R. Cuscó

Raman-scattering study of the InGaN alloy over the whole composition range

We present Raman-scattering measurements on InxGa1−xN over the entire composition range of the alloy. The frequencies of the A1(LO) and E2 modes are reported and show a good agreement with the one-mode behavior dispersion predicted by the modified random-element isodisplacement model. The A1(LO) mode displays a high intensity relative to the E2 mode due to resonant enhancement. For above band-gap excitation, the A1(LO) peak displays frequency shifts as a function of the excitation energy due to selective excitation of regions with different In contents, and strong multiphonon scattering up to 3LO is observed in outgoing resonance conditions.

Isotopic study of the nitrogen-related modes in N+-implanted ZnO

Micro-Raman measurements were performed to study the nitrogen-related modes in ZnO samples implanted with N+. The two stable N isotopes, N14 and N15, were implanted. Distinct peaks at 277 and 512cm−1 are observed irrespective of the implanted isotope, both before and after rapid thermal annealing. The insensitivity of the mode frequencies to the implanted isotope rules out the explanation of these modes as local vibrational modes involving N motion. These modes were not detected in ZnO samples implanted with Zn+, O+, or P+, which suggests that they may be associated with distortions∕defects favored by the presence of N.

Raman scattering of cadmium oxide epilayers grown by metal-organic vapor phase epitaxy

We present a lattice-dynamics study of rocksalt cadmium oxide (CdO). We use Raman scattering to investigate a series of high-quality CdO epilayers grown on sapphire substrates by means of metal-organic vapor phase epitaxy. Two main features are found to dominate the Raman spectra of CdO: a sharp peak at ∼265 cm−1 and a broad feature at ∼390 cm−1. To assign these and other weaker second-order features that appear in the Raman spectra, we have carried out an ab initio calculation of the phonon dispersion in CdO. From the calculated two-phonon density of states (PDOS) we tentatively assign the peak at 265 cm−1 to a 2TA(L) mode. The broadband centered at ∼390 cm−1 seems to reflect the high two-PDOS in the 300–450 cm−1 spectral region.

Performance of bulk SiC radiation detectors

SiC is a wide-gap material with excellent electrical and physical properties that may make it an important material for some future electronic devices. The most important possible applications of SiC are in hostile environments, such as in car/jet engines, within nuclear reactors, or in outer space. Another area where the material properties, most notably radiation hardness, would be valuable is in the inner tracking detectors of particle physics experiments. Here, we describe the performance of SiC diodes irradiated in the 24 GeV proton beam at CERN. Schottky measurements have been used to probe the irradiated material for changes in I–V characteristics. Other methods, borrowed from III–V research, used to study the irradiated surface include atomic force microscope scans and Raman spectroscopy. These have been used to observe the damage to the materials surface and internal lattice structure. We have also characterised the detection capabilities of bulk semi-insulating SiC for α radiation. By measuring the charge collection efficiency (CCE) for variations in bias voltage, CCE values up to 100% have been measured.

Probing the electron density in undoped, Si-doped, and Mg-doped InN nanowires by means of Raman scattering

We report a Raman scattering determination of the electron density in InN nanowires from the analysis of longitudinal optical-phonon-plasmon coupled modes. A Raman peak assigned to the L− coupled mode is observed in both undoped and doped InN nanowires. This peak exhibits a shift to higher (lower) frequencies in the Si-doped (Mg-doped) nanowires and allows us to estimate the electron density in the nanowires. A significant residual electron density is found in the undoped nanowires, which increases in Si-doped nanowires and is partially compensated in Mg-doped nanowires.

Far-infrared transmission in GaN, AlN, and AlGaN thin films grown by molecular beam epitaxy

We present a far-infrared transmission study on group-III nitride thin films. Cubic GaN and AlN layers and c-oriented wurtzite GaN, AlN, and AlxGa1−xN (x<0.3) layers were grown by molecular beam epitaxy on GaAs and Si(111) substrates, respectively. The Berreman effect allows us to observe simultaneously the transverse optic and the longitudinal optic phonons of both the cubic and the hexagonal films as transmission minima in the infrared spectra acquired with obliquely incident radiation. We discuss our results in terms of the relevant electromagnetic theory of infrared transmission in cubic and wurtzite thin films. We compare the infrared results with visible Raman-scattering measurements. In the case of films with low scattering volumes and/or low Raman efficiencies and also when the Raman signal of the substrate material obscures the weaker peaks from the nitride films, we find that the Berreman technique is particularly useful to complement Raman spectroscopy.

Strain relaxation in stacked InAs/GaAs quantum dots studied by Raman scattering

We report a Raman scattering investigation of InAs vibrational modes in multiple layers of InAs self-assembled quantum dots in a GaAs matrix. The Raman peak associated with quantum-dot phonons shows a downward frequency shift as the interlayer spacing decreases. We attribute this frequency shift to the relaxation of the elastic strain in the stacked quantum-dot layers. From the phonon frequency shift, we estimate the magnitude of the strain in the quantum dot layers, which we relate to the energy of the photoluminescence emission of the dots.

Quantum-dot phonons in self-assembled InAs/GaAs quantum dots: Dependence on the coverage thickness

We study the phonons of self-assembled InAs/GaAs quantum dots for different coverage thicknesses L. The additional Raman feature detected between the GaAs transverse optical and the InAs longitudinal optical modes, which we assign to phonons of the dots, exhibits an upward frequency shift with L. This shift is attributed to compressive strain in the dots and, on the basis of its dependence on L, we show that these phonons arise from the quantum dots and not from the wetting layer.

Raman‐scattering assessment of Si+‐implantation damage in InP

We studied the lattice damage caused by Si+ implantation into semi‐insulating InP, with doses in the range of 1012 to 5×1014 cm−2, and the subsequent lattice recovery achieved by rapid thermal annealing (RTA), by means of Raman spectroscopy. With increasing implantation dose, an intensity reduction of the first‐ and second‐order Raman peaks characteristic of crystalline InP is observed, together with the enhancement of disorder‐activated modes. In samples implanted with doses higher than 1014 cm−2 the Raman spectra resembles that of amorphous InP, and the samples can be considered as fully amorphized. By RTA at 875 °C for 10 s, sample crystallinity is recovered, even in the case of those samples implanted with the highest dose. After annealing, the Raman spectra show no evidence of disorder‐activated modes, and the intensity of the characteristic second‐order peaks approaches the value found in unimplanted InP.

Crystal damage assessment of Be+-implanted GaN by UV Raman scattering

We present resonant UV Raman-scattering measurements on GaN epilayers implanted to different doses of Be+ ions. The Raman spectra are dominated by outgoing multiphonon resonant Raman lines, the intensity of which is very sensitive to the implantation dose. We discuss the criteria, based on the intensities and widths of the observed lines, which allow one to assess the crystal quality of the implanted samples.