Effect of Material Properties on the Performance of a-Si,Ge:H,F Photodetectors
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EFFECT OF MATERIAL PROPERTIES ON THE PERFORMANCE OF a-Si,Ge:H,F PHOTODETECTORS *) D.S. SHEN, J.P. CONDE, V. CHU, S. ALJISHI* AND S. WAGNER Department of Electrical Engineering, Princeton University, Princeton, NJ 08544
ABSTRACT We report a study of the transient photoconductivity of a-Si,Ge:H,F in reversebiased Schottky barrier detectors. The barriers were used to detect light pulses from commercial (AI,Ga)As laser diodes. The photodetectors work at a modulation frequency of up to 10MHz. Lowering the optical gap, Eopt, reduces the optical absorption length but raises the dark current and thus reduces the specific detectivity D*. The charge collection efficiency is determined by deep trapping of electrons in the conduction band tail. The frequency response is controlled by the electron drift mobility. The performance of the photodetector is discussed in terms of the optical and transport properties of the aSi,Ge:H,F alloys.
INTRODUCTION Several groups have reported photodetectors based on un-alloyed a-Si:H. Johnson et al reported a-Si:H photoconductive detectors with picosecond response to single pulses [1]. Wei et al developed an a-Si:H based X-ray sensor sensitized with an X-ray fluorescent overlayer [2]. Deimel et al reported a-Si:H pin diode detectors [3]. In this paper, we describe a study of a-Si,Ge:H,F alloys for photodetection. The advantage of this alloy is that the wavelength response can be tailored to specific applications. a-Si,Ge:H,F is under development for the narrow-gap layer in a tandem solar cell [4,5]. These alloys also are used to improve the long wavelength response of electrophotographic receptors [6,7]. By changing the Ge content, we can adjust the optical gap (Eg) from --.1.7eV (a-Si:H) to - 1.25eV (at present) with satisfactory transport properties. aSi,Ge:H,F alloys have absorption coefficients of the order of 104 to 105 cm- 1 for photon energies 0.1-0.2eV above their optical gap. For efficient light absorption we need only 0.5-1lm thick films. Other advantages are that a-Si,Ge:H,F alloys can be deposited at low temperature (- 250 C) and over a large surface. The results reported here follow our early work on the transport and the d.c. light detection properties of a-Si,Ge:H,F [8-
10]. MODEL The optical absorption spectrum a(hv) of a-Si,Ge alloys above the optical gap Eg can be written as [11]: (ahv)' = B(hL - Eg)
*) This work is supported by the Electric Power Research Institute.
Mat. Res. Soc. Symp. Proc. Vol. 118.
1980 Materials Research Society
(1)
458
Here B is approximately 700cm-1/2eV-1/2, and hv is the photon energy. B depends little on Eg and on the alloy composition. Once a(hv) is known, one can determine the film thickness ('-. 1/a) needed to collect photons with energy hv. The high frequency response is determined by the transit time tT for electrons through the entire sample, which is twice the average time which an electron needs to drift from the location where it is generated to the collecting electrode. The transit time is tT=d/PF. Here d is the film thickness, F
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