Je. Van Nostrand et al., Cathodoluminescence, microstructure, and morphology of tensile-strained AlxGa(1-x)N epitaxial films grown by gas source molecular beam epitaxy, J APPL PHYS, 86(6), 1999, pp. 3120-3128
Tensile-strained AlxGa(1-x)N (0 less than or equal to x less than or equal
to 0.4) alloys are deposited on 2-mu m-thick GaN on Al2O3(0001) by gas sour
ce molecular beam epitaxy using ammonia as the nitrogen source. The evoluti
on of the surface morphology of AlxGa(1-x)N epitaxial films as a function o
f mole fraction is studied using atomic force microscopy. Surface roughness
initially increases with Al mole fraction, but then decreases with a corre
sponding onset of defects. Extended ridge-like defects are observed along t
he < 0010 > of the AlxGa(1-x)N for high Al mole fractions. Band-edge and su
bband-edge emission at 6 K is investigated using cathodoluminescence. Stron
g emission, ascribed to donor-bound excitons, shallow donor to shallow-acce
ptor pair emission, and deep emission associated with the "yellow" band of
GaN are observed. The energy level of emission from donor-bound excitons is
found to exhibit a linear dependence on Al more fraction, suggesting a lac
k of band bending in this material system. Finally, microstructure is inves
tigated using a triple axis x-ray diffraction system. X-ray results are com
bined with x-ray photoelectron spectroscopy data to determine the extent of
relaxation of AlxGa(1-x)N epitaxial films. For 0.5 mu m AlxGa(1-x)N films
deposited on thick GaN on Al2O3(0001), a "critical" Al mole fraction of x =
0.20 is determined, beyond which significant film relaxation and extended
defect formation is observed. (C) 1999 American Institute of Physics. [S002
1-8979(99)07518-0].