Thesis (M. Phil.)--University of Hong Kong, 2002. / Includes bibliographical references (leaves 75-77).

The energy requirements of our planet will continue to grow with increasing world population and the modernization of currently underdeveloped countries. This will force us to search for environmental friendly alternative energy resources. Solar energy by far provides the largest of all renewable energy resources with an average power of 120 000 TW irradiated from the sun which can be exploited through solar electricity, solar fuel, and biomass. Nanostructured materials have been the subject of extensive research as the building block for construction of solar energy conversion devices for the past decades. The nanostructured materials sometimes have peculiar electrical and optical properties that are often shape and size dependent and are not expected in the bulk phase. Recent research has focused on new strategies to control nanostructured morphologies and compositions of semiconductor materials to optimize their solar conversion efficiency. In this dissertation, we discuss the synthesis and characterizations of one dimensional nanostructured TiO₂ based materials and their solar energy conversion applications. We have developed a solvothermal synthesis method for growing densely packed, vertical, single crystalline TiO₂ rutile nanowire arrays with unprecedented small feature sizes of 5 nm and lengths up to 4.4 [mu]m. Because of TiO₂'s large band gap, the working spectrum of TiO₂ is limited to the ultra violet region with photons shorter than 420 nm. We demonstrate that the active spectrum of TiO₂ can be shifted to ~ 520 nm with incorporation of N via nitridation of TiO₂ nanowires in NH₃ flow. In addition, we demonstrate a synergistic effect involving hydrogenation and nitridation cotreatment of TiO₂ nanowires that further redshift the active spectrum of TiO₂ to 570 nm. The Ta and N co-incorporated TiO₂ nanowires were also prepared and showed significant enhancement in photoelectrochemical performance compared to mono-incorporation of Ta or N. This enhancement is due to fewer recombination centers from charge compensation effects and suppression of the formation of an amorphous layer on the nanowires during the nitridation process. Finally, we have developed hydrothermal synthesis of single crystalline TiO₂ nanoplatelet arrays on virtually all substrates and demonstrated their applications in water photo-oxidation and dye sensitized solar cells. / text

Not available / text

Not available / text

Monte Carlo modelling is a common technique in numerous fields, and is widely used in semiconductor device simulation. This thesis describes the application of Monte Carlo modelling to the simulation of Gunn diode devices, focusing on devices composed of Gallium Arsenide (GaAs) and Gallium Nitride (GaN). Gunn diodes are simple structures that take advantage of negative differential resistance to act as a source of high frequency radiation, from 10 GHz to over 100 GHz in GaAs devices. It has been theorised that GaN should exhibit negative differential resistance and a GaN Gunn diode could produce radiation of even higher frequency, within the terahertz band. Gunn diodes have the advantage of being cheap and portable, and so are worth exploring as such a source. Unfortunately, GaN devices have a high electron density and so they tend to generate heat quickly. It therefore becomes important to include modelling of heat generation and flow in simulations of these devices. This is uncommon in Monte Carlo models of Gunn diodes, as in less highly doped devices thermal effects can usually be assumed to result in the device reaching an equilibrium temperature of about 100 K above the ambient. This thesis describes the creation of a model to track the generation and distribution of heat during operation of a GaN device. Simulations found that thermal effects within the device were significant. Heat generation occurred to the extent that the device could only be operated in pulsed mode, with on pulses of 2 ns requiring 50 ns of cooling for sustainable operation. The increased temperature within the device also lead to deleterious changes in the Gunn diode's operating frequency. In the simulated device, a 150 K change in temperature lead to a decrease in operating frequency of 40 GHz, from an initial frequency of 280 GHz. At the end of 2 ns of operation, the mean temperature within the device had increased by 120 K. The high accidental doping level in GaN also means the use of a doping notch to act as a nucleation point for dipoles within a Gunn diode, a common technique in other materials, becomes less feasible. As an alternative to a notch, a device was simulated incorporating a doping spike to nucleate the dipole. The use of a doping spike is not novel, however its use in GaN has not been previously explored. Simulations found that a fully-depleted p-type doping notch of length 2.1 nm, doped at 1x1024 m-3 would act as a nucleation point for dipole operation. The device was compared to a simulated device incorporating a doping notch of width 0.25 µm doped at 0.5x1023 m-3 and found to operate at a similar frequency and RF efficiency, making it a viable substitute. One limitation of Gunn diodes is that when operated in transit-time mode, the operating frequency is determined by the length of the diode's transit region and so is well-defined and fixed. This means that traditional Gunn diodes are not as useful a source of radiation for spectroscopic applications as might be desirable. Recent experimental results for planar devices have shown a broadening in operation frequency and even multiple frequencies. This thesis explores the hypothesis that such a broadening might be achieved in a vertical structure via the incorporation of an additional notch into the Gunn diode's transit region, effectively incorporating two transit regions into the device. Results showed that this novel device structure did show multiple modes of operation. Under a DC applied voltage, the device showed spontaneous switching behaviour, oscillating between dipole and accumulation layer operation from the second notch. Changes in the frequency of an applied RF voltage would shift the device from operating from the first or second notch, in dipole and accumulation layer mode respectively.

published_or_final_version / Physics / Master / Master of Philosophy

The present study described the daily moods of male bodybuilders who self-administered large doses of anabolic-androgenic steroids (AS) through a full cycle of steroid use. Male bodybuilders (N = 13) who had been self-administering AS for 2.5 to 12 years served as subjects and participated in a 14 to 16 week experience sampling procedure wherein brief mood questionnaires were filled out twice daily. / Findings revealed that 11 of the 13 subjects experienced self-reported mood changes in association with AS use. In particular, 2 subjects (subjects 4 & 11) experienced quite dramatic changes in mood. It is concluded that there is much variability with regards to the psychological effects that humans may display in association with AS use. / Data are discussed in terms of the effects that AS use may have on mental health.

On-chip characterization devices have been used to extract the Young’s modulus, average stress and stress gradient of polysilicon doped with phosphorus using thermal diffusion. Devices for extracting the Young’s modulus, average stress and stress gradients have been designed to work within the range of expected material property values. A customized fabrication process was developed and the devices were fabricated. Static and resonant tests were performed using clamped-clamped and cantilever beams in order to extract material properties. The experimental setup and detailed experimental results and analysis are outlined within. Several doping concentrations have been studied and it has been concluded that the Young’s modulus of polysilicon doped for 2 hours increases by approximately 50GPa and the average stress of polysilicon doped for 2.5 hours becomes more tensile by approximately 63 MPa. It has also been found that short doping times can introduce a large enough stress gradient to create a concave up curvature in free-standing structures. This work was performed in order to determine the usability of doping as a means to increase the sensitivity of temperature and pressure sensors for harsh environments. It has been concluded that doping is a promising technique and is worth studying further for this purpose.

This dissertation investigates the growth of high quality GaN and InN thin films by plasma assisted molecular beam epitaxy (PAMBE). It also explores the growth of self-seeded GaN branching nanowires and p-type doping of InN, two topics of particular interest at present. The growth of high quality III-Nitride semiconductor thin films have been shown to be dependent on the group-III (metal) to nitrogen ratio. A metal-rich growth environment enhances the diffusion of the group-III adatoms through the formation of a group-III adlayer. By using a metal-rich growth environment, determined by growth rate studies using laser reflection interferometry or RHEED analysis of the surface, both GaN and InN films have been grown with a smooth surface morphology. Additionally the smooth surface morphology has beneficial effects on the electrical and optical properties of both materials. However, with the growth using a metal-rich environment, group-III droplets are present on all film surfaces, which can be an issue for device fabrication, as they produce facets in the crystal structure due to enhanced growth rates. MBE growth of GaN nanowires via the vapour liquid solid (VLS) and vapour solid (VS) growth techniques have so far been based on the N-rich growth regime. However, we have shown that the Ga-rich growth regime can be used to grow self-seeded one dimensional and hierarchical GaN nanowires. 7 µm long hierarchical GaN nanowires with at least three branches were grown and shown to have a high crystalline quality. The suggested growth mechanism is a self-seeding VLS process driven by liquid phase epitaxy at the nanoscale, while the branching growth was nucleated due to the Ga-rich growth regime by excess Ga droplets forming on the trunk during growth. The growth of vertical GaN nanowires has also been achieved using the same self-seeding process and the critical parameter seems to be the Ga to N ratio. Also, the growth rate of the Ga-rich grown GaN nanowires can supersede the growth rates reported from N-rich grown GaN nanowires by at least a factor of two. The fabrication of vertical and planar GaN nanowire devices has been demonstrated in this study. Two point and three point contacts were fabricated to the branching GaN nanowires in the planar direction with resistive measurements ranging from 200 - 900 kΩ, similar to chemical vapour deposition and MBE grown GaN nanowires. The nonlinear current-voltage characteristics from the three point contacts may lead to unique nano-devices. The planar nanowires have also shown to have potential as UV detectors. Schottky diodes were fabricated on the vertical nanowires, with values for the barrier heights consistent with bulk diodes. Mg and Zn doping studies of InN were also performed. Both InN:Mg and InN:Zn have strong photoluminescence only at low doping concentrations. However, the InN:Mg films have reduced mobilities with increased Mg content, whereas the mobility determined from the InN:Zn films is independent of Zn. When the InN:Zn film quality was improved by growing under the In-rich growth regime, electrochemical capacitance-voltage results suggest n{type conductivity, and strong photoluminescence was obtained from all of the films with four features seen at 0.719 eV, 0.668 eV, 0.602 eV and 0.547 eV. The features at 0.719 eV and 0.668 eV are possibly due to a near band edge to valence band or shallow acceptor transition, while the 0.547 eV has an activation energy of 60 meV suggesting a deep level acceptor.

Semiconductors represent the enabling technology that underpins the many advances that define modern society. One semiconductor that shows considerable promise in the fabrication of new devices is zinc oxide (ZnO). A fundamental understanding of the properties of a material is required in order to exploit its properties. The behaviour of dopants and defects relevant to optoelectronic device fabrication is of particular interest. However, acceptor doping of ZnO is currently controversial, as successful and reproducible acceptor doping has not yet been achieved. Acceptor doping of ZnO using silver (Ag) is explored in this thesis to contribute towards the understanding of defect introduction in ZnO. In addition, there is also increasing interest in exploring materials with unconventional properties, commonly referred to as metamaterials, particularly for optical applications. The previously unexplored unique combination of Ag and ZnO may enable the fabrication of those devices. Several key factors that affect heteroepitaxy film quality, and ultimately its properties, are buffer layers and substrate temperature. A lattice match between sapphire and ZnO was provided by using buffer layers of 1 nm magnesium oxide (MgO) and 7.9 nm low temperature ZnO. The highest quality film was grown at the highest temperature (800°C), with rms roughness of 2.9 nm, carrier concentration of 3.6x10¹⁶ cm⁻³, and mobility of 105 cm²/Vs. In contrast, dopant (Ag) incorporation occurs more readily below 600°C, with dopant incorporation of up to 1020 cm⁻³ measured. Ag manifests as a deep acceptor (up to 94% substitutionally on Zn lattice sites), as evident from decreasing carrier concentration with increasing Ag flux, and DLTS measurements indicating an acceptor trap at 319 meV. This suggests that Ag is suitable for introducing compensation in ZnO, but Ag acceptors are not sufficiently shallow to result in p-type material. However, the unique combination of ZnO and Ag also enables the fabrication of a novel device, namely a superlens. Initial experimental results show the possibility of imaging a 100 nm line as 132 nm, compared with the diffraction-limited resolution of 332 nm for the same line feature.