J. C. Galzerani

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.

Structural and optical analysis of GaAsP/GaP core-shell nanowires

The structural and optical properties of GaAsP/GaP core-shell nanowires grown by gas source molecular beam epitaxy were investigated by transmission electron microscopy, Raman spectroscopy, photoluminescence (PL), and magneto-PL. The effects of surface depletion and compositional variations in the ternary alloy manifested as a redshift in GaAsP PL upon surface passivation, and a decrease in redshift in PL in the presence of a magnetic field due to spatial confinement of carriers.

Capacitance spectroscopy of InAs self-assembled quantum dots embedded in a GaAs/AlAs superlattice

The characteristics of the InAs self-assembled quantum dots embedded both in a GaAs bulk matrix and in a GaAs/AlAs superlattice were investigated. Evidences of electrons confinement inside the InAs quantum dots were obtained using both capacitance–voltage measurements and Raman spectroscopy. A much stronger electron localization was detected for the quantum dots embedded in the superlattice in comparison with those embedded in bulk GaAs. As a consequence, the electrical characteristics of the structures with quantum dots grown in superlattices were found to be significantly thermo-stabilized. The origins of these effects are discussed in connection with the differences between the electronic features of the two kinds of structures.

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.

CHARACTERIZATION OF GAAS WIRE CRYSTALS GROWN ON POROUS SILICON BY RAMAN SCATTERING

We measured the Raman spectra of GaAs wirelike crystals grown on porous silicon (PS) using two different excitation radiations which probe the near surface and the bulk. The transverse optic and longitudinal optic vibrational bands appear redshifted and broadened when compared to bulk GaAs, and with shoulders on their low frequency sides. These features are attributed to a disorder-induced relaxation in the selection rules for Raman activity arising from point defects. In addition, the bands show a strong frequency and bandwidth dependence on excitation energy. Differences in penetration depth together with a gradient in defect density are invoked to account for the observations. This analysis yields a picture in which there is a predominantly amorphous GaAs region buried at the PS–GaAs interface followed by microcrystalline GaAs cylindrical structures that become less defective as they grow farther from the interface. The near surface tends to approach the low defect density of crystalline GaAs.

Raman study of the topology of InAs/GaAs self-assembled quantum dots

The topology of self-assembled InAs/GaAs quantum dots was studied by resonant Raman scattering caused by the interface modes localized near the edges of the dots. Evidences were found that on both sides of the InAs layer containing the dots, their topologies show some resemblances. In addition, in the multilayered systems the evidence of the coalescence of the dots (which form vertical columns) in neighbor layers separated by the distance smaller than 25 monolayers was obtained.

Properties of Au‐Zn Ohmic contacts to p‐GaSb

Ohmic contacts to p‐GaSb were prepared by the deposition of (100 A Au+100 A Zn+800 A Au) and their characteristics were analyzed. Measurements of specific contact resistance as a function of annealing temperature show a minimum value of ∼1×10−5 Ω cm2 for alloying at 300 °C for 15 min. Auger electron spectroscopy depth profiles and Rutherford backscattering spectroscopy analysis for samples annealed at different temperatures show the diffusion of Au into GaSb and also give evidence of Ga outdiffusion and Zn in‐diffusion. The presence of oxygen in the film surface and at the interface is discussed.

Carrier confinement in an ultrathin barrier GaAs/AlAs superlattice probed by capacitance–voltage measurements

Abstract The capacitance–voltage characteristics of (GaAs)m/(AlAs)n superlattices and of a GaAs/AlGaAs multiple quantum well system were used as a tool to probe the homogeneity of the studied samples. Evidences of a strong electron localization in the superlattices even with the presence of minibands were found. We interpret this result taking into account the presence of local inhomogeneities in the superlattices which causes the breakdown of the coherence of the miniband transport and therefore, give rise to the electron localization. In order to support this conclusion we numerically calculate the capacitance of the superlattices assuming a localization center near the region where electron confinement takes place and the results were found in good agreement with the measured capacitance.

Espectroscopias de infravermelho, Raman e de fotoluminescência: potencialidades e complementaridades

It is quite common that the student engaged in the analysis of properties of materials using spectroscopic techniques based on the effects of infrared absorption, luminescence and Raman scattering, faces difficulties such as the interpretation of the information that can be accessed in each case, the choice of the best technique in order to study a specific phenomenon and, especially, the cross-checking of the information obtained in each one of the three techniques. Trying to make easier the apprenticeship of the rational use of these techniques, we summarize in this paper the fundamental physical principles related to each one of them, and how these principles determine their employment - individually or in a complementary way - to explore their potentiality towards better characterizing the desired properties.

Resonant Raman scattering in GeSi/Si superlattices with GeSi quantum dots

The resonant Raman scattering in GeSi/Si structures with GeSi quantum dots has been analyzed. These structures were formed at various temperatures in the process of molecular-beam epitaxy. It has been shown that Raman scattering spectra recorded near resonances with the E0 and E1 electronic transitions exhibit the lines of Ge optical phonons whose frequencies differ significantly from the corresponding values in bulk germanium. In the structures grown at low temperatures (300–400°C), the phonon frequency decreases with increasing excitation energy. This behavior is attributed to Raman scattering, which is sensitive to the size of quantum dots, and shows that quantum dots are inhomogeneous in size. In the structures grown at a higher temperature (500°C), the opposite dependence of the frequency of Ge phonons on excitation energy is observed. This behavior is attributed to the competitive effect of internal mechanical stresses in quantum dots, the localization of optical photons, and the mixing of Ge and Si atoms in structures with a bimodal size distribution of quantum dots.