Low-temperature metallic behavior of amorphous MoO3–TeO2 thin films

Abstract

We report that vacuum-deposited amorphous MoO3–TeO2 films of different thickness (469–1185 nm) and compositions show metal-insulator transition (MIT) in the temperature (Tp) range 341–231 K (depending on thickness and composition). Interestingly, the corresponding thick bulk glass plates (∼0.2–0.5-mm thickness) of same compositions behave like the usual semiconducting transition metal oxide glasses over the entire range of temperature (80–400 K) and follow small polaron hopping (SPH) conduction mechanism. Temperature-dependent resistivity curves of the films showed peaks (maxima) around the respective MIT temperature Tp, which shifted to the lower-temperature region with the increase of MoO3 content. Fitting of the conductivity data of the film in the high-temperature (T>Tp) semiconducting phase shows a crossover around Tc(>Tp) from the Mott variable range hopping to SPH behavior, which is in sharp contrast to the behavior of the bulk glass. In these films, the estimated localization length (ξ) diverges as MIT is approached from the high-temperature phase. Low-temperature (T<Tp) magnetic-field-independent metallic part of the resistivity (ρ) data of higher (⩾60%)TeO2 containing films can be fitted with ρ=ρ0+ρ2T2 indicating the importance of electron-electron scattering (second term) mechanism of conduction. The corresponding metallic behavior of the comparatively high-resistive and lower (⩽50%)TeO2 containing film is, however, more complicated. All these different features of these glassy films suggest that their glass network structure is different from that of the corresponding bulk glass (indicating a change of glass network structure in the film phase), which is considered to be responsible for the metallic transition in the films.