ON THE ELECTRICAL AND OPTICAL PROPERTIES OF NANOCRYSTALLINE CdTe THIN FILMS∗

In the paper, the experimental results regarding the temperature dependence of the electrical conductivity in the temperature range 300 – 550 K, the transmission spectra in the 500 – 1400 nm wavelength range and reflexion spectra in the 1.5 – 6.5 eV photon energy range, as well as the effect of the heat treatment on the spectra, for CdTe thin films having a nanocrystalline structure are presented. The CdTe films (d = 100 nm – 1700 nm) were prepared by the quasi-closed volume technique. The substrate temperature ranged between room temperature and 600 K and the source temperature ranged from 1000K to 1270 K. The XRD investigations revealed that, depending on the deposition conditions, both the textured and quasiamorphous CdTe films may be obtained. The films deposited onto unheated substrates with greater source temperature present a quasiamorphous structure and an excess of tellurium atoms, which precipitate in crystalline form during the heat treatment. It was found that these films present an irreversible temperature dependence of the electrical conductivity and a decrease of film transmitance. These results are correlated with the film recrystallisation during the annealing process. The effect of the heat treatment on the transmission and reflection spectra of the films is also investigated