SOLAR-BLIND ultraviolet (UV) photodetectors (PDs)

Transcription

1 2086 IEEE SENSORS JOURNAL, VOL. 12, NO. 6, JUNE 2012 Ultra-Low Dark Current AlGaN-Based Solar-Blind Metal Semiconductor Metal Photodetectors for High-Temperature Applications Feng Xie, Hai Lu, Dunjun Chen, Member, IEEE, Xiaoli Ji, Feng Yan, Rong Zhang, Youdou Zheng, Liang Li, and Jianjun Zhou Abstract Solar-blind metal semiconductor metal (MSM) photodetectors (PDs) with Ni/Au semi-transparent interdigitated contact electrodes are fabricated on Al 0.4 Ga 0.6 N epi-layer grown by metal organic chemical vapor deposition on sapphire substrate. The PD exhibits ultra-low dark current in fa range at room temperature (RT) under 20-V bias and a corresponding breakdown voltage higher than 300 V. The PD also shows a maximum RT quantum efficiency of 64% at 275 nm under 10-V bias with a solar-blind/ultraviolet (UV) rejection ratio up to four orders of magnitude. Even at a high temperature of 150 C, the dark current of the PD is still in fa range with a reasonable rejection ratio of more than 8000, suggesting its potential applications for high-temperature deep-uv detection. The ultra-low dark current achieved is believed to be related to the high-temperature AlN buffer layer used in the epi-structure as well as the coplanar configuration of the MSM PD itself. Index Terms AlGaN, metal semiconductor metal, photodetector, solar-blind. I. INTRODUCTION SOLAR-BLIND ultraviolet (UV) photodetectors (PDs) have a variety of potential applications including missile plume sensing, flame detection, environmental monitoring, chemical/biological agent detection, and covert space-to-space communications [1]. Solar-blind photo-detection is traditionally accomplished by photomultiplier tubes (PMTs) and silicon photodiodes. However, these detectors require complex and expensive filters for solar-blind operation. In addition, PMTs which require a high-voltage power supply are generally Manuscript received December 15, 2011; accepted January 8, Date of publication January 16, 2012; date of current version April 26, This work was supported in part by the State Key Program for Basic Research of China under Grant 2010CB327504, Grant 2011CB922100, and Grant 2011CB301900, in part by the National Natural Science Foundation of China under Grant , Grant , Grant , and Grant BK , and in part by the Fundamental Research Funds for the Central Universities under Grant and Grant The associate editor coordinating the review of this manuscript and approving it for publication was Dr. Alexander Fish. F. Xie, H. Lu, D. Chen, X. Ji, F. Yan, R. Zhang, and Y. Zheng are with the Jiangsu Provincial Key Laboratory of Advanced Photonic and Electronic Materials, and School of Electronic Science and Engineering, Nanjing University, Nanjing , China ( hailu@nju.edu.cn). L. Li and J. Zhou are with the National Key Laboratory of Monolithic Circuits and Modules, Nanjing Electron Devices Institute, Nanjing , China. Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /JSEN X/$ IEEE bulky, fragile, and expensive. Thus, there is an urgent need for developing high performance solid-state PDs that can replace traditional PMTs and silicon photodiodes for solarblind sensing. Recently, AlGaN-based wide-bandgap semiconductors appear as an excellent candidate for deep UV detector applications due to their intrinsic solar-blindness, superior radiation hardness and high-temperature resistance [2]. A few AlGaN-based solar-blind PDs with different device structures have already been demonstrated, including photoconductors [3], metal semiconductor metal (MSM) PDs [4], [5], Schottky barrier photodiodes [6], [7], and p i n photodiodes [8]. Among these structures, MSM PDs have many attractive advantages for practical applications, such as no p-type doping required, intrinsically high speed, fabrication simplicity and compatibility with field effect transistor technology. Meanwhile, for many harsh environmental applications, such as flame detection for gas turbines, PDs capable of operating at high temperature are greatly needed. To develop thermally stable PDs, one should first significantly reduce the reverse leakage current. However, due to the high-density defects existing within the hetero-epitaxial structure, III-nitride based devices commonly suffer from enhanced leakage current at elevated temperatures [9], making the development of AlGaN-based PDs toward high temperature applications face many difficulties. As a result, until recently there is still no report on high temperature characterizations of AlGaN-based solar-blind PDs. In this work, we report the first fabrication and characterization of high performance AlGaN-based solar-blind MSM PDs at high temperature. The PDs exhibit ultra-low dark current, high solar-blind/uv rejection ratio, and high quantum efficiency at room-temperature (RT). Even at 150 C (the high temperature limit of conventional plastic package), these PDs still exhibit reasonably high performance, especially in terms of low dark current. II. EXPERIMENT The device structure was grown by metal-organic chemical vapor deposition on 2-inch sapphire substrate under optimized growth conditions, which consists of a 0.3-µm unintentionally doped Al 0.4 Ga 0.6 N layer on top of a 0.5-µm highly insulative AlN buffer layer. The AlN buffer was grown with our proprietary high temperature recipe, while the AlGaN active layer was grown at 1100 C with an equivalent III/V ratio 转载

2 XIE et al.: ULTRA-LOW DARK CURRENT AlGaN-BASED SOLAR-BLIND MSM PDs FOR HIGH-TEMPERATURE APPLICATIONS 2087 Fig. 2. (a) Schematic of the device structure and (b) top view image of one fabricated PD with an effective device area of µm 2. Fig. 1. Optical transmission spectra of the epi-structure and the Ni/Au semitransparent contact layer. The RT CL spectrum of the AlGaN active layer is also shown. of The chamber pressure during the AlGaN growth was kept at 100 mtorr. Optical transmission spectroscopy was used to confirm the Al content of the AlGaN active layer. As shown in Fig. 1, the optical transmission curve exhibits a sharp cutoff at 272 nm, corresponding to an Al-mole fraction of 0.40 for AlGaN alloy. The spectrum also shows regular Fabry-Pérot oscillations in long wavelength region, suggesting high interface quality of the epi-structure. Cathodoluminescence (CL) spectrum of the AlGaN layer taken at RT was also shown in Fig. 1 for comparison, which peaks at 279 nm with a full width at half maximum (FWHM) of 15 nm. The slight red-shift of the CL peak relative to the optical absorption edge is likely caused by the contribution of light emission through near band-edge states. It is important to note that no discernible parasitic emission is observed in the yellow band range, suggesting low impurity and point defect density within the AlGaN active layer [10]. Observation with a Nomarski microscope indicates that the whole wafer surface is crackfree and the surface roughness of the AlGaN layer determined by atomic force microscopy is 0.47 nm. In addition, high resolution x-ray diffraction measurement indicates that the AlGaN active layer is fully relaxed with a FWHM of 680 arcsec for the (002) rocking curve. Considering the large lattice mismatch between AlN and Al 0.4 Ga 0.6 N as well as the fact that the AlGaN layer is quite thin (0.3-µm), the AlGaN layer obtained has reasonable crystalline quality. The fabrication process started with deposition of 5 nm/5 nm thick semi-transparent Ni/Au Schottky contact by e-beam evaporation. Standard optical lithography and liftoff techniques were used to define the interdigitated contact electrodes. The fingers of the contact electrodes are 5 µm wide and 400 µm long with a spacing of 5 µm. The optical transmittance of the semi-transparent Ni/Au contact layer in wavelength range of nm is about 35-43% (Fig. 1). Next, a 200-nm thick Ti/Au bi-layer was evaporated and patterned to form the contact pads. Finally, the PDs were annealed at 300 C for 3 min in N 2 ambient by a rapid thermal annealing equipment. Figure 2 shows a schematic and a topview image of one finished PD with an effective device area of µm 2. Fig. 3. (a) I V characteristics of the PD measured under dark and under 254-nm UV illumination at RT and 150 C, respectively. (b) RT electrical breakdown characteristics of the PD. The optical power density of the 254-nm UV illumination is 5.8 µw/mm 2. Current-voltage (I V ) characterizations of the PDs are measured by using a Keithley 4200 semiconductor parameter analyzer and a Keithley 237 high-voltage source measurement unit. The light output from a 500-W Xe lamp is directed into a monochromator for single wavelength selection. The PDs under test are illuminated by monochromic light transmitting through an UV optical fiber, which is coupled into the output port of the monochromator. The incident optical power density is calibrated by using a UV-enhanced Si photodiode. In addition, linearity characteristics of the PD is measured by powering a 280 nm UV light-emitting diode with varied injection current levels and measuring the detector s photoresponse simultaneously. III. RESULTS AND DISCUSSION Figure 3(a) shows the dark and photocurrent curves of the MSM PD at RT and 150 C, respectively. For bias below 20 V, the measurable RT dark current of the PD is limited

3 2088 IEEE SENSORS JOURNAL, VOL. 12, NO. 6, JUNE 2012 by the experimental setup and should be less than 1 fa, which corresponds to an ultra-low dark current density of < A/cm 2. Even at a high temperature of 150 C, the dark current of the PD is still in fa range. Moreover, as shown in Fig. 3(b), the PD exhibits a RT breakdown voltage higher than 300 V. Meanwhile, the photocurrent curves of the PD first rise rapidly in low bias range and then increase much slower after 2 V. This trend should be caused by full depletion of the active region between the two interdigitated contact electrodes. The photocurrent density measured at 150 C is 20-40% lower than that measured at RT depending on bias levels, which can be readily explained by the enhanced carrier recombination loss at high temperature. Besides the high Schottky barrier height between the Ni contact and the AlGaN layer, the ultra-low dark current obtained in this work is believed also related to the high temperature AlN buffer layer used in the epi-structure as well as the coplanar configuration of the MSM PD itself. Although the AlGaN epi-layer has reasonable crystalline quality, there are still high-density threading dislocations existing within the material. It has been determined that structural defects, especially dislocations with a screw component, are the primary leakage path in III-nitride semiconductors [11]. In the present device, although there are dislocations existing within the AlGaN active layer serving as leakage path, the leakage path would terminate at the AlGaN/AlN interface if the dislocations are misfit dislocations [12]. Even some of the dislocations are originally extended from the misfit dislocations at the AlN/sapphire interface and are continuous at the AlGaN/AlN interface, because of the highly insulative nature of the AlN buffer, there is still no lateral conductive path formed between the two contact electrodes. Thus, the high temperature AlN buffer here could work as a high energy barrier preventing the leakage current from flowing laterally. Figure 4(a) and (b) show the spectral response characteristics of the PD measured at RT and at 150 C, respectively. The photoresponse curves show a sharp cutoff at 280 nm and peak at 275 nm, which is in good agreement with the band edge absorption of the AlGaN active layer. The RT peak responsivity gradually increases as a function of bias and reaches 143 ma/w at 10 V, corresponding to a maximum quantum efficiency of 64%. Meanwhile, also at 10-V bias, the solar-blind/uv rejection ratio of the PD could be as high as at RT, and is still higher than even at a high temperature of 150 C. Here, the solar-blind/uv rejection ratio is defined as the responsivity measured at 275 nm divided by that measured at 350 nm. The ultra-low dark current and high rejection ratio at 150 C suggest that such PDs are suitable for high temperature applications. Figure 5 shows the measured peak responsivity of the PD at 275 nm as a function of applied bias at RT and 150 C, respectively. Clearly the responsivity gradually rises as bias increases and the corresponding RT quantum efficiency of the PD would eventually exceed 100% (>27 V, not shown), indicating that a slight internal gain exists within the PD. Such kind of gain is not necessarily linked to the crystalline quality of the semiconductor, as it is also frequently present in MSM PDs based on much maturer semiconductors like GaAs [13]. Fig. 4. Bias-dependent spectral response of the MSM PD measured at (a) RT and (b) 150 C. Fig. 5. Measured peak responsivity at 275 nm as a function of applied bias at RT and 150 C, respectively. The optical power density of the 275-nm UV illumination is 1.7 µw/mm 2. Fig. 6. Photocurrent as a function of optical power density measured at 280 nm under different biases. The inset shows the corresponding responsivity as a function of optical power density. In our past study, we have determined that such internal gain is caused by photogenerated holes trapped at the semiconductor/metal interface, as well as the field-induced image-force lowering effect [14]. Here it should be noted that photoconductive gain is desirable for applications requiring high responsivity but could limit PD s bandwidth simultaneously.

4 XIE et al.: ULTRA-LOW DARK CURRENT AlGaN-BASED SOLAR-BLIND MSM PDs FOR HIGH-TEMPERATURE APPLICATIONS 2089 Finally, linearity characteristics of the MSM-PD are shown in Fig. 6. Although the photocurrent under different bias increases approximately linearly with incident optical power density in log-log scale, exact calculation indicates that the corresponding photo-responsivity actually varies as a function of optical power density. As shown in the inset of Fig. 6, the responsivity gradually increases by 50 % as the optical power density increases from to W/mm 2. This observation is another evidence for the suggested weak internal gain of the PD, as if the gain is indeed partly caused by interface hole trapping mechanism, more trapping states would be filled at higher photon flux, leading to more severe Schottky barrier lowering. IV. CONCLUSION In summary, AlGaN-based solar-blind MSM PDs grown by MOCVD have been fabricated on sapphire substrate. The PDs exhibit extremely low dark current density and high solarblind/uv rejection ratio at RT as well as at 150 C, suggesting that these devices are suitable for high temperature deep-uv sensing applications. The ultra-low dark current achieved is explained as a result of the high temperature AlN buffer layer applied in the epi-structure. REFERENCES [1] M. Razeghi, Short-wavelength solar-blind detectors-status, prospects, and markets, Proc. IEEE, vol. 90, no. 6, pp , Jun [2] M. A. Khan, M. Shatalov, H. P. Maruska, H. M. Wang, and E. Kuokstis, III-nitride UV devices, Jpn. J. Appl. Phys., vol. 44, no. 10, pp , Oct [3] D. Walker, X. Zhang, P. Kung, A. Saxler, S. Javadpour, J. Xu, and M. Razeghi, AlGaN ultraviolet photoconductors grown on sapphire, Appl. Phys. Lett., vol. 68, no. 15, pp , Apr [4] S. V. Averine, P. I. Kumetzov, V. A. Zhitov, and N. V. Alkeev, Solarblind MSM-photodetectors based on Al x Ga 1 x N/GaN heterostructures grown by MOCVD, Solid State Electron., vol. 52, no. 5, pp , May [5] C.H.Chen,S.J.Chang,M.H.Wu,S.Y.Tsai,andH.J.Chien, AlGaN metal-semiconductor-metal photodetectors with low-temperature AlN cap layer and recessed electrodes, Jpn. J. Appl. Phys., vol. 49, no. 4, pp. 1 3, Apr [6] H. Jiang and T. Egawa, High quality AlGaN solar-blind Schottky photodiodes fabricated on AIN/sapphire template, Appl. Phys. Lett., vol. 90, no. 12, pp , Mar [7] J. Y. Duboz, N. Grandjean, F. Omnes, M. Nosca, and J. L. Reverchon, Internal photoemission in solar blind AlGaN Schottky barrier photodiodes, Appl. Phys. Lett., vol. 86, no. 6, pp , Feb [8] T. Tut, T. Yelboga, E. Ulker, and E. Ozbay, Solar-blind AlGaN-based pin photodetectors with high breakdown voltage and detectivity, Appl. Phys. Lett., vol. 92, no. 10, pp , Mar [9] H. Zhang, E. J. Miller, and E. T. Yu, Analysis of leakage current mechanisms in Schottky contacts to GaN and Al 0.25 Ga 0.75 N/GaN grown by molecular-beam epitaxy, J. Appl. Phys., vol. 99, no. 2, pp , Jan [10] N. Yamamoto, H. Itoh, V. Grillo, S. F. Chichibu, S. Keller, J. S. Speck, S. P. DenBaars, U. K. Mishra, S. Nakamura, and G. Salviati, Cathodoluminescence characterization of dislocations in gallium nitride using a transmission electron microscope, J. Appl. Phys., vol. 94, no. 7, pp , Oct [11] J. W. P. Hsu, M. J. Manfra, D. V. Lang, S. Richter, S. N. G. Chu, A. M. Sergent, R. N. Kleiman, L. N. Pfeiffer, and R. J. Molnar, Inhomogeneous spatial distribution of reverse bias leakage in GaN Schottky diodes, Appl. Phys. Lett., vol. 78, no. 12, pp , Mar [12] F. A. Ponce, J. S. Major, W. E. Plano, and D. F. Welch, Crystalline structure of AlGaN epitaxy on sapphire using AlN buffer layers, Appl. Phys. Lett., vol. 65, no. 18, pp , Oct [13] J. Burm and L. Eastman, Low-frequency gain in MSM photodiodes due to charge accumulation and image force lowering, IEEE Photon. Technol. Lett., vol. 8, no. 1, pp , Jan [14] F. Xie, H. Lu, X. Q. Xiu, D. J. Chen, P. Han, R. Zhang, and Y. D. Zheng, Low dark current and internal gain mechanism of GaN MSM photodetectors fabricated on bulk GaN substrate, Solid State Electron., vol. 57, no. 1, pp , Mar Feng Xie received the B.S. and M.S. degrees from the School of Technical Physics, Xidian University, Xi an, China, in 2006 and 2009, respectively. He is currently pursuing the Ph.D. degree with the School of Electronic Science and Engineering, Nanjing University, Nanjing, China. His current research interests include fabrication and characterization of III V optoelectronic devices. Hai Lu received the B.S. and M.S. degrees in physics from Nanjing University, Nanjing, China, and the Ph.D. degree in electrical engineering from Cornell University, Ithaca, NY, in 1992, 1996, and 2003, respectively. He was with GE Global Research Center, Niskayuna, NY, from 2004 to In 2006, he joined Nanjing University and is a Full Professor of microelectronics. He is currently a Principle Investigator with the Nanjing National Laboratory of Microstructures, Nanjing. His particular interest has been in the correlation of device performance with material growth and processing parameters. He has published more than 200 articles, book chapters, and conference papers. His current research interests include growth and characterization of III-nitride semiconductors, photonic devices, and high-power devices. Dunjun Chen (M 09) received the B.S. and Ph.D. degrees in materials engineering from Northwestern Polytechnical University, Xi an, China, in 1991 and 2001, respectively. He joined the Physics Department, Nanjing University, Nanjing, China, in September He is currently a Professor with the School of Electronic Science and Engineering. From September 2007 to July 2008, he was a Visiting Research Fellow with Harvard University, Cambridge, MA, funded by the Graduate School of Arts and Science. His current research interests include GaN-based electronics and optoelectronic devices. Xiaoli Ji received the Ph.D. degree from Tsukuba University, Ibaraki, Japan, in She is currently with the School of Electronic Science and Engineering, Nanjing University, Nanjing, China, as an Associate Professor. Her current research interests include physics and reliability of light-emitting diodes and complimentary metaloxide-semiconductor devices.

5 2090 IEEE SENSORS JOURNAL, VOL. 12, NO. 6, JUNE 2012 Feng Yan, photograph and biography not available at the time of publication. Rong Zhang received the M.S. and Ph.D. degrees in physics from Nanjing University, Nanjing, China, in 1986 and 1995, respectively. He was a Visiting Scholar with the University of Maryland, College Park, from 1995 to From 1996 to 1998, he was a Visiting Scholar with the University of Wisconsin-Madison, Madison. He is currently a Full Professor with the School of Electronic Science and Engineering, Nanjing University. His current research interests include epitaxial growth of wide bandgap semiconductors, opto-electronic devices, and spintronics. Liang Li was born in Nanjing, China, in He received the Ph.D. degree from the Department of Physics, Nanjing University, Nanjing, in He is currently a Senior Engineer with the National Key Laboratory of Science and Technology on monolithic integrated circuits and modules, Nanjing Electronic Devices Institute, Nanjing. He specializes in metalorganic chemical vapor deposition growth of III-nitride semiconductors and fabrication of nitride-based optoelectronic devices. Jianjun Zhou was born in Ningxia, China, on May 10, He received the Ph.D. degree from the Department of Physics, Nanjing University, Nanjing, China, in He joined Nanjing Electronic Devices Institute, Nanjing, in His current research interests include optoelectronic devices and microwave semiconductor devices. of Sciences. Youdou Zheng received the B.S. degree in physics from Nanjing University, Nanjing, China, in He is currently a Full Professor with the School of Electronic Science and Engineering, Nanjing University. He has several national awards for his achievement in the field. His current research interests include electronics and optical properties of semiconductors, with a focus on wide bandgap semiconductors and group-iv semiconductor-based hetero-structures. Prof. Zheng is a member of the Chinese Academy