V. Lemos

Photoluminescence on CdSe and CdTe under hydrostatic pressure

Abstract Photoluminescence measurements on CdSe and CdTe samples have been made under hydrostatic pressure up to the phase transition. Transition pressure values are reported. Nonlinear pressure dependence of the direct gap is observed for both materials. Pressure coefficients are in good agreement with values calculated from the dielectric theory of the chemical Bond.

Evidence of phase separation in cubic InxGa1−xN epitaxial layers by resonant Raman scattering

Phase separation effects in cubic InxGa1−xN epitaxial layers were investigated by means of resonant Raman scattering. The alloy epilayers were grown by radio-frequency plasma-assisted molecular beam epitaxy on GaAs (001) substrates. The results, which are confirmed by x-ray diffractometry (XRD) experiments, show the presence of In-rich inclusions in c-InGaN layers with x=0.19 and 0.33. In-rich inclusions were also found by XRD in a lower In-content layer with x=0.07. Compositional inhomogeneity of about 10% was observed through selective resonances of localized regions in the In-rich separated inclusions. We find that the In-rich separated phase has nearly the same composition in all analyzed samples (x≅0.8).

Optical and structural properties of polycrystalline CdSe deposited on titanium substrates

We report studies of photoluminescence, Raman scattering and x-ray diffraction performed on CdSe polycrystalline films deposited on titanium substrates by two different methods: chemical deposition and electroplating. We discuss the changes observed in these films as they are subjected to heat treatments. The differences observed in the energy gap of both types of film and their evolution as a function of annealing temperature are tentatively explained in terms of quantum confinement produced by the small grain size of the films.

Resonant Raman scattering of TO(A1), TO(E1) and E2 optical phonons in GaN

Abstract Resonant Raman Scattering by optical phonons in GaN was observed under conditions that permitted comparison with the ‘bare exciton’ theory. Excellent agreement was obtained. Results covered an energy range from ≈ 800 to 300 meV below the gap energy, which shows that even at such energies exciton effects are dominant for Raman scattering.

Raman scattering from three phases in LiIO3

Abstract We studied the temperature dependence of the Raman spectrum of LiIO3, from room temperature up to t ∼ 350°C. Two discontinuous changes in the spectrum are observed as temperature increases. The first one is reversible and occurs in a temperature range between 215°C and 260°C, depending upon sample origin (single crystal or powder) and thermal history. The second occurs at t ≳ 290°C and becomes irreversible once the samples are heated above 340°C. Each phase has a characteristic spectrum, distinct from that of the other two. Although the occurence of these phase changes are in complete agreement with studies made with X-ray diffraction and differential thermal analysis (DTA), it is at variance with previous Raman and infrared work which report no qualitative change in spectrum at the α ⇄ γ phase transition. We believe this disagreement comes about because our measurements are the first ones so far to have actually passed through the transition.

Micro-Raman analysis of cubic GaN layers grown by MBE on (001) GaAs substrate

Cubic GaN layers are grown by molecular beam epitaxy on (001) GaAs substrates. Optical micrographs of the GaN epilayers intentionally grown at Ga excess reveal the existence of surface irregularities such as bright rectangular structures, dark dots surrounded by rectangles and dark dots without rectangles. Micro-Raman spectroscopy is used to study the structural properties of these inclusions and of the epilayers in greater detail. We conclude that the observed irregularities are the result of a melting process due to the existence of a liquid Ga phase on the growing surface.

Pressure dependence of the bowing parameter of direct band gap of CdSexTe1−x

Abstract The variation of the lowest direct band gap of compounds CdSe x Te 1-x with hydrostatic pressure has been measured by photoluminescence at room temperature. This allowed determination of pressure dependence of the bowing parameter. Our data compared to theoretical calculations based on the dielectric model indicates that the average ionic potential is constant with pressure. This result has been previously assumed in the calculation of energy-band pressure coefficients and proved fundamental for the agreement with experimental values for a wide variety of semiconductors.

A Raman study of the β-phase of LilO3

Abstract Polarized Raman spectra of LilO3 in its β-phase were taken at T = 10K, using single - crystalline samples. The spectrum breaks up into two distinct regions: high (ω > 760 cm−1) and low (ω

Electroreflectance in GaSe1−xTex solid solutions☆

Abstract Room temperature electroreflectance measurements in the region of the lowest direct exciton were performed on the system GaSe1−xTex in the full composition range. A discontinuous jump in the dependence of exciton energy and broadening parameter occurs in the molar fraction range 0.26 ⩽ x ⩽ 0.60. This supports previous Raman measurements on these solid solutions which suggested the existence of two distinct phases: an hexagonal phase for x ⩽ 0.26 and a monoclinic phase for x ⩽ 0.60. Detailed study of broadening parameter as a function of molar fraction in the hexagonal phase suggests that the alloys have an indirect edge for low tellurium concentration but become direct as the latter increases. Line width considerations locates the crossing point between direct and indirect gaps at x ⋍ 0.11 , in good agreement with predictions made on volume change using the behaviour of direct and indirect edges in the alloy system GaSe1−xSx.

Pressure dependence of the Raman modes of LiIO3

Abstract The pressure dependence of the Raman modes of LiIO 3 at room temperature was studied up to 78 kbar in a gasketed sapphire-anvil cell. Nonlinear frequency increases were observed in all modes involving translation of the light Li + atom. This behavior is more marked in the lowest frequency A mode, which involves translations along the c-axis. Other modes slow smaller linear frequency increases, with increasing pressure. The TO-LO splittings of all but the highest frequency modes are seen to be independent of pressure. This indicates that charge transfer between ions is negligible. For modes derived from internal vibrations of IO 3 - ions an increase in this splitting with increasing pressure is observed, indicating some intra-ion redistribution of charge. Mode Gruneissen parameters are obtained for representative modes of all allowed symmetries in each of the three frequency ranges in which the Raman spectrum of the material can be divided.