Morphology, polarity, and lateral molecular beam epitaxy growth of GaN on sapphire

Gallium nitride was grown on sapphire (0001) substrates by radio frequency plasma assisted molecular beam epitaxy. The surface morphology was characte rized during growth by reflection high energy electron diffraction, and ex situ by scanning electron microscopy (SEM), atomic force microscopy (AFM) a nd x-ray diffraction. It is found that surface morphological features are l inked to domains of specific wurtzite crystal polarity, (0001)Ga face or (0 00 (1) over bar)N face, for Ga-rich growth. For growth on ALN buffer layers , we commonly observe films which consist of largely (0001)Ga polarity mate rial, as confirmed by selective etch tests, with a varying coverage of (000 (1) over bar)N-face inversion domains threading along the growth direction . For growth near stoichiometric conditions, the growth rate of the N-face domains is slightly lower than that for the Ga-face matrix, which results i n the formation of pits with inversion domains at their centers. For sample s grown by first depositing GaN under N-rich conditions, followed by growth under Ga-rich conditions, a different morphology is obtained, exhibiting l arge hexagonal flat terraces observable by SEM and AFM. The apparent grain size of these films is increased substantially over films grown using a sin gle step approach. The cross sectional SEM images of the two-step films sho w a network of voids and columns at the interface between the N-rich and th e Ga-rich layers, above which micron-scale islands form and coalesce via la teral growth. Lateral growth may result in reduced defect density and impro ved crystal quality. The asymmetric x-ray peak (11 (2) over bar 4) width is reduced to approximately 280 arcsec in the two-stage GaN films. (C) 1999 A merican Vacuum Society. [S0734-211X(99)02303-3].