Table of contents

Ultra-thin (20 AA) In and Pt layers have been incorporated in amorphous silicon by a laser quenching technique. With this method it is possible to determine the diffusivities of such dopants in alpha -Si. The values obtained are 4.8*10^-17 and 8.9*10^-17 cm² s^-1 for In at 220 and 300 degrees C respectively and 1.8*10^-16 cm² s^-1 for Pt at 300 degrees C. As for slow diffusers in crystalline Si, impurity diffusion in amorphous silicon is found to be rapid at high concentrations and very slow at low concentrations.

With optically detected electron spin resonance the spectra of the two charge states of vanadium V²⁺(3d³) and V³⁺(3d²) in GaAs were investigated. A novel magneto-optical technique was used for the first time to determine the spin states of both configurations. The spin of the 3d³ configuration of V²⁺ is found to be a low spin S=¹/₂ and not a high spin S=³/₂ according to Hund's rule. This confirms recent theoretical predictions. The optical transitions of each charge state could be determined separately and assigned to each spin state.

A detailed model is developed to study the influence of the sample RC product on capture cross section measurements and to enable the determination of absolute values of capture cross sections from such measurements. The model is tested on the deep double donor of selenium in silicon. It is shown that the value of the RC product should be at least a factor of ten smaller than the time constant of the capture cross section in order to avoid appreciable errors in the determination of capture cross sections. Such errors remain easily undetected in an ordinary analysis of capture cross section measurements.

Absolute values of electron capture cross sections for the charged selenium centre in silicon are reported. The cross sections have been measured in the temperature range between 200 and 300 K. Electronic parameters such as the enthalpy and the Gibbs free energy are for the first time presented and discussed. It is shown that the electron capture is governed by some kind of cascade process. All cross sections have been calculated by taking the effect of the RC product into account. It is demonstrated that incorrect results are obtained when the effect of the RC product is neglected.

Oxide layer growth on GaAs, kept in methanol and without applying an external voltage, up to a thickness of about 20 nm is reported. A growth law is found indicating that diffusion (with a diffusion constant approximately=10^-22 m² s^-1), probably of a water or a water related species, is the dominating growth mechanism. The observed light-dependence of the growth on n-type material can be explained by the participation of holes in the first reaction step. Auger depth profiles show that the As-content of the layers grown in methanol is higher than that of layers grown in water, explaining the observed difference in the layers conductivity.

A new approach to the problem of including non-parabolic band structure in calculations of Auger recombination rates is presented, giving results for quantum wells (QW) that are readily generalised to the cases of bulk semiconductors and quantum well wires (QWW). The band structure in the region of the excited Auger carrier is described using non-local pseudopotential band structure, whereas the other carriers, being close to their respective band edges, are described using parabolic bandstructure. The authors' approach differs from earlier work in that the energies and wavevectors of carriers close to the band edges are not neglected, but simple analytic results are obtained. Numerical results for the effects of non-parabolic band structure on bulk Auger transition rates in GaAs and GaSb, using the simple expressions derived in this paper, are found to be in good agreement with other calculations.

Non-homogeneous doping of semiconductor devices and the calculation of the corresponding sub-band structure is an expanding field of great interest. This paper is an attempt to calculate the sub-band energies in the case of a Lorentzian doping concentration within the harmonic approximation. The major purpose is to find a handy, mainly analytical, solution to a complex problem. When using the Lorentzian shape, the strength and the width of the peak can be chosen randomly instead of the restricted Dirac delta distribution. Furthermore, the combined effect of a Lorentzian peak and a homogeneous background is investigated, which is important, as one can get an indication of the most favourable geometry of the device.

Multiple-quantum-well (MQW) samples of GaInAs/AlInAs, with 100 AA, barriers and 50 AA wells, were grown by MBE. The samples contained 10, 20 or 50 periods and were either nominally undoped throughout or consisted of nominally undoped MQW region sandwiched between thick doped layers. Measurements of optical absorption yielded information about the sub-band structure of the superlattice and gave a measure of the absorption coefficients. Non-parabolic effective-mass theory was used to compare calculated energy levels with observations and satisfactory agreement was obtained.

The authors calculate the self-consistent potential and electronic eigenstates of doped heterostructure superlattices. The minibanding effects, nonparabolicity of the conduction band and exchange-correlation effects are included. They calculate the transition energy shifted by many body effects and the dipole matrix element of the inter-sub-band excitations. The miniband energies of the excited states calculated as a function of the quantum well width or of spacer width show a level crossing. Thereby they can explain the dependence on quantum well width of sub-band resonance experiments using narrow quantum wells. Numerical results are given for the InGaAs/InP system. They find quantitative agreement with recently measured transition energies.

In this paper two and three-dimensional Si MOSFET simulations based on a new carrier dynamical model of transport in semiconductors are presented. The simulation results derive from numerical solution of Poisson's equation, the carrier continuity equation and carrier momentum and energy conservation equations. The modelling equations differ from those used previously in that they assume the carrier distribution is quantified by a perturbation approximation rather being a drifted Maxwellian. Momentum and energy relaxation times are taken as being dependent upon the average carrier energy, which is strictly proportional to temperature in the new transport model. This overcomes the problem, inherent in drift-diffusion-based device simulation, of mobility being modelled as a local electric field-dependent parameter. It is shown that no boundary conditions are needed for the carrier energy conservation equation and that a lattice energy conservation can be easily appended to the carrier dynamical transport model, allowing lattice temperature to be readily calculated. Carrier temperature profiles within MOSFETS are presented, as are lattice temperature profiles for the two-dimensional simulations. It is demonstrated that carrier heating effects can be significant, indicating that carrier diffusion, which is directly proportional to the carrier temperature, may be underestimated by conventional (isothermal carriers and lattice) modelling and simulation techniques. The three-dimensional simulations show that the hottest carriers are not generated in the middle of the channel with respect to the width of a MOSFET.

Hot-electron magnetophoton resonances in micrometre size n⁺n^-n⁺ InP structures are reported in crossed electric and magnetic fields in the temperature range from 110 to 230 K. The full series of quantum oscillation was studied up to the quantum number N=1 using pulsed high magnetic fields up to 40 T. The sign reversal of the oscillatory magnetoconductivity extrema from maxima to minima was observed with increasing electric field up to 3*10⁴ V cm^-1. The physical mechanism for the reversal of the extrema is interpreted in terms of the electric-field-induced inter-Landau level inelastic scattering of carriers by longitudinal optic phonons.

Electrical charge transport in a thin off-axis, sample of n-Si is considered within the framework of the phenomenological transport equations. A finite space charge is included through the use of Poisson's equation. In addition to calculating the sample transport coefficients for uniform electric fields applied in the principal directions, expressions are obtained for the space charge densities induced by these fields. The surface space charge density induced by a longitudinal applied field is found to be comparable in magnitude with that induced by a field applied normal to the sample surface.

The authors consider perpendicular transport in a 'superlattice' miniband in the presence of an electric field perpendicular and a magnetic field parallel to the layers, in the framework of a one-band tight-binding model. Using the transfer matrix technique they derive rigorous asymptotic results for the one-dimensional resistance in the presence of diagonal and off-diagonal disorder. They deduce field-dependent localisation lengths and predict oscillatory magnetoresistance.

For the first time a method is given to determine the Hall conductivity on a Corbino disc with an electrical transport experiment. Use has been made of a time-modulated magnetic field to induce currents in the disc. First measurements were carried out on GaAs/AlGaAs heterojunctions in the quantised Hall regime. Instead of the expected plateaus, oscillations as a function of the magnetic field were found. It is shown that an inhomogeneous electron gas density can account for the observations.

Measurements have been made of the 1/f noise in MOS enhancement and depletion devices in strong inversion, biased in the Ohmic region between 77 and 360 K. Measurements of the DC characteristics have enabled the results to be related to a clear model of the device operation. The data do not fit mobility or quantum 1/f noise models. Good agreement is found with a number fluctuation model if the noise is assumed to be produced by two types of trapping process.

The formation of Schottky barriers at Cr/n-CdTe interfaces has been investigated in detail using angle-resolved ultraviolet photo-emission, soft X-ray photo-emission and current-voltage techniques. The CdTe surfaces studied were prepared by cleaving in ultra-high vacuum, cleaving in air, and by chemical etching. The results indicate strong chemical reactions between both CdTe and its native oxide (TeO₂) with evaporated Cr layers. A typical high-coverage Cr/clean-CdTe interface consists of a mixture of cadmium chromium telluride and metallically bonded cadmium. In the case of oxidised CdTe surfaces, oxides of chromium and chromium telluride also exist at the interface. The resulting barriers are less than about 0.35 eV, showing Ohmic behaviour, and are almost independent of the surface preparation. These chemical reactions at the interface are discussed on the basis of the published bulk thermodynamic data.

In the graded-gate FET (GFET), a high-resistance gate, with Ohmic end contacts, is biased to produce a longitudinal channel field. The gate voltage varies linearly along the gate. In this paper the authors develop a first-order theory to model the structure. The effects of a linearly varying gate voltage on the surface potential, the inversion layer and bulk charges, and the current-voltage characteristics for a single-drain GFET are predicted. The cases of positive, negative and zero voltage gradient are included for comparison.

A photoemission survey is presented for the effects of metal deposition on the cleaved, sub-monolayer oxidised Si(111) surface, Al reacts with the oxidised surface; In and Ag show minimal reactivity; Au and Ni react only with the clean Si, and Mn and V react with both clean and oxidised Si. The experimental reactivities are shown to follow the predictions of a bulk thermodynamic model.

Spatially resolved experiments have been performed on the luminescence of bevelled Al_xGa_1-xAs structures at different temperatures as a function of the alloy composition. The binding energy of the Ge-acceptor E_A^Ge relative to the valence band edge was determined in the composition range 0<or=x<or=0.4 analysing the radiative decay of free electrons with holes bound to the neutral acceptor or the DA-pair transition at T=77 K. With regard to the AlAs content the activation energy of the lightly doped material obeys the relation E_A^Ge=40.4+5.79*10²x^2.23 (E_A^Ge in meV). This strong x-dependence cannot be explained by a simple effective-mass-like expression. Computations of the hole state density for highly doped material (N_A=1*10¹⁸ cm^-3) confirm the lowering of the acceptor ionisation energy especially for small AlAs contents, which is in fairly good qualitative agreement with the features experimentally obtained. In contrast to previous investigations two low-energy broad emission bands with different properties associated with incorporated Ge in Al_xGa_1-xAs have been obtained. While the first band is due to a DA-pair transition involving a deep donor and the Ge-acceptor, the second one is suggested to be caused by recombination connected with an impurity-vacancy complex.

Analysis of the thermal diffusion contribution to laser-irradiation-induced dissolving in semiconductors of inhomogeneities enriched in metallic components has been carried out for the laser spectral region h(cross) omega <E_g, where E_g is the energy band gap of the semiconductor. Experimental dependences of the rates of variation of free-carrier concentrations in narrow-gap Pb_1-xSn_xTe(Se) semiconductors, associated with filling of electrically active metal vacancies with atoms from inclusions formed in the process of semiconductor growth, have allowed evaluation of the parameters of the thermodiffusion and diffusion mechanisms, and of the criteria for predominance of the former of these. The experimental values of the times for variation of the electrical characteristics of Pb_1-xSn_xTe(Se) crystals are close to the theoretical predictions.

The authors investigate the implications of the neutrality condition for the steady-state photoconductivity of a material, such as a-Si:H, whose gap density of states is skewed towards the lower half of the gap. They find that the asymmetry in the state density leads to asymmetry in the electron and hole concentrations n and p. For undoped material p<<n by several orders of magnitude. For p-type materials they show that the condition p>n is difficult to maintain as the illumination level is increased, and they predict the occurrence of a type change (n>p) as the photoconductivity reaches a certain value.

Exciton binding energies and wavefunctions are calculated for spherical quantum dots of GaAs embedded in GaAlAs for a range of radii from 10 to 200 AA. The calculations use the quantum Monte Carlo method which allows the finite potential barriers in this system to be treated in a straightforward manner. The exciton binding energy and absorption intensity are found to pass through a maximum for dots of approximately 30 AA radius; strong optical non-linearities would be expected in dots of this size.

Transition rates for electron tunnelling between interface states and the metal in Cr-SiO₂-n-Si structures are determined from the temperature dependence of the barrier height measured by steady-state 1/C²-V capacitance-voltage characteristics. The tunnelling attempt-to-escape frequency 10^15.2+or-1.5 s^-1 of interface states and the attenuation coefficient 0.88+or-0.11 AA^-1 in the oxide are obtained in agreement with previous data from capacitance transient spectroscopy and theory.