Morphology, electrical, and optical properties of heavily doped ZnTe:Cu thin films

摘要

We report on a study of the physical properties of ZnTe:Cu films with Cu content up to ～12 at. % prepared using rf magnetron sputtering. The composition and lateral homogeneities are studied using X-ray photoelectron spectroscopy (XPS). Atomic force microscopy measurements on films deposited at different substrate temperatures (up to 325 ℃) yielded activation energy of 12kJ/mole for the grains growth. The results of XPS and electrical and optical measurements provide evidence for the formation of the ternary zinc copper telluride alloy in films containing Cu concentration above ～4 at. %. The XPS results suggest that copper is incorporated in the alloy with oxidation state Cu~(1+) so that the alloy formula can be written Zn_(1-y)Cu_y Te with y=2-x, where x is a parameter measuring the stoichiometry in the Cu site. The formation of this alloy causes appreciable shift in the binding energies of the XPS peaks besides an IR shift in the energy band gap. Detailed analysis of the optical absorption data revealed the presence of two additional transitions, besides the band gap one, originating from the Γ_8 and Γ_7 (spin-orbit) valence bands to a donor level at ～0.34 eV below the Γ_6 conduction band. This interpretation yields a value for the valence band splitting energy Δ ≌ 0.87 eV independent of copper concentration. On the other hand, the mechanism of formation of the alloy is tentatively explained in terms of a point defect reaction in which substitutional Cu defect Cu_(Zn) is also created. Assuming that substitutional Cu is the dominant acceptor in the Zn rich alloy as in ZnTe, its formation energy was determined to be 1.7eV close to the theoretical value (1.41 eV) in ZnTe.