ge (410), Ge (413), GeO2 (102), GeO2 (114), GeO2 (311), GeO2 (204), and SiO2 (203) peaks. That means the deposited Ge has polycrystalline structure. Additionally, the Si (400) peak position is well known at 69 [33]. However, the Si (211) peak and Ge (201) peak are situated in nearly the same position. For clearly specification of that, the XRD spectra is plotted between 32.93 and 33.00 and given in the Fig. 3. If the XRD characteristics of Ge/n-Si structures given in the Fig. 3 are examined, it is clearly seen that this broadening behavior of the peaks cannot be arisen from the Si substrate. Generally, Si substrate peaks appear very sharply. So, this broadening behavior can be attributed to the Ge (201). In addition, the full width at half maximum (FWHM) values of Ge (201) peaks for the N340, N341, and N342 structures were determined as 0.018 , 0.025 and 0.010 , respectively. Although, these lower FWHM values indicate that there is no sharpness on the peaks. So, it can be said that the rough changes on the peaks can represent the amorphous morphology. The structural parameters of the three samples such as the grain size (D), dislocation density (d) and strain (ε) were calculated by using the equations D ¼ (0,9l)/(bcosq), d ¼ 1/(D2) and ε ¼ ((l/Dcosq)b)$(1/tanq), where l is the X-Ray wavelength (1.540 Å), q is the peak angle value in the u-2q scans, hkl are the Miller indices, and b is the FWHM value of the Ge (201) peaks [33e35]. The calculated structural parameters are given in Table 3. It is seen from Table 3 that the grain size of the N341 is lower than the others. However, N341 has higher dislocation density and strain values. Additionally, the electrical results of devices which was fabricated onto N341 may be affected by the higher dislocation density and strain. The surface morphology is also very important for the optoelectronic devices such as MSM IR photodetectors. For example, a non-uniform surface can cause breakdown or shortening with upper layers. So, this can result in a reduction
Fig. 1. Schematic representation of the MSM device.
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Table 2 The identification of the Ge MSM IR photodetectors. Device
Metallization
Description
MSM1 MSM2 MSM3
Au/Cr/N340 Au/Cr/N341 Au/Cr/N342
N340 is Ge on n-Si with 100 W Sputter Power N341 is Ge on n-Si with 75 W Sputter Power N342 is Ge on n-Si with 50 W Sputter Power
of the performance of optoelectronic devices. Thus, some measurement methods have been developed to analyze the surface morphology. AFM is one of these methods which can be used to characterize the surface and also to estimate grain size and surface roughness. Fig. 4 shows that three-dimensional (3D) and two-dimensional (2D) AFM images with 5 5 mm2 scan area of the samples. It can be seen in the Fig. 4 that the crystallinity of the films is changed by the sputter power. The grain sizes were evaluated as 410.23 nm, 371.42 nm, and 449.35 nm from AFM images for N340, N341, and N342, respectively. If the grain sizes are compared with each other, it is clearly seen that the grain size of N341 is smaller than the others. This trend of grain size was nearly matched with calculated ones from the XRD measurements as seen in Table 4. Also, the root-mean square (RMS) values which were obtained from the samples' surfaces are listed in Table 4. The RMS value decreases from 21.03 nm to 18.91 nm as the sputter power is increased from 50 W to 100 W. So, it is thought that the changes on roughness and grain size are associated with the performance of the Ge MSM devices. The electro-optic performance of Ge MSM IR photodetectors were also determined. Recombination lifetime (t) and diffusion length (L) for Ge MSM devices were calculated by using the carrier density (N) and mobility (m) obtained from the room temperature Hall Effect measurements. The total recombination lifetime is given as [36].
1 1 1 1 ¼ þ þ : t tSRH tR tA
(1)
In terms of doping concentrations N, the Eq. (1) can be rewritten as
i1 h 2 t ¼ t1 : SRH þ BN þ CN
(2)
As seen from the equations, the total recombination lifetime depends on the N separate into the three parts as ShockleyeReadeHall (SRH), Auger and radiative recombination [30]. However, Auger recombination dominates the others at the doping level that is higher than 1018 cm3, and the lifetime varies with the inverse square of the doping concentration [37,38]. Then, the total lifetime can be given as
i1 h t ¼ CN 2 :
(3)
Here, the Auger recombination coefficients (C) of the Ge and Si are taken as nearly equal (2 1031 cm6 s1) [37,39,40]. The diffusion length (L) leads the distribution of carrier in the devices depending on the minority carrier injection and diffusion. The diffusion length can be derived from the carrier lifetime t as
Ln;p ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffi t$Dn;p ;
(4)
where the diffusion coefficient Dn,p is given by the Einstein relation as
Dn;p ¼ mn;p
kT : q
(5)
Here, mn,p is the carrier mobility, k is the Boltzmann's constant and q is the electrical charge on electron [41]. Finally, the carrier lifetime and the diffusion length calculated from the theoretical modeling are given in Table 5. Important recombination mechanisms in indirect bulk semiconductors are processes in which intermediate trap levels are evoked [42]. In bulk Si, for instance, the impurity dominated Auger process gives rise to low temperature carrier lifetimes in the range of 1 nse1 ms depending on the binding energy of the impurity [43,44]. The recombination statistics of these processes in bulk indirect semiconductors were comprehensively studied in the literature [42,44]. A common feature to all these processes is the saturation of the recombination as the photoinduced carrier density exceeds the trap density [45]. The forward bias dark IeV characteristics of Ge MSM devices were measured to investigate the electrical properties of structures such as the ideality factor (n), Schottky barrier height (ФB), saturation current (I0) and series resistance (Rs). Fig. 5
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Fig. 2. The u-2q XRD spectra of Ge/n-Si structures.
shows the semi-logarithmic IeV characteristics of the Ge MSM devices at room temperature. According to the thermionic emission theory, the relationship between the applied forward voltage (V kT/q) and the current can be expressed as shown below [46,47]:
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Fig. 3. The Si (211) and Ge (201) XRD characteristics of Ge/n-Si structures.
Table 3 The structural parameters of the samples. Structure
2q (deg)
b (deg)
D (nm)
d (109 cm2)
ε (103)
N340 N341 N342
32.9655 32.9636 32.9649
0.0183 0.0253 0.0095
452.7179 327.4584 872.0764
0.4879 0.9326 0.1315
1.1990 1.6580 0.6230
I ¼ Io exp
qV nkT
qV 1 exp ; kT
(6)
where, V is the forward bias voltage, q is the electronic charge, n is the ideality factor, k is the Boltzmann constant, T is the temperature in K and Io is the saturation current. The series resistance (Rs) of devices were determined from the resistance ðRi ¼ dVi =dIi Þ vs. applied bias voltage (Vi) plots obtained from the IeV characteristics. It was observed that at sufficiently high forward bias voltage region the series resistance of devices approaches to a constant value. The Rs values of the MSM1, MSM2 and MSM3 were found as 9.45 kU, 35.32 kU and 11.47 kU and given the Table 6. In Fig. 5, the saturation current I0 is derived from the straight-line intercept of lnI at zero bias. On the other hand, the saturation current I0 is defined as
qF I0 ¼ AA* T 2 exp B ; kT
(7)
where A, A*, q and ФB are the diode area, the effective Richardson constant (for Ge 143 A/cm2 K2) [61], the electronic charge, and the zero-bias Schottky barrier height, respectively. From Eq. (6), the ideality factor can be written as
n¼
q dV : kT dlnðIÞ
(8)
n is used to calculate the deviation from the ideal thermionic model. ФB was calculated using the theoretical value of A* and extrapolated I0 at room temperature according to Eq. (9),
FB ¼
* 2 kT AA T ln : q I0
(9)
The calculated values of n and ФB obtained from Eqs. (8) and (9) are shown in Table 6. As seen from the Table 6, the ФB values of MSM1 and MSM3 devices were calculated as 0.61 eV. Additionally, the fB value of MSM2 device were calculated as 0.73 eV. These values correspond the infrared wavelengths of 2.032 mm, and 1.699 mm. Thus, it can be said that the Ge MSM IR photodetectors can work at these wavelengths. Actually, the ФB is as low as 0.09 eV for
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Fig. 4. 3D and 2D AFM images of the structures. The scan area is 5 5 mm2 in the all images.
p-Ge and 0.54 eV for n-Ge in conventional Ge MSM photodiodes [48]. This is due to the small band-gap energy of Ge and Fermi level pinning between 0.54 eV and 0.61 eV below the conduction band edge, independent of the contacting metallization [22]. However, in this study, the fB was increased to 0.73 eV. This increment in the Schottky barrier height resulting from Ge and Cr/Au contacts led to a reduction of the dark current. It can be seen in Table 6 that MSM2 produced lower dark
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Table 4 The RMS and grain size values of the samples. Sample
Grain size (nm)
N340 N341 N342
XRD results
AFM results
452.72 327.46 872.08
410.23 371.42 449.35
RMS (nm)
Sputter power (W)
18.91 17.96 21.03
100 75 50
Table 5 Carrier density, mobility, diffusion coefficient, carrier lifetime and diffusion length of the MSM devices. Structure
T (K)
N (1/cm3)
m (cm2/V.s)
Rs (ohm.cm)
D (cm2/s)
t (ns)
L (mm)
MSM1 MSM2 MSM3
302.925 302.744 302.703
1.875 1018 2.805 1018 1.558 1018
1239.200 780.610 1298.200
0.0027 0.0029 0.0031
32.348 20.365 33.863
1422.526 635.348 2059.055
67.835 35.971 83.503
Fig. 5. Measured forward bias dark currentevoltage characteristics of Ge MSM devices.
Table 6 Electrical parameters for the Ge MSM devices. Device
n
ФB (eV)
Io (A)
Rs (kU)
MSM1 MSM2 MSM3
4.200 2.110 3.580
0.610 0.730 0.610
8.590 106 8.460 108 9.660 106
9.450 35.320 11.470
current than the others. The relatively high dark current of the MSM1 and MSM3 is due primarily to the high surface field in the samples. It is the fact that the Ge surface cannot be effectively passivated with Ge oxides. In fact, the unsteady Ge oxides can induce point defects at the surface [22]. These surface states can act as recombination-generation centers, which increase the surface leakage current. The effectiveness of Ge at reducing the dark current is believed to be due to the enhanced fB by increasing the separation between the mobility edges in Ge, which plays a role equivalent to the band-gap energy in a crystalline semiconductor [22]. The separation between the mobility edges in a-Si:H was reported to be about 1.8 eV, which is higher than the band gap energy of crystalline Si [49,50]. In addition, it can be seen from Table 6 that the MSM2 Ge device has a lower saturation current compared with that of MSM1 and MSM3. Also in agreement with XRD, AFM, and IeV results, the MSM2 with lower Ge grain size shows a lower saturation current than that of both MSMs with larger Ge grain size. It can be related that the size of Ge grain plays a major role on operating the MSM. Therefore, the optimizing of Ge layer may cause a decrease in the saturation current. The literature result for dark currents of Ge MSM suggests that the presence of Ge islands beneath the metal electrode has suppressed the flow of carriers since there are two interfaces to traverse across, Metal/Ge interface and Ge/Si interface [51]. This causes lower the dark currents than Si MSM. This reason can be used to explain why MSM2 with small grain has lower saturation current than that of MSM1 and MSM3 with large grains. For same electrode design, lots of clusters with small Ge grains provide more
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rooms for carriers to traverse across metal/semiconductor interface which acts as the easiest path. On the other hand, MSM with large Ge grains has less room for carriers to traverse across. The carriers in these type MSM can endure much more collisions with each other. This may be the cause of the series resistance, and can increase the saturation current. It may be resulted with reduction of output current. So, it can be said that MSM2 with small Ge grains has more rooms for carriers hence resulting in the suppression of saturation current. 4. Conclusions It has been shown that the barrier enhancement of Ge MSM IR photodetector has been investigated by the optimization of Ge layer. The grain sizes of the structures were evaluated as 452.72 nm, 327.46 nm, and 872.08 nm from XRD analyses, and 410.23 nm, 371.42 nm, and 449.35 nm from AFM images for N340, N341, and N342, respectively. It was seen that the grain size firstly decrease and then increase with the increasing sputter power. The grain size of N342 which had lowest sputter power was the biggest one. However, MSM3 which was fabricated on N342 did not reveal the best electrical results. So, it can be said that the optimized grain size is so important for such MSM devices. This behavior can affect the MSM IR photodetector properties. Also, the RMS values which were extracted from AFM results decreases from 21.03 nm to 18.91 nm when the sputter power increase from 50 W to 100 W. In addition, the carrier lifetime and diffusion length in the Ge MSM IR photodetectors were calculated by using the mobility and carrier density data obtained from Hall Effect measurements. These values were 1422.526 ns, 635.348 ns, 2059.055 ns, and 67.835 mm, 35.971 mm, 83.503 mm for MSM1, MSM2, and MSM3, respectively. Important device parameters such as ideality factor, Schottky barrier height and saturation current of fabricated Ge MSM IR photodetectors were obtained by IeV measurements. The values of fB were calculated as 0.61 eV for MSM1 and MSM3, and 0.73 eV for MSM2 from room temperature forward bias dark IeV measurements. It was seen that the Schottky barrier height fB was increased from the conventional Schottky barrier height to 0.73 eV. These results showed that an improvement of the Schottky barrier height was obtained by the optimizing of the Ge layer deposition with sputter power adjustment. This improvement can be attributed to obtain less carrier lifetime and diffusion length. Besides these parameters, the electrical properties of the devices can be improved with the good quality of fabrication. After all, it can be said that N341 type structure which was deposited at 75 W sputter power at room temperature can be more convenient for MSM IR photodetector applications especially due to easy deposition and fabrication processes at room temperature. Thereby, this type MSM IR photodetectors can be produced with low production cost.
Acknowledgments This work is supported by the Ministry of Development of Turkey under project number: 2011K120290.
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