Experimental observation of the post-annealing effect on the dark current of InGaAs waveguide photodiodes

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1 Solid-State Electronics 50 (2006) Experimental observation of the post-annealing effect on the dark current of InGaAs waveguide photodiodes Hansung Joo a, Su Chang Jeon a, Yong Hwan Kwon b, Joong-Seon Choe b, Ilgu Yun a, * a Semiconductor Engineering Laboratory, Department of Electrical and Electronic Engineering, Yonsei University, 134 Shinchon-Dong, Sudaemun-Ku, Seoul , Republic of Korea b IT Convergence and Components Laboratory, Electronics and Telecommunications Research Institute, Daejon , Republic of Korea Received 29 July 2005; received in revised form 10 February 2006; accepted 22 July 2006 Available online 9 October 2006 The review of this paper was arranged by C. Tu Abstract The post-annealing effect on the dark current of the InGaAs waveguide photodiodes, which are developed for 40-Gbps optical receiver applications, is experimentally investigated. The interesting experimental phenomena were observed that the dark current is significantly decreased and the breakdown voltage is slightly increased after annealing at 250 and 300 C whereas the dark current and the breakdown voltage are almost constant after annealing at 200 C. Based on the experimental results, the post-annealing is more effective for the dark current improvement than the conventional curing process. Ó 2006 Elsevier Ltd. All rights reserved. PACS: Photodiode (85.60.D) Keywords: Waveguide photodiodes; Long-term annealing; Dark current; Optical receivers 1. Introduction Recently, the InGaAs waveguide photodiodes (WGPDs) are widely developed for high-speed optical receiver module [1]. The conventional high-speed photodetectors, such as avalanche photodiodes, have the bandwidth limitation by resistance capacitance time constant and carrier-transit time due to an intrinsic layer causing the substantial bandwidth reduction. However, the quantum efficiency of WGPDs is just dependent on the length of waveguide structure so that it becomes a promising candidate to solve the problem of bandwidth limitation. In addition, mesa-type side-illuminated WGPDs are advantageous for surface hybrid integration because they can be easily flip-chip mounted on the planar lightwave circuit (PLC) platforms * Corresponding author. Tel.: ; fax: address: iyun@yonsei.ac.kr (I. Yun). in the same way as laser diodes, and they can directly be coupled to planar waveguides without requiring optical components such as lenses or mirrors [2]. However, mesatype WGPDs are struggled with a high level of dark current at exposed junction, which results in poor device reliability. The dark current enhancement of WGPDs is thus crucial for reliable operation of optical receiver modules [3]. Several researches have been conducted on the study of dark current with respect to the reliability of photodiode. Kuhara et al. fabricated p i n photodiodes with polyimide passivation and investigated the dark current variation by long-term reliability testing [4]. Shishikura et al. conducted reliability testing on InGaAlAs mesa-waveguide photodiodes in a humid ambient [5]. Mawatari et al. analyzed the dark current degradation of the planar waveguide photodiodes for optical subscriber systems [6]. In this paper, the temperature annealing effect on the dark current of WGPDs is experimentally investigated. The test structures /$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi: /j.sse

2 H. Joo et al. / Solid-State Electronics 50 (2006) of InGaAs WGPDs are fabricated, processed and annealed at the three different ambient temperature levels: 200, 250, and 300 C. After the testing, the dark current and the breakdown voltage are extracted from the current voltage (I V) characteristics and the variations of dark current and breakdown voltage are investigated. 2. WGPD structure and fabrication processes Table 1 Structural parameters of the WGPD Doping Material Wavelength Dimension Layer (cm 3 ) P-InP 500 A Capping layer >10 19 P A P contact InGaAs P + -InP 4000 A P clad u A Absorption InGaAs u-inp 100 A Etchstop >10 18 N Q 5000 A Coupling guide and n contact u-inp 6000 A Fiber guide u- 1.24Q 500 A u-inp 6000 A u- 1.24Q 500 A u-inp 6000 A u- 1.24Q 500 A u-inp 3000 A Buffer S.I.-InP Substrate The schematic diagram and microscopic image of sideilluminated InGaAs WGPD test structure are shown in Fig. 1. The structural parameters are summarized in Table 1. The initial epi-layers were grown by metal organic chemical vapor deposition (MOCVD) growth technique at the Electronics and Telecommunication Research Institute in Korea. The epitaxial structure used in this study was as follows: The fiber guide section consists of three 600-nm-thick InP layers and three 50-nm-thick (kg = 1.24 lm) layers, which were grown in turn on the semi-insulated InP substrate. n-doped coupling guide layer was grown on top of fiber guide section, which was followed by undoped 500-nm-thick InGaAs absorption layer. As the initial process step, ridge-typed absorption layer was defined via pattern transferring. 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 an etchant. Tapered shape coupling guide section was then formed using pattern transferring and etching process. This layer plays a major role of changing mode size as well as role of a n-type contact layer. After the coupling guide is completed, a fiber guide section was formed. It is 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 curing process was then performed. At first, heating in the air from 0 to 100 C environment was performed and heating with the rate of 100 C/1 min from 100 to 365 C was followed. Finally, 1-h curing was executed at 365 C. Then, it was followed by Si 3 N 4 thin film deposition for protecting the polyimide layer from absorbing water vapor in the air. Then p-type and n-type electrodes were deposited using Ti/Pt/Au alloy at 400 C for 30 s and Cr/ Au alloy at 380 C for 30 s, respectively. Finally, GSG (ground signal ground) coplanar typed electrode was deposited followed by the sintering process at 380 C for 30 s. 3. Results and discussion Fig. 1. Schematic diagram and microscopic image of WGPD: (a) schematic diagram and (b) microscopic image. Post annealing tests for InGaAs WGPDs were performed at three different ambient temperature levels: 200, 250, and 300 C. Annealing was performed in high temperature at the inert ambient not in high humidity environment. For each temperature level, three sample devices were used for the testing. During the annealing tests, dark current and breakdown voltage were measured at room temperature (300 K) after temperature annealing using Keithley SMU 236. The breakdown voltage was defined as the voltage when the dark current level is 100 la and

3 1548 H. Joo et al. / Solid-State Electronics 50 (2006) E-4 1E-5 a 1E-6 1E-7 Current [A] 1E-8 1E-9 1E-10 1E-11 Initial at 200 o C After 300-h testing at 200 o C 1E-12 Initial at 250 o C After 80-h testing at 250 o C 1E Reverse Bias [V] Fig. 2. Room temperature I V characteristics of InGaAs WGPDs. b it was obtained from the I V curve. The room temperature I V characteristics of InGaAs WGPDs annealed at 200 and 250 C is shown in Fig. 2. The initial data is considered as the I V characteristics after curing process at 365 C for 1hr and subsequent short time (30 s) annealing process at 400 C indicating the dark current levels are a few tens of nano-amperes biased at 3 V. It is observed that dark current level remains constant after 300-h annealing at 200 C, whereas the dark current is significantly decreased after about 80-h annealing at 250 C. The dark current and breakdown voltage variations of WGPDs after annealing at 200 and 250 C are illustrated in Fig. 3. Although the sample size is small, the data show the same tendency in each case. Fig. 3(a) shows that the dark current and the breakdown voltages remain almost constant during the annealing test at 200 C. However, it is observed that the dark current level was decreased remarkably and the breakdown voltages are slightly increased after 80-h annealing at 250 C in Fig. 3(b). The dark currents of WGPDs after 80-h annealing at 250 C are in the range of na. In addition, the annealing results at 300 C showed the similar behavior with the annealing results at 250 C. The dark currents of WGPDs after about 40-h annealing at 300 C are in the range of na. These results are superior to the similar structure of the 40-Gbps coplanar waveguide photodiodes manufactured by commercial vendors, such as Opto Speed Co. [7]. The temperature dependence of dark current for WGPD is shown in Fig. 4. The activation energy (E a ) calculated from the results was approximately in the range of ev regardless of the post-annealing process. It is also observed that the dark current levels of the post-annealing test devices at the temperature measurement range are decreased almost the same amount of magnitude compared with those of the before-annealing test devices. It can be concluded that the dark current levels for both beforeannealing and post-annealing WGPDs have the similar tendency depending on the temperature variation and the Fig. 3. Dark current and breakdown voltage variations of WGPDs after annealing at: (a) 200 C and (b) 250 C. Dark Current [A] 1E -5 1E -6 1E -7 1E -8 1E -9 Before annealing After annealing 1E /T [1000/K] Fig. 4. Temperature dependence of dark current for WGPD. mechanism for dark current reduction is almost consistent with all measured temperature levels. The temperature versus time dependence on annealing effect is shown in Fig. 5. The mean time to improve dark current level to 1 na is computed to be about 80 h and

4 H. Joo et al. / Solid-State Electronics 50 (2006) p 1.8p p Before Annealing Time [hour] 10 1 Capacitance [F] 1.4p 1.2p 1.0p f o C curing for 1hr 200 o C annealing for 300hr /T [1000/K] Fig. 5. Temperature time dependence of dark current at 250 C and 300 C f After Annealing 400.0f Voltage [V] Fig. 6. Capacitance voltage (C V) characteristics of InGaAs WGPDs. 40 h for the annealing at 250 and 300 C, respectively. The activation energy (E a ) obtained from Arrhenius relationship was 0.41 ev. Based on the experimental results shown in Fig. 5, there is no dark current reduction after 300-h post-annealing at 200 C. Therefore, it can be concluded that the temperature threshold for this effect on the dark current reduction can be existed in the temperature range between 200 and 250 C. In addition, from Fig. 3(b), it can be also concluded that time threshold exists around 40 h at 250 C process. The previous researches on the planar-type photodiodes reported that the dark current is a mostly bulk R G current in sub-nano-ampere ranges at the reverse bias of 10 V [8]. Since the dark current is about a few 100 na at the reverse bias of 2 V in WGPD test devices, it can be concluded that the higher dark current levels are attributed from between the polyimide passivation film and the dry etched semiconductor. Though we have limited number of test devices, the results show the similar tendency in the tolerant deviations indicating that the annealing effect can be determined by these experiments. Based on the results shown in Figs. 4 and 5, the possible mechanism for dark current reduction can be explained as follows: For the annealing process at 250 and 300 C, the dark current levels are remarkably decreased. It can be explained that the mobile ions were sufficiently excited by the thermal energy and annealed trap centers at the surface between the polyimide passivation film and the semiconductor causing the reduction of the surface leakage current. In addition, the decreased number of trap centers in the surface of the exposed junction area can slightly increase the breakdown voltage since the probability of the trapassisted ionization process is decreased. In addition, the responsivity of WGPD test structure is about A/ W and it is not much changed after annealing and frequency response dose not show the significant change after the annealing process. However, the capacitance voltage (C V) measurement results, shown in Fig. 6, also support the mechanism for the trapping of the mobile ions in the polyimide. The capacitance values of the WGPDs are decreased after the post-annealing process, which can come from the reduction of the mobile ions. 4. Conclusion The experimental observation of the post-annealing effect on the device performances of high-speed InGaAs WGPDs has been presented. The WGPDs sample devices were fabricated and tested at 200, 250 and 300 C, respectively. It is observed that the dark current is significantly decreased and the breakdown voltage is slightly increased after post-annealing at 250 and 300 C whereas the dark current and the breakdown voltage are almost constant after post-annealing at 200 C. It is also observed that the capacitance is also decreased after post-annealing processes. These phenomena can be explained by the trap annealing in the passivation layer at the exposed junction boundary near the surface area. Based on the experimental observation, the threshold temperature is crucial for the annealing effect and the annealing above the threshold temperature can be more effective for improving the dark current characteristics than conventional curing process. References [1] Takahata K, Miyamoto Y, Muramoto Y, Fukano H, Matsuoka Y. 50- Gbit/s operation of monolithic WGPD/HEMT receiver OEIC module. In: Proceedings of the 24th European conference on optics communications, Madrid, Spain, vol. 3; p [2] Nakamura H, Shishikura M, Tanaka S, Matsuoka Y, Ono T, Tsuji S. Highly reliable operation of InGaAlAs waveguide photodiodes for optical access network systems. Jpn J Appl Phys 1998;37: [3] Islam MS, Nespola A, Yeahia M, Wu MC, Sivco DL, Cho AY. Correlation between the failure mechanism and dark currents of high

5 1550 H. Joo et al. / Solid-State Electronics 50 (2006) power photodetectors. In: IEEE 13th annual meeting lasers and electro-optics society, Rio Grande, Puerto Rico, vol. 1; p [4] Kuhara Y, Terauchi H, Nishizawa H. Reliability of InGaAs/InP longwavelength p i n photodiodes passivated with polyimide thin film. IEEE J Lightwave Tech 1986;LT-4: [5] Shishikura M, Tanaka S, Nakamura H, Matsuoka Y, Kikuchi S, Nagatsuma K, et al. Highly reliable operation of InGaAlAs mesawaveguide photodiodes in a humid ambient. In: 23rd European conference on optics communications, vol ; p [6] Mawatari H, Fukuda M, Kato K, Takeshita T, Yuda M, Kozen A, et al. Reliability of planar waveguide photodiodes for optical subcarrier systems. IEEE J Lightwave Tech 1998;16(12): [7] Opto Speed Datasheet, PDCS12T (Rev. September 2001). Available from: < [8] Jung J, Kwon YH, Hyun KS, Yun I. Reliability of planar InP-InGaAs Avalanche photodiodes with recess etching. IEEE Photon Technol Lett 2002;14(8):