J. Ibáñez

Molecular beam epitaxy of GaBiAs on (311)B GaAs substrates

We report the growth by molecular beam epitaxy of GaBixAs1−x epilayers on (311)B GaAs substrates. We use high-resolution x-ray diffraction (HRXRD), transmission electron microscopy, and Z-contrast imaging to characterize the structural properties of the as-grown material. We find that the incorporation of Bi into the GaBiAs alloy, as determined by HRXRD, is sizably larger in the (311)B epilayers than in (001) epilayers, giving rise to reduced band-gap energies as obtained by optical transmission spectroscopy.

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.

Carrier localization in GaBiAs probed by photomodulated transmittance and photoluminescence

The carrier localization phenomenon has been investigated for GaBiAs by photomodulated transmittance (PT) and photoluminescence (PL). In the case of PT measurements, a decrease in the energy-gap related PT signal has been clearly observed below 180 K. In PL spectra a broad emission band very sensitive to the excitation power has been found. In comparison to the energy-gap related transition, this band is shifted to red. The recombination time for this band at low temperature decreases from 0.7 to 0.35 ns with the increase in the emission energy. All the findings are clear evidences for strong carrier localization in this alloy.

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.

Nitrogen incorporation into strained (In, Ga) (As, N) thin films grown on (100), (511), (411), (311), and (111) GaAs substrates studied by photoreflectance spectroscopy and high-resolution x-ray diffraction

We have used photoreflectance (PR) and high-resolution x-ray diffraction (HRXRD) measurements to assess the composition of InxGa1−xAs1−yNy thin films (x∼20%, y∼3%) grown by molecular beam epitaxy on GaAs substrates with different surface orientations. The aim of our work is to investigate the effect of substrate orientation on the incorporation of N and In into the films. Whereas in principle the composition of the InxGa1−xAs1−yNy films cannot be characterized by HRXRD alone because lattice spacings depend on both x and y, we show that a combined analysis of the PR and HRXRD data allows us to determine the sample composition. Our data analysis suggests that the incorporation of N in (In, Ga)(As, N) exhibits some dependence on substrate orientation, although not as strong as previously observed in Ga(As, N). We determine shear deformation potentials for our samples that are sizably different than those obtained by linearly interpolating from the values of the pure binary compounds, which has already been o...

Thermal quenching of single localized excitons in GaInNAs layers

The authors applied the temperature-dependent microphotoluminescence to study the thermal quenching of exciton lines related to localized emission from GaInNAs. It has been observed that the dispersion of localization energy is very strong (∼0–150 meV) whereas the activation energy of each individual line is the same within the experimental error (∼6 meV). This suggests that the main source of sharp line features are excitons localized on deep donor(acceptor)like states. At low temperatures these states can participate in radiative recombination due to the coulomb attraction between electrons and holes whereas at higher temperatures they still trap carriers but these carriers recombine nonradiatively.

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.

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.

High-pressure Raman scattering in wurtzite indium nitride

We perform Raman-scattering measurements at high hydrostatic pressures on c-face and a-face InN layers to investigate the high-pressure behavior of the zone-center optical phonons of wurtzite InN. Linear pressure coefficients and mode Gruneisen parameters are obtained, and the experimental results are compared with theoretical values obtained from ab initio lattice-dynamical calculations. Good agreement is found between the experimental and calculated results.