C. H. Chen, S. J. Chang, Y. K. Su, Senior Member, IEEE, G. C. Chi, J. Y. Chi, C. A. Chang, J. K. Sheu, and. J. F. Chen, Member, IEEE. AbstractâIndiumâtinâoxide ...
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 13, NO. 8, AUGUST 2001
GaN Metal–Semiconductor–Metal Ultraviolet Photodetectors With Transparent Indium–Tin–Oxide Schottky Contacts C. H. Chen, S. J. Chang, Y. K. Su, Senior Member, IEEE, G. C. Chi, J. Y. Chi, C. A. Chang, J. K. Sheu, and J. F. Chen, Member, IEEE
Abstract—Indium–tin–oxide (ITO) layers were deposited onto n-GaN films and/or glass substrates by electron-beam evaporation. With proper annealing, we found that we could improve the optical properties of the ITO layers and achieve a maximum transmittance of 98% at 360 nm. GaN-based metal–semiconductor–metal (MSM) photodetectors with ITO transparent contacts were also fabricated. A maximum 0.12-A photocurrent with a photo current to dark current contrast higher than five orders of magnitude during ultraviolet irradiation were obtained for a photodetector annealed at 600 C. We also found that the maximum photo responsivity at 345 nm is 7.2 and 0.9 A/W when the detector is biased at 5 and 0.5 V, respectively. Index Terms—GaN, ITO, MSM photodetector, transparent contact.
O DETECT and give early warning against ground-to-air, air-to-air, air-to-ground, and ground-to-ground threats, airborne, sea borne, and ground-based equipments require ultraviolet (UV) detectors. Other applications for such UV detectors include space communications, ozone layer monitoring and flame detection. Gallium nitride (GaN) is one of the most promising materials for the fabrication of high-responsivity and visible-blind UV detectors, since it has a large direct bandgap energy (3.41 eV at room temperature) and a high saturation electron drift velocity (3 10 cm/s) . The superior radiation hardness and high temperature resistance of GaN also make it the suitable material for UV detectors working in extreme conditions. In the past few years, various types of GaN-based photodetectors have been proposed, such as p-n junction diode , p-i-n diode , , p- -n diode , Schottky barrier detector , and metal–semiconductor–metal (MSM) photodetector . Among these devices, MSM photodetector has an Manuscript received November 27, 2000; revised May 1, 2001. This work was supported by the National Science Council under Contract NSC-89-2215-E-006-095. C. H. Chen, S. J. Chang, Y. K. Su, and J. F. Chen are with the Institute of Microelectronics, Department of Electrical Engineering, National Cheng Kung University, Tainan, Taiwan 70101, R.O.C. G. C. Chi is with the Department of Physics, National Central University, Chung-Li, Taiwan, R.O.C. J. Y. Chi and C. A. Chang are with Opto-Electronics and Systems Laboratories, Industrial Technology Research Institute, Hsinchu, Taiwan, R.O.C. J. K. Sheu is with the Institute of Microelectronics, Department of Electrical Engineering, National Cheng Kung University, Tainan, Taiwan 70101, R.O.C. and also with the Department of Physics, National Central University, Chung-Li, Taiwan, R.O.C. Publisher Item Identifier S 1041-1135(01)06443-6.
ultralow intrinsic capacitance and its fabrication process is also compatible with field-effect-transistor (FET) based electronics. Thus, one can easily integrate GaN MSM photodetectors with GaN FET-based electronics to realize GaN-based optoelectronic integrated circuits (OEIC). However, the responsivity of the MSM photodetectors is low due to the blocking of incoming light by the metal electrodes. Although this problem can be solved by backside illumination , however, backside illumination will also create critical problems in optical lithography alignment, device processing, and chip packaging. Therefore, it should be very attractive if we could use a transparent conductor, such as indium–tin–oxide (ITO), to replace metal contact as the electrodes of MSM photodetectors , . Previously, we have reported a detailed study of ITO on n-GaN films deposited by electron-beam evaporation . In that report, we found that post deposition anneal will result in a larger effective Schottky barrier height due to interfacial reaction between ITO and GaN. We found that the measured effective Schottky barrier height was 0.68, 0.88, 0.94, and 0.95 eV for the nonannealed, 400 C annealed, 500 C annealed, and 600 C annealed ITO on n-GaN, respectively . In this study, we report the fabrication and characterization of GaN-based UV MSM photodetectors using ITO as the transparent contact electrodes. The GaN samples used in this study were all epitaxially grown on c-face sapphire substrates by metal–organic chemical vapor deposition (MOCVD). Trimethylgallium (TMGa) and ammonia (NH ) were used as the source materials of Ga and nitrogen, respectively. During MOCVD growth, hydrogen was used as the carrier gas and Si H was used as the n-type dopant source. Before growing the thicker high-temperature GaN epitaxial layers, a 300-Å low-temperature GaN buffer layer was deposited at 550 C. The growth temperature of the high-temperature GaN epitaxial layers was 1050 C and the growth rate was 3 m/h. The thickness of the GaN epitaxial layers used in this study was 2 m. The typical room temperature carrier concentration of the n-GaN epitaxial layers was 1 10 /cm , and the electron mobility was about 210 cm /Vsec. The ITO films were deposited onto n-GaN films and/or glass substrates by electron-beam evaporation from a target composed of 90% In O and 10% SnO . During ITO deposition, we kept the evaporation pressure at 1 10 torr and the substrate temperature at 300 C. The thickness of the ITO layers was kept at 1000 Å throughout this study. A xenon arc lamp was used as the optical source for the spectral responsivity measurements
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CHEN et al.: GaN MSM ULTRAVIOLET PHOTODETECTORS
Fig. 1. Optical transmittances characteristics variations with different annealing temperatures for 1000-Å-thick ITO films under N atmosphere for 15 min.
Fig. 2. Schematic structure of the MSM photodetector using ITO as transparent contact.
and a standard synchronous detection scheme was employed to measure the front-side illuminated detector signal. A calibrated UV-enhanced Si detector was used for signal detection. We first deposited ITO on glass substrates and measured the optical transmittance before and after thermal annealing. Fig. 1 shows the optical transmittance as a function of wavelength for as-deposited and thermally annealed ITO layers. In this figure, the transmittance of each film was normalized with respect to the transmittance of the glass substrate. From Fig. 1, we can see that the as-deposited ITO film has a peak 85% transmittance at around 390 nm, and a 15-min thermal annealing at 400 C and/or 500 C in N atmosphere has no influence on its transmittance characteristic. However, when we increased the annealing temperature to 600 C, we observed a significant improvement in its optical property. We found the ITO become more transparent in all wavelengths of interest. We also found that the peak transmittance increased from 85% to 98%. The wavelength where the peak occurs was also blue-shifted from 390 nm to 360 nm. We then deposited ITO contacts onto n-GaN epitaxial layers to fabricate GaN-based MSM photodetectors. Fig. 2 shows the schematic structure of the GaN MSM photodetectors used in this study. The devices consist of two interdigitated electrodes. The ITO fingers were 2 m wide and 100 m long with a spacing of 2 m. Prior to ITO deposition, the wafers
Fig. 3. The dark and illuminated I –V characteristics of the 400 600 C annealed ITO MSM photodetector on n-GaN.
were dipped in a diluted hydrochloric acid water solution (HCl : H O 1 : 1) for 3 min to remove the native oxide. After ITO deposition, Cr–Au contacts were deposited onto ITO films to serve as bonding pads. In order to obtain stable ITO Schottky contacts, the samples were thermally annealed in nitrogen atmosphere at 400 C, 500 C, 600 C for 15 min, respectively  to complete the fabrication of GaN-based MSM photodetectors. Fig. 3 shows the current–voltage ( – ) characteristics of the 400 C annealed and 600 C annealed ITO GaN MSM photodetectors measured in dark (dark current) and under illumination (photocurrent). A deuterium lamp was used as the UV light source for photocurrent measurement. From this figure, we can clearly see that the 600 C annealed detector has a larger photo current and a smaller dark current as compared with the 400 C annealed detector. The larger photo current is due to the higher contact transparency for the 600 C annealed detector, as can be seen from Fig. 1. On the other hand, the difference in dark current can be attributed to different Schottky barrier height for the two ITO contact electrodes. Previously, it has been reported that the effective Schottky barrier heights were 0.88 eV and 0.95 eV for the 400 C annealed and 600 C annealed ITO on n-GaN, respectively . The lower dark current of the 600 C annealed detector can thus be attributed to a larger Schottky barrier high between ITO and the n-GaN epitaxial layer. We also found that the dark currents increase almost linearly when the reverse bias voltage is small. As can be seen from Fig. 3, the dark current was A at a bias voltage of 0.5 V for the 600 C anonly 1 10 nealed detector. The photocurrents also increase almost linearly initially and then start to saturate at about 2 V. Such a saturation phenomenon may be due to generation-recombination centers and is limited by carrier lifetime ( ) . For the 600 C annealed detector, we found that the maximum photocurrent is 0.12 A with a photocurrent to dark current contrast higher than five orders of magnitude during UV irradiation. The high photocurrent and high photocurrent to dark current contrast may be attributed to the transparency of the ITO electrodes, since the transparent ITO contacts could increase effective lighted areas and, thus, increase the number of photogenerated carriers.
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 13, NO. 8, AUGUST 2001
with ITO transparent contacts were also fabricated. It was found that we can achieve a maximum 0.12-A photocurrent with a photocurrent to dark current contrast higher than five orders of magnitude during UV irradiation for the 600 C annealed photodetector. We also found that the maximum photo responsivity at 345 nm is 7.2 A/W and 0.9 A/W when the detector is biased at 5 and 0.5 V, respectively. ACKNOWLEDGMENT The authors would like to thank Opto-Electronics and Systems Laboratories of Industrial Technology Research Institute for their technical support. REFERENCES
Fig. 4. Spectral responsivity of the ITO MSM photodetectors, (a) and (b) at 5 and 0.5 V bias, respectively.
The responsivity as a function of wavelength for the 600 C annealed ITO GaN MSM photodetector is shown in Fig. 4. The sample was illuminated from the frontside by a xenon arc lamp. We found that a sharp cutoff occurs at 351 nm. Such a spectral response is typical for the visible-blind GaN-based UV photodetector. The fact that the detector response drops by more than four orders of magnitude across the cutoff region suggests that the quality of the n-GaN epitaxial layer is good. We also found that the response is nearly constant for wavelengths between 250 and 325 nm. Furthermore, we found that the maximum photo responsivity at 345 nm is 7.2 and 0.9 A/W when the detector is biased at 5 and 0.5 V, respectively. These values are much larger than its theoretical limit, which suggests that there exists photoconductive gain when the detector is under illumination. Such a photoconductive gain might significantly degrade the response speed of the detector. However, such a detector could still be used in applications such as ozone layer monitoring and flame detection. In summary, ITO layers were deposited onto n-GaN films and/or glass substrates by electron-beam evaporation. With proper annealing, we found that we can improve the optical properties of the ITO layers and achieve a maximum transmittance of 98% at 360 nm. GaN-based MSM photodetectors
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