Thickness dependence of photoelectrical properties of intrinsic amorphous silicon

The stable and transient photoconductivities σ_ph are measured in intrinsic amorphous silicon a-Si:H of different thicknesses. The results show that the surface layer is more photosensitive than the bulk. The optical properties of a-Si:H with varying thickness of film are also determined. In a certain range of thickness of film, the absorption coefficient α and optical gap E_opt increase with reducing thickness. Both create the effect of increasing photoconductivity. Experiments indicate that thinner intrinsic samples have a higher activation energy E_a and lower dark conductivity σ_D, owing to surface band bending and the existence of a surface layer. The short decay time of electrons, about 10^-8 s, confirms the carrier loss from deep states, as revealed by the transient photoconductivity data. This result is also confirmed by photoluminescence.