Properties of flash evaporated thin films of In 2Se 3

Abstract

In 2Se 3 thin films were formed by flash evaporation of powdered In 2Se 3 single crystals onto pyrex or mica substrates whose temperature T s during film deposition varied between 20° and 250 °C. The influence of T s and of the postdeposition vacuum heat treatment on the properties of the films were examined. The results of electrical resistance measurements led to the conclusion that reordering of the film structure took place in the first cycle of thermal treatment; electrical conductivity σ versus temperature data then became reproducible, the relationship between σ and T being of the intrinsic form: σ = σ 0 exp(− ΔE/2 kT). An explanation is given of why the activation energy Δ E is not always equal to the optical energy gap value. Large agglomerates of defects present in the films formed at lower substrate temperatures, which cannot be removed by post-deposition heating up to 260 °C, may be assumed to account for these differences. It can also be assumed that the transition from the α- to the β-phase in In 2Se 3 thin films occurs at 200 °C, as in the case of single crystals. The films presumably remain in the β-phase when cooled back to room temperature. The optical gap of β-In 2Se 3 films determined from optical absorption data was found to be 1.39 eV, which is close to the single-crystal value.