Uniform and High Performance of Monolithically ... - IEEE Xplore

IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 44, NO. 4, APRIL 1997

559

Uniform and High Performance of Monolithically Array of Planar GaInAs Photodiodes Integrated Wen-Jeng Ho, Meng-Chyi Wu, and Yuan-Kuang Tu, Member, IEEE Abstract— In this article, we describe the fabrication of a monolithically integrated 1 2 12 array of GaInAs/InP planar photodiodes, which has highly uniform characteristics in dark current, capacitance and crosstalk capacitance, quantum efficiency, and the frequency bandwidth at 3-dB reduction with a deviation of 61%. Besides, each diode on the array exhibits an extremely low dark current of 75 pA, a low capacitance of 2.3 pF and a crosstalk capacitance between adjacent diodes of 0.36 pF, a high quantum efficiency of 95% at 1.3 m and 89% at 1.53 m, the 3-dB frequency of >2 GHz, and a small 1/f noise component over a wide operating voltage range. Also, the diode on the array has a negligible degradation after the burn-in test of 020 V, 200  C, and 20 h.

Fig. 1. A schematic cross section of the monolithic 1 photodiode array.

II. STRUCTURE

I. INTRODUCTION

L

INEAR ARRAYS of photodetectors and emitters are useful in areas of high-fiber density and parallel data-bus transmission systems because of their advantages in performance, reliability, compactness, and cost. In the receiver circuit of parallel optical transmission, it is important to reduce the junction capacitance of a photodiode, the crosstalk, parasitic capacitance, and the variation in the characteristics among photodiodes. The receiver approach, using p-i-n photodiodes constructed from the GaInAs/InP material, is most preferred in the silica-based fiber low-loss window [1]. However, the main difficulty with the photodiode array is that the materials with very uniform characteristics along with a well-controlled processing technology must be highly expected. Nevertheless, there has been a considerable effort to fabricate the good performance of GaInAs p-i-n photodiodes by monolithically integrated arrays [2], arrays used in a common arrays [4]. cathode circuit [3], and flip-chip integrated In this work, we report on the monolithic arrays of planar GaInAs p-i-n photodiodes, which have highly uniform characteristics and good performance in dark currents and photoresponsivities. These photodiode arrays are very suitable for the applications of parallel optical transmission and fiber communication. Manuscript received May 15, 1996; revised October 21, 1996. The review of this paper was arranged by Editor P. K. Bhattacharya. This work was supported by the National Science Council and Telecommunication Laboratories, Ministry of Communications, under Contract NSC 85-2215-E-007-001. W.-J. Ho is with the Research Institute of Electrical Engineering, National Tsing Hua University, Hsinchu 300, Taiwan, R.O.C., and the Telecommunication Laboratories, Ministry of Transportation and Communications, Yang-Mei 32617, Taiwan, R.O.C. M.-C. Wu is with Research Institute of Electrical Engineering, National Tsing Hua University, Hsinchu 300, Taiwan, R.O.C. Y. K. Tu is with Applied Research Laboratory, Telecommunication Laboratories, Ministry of Transportation and Communications, Yang-Mei 32617, Taiwan, R.O.C. Publisher Item Identifier S 0018-9383(97)02361-7.

AND

2 12 GaInAs/InP

FABRICATION

Fig. 1 shows a schematic diagram of the monolithic GaInAs/InP photodiode array. The p-i-n structure grown on the -doped InP substrate by metalorganic chemical vapor deposition (MOCVD) consists of three layers, i.e., an doped InP buffer layer (0.5 m, cm ), an unintentionally doped Ga In As absorbing layer (2.5 m, cm ) and an -doped InP window layer (1 m, cm ). The p-n junction was formed at a depth of 0.2 m into the GaInAs absorbing layer by Zn-diffusion through a 170- m-diam circular opening in the SiN mask. Surface passivation was made of 200 nm SiN by the plasma-assisted chemical vapor deposition, and a 150- m-diam circular opening is etched for the contact to the p region. The p contact consisted of a ring by patterning Au/AuZn/Cr around the active area for top illumination and tracks leading to a remote bondpad of m m on the SiN for wire bonding. A 180-nm SiN film with a refractive index of 2.03 was deposited on the p-InP surface for an antireflection (AR) coating. Finally, Au/AuGeNi was evaporated to the lapped n-substrate side as the n-contact followed by annealing at 425 C for 20 s. The array elements were fabricated with a 250- m spacing, which matches the center-to-center spacing of the optical fiber ribbon. The monolithic array with a size of mm mm was mounted on a ceramic submount and the photodiodes were alternately connected with Au wire to leads in the respective line of the package. III. EXPERIMENTAL RESULTS AND DISCUSSION Reverse current–voltage ( – ) characteristics of a typical photodiode on the array measured at different temperatures is shown in Fig. 2. The dark currents at room temperature are 56 pA A/cm at 5 V, 118 pA at 10 V, and 619 pA at 25 V. At 30 C, the dominant component of the

0018–9383/97$10.00  1997 IEEE

560

IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 44, NO. 4, APRIL 1997

Fig. 3. The dark current as a function of reverse bias voltage for each diode on an array.

Fig. 2. The reverse current–voltage characteristic of a typical diode on an array measured at different temperatures. The inset of this figure shows the temperature dependence of dark current for a photodiode biased at 5 and 20 V.

0

0

dark current is the generation–recombination (G–R) current in the depletion layer which increases with the bias voltage at the moderate bias region. At a high voltage over 45 V, the sudden increase in the dark current is interpreted as being due to excess current resulting from field emission through the interface states in the depletion region. In the absence of abrupt breakdown, the breakdown voltage, defined as the voltage required to make 10- A dark current flow, ranges from 65 to 68 V. The temperature dependence of the dark current for a photodiode biased at 5 V and 20 V is also shown in the inset of Fig. 2. The dark current in the photodiode at with – , as 30–90 C varies as expected for surface current or G-R current [1], [5], where is the energy gap of GaInAs, is Boltzmann’s constant, is temperature in Kelvin. At elevated temperatures and ( 100 C), however, the dependence becomes proportional to , indicative that the dark current at higher temperatures is dominated by diffusion current [5]. Fig. 3 shows the dark current as a function of reverse bias voltage for each diode in a typical array. The dark current for the diodes biased at 5 V falls around 75 pA A/cm with a standard deviation of 4 pA, which is slightly larger than that of 56 pA in a discrete diode. It can be seen that no significant degradation in the reverse characteristics occurs after packaging the array. The dark current of 75 pA is much lower than those of 20–25 nA at the same bias voltage for the reported or arrays [2], [6]. The low dark current in the array is attributed to the extremely low defect density 100 cm of the epitaxial layers, nearly lattice-matched heterointerface between GaInAs and InP layers, and high-quality SiN surface passivation. The average breakdown voltage is greater than 65 V with a deviation less than 1% in array.

Fig. 4 shows the capacitance as a function of reverse bias voltage measured at 1 MHz for each diode with a 170m diam in the array. The capacitance is 3.2 pF at 0 V, decreases with bias voltage, and then approaches to a constant value of 2.2 pF at a 8 V bias. The calculated indicates a uniform carrier concentration in the depletion region of cm . The diffusion potential voltage is estimated to be 0.41 V—in good agreement with forward “turn-on” voltage of the diodes measured from the – characteristic. It is further inferred that there is no “intrinsic” region of lower net carrier concentration near the junction interface caused by Zn diffusing from the p side for the compensation [7]. The average capacitance is 2.3 pF at 5 V with a standard deviation of 0.03 pF. The capacitance value of 2.3 pF at 5 V is expected to yield a speed response of 1.5 GHz. The average crosstalk capacitance between adjacent diodes biased at 5 V varies from 0.21 pF before bonding to 0.36 pF after bonding, as shown in the inset of Fig. 4. The electrical (power) crosstalk between channels for a sinusoidal input signal , is calculated as 30.06 dB from the following expression [8]

where is the frequency ( MHz), is the load resistance ( k ), is the crosstalk capacitance after bonding ( pF), and is the parallel combination of the diode capacitance and all other parasitic capacitances ( pF in our case). The photodiode arrays having the low electrical crosstalk are very suitable for use in highbit-rate applications. The spectral response of the GaInAs photodiodes was measured using a halogen lamp as a light source with a silicon filter to eliminate the transmission of higher orders through the 0.275-m grating monochrometer. The light was coupled into the photodiode via a multimode fiber. The absolute quantum efficiency was determined by comparing the photodiode output to that of a calibrated GaInAs photodetector. Since the absorption coefficient of Ga In As is on the order of 10 cm at wavelengths near its bandgap, almost all photocarriers are generated within 1–2 m from

HO et al.: UNIFORM AND HIGH PERFORMANCE OF PLANAR GaInAs PHOTODIODES

561

Fig. 4. The junction capacitance as a function of reverse bias voltage for a typical diode on an array. The inset of this figure shows the crosstalk capacitance at 5 V in an array.

0

the diode surface. Because of the slightly higher absorption coefficient at shorter wavelengths, the number of minority carriers which reach the junction and contribute to the photocurrent declines and thus the surface recombination limits the quantum efficiency. The effect of surface recombination on the quantum efficiency can be eliminated by growing a higher bandgap heterstructure window over the absorbing region, as the structure of InP/GaInAs in the Fig. 1. Fig. 5 shows the photoresponsivity and external quantum efficiency of the typical GaInAs diode in the array at zero bias for the front-illumination design. It is characterized by the relatively flat response over the whole sensitive wavelengths and a sharp cut-off in the short and long wavelengths. The photoresponsivity at 1.3 and 1.53 m is 0.96 and 1.02 A/W, respectively, which corresponds to that quantum efficiency of 92% and 83% for the diode with AR coating of 180-nm thick SiN . In addition, the quantum efficiency slightly increases with the bias voltage. At bias voltages near 0 V, the depletion layer does not extend fully, and quantum efficiency degrades due to recombination of photogenerated carriers in the neutral region. A histogram of quantum efficiency at 1.3 and 1.53 m for 12 diodes biased at 5 V of a packaged array is shown in the inset of Fig. 5. The average quantum efficiency at 1.3 and 1.53 m is 95% and 89% which is corresponding to 1.0 and 1.1 A/W of photoresponsivity, respectively. The deviation of quantum efficiency in one array is less than 1%. These values of photoresponsivity or quantum efficiency are much better than those of photodiode arrays reported previously [2], [4], [6]. On the other hand, these arrays also exhibit a good linear photoresponse with increasing input light power at the measured wavelengths 1.3 and 1.53 m. It is attributed to the low and homogeneous electron concentration throughout the n -GaInAs absorbing layer of the photodiode. The photodiodes were packaged onto SMA connector using single bondwire. Frequency response was measured using a

Fig. 5. The photoresponsivity and quantum efficiency of the GaInAs photodiode array at zero bias voltage. The inset of this figure shows a histogram of quantum efficiency at 1.3 and 1. 53 m for each diode on an array biased at 5 V.

0

Fig. 6. The 3-dB bandwidth-voltage characteristic of a typical diode on an array measured at a 1.3-m laser diode. The inset of this figure shows the pulse response of a photodiode biased at 5 V.

0

1.3- m GaInAsP laser diode as optical source, and with the photodiode output monitored using a lightwave component analyzer HP8703A. The applied bias voltage of the device using HP11612A bias network resulted in a small output power

562

IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 44, NO. 4, APRIL 1997

Fig. 7. The room-temperature current noise spectra of a typical diode on an array at the operating biases of 0, shows the noise current as a function of dark current for a photodiode on an array.

variations over a 26.5 GHz frequency sweep range. Fig. 6 shows the 3-dB bandwidth characteristic as a function of reverse bias voltage for a typical GaInAs diode in the array. The diodes function well with a bias voltage as small as 0.5 V. The frequency bandwidth at 3-dB reduction is around 2 GHz over a wide operating voltage range. For the pulse response measurements, the diode were tested by using the 1.3- m GaInAsP injection laser which was driven by a Hamamatsu PLP-01 picosecond light pulser to generate the light pulses with a pulsewidth of 60 ps and a rise time of 40 ps at a repetition rate of 10 MHz and a 50- load resistance. The inset of Fig. 6 shows the measured pulse reponse of a photodiode in the array biased at 5 V, indicating a rise time of 123 ps, a fall time of 217 ps and a pulsewidth (FWHM) of 253 ps. The rise time of sampling system itself (Tektronics 7854 sampling oscilloscope/HP11612A bias network) is estimated to be about 25 ps for our measurement system. Therefore, the true pulse response will be faster than the measured, and the true rise time will be shorter than 100 ps. The corresponding baseband frequency width at 3-dB reduction is broader than 2 GHz which is in good agreement with the calculated value of 2.8 GHz. As compared to the top-illuminated GaInAs p-i-n homostructure [7], [9], [10], the trailing edge of the pulse response, which is resulted from the slow diffusion of photogenerated in the neutral region, is much suppressed by employing the wider bandgap p -InP window to absorb the photons in the GaInAs depletion region. During the electrical noise measurements the diodes were subjected to reverse-bias voltages of 0– 20 V. To be able to measure the voltage noise due to the current fluctuations in the diode, a 10-k resistor was connected in series with

05, and 010 V. The inset of this figure

the diode. The signal was fed into a low-noise amplifier and passed on to a PC-controlled HP3561A dynamic signal analyzer. The current noise spectra were measured in the range from 10 Hz to 100 kHz. Fig. 7 shows the room-temperature current noise spectra of a typical diode on an array at the operating biases of 0, 5, and 10 V. It shows the shot noise plus the diode thermal noise at high frequencies and a weak frequency dependence with 1/f at low frequencies which is associated with the G-R centers in the devices or the density of dislocations at the heterojunction [11]. The shot noise and the thermal noise is associated with the carriers crossing the p-n junction, and the diode series resistance, respectively [11]. In addition, the current noise spectra are also characterized by the sharp 60-Hz harmonics in the frequency range of 10 Hz–2 kHz. A surface degradation mode through spatially localized leakage sites (SLLS’s) has been found to be the origin of the 1/f noise in the planar devices [12]. It is supposed that the formation of diffusion spikes and/or dislocation movement during thermal processing can result in deterioration of SiN -InP interfacial layer at the perimeter of the p-n junction, thus create the SLLS’s. The small 1/f A /Hz at component and the low noise spectra of 10 10 Hz and 10 V can result from the low SLLS’s at SiN /InP interfaces and the low dislocations at the heterojunction of the photodiode array. The inset of Fig. 7 shows the noise current as a function of dark current for a typical diode on an array. The noise at the operating bias of 10 V is around pA/Hz which is much lower than that of 0.2 pA/Hz reported by Webb and Olsen [13]. In addition to outstanding performance, however, the photodiode arrays used in parallel optical transmission and fiber

HO et al.: UNIFORM AND HIGH PERFORMANCE OF PLANAR GaInAs PHOTODIODES

563

ACKNOWLEDGMENT The authors wish to express their deep gratitude to C. J. Hwang, Z. M. Chuang, and J. G. Chen for their beneficial discussions, and J. Gong for noise measurements. REFERENCES [1] G. E. Stillman, V. M. Robbins, and N. Tabatabaie, “III-V compound semiconductor devices: Optical detectors,” IEEE Trans. Electron Devices, vol. ED-31, pp. 1643–1655, 1984. [2] M. G. Brown, P. H. S. Hu, D. R. Kaplan, Y. Ota, C. W. Seabury, M. A. Washington, E. E. Becker, J. G. Johnson, M. Koza, and J. R. Potopowicz, “Monolithically integrated 1 12 array of planar InGaAs/InP photodiodes,” J. Lightwave Technol., vol. LT-4, pp. 283–286, 1986. [3] Y. Liu, S. R. Forrest, G. L. Tangonan, R. A. Jullens, R. Y. Loo, V. L. Jones, D. Persechini, J. L. Pikulski, and M. M. Johnson, “Veryhigh-bandwidth In0:53 Ga0:47 As p-i-n detector arrays,” IEEE Photon. Technol. Lett., vol. 3, pp. 931–933, 1991. [4] M. Makiuchi, M. Norimatsu, T. Sakurai, K. Kondo, and M. Yano, “Flip-chip planar GaInAs/InP p-i-n photodiode array for parallel optical transmission,” IEEE Photon. Technol. Lett., vol. 5, pp. 518–520, 1993. [5] T. P. Lee, C. A. Burrus, Jr., and A. G. Dentai, “InGaAs/InP pi-n photodiodes for lightwave communications at the 0.95–1.65-m wavelength,” IEEE J. Quantum Electron., vol. QE-17, pp. 232–238, 1981. [6] K. Takahashi, T. Murotani, M. Ishii, W. Susaki, and S. Takamiya, “A monolithic 1 10 array of InGaAsP/InP photodiodes with small dark current and uniform responsivities,” IEEE J. Quantum Electron., vol. QE-17, pp. 239–242, 1981. [7] T. P. Pearsall, “Ga0:47 In0:53 As: A ternary semiconductor for photodetector applications,” IEEE J. Quantum Electron., vol. QE-16, pp. 709–720, 1980. [8] D. R. Kaplan and S. R. Forrest, “Electrical crosstalk in p-i-n arrays—Part I: Theory,” J. Lightwave Techol., vol. LT-4, pp. 1460–1469, 1986. [9] N. Susa, Y. Yamauchi, and H. Kanbe, “Punch-through type InGaAs photodetector fabricated by vapor-phase epitaxy,” IEEE J. Quantum Electron., vol. QE-16, pp. 542–545, 1980. [10] W. J. Ho, M. C. Wu, Y. K. Tu, and H. H. Shih, “High-responsivity GaInAs PIN photodiode by using erbium gettering,” IEEE Trans. Electron Devices, vol. 42, pp. 639–645, 1995. [11] A. van der Ziel, “Unified presentation of 1/f noise in electronic devices: Fundamental 1/f noise sources,” in Proc. IEEE, vol. 16, pp. 233–258, 1988. [12] D. Pogany, S. Ababou, and G. Guillot, “Noise and current characterization of lattice-mismatched LP-MOCVD grown InP/InGaAs/InP photodetector arrays,” in 5th Int. Conf. Indium Phosphide and Related Materials, Apr. 1993, pp. 611–614. [13] P. P. Webb and G. H. Olsen, “Large-area and visible response VPE InGaAs photodiodes,” IEEE Trans. Electron Devices, vol. ED-30, pp. 395–400, 1983. [14] Y. Tashiro, K. Taguchi, Y. Sugimoto, T. Torikai, and K. Nishida, “Degradation mode in planar structure In0:53 Ga0:47 As photodetectors,” J. Lightwave Techol., vol. LT-1, pp. 269–272, 1983. [15] Y. Matsushima, Y. Noda, and Y. Kushiro, “Bias-temperature life tests for planar-type VPE-grown InGaAs/InP heterostructure APD’s,” IEEE J. Quantum Electron., vol. QE-21, pp. 1257–1263, 1985.

2

Fig. 8. The dark current variation of each diode on an array in the burn-in test at 20 V, 200  C, and 20 h.

0

2

communication must above all have extremely high reliability. Fig. 8 shows the dark current variation of each diode of the packaged array in the burn-in test at 20 V, 200 C, and 20 h. Each diode of the array shows a slight increase in the currents of 20–30 pA after the burn-in test. In addition, the breakdown voltage of each diode almost remains constant as measured before the burn-in test. The slight increase of dark current after burn-in tests may be attributed to the surface degradations. The accelerated interface deterioration between the passivation film and the semiconductor will result in a hole trapping at the interface and/or a hole injection into the SiN film [14], [15]. It induces an electron accumulation which becomes a leakage current path. The degradation observed in the diodes is not considered to be due to the contact failure [13], [14]. These diodes have Au/AuZn/Cr contacts in which a Cr layer acts as a barrier to the Au migration during the contact formation and/or the high-temperature aging tests. Thus, the test results are considered to be encouraging as to achieve highly reliable photodiodes. IV. CONCLUSION We have demonstrated the uniform and high performance of monolithically integrated array of GaInAs/InP planar photodiodes. For each diode of the packaged array at 5 V, the dark current is 75 pA, the capacitance is 2.3 pF and the crosstalk capacitance between adjacent diodes is 0.36 pF, the quantum efficiency is 95% at 1.3 m and 89% at 1.53 m, and the frequency bandwidth at 3-dB reduction is broader than 2 GHz. The diode of the packaged array also exhibits a small 1/f noise component and a very low noise current over a wide operating voltage range. In addition, the diode on the array has a small dark current variation after the burn-in test of 20 V, 200 C, and 20 h. These results confirm that these photodiode arrays can be useful in the applications of parallel optical transmission and fiber communication.

Wen-Jeng Ho was born on August 10, 1957, in Hsinchu, Taiwan. He received the B.S. degree in electronics engineering from the National Taiwan Institute of Technology, Taipei, in 1987, and the M.S. degree in electronics engineering from the National Chiao-Tung University, Hsinchu, in 1991. He is currently pursuing the Ph.D. degree in the Department of Electrical Engineering at National Tsing-Hua University, Hsinchu. Since 1981, he has been a staff member at the Telecommunication Laboratories, Ministry of Transportation and Communications, Yang-Mei, Taiwan. He has worked on hybrid microelectronics, integrated optical devices, and optoelectronic devices. Currently, he is the Researcher of the Integrated Optoelectronics of Applied Research Laboratory, Telecommunication Laboratories. His research interests include device fabrication and characterization of semiconductor photodetectors and lasers.

564

IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 44, NO. 4, APRIL 1997

Meng-Chyi Wu was born in Taipei, Taiwan, R.O.C., on November 17, 1957. He received the B.S., M.S., and Ph.D. degrees in electrical engineering from National Cheng Kung University, Tainan, Taiwan, in 1981, 1983, and 1986, respectively. He was appointed Associate Professor of Electrical Engineering at the National Tsing Hua University in 1988 and Full Professor in 1993. He is engaged in research on III–V compound semiconductors, material characterization, optoelectronic devices, and epitaxial techniques consisting of liquid-phase epitaxy, metalorganic chemical vapor deposition, and molecular-beam epitaxy.

Yuan-Kuang Tu (M’91) was born on October 13, 1955, in Taiwan, R.O.C., and received the B.S., M.S., and Ph.D. degrees from National Taiwan University, Taipei, in 1977, 1979, and 1988, respectively, all in electrical engineering. He joined Telecommunication Laboratories, Yang-Mei, Taiwan, in 1981, and worked for hybrid microelectronics, integrated optics, optoelectronic devices, and optical fiber communications. From 1990 to 1996, he was the Project Manager of Photonic Technology Research in the Applied Research Laboratory, Telecommunication Laboratories, taking charge of the research on optical communication technologies, fiber components, optoelectronic devices, and optical module technologies. Currently, he is the Acting Managing Director of the Outside Plant Technology Laboratory, Telecommunication Laboratories, and is in charge of the technology developments for access network. Dr. Tu is a member of IEEE/LEOS, ISHM R.O.C. Chapter, Chinese Institute of Engineering, Optical Engineering Society of R.O.C., and the Electronic Devices and Materials Association (EDMA). In 1995, he received the Distinguished Youth Award from EDMA.