Phase transformation and disorder effect on optical and electrical properties of Zn3P2 thin films

Abstract

The phase transformation of zinc phosphide (Zn3P2) thin films was detected through isochronal annealing process. The effects on isochronal annealing on the internal structural, optical and electrical properties of deposited Zn3P2 thin films have been discussed. The films were prepared by thermal evaporation under constant preparation conditions of vacuum 1.3×10−5Torr, substrate temperature (300K), rate of deposition (∼1nm/s) and film thickness (480nm). The annealing process was carried out under vacuum for 2h at different temperatures ranging from 373 to 623K. X-ray diffraction patterns showed that the as-deposited films and those annealed at temperatures less than 623K exhibit amorphous structure, while the films annealed at 623K showed tetragonal polycrystalline structure. The optical transmission and reflection spectra were measured at the wavelength range of 190–2500nm. The absorption coefficient spectra and the degree of disorder as measured from the absorption edge were determined. The indirect and direct optical energy band gaps were evaluated for indirect allowed and direct allowed transitions for amorphous and polycrystalline films, respectively. The refractive index no increases with raising the annealing temperature which refers to more condensation in the material. The electrical resistivity for Zn3P2 films decreases exponentially with raising the annealing temperature up to 623K as influenced by structure transformation and decreasing the degree of disorder in the films.