Cathodoluminescence, Photoluminescence, and Optical Absorbance Spectroscopy of Aluminum Gallium Nitride (AlxGa1−xN) Films

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Abstract
Aluminum gallium nitride (AlxGa1−xN) films, grown by metalorganic chemical vapor deposition on sapphire, were characterized by low-temperature cathodoluminescence (CL) and photoluminescence (PL), and room-temperature optical absorbance. The aluminum fractions are estimated to range from x = 0 to x = 0.444. Most films were silicon-doped. The absorption spectra have a Urbach (exponential) form below the bandgap. The width of the Urbach edge, EU, increases with Al fraction, x, as EU = (0.045 +1 0.104x) eV. The luminescence (CL or PL) spectra show a relatively narrow band-edge peak and a broad deep-level peak. The full-widths at half-maximum of the band-edge CL peaks (measured at T = 15 K) are remarkably similar to the Urbach absorption widths, EU (measured at T = 300 K). PL spectra were obtained from the top surfaces and the film-substrate interfaces of several films. The interface PL spectra of some films show an extra peak 0.15 eV to 0.45 eV below the bandgap, which is ascribed to structural defects or impurity phases localized near the interface. The energy of the band-edge luminescence peak shifts with excitation mode (CL, top-surface PL, or interface PL). This effect is attributed to the variation of the excitation depth, between the top surface and film-substrate interface, with excitation mode, together with the depth variation of film properties such as residual stress or aluminum fraction.