Table of contents

A model is presented that describes the hot-electron transport in the electric field in zero-gap semiconductors, such as HgTe and Cd_xHg_1-xTe, at low lattice temperatures. Increase in the carrier concentrations in the process of heating is caused by impact ionisation, whereas recombination is due to both the Auger mechanism and interband transitions with emission of LO phonons. The LO-phonon scattering also serves as an efficient channel of the carrier energy relaxation. At the initial stage of heating the influence of DA interaction on the energy relaxation is also important. The momentum relaxation of the electrons is connected with scattering by the charged impurities and by the holes in the valence band. The model is based on the Boltzmann equation within the electron temperature approximation. The results of calculations agree well with the existing experimental data obtained for HgTe at the liquid helium temperature.

By reverse-bias capacitance-voltage measurements on Schottky diode samples the authors have determined the immobile-charge concentrations N⁺ at 85 K and 120 K on n-type VPE GaAs, before and after 1.0 MeV proton irradiation at the low temperature and after progressive heating to 300 K, so as to study the proton-irradiation-induced defects that anneal at low temperatures in this semiconductor and to obtain information on their carrier-trapping energy levels. They find that the irradiation-induced decrease in the N⁺ value is considerably removed, by a total of about 40%, in three defect-annealing stages, A, B and C, centred at approximately 180, 235 and 280 K respectively. The annealing kinetics of stages B and C are consistent with those of the annealing stages I and II found by Thommen (1970) in studies of resistivity changes in electron-irradiated bulk n-GaAs. In contrast, they report stage A as a new annealing stage at a temperature lower than any previously known for defects in gallium arsenide. By consideration of the sensitivity of the N⁺ value at low measurement temperature to the natures and energy levels of electron trapping defects they deduce that the three stages A, B and C are all predominantly due to the removal of acceptor levels that lie below about E_c-0.2 eV. They deduce also that the stage C levels are close to or below the middle of the energy-band gap. They suggest that the stage A, B and C defects may include defects produced by the displacement of gallium atoms.

For a proper description of growth by metal organic vapour phase epitaxy the three-dimensional Navier-Stokes partial differential equations need to be solved which govern the following series of processes: (i) transport by diffusion and flow through the gas phase, (ii) reactions which take place in this gas phase, (iii) reactions which take place at the surface. The authors are at first interested in the medium- and higher-temperature regions, which cover the growth determined by diffusion through the gas phase (medium temperature) and the growth that is determined by the desorption of growth species (higher temperature). Using a number of well justified assumptions one can reduce the problem to a two-dimensional one. For the diffusion-limited region (i.e. medium-temperature region) the effect of different flow profiles (plug flow, parabolic flow, linear increasing velocity and combination of plug and linear profile) on the growth rate has been studied under isothermal conditions. It was found that all profiles yield the same growth rate within a few per cent, so that it suffices to use the simple plug flow profile in growth rate calculations. It is also shown that axial diffusion is an important effect only at the end of long reactors. Finally a model is derived in which surface reaction kinetics is combined with the diffusion-limited model for the isothermal case.

For pt.I see ibid., vol.5, p.16 (1990). Analytical descriptions are given of the growth rate in metal organic vapour phase epitaxy processes in the medium- and higher-temperature regions, where growth is controlled by diffusional transport through the gas phase and desorption of growth species, respectively. In continuation of part I, where only isothermal problems were considered, the temperature-dependent hydrodynamic behaviour is now investigated. A temperature gradient is introduced and the effect of thermal diffusion on the growth rate is studied, which is significant: a much ( approximately 40%) lower depletion is calculated if thermal diffusion is included. Furthermore the influence of surface reactions at high and low temperatures is investigated, resulting in a model in which the CVD number plays an important role. This model allows for the calculation of growth rates in both the diffusion-limited as well as the high-temperature region, where desorption dominates.

The operation of a new type of thermally cycled gas sensor is examined. Taguchi-type tin oxide semiconductor gas sensors are mounted within a small chamber, along with a platinum ribbon, and access to the outside atmosphere is restricted to a narrow opening. During the thermal cycling of either the sensor heater voltage or the platinum ribbon current, gas will 'breathe' in and out of the chamber. The outside atmosphere contains the reducing gas to be tested mixed in normal air. When cycling the sensor temperature, gas will enter as the tin oxide cools, and adsorb onto the surface. When the tin oxide is reheated the adsorbed gas will quickly oxidise, producing a sharp rise in conductance. Increasing the rate of bulk gas consumption in the enclosed chamber, by increasing the constant current supplied to the platinum ribbon, the authors are able to examine this process in more detail. In other experiments the sensor heater is kept at a constant voltage and the gas is drawn in by cycling the platinum ribbon current. In this case low-temperature surface adsorption does not take place and direct comparison of gas consumption rates can be made. They examine the response of the TGS Hash 812 and the TGS Hash 813 to ethanol, propane and carbon monoxide gas.

The authors report on a detailed photoluminescence study of a series of GaAs/AlAs type II quantum well structures. In these structures the thickness of the GaAs layers was kept constant at 25 AA and the thickness of the AlAs layers was varied between 28 and 114 AA. As the AlAs layer thickness was increased they observed a large change in the relative strengths of the zero-phonon and phonon-assisted type II exciton recombinations. They have interpreted this phenomenon as being due to a change in the nature of the lowest lying AlAs electron state from X_xy (k perpendicular to (001)) to X_z (k parallel to (001)) as the AlAs thickness was decreased, where (001) is the growth direction. This changeover is due to the competing effects of electron confinement and strain produced by the small but finite lattice mismatch between GaAs and AlAs. They have estimated the strain splitting of the different X states to be 23 meV. Calculations using this figure along with an envelope function treatment of the effects of electron confinement predict that the lowest confined X_z and X_xy states should cross at an AlAs thickness of the order of 60 AA. For the samples with the thickest AlAs layers (94 and 114 AA) they were able to resolve two zero-phonon lines which they attribute to the different X states, the measured splittings are in good agreement with the calculations.

The heights of CoSi₂/- and TiSi₂/n-GaAs Schottky barriers are determined, by various methods, to be 0.77 and 0.64 eV respectively. These values are discussed in view of the recently developed Schottky barrier theories such as the advanced unified defect model and the effective workfunction model. The thermal stability of these contacts is investigated and it is found that the CoSi₂/n-GaAs barrier height remains reasonably stable up to 600 degrees C for 10 min anneals and up to 750 degrees C for 1 min anneals. Under oxygen-free conditions and/or for shorter anneal times, this limit value may probably be increased. Consequently, CoSi₂ may be an interesting contact material for high-temperature applications such as self-aligned GaAs MESFET fabrication.

The mechanism behind the formation of alloyed ohmic contacts to p-type InP doped in the 10¹⁶ cm^-3 range has been investigated. Techniques used for characterising the contacts include the scanning electron microscope (SEM), specific contact resistance measurements, X-ray diffractometry (XRD), energy dispersive X-ray analysis (EDAX) and Auger electron spectroscopy (AES). Both temperature and time of annealing were varied during the study. The results obtained using these annealing procedures indicate that the optimum contact resistance is associated with the formation of an Au-In phase. The atomic percentage of indium increases with increasing annealing temperature whilst that of gold drops. Zinc phases appear only at the higher annealing temperatures used.

The authors calculate the subband states in n-inversion layers on p-Hg_1-xCd_xTe at finite temperature using a numerical self-consistent procedure based on the six band k.p model for the band structure near the Gamma point. Results from the calculations are presented for different values of the physical parameters, and the applicability limits of the model are discussed.

The stability of polycrystalline silicon thin film transistors formed at low temperatures has been studied. Charge trapping in the deposited gate oxide is observed over a wide range of conditions. At high gate biases this is due to the passage of a tunnel current, and at lower biases to direct tunnelling into the traps. Trap concentrations, cross sections and energy levels have been deduced based upon a simple trapping model, and an empirical device lifetime model is presented.

EPR measurements have been made of defects produced by implanting (100) silicon wafers with 200 keV C⁺ ions in the dose range (0.75-2.29)*10¹⁸ C⁺ cm^-2 at an implantation temperature between 510 and 700 degrees C. A single isotropic nearly Lorentzian line is observed. The g value drops from 2.0037(2) to 2.0029(2) and the spin concentration increases from about (1.2 to 5.7)*10¹⁵ cm^-2 as the dose is increased from (0.75 to 2.29)*10¹⁸ C⁺ cm^-2. It is suggested that the signal arises from Si and C dangling bonds but that, at least for a dose >or=1.4*10¹⁸ C⁺ cm^-2, the C ones predominate. The results of annealing the samples are also reported.

Recent Raman scattering studies of Ga_0.47In_0.53As-InP quantum wells have shown that the form of the alloy optical phonons can be quite different from that observed in the bulk due to coupling between the phonons and the two-dimensional electron gas (2DEG). However, the limited range of samples studied so far has made the elucidation of the precise coupling mechanism difficult. The authors report here a Raman scattering study of the GaInAs optical phonons in a wide range of undoped GaInAs-InP quantum wells with well widths in the range 50-300 AA. The frequencies and intensities of the two LO phonons of the alloy are studied as a function of both well width and photoexcited carrier density and a resonant coupling between the electrons and the GaAs-like LO phonon is observed when the lowest electronic inter-subband spacing approaches the phonon energy. These results are discussed in terms of the coupling between phonons and intersubband collective excitations.

A detailed study of direct Auger processes in 1.3 mu m InGaAsP/InP quantum wells and quantum well wires is presented. CHCC, CHLH, and CHSH Auger processes are considered, as are transitions involving confined states only, and transitions in which the excited carrier is in an unbound state of the well, with the other carriers confined. The effects of using realistic confined state wavevectors and the inclusion of evanescent tails for the confined states are examined. The approximations of Boltzmann statistics and parabolic subbands are examined, and it is found that these assumptions do not unduly affect the results (in contrast to the bulk case). A comparison of numerical results with present experimental estimates of Auger coefficients is given.

A novel method for measuring the carrier recombination time of a semiconductor which requires only a single semiconductor laser source is described.