Atomic force microscopy (AFM) in air has been used to study various III-V semiconductor heterostructures. Topography of the (110) cleaved cross-sections has been examined where oxidation processes modify the surface and allow the structures to be investigated. This research aims to establish the potential of this technique as a metrology tool for use in an industrial environment. AlGas/GaAs was used as the prototypical system, and a test structure grown in order to establish how differences in oxidation rates between the different material compositions may be used to establish composition and layer thickness for heterostructure devices. The dependence of oxidation rate on layer composition and thickness has been confirmed. The mechanisms of field-aided and diffusion limited growth have been determined to be responsible for the oxidation of the AlGas layers within the test structure, with field aided being the dominant mode for materials x < 0.8 and diffusion limited dominating for layers x > 0.8 observed. In addition, a new effect of interface enhanced oxide growth have been observed and quantified in terms of layer composition. It is found to be most important for Al09Gao, As layers of 50--100 nm thickness during relatively early stages of oxidation. The use of phase detection microscopy has also been applied for the first time to determine the presence of layers where no measurable step heights are present. Unlike previous reports, there has been no observed difference in oxidation rate between p- and -type materials. These findings have been applied to real device structures, where the material composition of Al/JaAs can be determined to within x 0.02 and layer widths may be determined to within 3 nm at best. It has been shown that step height differences of as little as 0.1 A are sufficient to distinguish between layers, and that quantum wells of as little as 42 A in width are detectable.

In this PhD project the effect of p doping on the carrier dynamics in InGaAs quantum dot amplifiers emitting near 1.3/mi at room temperature has been investigated by transient differential transmission spectroscopy (DTS) and four-wave mixing (FWM) experiments in a heterodyne detection scheme. From DTS experiments, an absorption bleaching on the order of few hundreds of ps and an ultrafast gain recovery were measured at operating condition, i.e. room temperature and with current injection. The faster absorption bleaching recovery observed in p doped amplifiers has been attributed to the carrier-carrier scattering due to built-in holes. Conversely, the gain compression recovery is limited by the lack of an electron reservoir in the dots which has been demonstrated in doped amplifiers. These findings should help in elucidating the role of p doping in the design of QD-based devices with high-speed performances. Temperature dependent DTS measurements have confirmed this interpretation and lead to a microstate model developed at 20 K to represent the gain dynamics. At room temperature the combined study of the gain and refractive index dynamics allows us to measure the line width enhancement factor. We observed that p doping is effective in reducing this parameter. By FWM experiments, the polarization decay of ground state excitonic transitions in the temperature range from 5K to 210 K has been measured to obtain the zero-phonon line (ZPL) width and its contribution to the homogeneous line shape. The temperature-dependent ZPL width is reproduced by a thermally-activated behaviour. This finding has been discussed in the framework of exciton-phonon interactions. Coulomb interaction is investigated by measuring the dephasing time versus injected current at 20 K. From measurements of the homogeneous broadenings of exciton and biexciton transitions we demonstrated that the carrier capture dominates on pure dephasing in these strongly confined dots. Moreover, a much faster dephasing is observed in p doped devices due to Coulomb interaction between built-in holes.

InGaN/GaN based quantum well structures are strained and due to the lack of a centre of symmetry large internal fields are present which skew the potential of the quantum wells, this has a large effect on the properties of these structures. InGaN/GaN based quantum well structures are studied using a theoretical method based on the Pade model and comparison made with experimental results. The importance of using a correct description for the depletion widths of p-i-n structures for use in interpreting measurements of the internal field is established in this work. Interpreting the results from reverse bias photocurrent absorption measurements of an In0.1Gao.9N quantum well structure, a value of-1.9 MVcm-1 for the internal field has been determined, which is within 5 % of the field of -1.8 MVcm-1 calculated using piezoelectric constants interpolated from the binaries. Comparison of the results of the Pade model and experiment demonstrate that the experimental absorption magnitude is an unreliable indication of the internal field due to the carrier extraction efficiency at low bias, hence the absorption peak energy should be used. Using a theoretical model governed by pumping and recombination processes time-resolved photoluminescence experiments performed at Sheffield University have been modelled. It was found that the observed shift of the emission peak arises from a delicate balance between the contributions from bandgap renormalisation, screening of the internal field and the Coulomb interaction. Comparisons between the free carrier and Pade models, found that the energy shift and magnitude of the peak intensity are underestimated when using the free carrier model. Although the internal field is strongly screened at high carrier density, the increase in the dipole matrix element is small. It was found that at threshold the screened internal field is still of the order of -1 MVcm-1, hence inclusion of the screened internal field is essential in laser gain-current calculation.

The twelve components of the magnetoresistivity tensor in antimony up to second order in magnetic field have been measured at 273ºK, 225ºK, 183ºK, 139ºK and 77ºK. Results are interpreted in terms of a two carrier, multi-valley band model to obtain carrier mobilities and densities as a function of temperature and details of the nature of the Fermi surface. In agreement with recent theoretical band structure calculations the electrons are shown to be sited in pockets with a small tilt angle (5º) away from the trigonal axis, while the extrema containing holes have a large tilt angle (24º). The temperature dependence of mobilities of electrons and holes are found to be T(^-1.42) and T(^-1.48) respectively. Carrier densities are almost independent of temperature, ranging from 3.9 x 10(^19) cm(^-3) at 77ºK to 4.2 x 10(^19) cm(^-3) at 273ºK for both electrons and holes. The Seebeck coefficient of antimony can be interpreted by inserting the mobility data obtained in a two parabolic band model. Results suggest that the Fermi energies are equal to 0.098 eV for electrons and 0.067 eV for holes giving a band overlap energy of 0.165 eV. These energy parameters are essentially temperature independent between 77ºK and room temperature, but increase markedly at higher temperatures. Holes probably occupy six and electrons three pockets. Conductivities, Hall coefficients and some of the magneto resistivity coefficients have been measured in t in - antimony alloys of compositions 1.7 at. % , 2.0 at. %, 2.5 at .% 3.0 at % and 8 at .% t in at 77 K, 183 K and 273 K to obtain further knowledge of the valence band structure. Results cannot be explained quantitatively by a simple tilted ellipsoidal band model, although they evidence that holes are to be assigned to the highly tilted ellipsoids.

Ion sensitive organic field-effect transistors (ISOFETs) with a metal–oxide–semiconductor field-effect transistor (MOSFET) architecture have been fabricated by using poly(3-hexylthiophene) (P3HT) and poly(methyl methacrylate) (PMMA) as the semiconductor and dielectric layers, respectively. To avoid any pin-holes in the dielectric layer, the ISOFET was coated by two separate PMMA layers. An Ag/AgCl double-junction reference electrode was used as the gate. The results show that the uncoated ISOFET exhibited transistor behaviour in aqueous solutions. However, these devices possessed a small sensitivity of about 0.5 nA dec-1 to H+, K+ and Na+ ions. Langmuir-Blodgett membranes were then used to improve the ISOFET response to the target ions in solution. By coating the gate dielectric (PMMA) with an LB membrane of pure arachidic acid (AA), the ISOFETs showed a significantly higher sensitivity to H+ ions of about 3.5 nA pH-1, but no improvement in the pK response (< 0.5 nA dec-1). The compact ionised layers of carboxylic acid head groups were thought to lead to the improvement in the pH sensitivity; however, the layers of long hydrocarbon chains prevented large monovalent ions, such as K+ and Na+, from interacting with the ionised carboxylic acid head groups. ISOFETs coated with an arachidic acid/valinomycin (AA/val) mixture did not show any selectivity to K+ ions, but exhibited enhanced sensitivities to both K+ and Na+ ions. Instead of trapping K+ ions, the valinomycin molecules in the AA membrane were thought to disrupt the membrane architecture and provide ion-leakage channels. Pure valinomycin-coated ISOFETs also revealed enhancements in both sensitivity and selectivity to K+ ions over Na+. This may be due to the fact that the cavity in the valinomycin molecules can accommodate a K+ ion but not a Na+ ion. To study facilitated K+ transport across the membrane, LB films of AA/val mixture and pure valinomycin were coated on porous supports. The responses of both uncoated and coated membranes were similar. After deposition, collapse of the LB film into the pores may provide leakage channels. This probably led to the observed gradual decrease of the potentials across the membranes.

A number of single crystals of barium titanate have been grown by the flux melt process. These have been rendered semiconducting by reduction in hydrogen at 800 C for times up to 15 min. The resistivity, Seebeck coefficient and optical absorption coefficient have been measured in these crystals to obtain information about the conduction processes. The temperature of the tetragonal to cubic transition in the reduced crystals has been measured and is shown to be consistent with the results of the resistivity and Seebeck data. The results obtained are consistent with the theory that electrical conduction in these crystals is in the small polaron hopping mode with a drift mobility at room temperature of about 4 X 10(^-4)cm(^2)/V.s. with polaron concentrations reaching a peak of 4.18 X 10(^20)cm(^-3).The variation of polaron concentration with reduction time is similar to that obtained by Arend et al. by chemical analyses.

Magnetic measurements have been carried out on compounds of the form Gd(_x)Y(_1-x)Co(_2), where x varies from 1 to 0. The measurements were carried out over a wide range of temperatures and applied magnetic fields on a vibrating sample magnetometer. The results show that these compounds with high gadolinium content are strong magnetic, and their Curie points range from 400ºK inGdCo(_2) down to near zero for x 0.l. The compound YCo(_2) is shown to be antiferromagnetic, with, a Keel point of 190 K. The magnetisation versus temperature results show an anomaly im the form of a "kink" in the curves for those compounds, with x greater than 0.33, and the presence of this kink is shown to be dependent on the strength of the applied magnetic field, a minimum, or critical field Being required before the anomaly appears. The model proposed to explain, this: behaviour is. an adaptation of one proposed by Lotgering, for which an. antiferromagnetic Gd-Go coupling, an antiferromagnetic Co-Co coupling, and a ferromagnetic Cd-Cd coupling are required, Given these conditions, it is shown that a triangular configurations of moments can; exist, in which the Gd moments lie parallel to the applied magnetic field, and the cobalt moments; lie antiparallel to the applied field, but tilted alternately right and left at an angle so as to form a triangle with the Gd moment. It is shown, that such, a condition cam exist only; below a certain critical temperature, and at fields above a certain critical value. In all respects this model appears to fit the observed results well, but confirmation of the existence of such a configuration not only: in these compounds, but probably: in rare-earth - (cobalt)(_2) and rare-earth - (iron)(_2) compounds also, must await neutron diffraction measurements with a moderately high magnetic field applied to the specimens.

No description available.

GaN-based semiconductors show promise for the fabrication of electronic components capable of high power, high frequency operation. This has lead to the development of both AlGaN/GaN heterostructure field effect and bipolar junction transistors. In order to increase the efficiency of these devices novel fabrication technologies are examined. Contact resistances to buried p-type layers, exposed using a novel wet etch, were found to be superior when compared to those formed on dry etched structures. Utilising this technology in conjunction with a novel inverted n-p-n AlGaN/GaN HBT is found to increase the emitter-base heterojunction quality and also reduces growth complexity. Replacing the n-type collector with a Schottky diode was found to significantly reduce leakage in the component and enable normal operation under common emitter bias conditions. Bulk and surface trapping effects in AIGaN/GaN heterostructures are independently identified. Bulk traps are found to be located close to or at the me~-semiconductor interface which results in a modification to the HFET band structure. Gate leakage current along the AIGaN surface is found to be controlled by injection from the gate and a surface hopping conduction mechanism which dominates at high and low temperature respectively. Passivation of the structure using SiN reduces current flow at the AIGaN surface. Employing a plasma pre-treatment, contamination at the surface of the device can be removed allowing intimate contact between the passivation and AlGaN films. Using a CF4 plasma treatment is found to remove these contaminants and deposits a thin film of AlF3 on the AlGaN surface which reduces current collapse in AlGaN/GaN HFETs to negligible levels. GaN capping layers on HFETs can reduce both parasitic contact resistances and also reduce current collapse. Methods of selectively etching through the GaN capping layer are presented in order to develop a suitable self-aligned gate recess process.

Over recent years, research into organic semiconductors has intensified considerably due to the increasing commercial viability of inexpensive, flexible, large area electronic applications. In particular, the introduction of a new generation of small molecule based organic semiconductors has increased the possibility of achieving high field effect mobilities. So far pentacene seems to be the most promising candidate since it yields field effect mobilities that are comparable to that of hydrogenated amorphous silicon (a-Si:H). Recently, a mobility larger than 1.2 cm2y-fs-l and an on/off ratio greater than 108 was reported for a thin film transistor (TFT) made with triisopropylsilyl pentacene (TIPSpentacene) as the active material. The solution processability of TIPS-pentacene is advantageous since solution-processed organic TFTs are needed to pave the way for low cost manufacturing approaches such as inkjet printing and roll-to-roll processing. One ofthe main potential applications for organic materials is in low cost radio frequency identification (RFID) tags operating at a frequency of 13.56MHz. The high frequency op~ration of RFID tags will be most demanding on the rectifying component of the circuit which can be based either on a TFT or a rectifying diode. This thesis is primarily concerned with analysing metal semiconductor barriers made with a variety of organic semiconductors such as highly regioregular poly(3-hexylthiophene) (p3HT), polytriarylamine (PTAA) (S 1105) and vacuum-deposited pentacene. The work includes a review of the various charge transport models proposed along with a discussion on the Meyer Neldel Rule (MNR) which is a commonly occurring phenomenon in organic semiconductors. A simple analytical model that demonstrates the empirical relationship between mobility and carrier density is also developed. This general dependency is formally known as the Universal Mobility Law (UML). The electrical characteristics of Schott)..)' junctions made between aluminium and various organic solids are investigated. Both P3HT and PTAA are highly disordered semiconductors whilst pentacene is a small molecule organic semiconductor akin to polycrystalline inorganic solids. The analysis of the pentacene vertical diode is thus extensively based on the model developed by Eccleston. The AI-PTAA Schott)..)' diode is found to not only yield a high rectification ratio but also an extremely low off current which suggests that the device is most suitable for low current circuit operations. In contrast, diodes based on P3HT and pentacene demonstrate much weaker rectifYing properties. Nevertheless, in these diodes, the non-saturation of the reverse currents allows the dopant densities ofthe materials to be determined. The new current density expression developed for organic based Schottk.')' diodes allows the characteristic temperature of the exponential distribution of intrinsic carriers (To) and states (Te) to be determined directly from the forward current density voltage characteristic. The Meyer Neldel energy representing the exponential density of states (DOS) is then obtained using the value of Te• In general, the !\tIN energy estimated from the exponential current regime ofthe diodes range between 30 meV and 35 meV. Special attention is given to the saturation current region of the diodes as relatively high currents are needed to satisfY the demands ofRFID related circuits. The saturation current of the as-synthesised P3HT diode was found to obey Ohm's law over the entire applied voltage range. In contrast, the PTAA and pentacene diodes demonstrate a transition from ohmic to space charge limited (SCL) conduction with increasing applied bias. The saturation current regime of these diodes is modelled using the new SCL current expression developed for disordered materials. The intrinsic value of Te determined for both diodes suggests the absence of dopant states at higher energies. The effects of changing the back metal/organic interface and further with doping with 2,3-dichloro-5,6-dicyano-I,4-benzoquinone (DDQ) on the saturation region ofAI-PTAA Schottk.')' diodes are also studied. Finally, the temperature variation of the current density voltage characteristics of assynthesised P3HT and PTAA Schottk.')' diodes are analysed. Below room temperature, the distinct fall in the forward exponential slope leads to much larger ideality factors 1/. This is attributed to the non-ideal behaviour of the Schottk.')' barrier interface with decreasing temperature. The anomalous rise in 1/ is attributed to a number of possibilities including the presence of extrinsic trapping states, carriers taking alternative routes to other potential barriers and the lack of validity of the flat quasi Fermi level approximation. Modelling the saturation current region of the PTAA diode with the new SCL current expression yields a self-consistent value of Te at relatively high temperatures. The steep decline in the Meyer Neldel energy observed at lower temperatures is mainly attributed to the formation of a potential barrier at the back metal/organic interface.