Theory of the steady-state-photocarrier-grating technique for obtaining accurate diffusion-length measurements in amorphous silicon

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Abstract
This paper presents a theory that is the basis of the steady-state-photocarrier-grating (SSPG) technique, as a means of determining the diffusion length of photocarriers in amorphous semiconductors. Solving the SSPG transport problem, including the formulation of small-signal photocurrent, by a second-order perturbation approach reveals deficiencies in the existing SSPG theory, which is based on first-order perturbation theory. It is also shown that the SSPG data analysis done routinely, assuming a priori that local space-charge neutrality prevails, yields a severe overestimate of the diffusion length when the condition is not met experimentally. An extension of the theory to measure accurately the diffusion length by removing these defects inherent in the original SSPG formulation is demonstrated by its successful application to hydrogenated amorphous silicon. The experiments carried out at various illumination levels show that the correct value of the diffusion length and its light-intensity dependence indeed differ to a significant degree from the results obtained with use of the previous method. A physical interpretation of the measured intensity dependence is also given, assuming a trap-controlled photocarrier transport model.