Growth and characterization of Ga-doped ZnO layers on a-plane sapphire substrates grown by molecular beam epitaxy

Gallium-doped ZnO epitaxial layers were grown on a-plane sapphire substrates by molecular beam epitaxy (MBE) at various Ga cell temperatures from 350°C to 450°C. The ZnO layers grown on a-plane sapphire were c-oriented without any trace of the 30° rotation domains often observed in ZnO on c-plane sapphire. The Ga concentration in Ga-doped ZnO increased from 4×1016 to 7×1018 cm−3 with increasing Ga cell temperature. The activation ratio of Ga was about unity when the Ga concentration exceeded 3×1017 cm−3. The photoluminescence (PL) spectra of Ga-doped ZnO were dominated by an emission at 3.362 eV which can be assigned to emission of exciton bound to Ga-related neutral donors. The intensity of this emission was maximum when the Ga concentration was 2×1018 cm−3. The high crystalline quality of the Ga-doped ZnO epilayers was confirmed by X-ray diffraction (XRD), Hall effect measurement and Rutherford backscattering spectrometry. Our results show that high-quality Ga-doped n-type ZnO can be grown on a-plane sapphire substrates by using MBE.