Showing papers by "Paulo V. Santos published in 1995"

Hydrogen‐induced modification of the optical properties of the GaAs(100) surface

Abstract: The influence of hydrogen adsorption on the surface order, dielectric function, and dielectric anisotropy was investigated for the three main reconstructions of the GaAs(100) surface [c(4×4), (2×4), (4×2)] by low‐energy electron diffraction, spectroscopic ellipsometry, and reflectance difference spectroscopy. For all reconstructions, low hydrogen exposure removes the surface dimers, whereas surface roughening is observed at high hydrogen exposures. However, detailed differences can be resolved depending on the surface reconstruction. On the As‐rich c(4×4) surface, the outermost As‐dimers are removed at low H‐exposures leaving behind a still As‐rich surface of (1×1) symmetry. Thereafter, more As is removed and a new surface anisotropy develops due to the exposed Ga‐dimers arranged in a mixture of (1×2) and (√2×√2)‐reconstructions. For H‐adsorption on the (2×4)‐ and (4×2)‐surfaces, in contrast, only the subsequent reduction of the surface reconstruction is observed with increasing exposure, but no structura...

Theoretical study of band-edge states in Sn 1 Ge n strained-layer superlattices

Abstract: Recently, ${\mathrm{Sn}}_{1}$${\mathrm{Ge}}_{\mathit{n}}$ (ng10) strained-layer superlattices grown along the [001] direction have been fabricated in spite of the large lattice mismatch (13%) between the two constituents. Sn/Ge superlattices, with a small positive band gap, could have applications in far-infrared optoelectronics. In this paper, we present a detailed study of spin splittings in ${\mathrm{Sn}}_{1}$${\mathrm{Ge}}_{\mathit{n}}$ strained-layer superlattices without inversion symmetry. The effective masses for the ${\mathrm{Sn}}_{1}$${\mathrm{Ge}}_{3}$ structures have also been calculated. These band-edge properties are correlated with the nature of the superlattice states in both real space and wave-vector space and are compared with those of GaAs/AlAs and Si/Ge superlattices. Our calculations are based on the linear-muffin-tin-orbital and the empirical tight-binding methods.

Hydrogen interaction with Sb-terminated GaAs and InP (110) surfaces.

Abstract: The interaction of atomic hydrogen with InP(110) and GaAs(110) surfaces terminated with a well-ordered antimony monolayer was investigated using spectroscopic ellipsometry and Raman spectroscopy. The characteristic features of the surface dielectric function associated with the Sb monolayer and the Raman lines from the excitation of surface vibrational modes disappear with increasing hydrogen exposure, indicating the etching of the Sb monolayer. No additional structure in the surface dielectric function and no shift in the phonon frequencies were detected during the etching process. By comparing the experimental results with ab initio calculations we conclude that the etching of the Sb monolayer starts at defect sites of the Sb monolayers and then propagates through the Sb surface chains.

Optical anisotropy in [311]-oriented GaAs/AlAs superlattices

Abstract: [311]-oriented GaAs/AlAs superlattices have a monoclinic unit cell and are thus anisotropic in the plane of the superlattice layers. The superlattice interfaces have a periodic corrugation along the [233] in-layer direction in form of macrosteps with a height of 10.2 A. The effects of the interface corrugation on the optical anisotropy are investigated in the energy range of the bulk GaAs E 1 and E 1 + Δ 1 using ellipsometry and reflection difference spectroscopy. The experimental results are compared to an effective medium calculation of the optical anisotropy. It is found that the measured anisotropy is considerably smaller than that expected from the interface corrugations. Reasons for these discrepancies are discussed in the text.