Effect of substrate temperature on the crystalline phases of Cu2-xTe films grown by RF sputtering

The effect of the substrate temperature (Ts) on the growth of Cu2-xTe films using the RF sputtering technique has been studied through a structural assessment of the different trigonal (space group P3m1) phases encountered in the range 100–350 °C. A new stoichiometric Cu7Te4 phase (δ′) with Z = 1 is proposed, and it is present in all the temperatures studied. Low-angle peaks (2θ < 20°) of Glancing Incidence X-ray Diffraction (GIXD) patterns are consistent with the formation of commensurate phases (forms α′) through variations in the c parameter whose value can be as large as 25.47 Å. The base modulation vector is the c parameter of the unit-cell of phase δ′ that, in this work, resulted in a value of 3.61 Å. The origin of the observed commensurate phases has been ascribed to an occupancy-wave modulation. Whereas at high temperatures (Ts > 250 °C), the major phase is δ' (Cu7Te4 or Cu2-xTe with x = 0.25), the predominant phase, for Ts < 250 °C, is the commensurate superstructure 7C with higher [Cu]/[Te] ratio (x < 0.25). The transition between these two thermal regimes is stated at a temperature of about 250 °C. Maintaining a constant global composition, the devaluation of [Cu] in films, with the temperature above the transition range, is explained in terms of diffusion of copper downward the film and towards its edges.