Localized states and carrier transport in amorphous chalcogenide semiconductors

Abstract

Electronic transport has been investigated in ${\mathrm{As}}_{2}$(${\mathrm{Se}}_{1/3}$ ${\mathrm{Te}}_{2/3}$${)}_{3}$ and Ge(${\mathrm{Se}}_{1/3}$${\mathrm{Te}}_{2/3}$ ${)}_{2}$ amorphous semiconductors through measurements of photoconductivity, optical absorption, dark conductivity, thermoelectric power, and field effect. Comparison of surface and volume measurements on bulk glassy material with surface measurements on thin sputtered films indicates no electronically detectable difference between surface and volume properties. Analysis of photoconductivity data in terms of a previously developed model indicates a density of localized recombination centers near the band edges with a density of about ${10}^{19}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ ${\mathrm{eV}}^{\ensuremath{-}1}$ within 0.1-0.2 eV from the edges. Combination of thermoelectric power and conductivity data indicates $p$-type conductivity with a thermally activated mobility. Correlation of photoconductivity decay with field-effect measurements indicates a density of states of about ${10}^{19}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ ${\mathrm{eV}}^{\ensuremath{-}1}$ near the Fermi level, decreasing by at least one order of magnitude for energies 0.15 eV closer to the band edge.