CdTeO x to CdTeO3 structural phase transition in as-grown polycrystalline films by reactive sputtering

Abstract

Polycrystalline thin films of CdTeOx (x<3) and CdTeO3 were grown at 400 °C by rf-reactive sputtering. The CdTe oxide films were elaborated in an Ar–O2 atmosphere using different oxygen flow rates in the 5–11 SCCM (SCCM denotes cubic centimeter per minute at STP) range. The structural properties of the samples were studied by x-ray diffraction and micro-Raman spectroscopy. The diffraction patterns show that the films grown with oxygen flows between 5 and 10.5 SCCM present a crystalline structure similar to that of pure CdTe films, that is, a mixture of cubic zinc-blende-type and hexagonal wurtzite-type structures. However, the diffraction patterns of samples grown under an oxygen flow rate of 11 SCCM are strikingly different and in agreement with that of cubic CdTeO3, indicating that a structural phase transition was achieved. These results are in accordance with those obtained by micro-Raman spectroscopy, where the spectra of the CdTeOx samples grown with oxygen flows below 11 SCCM show an intense LO CdTe-like peak at 166 cm−1, which is no longer observable for the CdTeO3 sample. Instead, the Raman spectrum in this case is dominated by two broad bands in the 550–800 cm−1 range, which have been ascribed to the vibrational modes of TeO3 structural subunits. The optical properties, as determined by optical absorption and photoreflectance spectroscopies, are in correspondence with the structural and chemical changes induced by the incorporation of oxygen. The band gap varied between 1.55 and 3.3 eV, the last value corresponding to CdTeO3 films.