Structure and composition of GaN(0001) A and B surfaces

Homoepitaxial, GaN films on both c-plane surfaces of bulk GaN crystals were examined using reflection high-energy electron diffraction (RHEED). Differ ences in the RHEED pattern, time development of the RHEED intensity, and su rface reconstructions were observed. The substrate surfaces were prepared e ither by mechanical polishing [GaN(0001)A] or by chemo-mechanically polishi ng [GaN(000 (1) over bar)B]. Then films were grown by molecular beam epitax y; Ga was provide by a Knudsen cell and nitrogen from NH3. On the B surface , the Ga rich reconstructions reported by Smith and co-workers [Phys. Rev. Lett. 79, 3934 (1997)] were observed. On the A surface, a (2 X 2) reconstru ction was observed. Both reconstructions were much sharper than those seen on GaN films grown on sapphire. RHEED measurements of the specular intensit y vs time showed that two different surface terminations could be maintaine d on the B surface, one of which is a stable, gallided surface, while the o ther is a nitrided surface, which is unstable in vacuum. If the nitrided su rface is heated in vacuum it changes to the gallided surface in several min utes at 800 degrees C. Only one termination was detected on the A surface. The results are complemented by desorption mass spectroscopy measurements, and the resulting surfaces were then investigated using atomic force micros copy and scanning tunneling microscopy. We were able to distinguish the two surface terminations on the B surface, and a unique annealing process unde r NH3 will be documented. Preliminary investigation of the A surface reveal ed decorated step edges. The results were compared to films grown on sapphi re with different nucleation layers, which can be grown to yield either pol arity. (C) 1999 American Institute of Physics. [S0021-8979(99)04411-4].