Molecular Beam Epitaxial Growth of GaN on (0001) Al2O3 Using an Ultrathin Amorphous Buffer Layer Deposited at Low Temperature

The effects of buffer layer quality and/or deposition conditions on the molecular beam epitaxy of GaN on c-Al2O3 substrates were investigated. In order to precisely determine the effects of a thin buffer layer, atomically flat and monolayer-stepped Al2O3 substrates were prepared by conventional chemical etching with a H3PO4: H2SO4 solution. A significant improvement in the crystalline quality of the GaN epilayer was achieved when ultrathin amorphous buffer layers deposited at temperatures as low as 125°C were used. It was found that low-temperature deposition leading to an amorphous buffer layer is important for the optimization of the subsequent GaN epitaxial layer. The thickness of the buffer layer is critical; only a very thin amorphous layer can be well crystallized during the thermal annealing process before epilayer growth. It was found that GaN epilayers grown on this buffer layer gave rise to band-edge emissions (357 nm/FWHM=19.7 meV) without any deep emissions in low-temperature photoluminescence and led to (0002) X-ray rocking curves with a full-width at half maximum of about 9.8 arcmin.