Zinc oxide (ZnO) is a promising photoanode material which has been used in quantum dot-based depleted heterojunction solar cells. The specific influence of the defect chemistry of ZnO on its n-type conductivity remains a focus for research. This thesis presents results from a series of near-ambient pressure (NAP) XPS experiments (at The University of Manchester, UK), used to characterise surface adsorption of O2 and H2O on ZnO(10-10) surfaces in high pressure environments. Water dosing is shown to lead to surface hydroxylation and a change in the surface band bending consistent with an increase in the surface conductivity. Oxygen dosing is also observed to lead to the formation of surface species on the ZnO surface, revealing that ZnO is prone to hydroxylation even in oxygen-rich environments. The role of surface OH on influencing the transient surface photovoltage (SPV) of the ZnO(10-10) surface is probed through a series of time-resolved, pump-probe XPS experiments (at SOLEIL synchrotron, France). It is shown that increasing the degree of surface hydroxylation leads to a decrease in surface band bending, leading to longer-lived transient SPV. Other factors influencing the SPV dynamics are explored, such as the role of the oxygen vacancy concentration. The transient SPV decay lifetime is shown to increase with increasing oxygen vacancy concentration, consistent with the presence of persistent photoconductivity (PPC) in ZnO, mediated by oxygen vacancy-related hole traps. The influence of the concentration of thermally excited carriers in ZnO on the surface band bending is also described, showing that the equilibrium band bending and the surface photovoltage are both reduced at low temperature. It is shown that thermal excitation of carriers from the valence band of ZnO and from neutral oxygen vacancies have negligible influence on the magnitude of equilibrium band bending at the surface. The energy regime consistent with the observed temperature dependence is also consistent with a perturbed-host state 0.2 eV below the conduction band minimum. This meta-stable state is associated with doubly-ionised oxygen vacancies, that mediate the PPC in ZnO. However this does not rule out the contribution from other shallow donor levels such as those associated with hydrogen impurities. The influence of hydrogen on the SPV dynamics in ZnO is explored, through angle-resolved photoemission spectroscopy (ARPES) after implanting hydrogen atoms into the ZnO surface. H implantation is shown to lead to the formation of a 2D electron gas (2DEG) at the surface, consistent with an increase in conductivity at the surface large enough to change the nature of the space-charge region at the ZnO surface from depletion to accumulation.

A surface acoustic wave (SAW) is a combination of a mechanical wave and a potential wave propagating on the surface of a piezoelectric substrate at the speed of sound. Such waves are widely applied in not only the communication industry, but also in quantum physics research, such as nanoelectronics, spintronics, quantum optics, and even quantum information processing. Here, I focus on low-dimensional electron transport and SAWs in GaAs and ZnO semiconductor heterostructures. The ability to pattern quantum nanostructures using gates has stimulated intense interest in research into mesoscopic physics. We have performed a series of simulations of gate structures, and having with the optimised boundary conditions and we find them to match experimental results, such as the pinch-off voltage of one-dimensional channels and SAW charge transport in induced n-i-n and n-i-p junctions. Using the improved boundary conditions, it is straightforward to model quantum devices quite accurately using standard software. With the calculated potential, we have modelled the process how a dynamic quantum dot is driven by a SAW and have analysed error mechanisms in SAW-driven quantisation (I=Nef, where N is the number of electrons in each SAW minimum, and f is the SAW resonant frequency). From energy spectroscopy measurements, we probe the electron energy inside a SAW-driven dynamic quantum dot and find that the small addition energy, which is around 3meV, is the main limitation for the SAW quantisation. To increase the confinement of SAW-driven quantum dots, we deposit a thin ZnO film, with a better piezoelectric coupling than GaAs, on a GaAs/AlGaAs heterostructure using high-target-utilisation sputtering (an Al2O3 buffer layer is deposited to protect the 2DEG during sputtering). With the ZnO, the SAW amplitude is greatly improved to 100 meV and the RF power required for pumping electrons using a SAW is greatly reduced. Finally, we have studied low-dimensional electron transport in a MgZnO/ZnO heterostructure. We have developed a technique for patterning gates using a parylene insulator, and used these to create one-dimensional quantum wires and observe electron ballistic transport with conductance quantised in units of 2e2/h The increasing electron effective mass as the 1D electron density decreases indicate that the electron-electron interaction in this MgZnO/ZnO heterostructure is strong. Because of these strong interactions, the 0.7 anomaly is observed just below each quantised plateau, and are much stronger than in GaAs quantum wires. Furthermore, we have also calculated the SAW-modulated spontaneous and piezoelectric polarisation in the ZnO heterostructure, and have observed a sign of this SAW-modulation in 2DEG density, which is different from the classical SAW-pumping mechanism. Our results show that a ZnO heterostructure should provide a good alternative to conventional III-V semiconductors for spintronics and quantum computing as they have less nuclear spins. This paves the way for the development of qubits benefiting from the low scattering of an undoped heterostructure together with potentially long spin lifetimes.

Ce travail de thèse porte sur l'étude d'oxydes (ZnO et SiO2) fabriqués par voie sol-gel. Dans un premier temps, nous avons étudié la cinétique de croissance des nanoparticules de ZnO en milieu aqueux. L'étude in-situ de la croissance d'oxyde de zinc par la technique de croissance en milieu aqueux (ACG) a été réalisée grâce à la radiation synchrotron de l'ESRF sur la ligne ID10B. Nous avons réalisé la diffraction de rayons X en incidence rasante sur un substrat plongé dans une solution constituée de nitrate de zinc hexahydraté [Zn(NO3)2,6H2O], d'hexamethylenetetramine (HMT) (C6H12N4) et de l'eau dé-ionisée comme solvant. Nous avons réussi à mettre en évidence l'existence de nanoparticules de ZnO dans la solution et suivre la cinétique de formation. La deuxième partie de ce travail est consacrée à l'étude des films minces de silice mésoporeux structurés par un tensioactif connu sous l'acronyme Brij58. Nous avons commencépar établir le diagramme de phase du Brij58 par la diffusion centrale des rayons X (SAXS), complété par l'établissement d'un diagramme d'état solide-liquide par la rhéologie. Grâce à ce diagramme de phase nous avons pu fabriquer des films minces de silice. L'étude de la structuration des films a été réalisée par la diffusion des rayons X en incidence rasante (GISAXS) et la réflectivité des rayons X. Nous avons finalisé ce travail par l'utilisation des films mésoporeux comme matrice hôte à deux fluides : l'eau et le CO2, et par l'insertion de nanocristaux de ZnO dans les pores de la matrice mésoporeuse.

The purposes of this thesis are to prepare ZnO and tin-doped In2O3 (ITO) films at low temperature and study their microstructure and optoelectronic properties. Low-temperature growth of undoped ZnO films with high transparency and low electrical resistance was prepared by ion beam sputtering. After systematic testing, a sheet resistivity as low as 2.95 x 10-3 £[-cm was obtained at a substrate temperature of 150 oC, ion source voltage of 850 V, and ion beam current of 30 mA. The transmittance of the ZnO films was in the range of 85-90%. Hall measurements showed that a high mobility of 21.41 cm2/Vs was obtained for films less than 200 nm thick. The related microstructures and physical properties were measured and discussed. ZnO nanofilm of (2-1-10) and (01-11) surfaces were prepared on NaCl (001) surface at 200 oC and 400 oC to produce nearly pure (2-1-10) and (01-11) textures respectively and the orientation relationships were determined and the interface discussed. By dissolving the NaCl substrate, the ZnO (2-1-10) and (01-11) surfaces several cm2 in area, which may have some useful applications, can be easily prepared. The photoluminescence spectrum from the (2-1-10) surface showed only a near-band-edge UV emission peak while the (01-11) surface showed a near band-edge UV emission and a broad green emission. Low-temperature preparation of transparent conducting electrode is essential for flexible optoelectronic devices. ITO films of high transparency and low electrical resistance were prepared at room temperature with a radio-frequency ion beam sputtering system. Specimens with a low sheet resistivity of 10-4 £[-cm and a high visible-light transmittance of 85-90% were obtained. Hall measurement was used to measure the mobility and carrier concentrations and the effects on resistivity were discussed. ITO films were deposited on glass substrates at 200 oC at various oxygen flow rates. At low oxygen flow rate the film surface has ITO whiskers with metallic In tips and a crystallographic relationship of (010)In//(110)ITO and (001)In//(001)ITO is present between them. The In tips act as the seeds for the growth of ITO whiskers by a vapor-liquid-solid growth mechanism. As the oxygen flow rate increases, the crystallinity of the ITO film decreases till an amorphous phase is formed. The microstructure, resistivity and transmittance of the films were studied as a function of oxygen flow rate. Thin films of high transmittance (~90%) and low resistivity (6 x 10-4 Ω-cm) were prepared at an intermediate oxygen flow rates.

In recent years, even though the light output of GaN-related LED continues to increase, the brightness is still low compared to conventional lighting systems and it is necessary to further improve the light extraction of LEDs. In this study, we utilize the ZnO nanotip with aqueous solution and flip-chip technique to increase the light extraction of GaN LEDs. Electroluminescence (EL) and angular optical distribution are used to measure the light output intensity of LED. In the results, ZnO nanotip after thermal annealing with N2O ambiance decrease the ZnO defects. Flip-chip LED has higher light intensity ( 1.25 times) than conventional one in vertical emitting area ( at 0 angles). The enhancement of light output is duo to the reduction of light absorption from the metal contact and Fresnel¡¦s transmission losses. Finally, we fabricate a high brightness LED with above light enhancement design. EL intensity of LED is increased about 1.38 times than conventional one. Therefore, we can manufacture a LEDs array with above designs to obtain high light output for future solid-state illumination.

The length of ZnO nanotip can be controlled by the deposition time, and the crystal of ZnO nanotip can be enhanced by a thermal annealing at 300oC in this study.The thickness of TiO2 on ITO/glass also can be controlled by the deposition time in this investigate. There are three major parts in this study : 1. (1). The control of thickness of TiO2 film and length of ZnO nanotip and (2). the difference of their photocatalytic activities are two major parts. 2. The relationship between the surface area and the photocatalytic activities of TiO2 powder (P25) and film. 3. The improvement of photocatalytic activity was utilized by the hetrojunction of ZnO nanotip/TiO2 and TiO2/ZnO nanotip, and the P25 is used as a reference for all measurements.

This study presents a high-performance ZnO piezoelectric transducer integrated with the flexible stainless steel substrate. The ZnO piezoelectric film of 1.08nm was deposited on the flexible stainless steel substrate using a RF magnetron sputtering system. The cantilever length of 1cm and the vibration area of 1cm2 were designed for low-frequency environment according to the Cantilever Vibration Theory. The effects of various sputtering parameters such as substrate temperature, RF power and sputtering pressure were investigated to improve the piezoelectric characteristics of ZnO thin films. It was also discussed the unit thickness of open voltage values, and then the optimal sputtering parameters were determined. The physical characteristics of ZnO thin films were obtained by the analyses of the scanning electron microscopy (SEM) and X-ray diffraction (XRD) to discuss the surfaces, cross section and crystallization of ZnO thin films. The voltage analysis were measured the open and load voltage by the measurement system. The optimal deposition parameters for ZnO thin films are substrate temperature of 300¢J, RF power of 75W, sputtering pressure of 9 mTorr and oxygen concentration of 60%, which were determined by physical characteristics and voltage analysis. The study employs a precise mass loading of 0.57g on the cantilever to increase the vibration amplitude. The vibration source from 1~150Hz was provided to the piezoelectric transducer, and then the experimental results were showed resonance frequency of 75Hz by oscilloscope. When the optimal thickness of ZnO films is 1.08£gm and vibration amplitude is 1.19mm, the open circuit voltage of the power generator is 5.25V.After rectifying and flitting with a capacitor of 33nF,the maximum power of 1.0£gW/cm2 was achieved with the load resistance of 5M£[.

In this study, the tansparent conductive oxide (TCO) multilayer film AZO/Ag-Ti/AZO was fabricated with Ag-Ti alloy as conducting layer and AZO as anti-reflective material. The metal alloy was deposited by DC magnetron sputtering, and the AZO film deposition was performed by spin-coating technique and dried at suitable temperature. The thicknesses of Ag-Ti and AZO thin films were varied to fabricate AZO/Ag-Ti/AZO multilayer films. The microstructures of the multilayer films were observed by SEM and AFM. Sheet resistance was measured by using four-point probe. Optical transmittance was measured in the visible range by uv-vis spectrophotometer. The results show that as the top of AZO thickness is 50 nm, intermediate Ag-Ti metal laminated to 9 nm, and the bottom of the AZO is 35 nm, the transmittance of multilayer film AZO/Ag-Ti/AZO can reach 78.92%, and the sheet resistance is 1.86£[/¡¼. When thermal annealing process was carried out to the bottom AZO film, the worse characteristics of the transmittance and resistance of the performed multilayer film were resulted.

The contactless electroreflectance¡]CER¡^ spectra of ZnO bulk has been measured at 300K. It was observed the difference between the CER spectra by using the polarization E of probe light perpendicular ( E¡æc) and parallel ( E¡üc) to the c-axis of the m-plane ZnO. In addition, a mercury lamp was focused on the sample to reduce its strength of electric field. It was observed that the CER spectrum was blue-shifted with Hg lamp being on. Hence, the observed features were attributed to excitonic transitions. The experimental spectra were fitted by Lorentzian lineshapes. The energies of the A ( B¡^, B¡]A¡^, and C excitonic transitions were determined as 3.329eV, 3.343eV and 3.387eV, respectively.

In the thesis, epitaxial ZnO films were grown by chemical vapor deposition method (CVD) on LiGaO2 (001) substrate. Zinc 2,4-pentanedionate monohydrate [Zn (C5H7O2) 2. H2O] , used as the zinc precursor, was vaporized at the temperature between 130 to 140¢J. Then the vapor was carried by a mixture of N2/O2 gas flow into the furnace where the (001) LGO substrate located. The parameters of growth temperature, growth pressure and growth time were adjusted to found the best ZnO films growth conditions. After CVD growth, the crystal structure, crystal quality, surface morphology, optical properties, surface roughness and micro-structure properties of the specimens were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL), atomic force microscopy (AFM) and transmission electron microscopy (TEM). This study was divided into three parts. In the first parts, the dependence of growth characteristics on the different growth temperatures was investigated. The best surface morphology and crystal quality of ZnO films were grown under the growth temperature of 550 ¢J. In the second parts, the growth of ZnO films under various pressures was investigated. High orientation (0002)ZnO films were grown at lower growth pressure of 50 torr. In the third parts, the dependence of growth time on the ZnO films quality was investigated under growth temperature of 550 ¢Jand growth pressure of 50 torr.