Theoretical study of adatom stability on polar GaN surfaces during MBE and MOVPE

GaN is a key material for realizing an energy saving society owing to its superior properties for power devises. Understanding the physics of the crystal growth can accelerate the development of GaN-based devices. There have been multiple theoretical studies that investigate the surface reconstructions with minimum energy from GaN during growth. However, although the most stable reconstruction is dominant on the surface, other reconstructions could also appear probabilistically due to the entropic effect. Considering the growth rate of GaN is typically limited by the Ga precursor flow rate, the appearance probability of Ga-adsorbed reconstruction should be small. Little is known about the relative stabilities of such minor reconstructions. Here, we propose a new scheme to calculate the appearance probabilities of reconstructions. Our calculation reveals that the N density increases and Ga density decreases on GaN(0 0 0 1) during metalorganic vapor phase epitaxy (MOVPE) with decreasing H₂ partial pressure, resulting in a highly N-rich surface. In contrast, for GaN(0 0 0 −1), both N and Ga densities increase with decreasing H₂ partial pressure, resulting in a Ga-rich surface. The results well explain the reported growth characteristics of polar GaN.