Heteroepitaxial ZnO Films Grown by Plasma-assisted Molecular Beam Epitaxy

Heteroepitaxial ZnO films were grown on GaN templates by plasma-assisted molecular beam epitaxy. The epitaxial GaN templates were deposited by metalorganic chemical vapor deposition on c-plane sapphire substrates. Photoluminescence spectra of ZnO epilayers excited by a He-Cd laser exhibit exciton emission at 376 nm with a full width at half maximum (FWHM) of 10 nm (90 meV) at room temperature. The exciton emission intensity of stoichiometric condition is 2 times greater than that of O-rich and Zn-rich conditions. Samples grown under stoichiometric and Zn-rich conditions do not exhibit defect-related green luminescence, but samples grown under O-rich condition do. In these heteroepitaxial ZnO layers there exists interstitial Zn and Zn vacancies. X-ray diffraction measurements revealed that there exists a residual compressive strain, ε ~ -0.2%, in the [0002] direction of the ZnO epilayer. The residual strain might be attributed to grain boundaries of ZnO.