CRYSTAL-STRUCTURE AND ENERGY-GAP OF CDTE THIN-FILMS GROWN BY RADIO-FREQUENCY SPUTTERING

We have investigated the influence of structural characteristics on the band gap of rf sputtered CdTe thin films grown at substrate temperatures in the 69-232-degrees-C range. The results of scanning electron microscopy and x-ray diffraction studies indicated that the films are a polycrystalline mixture of cubic and hexagonal phases with preferential growth cf columnar type parallel to the cubic [111] direction. The band gap of the films was obtained from photoreflectance spectroscopy experiments carried out at room temperature. It was found that the films had a band gap larger than that of CdTe single crystals. This result has been correlated with the existence of lattice strain, quantum size effects, and hexagonal phase regions. By using theoretical models it was possible to estimate the contribution to the band gap shift due to strain and quantum size effects obtaining results in good agreement with the experiment. The study of annealed samples indicated that the effects of thermal treatments were to promote the change of the hexagonal phase to cubic, increase grain size, and shift the band gap towards lower energies reducing its difference with respect to that of single crystals.