Bulk photoconductivity of a-Ge:H films from spectral measurements

The authors prepared a-Ge:H films with different thicknesses and measured their spectral photoconductivity, which was found to increase with both film thickness and light wavelength. As an explanation, reduced photoconductivity under the illuminated surface is assumed. The ημτ-product of a film is given by its photoconductivity measured between ohmic contacts and represents a property of the photoelectrons. A two-layer model ascribes different ημτ-products to a front layer of the film and to the rest of it, called the bulk layer. By fitting the two-layer model to the present experiments the authors find a front-layer thickness of 55 nm, a surface ημτ-product of 1×10^-8 cm²/V, and a bulk-layer ημτ-product of 5×10^-7 cm² /V. The effective ημτ-product, determined under the assumption of a homogeneous film, approaches the bulk value when both film thickness and light wavelength reach a value of 1 μm. No light degradation of the a-Ge:H films is found