Thermal stability of rhombohedral α- and monoclinic β-Ga2O3 grown on sapphire by liquid-injection MOCVD

We investigate the thermal stability of single-phase undoped epitaxial rhombohedral alpha-and bi-axially-symmetric, monoclinic beta-Ga2O3 thin films grown on sapphire substrates by liquid-injection metalorganic chemical vapor deposition (LI-MOCVD) by means of in-situ high-temperature X-ray diffraction (HT-XRD) and room temperature (RT) cathodoluminescence (CL). Ga2O3 layers were vacuum annealed up to 1100 degrees C, while monitoring the respective 3030 and 603 reflections of alpha-and beta-phase Ga2O3. alpha-Ga2O3 layers were found to withstand the vacuum annealing at 700 degrees C/20 min, 750 degrees C/10 min, and 800 degrees C/8 min without notable degradation, however an indication of reversible biaxial in-plane compressive strain was observed for annealing temperatures close to 800 degrees C. Annealing at temperatures above 825 degrees C led to complete layer degradation and partial phase trans-formation to (010) beta-Ga2O3. beta-Ga2O3 layers showed no notable degradation or structural changes after the annealing at 1100 degrees C/30 min, however an indication of reversible biaxial in-plane tensile strain was observed for prolonged annealing at 1100 degrees C. RT CL spectra revealed the Ga2O3 emission consisted of several distinctive luminescence bands (red, green, blue, and two UV bands). Relatively weak but distinctive signal maxima with energy close to the expected bandgap energies of alpha-and beta-Ga2O3, at 5.3 and 5.0 eV, respectively, were observed and attributed to their near band-edge (NBE) emissions.