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

Abstract

We study the optical properties of ZnO films grown using plasma-assisted molecular beam epitaxy. We employ three different growth temperatures (Tgrowth) of 298, 373, and 423 K. The XRD patterns and field-emission SEM images indicate that Tgrowth increment linearly increases the c-axis strain and average growth rate. The photoluminescence spectra show that Tgrowth increment from 298 to 373 K induces the blue-violet emission promoted by Zn interstitials (Zni). We also observe the blue emission promoted by the few O interstitials (Oi). However, this Tgrowth increment significantly reduces oxygen antisites (OZn) and eliminates oxygen and zinc vacancies (VO, VZn). We find that Tgrowth increment to 423 K enhances Oi and eliminates Zni inducing the excellent blue emission the O-rich condition. Notably, the point defects suppress the exciton-exciton scattering emission for Tgrowth = 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.