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Optical and electronic properties of Bi-modified amorphous thin films of Ge20Te80-xBix

An investigation of the effect of addition of Bi impurities on the electrical, optical and thermoelectric properties of vacuum evaporated amorphous thin films of Ge20Te80-xBix (x=0, 0.19, 2.93 and 7.35) has been carried out. The synthesized thin films were characterized by X-ray diffraction and electron probe microanalysis. Analysis of the results revealed that Bi modification of the amorphous semiconducting thin films does not induce a clear-cut carrier sign reversal. This is in contrast to the observation of a p-n transition in bulk glassy compositions Ge20Te80-xBix at x=3.5. This dissimilarity may be explained by the fact that the bonding in the bulk glass and the corresponding thin film may be different. This fact is possibly responsible for creation of some additional Bi-induced p-type defects with increase in Bi concentration. The optical energy gap has been found to be approximately equal to twice the thermal activation energy in all the four compositions studied in this work.