Transient photoconductivity for the identification of spatial inhomogeneities in the defect density in amorphous silicon

We achieved the probing of near-surface regions in hydrogenated amorphous silicon by directing green-pulsed laser light with a short absorption depth either through the air/silicon or the glass/ silicon interface. Transient photoconductivity under these conditions results in different decay behaviors of the photocurrent when the sample is illuminated from either side. By application of a Fourier transform technique on the photocurrent decay data we identify spatial inhomogeneities in the deep-defect density that are responsible for these differences. When compared with the defect density in the bulk of the film, obtained from probing the sample with homogeneously absorbed light, we can thus reveal a possible increase in the defect density towards either the air/silicon or silicon/substrate interface which remains undiscovered by other experimental techniques.