Strain dependent anisotropy in photoluminescence of heteroepitaxial nonpolar a-plane ZnO layers

Nonpolar a-plane ZnO layers with anisotropic in-plane strains were prepared on the three substrates of r-sapphire, a-GaN, and a-Al0.08GaN templates via a pulsed laser deposition system, to investigate the distinguishing anisotropic photoluminescence properties of a-ZnO grown on foreign substrates. The optical anisotropy of nonpolar ZnO grown on GaN and AlGaN templates was investigated via polarization-dependent photoluminescence (PL) measurement and polarization transmission spectra measurement. The 0.3 μm a-ZnO layer grown on the a-GaN template has significant anisotropic optical properties with a degree of polarization (DOP) of the photoluminescence (PL) spectrum of about 0.8907, larger than 0.8786 of ZnO on a-Al0.08GaN or 0.8408 of a-ZnO on r-sapphire, revealing that the a-GaN may be the best candidate for the fabrication of modulators and that the increase of the Al component x of p-AlxGa1-xN will attenuate the anisotropic properties of the heteroepitaxial a-ZnO layer, providing a valuable basis for the choice of appropriate substrate for nonpolar a-plane ZnO based polarized optoelectronic devices. Moreover, the relationship between crystal quality anisotropy and optical anisotropy was proposed.