Metalorganic chemical vapour deposition of GaN layers on ZnO substrates using α-Al2O3 as a transition layer
This work addresses the instability of a ZnO substrate during metalorganic chemical vapour deposition (MOCVD) growth of GaN by using Al2O3 films deposited by atomic layer deposition (ALD) as a stabilizing transition layer on the Zn face of ZnO (0 0 0 1) substrates. A systematic study of Al2O3 films of different thicknesses (2–90 nm) under different ALDs and post-annealing conditions was carried out. However, this paper focuses on as-deposited 20 and 50 nm Al2O3 films that were transformed to polycrystalline α-Al2O3 phases after optimal annealing at 1100 °C for 10 min and 20 min, respectively. GaN layers were grown on ZnO substrates with these α-Al2O3transition layers by MOCVD using NH3 as a nitrogen source. Wurtzite GaN was observed by high resolution x-ray diffraction only on 20 nm Al2O3/ZnO substrates. Field-emission scanning electron microscopy showed a mirror-like surface, no etch pits and no film peeling in these samples. Room temperature photoluminescence showed a red-shift in the near band-edge emission of GaN, which may be related to oxygen incorporation forming a shallow donor-related level in GaN. Raman scattering also indicated the presence of a well-crystallized GaN layer on the 20 nm Al2O3/ZnO substrate.
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