L. Artús

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.

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.

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.

Raman scattering in InAs/(AlGa)As self-assembled quantum dots : Evidence of Al intermixing

We use Raman scattering to study the composition of InAs self-assembled quantum dots (QDs) embedded in an AlxGa1−xAs matrix. When Al is introduced into the matrix (0.15<x<0.4), the QD phonon frequency exhibits a downward frequency shift with respect to x=0. This shift is attributed to the incorporation of Al into the QDs. For the samples with an Al-rich matrix (x≳0.6), the QD phonon frequency is higher than that for the samples with a Ga-rich matrix, which suggests that Al∕In intermixing is weaker than Ga∕In intermixing. These processes affect strongly the energy of the QD luminescence.

Raman-scattering criteria for characterization of anneal-restored zinc blende single crystals: Application to Si+-implanted InP

We have studied the lattice recovery by rapid thermal annealing of Si+-implanted InP using Raman spectroscopy. The crystallinity recovery for different annealing temperatures of samples totally amorphized by the implantation can be monitored by means of their Raman spectra. However, free-charge coupling with the LO mode and possible misorientation of the recrystallized material may alter substantially the first-order Raman spectrum, making it unreliable for a good characterization of the lattice recovery. The study of second-order Raman spectrum overcomes the problems present in the analysis of first-order Raman spectrum and provides suitable criteria to assess the recrystallization of the implanted and annealed samples. After rapid thermal annealing at 875 °C for 10 s, the intensity of the second-order peaks approaches 70% of its value in virgin InP, and third-order Raman peaks are also clearly detected, evidencing the good lattice recovery achieved.

Probing the intermixing in In(Ga)As∕GaAs self-assembled quantum dots by Raman scattering

We show that Raman scattering is a sensitive technique for probing the degree of Ga intermixing in In(Ga)As∕GaAs self-assembled quantum dots (QDs). The shifts of the QD phonon frequency that we observe are explained by the modification of the strain due to Ga incorporation into the QDs from the GaAs matrix during growth. Using an elastic continuum model, we estimate the average In content of the dots from the QD phonon frequency. The varying amount of intermixing in QDs grown with different In compositions, QD layer thicknesses, growth temperatures, and stacking spacer layer thicknesses are investigated. The Raman data indicate that Ga intermixing is larger for QD samples with low In(Ga)As coverage thickness and∕or high growth temperature and, in multilayered systems, for samples with small GaAs spacer layers.