Properties of Cu_x(CdTe)_yO_z thin films: Composition-dependent control of band gap and charge transport

Thin films of Cu_x(CdTe)_yO_z were grown by reactive rf co-sputtering using a simple method, in which all the growth parameters were kept constant except for the power applied to the copper target. This technique allowed controlling the relative concentrations of the chemical elements since the Cu addition to CdTe catalyzed, nearly in a one-to-one ratio, the incorporation of oxygen. As a result, it was possible to tailor the structural, optical and electrical properties of the CdTe host by adding Cu from 0 to 25 at.% for different runs. For films with 25 at.% of Cu and O, the crystalline structure changed from a hexagonal CdTe-like to a hexagonal Cu ₂Te-like. The optical band gap, obtained from UV-Vis spectroscopy, could be varied between 1.48 and 1.62 eV. All as-grown samples were p-type with a resistivity that could be controlled, depending upon the Cu/O concentration, over a wide range between 8 × 10^-4 and 2 × 10² Ω cm without any post-growth activation treatment. These results are in agreement with a synergy effect of Cu and O when modifying the CdTe properties. The chemical properties were analyzed by energy dispersive and X-ray photoelectron spectroscopies. The X-ray diffraction and high-resolution transmission electron microscopy experiments showed that the films consist of grains with CdTe and/or Cu₂Te structure coexisting with nanometric particles of binary or ternary metallic oxides such as TeO₂ and CdTeO₃, and for higher Cu concentrations, of Cu_2-xTe. Raman scattering experiments showed that the long wavelength phonons of the CdTe structure are not significantly affected by the incorporation of Cu and O since the frequency of the CdTe-like longitudinal optic mode remains basically unchanged.