This thesis presents experimental studies of InAs/InGaAs/GaAs quantum dot-in-awell infrared photodetectors (DWELL QDIPs) grown by molecular beam epitaxy (MBE). Detailed studies were carried out to investigate the effects of design parameters on the performance of DWELL photodetectors, along with fundamental studies to determine the intraband optical and electronic properties of such structures. Using the results of these studies, an optimised structure was designed. In addition, the observation of a strong bias dependent spectral photoresponse demonstrated the capability of post growth spectral tunability within the long wavelength IR (LWIR) atmospheric window. Various approaches were investigated for enhancing the performance of quantum dot (QD) based devices. The main shortcoming of QDIPs versus quantum well infrared photodetectors (QWIPs) has been addressed i.e. the low dot density, which prohibits the high doping of these structures. The use of an antimonide surfactant to enhance the dot density in DWELL QDIPs is presented here for the first time. Also a method for decreasing the dark current in QDIPs was investigated, via the use of wide band gap AIGaAs barriers. Another technique using GaP strain balancing layers to reduce the strain in multilayer structures and allow the growth of >20 layer QD devices was illustrated. The effects of intermixing via thermal annealing are also reported in for DWELL QDIPs. As part of this study, the possibility of using such a technique to shift the spectral photoresponse across the 8-12J.lm LWIR window is demonstrated, and it is shown that the performance still remains in a competitive range within the LWIR range. Non-linear two photon absorption in QDIPs was demonstrated and studied. As a result of this study, the capability of QDIPs to operate as quadratic detectors in the far-infrared was established, which could prove very significant, since detector availability is reduced in that range. Finally, a novel approach to photovoltaic QDIPs was investigated experimentally, using a purpose built design in order to provide an internal electric field, which preferentially drives carriers in one direction.

This thesis describes the forming process under <I>constant current </I>conditions of Cr/<I>p</I><SUP>+</SUP>/V thin film devices (a-Si:H denotes hydrogenated amorphous silicon). In the initial stages of electro-forming by constant current stressing, with increasing injection of charge via either increasing bias or time, the <I>J - V </I>characteristics of devices exhibit an instability, as shown by a decrease in the reverse current. This is interpreted in terms of the creation of defects in the a-Si:H. The defect generation rate, as measured by the voltage shift verse current in the <I>J - V</I> curves, is found to follow a square-root time-dependent law. Eventually, with further increasing current bias, the local current density reaches a critical value <I>J<SUB>F</SUB></I>, and a rapid 'runaway' processes occurs, which results in an irreversible change of the initial high-resistance state into a permanent 'formed' state of lower resistance. It has also been observed that formed samples sometimes show a metal-non metal (MNM) transition at low temperatures (60 - 100<I>K</I>). The electrical properties of these devices have been analysed in detail. The AC characteristic can be modelled using multi-component RC and RL equivalent circuits below and above the MNM transition region. An anomalous frequency dependence of the capacitance is explained in terms of a percolation-like critical behaviour of the dielectric constant <I><SUB>eff</SUB></I>, which diverges at the percolation threshold <I>p<SUB>c</SUB>. </I>A general approach for activated tunnelling in granular thin films is used to explain electron transport in formed devices. Analysis shows that the structure of formed Cr/<I>p</I><SUP>+</SUP>a-Si:H}/V devices structure can be modelled as a heterogeneous filamentary medium composed of metallic particles and an insulator host (e.g.a-Si:H). The metallic particles originate from the diffusion of the top vanadium electrode during the forming process. This model is consistent with memory characteristics, such as the change in activation energy with device resistance.

This thesis reports on the characterisation of platinum features which were deposited and patterned using UV (ultra violet) exposure of a solid photosensitive organometallic material through a mask. The work aims to evaluate the process for its use in present and future semiconductor technology. A range of devices including resistive and capacitive test structures as well as MOS transistors were produced using platinum deposited using the organometallic process and characterised by standard methods. The deposited films were found to be metallic and have a good adhesion to silicon dioxide, although their resistivity is higher than the one of bulk platinum. Measurements on platinum MOS capacitors showed their capacitance curves to agree well with curves from aluminium capacitors on the same substrate albeit the curves of capacitors on chips which incorporated MOS transistors indicated interface traps. Threshold and source-drain characteristics of platinum gate transistors are presented and these results are compared with aluminium gate transistors manufactured on the same substrate. Both sets of characteristics are very similar with the major difference being that the platinum gate devices have a lower sub-threshold slope, an effect which was attributed to trap charges at the silicon/silicon dioxide interface. The work also found that structures produced were generally larger than expected and spectroscopy methods showed contaminants to be present in the films.

A series of detailed experimental and computational studies on aspects of pressure-induced phase transitions in solids is described. The emphasis is on the nature of such transitions in tetrahedral semiconductors. It is found that the materials investigated exhibit previously unreported and very complex behaviour which is related to the structural transition. New high-pressure diffraction evidence suggests that the crystallographic structures of several III-V semiconductors are not in agreement with those suggested either by prior experimentation or total energy calculation. The high-pressure structures determined in this work are generally of lower symmetry than previously believed and in at least one case of much greater complexity. It is also argued that phase-transition-induced defects are created at the transition in many of these materials and that the transition may be sensitive to either the concentration or mobility of such defects. The development and implementation of a simple new x-ray detection method has allowed for the observation of these subtle processes for the first time. To a certain degree, these novel aspects of previously well-studied transitions are now amenable to theoretical and computational treatment through the advances made in modern computer technology and parallel algorithms. As a result, new <I>ab-initio</I> local density functional investigations of structural stability and possible transition routes in one of these materials have been performed and the results are in good agreement with observation. Also the issue of defects has been treated by Car-Parrinello-type pseudopotential calculations on defect energies. It is found, among other things that the defect formation energies are relatively low and that structural relaxation under the influence of Hellmann-Feynman forces is necessary in order to obtain accurate values of such energies. The remainder of the thesis is devoted to an exploration of pressure-induced metastable forms of group IV and III-V semiconductors. These studies unite and compare results from image plate experiments, <I>ab-initio</I> pseudopotential calculations as well as empirical calculations using pair potentials and are applied in an attempt to make speculations as to the relative structural stability of these phases at finite temperature.

The last decade has seen a tremendous improvement in high-pressure diffraction techniques. Among other things, this has led to a completely new understanding of the structural systematics of the group IV, III-V and II-VI semiconductors. Many phases have been shown to have more complex, lower-symmetry, high-pressure structures than previously thought. One of the most surprising discoveries has been the non-existence of the diatomic <i>b</i>-tin structure, long believed to be one of the principal high-pressure phases of the III-V and II-VI systems. However, most of the work to date has been performed at room temperature and in fact, very little is yet known about the high-pressure phases of these systems above room temperature. The work presented in this thesis centres on the use of high temperature under pressure to investigate further the absence of the diatomic, site-ordered, <i>b</i>-tin or <i>b</i>-tin-like phases have been found but appear to be site-disordered. Additionally, the P-T phase diagrams of these systems are explored above room temperature. This work has required the development the existing high-pressure facilities on the SRS synchrotron source of Daresbury Laboratory to allow routine high-pressure high-temperature (hp/ht) experiments. These technical developments are described. High-temperature studies of GaSb under pressure reveal a new, previously unknown phase. A detailed study of the structural ordering in the hp/ht phases of GaSb is performed by combining two different experimental techniques. It is shown, by x-ray powder-diffraction, that all the hp and hp/ht phases of GaSb are site-disordered over about two unit cells or less. A complementary high-pressure EXAFS study demonstrates the absence of complete order over nearest-neighbour distances.

This thesis presents a new technique which makes use of the ability of area detectors, such as image-plates, to record simultaneously data from crystallites in many different orientations to model texture from samples under pressure. Experimental methods have been developed to allow data to be collected of sufficient quality to allow preferred orientation of the sample to be accurately modelled. A new preferred orientation model is presented that describes the effect of preferred orientation on the whole 2-D powder pattern and is suitable for the transmission geometry used in high-pressure angle-dispersive diffraction. Techniques are developed which make use of the new model and the 2-D character of the effect of preferred orientation on the powder pattern to de-correlate the effects of texture and structure on the powder pattern. The model and these techniques are tested against diffraction data collected from a wide range of sample orientations from a sample of known texture. The techniques are applied to a study of the crystal structures of three high-pressure phases of the II-VI semiconductor HgTe. A detailed study of the crystal structure of the cinnabar phase of HgTe is presented, which shows the structure to be different from the prototype HgS cinnabar, with coordination much closer to 4-fold. Results on the pressure dependence of the structure are presented and discussed in relation to the transformation upon further pressure increase to the NaCl structure. A structural study of the previously unsolved HgTe phase IV is presented. This phase, which forms with particularly severe preferred orientation, is found to be an orthorhombic distortion of the NaCl structure. Also presented is a short study of HgTe phase V, which is observed to occur above 29 GPa and to have a disordered b.c.c. structure.

Existing inorganic materials for radiation dosimeters suffer from several drawbacks, including their inability to cover large curved areas, lack of tissueequivalence, toxicity, and mechanical inflexibility. As an alternative to inorganics, poly(triarylamine) (PTAA)-based devices have been evaluated for their suitability for detecting radiation via the direct creation of X-ray induced photocurrents. The device was prepared by sandwiching an active layer of PT AA between a transparent indium tin oxide (ITO) b()ttom contact and a metal top contact. The charge transport properties of the device were assessed using the spectral photo current method. Increased photocurrent sensitivity was observed for samples annealed at 150°C, consistent with solvent loss, which was confirmed by thermogravimetric analysis. A diode with an Al contact shows the highest quality of recti tying junction, and it produces a high X-ray photocurrent (several nA) that is stable during continuous exposure to 50 kV Mo Ka X-radiation over long timescales, combined with a high signal-to-noise ratio with fast response times of less than 0.25 s. To optimize the performance of the device, the X-ray stopping power and the charge carrier mobility of the active layer have been modified by the introduction of various concentrations of the high atomic number bismuth oxide nanoparticles and the high mobility TIPS-pentacene organic material, respectively. The increasing of the devices' sensitivity correlates with an increased charge carrier mobility which was measured by a time-of-flight photocurrent measurement. Good stability of Xray sensitivity was found as the dosimeter was exposed to the X-ray beam for a long period of time with no discernible reduction in performance. These results indicate that PTAA is a highly-promising material for the direct detection of X-rays and potentially other types of radiation.

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No description available.

The current transport mechanism usually assumed to be valid for metal silicon Schottky barriers is thermionic-emission. However, Crowell and Beguwala, using a thermionic-diffusion model, suggested that significant deviations from the behaviour predicted by the thermionic-emission theory shonld be observed on low barriers, especially those formed on silicon of low impurity concentration. The barrier height of titanium on n-type silicon is 0.1)0 volt which is lower than most other metals, and should make the effects predicted by the thermionic-diffusion theory more important for titanium contacts. Titanium contacts were prepared on n-type silicon with impurity concentratlon from 2 x 10²⁰ m⁻³ to 3 x 10²¹ m⁻³ • Most of the diodes showed nearly ideal behaviour at low applied voltages and the current-voltage characteristics could be represented by the relationship [handwritten equation impossible to transcribe] with n values as low as 1.01. It was concluded that tunnelling interfacial layer and surface effects were insignificant for such diodes. However, at higher current densities, many of the same diodes exhibited deviations from ideal behaviour which were equivalent to n values as high as 1.25, or which could be interpreted in terms of a rapidly decreasing saturation current IS. Similar effects were observed on magnesium and aluminium contacts on silicon, but at bigher applied voltages, corresponding to the higher barrier heights of 0.55 and 0.72 volts. The main features of the experimental results agreed well with the predictions of the thermionic-diffusion theory for band bending between β = 9 and β = 2. At the upuer value of β the predictions of the thermionic-emission and thermionic-diffusion theories were almost identical and the diode behaviour was closest to ideal with n = 1.01. The lower value of β represents the limit of agreement between the measurements and tho thermionic-diffusion theory. Two possible mechanisms are outlined which could explain the discrepancy below β = 2. These are phonon scattering of electrons between the barrier maximum and the metal, and the effect of the reserve layer on the shape of the potential barrier at very low band bending. The results demonstrate the concli tions under which the thermionic-diffusion theory rather than the thermionic-emission theory should be applied, and suggest a practical lower limit on β for the range of applicability of the thermionic-diffusion theory.