Photocurrent multiplication in GaAs Schottky photodiodes

Solid-Slate Electronics Vol. 30, No. I, pp. 93-96, 1987 Printed

in Great

Britain.

All rights reserved

Copyright

PHOTOCURRENT MULTIPLICATION SCHOTTKY PHOTODIODES J.

SAFRANKOVA

and P.

8

0038-I lOI/ 1987 Pergamon

$3.00 + 0.00 Journals Ltd

IN GaAs

KORDOS

Institute of Electrical Engineering, Centre of Electra-Physical Research, Slovak Academy of Sciences, 842 39 Bratislava, DlibravskSt cesta, Czechoslovakia (Received

3 February

1986; in revised form 30 May

1986)

attempts have been made to build fast, sensitive photodetectors which offer simple fabrication and ease of integration. Regarding this view, GaAs Schottky barrier photodiodes seem to be ideally suited for use in the near-infrared region. The fabrication of GaAs Schottky photodiodes and the investigation of their properties (mainly photocurrent multipication) are presented in this paper. A Abstract-Many

photocurrent gain of 10’ was achieved and dependences of gain on incident power level and position were observed.

1. INTRODUCTION High sensitivity and high speed photodetectors are required for optoelectronic applications. Various types of photodetectors have been reported for the detection of radiation in the 0.82pm wavelength range, namely, bipolar devices (p-n, P-i-n, or avalanche photodiodes, and phototransistors) as well as unipolar devices (Schottky photodiodes, metalsemiconductor-metal and photoconductive detectors and optical field effect transistors, so-called OPFETs). Theoretical and experimental values for gain and bandwidth of photodiodes, phototransistors, metal-semiconductor-metal and photoconductive detectors on GaAs were compared by Beneking[l]. It was shown that fast detectors with speed of response in the 100 ps range can be made using any above mentioned type. Therefore, the choice depends on other reasons than speed. Sensitivity and noise considerations as well as simple fabrication procedures and integrability may tend to favor one configuration. The choice of diodes leads to smaller leakage currents than photoconductors or phototransistors. For simplicity of fabrication the Schottky diode is preferable to the p-n diode. Recently, high speed GaAs Schottky photodiodes with bandwidths of 10-100 GHz were reported[2-71. In the earlier work of Lindley et a/@], a uniform avalanche GaAs Schottky photodiode was described. Photocurrent gain of 100 for optimal noise conditions was obtained, while at higher values of multiplication the noise increases faster than the signal. The ionization rates for carriers in GaAs have been determined from multiplication in Schottky barriers[9, lo]. In this paper the fabrication of Au(n)GaAs Schottky barrier photodiodes and the investigation of their properties are presented. The photocurrent dependences on reverse bias voltages for various incident radiation powers are investigated and the

avalanche gain values are determined. Further, the photo-response distribution by scanning a laser beam across the photodiode area is measured with the aim of analysing the influence of the Schottky junction periphery on the multiplication conditions.

2.

EXPERIMENTAL

The Schottky barrier photodiodes investigated in this study were formed on undoped n-type GaAs layers (free carrier concentration in the range of 1015cm-3) which were grown on low-resistivity Tedoped n+-GaAs substrates by vapor-phase epitaxy. The epitaxial layers were about 10 pm thick. Ohmic contact to the substrate was prepared by vacuum evaporation of Au:Ge (88: 12)/Au onto the backside and the wafers were annealed at 450°C for 1 min after deposition. Gold was chosen to create the Schottky barrier on GaAs because of high transparency of incident radiation into GaAs[l 11. Approximately 10 nm thick semitransparent Au film was deposited in ultra-high vacuum onto the chemically cleaned GaAs surface. 200 pm diameter Au dots were formed using standard photo-lithographic techniques and Al wire was ultrasonically bonded to the Au film. Neither guard-ring nor mesa etch were applied. Our samples were without an antireflection coating. A crosssectional view of the present Schottky diodes is given in the insert in Fig. 1. The quality of prepared Schottky barriers on GaAs was investigated using conventional I-V and C-V measurements in the dark. The exponential dependence of forward current on the voltage is believed to be due to the thermionic emission of electrons from GaAs to the metal. The ideality factor determined from the forward 1-V characteristic of the diodes was n = 1.05 and the Schottky barrier height was 48= 0.85eV which corresponds to the published values for Au-(n)GaAs contacts. The series 93

94

J.

SAFRANKOV.~

and P. KORDOS

from the measurement conditions (reverse bias voltage or radiation power) on the photocurrent gain. We measured the photocurrent of our photodiodes by flooding the entire surface of the samples with radiation of different intensity. The solid curves in Fig. 1 show the measured photocurrents Z, as a function of reverse bias voltage U, at various input radiation powers Pi which were obtained from a defocused 0.63 lrn He-Ne laser beam using calibrated filters (dashed curve marks the dark current Id). The photocurrent for higher radiation powers follows the UX/2-law near zero bias due to the depletion width increasing and at higher voltages below the onset of multiplication the photocurrent begins to saturate. The photocurrent curves for low radiation power are determined mainly by dark current Id. The photocurrent gain was determined from the relationship[ 121:

Ip - Id

10-10

1

0

I

I

I

I

40

80

120

160

U,(V)

Fig. 1. Photocurrent fp as a function of reverse bias voltage U, at different radiatron powers P, incident on the detector area (I. = 0.63 pm); the dashed curve is the dark current I,,. resistance R, was R, < 10 R. A typical reverse Z-V diode characteristic in the dark is shown in Fig. l&dashed curve. The breakdown voltage U, was about 136 V and the dark currents were higher than expected from the Schottky thermionic current and generation current in the depletion layer. This higher dark current can be due to leakage currents at the surface periphery of the diode. The maximum value of current under breakdown conditions is limited by the series resistance of contacts and bulk material as well as shunt resistance. The basic photoelectric properties of GaAs Schottky diodes were evaluated from the following measurements: (a) spectral response, (b) reverse I-V characteristic vs incident power, (c) speed of response. Spectral response (50% from maximum value) of prepared GaAs Schottky photodiodes is in the range from 0.47 to 0.89pm and the external quantum efficiency at 0.82 pm was found to be about r) = 60%. From the impulse response we have evaluated the rise and fall times measured at low reverse bias U, = 20 V having the values (l&90%) in the range of about 100~s.

and primary photowhere IpM. I, are multiplied currents and I dM, Z, are dark currents at reverse bias voltage corresponding to the multiplied and primary photocurrents, respectively. For our analysis we have taken the IP and I, values at the bias voltage U, = 50 V since only at higher voltages does photocurrent multiplication occur. As a result, we have obtained the photocurrent gain dependence on the incidence radiation power P, at different reverse bias voltages UR. Under conditions with reverse bias near breakdown voltage (U, = 0.99 U,) and for high radiation power Pi = 6 pW, the gain value of G = 20 has been achieved. With decreasing radiation power, the gain increased continuously. For low radiation power, when the primary photocurrent I, equals the dark current Id, in our case Pi = 5nW, a gain value of lo4

W IO2

IO’

I

IO0 10-g

3.

MEASUREMENTS

AND

RESULTS

We concentrated on the avalanche conditions of the GaAs Schottky photodiodes since our preliminary experiments had indicated a strong influence

10-a

I

1o-7 P, (W)

I

10‘6

I

10-5

Fig. 2. Photocurrent gain G dependence on radiation power P, incident on the detector area at different bias conditions (U,= 136V).

Photocurrent

multiplication

in GaAs Schottky photodiodes

95

using the focused laser beam falling on the periphery (curve A) and center (curve B) of the Schottky junction are shown in Fig. 4. For example, under reverse bias conditions U, = 0.76 U, the value of G = 30 was found at the periphery compared to G = 1.5 from center of the Schottky junction. 4. DISCUSSION

Fig.

3. Photocurrent

response as a function across the diode.

of position

was observed. The dependence of photocurrent gain on the radiation power at reverse bias voltages of U, = 61, 76, 83 and 99% of breakdown voltage are shown in Fig. 2. Thus the gain has a maximum at a certain value of radiation power which is typical for given reverse bias conditions. A distribution of photocurrent response was investigated because electric field enhancement can occur at the periphery of the Schottky contact. The photocurrent distribution was observed by scanning a focused 0.63 pm He-Ne laser beam across a photodiode 200pm in diameter. Figure 3 illustrates the photocurrent scan measurement for reverse bias voltages U, = 40, 70 and 1lOV. The dependence of photocurrent gain value on position of beam incidence is evident from Fig. 3. The photocurrent is much higher at the periphery than in the center of the Schottky junction. The photocurrent gains measured

IO2

I/I A

53

”

IO'

lo./ 50

-loo

U,(V) Fig. 4. Photocurrent gain G as a function of reverse bias voltage U, measured by focused laser beam; curve A when the beam is at the edge of the diode, curve B when the beam is in the center of the diode.

AND CONCLUSIONS

Published papers on GaAs Schottky barrier photodiodes deal mainly with high bandwidth conditions[3-71. Those photodiodes were not made with the aim of sensitivity enhancement by means of internal amplification. Using our Au-(n)GaAs Schottky photodiodes we have measured a photocurrent dependence on reverse bias voltages for 0.63 pm radiation. At voltages higher than 50 V, multiplication of photocurrent occurs. We observed a strong dependence of photocurrent gain on incident radiation power at various reverse bias voltages. As it follows from Fig. 2 these dependences have a maximum value for certain radiation power which is typical for given reverse conditions. For extremely low radiation power we observed a photocurrent gain as high as 104[13]. At reverse bias conditions near breakdown voltage the photocurrent gain decreases as the radiation power increases. The observed photocurrent gain dependence of GaAs Schottky photodiodes on radiation power can be explained from the relationship in Equation (1). For reverse bias voltages near breakdown and high radiation powers, the primary photocurrent Z,,is high compared with the dark current Z, and Equation (1) can be simplified to G = I,,/I,,. Thus, G is indirectly proportional to I,, . For low powers when the primary photocurrent Zpis approximately the same as the dark current Id then G = Z,,/Z,. To obtain a high value of G a dark current as low as possible is needed. Similar dependences of photocurrent gain on radiation power were observed on p-n AIGaAs/GaAs photodiodes[l4] and also on GaAs photoconductive detectors[l5,16]. Values of 104-lo6 were measured for the external steady-state gain on GaAs photoconductive detectors. Explanation for these observations is related to trapping effects at the surface. From photocurrent response mesurements using focused laser beams we have observed that the photocurrent exhibits a higher response mainly at the periphery of the Schottky junction. At low reverse bias voltages before multiplication occurs (e.g. at U, = 40 V, Fig. 3) the higher response is due to the direct incidence of radiation into the depletion layer at the junction periphery. At higher voltages the photocurrent response at periphery increases much more than in the center region owing to the higher electric field in the periphery region of the Schottky junction. From this it follows that the photocurrent is multiplied mainly at the periphery. The premature breakdown caused by the higher electric field at the

J. SAFRINKOVA and P. KORDOS

96

periphery must be eliminated in order to achieve substantial photocurrent gain in the center of the diode-a guard ring seems to be necessary. In conclusion, from our results it follows that Au-(n)GaAs avalanche Schottky barrier photodiodes can exhibit a photocurrent gain as high as G = 104. The gain value depends on radiation power and the photocurrent is multiplied mainly at the periphery of a Schottky junction with no guard ring. These results should be considered for optimal design of the photodiode structure, for example by using a guard ring or applying an interdigitated Schottky contact finger configuration. From the practical point of view the best operating conditions of GaAs avalanche Schottky photodiodes should be fixed depending on the expected level of detected radiation power.

3. 4. 5. 6. 7. 8. 9. 10. Il. 12.

13. 14. REFERENCES 15. I. H. Beneking, IEEE Trans. Electron. Deu. ED-29, 1420 (1982). 2. Yu. A. Vasil’ev, Yu. V. Dmitriev, P. G. Eliseev, I. A.

16.

Skopin and V. I. Stafeev, Quantum Elekrron. I, 2218 (1980). (in Russian). S. Y. Wang, D. M. Bloom and D. M. Collins, Appl. Phys. Left. 42, 190 (1983). S. Y. Wang and D. M. Bloom, Electron. Lett. 19, 554 (1983). H. Blauvelt, G. Thurmond, J. Parsons, D. Lewis and H. Yen, Appl. Phys. Left. 45, 195 (1984). Z. Rav-Noy, C. Harder, U. Schreter, S. Margalit and A. Yariv, Electron. Lett. 19, 753 (1983). D. G. Parker, Electron. Left. 21, 778 (1985). W. T. Lindley, R. J. Phelan, C. M. Wolfe, A. G. Foyt, Appl. Phys. Left. 14, 197 (1969). G. H. Glover, J. appl. Phys. 44, 3253 (1973). G. E. Stillman, C. M. Wolfe, J. A. Rossi and A. G. Foyt, Appl. Phys. Lett. 24, 471 (1974). H. J. Hovel, J. appl. Phys. 47, 4968 (1976). G. E. Stillman and C. M. Wolfe, Semiconductor and Semimetals (Edited by R. K. Willardson and A. C. Beer), Vol. 12, p. 291. Academic, New York (1977). Gains as large as 10’ were observed in GaAs Schottky photodiodes which did not have a guard ring [8]. J. Novak, M. Morvic, P. KordoS, Solid-St. Electron. 25, 82 (1982). J. P. Vilcot, J. L. Vatercowski, D. Decoster and M. Constant, Elecfron Lett. 20, 86 (1984). N. Matsuo, H. Ohno and H. Hasegawa, Jap. J. Appl. Phys. 23, L 299 (1984).