Optical and structural studies of phase transformations and composition fluctuations at annealing of Zn1-xCdxO films grown by dc magnetron sputtering

Ternary Zn1-xCdxO (x <= 0.12) alloy crystalline films with highly preferred orientation (002) have been successfully deposited on sapphire c-Al2O3 substrates using the direct current (dc) reactive magnetron sputtering technique and annealed at temperature 600 degrees C in air. The structural and optical properties of Zn1-xCdxO thin films were systematically studied using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), micro-Raman and photoluminescent (PL) spectroscopy. XPS measurements clearly confirmed Cd incorporation into ZnO lattice. XRD data revealed that the growth of wurtzite Zn1-xCdxO films occurs preferentially in the (002) direction. Also, when the Cd content is increased, the XRD peaks shift towards smaller angles and the full width at half-maximum of the lines increases. When the Cd content increases, A(1)(LO) (Zn1-xCdxO)-like Raman modes show composition dependent frequency decrease and asymmetrical broadening. The near band-edge PL emission at room temperature shifts gradually to lower energies as the Cd content increases and reaches 2.68 eV for the highest Cd content (x = 0.12). The analysis of NBE band emission and Raman A(1)(LO) (Zn1-xCdxO) mode shows that at a higher Cd content the coexistence of Zn1-xCdxO areas with different concentrations of Cd inside the same film occurs. The presence of CdO in annealed Zn1-xCdxO films with the higher Cd content was confirmed by Raman spectra of cubic CdO nanoinclusions. The XRD data also revealed phase segregation of cubic CdO in annealed Zn1-xCdxO films (T-ann = 600 degrees C) for x >= 0.013.