Epitaxial growth of nonpolar m-plane ZnO epilayers and ZnO/Zn0.55Mg0.45O multiple quantum wells on a LiGaO2 (100) substrate

Abstract

Nonpolar m-plane ZnO epilayers and 5-period ZnO/Zn_1−xMg_xO (x = 0.45) multiple quantum wells (QWs) have been grown successfully on a LiGaO₂ (100) substrate by plasma-assisted molecular beam epitaxy. The epilayer and QWs samples show different anisotropy of the X-ray rocking curves (RCs) and the surface stripes. The former exhibits a low FWHM value of 126 arcsec of the X-ray (100) RC along [110]_ZnO, and its surface is composed of fine stripes along [0001]_ZnO. The QW sample, in contrast, exhibits a low RC FWHM (169 arcsec) along [0001]_ZnO with surface stripes oriented along [110]_ZnO. These phenomena are attributed to an anisotropic stress relaxation. Moreover, combining the X-ray and TEM characterization techniques, both samples show an extremely low density of threading dislocations of less than 1 × 10⁸ cm⁻², but a high density of basal stacking faults of 1 × 10⁶ cm⁻¹. The room temperature cathodoluminescence spectrum of the ZnO/Zn_1−xMg_xO QWs shows a strong emission peak with an apparent blue shift to 3.56 eV, indicating the positive quantum confinement effect and hence the absence of induced polarization fields.