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Japanese Journal of Applied Physics Vol 43, No 4B, 2004, pp 2008–2010 #2004 The Japan Society of Applied Physics InGaN/GaN Multi-Quantum Well Metal-Insulator Semiconductor Photodetectors with Photo-CVD SiO2 Layers Ping-Chuan C HANG, Chin-Hsiang CHENÃ;1 , Shoou-Jinn C HANG, Yan-Kuin S U, Po-Chang C HEN2 , Yi-De J HOU, Chun-Hsing L IU2 , Hung HUNG and Shih-Ming W ANG3 Institute of Microelectronics & Department of Electrical Engineering, National Cheng Kung University, Tainan 701, Taiwan, R.O.C Department of Electronic Engineering, Cheng Shiu University, Kaohsiung 830, Taiwan, R.O.C Department of Electronic and Computering Engineering, Nan Jeon Institute of Technology, Yan-Hsui, Taiwan 737, R.O.C Department of Electronic Engineering, National Yunlin University of Science and Technology, R.O.C (Received September 18, 2003; accepted November 17, 2003; published April 27, 2004) InGaN/GaN multiple-quantum well (MQW) structure was epitaxial growth by metal-organic chemical vapor deposition (MOCVD) Their MIS photodiodes with SiO2 interlayer were fabricated successfully using photochemical vapor deposition The normal undoped-GaN metal-semiconductor-metal (MSM) potodiodes were also prepared to compare with them It was found that the minimum dark current of InGaN/GaN MQW photodiodes was 4:2  10À13 A with 88 nm-thick SiO2 layer under V reverse bias voltage Furthermore, it was found that we can significantly reduce the dark current of this photodiodes by inserting a thin SiO2 interlayer in between metal electrode and the underneath InGaN/GaN MQW With a 53 nm-thick SiO2 interlayer, it was also found that we could achieve a high 1:53  103 photo current to dark current contrast [DOI: 10.1143/JJAP.43.2008] KEYWORDS: InGaN, MQW, MIS photodiode, GaN, MOCVD Introduction III-V nitride semiconductors are attractive materials that can be applied in various optoelectronic devices such as photodiodes, light emitter diodes (LEDs) and laser diodes (LDs) In fact, nitride-based blue and green LEDs are already commercially available and have been extensively used in traffic light source and full color display However, relatively few reports regarding nitride-based blue/ultraviolet (UV) photodiodes could be found in the literature, as compared to nitride-based LEDs.1) Nitride-based photodiodes are important devices that can be used in various commercial and military applications For example, these devices can be applied in space, medical and environmental fields Depending on device structure, nitride-based p-n junction diodes,2) p-i-n diodes,3) p--n diodes,4) Schottky barrier detectors,5) metal-insulator-semiconductor (MIS) and metal-semiconductor-metal (MSM) photodetectors6,7) could all be used to detect blue/UV signal Among these devices, MSM and MIS photodetectors have an ultra-low intrinsic capacitance and their fabrication process is also compatible with field-effect-transistor (FET) based electronics Thus, one can easily integrate GaN MSM and/or MIS photodetectors with GaN FET-based electronics to realize a GaNbased optoelectronic integrated circuit (OEIC) These advantages all make nitride-based MSM and MIS photodetectors attractive for practical applications Although it is easier to fabricate MSM photodetectors, the leakage current of such MSM photodetectors will become very large if the Schottky barrier height at the metal/semiconductor interface is small Such a problem could be overcome by inserting a thin insulating layer in between metal and the underneath semiconductor We should be able to significantly suppress the dark current and thus achieve a much higher photo current to dark current contrast ratio by using such MIS photodetectors.8,9) Previously, it has been shown that photo-enhanced à E-mail address: chchen@csu.edu.tw chemical vapor deposition (photo-CVD) can be used to grow high quality SiO2 layers GaN.10,11) In using photoCVD to grow thin films, selecting proper light source with a radiation spectrum matching the absorption spectra of the reactance gases is very important In this study, we used a deuterium (D2 ) lamp as the excitation source It is known that D2 lamp emits strong ultra violet (UV) and vacuum ultra violet (VUV), which can effectively decompose SiH4 and O2; 12,13) In addition, such a photo-CVD system offers better control in the oxide region and selective growth is possible GaN MIS photodetectors with such photo-CVD SiO2 layers have also been demonstrated On the other hand, the multiquantum well (MQW) structure is frequently used as light emitter in LED and LD optoelectronic devices MQW based on the quantum-confined Stark effect (QCSE) are very promising as the modulators in the self-electro-optic effect device (SEED) technology14) because of their fast switching speed, relatively high photo current, better responsivity characteristic, high extinction ratio, and variable detected wavelength range Therefore, we can use InGaN/GaN (MQW) structure to shift cut-off wavelength and to enhance the responsivity of photodetectors In this paper, we report the fabrication and characterization of InGaN/GaN MQW MIS photodetectors using photo-CVD SiO2 as the insulating materials The electro-optical properties of the fabricated photodetectors will also be discussed Experiment Prior to the deposition of SiO2 layers, an InGaN/GaN MQW structure was grown on (0001) sapphire substrates by metal-organic chemical vapor deposition (MOCVD).15–21) The InGaN/GaN MQW structure consists a 25 nm-thick low temperature GaN nucleation layer, a mm-thick undoped GaN, a mm-thick Si-doped GaN, a 5-period Si-doped InGaN/GaN MQW and a 30 nm-thick GaN cap layer Each InGaN/GaN pair consists a 2.5 nm-thick In0:23 Ga0:77 N well layer and a 12 nm-thick GaN barrier layer The SiO2 films with various thicknesses were subsequently deposited onto the 30 nm-thick GaN cap layer by photo-CVD with a 150 2008 Jpn J Appl Phys., Vol 43, No 4B (2004) P.-C C HANG et al Ni/Au -2 10 Dark current (A) Ni/Au 2009 SiO2 InGaN/GaN MQW n-GaN SiO2 thickness -5 10 0nm 7nm 53nm 88nm -8 10 -11 10 u-GaN nucleation Layer Sapphire Watt D2 lamp During SiO2 deposition, the gas ratio was fixed at SiH4 /O2 =0.055 under various growth temperatures Ni/Au metal contacts were then evaporated on top of the SiO2 to serve as the electrodes Figure shows the schematic diagram of the fabricated InGaN/GaN MQW MIS photodetectors The device consists of two inter-digitated contact electrodes.6) The capacitance-voltage (C-V) characteristics of the fabricated MIS capacitors were then measured by an HP 4284B LCR meter An HP-4156 semiconductor parameter analyzer was used to measure the current-voltage (I-V) characteristics of the InGaN/GaN MQW photodiodes in dark and under illumination For photo current measurements, a He-Cd laser illuminating from the front side of InGaN/GaN MQW photodiodes was used as the light source Results and Discussion CHF /COX Figure shows C-V characteristics (1 MHz) of the photoCVD SiO2 grown at various temperatures It was found that no significant hysteretic was observed as the gate voltage varied at 0.1 V/s from ỵ5 V to 20 V and then backs to ỵ5 V The lack of hysteresis in these C-V curves indicated that the number of mobile ions in SiO2 layers is negligibly small Using the standard high frequency capacitance method,22) we found that the interface state density, Dit , 1.00 Tsub=500°C 0.95 Tsub=150°C Tsub=300°C 0.90 Ideal C-V curve 0.85 Tox=50 nm 0.80 Frequency=1 MHz -3 Area=1.26X10 cm 0.75 0.70 0.65 0.60 -25 -20 -15 -10 Reverse voltage (V) Fig Dark I-V characteristics of the InGaN/GaN MQW MIS photodetectors with various SiO2 layer thicknesses Fig Schematic diagram of the fabricated InGaN/GaN MQW MIS photodetectors -5 Gate Voltage (V) Fig C-V characteristics (1 MHz) of the photo-CVD SiO2 grown at various temperatures equals 1:2  1012 cmÀ2 eVÀ1 and 8:4  1011 cmÀ2 eVÀ1 for the photo-CVD SiO2 layers deposited at 150 C and 300 C, respectively The smaller Dit for the photo-CVD SiO2 layer deposited at 300 C could be attributed to the fact that a higher substrate temperature can significantly improve the SiO2 /GaN interfacial properties probably through supplying thermal energy to the Si and O atoms However, Dit was increased to 6:4  1012 cmÀ2 eVÀ1 when the substrate temperature was increased to 500 C The exact reason for the increase in Dit is not clear yet Possible reasons for such degradation in oxide quality include too fast an oxide growth rate and/or some interface reactions, which occur at high temperatures Figure shows dark I-V characteristics of the InGaN/ GaN MQW MIS photodetectors with various SiO2 layer thicknesses It was found that the dark current of the InGaN/ GaN MQW photodetectors without this SiO2 layer (i.e SiO2 thickness = nm) was higher than that with SiO2 layers This is mainly caused by sequential resonant electron tunneling through barriers of InGaN/GaN MQW structures Such a high dark current may increase detector noise, reduce detectivity, and consume power It is also indicated that the Schottky barrier height of this MQW photodetector was small In order to reduce the dark current, the SiO2 layer was inserted as the insulating materials to fabricate InGaN/GaN MQW MIS photodetectors by using photo-CVD system It was also found that dark current decreases with the increase of SiO2 layer thickness With a V applied bias, we could reduce the dark current from 8:6  10À3 A to 4:9  10À10 A with the insertion of a 53 nm-thick photo-CVD SiO2 layer In other words, we could reduce the dark current by more than orders of magnitudes Figure shows measured photo current of the InGaN/GaN MQW MIS photodetectors with various SiO2 layer thicknesses It was found that photo current also decreases as the SiO2 layer thickness increases However, the decrease of photo current was much less significant compared to the decrease in dark current With a V applied bias, we could only reduce the photo current by a factor of about 4:5  10À3 Such a result suggests that we could significantly reduce the dark current, while still maintain a reasonably large photo current by introducing the photo-CVD SiO2 layer into the photodetectors Figure shows photo current to dark current contrast ratio as a function of the SiO2 layer thickness under various applied biases It can be seen clearly that we could always achieve 2010 Jpn J Appl Phys., Vol 43, No 4B (2004) P.-C C HANG et al With a 53 nm-thick SiO2 layer, it was also found that we could achieve a high 1:53  103 photo current to dark current contrast ratio -1 Photo current (A) 10 SiO2 thickness 0nm 7nm 53nm 88nm -4 10 -7 10 -10 10 -13 10 Photo/dark current contrast Fig Measured photo current of the InGaN/GaN MQW MIS photodetectors with various SiO2 layer thicknesses Applied bias 1V 2V 3V 4V 5V 10 10 10 20 40 60 80 100 SiO2 thickness (nm) Fig Photo current to dark current contrast ratio as a function of the SiO2 layer thickness under various applied biases the highest photo current to dark current contrast from the sample with a 53 nm-thick photo-CVD SiO2 layer, regardless of the applied bias With a V applied bias, it was found that we could achieve a high 1:53  103 photo current to dark current contrast ratio from the photodetector with a 53 nm-thick photo-CVD SiO2 layer Such a value suggests that photo-CVD SiO2 layer can indeed be used to improve the performance of the InGaN/GaN MQW MIS photodetectors The authors would like to acknowledge the financial support from the National Science Council for their research grant of NSC 92-2215-E-230-002 and Ministry of Education for the program for promoting university academic excellence A-91-E-FA08-1-4 Reverse voltage (V) 10 Acknowledgements Summary In summary, InGaN/GaN MQW MIS photodiodes with photo-CVD SiO2 layers were fabricated successfully It was found that we could significantly reduce the dark current, while still maintain a reasonably large photo current by inserting a photo-CVD SiO2 layer in between metal electrodes and the underneath 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(V) Fig Dark I-V characteristics of the InGaN/ GaN MQW MIS photodetectors with various SiO2 layer thicknesses Fig Schematic diagram of the fabricated InGaN/ GaN MQW MIS photodetectors -5 Gate Voltage... shows dark I-V characteristics of the InGaN/ GaN MQW MIS photodetectors with various SiO2 layer thicknesses It was found that the dark current of the InGaN/ GaN MQW photodetectors without this