Structural and optical studies in a-CdTe:O annealed films

Thin films of stable crystalline phases of the Cd-Te-O ternary system were prepared by thermal annealing in an Ar flux of amorphous CdTe:O films. The composition of the annealed films depended on the initial oxygen content in the film, the temperature and duration of the annealing process. The annealed films were characterized by means of x-ray diffraction spectroscopy and by optical absorption spectroscopy. The results show that for low oxygen content in the as grown film and high annealing temperature, the CdTe cubic phase is the predominant crystalline phase in the annealed film. For intermediate oxygen content in the as grown film, the result obtained after the annealing process is a composite material consisting in the mixture of both CdTe and CdTeO₃ crystallites. The variation of the annealing temperature produces changes in the size of the crystallites of both crystalline phases. It also can change the CdTe to CdTeO₃ proportion in the films. For low annealing temperature, the size of the CdTe crystallites in the film is <14 nm, the exciton Bohr diameter in CdTe. The optical characterization of the CdTe-CdTeO₃ composite material shows that this system is very suitable for the study of optical transitions in the electronic band structure of CdTe. Furthermore, for the films with the smallest CdTe crystallites it is possible to observe the effects in the CdTe band structure produced by the reduction in the size of the CdTe crystallites to a dimension comparable to the exciton Bohr diameter. The annealing process in the as grown films with high oxygen content produces films composed by a mixture of the oxide insulating phases CdTeO₃ and CdTe₂O₅.