Study on the structural and optical properties of Cd_1-xZn_xO layers enhanced by rapid thermal annealing

Cd_1-xZn_xO alloy layers were grown by a radio-frequency magnetron co-sputtering system. By X-ray diffraction measurement, the Cd_1-xZn_xO layers were observed to have a single-phase cubic structure at x ≤ 0.21. However, polycrystalline having mixed phases was observed at x = 0.29. It seems that the high miscibility of Zn flux in CdO leads to a large fluctuation in phase segregation caused by the hexagonal ZnO and cubic CdO structure within CdZnO. Therefore, the maximum Zn-composition ratio without phase segregation was found to be x = 0.21. The bandgap energies of the Cd_1-xZn_xO layers between x = 0 and x = 0.21 were distributed from 2.574 eV to 2.892 eV, respectively, while the bandgap energy of x = 0.29 was a slightly smaller value of 2.834 eV. After rapid-thermal-annealing (RTA) treatment at 600°C, the bandgap energies of each x component decreased, especially the x = 0.29 layer, which converted into the double phase of a cubic-and-hexagonal structure existing in the cubic CdO (111) and hexagonal ZnO (0001) phases. Consequently, reliable formation and crystallinity improvement of the Cd_1-xZn_xO layers were achieved through an x composition ratio below x = 0.29 and an RTA method of short-time thermal-annealing below 600°C.