published_or_final_version / Physics / Doctoral / Doctor of Philosophy

Thesis (Ph. D.)--University of Hong Kong, 1999. / Includes bibliographical references.

Silicon-on-insulator (SOI) structures formed in the top region of silicon wafers by ion implantation of oxygen were characterized by RBS (Rutherford Backscattering Spectrometry), OPM (Optical Microscopy), SEM (Scanning Electron Microscopy), and TEM (Transmission Electron Microscopy). Specimens taken from these wafers were previously subjected to specific thermal treatment and silicon epitaxial growth. The results of this investigation show that homogeneous, stoichiometric buried SiO₂ layers were formed beneath the silicon wafer surfaces after high-dose oxygen ion implantation (2.0 x 10¹⁸ O⁺/cm², 180 keV/O⁺). No buried SiO₂ layers were observed in the low-dose wafers (1.0 x 10¹⁷ O⁺/cm², 180 keV/O⁺). Solid-phase epitaxial regrowth (SPE) is strongly temperature dependent. The transition from amorphous (caused by ion impact) to crystalline through the SPE process is completed in the high-dose-rate wafers (∼33 μA/cm²), but not in the low-dose-rate wafers (∼17 μA/cm²). Polysilicon layers were formed on both sides of the SiO₂ layer in the low-dose-rate wafers. Evidence shows that both post annealing (>1000°C, 2 hours in N₂) and in-situ annealing (wafer substrate heating at 500°C during oxygen ion implantation) lower the imperfection density of the top surface region of silicon wafers. A silicon epitaxial layer with low levels of crystalline imperfections was able to be grown on these annealed wafers. The results also show that in-situ annealing is more effective than post annealing. The major microdefects in SOI structures observed in this investigation are dislocations.

Thin layer deposits of cadmium sulfide (CdS) for photovoltaic purposes can be made by cathodic deposition from a nonaqueous solution. There were numerous parameters that were controlled in this electro-deposition. Several of these parameters, including temperature, current density, reactant concentrations and impurity level doping, were studied and optimized. The mechanism of this deposition process is not fully understood, mainly due to the complex chemistry of sulfur. Part of this complexity is the presence of S(,6) and S(,7) along with the major component of S(,8) in sulfur solutions. At 90(DEGREES), these minor species constitute 2% of the total sulfur. Electrochemical studies were made on these species with gold, porous carbon and CdS single crystal electrodes. These showed that S(,6) and S(,7) are electrochemically more reactive than S(,8). Furthermore, they may be the main reacting species in CdS formation, even though they are present at such low levels. Adsorption of all species of sulfur was noted at room temperature and this adsorption may be causing excess sulfur to be incorporated into the CdS deposits. There has been an important development in the measurement of impurity levels of semiconductors called electrochemical photocapacitance spectroscopy. It can be used to analyze impurity levels in a wide variety of semiconductors by shining subband gap light on the semiconductor in a photoelectrochemical cell while measuring the capacitance on the surface. Interpretation of these spectra has previously been qualitative. A quantitative model was developed here along with a computer program utilizing this interpretation. Several types of semiconductors were analyzed by this technique, including these CdS deposits which showed impurity levels that may be due to excess sulfur. Other types of compound semiconductors can also be made by this cathodic deposition. It is hoped that the information gathered here can be used to improve these other semiconductor depositions as well as CdS.

Graduation date: 2004

No description available.

Thesis (M.S.)--Ohio University, August, 1985. / Title from PDF t.p.

This thesis concerns the study of equilibrium and photo excited carriers in thin-film semiconductors using terahertz time-domain spectroscopy (THz- TDS) and optical- pump terahertz-probe (OPTP) spectroscopy. In particular, it focuses on the develop- ment and application of a MATLAB program to extract the sample complex refractive index (and thereby the complex conductivity) from THz-TDS data. The parameter extraction procedure was developed to be effective in the case of thin-film samples for which the transmission function contains a substantial contribution from multiple internal reflections in the sample. The program's effectiveness was demonstrated in the case of a 30 nm-thick film of gold deposited on silicon; the program successfully extracted the complex refractive index while the original method failed. The program was applied in a study of thin-film nanoporous indium phosphide (InP). The frequency-dependent complex conductivity-obtained via THz- TDS meas- urements-was not Drude-like, as observed for bulk InP, but was qualitatively con- sistent with a plasmonic response. The time-resolved photoconductivity was obtained from OPTP spectroscopy measurements, and revealed that the presence of pores substantially increases the photo carrier lifetime compared with the value measured for bulk InP. This behaviour is attributed to the presence of surface-bound electronic states that pin the Fermi level at the pore surface, bending the electronic bands upwards. This creates a depletion region which contains the carriers away from the recombination sites at the surface. The significance of the role played by surface states in determining carrier be- haviour is further evidenced by a study of carrier dynamics in etched and surface- passivated semi-insulating gallium arsenide (SI-GaAs). Compared with an untreated sample, surface-passivated SI-GaAs exhibited a greater change in photoconductiv- ity upon generation of photocarriers and an increased photo carrier lifetime. These observations are attributed to an increased photo carrier mobility and a decreased recombination rate resulting from the removal of surface states.

by Kim-chung Koon. / Chinese title: / Bibliography: leaves 108-112 / Thesis (M.Phil.)--Chinese University of Hong Kong, 1983

by Ong Chung Wo. / Bibliography: leaves 118-120 / Thesis (M.Ph.)--Chinese University of Hong Kong, 1986