Growth of stoichiometric indium sulfide films by thermal evaporation: Influence of vacuum annealing on structural and physical properties

摘要

The paper describes studies on the compositional, structural and optical characteristics of indium sulfide films deposited by thermal evaporation of chemically synthesized indium sulfide powders and subsequently annealed in vacuum in 523-773 K temperature range. Rutherford backscattering spectrometry measurements and depth profiling of sulfur by ~(32)S(p, p'γ)~(32)S nuclear resonance reaction analysis show that the pristine films possess the nominal atomic composition of indium sulfide (In_2S_3) which remains invariant during vacuum annealing. The pristine as well as vacuum annealed films do not possess, as measured by ~(16)O(α, α)~(16)O resonant scattering, any detectable oxygen contamination. The pristine films are amorphous but crystallize into β-In_2S_3 (tetragonal) phase on annealing. The pristine films and those annealed at high temperatures (＞673 K) exhibit about 65% transmission in visible region. These films are characterized by both indirect and direct band gaps which increase with increase in annealing temperature. The band gaps lie in 1.9-2.2 eV and 2.6-3.3 eV regions respectively. The electrical resistivity of the pristine films is about 5 × 10~5 Ω.cm which decreases by about an order of magnitude on annealing in vacuum in the 523-623 K region. It, however, increases with subsequent increase in annealing temperature. The structural and the physical properties of the films are explained in terms of the defective spinel structure of β-In_2S_3 and the inclusion of Na (from the glass substrate) into the lattice of In_2S_3 at higher annealing temperatures.