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

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Tensile-strained AlxGa(1−x)N (0⩽x⩽0.4) alloys are deposited on 2-μm-thick GaN on Al2O3(0001) by gas source molecular beam epitaxy using ammonia as the nitrogen source. The evolution of the surface morphology of AlxGa(1−x)N epitaxial films as a function of mole fraction is studied using atomic force microscopy. Surface roughness initially increases with Al mole fraction, but then decreases with a corresponding onset of defects. Extended ridge-like defects are observed along the 〈0010〉 of the AlxGa(1−x)N for high Al mole fractions. Band-edge and subband-edge emission at 6 K is investigated using cathodoluminescence. Strong emission, ascribed to donor-bound excitons, shallow donor to shallow-acceptor 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 lack of band bending in this material system. Finally, microstructure is investigated using a triple axis x-ray diffraction system. X-ray results are combined with x-ray photoelectron spectroscopy data to determine the extent of relaxation of AlxGa(1−x)N epitaxial films. For 0.5 μm 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.