Use of a Field Effect Transistor to Study Phototransport Properties of a-Si:H
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USE OF A FIELD EFFECT TRANSISTOR TO STUDY PHOTOTRANSPORT PROPERTIES OF a-Si:H
A. R. Grant and P. D. Persans Physics Department and Center for Integrated Electronics Rensselaer Polytechnic Institute, Troy, NY 12180 R. F. Kwasnick and G. E. Possin Corporate Research and Development, General Electric, Niskayuna, NY 12309
Abstract We report new measurements of the diffusion length of minority photocarriers in thin film amorphous silicon field effect transistor structures. We are able to vary the majority carrier photoconductance by more than four orders of magnitude while monitoring the effective ambipolar diffusion length using the photocarrier grating technique.
Introduction Recent phototransport studies on amorphous silicon hydride have focussed on the relationship between majority carrier and minority carrier parameters [1-5]. This new interest is in part due to the development of a new and powerful tool to study minority carrier properties, the photocarrier grating technique [6-9]. All of these studies have been carried out by either doping or by inducing metastable defects to move the Fermi level. Field effect transistors have been employed to study basic properties of amorphous silicon materials such as effective carrier mobility and the density of states. These devices permit us to move the Fermi level without introducing new states, as is the case with doping. In this paper we report new measurements of the transport properties of minority photocarriers in a-Si:H. This was accomplished by creating a photocarrier grating parallel to the source-drain electrodes of a field effect transistor and measuring the photocurrent transverse to the grating (the PCG technique). Analysis of the photocurrent measurement as a function of grating period allows us to extract the ambipolar diffusion length for carriers in the region of the sample which dominates the photoconductance of the transistor. The ambipolar diffusion length is controlled by the diffusion length of the minority photocarrier.
Experimental Details A schematic of the field effect transistors used in this study is shown in Fig. 1. It is of the inverted staggered gate structure [10] with a Ti gate electrode covered by 150 nm of
Mat. Res. Soc. Symp. Proc. Vol. 297.
1993 Materials Research Society
884
SILICON NITRIDE
BACK SURFACE
SOURC
SURFACE
XX X x×XXXXXIo x
n+ a-i
n+ a-Si:H
DRAIN
6
X a-S H
a-Si
X~x
X X XX
1X
GATE
SILICON NITRIDE
Figure 1.) Schematic of the thin film transistor structure used in this study. silicon nitride. Approximately 200 nm of intrinsic a-Si:H and a 50 nm n+ a-Si:H contact layer are then deposited. The n+ layer is removed from between the Mo source-drain contacts by a timed etch, which also removes about 80 nm of the intrinsic Si layer, leaving a 120 nm thick a-Si:H layer. The top surface is passivated by baking in air and encapsulated by deposition of a thin silicon nitride layer. This preparation leaves the top surface with a relatively high density of surface states which appears to pin the top chemical potential
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