Photoconductivity and density of states for amorphous GeTe

Abstract

GeTe films evaporated onto substrates held at room temperature are amorphous and exhibit properties more characteristic of chalcogenide glasses than of crystalline GeTe. The resistivity of such films increases exponentially with reciprocal temperature, the activation energy being 0.30-0.35 eV. We have measured both optical absorption and photoconductivity for these films over a wide range of temperatures. The absorption coefficient is found to vary as (hν-E)2hν at higher photon energies, Eg being 0.70 eV at 295°K and 0.77 eV at 77°K, while at lower photon energies the absorption follows an exponential variation with slope independent of temperature. These results are used to infer a density of states for amorphous GeTe. Photoconductivity is observed down to photon energies of 0.2 eV near room temperature; however, the strong temperature dependence of photoconductivity for photon energies below about 0.6 eV distinguishes the response in this region as arising from a different mechanism from that for photon energies greater than Eg. The observed magnitudes of photoresponse, rise times, and intensity dependences are seen to be consistent with a Mott-Cohen model using the inferred density of states. It is proposed that the observed low-energy photoconductivity may result from a perturbation of the mobility edge by trapped charge. © 1970 The American Physical Society.