Optimization of High-Quality AlN Epitaxially Grown on (0001) Sapphire by Metal-Organic Vapor-Phase Epitaxy

A systematic study is performed to optimize aluminum nitride (AlN) epilayers grown on (0001) sapphire by metal-organic vapor-phase epitaxy. Specifically, the impact of the AlN nucleation conditions on the crystalline quality and surface morphology of AlN epilayers is studied. Atomic force microscopy (AFM) and x-ray diffraction (XRD) results reveal that the nucleation layer plays a critical role in the growth of subsequent layers. The magnitude of the TMAl flow of AlN nucleation layer is found to have a strong effect on the crystalline quality and surface morphology of the high-temperature (HT) AlN epilayer. A simple Al adatom-diffusion-enhancement model is presented to explain the strong dependence of the crystalline quality and surface morphology on TMAl flow. Furthermore, ammonia flow, nucleation temperature, and growth time of the AlN nucleation layer are found to affect the surface morphology and the crystalline quality as well. A trade-off is found between surface morphology and crystalline quality; that is, we do not obtain the best surface morphology and the highest crystalline quality for the same growth parameters. For optimized AlN nucleation layers and HT AlN epilayers, a clear and continuously linear step-flow pattern with saw-tooth shaped terrace edges is found by AFM on AlN epilayers. Triple-axis x-ray rocking curves show a full-width at half-maximum (FWHM) of 11.5 arcsec and 14.5 arcsec for the (002) and (004) reflection, respectively. KOH etching reveals an etch-pit density (EPD) of 2 × 10⁷ cm⁻², as deduced from AFM measurements.