Enhancement in optical and structural properties of Zn_(0.85)Mg_(0.15)O nanorods over thin films synthesized by hydrothermal chemical treatment

摘要

We are reporting an enhancement in optical properties by changing the structure of Zn_(0.85)Mg_(0.15)O thin films through formation of crystalline hexagonal nanorods. Zn_(0.85)Mg_(0.15)O thin films were deposited on Si (100) substrate using dielectric sputter followed by annealing in oxygen ambient at temperatures of 700, 800 and 900°C for 10 seconds to reduce oxygen vacancies defects. Deposited thin film annealed at 900 °C (sample A) measured highest peak intensity and it was subjected to controlled the hydrothermal bath conditioning for forming hexagonal nanorods. Four samples were dipped in 2 different solutions with variable molar ratio of zinc nitrate hexahydrate and hexamethylentetramine for 2 and 3 hours, respectively. Samples processed in solution 1 (1:1) ratio for 2 and 3 hours were named B and C and those in solution 2 (2:1) were D and E, respectively. Photoluminescence measurement at 18K demonstrates exciton near-band-edge (NBE) emission peak at 3.61eV from Zn_(0.85)Mg_(0.15)O sample A whereas other samples exhibited slight blue shift along with bimodal peaks. The other peak observed at lower energy 3.43eV corresponds to transitions due to presence of ZnO phase in Zn_(0.85)Mg_(0.15)O. All samples compared to sample A exhibited more than 10 times increment in peak intensities with sample B producing the highest (～ 20 times). Nanorods formation was confirmed using cross-sectional SEM imaging. X-ray diffraction measurements revealed that all Zn_(0.85)Mg_(0.15)O samples had (002) preferred crystal orientation with peak position at 34.7°. All nanorods samples measured lower reflectance compared to sample A, indicating high absorption in nanorods due to high scattering of light at the nanorods surface.