Inelastic Light Scattering in Low Dimensional Semiconductors

[摘要] Raman scattering is a powerful technique with which to study the lattice vibrations of semiconductors. Investigations of the phonons of GaInAs-InP heterostructures have shown that although the phonons in GalnAs quantum wells resembled those of bulk GaInAs, they were screened by free carriers. Raman scattering and photoluminescence techniques were employed to estimate the plasma density at which plasmon-phonon coupling became significant. Triple crystal x-ray diffraction measurements complemented the Raman scattering data and provided information on the GaInAs alloy composition and state of strain. It was found that although nominally lattice-matched to the underlying InP, the epitaxially-grown layers were tetragonally distorted in the direction of growth. Assessment of sample damage produced by reactive-ion-etching (RIE) was undertaken as a prerequisite to the study of phonons in fabricated nanostructures. Structural damage to the crystal showed up as a relaxation of the crystalline selection rules allowing the observation of a symmetry-forbidden phonon. The intensity of this phonon correlated well with depth profiling of the damage. Optimised RIE conditions were found to produce negligible crystalline damage. The study of GaAs cylinders (with diameters of less than 100 nanometers) revealed an additional feature in the optical phonon region of their Raman spectra. This feature was identified as a surface phonon of the quantum cylinders. The experimentally-observed frequencies of the surface phonon peaks showed good agreement with calculated frequencies based on vibrations in small, geometrically-regular crystals. The main contribution of this work is the study of the surface phonons of the GaAs quantum cylinders. This is the first time that surface phonons have been observed in small fabricated samples: all previous work has involved specially-prepared crystalline powders or else comparatively large slab geometries. The conclusion that can be drawn from this work is that the cylinders are not only well-defined (as observed from the SEM micrographs) but they are also crystalline. The implication is that such structures can now be fabricated at a sufficiently high level to allow progress in prototype devices such as the quantum dot laser.

[发布机构] University:University of Glasgow

[关键词] Electrical engineering