Table of contents

The dynamics of growth of III-V compounds by molecular beam epitaxy can be evaluated experimentally using the temporal variation of the intensity of reflection high-energy electron diffraction patterns. The use of substrate surfaces which are slightly misoriented from an exact low-index plane (vicinal planes) enables direct measurements to be made of cation migration parameters, but a correct analysis requires both step anisotropy and nucleation on the terraces to be taken into account. The experimental results are strongly supported by Monte Carlo simulations of growth, especially with regard to growth mode changes and the anisotropy of cation incorporation at steps. The direct growth of quantum wires (structures giving quantum confinement of carriers in two dimensions) on vicinal planes has been achieved experimentally and it is shown here how the wire quality, as determined by its compositional integrity, can be obtained by the simulation technique. The effects of flux, temperature, misorientation direction and interruptions of growth on quality are demonstrated.

Dilute magnetic semiconductors such as Cd_1-xMn_xTe are of importance because of the strong enhancement of the magnetic properties of the electrons and holes that occur because of the spin exchange interaction with the magnetic ions. The exchange interactions lead to novel features such as magnetic tunability of the bandgap energy and the formation of magnetic polarons. Such materials can now be incorporated into multiple quantum well structures. Aspects of the resulting behaviour are discussed, together with details of the MBE growth of the layers on InSb substrates.

A brief review is given of recent work on the photoluminescence characterization of silicon grown by molecular beam epitaxy at temperatures less than 800 degrees C. Evidence is presented for the incorporation of carbon and nitrogen, which can form complexes during low-temperature growth which are not observed in material grown at higher temperatures. Although frequently observed, the suppression of donor and acceptor bound-exciton luminescence is shown not to be an inherent property of MBE silicon grown at low temperatures, and must be associated with the presence of contaminants which compete for exciton capture and give rise to non-radiative decay processes. The appearance of dislocation-related luminescence features is shown to need the presence of transition metal contamination.

The authors demonstrate two applications of Raman scattering for the characterization of low-dimensional structures: they measure the plasmon properties of a modulation-doped multiple quantum well with multiple subband occupation by Raman scattering and hence determine the electronic subband structure and the subband occupation. Secondly, they report work on the phonon properties of silicon-germanium strained layer superlattices. Using a fully three-dimensional model they calculate the 3D phonon dispersion of confined and extended superlattice phonons and measure the phonon spectra using Raman spectroscopy.

Recent advances in the growth technology of semiconductor materials using thin-film epitaxial methods (notably molecular beam epitaxy) have given a range of low-dimensional semiconductors (LDS) with new optical properties which are of great interest from the viewpoints of both novel physics and optical device applications. This paper presents a brief overview of just three of the optical research projects currently under way with a range of LDS structures in the Solid States Physics Group at Imperial College.

The paper describes a number of experiments on phonon emission and scattering from a two-dimensional electron gas which have been carried out in Nottingham as part of the NUMBERS project on low-dimensional structures.

A brief account is presented of the authors' recent work on one-dimensional ballistic transport in GaAs-AlGaAs heterojunctions in 1D. Quantization of the ballistic resistance occurs, resulting in values of 2e²/hi where i is the number of 1D subbands. Application of high-resolution electron beam lithography allows the fabrication of a number of structures which exploit various manifestations of ballistic transport and quantum interference and are described here.

The authors have measured the series resistance of two ballistic resistors, or quantum point contacts, in two configurations. First when the resistors are collinear they measure the series resistance and the intermediate voltage between the resistors. Using a simple model they calculate the ballistic transport coefficient from the data. They find that the highest values are obtained when one channel is much wider than the other. Poorest transmission is always obtained when the two channels are identical. Secondly, they investigate two quantum point contacts perpendicular to each other. They find that in zero field the resistances add classically but that the transmission coefficient increases if either the magnetic field or the width of the constriction in the ballistic resistors is increased.

Hydrostatic pressure provides a method to vary controllably the electronic band structure of low-dimensional semiconductor systems. It can be used to determine absolute band-offsets and their pressure dependence. Tunnelling structures can be brought into resonance and the intrinsic loss mechanisms in lasers can be studied. The variation of impurity levels with pressure can be compared with theory and naturally occurring low-dimensional molecular systems can show a variety of effects including metallic conduction. Examples of such processes are briefly reviewed.

Large biaxial strains may be imposed on thin layers by epitaxial growth on a mismatched substrate. Such strained layers offer advantages in semiconductor devices as well as interesting new physics. Some recent work on the structural and electronic properties of (InAlGa)As strained layers on GaAs is reviewed here, together with some results on the processing of device structures.

The recent applications of low-dimensional physics to semiconductor devices are reviewed, highlighting the physics that leads to the various types of improved performance that commend low-dimensional devices to potential users. The prospects for further low-dimensional devices during the 1990s are outlined.

An expression is derived for the third-order polarization of a two-level electronic system induced by a plane polarized light pulse, including the effects not only of recombination and dephasing, but also of the rise time of the pulse. It is shown that the expression encompasses all three main electronic mechanisms that lead to an intensity-dependent refractive index, namely the blocking of interband optical transitions by free carriers (the band filling effect), virtual interband transitions, and the motion of free carriers in the non-parabolic band. It also shows explicitly the regimes in which each mechanism dominates. It is also shown (i) how the transition from the virtual interband transition regime to the band filling regime is brought about by the frequency dependence of the dephasing time, (ii) that near resonance the virtual interband transition mechanism is just a casual consequence of the optical Stark effect and (iii) that the intensity-dependent refractive index resulting from the motion of free carriers is the causal consequence of the intensity-dependent shifts in the energy levels, leading to an intensity-dependent effective mass. How the non-linear response of the semiconductor changes as the frequency of the light pulse is changed is discussed in physical terms.

Wafers of undoped GaAs have been submitted to thermal treatments in various conditions of temperature, duration, environment and cooling rate. The electronic activity of dislocations is examined by cathodoluminescence in a scanning electron microscope. The results are that short treatments at 1050 degrees C give a fairly homogeneous luminescence all over the wafers, while defect contrasts are enhanced after 850 degrees C anneals.

A chemical bath deposition technique for obtaining photoconductive grade bismuth sulphide (Bi₂S₃) thin films of thickness approximately=0.05 to 0.3 mu m is described. These films show a photo-current to dark-current ratio of 100-500 and photoconductivity of approximately=1 Omega ^-1 cm^-1 at 25 degrees C, under 1200 W m^-2 tungsten-halogen illumination. Air annealing in the 100-250 degrees C range can drastically affect the photocurrent response of these films. The dark conductivity of the films can be increased by a factor of approximately=10⁷ during such an annealing process. This suggests the possibility of post-deposition processing of the films for specific optical and optoelectronic applications.

A new technique for growing SnO₂ thin films with high surface area is described, based on tin rheotaxial growth and its thermal oxidation (RGTO). Tin thin films, when grown, present a surface characterized by spheroidal agglomerates due essentially to the surface tension of the liquid metal; these agglomerates do not seem to have any electrical continuity. By means of a thermal oxidation, both the transformation of the metal into a semiconductor and the thin film continuity are obtained, owing to the volume increase during the above-mentioned phase transformation. After this annealing cycle, SnO₂ thin films are slightly oriented in the (101) direction and present an electrical resistivity equal to about 10² Omega cm. So, the surfaces of SnO₂ films appear to be formed by spongeous agglomerates, which are in electrical contact with the nearest agglomerates. SnO₂ thin films grown by this method show a high sensitivity (defined as the relative per cent conductance variation) to H₂; in fact, sensitivity to 200 PPM H₂ in synthetic air at ambient pressure is equal to 400%. It seems to be possible to prepare other metal oxide semiconducting thin films with a high surface area using this technique.