Growth Investigations of Nitrogen-Polar GaN Nucleation Layer Templates

This study aims at achieving highly crystalline and smooth Nitrogen-polar (N-polar) GaN layers deposited on c-plane sapphire by metal organic vapor phase epitaxy (MOVPE). The influence of nitridation, temperature and V/III ratio on the polarity, quality and coalescence of GaN is systematically investigated. It was observed that the initial nitridation of sapphire before GaN growth is critical for achieving N-polar growth and improving the crystal quality. Moreover, higher GaN deposition temperatures (above 1100 ◦C) result in lower coalescence and growth of vertical micro-hexagonal structures with a large degree of N-polarity, while lower temperatures (below 950 ◦C) resulted in a coalesced layer but lower degree of N-polarity and crystal quality. Growth under a low V/III ratio (below 50) at a temperature of 1000 ◦C showed complete coalescence with a layer thickness of 170 nm and a linewidth of 20meV for the GaN band-edge emission in low temperature photoluminescence, whereas a higher V/III ratio of 1100 resulted in GaN structures separated from one another (lower degree of coalescence). Finally, selectively grown vertical GaN micro-rods on masked nitridated sapphire proved to be exhibiting mainly N-polar crystal orientation.