S.W. da Silva

Comparative Raman studies of cubic and hexagonal GaN epitaxial layers

Hexagonal and cubic GaN layers are grown on (001) GaAs substrates by means of molecular beam epitaxy. First order Raman spectra are taken from these layers at various incident laser wavelengths and temperatures. The T2 transverse‐optical (TO) and longitudinal‐optical (LO) frequencies of cubic GaN are determined, as well as the A1 TO and LO, E1 TO, and E2 frequencies of hexagonal GaN. The T2 TO frequency of cubic GaN lies between the A1 and E1 TO frequencies of hexagonal GaN as one expects comparing the lattice dynamics of zincblende and wurtzite type crystals. The T2 TO frequency is close to the calculated value but disagrees with a recently reported experimental value. For the hexagonal layer, all frequencies are close to those previously measured. A broad Raman structure below the A1 LO peak is interpreted in terms of a disturbed long range order of the hexagonal layer.

The influence of cobalt population on the structural properties of CoxFe3−xO4

CoxFe3−xO4-based (x=0.05–1.6) nanoparticles prepared by combustion reaction were investigated using x-ray diffraction, Raman spectroscopy, and Mossbauer spectroscopy. The Mossbauer data revealed Co2+ in both tetrahedral (site A) and octahedral (site B) sites of the cubic ferrite structure. For x⩽0.4 the experimental data indicated the synthesis of a core-shell structure, with hematite as the shell and cobalt ferrite as the core of the nanoparticle. Differences in crystalline structure between the two phases support the evidences we found of a highly stressed core-shell interface, leading to symmetry reduction of the tetrahedral and octahedral sites.

Stability of citrate-coated magnetite and cobalt-ferrite nanoparticles under laser irradiation: a Raman spectroscopy investigation

In this paper, Raman spectroscopy was used to investigate the chemical and structural properties of citrate-coated Fe/sub 3/O/sub 4/ and CoFe/sub 2/O/sub 4/ nanoparticles. The Raman data obtained from the two samples investigated are compared with the Raman data from the literature of bulk maghemite and bulk hematite. The Raman data are used to discuss the higher structural stability of the CoFe/sub 2/O/sub 4/-based sample in comparison to the Fe/sub 3/O/sub 4/-based sample, when submitted to optical illumination at different intensities.

Raman study of ionic water-based copper and zinc ferrite magnetic fluids

Raman spectroscopy is used to investigate the OH-bending and OH-stretching modes of chemisorbed OH-groups in copper-ferrite and zinc-ferrite water-based magnetic fluid samples. Room-temperature Raman spectra were taken from diluted magnetic fluid samples and compared with the spectrum taken from liquid water. The suppression of the symmetric OH-stretching Raman modes from the ionic magnetic fluid sample spectra is discussed in terms of the replacement of a hydrogen atom from the water molecule by the nanoparticle surface. Moreover, changes on the area ratio between hydrogen bonded and nonhydrogen bonded OH-Raman peaks in the ionic magnetic fluid samples, as compared to the OH-Raman peaks from liquid water, are discussed in terms of changes on the hydrogen bond strength.

Effect of the Zn content in the structural and magnetic properties of ZnxMg1−xFe2O4 mixed ferrites monitored by Raman and Mössbauer spectroscopies

Samples of ZnxMg1−xFe2O4 (0≤x≤1) synthesized by the combustion reaction method were investigated by x-ray diffraction, Mossbauer spectroscopy, and Raman spectroscopy. All the samples are found to have a cubic spinel structure and the lattice parameter increases linearly with increasing Zn-content (x). The Mossbauer data showed that the replacement of Mg2+ ions for Zn2+ ions changes substantially the hyperfine parameter. Moreover, it was verified the presence of Fe3+ ions both in A and B sites. The Raman spectra showed five predicted Raman bands for the spinel structure and it was observed the splitting of the A1g Raman mode into tree branches, where each one have been attributed to peaks belonging to each ion (Zn, Mg, and Fe) in the tetrahedral positions.

Crystallization process of amorphous GaSb films studied by Raman spectroscopy

Thermal annealings of amorphous gallium antimonide films were accompanied using Raman spectroscopy, both for stoichiometric and nonstoichiometric compositions. The films were prepared by flash evaporation on silicon substrates. Structural changes were induced by the heat treatments: an increasing degree of crystallization as a function of the annealing temperature is observed. Sb clusters are found to crystallize before GaSb does, and the dependence of the corresponding Raman peak intensity with the annealing temperature (occurring in two regimes) is explained. A mechanism for the crystallization of the amorphous GaSb is proposed, based on the prior migration of the Sb excess outside the GaSb region to be crystallized.

SPATIAL AND TEMPERATURE DEPENDENCE OF CARRIER RECOMBINATION IN AN INGAAS/INP HETEROSTRUCTURE

The microluminescence surface scan technique has been used to investigate ambipolar carrier diffusion, photon diffusion, and photocarrier recombination in a nominally undoped InGaAs bulk layer lattice matched to InP grown by vapor levitation epitaxy. Measurements taken at different temperatures between 75 and 300 K are discussed in terms of the relative contribution of the two distinct mechanisms to the spectrally integrated luminescence intensity, namely, photon diffusion and photocarrier diffusion.

Raman spectroscopy of magnetoliposomes

In this study Raman spectroscopy was used to investigate monolayer and bilayer magnetite-based magnetoliposomes (MLs). The Raman probe is the hydroxyl (OH) group chemisorbed at the magnetite nanoparticle surface. Measurements were performed at room temperature in the typical OH stretching region. The data gathered for both samples are compared to each other and with those obtained for pure water. In comparison to liquid water (2.74 kcal/ mol), it was found that the hydrogen bond strength between the chemisorbed OH-group and the polar headgroup of the inner phospholipid layer was reduced in both the monolayer (2.22 kcal/mol) and the bilayer (1.83 kcal/mol) ML samples.

Carrier kinetics in quantum dots through continuous wave photoluminescence modeling: A systematic study on a sample with surface dot density gradient

A systematic study is presented of continuous wave (cw) photoluminescence (PL) of self-assembled quantum dots (QDs) grown on GaAs (001) by molecular-beam epitaxy as a function of excitation intensity and QD density. The sample used in this work was grown under nonisotropic indium flux that resulted in a QD density gradient across the sample surface ranging from 0 to 1.8×1011 cm−2. The carrier kinetics in the sample is described by a set of coupled rate equations through which the cw PL data from the GaAs barrier, wetting layer (WL), and QDs were simulated as a function of the excitation intensity and QD density. By comparing the PL data with our simulations we infer that carrier capture into the QD occurs directly from GaAs barrier. Auger and phonon-assisted carrier capture from the WL were found to give negligible contribution. With an increase of the QD density we observe an increase of the nonradiative recombination rates of the barrier and at the WL, which we tentatively correlate with the increase of...

Symmetric and asymmetric fractal diffusion of electron–hole plasmas in semiconductor quantum wells

Abstract The diffusion of photogenerated electron–hole plasmas in intrinsic InP/InGaAs/InP single quantum wells was investigated by measurements of the photoluminescence intensity profile around the illuminated area. Two kinds of heterostructures were investigated, one grown on InP substrate with the growth face orthogonal to the [001] direction, and the other grown on InP substrate two degrees off that orientation. The data show that the particles present a fractal diffusion described by the Levy distribution with the exponent parameter α =1.3. In the tilted heterostructure, the diffusion is asymmetric and the particles density obeys a generalized Levy distribution recently predicted by Chaves [Phys. Lett. A 239 (1998) 13].