Effect of buffer layer and substrate surface polarity on the growth by molecular beam epitaxy of GaN on ZnO

We present results on the effect of substrate surface polarity, oxygen and zinc faces, on the quality of GaN epitaxial layers grown on ZnO(0001) substrates by reactive ammonia molecular beam epitaxy. The possible effects dealing with the disparity in surface preparation of the two faces have been eliminated. Photoluminescence and reflectivity measurements demonstrate that the oxygen face leads to higher quality GaN on ZnO compared to the zinc face. We also present optical data obtained by using different low-temperature AlN, GaN, and ${\mathrm{In}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{N}$ buffer layers. The best result has been obtained with lattice-matched ${\mathrm{In}}_{\mathrm{0.20}}{\mathrm{Ga}}_{\mathrm{0.80}}\mathrm{N}$ buffer layer.