JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.18, NO.5, OCTOBER, 2018 ISSN(Print) 1598-1657 https://doi.org/10.5573/jsts.2018.18.5.645 ISSN(Online) 2233-4866 Gallium Nitride PIN Avalanche Photodiode with Double-step Mesa Structure Thu Thi Thuy Pham 1, Hyungsik Shin 1, Eugene Chong 2, and Ho-Young Cha 1,* Abstract Typical gallium nitride (GaN) PIN avalanche photodiodes (APDs) are fabricated using a beveled mesa structure due to the difficulty of the ionimplantation process for GaN. The bevel angle of mesa structure must be very small in order to suppress the localized electric field at the junction sidewall that limits the maximum APD gain. In this study, we proposed a double-step mesa structure that can suppress the high electric field at the junction sidewall while maximizing the electric field strength inside the active region under the avalanche bias condition. Index Terms Avalanche photodiode, gallium nitride, bevel, breakdown voltage, gain I. INTRODUCTION Recently, solid-state ultraviolet (UV) detectors have received great attention for various sensor applications due to their small size, light weight, and low production cost [1-4]. Gallium nitride (GaN) has a wide energy bandgap of 3.45 ev and a very low intrinsic carrier concentration of 1.9 x 10-10 cm -3 at 300K [5-8], which allow solar-blind photo response, high temperature operation, and low dark current characteristics [9]. An additional important feature is the tunable energy bandgap with adjusting Al x Ga 1-x N composition. Manuscript received Apr. 13, 2018; accepted Oct. 11, 2018 1 School of Electronic and Electrical Engineering, Hongik University, Seoul, Korea 2 Chem-Bio Division, Agency for Defense Development, Daejeon, Korea E-mail : hcha@hongik.ac.kr Therefore, GaN based UV detectors have benefits including small size, high signal-to-noise ratio, and temperature stability. Avalanche photodiodes (APDs) have an internal gain that amplifies the photocurrent and thus enable very weak signal detection. PIN structures are the most common eptitaxial structures for APD fabrication. Typical GaN PIN APDs employ beveled mesa structures [10] for device isolation due to the difficulty of the ion-implantation process for GaN. The bevel angle must be very small in order to suppress the localized high electric field at the junction sidewall. The high electric field at the sidewall causes early edge breakdown, so that the relatively lower electric field in the active region limits the APD gain due to insufficient multiplication effects [11]. Therefore, it is the most important design consideration to maximize the electric field in the active region under the avalanche bias condition. In this study, we proposed a double-step mesa structure that can suppress the electric field at the mesa sidewall while maximizing the electric field strength inside the active region under the avalanche bias condition. II. EXPERIMENTS AND DISCUSSION Cross-sectional schematics for a conventional singlebeveled mesa structure and a double-step mesa structure are shown in Fig. 1(a) and (b), respectively. The doping concentration and thickness of each layer are shown in Table 1. A bevel structure was employed to suppress the electric field at the junction sidewall [12]. The bevel angle must be sufficiently small in order to effectively suppress the electric field at the junction sidewall. The
646 THU THI THUY PHAM et al : GALLIUM NITRIDE PIN AVALANCHE PHOTODIODE WITH DOUBLE-STEP MESA Fig. 1. Cross-sectional schematics of PIN APDs with (a) conventional single-beveled mesa, (b) double-step mesa structures. Table 1. Doping concentration and thickness of each layer in GaN PIN APD Layer Doping concentration (cm -3 ) Thickness (mm) P + contact layer 1 10 19 0.15 P layer 5 10 17 0.15 N - intrinsic layer 1 10 16 1.3 N + contact layer 1 10 18 1.4 Fig. 2. Comparison of breakdown characteristics for ideal, single-beveled mesa, and double-step mesa structures. bevel angle of 10 o was used in this study considering the difficulties in fabrication. Device simulations were performed using a commercial two-dimensional simulator, TCAD (SILVACO ATLAS). The impact ionization coefficient used for GaN is given by [13] a e - 7 8 3.4 10 / E = 2.9 10 (1) where E is the electric field. The breakdown characteristics simulated for both structures are shown in Fig. 2 where the behavior of an ideal structure is also compared. The breakdown voltage for the single-beveled mesa and double-step mesa structures are 436 V and 495 V, respectively. The breakdown voltage of the double-step mesa structure is close to that of the ideal case. Two-dimensional potential and corresponding electric field distributions under the breakdown voltage conditions for single-beveled mesa and double-step mesa structures are compared in Fig. 3(a) and (b), respectively. The electric field profiles along the PN junction cutlines (a-a ) for both structures are compared in Fig. 3(c). It was found that the breakdown voltages were governed by the peak electric field strength at the sidewall PN junction; both structures exhibited the same magnitude of Fig. 3. Potential and corresponding electric field distributions under the breakdown conditions (a) single-beveled mesa structure (V BD = 436 V), (b) double-step mesa structure (V BD = 495 V), (c) Comparison of electric field distributions along the PN junction (a-a ). the peak electric field (~3.5 MV/cm) under the breakdown voltage conditions. However, it should be noted that the electric field strength inside the active region is significantly different between two structures. As compared in Fig. 3(c), the single-beveled mesa
JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.18, NO.5, OCTOBER, 2018 647 Fig. 5. Quantum efficiency characteristics of single-beveled mesa and double-step mesa structures. Fig. 4. Dark current, photo current with a wavelength of 340 nm, and gain characteristics of (a) single-beveled mesa, (b) double-step mesa structures. structure exhibited the electric field strength of ~2.2 MV/cm inside the active region whereas the doublestep mesa structure exhibited ~2.6 MV/cm under the breakdown conditions. According to the potential distributions shown in Fig. 3(b), removing the highlydoped p + contact layer at the edge of the active region in the double-step mesa structure resulted in a significant voltage drop along the thin p layer (~315 V). Therefore, the electric field at the junction sidewall can be mitigated by employing the double-step mesa structure and thus a higher voltage can be applied before the breakdown occurs. That is, the electric field strength inside the active region of the double-step mesa structure can be higher than that of the single-beveled mesa structure under the breakdown conditions. The simulated dark current and photocurrent characteristics along with the calculated gains for two structures are compared in Fig. 4 where the wavelength of the incident photon was 340 nm. It was assumed that the incident photon was absorbed only inside the active region (see Fig. 1). While both APDs exhibited similar characteristics in the low bias regime, significantly higher gain was achieved near the breakdown voltage for the double-step mesa structure. The relatively lower electric field inside the active region of the singlebeveled mesa structure cannot generate a sufficient multiplication process, which limits the APD gain. On the other hand, much higher gain can be achieved for the double-step mesa structure due to the sufficiently high electric field inside the active region. The double-step mesa structure effectively suppressed the electric field at the sidewall and thus allowed higher electric field strength in the active region under the avalanche condition. The higher electric field in the active region leads to higher impact ionization coefficients and subsequently higher APD gain characteristics. The quantum efficiency characteristics for both structures simulated at 5 V with unity gain are compared in Fig. 5. Since no avalanche process is generated under unity gain condition, no difference is observed in quantum efficiency characteristics between two structures. III. CONCLUSIONS A double-step mesa structure was proposed in order to mitigate the electric field at the junction sidewall of a PIN APD. Removing the top highly-doped contact layer at the edge resulted in a significant voltage drop outside the active region, which, in turn, suppressed the electric field at the mesa sidewall and increased the breakdown voltage. The increased breakdown voltage allowed higher electric field inside the active region under the avalanche bias condition, which enhanced the APD gain.
648 THU THI THUY PHAM et al : GALLIUM NITRIDE PIN AVALANCHE PHOTODIODE WITH DOUBLE-STEP MESA ACKNOWLEDGMENTS The authors thank Mr. Jong-Ik Kang for his technical assistance in device simulation. This work was supported by the Agency for Defense Development of Korea and Korea Electric Power Corporation (Grants: R17XA05-74, R18XA02). REFERENCES [1] M. A. Khan, J. N. Kuznia, D. T. Olson, J. M. V. Hove, M. Blasingame, High-responsibility photoconductive ultraviolet sensors based on insulating single-crystal GaN epilayers, Applied Physics Letters, 1992, 60, p. 2917. [2] H. Y. Cha, H. K. Sung, H. Kim, C. H. Cho, P. M. Sandvik, 4H-SiC Avalanche Photodiodes for 280nm UV Detection, IEICE Transactions on Electronics, 2010, E93.C, pp. 648-650. [3] D. Gedamu, I. Paulowicz, S. Kaps, O. Lupan, S. Wille, G. Haidarschin, Y. K. Mishra, R. Adelung, Rapid Fabrication Technique for Interpenetrated ZnO Nanotetrapod Networks for Fast UV sensors, Advanced Materials, 2014, 26, pp. 1541-1550. [4] J. I Kang, H. Kim, C. Y. Han, H. Yang, S. R. Jeon, B. Park, H. J. Cha, Enhanced UV absorption of GaN photodiodes with a ZnO quantum dot coating layer, Optics Express, 2018, 26, pp. 8296-8300. [5] J. Millan, P. Godignon, Wide Band Gap power semiconductor devices, Electron Device (CDE), 2013 Spanish Conference on, 2013, pp. 2163-4971. [6] L. F. Eastman, U. K. Mishra, The toughest transistor yet [GaN transistors], IEEE Spectrum, 2002, 39, pp. 28-33. [7] A. A. Burk Jr, M. J. O Loughlin, R. R. Siergiej, A. K. Agarwal, et al. SiC and GaN wide bandgap semiconductor materials and devices, Solid-State Electronic, 1999, 43, pp. 1459-1464. [8] W. Jian, Design and fabrication of 4H silicon carbide MOSFETs, DAI-B 70/03 Dissertation Abstract International, 2009. [9] H. Liu, D. Mcintosh, X. Bai, H. Pan, M. Liu, J.C. Campbell, H. Y. Cha, 4H-SiC PIN Recessed- Window Avalanche Photodiode With High Quantum Efficicency, IEEE Photonics Technology Letters, 2008, 20, pp. 1551-1553. [10] J. C. Campbell, S. Demiguel, F. Ma, A. Beck, X. Guo, S. Wang, X. Zheng, X. Li, J.D. Beck, A. Huntington, L. A. Coldren, J. Decobert, and N. Tscherptner, Recent advances in avalanche photodiodes, IEEE Journal of Selected Topics in Quantum Electronics, 2004, 10, pp. 777-787. [11] H. Y. Cha, Structural optimization of silicon carbide PIN avalanche photodiodes for UV detection, Journal of the Korean Physical Society, 2010, 56, pp. 672-676. [12] B. J. Baliga, Fundamentals of power semiconductor devices, New York, NY: Springer Science, 2008, p. 137-148. [13] K. Kunihiro, K. Kasahara, Y. Takahashi, Y. Ohno, Experimental Evaluation of Imapct Ionization Coefficients in GaN, IEEE Electron Device Letters, 1999, 20, pp.608-610. Thu Thi Thuy Pham received the B.S. degrees in Physical Engineering from Hanoi University of Science and Technology, Hanoi, Vietnam, in 2017. She is currently pursuing the M.S. degree at Hongik University. Her research interests include wide bandgap semiconductor devices. Hyungsik Shin received his B.S. degree in Electrical Engineering from Seoul National University in 2003. He received his M.S. degree in 2005 and Ph.D. degree in 2011 from Stanford University in Electrical Engineering, where he was a member of the Information Systems Laboratory (ISL). He worked at Docomo Communications Laboratories USA, Inc. as a research intern in 2010, and then joined Docomo Innovations, Inc. in 2011 where he was a Research Engineer in the Open Service Innovation Group. At Docomo Innovations, he was one of the lead engineers of machine learning and natural language processing projects. Hyungsik Shin is currently an assistant professor at the School of Electronic and Electrical Engineering, Hongik University, Seoul, Republic of Korea.
JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.18, NO.5, OCTOBER, 2018 649 Eugene Chong received the B.S. degrees in material science and engineering from Korea University of Technology and Education, Chenan, South Korea, in 2004, and Ph.D. degrees in nanomaterial science and engineering from Korea Institute of Science and Technology (KIST) and Korea Institute and University of Science and Technology (UST), Seoul, South Korea, in 2012. She is now with the ChemBio detection team, Agency for Defense Development (ADD), Daejeon, Republic of Korea. Ho-Young Cha received the B.S. and M.S. degrees in Electrical Engineering from the Seoul National University, Seoul, Korea, in 1996 and 1999, respectively, and the Ph.D. degree in Electrical and Computer Engineering from Cornell University, Ithaca, NY, in 2004. He was a Postdoctoral Research Associate with Cornell University until 2005, where he focused on the design and fabrication of wide bandgap semiconductor devices. He was with the General Electric Global Research Center, Niskayuna, NY, from 2005 to 2007, developing wide-bandgap semiconductor sensors and high-power devices. Since 2007, he is currently Professor in the School of Electronic and Electrical Engineering. His research interests include wide-bandgap semiconductor devices. He has authored over 100 publications in his research area.