In situ studies of low temperature atomic level processing of GaN surfaces for atomic layer epitaxial growth

In situ synchrotron x-ray studies were employed to develop a fundamental understanding of the low temperature atomic level processes (ALPs) for GaN substrates to develop in situ methods for preparation of epitaxy ready surfaces. An emulated gallium flash-off (GFO) ALP, followed by a hydrogen clean ALP, and a subsequent nitridation ALP are studied as a function of temperature and number of cycles. The results demonstrate that ideal GFO ALP results are achieved at a higher temperature, 500 degrees C, and that only ten GFO ALP cycles are needed to remove the surface oxide and result in an ordered GaN surface. Continued GFO ALP cycles at 500 degrees C roughen the GaN surface. GFO ALP executed at 400 degrees C only roughens the surface, while executing the GFO ALP at 250 degrees C causes uneven surface features presumably due to the incomplete removal of the oxide. The hydrogen clean ALP generally roughens the surface at all three temperatures after 30 cycles of the GFO ALP. Further, the nitridation ALP executed after 30 cycles of the GFO ALP, at any of the above temperatures, has little effect since the surface of the GaN has been roughened beyond recovery. These results provide insight into optimal GaN substrate surface preparation at temperatures consistent with the low temperature atomic layer epitaxy process.