IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 42, NO. 1 I , NOVEMBER 1995 1929

i -a-Si1-xGex:H

A Normal Amorphous Silicon-Based Separate

7000A \

iioooA

Absorption and Multiplication Avalanche Photodiode (SAMAPD) with Very High Optical Gain

K. H. Lee, Y. K. Fang, and G. Y. Lee

Abstract-A normal amorphous silicon-based separate absorp- tion and multiplication avalanche photodiode (SAMAPD) with very high optical gain of the avalanche photodiode has been devel- oped successfully by plasma-enhanced chemical vapor deposition (PECVD). Based on experimental results, using undoped a-Si:H as avalanche layer material and a-Si1-X GeX:H as absorption layer material, the hole-injection (HI) type SAMAPD yields a very high optical gain of 686 at a reverse bias of 16 V under an incident light power of P,, = 1 pW and has a rise time of 145 ps at a load resistance R = 10 kS1. Thus the amorphous silicon- based SAMAPD is a good candidate for the long-range optical communication applications.

I. INTRODUCTION OR long-range optical communications, where very low F levels of light are to be detected, it is desirable to use a

detector with large sensitivity, low dark current, fast response speed, and very high optical gain. To achieve these objec- tives, the separate absorption and multiplication avalanche photodiode (SAMAPD) had been developed successfully. The SAMAPD offers the advantages of low leakage current due to the avalanche layer being placed in the high-bandgap material; high sensitivity at long wavelength provided by the low-bandgap absorption layer material; high optical gain attained by high electric field being acrossed on the avalanche layer. Until now, most SAMAPD's were made by 111-V, II- VI materials [l], [2]. However, some problems still remain, e.g., high temperature process of the compound materials, and complicated preparation processes (i.e., MBE, LPE), thus raising the cost of the devices.

In this report, we propose an alternative structure based on amorphous silicon material with hole-injection (HI) type SAMAPD and electron-injection (EI) type SAMAPD for low cost and high gain applications. Fig. l(a) and (b) illustrates their schematic diagrams, respectively. The undoped cr- Sil-xGex:H material is selected as low-bandgap absorption layer due to its advantages of no lattice mismatch and variety of peak response wavelength is achieved easily by changing the composition [3]. The undoped a-Si:H material is used as high-bandgap avalanche layer due to its advantages of very low temperature process (25OoC), simple preparation process

Manuscript received August 30, 1994; revised March 20, 1995. The review of this paper was arranged by P. K. Bhattacharya. This work was supported by the Science Council of ROC under contract of NSC 83-0417-E-006-001.

The authors are with the VLSI Technology Laboratory, Department of Electrical Engineering, National Cheng Kung University, Tainan, Taiwan, R.O.C.

IEEE Log Number 9414581.

I p+-a-Si:H I T 400A m i-a-Si:H I t

n+-a-Si:H I 1 iooA I t

i-a -Sii-xGex:H

(a)

I n+-a-Si:H I 1 400A

42fA i-a-Si:H

p+ -a -Si:H

I G l a s s I (b)

Fig. 1. Schematic cross sections of (a) HI SAMAPD and (b) El SAMAPD.

of plasma-enhanced chemical vapor deposition (PECVD) system, large area device feasibility, and very low cost. The same idea has been employed successfully to raise the optical gain in the previously reported amorphous silicon based APD [4], [ 5 ] . For the purpose of comparison, some performances of conventional PIN APD are also reported. In the following sections, the fabrication process, operation mechanism, I-V curves, the effect of different SAMAPD structure on the optical gain, the effect of avalanche layer material on the

0018-9383/95$04.00 0 1995 IEEE

1930

optical gain, the effect of reverse bias voltage on the optical gain and the dark current, spectral response, and photoresponse speed are discussed in detail.

IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 42, NO. 1 I , NOVEMBER 1995

11. DEVICE STRUCTURE AND FABRICATION Fig. l(a) shows the schematic cross section of an HI

SAMAPD. An ITO-precoated glass plate was used as substrate. The 450 8, n+-type a-Si:H layer, 7000 N 11 000 8, undoped a 3 i l -X Gex:H layer, 100 8, n+-type a-Si:H layer, 420 N 840 A undoped a-Si:H layer and then 400 8, p+-type a-Si:H layer were deposited sequentially by using a PECVD system. Finally, a 5000 8, A1 layer was deposited by thermal evaporation and used as the contact electrode. Also, Fig. l(b) shows the schematic cross section of an E1 SAMAPD. The device was fabricated by using an ITO-precoated glass plate as substrate. The 450 8, p+-type a-Si:H layer, 7000 - 11 000 8, undoped a-Sil-xGex:H layer, 100 8, p+-type a-Si:H layer, 420-840 8, undoped a-Si:H layer and then 400 8, n+-type a-Si:H layer were deposited sequentially by using a PECVD system. The area of the contact hole is 1.8 mm'. The RF power, substrate temperature and total pressure during deposition of a-Si:H are 50 W, 25OoC, and 1 Torr respectively; and during deposition a-Sil-xGex:H are 40 W, 25OoC, and 0.5 Torr, respectively. The gases used are SiH4 (25% in Hz) and SiH4 (25% in Hz) + Ge H4 (47.8% in H2). The thickness of each layer was estimated and controlled by growth rate, which is 60 k m i n for a-Si:H deposition and 160 k m i n for a-Sil-xGex:H under these condition.

The energy band diagram and electric field profile of the HI SAMAPD is shown in Fig. 2(a). All the band gaps shown in Fig. 2(a) and (b) are optical gaps. The optical gap and electron affinity of a-Si:H are Eg,opt = 1.7 eV [6] and x = 3.7 eV [6], respectively; and those of a-Si0.7Ge0.3:H are Eg,opt = 1.45 eV [6] and x = 3.9 eV [6], respectively. So, there will be a band gap offset between the n - a-Si:H layer and i - a-Sil-xGex:H layer. The conduction band offset AE, is 0.2 eV, and the valance band offset AE, is 0.1 eV. By the Poisson equation, most of the applied bias will drop across undoped a-Si:H layer (i.e., the avalanche layer) under reverse bias condition. The operation mechanism of the HI SAMAPD is proposed as follow. As shown in Fig. 2(a), the electron- hole pairs were generated as the incident light entering the low-field undoped a-Sil-xGex:H layer (i.e., the absorption layer) and were separated by the electric field in this layer. The holes generated in the absorption layer were injected into the high-field avalanche layer, and generated more electron-hole pairs by the avalanche multiplication process. The generated holes will transfer to the p-type a-Si:H and the generated electrons will transfer to the n-type a-Si:H and thus constitute the total multiplied current. The operation mechanism of the E1 SAMAPD is similar as that HI SAMAPD, as shown in Fig. 2(b), only the carriers injected into the avalanche layer are replaced by electrons now.

111. EXPERIMENTAL RESULTS AND DISCUSSIONS

Fig. 3(a), (b), and (c) shows the I-V curves of the HI SAMAPD, E1 SAMAPD, and conventional PIN APD at dif-

X

E I

=k h u

X

\ /

Fig. 2. The energy band diagrams and electric field profiles of (a) HI SAMAPD, (b) E1 SAMAPD. The conduction band offset LIE, is 0.2 eV, and the valance band offset AE,. is 0.1 eV.

ferent incident light power, respectively. The incident light power is emitted from a He-Ne laser with wavelength X = 630 nm and intensity of 5 mW. As the results show, the optical gain value increases with decreasing the incident light power, which is a unique feature of majority carrier photodectors. Similar behavior has been observed in previously reported photodectors [7]-[9]. With incident light power P,, is 1 pW and dark current is set at 25 pA, the optical gain values of PIN APD, E1 SAMAPD, HI SAMAPD are 132, 80,686, as shown in Fig. 4, respectively. Obviously, the optical gain of the HI SAMAPD is much larger than the E1 SAMAPD. We suppose that the reasons are: the band gap offset of the conduction band is much larger than that of the valance band at the interface between the absorption layer and intemal undoped a- Si:H layer. This will contribute to the higher potential barrier for the injected electrons into the avalanche layer than that for the injected holes. In other words, the holes generated in the absorption layer can be injected into avalanche layer more easily than the electrons. In addition, the optical gain of the E1 SAMAPD is smaller than the conventional PIN APD. We think that this is due to the bandgap offset of the conduction band is too large and this will contribute to the accumulation of the injected electrons at the interface between the undoped a-Sil_xGex:H layer and intemal p+a-Si:H layer. Since the numbers of injected electrons are fewer, the optical gain of

1931 LEE et al.: NORMAL AMORPHOUS SILICON-BASED SEPARATE ABSORPTION

holes at the avalanche layer will transfer from the avalanche layer to the undoped a-Sil-xGe,y :H layer and recombine with the accumulated electrons at the interface and this will E

(C) Fig. 3. The traces sequentially correspond to 100 p W , 50 pW, 20 pW, 10 pW, 5 p W , 0 pW.

Dark and photo I-V characteristics of (a) HI SAMAPD, (b) E1 SAMAPD, and (c) conventional PIN APD at various incident light power levels.

-. A-.-

A.I. SAMAPD * * - *=A. /

*.

- .

material, we proposed the are: first, because the bind gap of a-SiC:H is larger than a-Si:H, the band gap offset

the HI SAMAPD and EI SAMAPD.-A~SO the optical gain of the conventional PIN APD is shown for comparison.

of the valance band with a-SiC:H is much larger to reduce more injected holes which can across the potential barrier; the

other is that the interface defects between the absorption layer and internal n+-type a-Si:H layer or between internal n+-

1932 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 42, NO. 11, NOVEMBER 1995

0.45 0.40

1000 I 1

- a- Si,.,Ge,,,:H - A Thickncss=lOOOA

1 0 0

1 0 0.5 1 10 1 0 0

Incident Light Power (pW) Fig. 5. different avalanche layer materials.

The relations of the optical gain and the incident light power with

.* d loo0G 1 0 0

'1 0-

V. I

5 7 9 1 1 13 1 5 17

Reverse Bias (V) Fig. 6. reverse bias.

The optical gain of the HI SAMAPD measured under different

type a-Si:H layer and avalanche layer are too much and these defects will lower the breakdown voltage of these interface. As the HI SAMAPD is reverse biased, these interfaces will first breakdown before the avalanche layer start to avalanche breakdown and will contribute to large leakage current.

Fig. 6 shows the effect of the reverse bias on the optical gain. The optical gain increased with increasing the reverse bias due to the higher impact ionization rates on the pho- togenerated carriers in the avalanche layer. In addition, the higher impact ionization probability, which in turn increases the dark current of the devices, as shown in Fig. 7. Under the reverse bias of 16 V, the optical gain of the HI SAMAPD can be attained as high as 686 at a incident light power P,, = 1 pW. The dark current under this condition is only 25 pA, which is good enough for actual applications.

In determining the diodes' responsivity, a Bausch and Lomb monochrometer with appropriate grating was used as the light source. As mentioned above, the undoped a- Sil-xGex:H layer was selected as absorption layer for its variety of peak response wavelength is achieved by changing the composition. As experimental results show, with a 7000 8, undoped a- Sil_xGex:H layer, the peak is shifted from 525 nm to 820

lo-. t lo-'

10'.

i o - ' 5 7 9 1 1 1 3 15 1 7

Reverse Bi is (V)

Fig. 7. The dark current of the HI SAMAPD measured under different reierse bias.

.:./ 0.1 5 ]\ 0.1 0 0.05 0.00

400 500 600 700 800 900 1000

Wavelength (nm)

The spectral response of the HI SAMAPD with 7000 8, undoped Fig. 8. a-Sio.7Geo.3:H layer.

nm as the Ge atomic ratio 2 increased from 0 to 0.48. Fig. 8 shows a typical photoresponse of the HI SAMAPD with 7000 8, undoped a-Sio.i.Geo.3:H layer, which the peak response wavelength of the diode is at 700 nm, i.e., red light. The experimental results show that the amorphous silicon-based SAMAPD is suitable for long-wavelength optical communica- tion applications with a-Sil-xGezy:H as the absorption layer material.

Finally, the photoresponse speed of the HI SAMAPD was measured under the illumination of infrared LED (A = 820 nm) with an average power intensity of 100 pW and square wave of 1 kHz, as shown in Fig. 9. The diode was also in series with a load resistance R = 10 k a . The measured rise time of the diode is about 145.3 ps at 14 V reverse biased, which is far smaller than the fall time (about 385.5 ,us) of the same diode. The difference may be due to three reasons: 1) minority carriers recombination time is too long in the ohmic contact region. 2) The high density of traps in the amorphous silicon material. Under the operation in the rise time, as the number of injected holes increases, the injected holes will first be trapped and the rest of the injected holes will then inject into avalanche layer. Therefore, the traps will enlarge the rise time. In the fall time, a negative step has the opposite behavior. Hence, a slower recovery would be expected, however, since it

LEE er al.: NORMAL AMORPHOUS SILICON-BASED SEPARATE ABSORF’TION 1933

141

151

[91

a-SiC:H barrier enhancement layer for high gain IR optical detector,” IEEE Trans. Electron Devices, vol. 40, p. 74, 1993. J. W. Hong, W. L. Laih, Y. W. Chen, Y. K. Fang, C. Y. Chang, and I. Gong, “Optical and noise characteristics of amorphous Si/SiC su- perlattice reach-through avalanche photodiodes,” IEEE Trans. Electron Devices, vol. 37, no. 8, pp. 18W1809, 1990. S. C. Jwo, M. T. Wu, Y. K. Fang, Y. W. Chen, J. W. Hong, and C. Y. Chang, “Amorphous silicodsilicon carbide superlattice avalanche pho- todiodes,” IEEE Trans. Electron Devices, vol. 35, no. 8, pp. 1279-1283, 1988. J. Kanicki, Amorphous & Microcrystalline Semiconductor Devices, Vol. II: Materials and Device Physics. C . Y. Chang, B. S. Wu, Y. K. Fang, and R. H. Lee, “Optical and elec- trical current gain in an amorphous silicon bulk barrier phototransistor.” IEEE Electron Device Lett., vol. EDL-6, pp. 149-150, 1985. K. C. Chang, C. Y. Chang, Y. K. Fang, and S. C. Jwo, “The amor- phous silicon Si/SiC heterojunction color-sensitive phototransistor,” IEEE Electron Device Lett., vol. EDL-8, pp. 64-65, 1987. J. W. Hong, Y. W. Chen, W. L. Laih, Y. K. Fang, and C. Y. Chang, “The hydrogenated amorphous silicon reach-through avalanche photodiodes (a-Si:H RAPD’s),” IEEE J . Quantum Electron., vol. 26, pp. 280-284,

Boston: Artech House, 1992.

1990. Fig. 9. illumination of infrared LED (A = 820 nm) with intensity of 100 pW.

The photoresponse of the HI SAMAPD under 14 V reverse bias and

K. H. Lee was born In Taiwan, R.O.C., on Septem- ber 16, 1969 He received the B.S. and M.S. degrees in electrical engineenng from the National Cheng Kung University, Taiwan, in 1991 and 1993, respec- tively. He is worklng toward the Ph.D. degree in electrical engineering at the National Cheng Kung University.

His current research interests are in high optical gain a-Si:Wa-SiGe:H heterojunction photodetec- tors.

takes more time for the traps to release a hole then to capture one. This is why the fall time of the diode is much longer

in amorphous silicon (pP in a-Si:H is 0.67 cm2N-S) is much slower than that in crystal silicon (pP in c-Si is 450 cm’N-S). Therefore, the response time of amorphous silicon material is much longer than that of crystal silicon.

than the rise time of the same diode. 3) The hole mobility

IV. CONCLUSION The structure, operation mechanism, optical gain and other

characteristics of the amorphous silicon based SAMAPD have been reported in detail. By comparison with the compound APD, the major features of the amorphous silicon-based HI SAMAPD are lost cost, simple preparation process, large area device feasibility, and very high optical gain, as high as 686. In addition, by changing the Ge atomic ratio X, the amorphous silicon-based SAMAPD also can absorb long- wavelength light, i.e., red light or infrared light. Based on the features mentioned above, especially very high optical gain and long-wavelength absorption, the developed normal amorphous silicon-based SAMAPD is a candidate for long- range, long-wavelength optical communication applications.

Y. K. Fang was bom in Tainan, Taiwan, R.O.C., on October IO, 1944. He received the B.S. and M.S. degrees in electronics engineering from Na- tional Chaio Tung University in 1957 and 1959, respectively, and the Ph.D. degree in semiconductor engineering from the Institute of Electrical and Computer Engineering, National Cheng Kung Uni- versity, in 1981.

From 1960 to 1978, he was a Senior Designer and Research Engineer in the private sector. From 1978 to 1980, he was an Instructor, then became an

Associate Professor in 1981 and a Professor in 1986 in the Department of Electrical and Computer Engineering, National Cheng Kung University.

Dr. Fang is a member of Phi Tau Phi.

REFERENCES G. Y. Lee was born in Taiwan. R.O.C., on October

[ I ] R. Kuchibhotla and I. C. Campbell, “Delta-doped avalanche photodiode for high bit-rate lightwave receivers,” J. Lightwave Techno/., vol. 9, no. 7, pp. 900-905, 1991.

[2] F. Capasso, A. Y. Cho, and P. F. Foy, “Low-dark-current low-voltage 1.3-1.6 p m avalanche photodiode with high-low electric field profile and separate absorption and multiplication regions by molecular beam epitaxy,” Electron. Leu., vol. 20, no. 15, pp. 635437, 1984.

[3] S. B. Hwang, Y. K. Fang, K. H. Chen, C. R. Liu, J. D. Hwang, and M. H. Chou, “An cu-Si:Wa-SiGe:H bulk barrier phototransistor with

23, 1970. He received the B.S. and M.S. degrees in electrical engineering from the National Cheng Kung University, Taiwan, in 1992 and 1994. respec- tively. He is working toward the Ph.D. degree in electrical engineering at the National Cheng Kung University.

His current research interests are in polysilicon thin film transistors.

Mr. Lee is a member of Phi Tau Phi.