Surface effects on photoconduction

The decrease in the normalized photocurrent at large a is probably caused by surface and interface recombination. The effective surface recombination states are, in turn, changed by band bending and surface treatments. It is not possible to exclude an influence of inhomogeneities along the film thickness. 

The analysis of the spectral dependence of /p in the presence of surface recombination should yield the minority-carrier diffusion length Lp and S, the ratio of the surface recombination velocity to the bulk diffusion velocity. DeVore (1956) presented an analysis using a single carrier model in which local charge neutrality is maintained by fast dielectric relaxation. Schetzina 

These experiments emphasize that an analysis of the photoconductivity requires consideration of surface recombination particularly when the film is thin and of high quality (large minority-carrier diffusion length). An example of the thickness dependence of the normalized photocurrent is shown in Fig. 14(Persans, 1980). 

2 X 10 electrons cm . The sample had been previously illuminated to prevent light-induced changes (Staebler-Wronski effect) from occurring during the measurements. 

Depending on the sign of F, one can produce an electron or a hole accumulation layer in the same sample. The small shift of the minimum conductance towards — F, shows that the ratio of the electron to the hole contribution to the conductivity under flat-band conditions increases 

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