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Keywords:

  • chalcogenides;
  • electrical properties;
  • microstructure;
  • optical properties;
  • phase-change materials;
  • phase transitions

Abstract

Thin films of GeTe, Ge2Sb2Te5 and Sb2Te3 have been synthesized from their respective polycrystalline bulk by thermal evaporation. X-ray diffraction study of the films confirms the amorphous nature of GeTe and Ge2Sb2Te5 films, but as-deposited Sb2Te3 film was in the crystalline form. Structural analysis has been performed for the annealed films above their crystallization temperature by XRD, which was evaluated by temperature-dependent sheet resistance measurements. The Sb2Te3 film, which was already crystalline, undergoes a small transition at 110 °C. For optical studies, the transmittance and the reflectance spectra were measured over the wavelength ranges 400–2500 nm using UV-VIS-NIR spectroscopy. The optical bandgap, reflectance and optical contrast are also presented for thermally evaporated thin films. The dc electrical conductivity of the as-deposited films has been measured as a function of temperature below the phase-transition temperature, and increases exponentially with temperature. The value of the activation energy, calculated from the slope of lnσ versus 1000/T, is found to decrease from one end binary GeTe to pseudobinary Ge2Sb2Te5 and then to the other end binary Sb2Te3 films. On the basis of the pre-exponential factor, the type of conduction in these films has also been discussed in the measured temperature range.