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 degrees C. A new stoichiometric Cu7Te4 phase (delta') with Z = 1 is proposed, and it is present in all the temperatures studied. Low-angle peaks (2 theta < 20 degrees) of Glancing Incidence X-ray Diffraction (GIXD) patterns are consistent with the formation of commensurate phases (forms alpha') through variations in the c parameter whose value can be as large as 25.47 angstrom. The base modulation vector is the c parameter of the unit-cell of phase delta' that, in this work, resulted in a value of 3.61 angstrom. The origin of the observed commensurate phases has been ascribed to an occupancy-wave modulation. Whereas at high temperatures (Ts > 250 degrees C), the major phase is delta' (Cu7Te4 or Cu2-xTe with x = 0.25), the predominant phase, for Ts < 250 degrees 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 degrees 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.

Automated summary

The study investigated how the substrate temperature affects the growth of Cu2-xTe films using the RF sputtering technique, discovering different structural phases at various temperatures. A new stoichiometric Cu7Te4 phase, delta', was identified at all temperatures, while higher temperatures favored the delta' phase with Cu7Te4 or Cu2-xTe composition and lower temperatures favored the commensurate superstructure 7C with higher Cu/Te ratio. The transition between these phases was observed at around 250 degrees C.