Conduction mechanisms in amorphous tellurium films

Abstract

In situ dc measurements are made on UHV deposited 100 nm thick tellurium films in sandwich configurations between two copper electrodes. Space-charge-limited conduction through amorphous films held at 125 K is believed to be a dominant mechanism, partially perturbed by the effects due to the velocity saturation of carriers and the migration of electrode material into the Te film. The high value of 5.3×1028 eV−1 m−3 for the density of states, N(Ef), in the energy gap is indicative of the existence of degeneracy in the band structure of the film. On the basis of the temperature-conductivity characteristics, three temperature regimes are assumed to exist, each attributed to a characteristic conduction mechanism: (1) variable range hopping over an optimum distance of 0.55 nm at T⩽225 K, (2) hopping between states localised at Te ions 225 K < T < 350 K and (3) band-to-band conduction with an activation energy of 0.32±0.01 eV at temperatures > 350 K. The high temperature activation energy of 0.32 eV compares reasonably well with a value of 0.33 eV for the optical band gap of polycrystalline tellurium films.