Tuning the point-defect evolution, optical transitions, and absorption edge of zinc oxide film by thermal exposure during molecular beam epitaxy growth

We study the optical properties of ZnO films grown using plasma-assisted molecular beam epitaxy. We employ three different growth temperatures (T-growth) o f 298, 373, and 423 K. The XRD patterns and field-emission SEM images indicate that T-growth increment linearly increases the c-axis strain and average growth rate. The photoluminescence spectra show that T-growth increment from 298 to 373 K induces the blue-violet emission promoted by Zn interstitials (Zn-i). We also observe the blue emission promoted by the few O interstitials (O-i). However, this T-growth increment significantly reduces oxygen antisites (O-zn) and eliminates oxygen and zinc vacancies (V-o,V-zn). We find that T-growth increment to 423 K enhances O-i and eliminates Zn-i inducing the excellent blue emission the O-rich condition. Notably, the point defects suppress the exciton-exciton scattering emission for T-growth = 373 and 423 K. Furthermore, the Kubelka-Munk spectra show that the strain provokes the free-excitonic absorption edge redshift. Our result emphasizes the significant thermal dependences of point-defect evolution, blue emission, and excitonic absorption in ZnO film. This study opens a possible application of ZnO in tunable-emission blue LEDs.