Cathodoluminescence, microstructure, and morphology of tensile-strained AlxGa(1-x)N epitaxial films grown by gas source molecular beam epitaxy

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].