published_or_final_version / Physics / Master / Master of Philosophy

Thesis (M. Phil.)--University of Hong Kong, 2007. / Title proper from title frame. Also available in printed format.

No description available.

Thesis (Ph. D.)--University of Hong Kong, 2008. / Also available in print.

Thesis (M. Phil.)--Hong Kong University of Science and Technology, 2002. / Includes bibliographical references (leaves 76-77). Also available in electronic version. Access restricted to campus users.

Thesis (M. Phil.)--University of Hong Kong, 2005. / Title proper from title frame. Also available in printed format.

The III-V semiconductor compounds (i.e. In Ga As x 1-x , 1 x x InAs Sb - , In Ga Sb x 1-x and Al Ga As x 1-x ) have been studied using room temperature Raman spectroscopy. X-ray diffraction has been used as a complementary characterization technique. In this study all the III-V semiconductor compounds were grown by metal organic chemical vapour deposition (MOCVD) on GaAs and GaSb substrates. The layers were studied with respect to composition, strain variation and critical thickness. Raman spectroscopy has been employed to assess the composition dependence of optical phonons in the layers. The alloy composition was varied, while the thickness was kept constant in order to investigate compositional effects. A significant frequency shift of the phonon modes were observed as the composition changed. The composition dependence of the phonon frequencies were described by linear and polynomial expressions. The results of this study were compared with previous Raman and infrared work on III-V semiconductor compounds. Strain relaxation in InGaAs and InGaSb has been investigated by Raman and X-ray diffraction. Measurements were performed on several series of layers. For each series, the thickness was varied, while keeping the composition constant. For a given composition, the layer thicknesses were such that some layers should be fully strained, some partially relaxed and some fully relaxed. The Raman peak shifts and XRD confirm that a layer grows up to the critical thickness and then releases the strain as the thickness increases. Critical layer thickness values measured in this study were compared with published data, in which various techniques had been used to estimate the critical thickness.

published_or_final_version / abstract / Physics / Doctoral / Doctor of Philosophy

Generally, nonlinear optics studies investigate optically-induced changes in refraction or absorption, and their application to spectroscopy or device fabrication. The photorefractive effect is a nonlinear optical effect that occurs in solids, where transport of an optically-induced free-carrier population results in an internal space-charge field, which produces an index change via the linear electrooptic effect. The photorefractive effect has been widely studied for a variety of materials and device applications, mainly because it allows large index changes to be generated with laser beams having only a few milliwatts of average power.Compound semiconductors are important photorefractive materials because they offer a near-infrared optical response, and because their carrier transport properties allow the index change to be generated quickly and efficiently. While many researchers have attempted to measure the fundamental temporal dynamics of the photorefractive effect in semiconductors using continuous-wave, nanosecond- and picosecond-pulsed laser beams, these investigations have been unsuccessful. However, studies with this goal are of clear relevance because they provide information about the fundamental physical processes that produce this effect, as well as the material's speed and efficiency limitations for device applications.In this dissertation, for the first time, we time-resolve the temporal dynamics of the photorefractive nonlinearities in two zincblende semiconductors, semi-insulating GaAs and undoped CdTe. While CdTe offers a lattice-match to the infrared material HgxCd1-xTe, semi-insulating GaAs has been widely used in optoelectronic and high-speed electronic applications. We use a novel transient-grating experimental method that allows picosecond temporal resolution and high sensitivity. Our results provide a clear and detailed picture of the picosecond photorefractive response of both materials, showing nonlinearities due to hot-carrier transport and the Dember space-charge field, and a long-lived nonlinearity that is due to the EL2 midgap species in GaAs. We numerically model our experimental results using a general set of equations that describe nonlinear diffraction and carrier transport, and obtain excellent agreement with the experimental results in both materials, for a wide variety of experimental conditions.

published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy