Table of contents

This review paper presents the applications of X-ray diffraction to routine measurements and the procedures for extending its capabilities of analysis to undertake a detailed structural investigation of low-dimensional structures. The uses and limitations of the familiar double-crystal diffractometer are discussed as are the advantages of 'reciprocal space mapping' with a multiple-crystal diffractometer. In general X-ray diffraction has been used for composition and thickness measurement in low-dimensional structures (LDS) and these aspects are covered, as well as the avoidance of the pitfalls associated with their determination. The possibilities for the use of X-ray diffraction methods to determine interface quality, the evolution of lattice relaxation, the detailed microstructure, etc, are discussed, with an indication of the limits of the techniques.

A theoretical description of chloride vapour-phase epitaxy (CVPE) has been proposed which contains two-dimensional (2D) gas-dynamic equations for transport of reactive components and kinetic equations for surface growth processes connected by nonlinear adiabatic boundary conditions. No one of these stages is supposed to be the limiting one. Calculated variations of growth rate and impurity concentrations along the growing layer fit experimental data well.

We report a comprehensive description of direction-dependent impact ionization threshold/anti-threshold calculations in GaAs and Al_0.3Ga_0.7As using a realistic pseudopotential band structure model. Both electron- and hole-initiated processes are studied. In the case of GaAs rate calculations are presented for some electron- and hole-initiated processes, while for Al_0.3Ga_0.7As results of rate calculations for some electron-initiated processes are given.

In this paper we give exact wavefunctions in the envelope-function approximation for a model system consisting of a symmetric two-band semiconductor in a uniform electric field. The method of solution proceeds by analogy with the relativistic electron (Dirac equation). The exact solutions, which are Weber functions, are characterized with respect to their limiting forms for narrow bandgaps, wide bandgaps and low fields. Results of this analysis include re-derivation of the interband tunnelling probability and a corrected plane-wave approximation for the wavefunctions. A simple expression for the current density is derived from the effective Hamiltonian and its eigenfunctions. The heterostructure transport problem is formulated, leading to an analytical expression for the transmission through an arbitrary piecewise-linear potential. As an application of the results, the quantum-confined Stark effect is computed and compared with that obtained in the one-band model. The interband coupling further lowers the conduction subbands as the field is increased.

We have studied unintentionally and deliberately Si- or Mg-doped indirect-gap Al_xGa_1-xAs (0.38<or=x<or=0.835) prepared by a variety of growth techniques, by means of photoluminescence in the temperature range 4.2 K<or=T<or=300 K. At 4.2 K two high-energy transitions were determined to be due to the radiative decay of donor and acceptor bound excitons. The zero-phonon bound exciton transitions are accompanied by phonon replicas due to the emission of one or two AlAs- or GaAs-like phonons. The peak energy of the indirect donor bound exciton increases in the range 0<or=x<or=1 corresponding to the relation (T<or=30 K) h nu _c(D⁰X,x)=1.9690+0.1975x+0.0517x^7.4 (in eV). The Al content was determined by electron beam microanalysis using different standards. The donor bound exciton binding energy was estimated using the intensity quenching versus the temperature. Including these values the indirect gap-composition relation is given. The direct-indirect crossover point was determined in excellent agreement with earlier estimations. Besides the radiative decay of bound excitons free exciton recombination was observed at elevated temperatures.

Detailed studies of the absorption kinetics of the DX centre are presented. These studies were performed on unique thick AlGaAs:Te layers and clearly demonstrated the participation of the neutral charge state of DX as an intermediate state in the photoionization process. The neutral state of the DX observed here is the localized relaxed energy state similar to the ground state of the DX centre.

We investigate the electrical properties of up to 12 GaAs/AlAs superlattices doped uniformly or selectively with silicon. We determine for each sample the apparent energy of the DX silicon donor by performing Hall-effect measurements. We find this energy depends on the discrete values of the energy assigned to the deep donor states present in GaAs, AlAs and interface layers respectively. Interdiffusion of silicon in the adjacent layers occurred in selectively doped samples. This interdiffusion effect appears to be responsible for shoulders in the thermal annealing of persistent photoconductivity. We determine the energies of both DX states with an all-Ga and all-Al environment of the silicon donor. They differ by more than 55 meV. Finally we give Hall and photo-Hall data under hydrostatic pressure which we interpret by taking into account the crossing of the Gamma conduction miniband over the X pseudo-direct miniband of the superlattice with applied pressure.

Carrier concentration and mobility were investigated by Hall measurements over a wide temperature range (7-700 K) in nominally undoped p-GaSb grown from the melt. The main parameters relating to levels controlling the conductivity and to scattering mechanisms limiting the mobility were derived by a self-consistent fit of carrier density and mobility versus temperature. In the temperature range 20-500 K an approach using two acceptor levels successfully explained the temperature variation of the hole density. A negligible influence of non-polar optical phonons resulted from the analysis of the high temperature mobility, whereas an impurity band conduction dominates the electric transport at low temperature. Photoluminescence spectra were measured in the interval 2-300 K to confirm the energy location of the main localized level: by considering a correction factor for the compensation effect, complete agreement between optical and thermal ionization energies was found.

The semianalytic trajectory method for hot carrier transport is combined with a new model for impact ionization to study breakdown in ultra-thin pin diodes where the dead space and spatial transient can dominate the ionization physics. Good agreement with the published data of Gaul et al (1991) is achieved.

The Schottky barrier height of Cr-Si and CrSi₂-Si junctions for both nand p-type samples has been measured by using I-V, C-V and activation energy techniques in a temperature range of 170-300 K. In most cases samples showed near ideal I-V characteristics. The transition from Cr-Si to CrSi₂-Si upon annealing at 440 degrees C was observed from the variation of the barrier height. The barrier height and the ideality factor were found to be temperature independent for p-type samples. The deviations from the ideality in n-type samples are discussed in terms of the current mechanisms that might contribute to the total measured value.

We present the results of a computational study to investigate the effects of interface disorder on the no-phonon optical transitions in ultrathin Si_m-Ge_n superlattices. The calculations were performed using local empirical pseudopotentials with a supercell containing 8000 atoms. We find that randomly ordered alloy layers at the interfaces between the Si and Ge layers degrade the strength of the direct transition to the zone-folded conduction states by up to two orders of magnitude. The perturbation in the planes parallel to the interfaces also produces allowed transitions of similar strength to states derived from the in-plane conduction minima. A reconstruction of the chemically driven ordering observed by Jesson et al (1992) indicates that the degradation is not so severe as for randomly ordered interfaces.

A systematic comparison of quantum well infrared photodetectors (QWIPS) made by different molecular beam epitaxy (MBE) systems is presented. QWIPS with identical layer specifications were requested from three different MBE facilities. Measurements of dark current, detector responsivities, infrared absorption and detectivities were carried out. Although good uniformity between detectors was obtained within the same MBE wafer, variations were observed between samples with the same specifications from different MBE systems. The results of this study illustrate the present accuracy of MBE technology.

By applying some post-pore-forming treatments, we have achieved solid state electroluminescence of an Au/PS/Si structure. Stable visible light emission is observed under forward bias with a threshold voltage and current density down to 6 V and 30 mA cm^-2.

Chemical vapour deposition of silicon from silane is commonly accepted to proceed by a Langmuir-Hinshelwood adsorption mechanism followed by a chemical decomposition reaction. The silicon growth rate, as known, is decreased by the addition of hydrogen to the reaction gas. Axial distribution of silicon growth rate in a hot-wall reactor tube is discussed in the case of successive increases in hydrogen input partial pressure, and it is shown that a satisfying mathematical solution of the problem may be obtained when a Freundlich isotherm in its square-rooted form is chosen for silane adsorption instead of the Langmuir isotherm.

The effect of thermal processing on salicided (self-aligned TiSi₂) CMOS structures with phosphorus-doped gates has been investigated in the temperature range of 800-1000 degrees C. The salicided layer in the submicrometre structures reported in this paper was formed by rapid thermal processing of the titanium film on doped substrates in a nitrogen ambient. The interface contact resistance as a function of temperature (i.e. TiSi₂/Si contact resistance) increases for both n-(As) and p-(BF₂) implanted junctions, the increase is greater for the case of p (BF₂) junctions. The sheet resistances measured on Van der Pauw patterns (large area) are affected by annealing temperature, time and dopant species, in addition to the substrate microstructure. The BF₂-implanted phosphorus-doped polysilicon shows the maximum increase in sheet resistance on Van der Pauw patterns with thermal processing, as compared with any other case. The narrower (1.0 mu m) runners of polysilicon show greater increase in sheet resistances on thermal processing than do large-area Van der Pauw patterns.

In order to delay the occurrence of the 2D-3D growth mode transition in highly strained In_xGa_1-xAs epitaxial layers grown on GaAs (001) by molecular beam epitaxy, the mass transport at the surface should be reduced. This can be achieved by increasing the growth rate. It is demonstrated in this letter that this is an efficient way to increase the critical thickness for which the 3D growth appears.