Effect of Bi addition on the electrical properties of a-Ge-Se-In thin films

This paper reports on the steady state and transient photoconductivity of Ge20Se70-xIn10Bix (x = 2, 4, 6, 8, 10) thermally evaporated thin films. The dark conductivity (sigma(d)) increases with the increase in the Bi content. The value of sd increases slowly from x = 2 to x = 6, but for x >= 8 the increase is quite abrupt. Photoconductivity (sigma(ph)) also increases with the increase in the Bi content. The dark activation energy (Delta E-d) decreases with the increase in the Bi content. There is a drop in the photosensitivity (sigma(ph)/sigma(d)) of films with the increase in Bi addition. The result of the intensity dependent steady state photoconductivity (sigma(ph)) follows the power law with intensity (F), i.e. sigma(ph)alpha F-gamma where the value of gamma is close to 0.5, suggesting the predominance of bimolecular recombination in these materials. The rise and decay of photocurrent show the increase in the differential lifetime (tau(d)), with the increase in the Bi content, indicating a delay in the recombination rate of photo-generated charge carriers. This change is governed by the photo-generated carriers trapped in the intrinsic defects or light induced defect creation through structural changes. One can therefore also expect that the density of states increases with the increase in the Bi content in amorphous thin films. The dispersion parameter (alpha) of the localized state energy distribution, determined from the decay curves, is also reported.