Carrier-type reversal in metal modified chalcogenide glasses: Results of thermal transport measurements

Abstract

It is known that chalcogenide glasses, in general, are p-type semiconductors. This is attributed to the following two reasons. (i) The number of electrons excited above the conduction band mobility edge is smaller than the number of holes excited below the valence band mobility edge. (ii) The lifetime of the free holes excited from positive defect states is higher than the lifetime of free electrons excited from negative defect states. Moreover, chalcogenide glasses are rather insensitive to impurity doping because their Fermi levels are pinned near the middle of the band gap by valence alternation pairs. However, it is found that the chemical modification of certain chalcogenide glasses by metallic elements such as lead and bismuth changes the conduction to n type at specific metal concentrations. This phenomenon, called carrier-type reversal (CTR), is explained in terms of the unpinning of the Fermi level and the consequent enhancement in electron concentration in the medium. In this work we report the results of our measurement of the thermal properties-thermal diffusivity, effusivity, conductivity and heat capacity—on four glass systems that are known to undergo CTR at specific metal concentrations. The photopyroelectric technique has been used to make these measurements on bulk samples prepared by melt quenching. Four series of glasses, PbxGe42−xSe58 (x=0-20), Pb20GeySe80−y (y=17-24), BixGe20Se80−x (x=2-12) and PbxIn25−xSe75 (x=0-15) which are known to undergo CTR at specific metal concentrations, have been subjected to the investigations reported in this work. It is found that all the above thermal properties show anomalous variations at the metal concentrations at which CTR occurs. The results are explained in terms of the enhancement in electron concentration during CTR in the material.