Control of GaN surface morphologies using plasma-assisted molecular beam epitaxy

The characteristic surface morphologies of GaN grown by plasma-assisted molecular beam epitaxy under various growth conditions have been investigated. Three growth regimes (one N stable and two Ga stable) are identified on a surface structure diagram (Ga/N ratio versus substrate temperature). The boundary between the N-stable regime (low Ga/N ratios) and the two Ga-stable regimes (high Ga/N ratios) is determined by the growth rate of the films and is constant over the range of substrate temperatures investigated. The boundary between the two Ga-stable regimes (the Ga-droplet regime and the intermediate regime) is determined by the formation of Ga droplets and has an Arrhenius dependence with substrate temperature. The characteristic morphologies of films grown within each of these regimes are investigated using atomic force microscopy and transmission electron microscopy. N-stable films have rough, heavily pitted morphologies. Films grown within the intermediate phase have areas of flat surface between large, irregularly shaped pits. The pits observed for films grown within both regimes are found to initiate from threading dislocations and to decrease in density with increasing Ga/N ratio at constant temperature. Ga-stable films, grown within the Ga-droplet regime, exhibit atomically flat surfaces with no pit features. The morphology transitions are associated with changes in the growth kinetics caused by variations in the coverage of the GaN surface by excess Ga.