This work has focused on the design, construction and testing of a close space sublimation system for CdTe deposition. In addition, it also focused on variations to the treatment and fabrication procedures of the transparent conducting oxide and CdS layers prior to the CdTe deposition, in order to influence the structure and electrical properties of the CdTe/CdS interface. CdTe was deposited by the physical vapour process, close space sublimation. The equipment used was custom built for this work and is therefore described in detail. Some of the deposition parameters of the equipment were varied in order to allow a comparison between the equipment in this work and those reported in the literature. Bilayers of CdS have been created by depositing two individual CdS layers, on top of one another, whilst also varying the annealing treatment of the first layer. These bilayers were included in devices and which subsequently underwent a series of etches to allow analysis of the materials. The bilayers were shown to effect the preferred orientation of the CdTe layer deposited on top, but the results suggested the bilayers had a negative effect on the amount of current recombination in the depletion region. The roughness of the transparent conducting oxide coated glass substrates used in this work, was proposed as a way of influencing the roughness of the CdS layer deposited on top and therefore the roughness of the layer the CdTe is deposited on. It was proposed that the CdTe material deposited on a roughened substrate may have an increased grain size; no evidence for this was found for samples including CdS layers. A second phase of CdTe was observed and assigned to the presence of a CdSxTe1−x alloy. The sulphur content of the alloy was found to increase as the roughness of the substrate increased. CdS was deposited by the wet chemistry process chemical bath deposition. In situ monitoring of the deposition was carried out by studying the re ection of light from the interface at the end of an optical fibre placed within the solution. The results indicated that the in situ monitoring was sensitive to the initial deposition but the sensitivity reduced as the film thickness approached 150 nm. The in situ monitoring technique and ellipsometry measurements of the CdS showed good agreement with the theory of two deposition mechanisms occurring during the deposition. The results showed that CdS grown by chemical bath deposition has a two layer structure consistent with; a low density CdS grown by homogeneous deposition, on top of dense CdS grown by ion by ion deposition. An ellipsometry model for the CdS and CdTe layers is presented, along with its optical properties over the wavelength range 245 to 1690 nm.

In this work, the growth of metal nanoparticles on domain patterned ferroelectric Pb(ZrxTi1-x)O3 and LiNbO3 by photochemical reaction is demonstrated. The photochemical properties and phenomena occurring on the surface of Pb(ZrxTi1-x)O3 and LiNbO3 under ultraviolet illumination are investigated. Ferroelectric materials possess a reversible spontaneous polarisation that has an effect on photochemical reactivity of a surface. Since the spontaneous polarisation is reversible, a desired pattern can be drawn on a ferroelectric surface in the form of domains. A combination of domain patterning and domain specific surface reactions can lead to fabrication of complex nanostructures. It is found that on a PZT (30/70) thin film, under UV irradiation, metal deposition occurred only on C + domains and no deposition occurred on C - domains. Hence, the chemical reactivity of ferroelectric surface was found to be dependent on the polarisation of domain that is underlying the surface. Annealing of PZT samples at high temperatures alters the defect concentration of the PZT as shown by an increase in the deposition of silver on the surface. When the PZT samples were annealed in air at temperatures ranging from 530-690°C the silver deposition increased by more than 150% and the size of deposited silver clusters increased by four times. The photochemical properties of PZT thin films of different compositions PbZr0.3Ti0.7O3, PbZr0.52Ti0.48O3 and PbZr0.7Ti0.3O3 were investigated by undertaking silver nanocluster deposition experiments. The composition of PZT film (Zr/Ti ratio) affects silver deposition such that on PbZr0.3Ti0.7O3 silver deposits only on C + domains, whereas PbZr0.52Ti0.48O3 and PbZr0.7Ti0.3O3 experience deposition on both C + and C - domains. This difference in silver deposition pattern is shown to be due to the difference in width of the space charge region and bandii gap of the three samples. The impact of size of poled pattern on silver deposition at the surface is shown. It is found that for smaller size of C + domains (smaller than 1 μm) the amount and size of deposited silver decreased with the decrease in domain size. On a PZT surface, formation of spherical, triangular and hexagonal nanoplates of gold via photoreduction has also been demonstrated. Silver cation reduction on C + and C - domains of ferroelectric lithium niobate (LN) by photochemical and photoelectric process is demonstrated. The interaction of photoelectric and domain dependent influences can be observed in LiNbO3 due to the low electron affinity (ca 1.1-1.5eV). The impact of composition of LN on the photoreduction of silver has been shown. It is found that the photochemical reactivity of MgO-doped LN is significantly higher as compared to non-doped LN. The energy and intensity of incident photon is also found to have an impact on the photoreduction of silver on LN surface. In addition to this, deposition of Al and Mn nanoparticles on LN surface by means of photoreduction reaction has been demonstrated. It is shown that for a metal to photoreduce on C + domain of a ferroelectric material, the reduction potential of the metal has to be within the band-gap of the material. It is shown that under atmospheric conditions, it is possible to selectively adsorb molecules of opposite charge on the surface of LN due to uncompensated polarisation charge.

Ferroelectric perovskite thin films for voltage tunable applications, namely (Ba,Sr)TiO3 (Barium Strontium Titanate or BST) and (Pb,Sr)TiO3 (Lead Strontium Titanate or PST), are synthesized via the so-called sol-gel route. While BST shows the tendency to severe film cracking, PST can be grown crack free onto platinised Si standard substrates and even directly onto SiO2, SiNx or bare Si. The growth kinetics of PST on platinised SiO2/Si and directly on SiO2/Si are studied in detail using X-ray diffractometry (XRD), scanning electron and atomic force microscopy, SEM and AFM respectively. It is shown that PST begins to crystallise at 500°C on Ti/Pt and 550°C directly on SiO2. After a thermal treatment of 650°C for 15 min both films are fully crystallised with random (100) and (110) orientation and a smooth surface. The dielectric properties, e.g. dielectric constant, loss and tunability, of PST 50/50 are measured using a standard Ti/Pt bottom electrode with Cr/Au top electrodes and a TiW/Cu bottom electrode on which the PST thin film was bonded with TiW/Cu top electrodes. The Cu/PST/Cu system shows an enhanced performance in terms of loss resulting in a larger device quality factor and a figure of merit (FOM) of 18.25 compared to 16.6 for the configuration using a Pt bottom electrode. The maximum tunability is 73% with an applied voltage of 35V and the dielectric constant at zero bias is ~ 420 with a loss < 4 %. (Pb0.4Sr0.6)(MnxTi1-x)O3 (Mn doped PST 40/60) thin films with x = 0, 0.01, 0.03, and 0.05 are grown on Ti/Pt coated SiO2/Si substrates. The surface morphologies, dielectric and tunable properties of these films are investigated as a function of Mn content (x). It is found that the grain size/roughness, dielectric constant, loss, tunability and figure of merit are affected by the Mn doping level. Further on it is found that the ferroelectricity and the transition temperature between the cubic paraelectric and tetragonal ferroelectric state increase with Mn content. The dielectric constant at zero bias reaches a maximum of 1100 and the maximum FOM is 23.96 with 3 mol% Mn; whereas the maximum value of the tunability is 76.72% at 10 V with 1 mol% Mn. A detailed understanding of the effect of Mn doping is developed and presented. It is found and explained that a doping level of 2 mol% Mn results in optimal properties in terms of tunability and loss. Auger spectroscopy is used to study the compositional change in the interfacial region between PST and PZT thin films and SiO2/Si substrates to understand the growth kinetics of PST directly onto SiO2 in more detail. The thin films from both materials are annealed under the same conditions (temperature and time). It is found that strontium stops the lead diffusion into SiO2 by forming SrSiO3/Sr2SiO4 and/or SrO, maintaining a well defined SiO2 region, while PbSiO3 is formed in the PZT/SiO2 system. It is shown that SrO covalently saturates all Si dangling bonds by forming SrSiO3 and/or Sr2SiO4. This provides the necessary ionic template towards the perovskite SrO-terminated SrTiO3, on which PST can grow further on. A single layer of PST is finally used as a buffer layer for the growth of piezoelectric PZT directly onto SiO2 to replace the common Ti/Pt bottom electrode. The initial characterisation of PZT device structures shows that PZT films with PST as a diffusion buffer had fully crystallized in the perovskite phase exhibiting good dielectric and ferroelectric behaviour. Although the piezoelectric coefficients of the PZT films were not measured directly in this study, it is envisaged from the experimental data of the dielectric constant and hysteresis loop that the PZT/PST composite has the potential to provide good and comparable piezoelectric performances as typically observed in PZT device structures grown on commonly used Ti/Pt.

No description available.

The possibilities of applying cold neutron scattering to the study of sorbed molecules are discussed. Atom-atom pair calculations have been carried out for the interaction of methane and hydrogen molecules with the basal plane of graphite and the results used as a guide to the best scattering law for the sorbed molecule dynamics. As the sorbents used are powders, a method of averaging the two dimensional scattering law over three dimensions has been developed. Neutron time of flight spectra were obtained for methane, ethylene and ammonia adsorbed on Graphon. For the surface coverages studied, 0.6-0.2, methane appears to change from a two dimensional Fickian liquid, to an unidentified phase at the lower surface coverage. The data for ethylene at around one monolayer approaches the two dimensional liquid model, but at higher surface coverages there are significant deviations. Whilst methane and ethylene appear to wet the Graphon surface, ammonia does not seem to do so. The spectra for ammonia show a liquid phase characterised by a diffusion coefficient and activation energy for diffusion, 2.4 kcals/mole, close to those measured for bulk ammonia A solid phase is observed at lower temperatures. Neutron time of flight spectra were obtained for methane absorbed in a lanthanum exchanged Y type zeolite. The spectra show that the methane has lost translational motion but retained rotational freedom. A vibration frequency of 19±1 cm^-1 is observed and this is ascribed to a vibration of the methane molecule within the cavity. The sorption of simple molecules on catalytically active surfaces was investigated by beryllium filter spectroscopy. A range of interactions was discovered, varying from the gas like sorption of hydrogen and methane on La-Y, to the dissociative sorption of formic acid on γ alumina. Intermediate behaviour was observed when dichloromethane and acetonitrile were sorbed on La-Y.

The work described in this thesis is concerned with the growth and examination of thin Si and Ge films. These were deposited by sublimation and evaporation techniques in UHV, using apparatus constructed by the author, The source of deposited material was heated by electron bombardment, while the substrates were heated either directly by the passage of an electric current or indirectly upon a Si bar, depending upon their electrical conductivity. The various substrates used were Si and the insulators sapphire, diamond and &beta;-SiC. The grown films were studied by use of the optical microscope, the transmission electron microscope and the scanning electron microscope. The main object of the work was to investigate the structural perfection of these films, to deduce information concerning their growth mechanisms and to compare the observations with suitable theoretical models. The initial growth of Si films on unpreheated (111) Si substrates, held at temperatures around 900&deg;C, was deduced to have been by nucleation (after an initial induction period), since irregular Si growth centres were produced. These eventually overlapped to give a channelled film containing few crystallographic defects. The dependence of the number density of Si growth centres upon deposition conditions was correlated with different theoretical relationships derived separately by Joyce et al (1967) and Logan (1969). It was deduced that the smallest stable nuclei contained 3 Si atoms, and that the activation energy for surface diffusion of Si was ~0.55 eV (Joyce et al) or ~1.1 eV (Logan). These observations and deductions were compared with analogous results obtained by Joyce et al, who grew Si films by silane pyrolysis. All the results were discussed in relation to initial growth processes, and various suggestions were put forward to account for certain unexpected features. Possible impurity effects were considered in detail and related to observed film growth characteristics. When (111) Si substrates were cleaned using heat treatment at 1250°C in UHV surface steps were generated, and these moved across the substrates in a [112] type direction. When Si deposition was initiated the direction of step motion reversed as growth took place. The steps were pinned in many places by impurity particles, which caused the production of deep pits in the grown films. Transmission electron energy loss analysis showed that the particles were SiC. The dependence of the Si film growth upon deposition conditions, the state of the substrate surface and the presence of contaminating gases was also investigated. The nature of both sublimation and growth step motion across substrate surfaces was analysed using theoretical models, and this showed that in the absence of pinning effects, the motion was generally that expected for a clean surface. Ge films were deposited on to (111) Si substrates at temperatures around 800°C. The thinnest films were composed of growth centres which were mostly triangular in shape, and this showed that initial growth had been by nucleation. A tentative comparison of experimental data with theoretical models indicated that the smallest stable Ge nucleus may have been composed of 4 atoms. TEM examination of the Ge films showed that they grew with the same crystallographic orientation as their Si substrates. However, the films contained many crystallographic defects, and some of the most prominent were continuous networks of misfit dislocations, which occurred over each entire Ge/Si interface. The spacings of the dislocations in such networks were non-ideal, and this was taken to indicate that considerable Ge/Si alloy formation had taken place. Deposition of Si on to heated (0001) sapphire substrates gave films which grew by initial nucleation with the epitaxial relationships (111)_Si//(0001)_Al₂O₃ and [110]_Si//[1120]_Al₂O₃. The thinnest films were composed of equal proportions of two 60&deg; rotational twins, and often contained large numbers of planar defects. However, films a substantial fraction of a micron in thickness contained misorientations of a more general nature, and with increasing thickness ultimately became polycrystalline. High resolution TEM studies provided no evidence of an interfacial misfit dislocation network, but instead indicated that a chemical reaction had taken place between the Si and the sapphire during deposition. Si films were also grown upon both (111) diamond and (111) &beta;SiC substrates, and once again initial growth was by nucleation. Thicker films on diamond substrates exhibited significant orientation retention despite the very large crystallographic mismatch. Work was also carried out involving the high resolution TEM studies of closely spaced misfit edge dislocation networks at the Ge/(111)Si interface. It was generally possible to obtain either fringe or structural images depending upon the nature of the diffraction conditions, and this behaviour was examined in detail and discussed with special reference to the work of Th&ouml;l&eacute;n (1970). Use of the weak-beam technique (Cockayne et al, 1969) enabled the network structures to be investigated in detail. While it was observed that all the intrinsic dislocation nodes had a relatively uniform extension, the extrinsic nodes had mostly a much smaller extension and approximately half appeared to be completely contracted. Also, using 111 type Bragg reflections, the intensity of the TEM contrast exhibited by the different kinds of node stacking fault were markedly dissimilar, and depended upon the sense of the deviation from the exact Bragg position. These studies were extended to observations of dislocation networks produced by deformation in single crystal Si. Measurements of the dimensions of intrinsic nodes, which were all extended, yielded values for the intrinsic stacking fault energy in Si of ~58 or ~71 ergs cm^-2, depending upon the type of measurement employed. Only half the extrinsic nodes were extended, and this behaviour was interpreted in terms of an energy barrier to node extension. Using 111 type reflections, the node contrast behaviour was analogous to that observed in the Ge/(111)Si system. This was correlated with the contrast behaviour of inclined stacking faults, and the image intensity appeared to depend upon the sign of (g.R)s. Using 224 type reflections, dissociated dislocation contrast was interpreted in terms of interactions between the strain fields of the bounding partial dislocations. In conclusion, suggestions were put forward concerning possible extensions of the present work, and these involved both film growth studies and investigations of TEM contrast behaviour.

The primary aim of this work is to study optically excited spin injection over a ferromagnetic metal-semiconductor Schottky barrier. A novel experimental approach has been introduced which enables the determination of the sign in the magnetic asymmetry of the photocurrent, hence the sign in the spin polarisation. The magnetic ultrathin films are grown on GaAs substrates using an e-beam evaporation technique in an ultrahigh vacuum system. The elemental and chemical characterisation of the Fe/GaAs samples has been conducted using x-ray absorption spectroscopy (XAS) whilst x-ray magnetic circular dichroism (XMCD) is utilised to obtain the magnetic information. The temperature-dependent magnetic properties of the samples are examined and the orbital, spin moments are estimated with classical optical sum rules. The magneto-optical Kerr effect (MOKE) is also employed to characterise the magnetic properties of ultrathin Ni, Fe films on GaAs substrates. The study of the spin injection processes is carried out with photoexcitation of electrons using circularly polarised light. Pure left and right circularly polarised light obtained using a Soleil-Babinet compensator plays a key role in the study of the spin injection processes. The influence of the magnetization of the ultra thin magnetic films on the chirality-dependent photocurrents is attributed to the effect of the spin-polarised density of states of the ferromagnetic films. Interestingly, under appropriate experimental conditions, a negative magnetic asymmetry in the photocurrent can be observed with Ni films, which are known to have a negative spin polarization for the electrons near the Fermi level. The study focuses on the use of a non-normal incidence experimental geometry in which various experimental factors are carefully explored. The influences of photon energy on the magnetic asymmetry in photocurrent have been examined providing a better understanding of the spin injection process.

Atmospheric pressure glow discharge (APGD) plasma CVD was used to deposit thin films of titania at 200 °C using two different precursors. The resulting films were characterised using techniques including XPS, RBS and XRD. It was established that annealing at temperatures as low as 275 °C produced crystalline films that were photocatalytically active. When annealed at 300 °C, the photoactivity was greater than that of a commercially available "self-cleaning" titania film. The effects of the different precursors, annealing times and temperatures on the crystallinity and photoactivity are discussed. This thesis also describes first reported deposition of aluminium oxide thin films by APGD, plasma-enhanced CVD. This approach allows deposition at substantially lower substrate temperatures than normally used in atmospheric pressure based processing. The films are analysed by SEM, XPS, RBS, XRD, and optical properties. It is demonstrated that the APGD approach yields films which are essentially smooth, conformal and free from pinholes or other imperfections. Further novel work was undertaken exploring the deposition of composite metal/ metal oxide thin films using APGD CVD. The described approach employs a parallel- plate dielectric barrier configuration, and the deposition of such materials is discussed with respect to their influence on discharge conditions. Controlled and variable composition films were produced based on aluminium which showed metallic-like reflection (up to 60% visible), and were conductive (~1 O per square). The films were analysed by RBS, SEM, AFM and optical spectroscopy. This new class of APGD-CVD derived thin film material, when combined with the associated low thermal load and attractions for industrial scaling, offers significant potential for new applications. The studies resulted in three full papers, and four posters.

In this thesis the influence of processing conditions on the structure and optoelectronic properties of conjugated polymer:methanofullerene blend thin-films has been investigated. These conditions strongly impact upon the efficiency with which blend films may produce a photocurrent when fabricated into an Organic Photovoltaic (OPV) device. Using the model system P3HT:PCBM, it is shown that films undergo a three-stage drying process upon casting. Heterogeneous growth of P3HT crystallites occurs once the solid fraction in the film exceeds 50 wt%. Measurements from spectroscopic ellipsometry (SE) and grazing-incidence wide-angle X-ray scattering (GIWAXS) suggest a correspondence between enhanced polymer crystallinity and the strength of the π-π* electronic transition in the polymer absorption spectrum. In-situ measurements of a blend during thermal annealing evidence the evolution of residual solvent loss upon heating, volume relaxation, phase separation and increased electronic conjugation of P3HT upon cooling. The glass transition of P3HT:PCBM blend films, measured in a thin-film geometry, is found to correlate with the minimum effective annealing temperature for improving the power conversion efficiency of thermally annealed OPVs. As-cast films with 20 to 60 wt% PCBM exhibit two glass transitions, an observation that may indicate the existence of two compositionally distinct amorphous phases. Studies on a different polymer:fullerene blend system (PCDTBT:PC71BM), indicate a greater miscibility between materials compared to blends of P3HT:PCBM. In this system, thermal annealing is found to result in increased disorder in the polymer phase of the film, and also to drive excessive phase separation of PC71BM. It is argued that thermal annealing is unlikely to be an appropriate treatment for optimising the efficiency of OPVs based on PCDTBT:PC71BM blends. Finally, Helium Ion Microscopy (HeIM) is used to image the chemical composition of OPV applicable blend films with nanometer resolution, providing a powerful technique to correlate film morphology with device functionality in a range of organic opto-electronic devices.

Zinc oxide (ZnO) thin films have great promise for a wide range of optoelectronic applications, however controlling crystallinity and stoichiometry at low processing temperatures remains a challenge. Pulsed laser deposition (PLD) is a versatile technique that allows precise control the film properties. The crystallinity and electrical properties can, theoretically, be tuned by altering a wide variety of deposition parameters. However, until now there has been little work performed exploring PLD as a technique for the preparation of thin films at low temperature, for use in optoelectronic applications. In this thesis, PLD is demonstrated as a highly appropriate technique for the preparation of semiconducting and electrically conducting transparent films, over a wide range of substrate temperatures applicable for optoelectronic grade substrates. Deposition conditions are identified allowing the low temperature deposition of ZnO directly onto functional organic poly(3-hexylthiophene) (P3HT) coated substrates. To demonstrate the applicability of this methodology the preparation of conventional architecture hybrid (inorganic:organic) photovoltaic devices is outlined with no degradation to the microstructure, optical or electrical properties of the P3HT observed. The methodology is widely applicable for depositing oxide interlayers multilayer organic devices. In this thesis, the role of ZnO is investigated as i) an exciton dissociation and electron transporting layer in hybrid devices, ii) an optical spacing layer in organic bulk heterojunction photovoltaic devices and iii) as a transparent conducting oxide (when doped with A1) as a top contact for organic optoelectronic devices. Device performance is optimised through careful control of PLD parameters. In each device and in free-standing thin films the microstructure, morphology and crystallographic nature of the as-deposited ZnO is studied by scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD). The electrical properties are studied in both operational devices and by 4-point probe measurements.