Properties of r.f. sputtered cadmium telluride thin films

Abstract

CdTe thin films were prepared using r.f. magnetron sputtering in an Ar atmosphere. Substrate temperatures in the range 100–320 °C were used. XRD results showed that the films are amorphous below 200 °C while above 200 °C the firms were polycrystalline with cubic structure and grains preferentially oriented along the [1 1 1] crystallographic direction. SEM measurements showed significant enhancement of crystallite size with increase of T _s or with post-preparation annealing above 400 °C. The 5 K photoluminescence spectrum showed a broad (FWHM=80 meV) band with a maximum at 1.538 eV. This band showed significant narrowing after annealing above 400 °C suggesting that it originates from transitions involving grain boundary defects. The refractive index n was determined from the interference pattern of the optical transmission. The results agree with the values of n calculated using the Jensen theory. The absorption coefficient was determined for photon energies hν≥E _g (the energy bandgap) from the optical transmission spectra in the absorption region using the Swanepoel theory. Several direct and indirect allowed optical transitions were identified. It was found that the transitions can be grouped into four main allowed transitions (two direct; E _o, E ₃ and two indirect; E ₁, E ₂) whose energy values vary from one sample to another due the quantum size effect associated with small grain size. The main transitions are: E _o (1.50–1.77 eV) assigned to Γ₈ valence band (VB)→Γ₆ conduction band (CB) transition, E ₁ (1.84–2.05 eV) assigned to L_4,5(VB)→Γ _i transition where Γ _i is an impurity level at 1.2 eV above the Γ₈ (VB), E ₂ (2.37–2.49 eV) assigned to L_4,5 (VB)→Γ₆ (CB) transition and E ₃ (2.25–2.55 eV) assigned to Γ₇ (VB)→Γ _i transition. The impurity is attributed to native centers or grain-boundary-related defects.