Surface compositional change of GaP, GaAs, GaSb, InP, InAs, InSb, GeSi and CdSe single crystals due to low keV noble gas ion beam bombardment has been investigated by combining X-ray Photoelectron Spectroscopy (XPS) and Low Energy Ion Scattering Spectroscopy (LEISS). The purpose of using this complementary analytical method is to obtain more complete experimental evidence of ion beam modification in surfaces of compound semiconductors and GeSi alloy to improve the understanding of the mechanisms responsible for these effects. Before ion bombardment the sample surfaces were analysed nondestructively by Angular Resolved XPS (ARXPS) and LEISS to get the initial distribution of surface composition. Ion bombardment experiments were carried out using 3keV argon ions with beam current of 1A for a period of 50 minutes, compositional changes in the surfaces of compound semiconductors and GeSi alloy were monitored with normal XPS. After ion bombardment the surfaces were re-examined with ARXPS and LEISS. Both XPS and LEISS results showed clearly that ion bombardment will change the compositional distribution in the compound semiconductor and GeSi surfaces. In order to explain the observed experimental results, two major theories in this field, Sigmund linear collision cascade theory and the thermodynamic models based on bombardment induced Gibbsian surface segregation and diffusion, were investigated. Computer simulation using TRIM code was also carried out for assistance to the theoretical analysis. Combined the results obtained from XPS and LEISS analyses, ion bombardment induced compositional changes in compound semiconductor and GeSi surfaces are explained in terms of the bombardment induced Gibbsian surface segregation and diffusion.

A time-resolved ellipsometry technique has been developed using a home- built and calibrated ellipsometer. The design and optimisation of the ellip- someter has been chosen carefully to study low absorption materials. The ellipsometric angles (Ψ and ∆) are measured in good agreement with commercial ellipsometry equipment. An optical model is constructed to find accurately the thin film thickness’ and dielectric function dispersions of the complex multi-layer samples. The values are then fit as a function of time allowing measurement of the transient change in dielectric function. The changes of complex dielectric function are modelled using a Drude approximation revealing interesting behaviour of the scattering processes and carrier concentration in samples of nc-Si:H and silicon nitride (SiN\(_x\)). In samples of nc-Si:H we find that the carriers adopt a classical distribution through analysis of the Fermi integrals and that recombination processes conserve the average temperature of electrons, suggesting that there is no preference for recombination of carriers with higher energies. This is contrary to current understanding of carrier dynamics in bulk semiconductors.

This work presents optical studies of electron spin dynamics in gallium arsenide (GaAs) quantum wells, focusing on the effect of inversion asymmetric confinement potentials on spin lifetimes in quantum wells grown on (110)-oriented substrates. Inversion asymmetry in the presence of the spinorbit interaction offers the possibility of complete control of electron spin dynamics in GaAs quantum wells. Symmetry arguments predict any inversion asymmetric two dimensional potential will reduce spin lifetimes via the Dyakonov-Perel spin relaxation mechanism. One aim of this work has been to make a comparison of the effect produced by an electric field to that from alloy engineering. The suppression of the Dyakonov-Perel spin relaxation mechanism in (110) quantum wells makes them ideal candidates for measuring increases in spin relaxation due to asymmetry. To measure temporal spin dynamics, the time-resolved Kerr rotation technique was adapted in order to compensate for reduced rotational symmetry in the (110) crystallographic direction. An investigation into the effect of a transverse electric field on electron spin lifetimes was conducted. By combining spin lifetime with electron scattering time measurements it was possible to provide the first direct measurement of the Rashba coefficient. There is good agreement with k.p theory at low temperatures; however there is an unexplained increase with temperature. Spin dynamics measurements were carried out on quantum wells with asymmetric alloy composition. Through the combination of electron spin lifetime and electron scattering time measurements it was shown that the effect of alloy engineering on electron spin dynamics is very small. This is consistent with theoretical predictions and highlights the importance of considering both conduction and valence band potentials for the understanding of the effect of asymmetry on electron spin dynamics in quantum wells.

In this thesis, polariton dynamics in GaAs semiconductor microcavities are investigated. Insertion of layers of active material within a Fabry-Perot resonator leads to strong coupling between excitons and photons, giving rise to new eigenmodes called excitonpolaritons. Bose-Einstein condensation, the ability for massive occupation of a quantum state is considered a fascinating property of polaritons, due to their bosonic character. A full study of polariton condensation in 2D and 0D microcavities is included in this thesis. Formation of a ground state condensation in a planar cavity is resolved by studying the spatial, angular, coherence, energy and transient dynamics of polariton photoluminescence, as well as the transition from the weak- to the strong-coupling regime in the time-domain. The role of longitudinal optical phonons in the relaxation dynamics is also investigated. Encouraging experimental results confirm the efficiency of this mechanism towards the formation of a ground state condensate. Polariton condensation in 0D GaAs quantum well microcavities is facilitated by etching the 2D semiconductor microcavity sample into pillars, removing the wavevector conservation. The spontaneous formation of a condensate in ground and non-ground states in 0D microcavities is investigated experimentally. A complete kinematic model that satisfactorily describes the spectral and temporal behaviour of polaritons in 0D structures completes this study. Finally, the observation of the all optical spin Hall effect, the separation of spin polarised carriers in real and momentum space, in a pure photonic cavity is included in this thesis. Experimental findings suggest that the excitonic contribution in similar observations in the strong coupling regime with polaritons acting as spin carriers, is not essential for the observation of the anisotropic polarisation flux.

Interactions of exciton-polaritons in semiconductor microcavities and the resulting polarisation dynamics are investigated theoretically. Within the coboson framework of polariton-polariton scattering, it is shown that the matrix element of direct Coulomb scattering is proportional to the transferred momentum, q, cubed in the limit of small q. In the same limit, the magnitude of superexchange/exchange interactions can be considered constant. These results are applied to the elastic circle geometry, where a system of equations describing the steady-state pseudospin components is derived. It is shown, that for this geometry, polariton-polariton scattering can account for the generation of circularly/linearly polarised final states from linearly/circularly polarised initial states, depolarisation and the generation of spin currents. In the low density regime polaritons are good bosons and the dynamics of polariton Bose-Einstein condensation (BEC) are investigated. A stochastic model is derived, resulting in a Langevin type equation describing the time dynamics of the condensate spinor order parameter. The build up in condensate polarisation degree is shown to evidence macroscopic ground state population, while the stochastic choice of polarisation vector evidences the symmetry breaking nature of the phase transition. The decrease of polarisation degree above threshold is demonstrated to be a consequence of polariton-polariton interactions, a result which is complemented by recent experimental work. The stochastic model is extended to include Josephson coupling of spatially separate condensates. The coupling results in polarisation and phase correlations between the condensates, explaining the polarisation locking and spatial coherence seen experimentally. Finally, the effect of polarisation pinning by local effective fields is examined.

This thesis is concerned with the deposition, and characterisation, of semiconductor thin films and microstructures deposited from a supercritical uid. Thin films of CdS, GaP, InP, InAs, and Ge were deposited using supercritical CO2 and CO2-solvent mixtures. Ge was deposited into macropores etched into crystalline silicon substrates. A variety of reactors were designed in order to achieve the successful deposition of the materials. The surface morphology and crystallinity of the films were characterised by scanning electron microscopy and X-ray diffraction. The chemical composition of the films was analysed by energy or wavelength dispersive X-ray spectroscopy, secondary ion mass spectroscopy, Auger electron spectroscopy, and X-ray photoelectron spectroscopy. The optoelectronic quality of the CdS and InP films was analysed by photoluminescence spectroscopy and mapping. The CdS films deposited were confirmed to be of hexagonal phase by X-ray diffraction and exhibited band edge luminescence. The InP and InAs films were determined to be of cubic structure and the InP films were found to exhibit weak band edge luminescence. The fabrication of macroporous silicon templates by photoelectrochemical etching is also discussed. Pores with diameters of between 60 nm and 2 m were fabricated, having aspect ratios of up to 100:1. Ge was successfully deposited into macropores etched into these crystalline silicon templates with near conformal coverage.

The aim of the research project presented in this thesis is to investigate the effects of electrostatic gating on the magnetic properties of carrier-mediated ferromagnetic Ga1-xMnxAs diluted magnetic semiconductors. (Ga,Mn)As can be regarded as a prototype material because of its strong spin-orbit coupling and its crystalline properties which can be described within a simple band structure model. Compressively strained (Ga,Mn)As epilayer with more complex in-plane competing cubic and uniaxial magnetic anisotropies is of particular interest since a small variation of these competing anisotropy fields provide a means for the manipulation of its magnetization via external electric field. An all-semiconductor epitaxial p-n junction field-effect transistor (FET) based on low-doped Ga0.975Mn0.025As was fabricated. It has an in-built n-GaAs back-gate, which, in addition to being a normal gate, enhances the gating effects, especially in the depletion of the epilayer, by decreasing the effective channel thickness by means of a depletion region. A shift in the Curie temperature of ~2 K and enhanced anisotropic magnetoresistance (AMR) (which at saturation reaches ~30%) is achieved with a depletion of a few volts. Persistent magnetization switchings with short electric field pulses are also observed. The magnitude of the switching field is found to decrease with increasing depletion of the (Ga,Mn)As layer. By employing the k . p semiconductor theory approach (performed by our collaborators in Institute of Physics, ASCR, Prague), including strong spin-orbit coupling effects in the host semiconductor valence band, a change in sign of Kc at hole density of approximately 1.5x1020 cm-3 is observed. Below this density, the [110]/[1⁻10] magnetization directions are favoured, consistent with experimental data. A double-gated FET, with an ionic-gel top-gate coupled with a p-n junction back-gate based on the same material, was also employed in an attempt to achieve larger effects through gating. It reaffirms the results obtained and demonstrates enhanced gating effects on the magnetic properties of (Ga,Mn)As.

In this thesis, a modal decomposition method and a time-frequency-domain formalism for the analysis of multimode dynamics of semiconductor ring laser are developed. The diffusion coefficient is suggested as a crucial parameter to take into account. The directional switching dynamics and dependence on the operation parameters has been studied. The lasing wavelength switching accompanied by directional flipping have also been studied. In this framework, a prior selection of the lasing mode is seen as a key factor for the numerical results.

Scaling of complementary metal-oxide-semiconductor (CMOS) technology has benefited the semiconductor industry for almost half a century. For CMOS devices with a physical gate-length in the sub-100 nm range, extreme device variability is introduced and has become a major stumbling block for next generation analogue circuit design. Both opportunities and challenges have therefore confronted analogue circuit designers. Small geometry device can enable high-speed analogue circuit designs, such as data conversion interfaces that can work in the radio frequency range. These designs can be co-integrated with digital systems to achieve low cost, high-performance, single-chip solutions that could only be achieved using multi-chip solutions in the past. However, analogue circuit designs are extremely vulnerable to device mismatch, since a large number of symmetric transistor pairs and circuit cells are required. The increase in device variability from sub-100 nm processes has therefore significantly reduced the production yield of the conventional designs. Mismatch models have been developed to analytically evaluate the magnitude of random variations. Based on measurements from custom designed test structures, the statistics of process variation can be estimated using design related parameters. However, existing models can no longer accurately estimate the magnitude of mismatch for sub-100 nm “atomistic” devices, since short-channel effects have become important. In this thesis, a new mismatch model for small geometry devices will be proposed to address this problem. Based on knowledge of the matching performance obtained from the mismatch model, design solutions are desired at different design levels for a variety of circuit topologies. In this thesis, transistor level compensation solutions have been investigated and closed-loop compensation circuits have been proposed. At circuit level, a latch-based comparator has been used to develop a compensation solution because this type of comparator is extremely sensitive to the device mismatch. These comparators are also used as the fundamental building block for the analogue-to-digital converters (ADC). The proposed comparator compensation scheme is used to improve the performance of a high-speed flash ADC.

The growing variability of electrical characteristics is a major issue associated with continuous downscaling of contemporary bulk MOSFETs. In addition, the operating conditions brought about by these same scaling trends have pushed MOSFET degradation mechanisms such as Bias Temperature Instability (BTI) to the forefront as a critical reliability threat. This thesis investigates the impact of this ageing phenomena, in conjunction with device variability, on key MOSFET electrical parameters. A three-dimensional drift-diffusion approximation is adopted as the simulation approach in this work, with random dopant fluctuations—the dominant source of statistical variability—included in the simulations. The testbed device is a realistic 35 nm physical gate length n-channel conventional bulk MOSFET. 1000 microscopically different implementations of the transistor are simulated and subjected to charge-trapping at the oxide interface. The statistical simulations reveal relatively rare but very large threshold voltage shifts, with magnitudes over 3 times than that predicted by the conventional theoretical approach. The physical origin of this effect is investigated in terms of the electrostatic influences of the random dopants and trapped charges on the channel electron concentration. Simulations with progressively increased trapped charge densities—emulating the characteristic condition of BTI degradation—result in further variability of the threshold voltage distribution. Weak correlations of the order of 10-2 are found between the pre-degradation threshold voltage and post-degradation threshold voltage shift distributions. The importance of accounting for random dopant fluctuations in the simulations is emphasised in order to obtain qualitative agreement between simulation results and published experimental measurements. Finally, the information gained from these device-level physical simulations is integrated into statistical compact models, making the information available to circuit designers.