The surface phenomena in glassy and nanocrystallyne chalcogenides at gas sensing

Abstract

The surface phenomena in chalcogenide glasses, including those caused by gas adsorption are extensively reviewed and discussed. A detailed quantitative analysis is made on experimental data taking on glassy and nanocrystalline thin films of As2S3 Ge8 – Te system, physically grown in vacuum. Particularly are reported the measurements of the frequency dependence of the AC conductivity of these films in the frequency range 5 Hz to 13 MHz, in both dry synthetic air and its mixture with a controlled concentration of different gases. The AC conductivity in dry synthetic air, being frequency independent until ~ 104 Hz, increases as σ (ω) ~ ωn at frequences between 104 <ω <106 Hz, with an exponent factor n ≈ 0,7. The dependence σ (ω) exhibits a shouldar at frequency ~ 106 Hz, followed by a sharp increasing of conductivity with an exponent factor n ≈ 1,8. Such behaviour of AC conductivity fits the generaly accepted model [1] of charge transport in desordered materials, which implies both extended states above mobility edges and localized states in the gap, including the states at the Fermi level with concentration estimated in this work as N(EF) = 1.3 •1021 eV-1 cm-3.