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Non-polar ZnO and Na-doped ZnO films were epitaxially grown on m-plane sapphire substrates by plasma-assisted molecular beam epitaxy. The films exhibit dominant ( $$ 10\overline{1} 0 $$ ) (m-plane) orientation as identified by the X-ray diffraction pattern. The quality of the obtained m-plane ZnO film is evidenced by X-ray diffraction rocking curves full width at half maximum of 1125 arcsec for the ( $$ 10\overline{1} 0 $$ ) reflection and 1427 arcsec for the ( $$ 10\overline{1} 1 $$ ) reflection, respectively. Hall-effect measurements show that the m-plane Na-doped ZnO film exhibits p-type conductivity with a hole concentration of 2.50 × 1017 cm−3, while the m-plane ZnO film exhibits compensatory conductivity. Na atoms substituting for Zn atoms are believed to be the origin of p-type conductivity. The Na-related acceptor level is deduced to be ~120 meV by temperature-dependent photoluminescence, indicating the superiority of m-plane ZnO film in p-type doping compared with the polar ZnO film.

This work investigates the structural and optical properties of non-polar ZnO/Zn0.81Mg0.19O multiple quantum wells (MQWs), which have been prepared on \(r\)-plane sapphire substrates by plasma-assisted molecular beam epitaxy (MBE). The MQWs are (\(11\bar{2}0\)) oriented (\(a\)-plane) as identified by the X-ray diffraction pattern. Structural properties are anisotropic and surfaces of MQWs show stripes running along the ZnO \(c\)-axis direction. Sharp interfaces between the well layers and barrier layers can be clearly resolved by the secondary ion mass spectroscopy (SIMS) analysis. The room-temperature photoluminescence (PL) resulting from the well regions exhibits a significant blueshift with respect to ZnO single layer. Exciton emission in the ZnO QW is resolved into two components in the temperature dependence of the PL spectra. Two types of excitons are responsible for this feature. The excitons trapped by the potential minima dominate at low temperature, and the excitons localized in the “free exciton states” dominate at relatively high temperature. An activation energy of 7.3 meV for quenching of the exciton emission is in good agreement with the transition of the two types of excitons.

P-type Zn1−x Mg x O (0≤x≤0.25) films are obtained through Na doping, as confirmed by repeated Hall-effect measurements and rectification behavior of p-Zn0.81Mg0.19O:Na/n-Zn0.8Mg0.2O:Al quasi-homojunctions. The absorption and photoluminescence spectra indicate the band gap shifts to higher energy in linearly proportional to Mg content. Critically, X-ray diffraction (XRD) patterns reveal that the strain induced by Na doping can be counteracted through an appropriate content of Mg alloying. Both XRD data and Hall-effect data imply that the solubility of Na acceptor should be enhanced by alloying with Mg. The X-ray photoelectron spectrum (XPS) result indicates the existence of Na. Microstructure structure of the film is confirmed by high-resolution transmission electron microscope.