The characteristic surface morphologies of GaN grown by plasma-assisted mol
ecular 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 bo
undary 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 an
d is constant over the range of substrate temperatures investigated. The bo
undary between the two Ga-stable regimes (the Ga-droplet regime and the int
ermediate regime) is determined by the formation of Ga droplets and has an
Arrhenius dependence with substrate temperature. The characteristic morphol
ogies of films grown within each of these regimes are investigated using at
omic force microscopy and transmission electron microscopy. N-stable films
have rough, heavily pitted morphologies. Films grown within the intermediat
e 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-dropl
et regime, exhibit atomically flat surfaces with no pit features. The morph
ology transitions are associated with changes in the growth kinetics caused
by variations in the coverage of the GaN surface by excess Ga. (C) 2000 Am
erican Institute of Physics. [S0021-8979(00)02416-6].