Xue-Shu Zhao

Carrier‐induced strain effect in Si and GaAs nanocrystals

New experimental results on Raman scattering from porous silicon and silicon and gallium arsenide nanocrystals are reported. In all of these systems, almost all vibrational modes become Raman active and are remarkably soft. A carrier‐induced strain model is proposed to explain the optical properties of these nanocrystal systems. According to this carrier‐induced strain model, the selection rule of crystal momentum conservation for Raman scattering is greatly relaxed in Si and GaAs nanocrystals due to the dilatation strain caused by coupling of excited free carriers with the particle lattice and the optical properties of such systems are dominated by multiphonon assisted free‐electron transition processes.

Brillouin and raman scattering from glasses under high pressure

Abstract Brillouin and Raman Scattering Spectra in SiO2 and GeO2 glasses have been measured in a diamond anvil cell up to pressures of 14 GPa. The elastic properties and equation of state for each glass type were obtained from the Brillouin scattering measurements with respect to pressure. Both elastic constants and compressibility of SiO2 and GeO2 showed anomalous behavior with respect to pressure. This anomalous behavior is reconciled with a model based on the pressure dependent bending of the oxygen angles in both glass types. The Raman measurements corroborate the conclusions from the Brillouin scattering results, namely that the SiO2 and GeO2 bond angles are changing with pressure or the oxygen angle distribution is changed without bond breaking.

Intrinsic Brillouin linewidths and stimulated Brillouin gain coefficients in glasses studied by inelastic light scattering

Rayleigh–Brillouin scattering measurements on various multicomponent halide glasses and some selected oxide glasses were performed. The Brillouin linewidth measurements, the Brillouin intensities, and Brillouin frequency shifts allowed us to calculate the phonon attenuation, Pockels elasto-optic coefficients, and the stimulated Brillouin scattering gain coefficients. Brillouin linewidths obtained experimentally range between the limit of 21 MHz for an 88BeF2–12ThF4 glass and 213 MHz for a zirconium barium lanthanum fluoride glass. The parameter obtained in this study shows that the threshold power for the onset of stimulated Brillouin scattering in some halide glasses is greater than that for silicate glasses, which has important ramifications as a selection criterion for halide-based glasses as single-mode optical waveguide materials.

Strained Quantum Dots in Porous Silicon

On the basis of Raman, photoluminescence, and absorption studies of porous and nanoparticle silicon we propose that the strong luminescence in porous silicon results from strained silicon quantum dots. A silicon nanoparticle is a special Jahn-Teller system induced by extended electron states rather than localized state. Thus Raman scattering and photoluminescence in porous silicon are multi-phonon assisted free electronic transition processes, all observed anomalous properties of porous silicon can be clearly explained by using this strained quantum dot model.

Interface Effects on Photocarrier Transport in a-Si:H

We discuss the absorption-length and film thickness dependence of photoconductivity in amorphous silicon using measurements of the ambipolar diffusion length and the Debye screening length.

Optical effects in II‐VI semiconductor nanocrystal colloids at high pressure

The Raman data of colloidal CdS shows that the LO and 2LO peaks persist far above 30 kbar which is an indication that the wurtzite to rock salt phase transition has not occurred as is the case for the bulk CdS. However the shifts of LO and 2LO peaks with pressure are about the same as the bulk CdS. We therefore conclude that stiffness or compressibility is the same as the bulk. From the photoluminescence data we observe two major peaks at 1.73 eV (peak 1) and 2.44 eV (peak 2) at 1 atm. By comparing the shifts of these two peaks as a function of pressure with that of the band gap shift in the bulk CdS, we conclude that peak 2 is either due to band edge emission or shallow defects and peak 1 is attributed to a deep level.

Optical high pressure studies of ternary semiconducting nanocrystals

Photoluminescence and Raman scattering as a function of pressure were used to study CdSxSe1−x composites. The phase transition from wurtzite to rocksalt structure in semiconducting nanocrystalline systems is quite different than in bulk material. They exhibit high pressure stability which may vary over a broad pressure range. The compositional dependence of deep levels are examined in terms of energy and pressure coefficients. Our results show that different behavior in energy is also reflected in the pressure coefficients.