D. Haddad

Temperature dependence of mobility and carrier density in InN films

We investigate the temperature dependence of Hall mobility μ and carrier density Ne for thin InN films grown by molecular-beam epitaxy and plasma source molecular-beam epitaxy over three orders-of-magnitude difference in their carrier density: for the low-density film Ne=5.8×1017∕cm3 and for the high-density film Ne=3.2×1020∕cm3. In both the films, for temperature up to 300 K, a large temperature-independent concentration of carriers is observed. For higher temperatures, however, carrier density increases with temperature. The characteristic behavior of the mobility for the low-density film is different from that of the high-density film, particularly for temperatures less than 300 K. The low-density film shows a peak behavior in the mobility around 250 K in contrast to the temperature-independent mobility observed for the high-density film for T<300K. We investigate theoretically the concentrations of donor, acceptor, and threading dislocations for both the films and also discussed various electron-scatt...

Phase transitional studies of polycrystalline Pb0.4Sr0.6TiO3 films using Raman scattering

Pb0.4Sr0.6TiO3 films with submicron size grains have been prepared on Pt substrates by the metalorganic decomposition method. X-ray diffraction analysis reveals that the films are polycrystalline with a perovskite crystal structure and negligible tetragonal splitting at room temperature. The room temperature Raman spectrum, however, shows all the characteristic phonon modes of a tetragonal ferroelectric phase. These modes lose their intensity with an increase in temperature but persist even up to 150 °C. This is in agreement with the dielectric permittivity versus temperature data which show a broad peak–room temperature, and ferroelectric hysteresis persisting up to ∼140 °C. Both Raman and dielectric data are interpreted as due to the presence of a distribution of phases in the film perhaps caused by a variation in Pb content along the film thickness and/or near grain boundary regions.

Room-temperature photoluminescence and resonance-enhanced Raman scattering in highly degenerate InN films

We report the results of room-temperature photoluminescence (PL) and resonance-enhanced Raman scattering studies on highly degenerate (carrier concentration >3×1020cm−3) InN films grown on c-plane sapphire substrates by plasma source molecular-beam epitaxy. Carrier concentration-dependent PL emission peak is observed in the 1.4–1.8 eV range. These films show strong resonance-enhanced first- and second-order Raman scattering under 785 nm (1.58 eV) excitation energy and not with 514.5 nm (2.41 eV) excitation, suggesting the existence of electronic states ∼1.5eV in these samples. The PL emission peak energies and their dependence on the carrier concentration are consistent with the observed optical absorption edges. These results are compared to the data on single crystalline, low degenerate InN film grown by molecular-beam epitaxy, which shows a band-gap energy of ∼0.6eV. The results imply a large shift in the optical absorption edges due to band filling effects in the highly degenerate InN samples.

Disorder effects on infrared reflection spectra of InN films

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 infrar...

Ultraviolet and visible resonance-enhanced Raman scattering in epitaxial Al1−xInxN thin films

We report the observation of ultraviolet and visible near-resonance enhanced Raman scattering in epitaxial wurtzite Al1−xInxN (0001) (0⩽x<0.7) thin films. The films (thickness∼150 nm) were grown by plasma source molecular beam epitaxy on sapphire (0001) substrates. A substantial spectral enhancement is seen for Al-rich samples using 244 nm (5.01 eV) radiation due to the closeness of their band gap energy to the excitation energy. On the other hand, samples with x∼0.6 (energy band gap ∼2.5 eV) show significant enhancement with 514.5 nm (2.41 eV) excitation. The A1(LO) and E2 zone center phonons have been observed for all the samples. The A1(LO) phonon frequency shows the expected decrease with increasing x. The E2 mode shows a two-mode behavior supporting the recent theoretical predictions. Due to increased resonance enhancement, strong second- and third-order spectra are seen in some films.

Optical Properties of Anatase, Rutile and Amorphous Phases of TiO 2 Thin Films Grown at Room Temperature by RF Magnetron Sputtering

Anatase (A), rutile (R) and amorphous phase TiO 2 thin films have been prepared by RF magnetron sputtering on unheated glass substrates by controlling the total pressure of sputtering gases (Ar + O 2 ) and the substrate bias. The crystal structures of the films were confirmed by x-ray diffraction and Raman scattering. The analysis of optical absorption data for A- TiO 2 film shows an energy bandgap (E g ) of 3.2 eV (indirect extrapolation) and ∼ 3.5 eV (direct extrapolation). On the other hand, R-TiO 2 film shows E g ∼ 2.9 eV (indirect) and 3.2 eV (direct). The latter film also shows the presence of amorphous regions with E g ∼ 3.0 eV (indirect) and 3.8 eV (direct). The bandgap of both the films, obtained using indirect extrapolation, has a value range consistent with the previous measurements.

Optical and Electrical Properties of Low to Highly-Degenerate InN Films

The optical and electrical properties of InN films with different levels of carrier concentrations have been investigated. Hall effect measurements at room temperature show that the InN films are n-type with carrier concentration, n e , ranging from ∼ 7 ×10 17 cm -3 to ∼ 3 × 10 20 cm -3 and corresponding mobility, //, of ∼ 1300 to 50 cm 2 V -1 S -1 . Optical absorption spectra of these films show a bandgap absorption edge ∼ 0.6 eV for the InN sample with the lowest n e , and 1.5 eV for the InN sample with the highest n e . However, after corrections for the degeneracy effects, all samples show an intrinsic E g ∼ (0.60 ± 0.05) eV. Temperature dependent (5 – 600 K) electrical measurements show that n e is nearly independent of temperature below 300 K, perhaps due to the presence of donor energy levels resonating with the InN conduction band. However, all the samples show an exponential increase in n e above 300 K due to excitation of other shallow donor like sources. Mobility versus temperature graph shows a maximum ∼ 200 K for InN film with n e = 7 × 10 17 cm -3 and moves towards lower temperature with increasing n e .