Optical Absorption, Raman, and Photoluminescence Excitation Spectroscopy of Inhomogeneous InGaN Films
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InxGa1-xN films were grown on (0001) sapphire substrates using a specially designed metal-organic chemical vapor deposition (MOCVD) reactor and growth conditions that have been described elsewhere (E.L. Piner4 et. al. and references therein). An AlN buffer layer and then a AlGaN graded to GaN prelayer were grown on each substrate before the InxGa1-xN layer; the InxGa1-xN layer thickness is estimated to be 0.3 µm to 0.5 µm, and the AlGaN/GaN prelayer thickness is estimated to be 0.1 µm. The growth temperature for the InxGa1-xN layer was between 690°C and 780°C. The In fraction, x, was controlled by varying the growth temperature and the hydrogen gas flow rate. Eleven films were examined, with x=0.06 to x=0.49 as determined from the shift of the 0002 XRD peak relative to pure GaN and InN, assuming a linear x-dependence of the lattice constant. All optical measurements were performed at room temperature. Transmittance spectra of the samples were measured with a Cary model 14 spectrophotometer.7 Metal-grid neutral density filters were inserted into the spectrophotometer reference channel to extend the lower end of the measured transmittance range to ~3x10-5. Raman spectra were excited by the 514.53 nm (2.409 eV) and 457.94 nm (2.707 eV) lines of an Argon ion laser and recorded by a Princeton Instruments intensified photodiode array detector attached to a Spex Triplemate monochromator.7 The wavelength resolution of the monochromator was 0.10 nm, corresponding to a wavenumber resolution of 3.7 cm-1 for the Raman spectra excited at 514.53 nm, and 4.7 cm-1 for the Raman spectra excited at 457.94 nm. The wavenumber accuracy was ~0.2 cm-1, calibrated with Ar, Kr and Xe line sources. In the DSR-PLX experiment, PL was excited by a wavelength-tunable source, and the PL intensity within a selected wavelength band was monitored as the excitation energy was scanned. The measurement was then repeated for several different emission wavelength bands, so that several excitation spectra were obtained for each sample, corresponding to different emission wavelengths (or photon energies). The excitation source was a 300 watt xenon arc lamp coupled to a Spex model 1680 double monochromator.7 The wavelength resolution of the spectrometer was 2 nm; the corresponding energy resolution varied from 0.006 eV at 1.9 eV, to 0.02 eV at 3.5 eV. Each emission band was selected by a bandpass interference filter, taken from a set of seven filters with center wavelengths every 50 nm from 450 nm to 750 nm and full bandwidths of 40 nm. To more completely remove the scattered excitation, each bandpass filter was paired with a longpass filter that had a cut-on wavelength slightly shorter than the bandpass center wavelength. The emitted PL was detected by a S20 type photomultiplier tube. The excitation source was mechani
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