CATHODOLUMINESCENCE, PHOTOLUMINESCENCE, AND OPTICAL ABSORBENCY SPECTROSCOPY OF ALUMINUM GALLIUM NITRIDE (ALXGA1-XN) FILMS

Aluminum gallium nitride (AlxGa1-xN) films, grown by metalorganic chem ical vapor deposition on sapphire, were characterized by low-temperatu re cathodoluminescence (CL) and photoluminescence (PL), and room-tempe rature optical absorbance. The aluminum fractions are estimated to ran ge from x = 0 to x = 0.444. Most films were silicon-doped. The absorpt ion spectra have a Urbach (exponential) form below the bandgap. The wi dth of the Urbach edge, E-U, increases with Al fraction, x, as E-U = ( 0.045 + 0.104x) eV. The luminescence (CL or FL) spectra show a relativ ely 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 som e 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 t he interface. The energy of the band-edge luminescence peak shifts wit h excitation mode (CL, top-surface FL, or interface FL). This effect i s 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.