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We have fabricated InN thin films using rf magnetron sputtering from an indium (In) metal target. Optical and electrical measurements show that these as-grown films are n-type with carrier concentrations ranging from 1020 to 1021 cm-3. This variation in carrier density is produced by controlling the conditions during the deposition. We used Rutherford backscattering spectrometry to identify possible sources for n-type carriers. We found that in addition to strong direct bandgap optical absorption ranging from 1.4 to 2.0 eV, a large plasmon absorption peak in the infrared region (0.45–0.8 eV) is also observed. This tunable IR absorption suggests that these highly degenerate InN films could be used for a number of applications, including optical filters and infrared devices. [ABSTRACT FROM AUTHOR]

We investigated the behavior of Raman modes for AlN thin films fabricated with plasma source molecular beam epitaxy method having high levels of oxygen contamination. Oxygen atoms occupy different lattice sites depending on their at. % value and, thus, strongly influence spectral features of certain Raman modes. We studied the variations in the width of nonpolar E2low and E2high modes which represent mainly the vibrations of Al sublattice and N atoms, respectively, in the AlN lattice. When oxygen occupies a N site, it affects the width of the E2high mode, and at the same time, the charge neutrality constraint creates an Al vacancy and, thus, simultaneously affects the width of the E2low mode. We found that for our films whose oxygen concentration vary from 1 to 10 at. %, the width of both the E2high and E2low modes varies linearly with the oxygen contamination levels suggesting that even at such high levels of oxygen contamination, oxygen atoms still prefer to occupy the N site. This is contrary to previous studies of single crystal AlN. The oxygen contamination in our films was determined by x-ray photoemission spectroscopy measurements. [ABSTRACT FROM AUTHOR]

The effect of plasmon and LO-phonon damping on the optical measurements of InN films is discussed. Phonon and plasmon damping dramatically modifies the spectral features of the optical spectra and destabilizes the coupled modes of the system. Phonon damping affects the optical properties in a qualitatively different way from plasmon damping. Increased phonon damping leads to a merging of the coupled modes for a certain range of carrier density. Plasmon damping broadens the spectral linewidths of both of the coupled modes when plasmon energies are of the order of LO-phonon energies. However, when plasmon energies are larger than LO-phonon energies, increasing plasmon damping makes the higher-energy mode completely degenerate with plasmon, and may even have lower energies than plasmon. In weakly damped situations, we also discuss the low-energy optical transmission region that forms in between the coupled modes. Finally, the effect of plasmon and LO-phonon coupling on the analyses of the experimental infrared reflection data of InN films (for different electron densities) is discussed. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Low energy optical modes of MBE-grown In1-xGaxN thin films with different values of x are investigated using Raman spectroscopy. We also studied the influence of Raman excitation frequency using red and green lasers on scattering intensity of various Raman modes. For those In1-xGaxN alloys whose bandgap energy is close to the red laser, a huge enhancement in the intensities of A1(LO) mode and its 2A1(LO) replica is observed when excited with red laser as compared to the green laser excitation. We found that the energies of longitudinal optical modes (A1(LO) and 2A1(LO)) vary nonlinearly unlike the E2 mode with increasing Ga atomic fraction. A Raman mode ~540 cm-1 was observed in all In1-xGaxN films with low energy red laser excitation but was absent with green laser excitation. We attribute this mode to A1(TO) mode of the underneath GaN buffer layer. [ABSTRACT FROM AUTHOR]

AlN thin films have been grown on a-plane sapphire (Al2O3(112̄0)) substrates. X-ray diffraction measurements indicate the films are fully c-plane (0001) oriented with a full width at half maximum of the AlN(0002) rocking curves of 0.92. The epitaxial growth relationships have been determined by the reflection high energy electron diffraction analysis as AlN[11̄00]//Al2O3[0001] and AlN[112̄0]//Al2O3[11̄00]. Angular dependence of important surface acoustic wave (SAW) characteristics, such as the phase velocity and electromechanical coupling coefficient, has been investigated on the AlN(0001)/Al2O3(112̄0) structure. While the SAW is excited at all propagation angles with an angular dispersion of the phase velocity in the range of 5503–6045 m/s, a higher velocity shear-horizontal (SH) mode is observed only at 0°, 105° and 180° off the reference Al2O3[11̄00] over a 180° angular period. The phase velocity of the SH mode shows dispersion (6089–6132 m/s) as a function of the SAW wavelength. Temperature coefficients of frequency are also demonstrated for both modes. [ABSTRACT FROM AUTHOR]

Low energy optical modes of molecular beam epitaxy-grown In1-xGaxN thin films with 0≤x≤0.6 are investigated using infrared reflectance measurements. We found that the reflectance of the films for wave vectors in the range from 600 to 800 cm-1 is determined by the high energy E1(LO)-plasmon coupled modes. In the higher energy regime of the UV-visible reflectance spectrum of InN, critical points with energies 4.75, 5.36, and 6.12 eV belonging to A and B structures are observed. The energies of these critical points increase with increasing values of x, similar to the band gap energy of these films. [ABSTRACT FROM AUTHOR]

We report the structural and optical properties of xSnO2–yFe2O3 nanocrystalline composite thin films. SnO2 and Fe2O3 exhibit strong phase separation instability and their particle size and crystallinity are tunable by changing their composition and annealing temperature. The bandgap for these composites continuously increases from 2.3 to 3.89 eV. We discuss the increasing bandgap values in terms of the quantum confinement effect manifested by the decreasing size of Fe2O3 crystallites. The method provides a generic approach for the tuning of the bandgap in nanocomposite systems. [ABSTRACT FROM AUTHOR]

We have investigated the properties of sputter deposited InN thin films prepared from an In-metal (InN-MT) and an In2O3 target (InN-OT). The excess oxygen present in the InN-OT films alters the microstructure by introducing additional disorder. Depth dependent x-ray photoelectron spectroscopy measurements indicate the presence of higher concentrations of oxygen in InN-OT. Raman spectra show evidence for the presence of an In2O3 secondary phase in both samples. Although the InN-OT film has a higher oxygen concentration, both films show similar electrical and optical properties. [ABSTRACT FROM AUTHOR]