Table of contents

The surface morphology of alpha -Sn(100) films grown pseudomorphically by MBE on InSb(100)-(4*2) surfaces prepared by an improved substrate cleaning method is investigated by RHEED. The authors report two new reconstructions for the alpha -Sn(100) surface, p(2*2) and c(4*4), together with the formation of 1D disorder and atomic steps as the thickness of the overlayer approaches 0.2 mu m. These surface reconstructions are very similar to those of the Si(100) surfaces and structural models and energetics for the two surfaces are compared. The presence of atomic steps on the alpha -Sn surface opens up the possibility of growing defect-free non-polar superlattices based on narrow band gap semiconductors.

A high-mobility two-dimensional electron gas at a GaAs/AlGaAs heterojunction has been exposed to low-energy Ga ion beams. The authors report the reduction in both the sheet carrier density and mobility at 44 K for ion doses and energies in the range 10¹¹-10¹² cm^-2 and 1-6 keV respectively. The sheet resistivity can be increased by a factor of 10¹⁰ with a dose of 2*10¹² cm^-2 ions at 6 keV. The ion irradiation also reduces the persistence of the low temperature photo-induced conduction and increases the amount of ionised impurity scattering. Reduced photoluminescence from wells close to the surface of a multiple quantum well structure extends the transport data to suggest a range of approximately=70 nm from the surface over which electronic properties are modified. An ultra-high vacuum compatible method for altering the surface electronic properties therefore seems viable.

The authors give an unambiguous identification of the Fe acceptor level in Ga_0.47In_0.53As and a coherent description of its electrical and optical properties by photoluminescence, thermal and photocapacitance spectroscopies. Deep-level transient spectroscopy measurements show a deep trap at E_c-0.34 eV which they identify as due to the single acceptor level Fe³⁺/Fe²⁺. This is confirmed by the observation of the internal luminescence transition of Fe²⁺(⁵T₂ to ⁵E) detected at 0.35 eV and by a well pronounced resonance peak at 0.34 eV due to the transition to the ⁵T₂ excited state in the sigma _n⁰(h nu ) spectrum of the photoionisation cross section of the E_c-0.34 eV level. Finally they discuss the validity of the empirical rule connecting transition metal levels to predict the band-gap discontinuity in the InP/InGaAs systems using the known positions of the Fe acceptor level in InP and in Ga_0.47In_0.53As.

A new method is described for studying the low-frequency anomalies which are observed in GaAs FETs. It consists of rapid device-characteristic measurements of the threshold voltage and channel and parasitic resistances, V_T, R_CO and R_S, at a temperature at which the transient is slow. The device parameters may be abstracted and their value and time dependence can be used for device modelling. The technique is verified by measurements on the properties of two hole-like traps. The effect of one is to cause a channel narrowing while the effect of the other is probably best described by an effective gate-length change.

The real part of the change in distribution function caused by the propagation of an acoustic wave through a semiconductor placed under a temperature gradient has been obtained from the linearised Boltzmann transport equation and the thermoelectric coefficients, which determine the drift velocity of the carriers, have been ascertained from the general equation for current density. The threshold temperature gradient for the onset of amplification of the acoustic wave has been obtained by equating the drift velocity to wave velocity. Numerical results for n-indium antimonide have been presented and compared with those obtained from the hydrodynamic approach and the scattering approach.

The authors have developed a formalism for the use of Fermi statistics in semiconductor equations so that they can be discretised and linearised for two-dimensional device modelling. They find it important to choose the variables and the form of the boundary conditions carefully in order to use Fermi statistics consistently in both the Poisson and current continuity equations. Their method has been implemented in a two-dimensional heterojunction devices program. Using the example of an Al_xGa_1-xAs/GaAs HIGFET, they show that Boltzmann statistics overestimate the carrier density and give rise to a greater variation of the potential distribution than Fermi statistics, and that Boltzmann statistics can cause significant errors in the currents at the electrodes.

A systematic study of zinc incorporation as a function of both time and temperature of heat treatment has shown that the surface zinc concentration in MOVPE p-type GaInAs can be increased from an as-grown maximum of 2*10¹⁹ to approximately 1*10²⁰ atoms/cm³ by diffusion from a spin-on glass source. Specific contact resistance values much lower than 1*10^-5 Omega cm² have been achieved after such diffusion. These values were found to depend on the type of p-doping elements used in the as-grown double heterostructures. Light-emitting devices incorporating such low-resistance contacts have predicted lifetimes in excess of 300 years. An advantage on the spin-on glass diffusion process is its suitability for large-scale device production.

SiO₂ layers that are applied in VLSI (very large scale integration) microelectronics (gate dielectric, insulation, etc) are prepared in different ways: thermal, chemical vapour deposition (CVD), thermochemical. It has been investigated whether the composition, thickness, film stress and disorder of thin silicon dioxide can be characterised in a simple manner by Fourier-transform infrared absorption spectroscopy (FT-IR). In addition to the main absorption bands at 460 cm^-1, 808 cm^-1 and 1075 cm^-1 (all transverse optical modes) spectral features at 1165 cm^-1, 1200 cm^-1 (LO₄-TO₄ pair) and 1230-1250 cm^-1 (LO₃ mode) have been analysed as a function of film thickness, preparation conditions, substrate and rapid thermal annealing. The position, full width at half maximum of the band at 1075 cm^-1, the position of the LO₃ mode and the intensity ratio of the LO₄-TO₄ pair and the LO₃ mode can be related in a qualitative fashion to film stress, composition and disorder of the thin SiO₂ films.

Photoconductivity and photoconductive relaxation measurements have been carried out on vacuum-evaporated, air-exposed CdTe and ZnTe films in the temperature range 330-130 K to probe into the mechanism of photoconduction in the films. An analysis is made of the long-term photoconductivity as a function of temperature and light intensity in the cases where drift and recombination are governed by the same or different barriers. It is shown that the experimental results can be interpreted in the light of the authors' analysis by assuming a recombination barrier different from the drift barrier, the height of which is not modulated under illumination. It is observed that both the recombination and drift barrier heights vary with the thickness of the film. The average value of the recombination and drift barrier heights are of the order of 0.22 eV and 0.70 eV for ZnTe, 0.04 eV and 0.45 eV for CdTe films respectively.

The problem of a consistent distribution function associated with a solution of the semiconductor device equations is addressed. It is shown that previously used distribution functions are of limited consistence with the balance equations. This can be overcome by a rigorous re-derivation of a recently published transport model using the method of moments. As a by-product, a consistent expression for the mobility is obtained which is valid in the general case including carrier degeneration. For this expression, a mobility reduction for high electron concentrations can be expected if acoustic or non-polar optic phonon scattering is dominant.

For the first time optically detected cyclotron resonance (ODCR) has been demonstrated using a CO₂ pumped far-infrared (FIR) laser instead of microwaves. Both the electron and the light-hole cyclotron resonances have been observed in GaAs, as well as the 1 s to 2p⁺ impurity transitions. Valence band quantum effects, well known in Ge, are resolved directly for the first time in GaAs and the electron cyclotron resonances show strong spin doublet in the highest quality MBE samples. The technique has remarkable resolution and sensitivity at low temperatures and, by contrast with other techniques that have been reported, the authors also observe the n=1 to 2 (polaron shifted) and higher spin doublet split resonances at helium temperatures and with low FIR laser power. The conduction band results are analysed on the five-band model and the implications of this model on the valence band results are discussed. The authors have determined the valence band inverse mass parameters to be: gamma ₁=7.5, gamma ₂=2.6, gamma ₃=3.1, kappa =1.0.

The potential of an electrochemical cell has been found to vary periodically as Si/Si_1-xGe_x superlattices are anodically etched at constant current. A simple anodic etching system has been used to characterise a range of structures, and both 5.6 AA germanium layers in silicon and 2.8 AA silicon layers in germanium have been resolved. A superlattice beneath a 0.5 mu m silicon layer has also been profiled, having removed the capping layer using a fast anodic etch.

The phase coherence length of excitonic polariton propagation in a waveguide incorporating a single quantum well is numerically evaluated using the correspondence between quantum field and classical electromagnetic field. The phase coherence length of such excitonic polariton propagation is found to be drastically increased to the order of 1 mm. This is achieved by controlling the boundaries of the quantum well and core, resulting in artificial control of the coupling between the exciton and photon fields. Such an extended phase coherence length will make it possible to fabricate new interference devices using polariton propagation.

Absorption coefficients have been calculated for the (4:4) superlattice on different SiGe buffers. It is shown that the buffer on which the superlattice is grown controls the skew of the spectral response compared with that for a SiGe alloy with a similar average composition. Compared with the absorption of the buffer, the presence of the superlattice always induces a knee in the band-edge response; this knee can exhibit significant polarisation dependence. The results for the (4:4) on Si are in good agreement with recent experimental results.