The Limitations of the Constant Photocurrent Method for Determining Subgap Absorption
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ABSTRACT The subgap absorption cxcpm measured by the constant photocurrent method (CPM) was studied between 4.2K and 300K for different dopings and defect concentrations N in hydrogenated amorphous silicon (a-Si:H). We found that Lxcpm is reduced by up to a factor 100 between 50K and 200K due to infrared selfquenching which changes the charge state of the defects and enhances recombination. The effect is diminished at high N and large doping. The relative change Aoxcx reflects AN/N both at very low and high temperatures. For undoped samples acpm is about four times larger at 4.2K than at 300K. For doped samples the two values are essentially the same.
INTRODUCTION The constant photocurrent method (CPM) is widely used for measuring the subgap absorption and thereby the density of defect states in hydrogenated amorphous silicon (a-Si:H) and related alloys. An advantage of CPM is its simplicity [1, 2]; one measures the photon flux F(hv), which yields the same photocurrent or photocarrier density n, as a function of photon energy hv. If a constant n ensures that the recombination lifetime T is constant and independent of hv, then the photocarrier generation rate G must also be constant since n=GT. In that case, the relation between G and F(hv), yields the desired optical absorption coefficient oX(hv). At photon energies corresponding to optical transition between the defects and the extended states,
cx(hv) can then be related to the defect density N either by modeling or by using a calibration factor. The reliability and validity of the CPM method have been questioned by several authors [3-8]. A major criticism was that optical transitions involving defect states must necessarily change their occupancy and hence change the recombination lifetime T, thus invalidating the basic premise of the method for determining t( hv) from F(hv). In addition, a considerable effort has been made to find suitable models and computational simulations for extracting the defect density N from or(hv) determined by CPM [5-8]. The major difficulty encountered with the latter task is the fact that these models are insufficient to even allow the calculation of cp(T), the temperature dependence of the photoconductivity at tone value of hv not to speak of the spectral dependence of aTp which is needed to obtain N from cL(hv). This paper addresses only the first problem, the determination of oa(hv) by means of CPM. We agree with the previous authors that the conditions for the success of the CPM method includes the spectral independence of the Rose factors y which is defined by aYpocGY, and that acpm must be independent of the chosen value of the constant photocurrent. It is the aim of this work to uncover the physical reasons for the failure of CPM and to find ways to cope with the difficulties. For this purpose, we have studied the dependence of the CPM results on temperature, doping, and the concentration N of defects in a-Si:H. 467 Mat. Res. Soc. Symp. Proc. Vol. 377 © 1995 Materials Research Society
EXPERIMENTAL DETAILS The a-Si:H sample
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