Surface, Structural, and Optical Properties in the ZnO Films Grown on Al2O3 and GaN Substrates by Using RF-Sputtering

We report the surface properties, structural properties, and optical properties of the ZnO films grown on Al2O3 substrates and GaN templates by using RF-sputtering. The surface flatness of the ZnO films on the Al2O3 substrates increases with an increase in substrate temperature, and the opposite for that of the ZnO films on the GaN templates. This is ascribed to the fact that the GaN templates have a rougher surface than the Al2O3 substrates, which facilitates 3-dimensional growth on the GaN templates and 2-dimensional growth on the Al2O3 substrates. The structural qualities increase with increasing substrate temperature and film thickness, irrespective of the substrate. These are ascribed to the facts that a higher substrate temperature supplies the sufficient chemical reaction energy, necessary for the formation of ZnO lattices, and a longer growth time enlarges the bulky region with a relatively small amount of defects, not influenced by the heterointerface. On the other hand, the ZnO films on GaN templates have 5 times smaller X-ray diffraction linewidth and 50 times larger photoluminescence intensity than those on the Al2O3 substrates. These are ascribed to the fact that the "ZnO on GaN" structure has a very small lattice mismatch of 1.8%, while the "ZnO on Al2O3" structure has a large lattice mismatch of 18%.