【International Papers】On the structural and bandgap properties of mist-CVD-grown κ-Ga₂O₃ post continuous temperature annealing

      Researchers from the King Abdullah University of Science and Technology have published a dissertation titled "On the structural and bandgap properties of mist-CVD-grown κ-Ga₂O₃ post continuous temperature annealing" in AIP Advances.

Abstract

      We investigate the continuous annealing of orthorhombic κ-Ga₂O₃ films on AlN/c-plane sapphire templates grown by Mist Chemical Vapor Deposition (mist-CVD) using in situ high-temperature x-ray diffraction (HT-XRD). Increasing the annealing temperature from 400 to 1100 °C in both vacuum and ambient air reveals a phase transition onset at 825 °C. High-resolution transmission electron microscopy and XRD demonstrated that annealing within the stability window of 650 to 775 °C effectively improves the crystal quality of the κ-Ga₂O₃ thin film. Optical transmittance and low-loss electron energy loss spectroscopy (EELS) show the pristine film’s bandgaps to be 4.96 and 4.67 eV, respectively, with reduced bandgaps in annealed films due to increased defect density. EELS-derived optical joint density of states indicates that air-annealing fosters sub-bandgap radiative processes, while vacuum annealing suppresses them, qualitatively correlated with the observed photoluminescence intensity variations. The results of this comprehensive high-temperature annealing study offer crucial insight into the influence of annealing ambient conditions on the crystallographic properties of κ-Ga₂O₃ films and the associated evolution of extended sub-bandgap states.

FIG. 1. (a) HR-TEM image showing the κ-Ga₂O₃ film and the γ-Ga₂O₃ nucleation layer along with the FFT diffraction patterns for both regions. (b) SAED patterns of κ-Ga₂O₃ film. (c) STEM image of the κ-Ga₂O₃ film and a magnified view in (d) showing the atomic distribution and matching with the modeled crystal structure in the figure inset.

FIG. 2. XRD coupled 2θ-ω scans of κ-Ga₂O₃ films on AlN/sapphire templates at high-temperature treatments under (a) vacuum and (b) ambient air, and (c-d) the corresponding intensity and FWHM values of the κ-Ga₂O₃ (004) peak.

DOI：

https://doi.org/10.1063/5.0233281