The Same Formation Mechanism of Surface Pits for both a- and c-plane GaN

摘要

Surface pits in a-plane GaN have long been considered as a result of island coalescence in literature, while pits in c-plane GaN are the surface termination of screw dislocations. It is found in the paper that small surface pits on a-plane GaN also terminate perfect screw dislocations and partial dislocations (PD), which is due to the intrinsic nature of the spiral growth of screw dislocations. This resulthas demonstrated that a-plane GaN also follows the prediction based on Frank's classical theory of dislocated crystals. Moreover, although caused by dislocation, the relative size of the small pits will be affected by the kinetic process of the atomic reactions on the surface through V/III ratio variation. Meanwhile, large hexagonal surface pits are observed on c-plane GaN grown under low V/III ratio, which is formed under the same mechanism as the large triangular or pentagonal pits on a-plane GaN surface. During the island coalescing, the lateral growth of the inclined facets are in competition with the vertical growth in the epitaxial orientation, so incomplete island coalescence will produce large surface pits for both a- and c-plane GaN. Under this mechanism, a triangular pit of a-plane GaN could be transformed into a pentagonal pit upon increasing temperature.