Temperature-dependent optical properties of hexagonal and cubic MgxZn1-xO thin-film alloys

Both hexagonal and cubic MgxZn1-xO (0 less than or equal to x less than or equal to 0.6) films were grown on c-plane (0001) sapphire substrate at low temperature. X-ray diffraction measurements show that the cubic MgxZn1-xO films grow along the [111] direction while the hexagonal ZnO films grow along [0001]. The temperature-dependent optical properties of MgxZn1-xO films were measured by ultraviolet optical transmission with temperature variation from 10 to 300 K and analysed by theoretically fitting the optical absorption spectra. For MgxZn1-xO with 0 < x less than or equal to 0.51, only stable hexagonal phase was observed and the optical absorption edge red shifts with temperature increase monotonically. For MgxZn1-xO with x greater than or equal to 0.55, the crystal structure is cubic at 300 K. However, as measurement temperature decreases from 300 to 10 K an abnormality of the optical absorption is observed, which is attributed to the possible phase transition from cubic to hexagonal structure. The underlying physical mechanism for the crystal phase transition is attributed to the interaction of stress with stacking faults in the cubic MgxZn1-xO.