Polymorphic transformation in epitaxial CdS films

Abstract

Changes in the structure of vacuum-evaporated CdS films grown epitaxially on single crystal substrates of rock salt, mica and silver have been investigated by electron diffraction at various substrate temperatures and at different values of laterally applied electric field. Epitaxial growth of the hexagonal (wurtzite) phase is obtained on mica and rock salt at room temperature, and on Ag at about 170 °C. As the substrate temperature is raised, epitaxial growth of the cubic (sphalerite) phase occurs. At substrate temperatures of about 200 °C for rock salt and 320 °C for mica, the structure is nearly completely cubic. Annealing at elevated temperatures subsequent to deposition at room temperature has little effect on the hexagonal structure of the film. At still higher substrate temperatures, and also for films thicker than about 1000 Å, mixed cubic and hexagonal structures are found, followed by the enhancement of the hexagonal phase. Only mixed structures are obtained on Ag (111) substrate at elevated temperatures. The cubic phase is also obtained at room temperature if an electric field ∼100 V/cm is applied laterally during the initial stages of the deposition of CdS films on rock salt. At thicknesses above 100 Å, the field has little effect, although the transformed cubic structure continues to grow mixed with the hexagonal. Thus the cubic phase grows epitaxially only in a limited range of substrate temperatures and film thicknesses and also under applied lateral electric field.