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No description available.

No description available.

No description available.

No description available.

No description available.

The parallel transport of electrons in pseudomorphic In(_z)Ga(_1-x)As/GaAs quantum wells is influenced by the degree of spacial confinement and by the effect of the indium concentration which determines, the amount of alloy scattering, the subband structure, and material parameters. The indium content changes the bandstructure and material parameters through both direct compositional and strain effects. We use the single particle and ensemble methods of Monte-Carlo simulation to investigate how the above phenomena influence the transport properties of electrons in In(_x)Ga(_1-x)As/GaAs quantum wells. To understand the effects of alloying and strain on the electron transport properties we first consider electrons in bulk In(_x)Ga(_1-x)As/GaAs. Alloying and strain are considered in artificial systems were the effect(s) of these factors on electron transport may be isolated. For a range of indium compositions, we consider independently the effects of alloying (with and without alloy scattering) and strain on the bandstructure and material parameters and, in turn, their effects on the electron transport properties. We show that increasing the indium concentration generally improves the carrier low field mobility and peak velocity of unstrained materials but has a detrimental effect on the saturation velocity. Strain reduces the low field mobility and peak velocity but gives a slightly higher saturation velocity when compared to GaAs, and the unstrained system. Comparison of transient and steady state transport phenomena is made for strained In(_0.15)Ga(_0.085)As/GaAs quantum well structures, at fields high enough for real- and reciprocal-space transfer to occur. An artificial case, called the unstrained system, where the strain effects on the bandstructure and material parameters are neglected is also considered. Differences between the strained and unstrained well results are small and mainly transient. At steady state, most of the electrons for almost all fields reside in unbound states. The strained and unstrained systems show higher low field mobilities when compared to bulk GaAs. Lattice vibrations are also affected by heterostructures and we have made a study of the effects on the low field transport of electrons in a 70Å Al(_0.3)Ga(_0.7)As/GaAs quantum well when the polar optical phonon modes which interact with the electrons are described by three phonon models which describe the lattice vibrations of the heterostructure; the Hydrodynamic Model (EDM), the Dielectric Continuum Model (DCM), and the Bulk Phonon Approximation (BPA). We show that the BPA and EDM predict similar transport effects and are in good agreement with experimental results. We conclude that, at present, the BPA is an adequate model to describe the phonon modes in heterostructure quantum wells for use in transport calculations.

Oxidation is a fundamental process in the fabrication of microelectronic devices and in order to promote the incorporation of SiGe into Si technology we have investigated the thermal oxidation of Si1-yGey alloy (y≈0.5) at high temperatures (mainly 900°C and 1000°C) using Rutherford backscattering spectroscopy, infrared transmission spectroscopy and X-ray photoelectron spectroscopy. It has been observed that three distinct regions form during oxidation of Si0.5Ge0.5 alloy, which are (I) a mixed oxide layer Si0.5Geg0.5O2, (II) a pure SiO2 layer and (III) a Si1-yGey (y≠0.5) alloy layer. These are formed during both wet and dry oxidation when the sample is not preheated, whilst only two regions (II) and (III) form when the sample is preheated in a non oxygen ambient up to the oxidation temperature prior to oxidation. Enhancement of the rate of wet oxidation of SiGe compared with bulk Si and the accumulation of Ge in region III just below SiO2 layer (region II) has also been observed. This behaviour is in good agreement with other results, however, some abnormal behaviour during wet and dry oxidation has been observed. The rate of oxidation during short wet oxidations (15 minutes) of Si0.5 Ge0.5 decreases as the oxidation temperature increases from 800°C to 1000°C for 15 minutes, which has not been previously reported, A reduced rate of oxidation of Si1-yGey (y≈0.5) in a dry environment has been observed and is discussed. In order to investigate the thermodynamics of the oxidation process new experiments have been carried out which involve the synthesis of a buried oxide layer, by O+ implantation followed by a high temperature anneal. The implanted oxygen atoms preferentially bond to silicon atoms and the oxygen atoms are found to bond to germanium atoms only after all of the silicon atoms are fully oxidised. Germanium tends to be rejected from the growing oxide during a subsequent higher temperature ( > 900°C) anneal. The different behaviour of the Si and Ge atoms during both thermal and internal oxidations is described in terms of the thermodynamics and kinetics of the SiGe alloy system. There are three important conclusions which emerge from these analyses: (i) in order to adequately control the composition of thermal oxides (T > 800°C) grown on Si0.5Ge0.5 material it is necessary to preheat the samples in a non oxidising atmosphere prior to oxidation. By so doing the entrapment of Ge in the near surface layer (region I) is inhibited; (ii) during the initial stage of dry oxidation the process is described by the reaction Si+O2?Si2 and we suggest that the rate is controlled by the availability of oxygen atoms and is numerically the same as for bulk Si; (iii) in contrast, during the initial stage of wet oxidation, the process is described by the reaction Si+2OH→SiO2+H2 and the rate is controlled by the areal density of Si atoms, which we propose is high due to the weak Si-Ge binding energy and thus an enhanced oxidation rate occurs.

The electrical characteristics of III-V semiconductors and their related structures have been investigated as a function of temperature and hydrostatic pressure. Bulk InP and GaAs samples have been examined over a wide range of impurity concentrations. The two dimensional electron gas (2DEG) confined in In0.53Ga0.47As at its interface with InP has been investigated in hetrojunction and multiple quantum well samples. The experimental Hall mobilities for nominally pure InP and GaAs are in good agreement with those calculated using an iterative solution of the Boltzman equation (ISBE). The activation energies of the shallow impurities present in the samples studied, as deduced from the temperature dependence of the carrier concentration, are in excellent agreement with theory. For the doped InP and GaAs samples the standard model was inadequate and could not describe the measured mobilities. For intermediately doped samples good agreement with experiment was obtained using a two band model, which included conduction via extended states as described by the ISBE together with hopping conduction in an impurity band. The experimental results for the heavily doped samples were found to fit a model for scattering from a correlated distribution of ionized impurities. The carrier concentration of the 2DEG in the hetrojunction samples had an unexpected pressure dependence at 300K. This data was found to be consistent with a pressure dependence of the conduction band offsets of -4+/-1meV/kbar. The temperature dependence of the pressure coefficient of the mobility for the 2DEG in InGaAs was in agreement with theory when the uncertainty in the pressure dependence of the effective mass was taken into account.

An experimental and theoretical analysis has been carried out into the effects of electrostatic discharge and constant power electrical overstress in GaAs MES structures. An experimental system has been set up to measure the electrical and physical characteristics of such devices when subject to electrical overstress. This system includes computer controlled equipment to analyse the electrical failure waveforms. The results from the experimental study have been analysed to establish any patterns which characterise ESD breakdown. Using a new thermal breakdown model, analytical predictions of the power required to degrade these devices, for both constant power, and electrostatic discharge breakdown, have been carried out.