Abstract
Combined transmission electron microscopy (TEM), x-ray diffraction, and cathodoluminescence spectroscopy measurements of AlxGa1−xN thin films grown by molecular-beam epitaxy reveal spontaneous modulation, phase separation, and band-gap reductions that vary systematically with AlN mole fraction across the full alloy series. At low AlN mole fraction (x⩽0.5), AlGaN epilayers display pronounced phase separation. With increasing AlN mole fraction, phase separation is strongly suppressed by the formation of spontaneous modulation, which high spatial resolution TEM techniques unambiguously determine to be an atomic-scale compositional superlattice. Superlattice-induced reductions from band gaps expected for compositionally disordered epilayers exceed several hundred meV for the Al-rich average alloy composition.
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