IEEE TRANSACTIONS ON ELECTRON DEVICES. VOL 40. N O 4. APRIL 1993 72 I

An a-Si : H/a-Si,Ge : H Bulk Barrier Phototransistor with a-Sic : H Barrier Enhancement Layer for

High-Gain IR Optical Detector Sheng-Beng Hwang, Y. K. Fang, Kuin-Hui Chen, Ching-Ru Liu, Jun-Dar Hwang, and Min-Hong Chou

Abstract-In this paper, the design and fabrication of a high-gain amorphous siliconlamorphous silicon germanium (a-Si : H/a-Si,Ge : H) bulk barrier phototransistor for infrared light detection application are reported. The a-Si,Ge : H mate- rial featured lower energy gap and is suitable for the absorp- tion of longer wave light, but it also leads to a low breakdown voltage and high dark current.

We used an additional a-Sic : H thin-film layer at the collec- tor/base interface in the conventional amorphous bulk barrier phototransistor to enhance the function of bulk barrier and ob- tain high optical gain.

Emittc i

dI

d3

d 2

Base

Col lccto

E -1

AI

n' a-Si:H

i a-Si:H

p' a-Si:H i a-SiC:H

~~

r-- GLASS ~

I. INTRODUCTION ECENTLY, hydrogenated amorphous silicon ger- R manium alloy (a-Si,Ge : H) has been studied as a ma-

terial for long-wavelength optical detectors [ 11-[6]. The optical gap of a-Si,Ge: H can be varied from - 1.75 to - 1.0 eV by changing the content of Ge, and make the material suitable for the detection of light emitted from

L- _____

(d)

3 0 0 A

ZOOA

1 so; 6 0 - 2 0 0 i

30008

7000;

1 S O L T C I -1

- commercial laser diodes or LED. However, the lower gap of a-Si,Ge : H also leads to lower breakdown voltage and higher leakage current. Thus it is difficult to obtain high gain from the device based on the a-Si,Ge: H material. Hence, in past, only the p-i-n or Schottky barrier structure have been constructed in using the a-Si,Ge : H material for infrared detection [3]-[6]. In this paper, we have suc- cessfully solved the problem, for the first time, by putting an additional amorphous silicon carbide (a-Sic : H) thin layer in the conventional amorphous bulk barrier photo- transistor as the barrier enhancement layer (see Fig. 1) to form a high-gain optical detector in infrared range.

The a-Si : H bulk-barrier-type phototransistor, which has a basic device structure different from those of usually empolyed photoconductor or p-i-n diodes, has been suc- cessful fabricated and exhibited high optical gain and high response speed in our laboratory [7]-[I 11. Thus we adopted the structure i.e., glass/TCO/n+-i(a-Si, Ge : H)/i-a-Sic : H/p+-i-n+(a-Si : H) bulk barrier het- erojunction phototransistor (BBHPT) again in this study.

The operation of the a-Si,Ge : H BBHPT is similar to

Manuscript received September 11, 1992; revised November 20, 1992. The review of this paper was arranged by Associate Editor J . J . Coleman.

The authors are with the VLSI Technology Laboratory, National Cheng Kung University, Tainan, Taiwan, ROC.

IEEE Log Number 9206972.

i-a-SiC:H

E C (b)

Fig. I . (a) Schematic cross section of the a-Si : H/a-Si,Ge: H BBHPT. (b) The energy band diagram of a-Si : H/a-Si,Ge : H BBHPT at V,, > 0 under dark (dash line) and illumination (solid line).

that of reported n-i-p-i-n a-Si : H bulk barrier phototran- sistor [7], [ 121. As illustrated in the band diagram under dark and illumination (see Fig. l(b)), a fraction of holes generated by the light in the undoped layer of the collector (i-a-Si,Ge : H) is accumulated at the thin base region and induces a large number of electrons to inject from the emitter. Part of the photo-generated holes are trapped at the a-Si,Ge : H/a-Sic : H interface, which build up a high electric field distributed across the thin a-Sic : H layer and raise the tunneling probability of electrons through the layer. Both effects give a current gain of infrared light absorption. In addition, the added i-a-Sic : H layer with a higher gap (2.0-2.15 eV) provides a higher barrier to sup- press the carriers' thermionic emission and reduce the dark

0018-9383/93$03.00 0 1993 IEEE

722 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 40, NO. 4, APRIL 1993

current, which in turn enhances barrier lowering and re- sults in a high optical gain (see (3)). Furthermore, a lower dark current implies a lower leakage current. Therefore, a higher reverse bias can be applied to the device. The higher field induced from the higher bias enhances the im- pact reaction of generated electron-hole pairs, and causes a larger barrier lowering. Hence a very high gain can be obtained with higher voltage.

It is expected that the optical gain, response time, spec- tral response peak, etc., would be varied by changing the layer thickness of i-a-Si : H(dl), i-a-Si,Ge : H(d2), and i-a-Sic : H(d3). The relations will be discussed in the fol- lowing sections, and the results are useful for developing a high-gain a-Si,Ge : H infrared detector.

11. DEVICE DESIGN, FABRICATION, AND MEASUREMENT The cross section of the studied device is shown in Fig.

l(a). TCO (transparent conductor oxide, Sn02 : F) pre- coated glass plate was used as the substrate. After the cleaning process, the substrate was put into an Anelva model PED 301 plasma enhanced CVD reactor. The vac- uum pressure was kept at02 X torr for initial heat cleaniag. Then, the 200-A n+-a-Si,Ge : H laye!, 3000- 7000 A of undoped a-Si,Ge : H layer, 60-200 A of i-a- S i c : H, and p'4-n' a-Si : H layers were deposited se- quentially. The deposition conditions are typical and have teen described previously [4], [7]-[ 151. Finally, a 5000- A A1 layer was deposited onto the n+-a-Si : H by thermal evaporation and used as the mask of plasma etching to define the device area. The device area is 2 mm2. During the deposition, in order to avoid C and Ge contamination, the glow discharge was turned off at the end of depositing the a-Si,Ge:H and a-Sic : H layers. The gases in the chamber were completely pumped out and the chamber was purged by H2 gas for 5 min prior to the deposition of the next layer.

To determine the sample's responsivity, a Bausch and Lomb monochrometer with appropriate grating was used as the light source. When the photoresponse speed of the diode was measured, the glass side was illuminated by a light beam generated from a Hitachi HL7802G GaAlAs laser diode ( A = 780 nm). To emphasize the operation under weak light input, the average optical power of the laser beam has been attenuated to 50 pW. The laser diode's bias was turned on/off by an ac pulse and the de- vice was in series with a 10-kn load resistor. The induced photoresponse waveforming on the load resistor was shown on an HP 54600A oscilloscope. For optical gain measurement, we used GaAlAs laser diodes of X = 780 nm and X = 830 nm with different light intensity. The dark/photo current/voltage ( I / V ) curves were displayed by Tektronix 370A.

The device has been designed in such a way that un- doped layers on both sides of the base region are com- pletely depleted of free carriers at any bias condition. Normally, the device is operated with the collector elec- trode biased positively with respect to the emitter elec-

trode (i.e., Vc- > 0). The thickness of the emitter i-layer (dl), collector a-Si,Ge : H layer (d2), and a-Sic : H bar- rier-enhancement layer (d3) are critical parameters for the optimization of the device performance. The dl should be as thin as possible to obtain high optical gain, but, if the dl is too thin, the n + / p junction would be tunnel!d and seriously reduce the optical gain. The dl = 200 A \yas chosen as an optimal practical thickness [7]. A thicker d2 is preferred to absorb the incident light, but this results in a large internal series resistance and small barrier lower- ing, which reduces the response speed and optical gain. Thicker d3 retards the transport of carriers, but, the thin- ner d3 destroys the integrity of the barrier and results in smaller optical gain. The Ge content in the collector i-layer and C content in the a-Sic : H barrier-enhancement layer also affect the characteristics of the device perfor- mance. In our early study [4], increasing Ge content in a a-SiGe : H reduces the optical gap thus enhancing the ab- sorption of longer wavelengths. However, too much of Ge will lead to lower breakdown voltage and large dark current. Therefore, in this study, the composite of the un- doped layer of the collector was optimized as a-Sio,57Geo,43 : H (E,,,, = 1.35 eV) for infrared detec- tion. All the performances affected by the parameters mentioned above will be discussed later.

111. RESULTS AND DISCUSSIONS For an optical detector, the optical gain G under uni-

(1)

(2)

form illumination is defined as

G = [(IC - Id)/q]/(Pi~~/~v)

IC = Id exp [(qA+L)/(kT)]

where

is the collector current under illumination [7], [12], Pi, is the incident light power, hv is the incident photo energy

Id = 7AA*T2 exp ( - q + B / k T > [exp (qVl/lnkT) - 11 (3)

is the dark thermionic emission current from emitter to collector under Vl bias between the base and emitter, A is the device area, A* is the Richardson constant, is the barrier hight of the emitter-base junction, 7 is the proba- bility of electron tunnel through the d3 enhancement layer, and

(4) is the induced barrier lowering due to accumulation of the photo-generated holes at the base valley [ 121, while qL is the photo-generated carrier flux from the collector i-layer.

Fig. 2 shows the I / V characteristics under illumination of GaAlAs laser diodes (Fig. 2(a): X = 780 nm; Fig. 2(b): X = 830 nm), with incident light powFr as parametefs. The device of Fig. 2(a) has d , = 200 A , d2 = 5000 A , and d3 = 100 A with EROPT of a-Sic : H 2.0 eV, and Fig. 2(b) has the same parameters except d2 = 7000 A. The collector current I, increases with increasing collector- emitter bias voltage V,, and the incident light power Pin.

A4L = (kT/q) In (q*L/Jd)

HWANG er U / . : BARRIER PHOTORESISTOR FOR HIGH GAIN IR OPTICAL DETECTOR 123

(b)

Fig. 2 . (a) Dark and photo-current-voltage (Ill') characteristics of the a-Si : H/a-Si.& : H BBHPT has d, = 5000 A and incident light wave- length X = 780 nm at various incident power levels. The traces sequentially correspond to 60, 41.5, 25.5. 12.5, 4 .5 , and 0 pW. (b) Dark and phot? I / V characteristics of the a-Si : H/a-Si.Ge : H BBHPT has dZ = 7000 A and X = 830 nm at various incident power levels. The traces sequentially correspond to 61.5, 46.5, 38.6. 23, 8.2, and 0 pW.

These figures reveal that the device optical gain depends on the bias voltage and incident light power. (The BBHPT of Fig. 2(a) has an optical gain G = 24.5 when VCE = 10 V (I,, = 3 PA), Pi, = 60 pW with h = 780 nm, and Fig. 2(b) has G = 17 at VcE = 10 V (Io = 5 PA), P,, = 61.3 pW with h = 830 nm.) The dependence is illustrated in detail in Fig. 3(a) and (b). The optical gain of a-Si, Ge : H/a-Si : H BBHPT gradually increases with increas- ing input power. This is related to the recombination of carriers in the device. At low incident light power, i.e., small collector current, the recombination current ac- counts for a significant fraction of the total current and thus degrades the injection efficiency and hence the cur- rent gain. This phenomenon is often observed in hetero- junction phototransistor [16], [ 171. Also, this figure re- veals that the device optical gain can be increased by increasing VcE. However, the dark current also increases with bias voltage, hence the bias should be optimized to obtain the best device performance.

The relative spectral response with different a-Si,Ge : H

3 0 I I

$ 1 IS V,, -8V. I d = 0 . 7 y A

5 1 .' d , = l o O K ,I. =780nm

0 ' I 0 10 2 0 3 0 4 0 5 0 60

Incident l ight power ( p W ) fa)

d, =7000A

d, = 100A

1 =830nm 0

5 1 5 2 5 3 5 4 5 5 5 6 5

Incident l ight power ( p W )

o " 5

(b)

Fig. 3. Optical gain of the a-Si : H/a-Si,& : H BBHPT plotted as a func- tion of incident light power with (a) dz = 5000 A , X = 780 nm, and (b) dz = 7000 A . X = 830 nm.

thickness and bias voltage of the device are presented in Fig. 4(a) and (b), respectively. The spectral response peak shifts from 7500to 820 nm when the d2 is changed from 3000 to 7000 A , but does not vary with bias voltage. Since the wider d2 implies a deeper region to collect light- generated electron-hole pairs, thus the absorption of longer wave light was enhanced [ 131, [ 141. Under fixed thickness of d2 , a higher bias results in a higher efficiency of collection or higher impact reaction of those generated electron-hole pairs, the responsivity is raised, but the spectral peak wavelength is not shifted. Although, the thicker d2 enhances the longer wave infrared light absorp- tion, owing to the larger internal resistance, too thick d2 will reduce the optical gain as shown in Fig. 5.

The optical gain decreases with inFreasing d3 thiFkness as shown in Fig. 6 (when d , = 200 A , d2 = 5000 A , and EgoPT of a-Sic : H = 2.0 eV). The figure also points out that the optical gain decrea$es with increasing EgoPT of a-SiC:H (d3 fixed at 100 A) . This is reasonable since thicker d3 or larger Ego,, of a-Sic : H will reduce the tun- neling probability [ 181 and hence the collector current.

724

5.0

' a-SiC:H-Z.OeV

0.5 -

5 0 0 6 0 0 7 0 0 8 0 0 9 0 0 1000 1

Wavelength (nm) (a)

d, - 2 0 0 b d, - 1 O O A . 7 5 0

. .------=

E&, of -

.-SiC:H=?.OoV

/ \ \ \

' \

IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 40, NO. 4, APRIL 1993

5 0 0 6 0 0 7 0 0 8 0 0 9 0 0 1000 1100

Wavelength (nm) (b)

Fig. 4. (a) Relative spectral response of the a-Si : H/a-Si,Ge: H BBHPT with various collector undoped layer thicknesses (d2) . (b) Relative spectral response of the a-Si : H/a-Si,Ge: H BBHPT with the different bias voltages Vc,.

0 d ." a U - a ." Y

0"

U 0

0 * e 4

t Y

U

I

Fig. 5 . The relative spectral response peak and optical gain of the a-Si : H/a-Si,Ge : H BBHPT versus the thickness d, of the collector un- doped layer.

However, if d3 = 0 or a-Sic : H is replaced by a-Si : H, the breakdown voltage will be less than 2 V and the op- tical gain will be reduced seriously. Moreover, it is wor- thy to mention, the a-Sic : H layer in our a-Si : H/a-Si, G e : H BBHPT structure plays a very important role; it

2.00 2.05 2.10 2.15 2.20 --c

301 ' 8 8 I .

0 ' " " " " 40 80 120 160 200

Thickness of a-SiC:H <A> Fig. 6. The relation between the optical gain and the thickness of a-Sic : H

(d3) , EgoPT of a-Sic : H.

Fig. 7. Response waveform of the device to the laser waveform ( X = 780 nm, light power = 50 pW). The device has the same parameters as Fig. 2(a) and V,, = 8 V.

acts as a hole-blocking layer between p+-a-Si : H and i-a- Si,Ge:H, reducing the leakage current at normal bias condition. In addition, refer to Fig. l(b), the a-Sic : H layer will offer a potential minimum (for holes) at the in- terface of a-Si,Ge : H/a-Sic : H. When the electron-hole pairs generated in the i-a-Si,Ge : H layer are separated by the existing electric field, holes will drift down to the po- tential minimum and find a potential barrier there. Some holes will cross the barrier into the emitter region and a fraction of these holes will accumulate at the minimum on the a-Si,Ge : H/a-Sic : H interface and the depleted thin- base region. The accumulated holes that entered the base region will reduce the base barrier, while holes accumu- lated at the a-Si,Ge : H/a-Sic : H interface will increase the voltage drop across the a-SiC:H layer and allow a larger electron tunneling probability. Thus, accumulated holes in the base region and potential minimum enhance electron emission under light illumination from the emit- ter and a large optical gain can be obtained.

Fig. 7 presents a photograph of the photoresponse of a typical a-Si : H/a-Si,Ge : H BBHPT under the illumina- tion of GaAlAs laser diode (A = 780 nm) with an average

HWANG et al . : BARRIER PHOTORESISTOR FOR HIGH GAIN IR OPTICAL DETECTOR 725

power intensity of 50 pW and bias voltage at VCE = 8 V. The rise time of the BBHPT is about 35 ps, which is al- most equal to an a-Si,Ge : H Schottky-diode infrared de- tector [4]. This means that if the thickness has been cho- sen properly, the additional a-Sic : H barrier enhancement layer does not affect the transport of carriers profoundly. The fall time is about 153.7 ps. The longer fall time may be due to the capture of the holes at the a-Si,Ge: H / a-Sic : H interface. Although this hole capture enhances the optical gain it also delays the recombination of car- riers after shutting the light source, therefore it slows down the response speed. However, the speed can be in- creased by reducing the device area and/or reducing cur- rent gain [7].

IV. CONCLUSION The design and fabrication of a high-gain a-Si : H/

a-Si,Ge : H bulk barrier phototransistor for infrared light detection application has been described in detail. The high gain is a result of large bulk barrier lowering under light illumination and the retardation of dark current. These functions have been enhanced by the additional a-Sic : H thin layer inserted at the interface of the collec- tor and base.

In addition, the thickness of the i-a-Sic : H barrier-en- hancement layer, the thickness of the i-a-Si,Ge : H layer for incident light absorption, and the thickness of the i-a- Si : H for emitterhase junction barrier are important pa- rameters for obtaining a high-performance sample. Their effects on the device’s performances have been described in detail and provide a good reference for the design a high-gain amorphous silicon-germanium alloy infrared detector.

REFERENCES

[ I ] Y. Yukimoto, “Hydrogenated a-SiGe alloy and its optoelectronic properties,” in JARECT vol. 6 , Amorphous Semiconductor Technol- ogies & Devices, Y. Hamakawa, Ed.

121 K. D. Mackenzie, J . R. Eggert, D. J . Leopold, Y . M. Li, S . Lin, and W. Paul, “Structure, electrical, and optical properties of a-Si, ~ ,Ge,: H and an inferred electronic hand structure,’’ Phys. Rev. B , vol. 31, no. 4, p. 2198, 1985.

[3] P. P. Deimel. B . Heimhofer, G. Kroiz, G . Muller, and J . Wind, “Amorphous silicon-germanium diodes for optical detection,” J . Non-Crystall. Solids, vol. 115, p. 186, 1989.

[4] Y. K. Fang, S . B . Hwang, K. H. Chen, C. R. Liu, and L. C. Kuo, “A metal-amorphous silicon-germanium alloy Schottky barrier for infrared optoelectronic IC on glass substrate application,” IEEE Trans. Electron Devices, vol. 39, p. 1350, 1992.

[5] D. S . Shen, J. P. Conde, V. Chu, S. Aljishi, J . Z. Liu, and S. Wag- ner, “Amorphous silicon-germanium thin-film photodetector array,” IEEE Electron Device Lett . , vol. 13. p. 5 , 1992.

[6] D. S. Shen, J . P. Conde, V. Chu, S. Aljishi, and S . Wagner, “Effect of material properties on the performance of a-Si,& : H, F photode- tectors,” Mat. Res. Soc. Symp. Proc . , vol. 118, p. 457, 1988.

[7] B . S . Wu, C. Y. Chang, Y . K. Fang, and R. H. Lee, “Amorphous silicon phototransistor on a glass substrate,” IEEE Trans. Electron Devices, vol. ED-32, p. 2192, 1985.

[8] C. Y. Chang, B . S . Wu, Y. K. Fang, and R. H. Lee, “Amorphous silicon bulk barrier phototransistor with Schottky barrier emitter,” Appl. Phys. Lett . , vol. 47, p. 49, 1985.

191 K. C. Chang, C. Y. Chang, Y. K. Fang, and S . C. Jwo, “The amor- phous Si/SiC heterojunction color-sensitive phototransistor.” IEEE Electron Device Lett . , vol. EDL-8, p. 64, 1987.

[IO] J . W. Hong, Y. W. Chen, K. C. Chang, Y. K. Fang, and C. Y.

Ohmsha Ltd., 1983, p. 136.

Chang, “Hydrogenated amorphous Si/SiC heterojunction phototran- sistor,” Solid-State Elecrron., vol. 32, p. 883, 1989.

[ I l l M. T. Wu, Y. K. Fang, J . W. Hong, and C. Y. Chang, “Hydrogen- ated amorphous Si / S i c superlattice phototransistor,” Solid-State Electron., vol. 34, p. 189, 1991.

[I21 C. Y. Chang, “Photogenerated and recombination in a bulk barrier phototransistor,” IEEE Trans. Electron Devices, vol. ED-33, p. 1829, 1986.

131 Y. K. Fang, S . B . Hwang, K. H. Chen, C. R. Liu, M. J. Tsai, and L. C. Kuo, “An amorphous SiC/Si heterojunction p-i-n diode for low-noise and high-sensitivity UV detector,” IEEE Trans. Electron Devices, vol. 39, p. 292, 1992.

141 Y. K. Fang, S. B . Hwang, Y. W. Chen, and L. C. Kuo, “A vertical type a-Si : H back to back Schottky diode for high speed color image sensor,” IEEE Electron Device Lett., vol. 12, p. 172, 1991.

IS] J . W. Hong, W. L. Laih, Y. W. Chen, Y . K. Fang, C. Y . Chang, and J . Gong, “Optical and noise characteristics of amorphous Si/SiC superlattice reach-through avalanche photodiodes,” IEEE Trans. Electron Devices, vol. 37, p. 1804, 1990.

[ 161 J . C. Campbell and K. Ogawa, “Heterojunction phototransistors for long-wavelength optical receiver,” 1. Appl. Phys., vol. 53, p. 1203, 1982.

1171 T. Mitsuyu, S . Fujita, and A. Sasaki, “lnGaAsP/InP wavelength- selective heterojunction phototransistors, ” IEEE Trans. Electron De- vices, vol. ED-31, p. 812, 1984.

New York: Wiley, 1981, pp. 540-548.

[I81 S . M. Sze, Physics ofSemiconducror Devices, 2nd ed.

Sheng-Beng Hwang was born in Taiwan, ROC, on May 4 , 1965. He received the B.S . and M.S. degrees in electrical engineering from National Cheng Kung University, Taiwan, ROC, in 1987 and 1989, respectively.

He is currently working toward the Ph.D. de- gree in electrical engineering at Cheng Kung Uni- versity. His research interests are focused on a-C : H/a-Si : H heterojunction and a-C : H TFT for HDTV application.

Y. K. Fang was born in Tainan, Taiwan, Repub- lic of China, on October 10, 1944. He received the B.S . and M.S. degrees in electronics engi- neering from National Chaio Tung University in 1957 and 1959, respectively, and the Ph.D. de- gree in semiconductor engineering from the Insti- tute of Electrical and Computer Engineering, Na- tional Cheng Kung University, in 198 1 .

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 Electrical and Computer Engineering Department, National Cheng Kung University.

Dr. Fang is a member of Phi Tau Phi.

Kuin-Hui Chen was born in Taiwan, ROC, on November 20, 1964. He received the B . S . and M.S. degrees in electrical engineering from the National Cheng Kung University, Taiwan, in 1988 and 1990, respectively.

He is working toward the Ph.D. degree in elec- trical engineering at the National Cheng Kung University. His current research interest is in the heterojunction between compound and amorphous materials.

726 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 40, NO. 4. APRIL 1993

Ching-Ru Liu was born in Taiwan, ROC, on March 12, 1966. He received the B.S and M.S. degrees in electncal engineenng from National Cheng Kung University, Taiwan, in 1988 and 1990, respectively.

Since 1990 he has been working toward the Ph.D. degree in the Institute of Electrical Engi- neering, National Cheng Kung University. His current study is to research and develop the amor- phous-crystalline silicon heterojunction devices for electrooptical system application.

Min-Hong Chou was born in Taiwan, ROC, on July 25 , 1968. He received the B.S. and M.S degrees in electrical engineering from the National Cheng-Kung University, Taiwan, in 1990 and 1992, respectively

Jun-Dar Hwang was born in Taiwan, ROC, on February 18, 1960. He received the B.S. degree in electrical engineering from National Taiwan Institute of Technology in 1986 and the M.S. de- gree from the National Cheng-Kung University, Taiwan, in 1990.

He is currently working toward the Ph.D. de- gree in electrical engineering at Cheng-Kung Uni- versity. His research interest is the heterojunction of silicon carbide/silicon application.