HIGH-PERFORMANCE m wavelength InP/In-

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1 262 IEEE TRANSACTIONS ON DEVICE AND MATERIALS RELIABILITY, VOL. 5, NO. 2, JUNE 2005 Reliability of InGaAs Waveguide Photodiodes for 40-Gb/s Optical Receivers Han Sung Joo, Su Chang Jeon, Student Member, IEEE, Bongyong Lee, Hongil Yoon, Member, IEEE, Yong Hwan Kwon, Joong-Seon Choe, and Ilgu Yun, Senior Member, IEEE Abstract The reliability of m wavelength InGaAs waveguide photodiodes (WGPDs) fabricated by metal-organic chemical vapor deposition is investigated for 40-Gb/s optical receiver applications. Reliability for both high-temperature storage and accelerated life tests obtained by monitoring both the dark current and the breakdown voltage is examined. The median device lifetime and the activation energy of the degradation mechanism are extracted for WGPD test structures. The device lifetimes are examined via statistical analysis which is highly reliable in predicting the device lifetime under practical conditions. The degradation mechanism for the WGPD test structures can be explained by the formation of leakage current path by ionic impurities in the passivation layer on the exposed p-n junction. Nevertheless, it can be concluded that the WGPD test structures exhibit sufficient reliability for practical 40-Gb/s optical receiver applications. Index Terms Optical receiver, reliability, waveguide photodiodes. I. INTRODUCTION HIGH-PERFORMANCE m wavelength InP/In- GaAs photodiodes are of interest as a receiver component for 40-Gb/s optical communication systems. In this application, the photodiode must have sufficient responsivity and bandwidth to enable the high-speed operation. The waveguide photodiodes (WGPDs) have recently been demonstrated as one of the potential candidates for 40-Gb/s optical communication systems [1], [2]. The conventional high-speed photodetectors, such as avalanche photodiodes with intrinsic layers, have bandwidth limitations due to the poor resistance-capacitance (RC) time constant and carrier-transit time. The WGPDs can surpass the bandwidth limitation beyond 40-Gb/s modulation because the quantum efficiency of WGPDs is just dependent on the length of the waveguide structure. In addition, the mesa-type, side-illuminated InGaAs WGPDs are advantageous for surface-hybrid integration because they can be easily flip-chip mounted on the planar lightwave circuit (PLC) and be directly coupled to Manuscript received April 27, 2004; revised December 15, This work was supported by the Ministry of Information and Communication (MIC), Korea, under the Information Technology Research Center (ITRC) support program supervised by the Institute of Information Technology Assessment (IITA). H. S. Joo, S. C. Jeon, H. Yoon, and I. Yun are with the Department of Electrical and Electronic Engineering, Yonsei University, Seoul , Korea ( iyun@yonsei.ac.kr). B. Lee is with PD_Product Engineering Group, Fairchild Korea Semiconductor Inc., Bucheon-si , Gyeonggi-do, Korea. Y. H. Kwon and J.-S. Choe are with the Telecommunication Basic Research Laboratory, Electronics and Telecommunications Research Institute, Daejon, Korea. Digital Object Identifier /TDMR planar waveguides without requiring optical components such as lenses or mirrors [3]. Despite these merits, the reliability of the mesa structure must be carefully evaluated for the dark current instability from the exposed junction at the edge region [4]. Several studies have been conducted regarding the reliability of WGPDs. Shishikura et al. studied the reliability of WGPDs in a highly humid ambient [5]. Nakamura et al. proposed degradation model on InGaAlAs waveguide photodiodes due to the trapped hole accumulation at the surface states [3]. Mawatari et al. investigated the early failure mode and wear-out degradation mode for the reliability of the planar WGPDs [6]. In this paper, the structure, fabrication, and reliability testing of InGaAs WGPDs fabricated by metal-organic chemical vapor deposition (MOCVD) are presented. The reliability characteristics are examined by accelerated life tests by monitoring the dark current. The activation energy of the degradation mechanism and device median lifetime are estimated to assess the feasibility of the device for practical 40-Gb/s optical communication system applications. II. WGPD STRUCTURE AND FABRICATION PROCESSES The schematic diagram and microscopic image of side-illuminated InGaAs WGPD test structure are shown in Fig. 1. The WGPD layer structure is presented in Table I. The initial epi-layers were grown by metal organic chemical vapor deposition (MOCVD). The epitaxial structure used in this study is based on the fiber guide section consisting of three stacks of 600-nm-thick InP layers and 50-nm-thick InGaAsP ( m) layers which were grown on a semi-insulated InP substrate. N-doped InGaAsP coupling guide layer was then grown on top of the fiber guide section and finally undoped 500-nmthickness InGaAs absorption layer was grown. As the initial processing step, ridge-typed absorption layer was defined via pattern transfer. Both dry and wet etching techniques were then used to realize the pattern and sulfuric or phosphoric acid was applied to the selective wet etching as etchants. The taper-shaped coupling guide section was then formed using pattern transfer and etching process. This layer plays a major role in changing the mode size as well as its usual role as n-type contact layer. After the coupling guide is completed, the fiber guide section was formed for coupling with the input optical fiber. Polyimide (PI2723 series manufactured by DuPont Co.) was used to passivate the exposed surface of p-n junction in the air and a curing process was performed at 365 C for 1 h, which was followed by Si N thin film deposition for protecting the polyimide layer from absorbing water vapor in the air. As p-type /$ IEEE

2 JOO et al.: RELIABILITY OF InGaAs WAVEGUIDE PHOTODIODES FOR 40-Gb/s OPTICAL RECEIVERS 263 TABLE I LAYER STRUCTURE FOR THE WGPD Finally, a coplanar ground-signal-ground (GSG) type electrode was deposited followed by the sintering process at 380 C for 30 s. III. RELIABILITY TESTING Initially, high-temperature storage tests (HTSTs) were performed in order to evaluate these devices under temperature stress. The HTSTs were performed at 200 C for 1000 h. After completing the HTSTs, the accelerated life tests (ALTs) for InGaAs WGPDs were performed in constant reverse voltage of 10 V at two different ambient temperature levels. Two sets of five samples were tested at 150 C and 200 C, respectively. In accelerated life tests, the failure rate is measured under stressful operating conditions. In order to maintain a constant reverse voltage, a Keithley 236 source measure unit (SMU) was used. The activation energy for the failure mechanism and the average device lifetime were subsequently computed. It was assumed that the temperature dependence of the device failure rate ( ) obeys the following Arrhenius law [7]: (1) Fig. 1 (a) Schematic diagram and (b) microscopic image of InGaAs WGPD test structure. and n-type electrodes, Ti/Pt/Au alloy and Cr/Au alloy, were deposited at 400 C for 30 s and at 380 C for 30 s, respectively. where is a temperature-independent pre-exponential failure acceleration factor, is the activation energy, is the absolute temperature, and is the Boltzmann s constant. During the life tests, dark current and breakdown voltage were measured at room temperature (300 K). The breakdown voltage was obtained from the device current-voltage (I V) curve using the tangential line method. Typical breakdown voltages for test devices were characterized to be V. The devices were classified as failing when the dark current at room temperature exceeded 1 A at the specified operating voltage of 3V.

3 264 IEEE TRANSACTIONS ON DEVICE AND MATERIALS RELIABILITY, VOL. 5, NO. 2, JUNE 2005 Fig. 2. Temperature dependence of the dark current for WGPD. Fig. 4. Room-temperature dark current variation of WGPDs for unbiased aging at 200 C. TABLE II SUMMARY OF THE ACCELERATED LIFE TEST RESULTS Fig. 3. Temperature dependence of the breakdown voltage for WGPD. IV. RESULTS AND DISCUSSION Fig. 2 shows the temperature dependence of the dark current characteristics. As the temperature varies from room temperature to 100 C, the dark current variation was confirmed to be within sub-micro ampere range at the bias voltage of 3V.It is observed that there is no excessive dark current increase in the temperature range from room temperature to 200 C and the activation energy of 0.35 ev is extrapolated from Fig. 2. However, the value of dark current is significantly increased over the micro-ampere range for temperatures from 150 Cto 200 C, indicating that these conditions can be regarded as a relatively high stress and leading to failure conditions. Based on the temperature-current relationship of WGPDs, the temperature is considered as an important factor for the device lifetime of the WGPDs. In addition, the temperature dependence of the breakdown voltage variation was shown in Fig. 3. It is observed that the WGPD shows the tendency of negative temperature dependence of the breakdown voltage [8]. The activation energy of ev is extracted from Fig. 3. It can be explained that as the temperature is increased, the avalanche-generated electron hole pairs can obtain the energy from the thermal excitation with the moderate electric field to produce the additional electron hole pairs, which lead to the breakdown voltage reduction. Therefore, the temperature-dependent WGPD characteristics limit the operating condition of InGaAs WGPD and they can also explain the device degradation mechanisms. After the investigation of temperature dependence of WGPD characteristics, the HTSTs were performed at 200 C. Fig. 4 shows the dark current variation of the WGPDs measured at room temperature after the HTSTs at 200 C. It is observed that the dark currents of the WGPDs have stable characteristics without drastic change after the 1000-h temperature aging. Since the HTSTs did not fully show the degradation mechanism of the device, accelerated life tests were then performed to estimate the device reliability and to efficiently analyze the failure mechanism. Fig. 5 shows the dark I V characteristics measured at room temperature before and after accelerated life testing at 150 C and 200 C. The results of the accelerated life tests for the WGPDs are summarized in Table II. The median lifetime was measured to be h at 150 C and 23.7 h at 200 C. The standard deviation based on the lognormal distribution was also calculated to be at 150 C and at 200 C, respectively. The percent of cumulative failures versus the lognormal projection of the device time-to-failure after accelerated testing is presented in Fig. 6. Although the sample size is small, the data display the expected linearity in each case. It is observed that failures obey the lognormal distribution relatively well and the failure mode is of the wear-out type. Furthermore, it is shown

4 JOO et al.: RELIABILITY OF InGaAs WAVEGUIDE PHOTODIODES FOR 40-Gb/s OPTICAL RECEIVERS 265 Fig. 6. Lognormal projection of time-to-failure versus percent of cumulative failure for InGaAs WGPDs after life testing at 150 C and 200 C ( =0:39). Fig. 5. Room-temperature I V curves of WGPD samples for biased baking at (a) 150 C and (b) 200 C. in Fig. 6 that the slopes of the two different temperature levels are almost the same indicating that the same degradation mechanism is involved with the failure mode at these temperature levels. The Arrhenius plot of median lifetimes as a function of reciprocal aging temperature is shown in Fig. 7. From this plot, the thermal activation energy of the device aging process is computed to be 0.84 ev. Using this activation energy level, the median lifetime under practical conditions can be estimated to be h at room temperature, with a lognormal standard deviation of Due to the lognormal distribution for degradation behavior of the WGPDs, failure probability of each device as a function of time can be expressed using the average device lifetime and its standard deviation in the following equation [7]: (2) Fig. 7. Arrhenius plot of median device lifetime for WGPD test structures as a function of reciprocal aging temperature. Along with the lognormal plot, this expression provides a quantitative method of evaluating the likelihood of failure for a given device as a function of its age. After the reliability testing of the WGPD test structures, the failure analysis on the thermally and electrically stressed WGPD test structures was carried out using the scanning electron microscopy (SEM) and the energy dispersive spectrometry (EDS). The degradation mechanisms for mesa-structure photodiodes have been reported by several researchers [3] [5]. It was reported that one of the main reasons for the increase in dark current, which causes the failure of the devices, is the leakage path at the exposed surface of the p-n junction between the semiconductor and the passivated dielectric film. It was proposed that the leakage path is formed by hole accumulation near lateral junction or accumulation of mobile ions in passivation film due to electric field [3]. Prior to the analysis, the presence of contaminants in passivating nitride and polyimide at the exposed junction was assumed as a possible cause for the dark current increase during the testing.

5 266 IEEE TRANSACTIONS ON DEVICE AND MATERIALS RELIABILITY, VOL. 5, NO. 2, JUNE 2005 TABLE III EDS ANALYSIS RESULTS FOR THE LOCAL DEFECT REGION Fig. 8. SEM image of WGPD test structure after accelerated life testing at 200 C. Fig. 8 shows the SEM image of the WGPD test structure after the accelerated life testing at 200 C. Based on the SEM image, a local defect at the junction region was found. In order to identify the elements of the local defect, EDS was performed around the defect region. Using the EDS analysis, the composition and the quantity of each element in a composite material can be obtained. In this case, EDS analysis confirmed the presence of ionic sodium and verified that the ionic sodium is the primary contaminant in these devices. The EDS test results are summarized in Table III. The identified material is considered as a common contaminant in silicon nitride passivation films. Such contamination, which is accumulated by electric field on the exposed p-n junction, generates a leakage path shorting the junction [9]. In addition, gold and potassium were also found in the region, which might have been introduced as residues after metal deposition and etching process in fabrication, respectively. V. CONCLUSION The long-term reliability testing of mesa-structured InGaAs WGPDs has been investigated using high-temperature storage and accelerated life tests. From the accelerated life test results, the activation energy of the degradation mechanism and median lifetime of these devices in room temperature were extracted from the lognormal failure model by using an average lifetime and the standard deviation of that lifetime as parameters in each test temperature. It was found that the WGPD devices exhibited a median lifetime which is longer than h at practical operating conditions. In addition, it was found that the degradation resulted from the presence of ionic contaminants in the passivation layer at the junction forming a leakage current path. With further work directed to remove this failure mechanism, the investigated WGPD structure should be well-suited as 40-Gb/s optical receiver modules in practical communication systems. REFERENCES [1] T. Takeuchi, T. Nakata, M. Tachigori, K. Makita, and K. Taguchi, Design and fabrication of a waveguide photodiode for 1.55-m-band access receivers, Jpn. J. Appl. Phys., vol. 38, no. 2B, pp , [2] F. Xia, J. K. Thomson, M. R. Gokhale, P. V. Studenkov, J. Wei, W. Lin, and S. R. Forrest, An asymmetric twin-waveguide high-bandwidth photodiode using a lateral taper coupler, IEEE Photon. Technol. Lett., vol. 13, no. 8, pp , Aug [3] H. Nakamura, M. Shishikura, S. Tanaka, Y. Matsuoka, T. Ono, and S. Tsuji, Highly reliable operation of InGaAlAs waveguide photodiodes for optical access network systems, Jpn. J. Appl. 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Electron Devices for Microwave and Optoelectronic Applications,Nov. 2002, pp [9] I. Yun, H. M. Menkara, Y. Wang, I. H. Oguzman, J. Kolnik, K. F. Brennan, G. S. May, C. J. Summers, and B. K. Wagner, Effect of doping on the reliability of GaAs multiple quantum well avalanche photodiodes, IEEE Trans. Electron Devices, vol. 44, no. 4, pp , Apr Han Sung Joo received the B.S. degree in the electrical engineering from Yonsei University, Seoul, Korea, in He is currently pursuing the M.S. degree in electrical engineering from Yonsei University. His research interest is the reliability of optodlectronic devices. Su Chang Jeon (S 03) was born in Seoul, Korea, on November 27, He received the B.S. degree in electrical engineering from Yonsei University, Seoul, in He is currently a graduate student at the Department of Electrical and Electronic Engineering, Yonsei University. His research interests are the modeling and simulation of semiconductor devices and integrated circuits (ICs) and the testing methodology including analog built-in self test (BIST). Bongyong Lee received the B.S. degree in electrical engineering and the M.S. degree in electrical and electronic engineering from Yonsei University, Seoul, Korea, in 2002 and 2004, respectively. He is currently an Engineer with PD_Product Engineering Group, Fairchild Korea Semiconductor Inc., Korea. His research interests include semiconductor device level and circuit modeling for III-V compound semiconductor devices, optoelectronic devices and high speed embedded passive components and interconnect of integrated modules, and process modeling, control, and simulation applied to computer-aided manufacturing of integrated circuits.

6 JOO et al.: RELIABILITY OF InGaAs WAVEGUIDE PHOTODIODES FOR 40-Gb/s OPTICAL RECEIVERS 267 Hongil Yoon received the B.S. degree in electrical engineering and computer sciences from the University of California, Berkeley, in 1991 and the M.S. and Ph.D. degrees in electrical engineering and computer science from the University of Michigan, Ann Arbor, in 1993 and 1996, respectively. From 1996 to 2002, he was with Samsung Electronics, Kiheung, Korea, involved in the design of dynamic random access memory. In 2002, he joined the Department of Electronic and Electrical Engineering, Yonsei University, Seoul, Korea, as an Assistant Professor. His research interests include low-voltage memory circuit and technology, high-frequency RF circuits and devices, and evolvable hardware design and test. Joong-Seon Choe was born in Seoul, Korea, on October 13, He received the B.S., M.S., and Ph.D. degrees in physics from Seoul National University, Seoul, Korea, in 1994, 1996, and 2001, respectively. He worked on GaAs/AlGaAs growth by MOCVD, the properties of wet-oxidized high Al content Al- GaAs, and 980 nm InGaAs/GaAs quantum well laser diodes. Since 2001, he has been a Senior Researcher with the Electronics and Telecommunications Research Institute, Daejon, Korea, where he has conducted research on high-speed photodetector for optical communication applications. Yong-Hwan Kwon received the B.S. degree in physics from the Seoul National University, Seoul, Korea, in 1993, and the M.S. and Ph.D. degrees in semiconductor physics from the Seoul National University, Seoul, Korea, in 1995 and 1998, respectively. His thesis research was on metal-organic vapor phase epitaxy growth and characterization of InP/InGaP self-assembled quantum dots. From 1998 to 2000, he was a Research Associate at the Center for Laser and Photonics Research, Oklahoma State University, Stillwater, where he was active in the field of GaN blue light emitting diodes. He is currently a Senior Researcher in the Basic Research Laboratory, Electronics and Telecommunication Research Institute, Daejon, Korea. His research interests include the design and fabrication of high-speed optoelectronic devices such as SAGCM InP/In- GaAs avalanche photodetectors, high-power high-speed waveguide photodetectors, and distributed-feedback laser integrated electroabsorption modulators. Ilgu Yun (S 93 M 97 SM 03) received the B.S. degree in electrical engineering from Yonsei University, Seoul, Korea, in 1990, and the M.S. and Ph.D. degrees in electrical and computer engineering, from the Georgia Institute of Technology, Atlanta, in 1995 and 1997, respectively. He was previously a Research Fellow with the Microelectronics Research Center, Georgia Institute of Technology, and a Senior Research Staff with the Electronics and Telecommunications Research Institute, Daejon, Korea. He is currently an Associate Professor of electrical and electronic engineering with Yonsei University. His research interests include reliability and parametric yield modeling for III-V compound semiconductor devices, optoelectronic devices and high-speed embedded passive components and interconnect of integrated modules, and process modeling, control, and simulation applied to computer-aided manufacturing of integrated circuits.