Thermoreflectance studies in CdNiTe nanocrystalline films

CdNiTe ternary semiconductor thin films were deposited using the cathodic erosion by radiofrequency technique (r.f. sputtering), on 7059 Corning glass substrates. Cd_1−xNi_xTe targets with different Ni compositions in the range 0<x<0.15 were used. Structural analysis in these samples using SEM and X-ray diffraction have shown that films are polycrystalline with grain sizes between 26 and 35 nm; for higher Ni-content, films have smaller grain sizes. As-grown samples and thermal-annealed films in an inert atmosphere at temperatures of 300 and 400 °C were studied using the thermoreflectance spectroscopy (TR) at room temperature. From these measurements the bandgap energy variation as a function of the nanocrystallite size and the annealing temperatures was obtained. From the TR spectra a systematic shift to higher energies of the E₀-point as the grain size decreases has been measured, and we have interpreted this result as due to the CdTe–Ni alloying process added to a quantum-size effect in which the nanocrystallites act as quantum dots. We discuss the TR lineshapes and their evolution with thermal annealing.