Atomic layer epitaxy of ZnO thin films on GaN templates

Zinc Oxide (ZnO) is a II-VI compound semiconductor with a wide direct bandgap of 3.37 eV at room temperature. The main advantage of ZnO for short wavelength optical devices is the large exciton binding energy of 60 meV. However, ZnO is naturally an n-type semiconductor because of a deviation from stoichiometry due to the presence of intrinsic point defects such as O vacancies and Zn interstitials. The growth technique of atomic layer epitaxy is thought to be effective to realize p-type ZnO, because impurity incorporation can be precisely controlled. The GaN templates we used were 1.5 μm thick GaN epitaxial films on the low temperature GaN buffer layer of 30 nm which were grown by MOCVD on (0 0 0 1) sapphire substrates. Diethylzinc and H2O reactant gases were alternately fed into the growth chamber with argon as a carrier gas. By reflection high-energy electron diffraction (RHEED) measurements, spotty pattern was observed for the ZnO film grown on sapphire substrate, while streaky pattern was observed for that grown on GaN template even at the low growth temperature of 247 C. In addition, full-width at half-maximum (FWHM) value of (0 0 0 2) X-ray diffraction (XRD) curve of ZnO film was greatly reduced from 6 to 0.1 degree by using GaN template. These results indicate that the GaN templates were effective to improve the crystal quality of the ZnO thin films. (Authors)