Low Temperature Growth of GaN Epitaxial Layers on Sapphire (0001) by Pulsed Laser Deposition Using Liquid Gallium Target

Smooth surface GaN epilayers have been grown on sapphire (0001) by pulsed laser ablation of liquid Ga target under radio frequency assisted atomic nitrogen ambient. Laser energy density, laser frequency and substrate-to-target distance were adjusted to optimize the Ga flux for a fixed nitrogen plasma condition. From in-situ reflection high energy electron diffraction and atomic force microscopy analyses, it is found that the Ga/N flux ratio plays a crucial role in determining the growth mode and that only the Ga-rich condition promotes a two dimensional GaN growth. Also, the increase of Ga flux by increasing the laser energy density above 8 J/cm(2) is observed to produce Ga droplets on the grown GaN surface even if the growth front is excess with nitrogen flux. Instead, the increase of laser frequency (up to 50 Hz, in this study) is preferable. The effect of growth temperature on the crystalline quality of GaN layers has also been investigated in the range from 300 to 700 degrees C. Highly c-axis oriented GaN layers, with a (0002) plane X-ray rocking curve full width at half maximum of 245 arcsec, have been obtained at 500-600 degrees C, which is about 150 degrees C lower than the conventional MBE growth.