Table of contents

The paper describes measurements of the velocity of electrons at electric fields up to 9 kV cm^-1 in GaAs and in GaAs/AlGaAs heterostructures. In order to avoid charge and field domain formation, this experiment uses a sinusoidally varying electric field with frequency of 35 GHz. The results of these measurements indicate that negative differential mobility is present in not only the undoped GaAs, but also in the heavily doped GaAs and in the selectively doped heterostructures. These results imply that such a measurement is necessary to correctly determine the velocity-field characteristics of electrons in this electric field range. The experimental results further indicate that in the case of the selectively doped heterostructures, the electrons have peak velocities lower than those found in undoped bulk GaAs and that the peak occurs at a lower electric field. At room temperature, this effect is primarily due both to the effective lowering of the Gamma -L energy difference which results in a more efficient k-space transfer of electrons and to real-space transfer of electrons into the AlGaAs. At 77 K, these mechanisms are still active; additionally, the two-dimensionality of the electrons results in an enhanced scattering of the electrons with the polar optical phonons. In the case of doped GaAs, because of the significantly increased ionised impurity scattering, the peak velocity is also lower than that found in undoped GaAs, but the peak occurs at higher electric fields.

Using time-resolved photoluminescence the authors have studied vertical transport in a series of superlattices with a large included 1 mu m away from the sample surface. With a streak camera, photoluminescence (PL) decays for excitation densities as low as a few 10¹³ cm^-3 photocarriers per pulse can be recorded. At such low densities, the electrons are the minority carriers in residual p-type MBE materials. They can move without being slowed down by the holes. By varying the excitation density one is able to observe the transition from an electron transport regime to an ambipolar one. The use of a one-dimensional diffusion model for the photocreated carriers and the excitons enables one to obtain the diffusion coefficients of electrons, holes and excitons in these structures at different temperatures.

The damage of a Si substrate processed in a 30 eV Ar magnetron discharge has been revealed, for the first time, from the non-destructive measurement of carrier bulk lifetime, carrier diffusion constant and surface recombination velocity by the photoinduced modulation absorption technique. It was found that slight damage occurring at substrate enhances the surface recombination velocity only little, while severe damage in the bulk much reduces the carrier diffusion. However, no significant change of the carrier bulk lifetime due to bulk damage was observed.

The dimensions of the hot wall deposition system (HWD) have been optimised for the growth of good crystalline quality Zn₃P₂ thin films. The growth conditions have been standardised for the optimal length to radius ratio of the system for film deposition on mica and glass substrates. The crystallinity of the films grown at various substrate temperatures ranging from 180 degrees C to 350 degrees C were checked using X-ray analysis. The electronic transport properties were studied as a function of growth conditions. The growth temperature was shown to influence strongly the electrical conductivity. The HWD system was suitably modified to incorporate the desired amount of dopant during growth. The resistivity was brought down to 10² Omega cm by silver doping. The transport properties of the doped films were studied. Schottky barriers were formed on doped Zn₃P₂ films grown on silver-coated glass substrates using Mg, Al and In. I-V and C-V measurements on these metal contacts were also carried out.

The growth kinetics of titanium silicide obtained from the reaction of titanium layers on silicon has been investigated on heating either by conventional thermal annealing or by rapid thermal annealing. Provided sufficient care has been taken to avoid oxygen contamination during titanium deposition and during prolonged annealing, the kinetics results are essentially the same for both treatments. This indicates that no acceleration of the process is induced by RTA. However, RTA has the effect of reducing the influence of oxygen and revealing even on contaminated samples, the real kinetics which would otherwise be masked by the presence of oxygen.

The defect parameters of substitutional silver in germanium have been investigated with DLTS. Silver, when diffused into germanium introduces four occupancy levels in the band gap. Three of them are already known from Hall experiments: the acceptor levels with ionisation enthalpies E_v+0.116 eV, E_c-0.261 eV and E_c-0.113 eV. The fourth level, a very deep donor situated at E_v+0.035 eV, has never been reported before. This new level implies that silver is the third known amphoteric impurity in germanium, besides gold and cobalt.

Zn₂In₂S₅(II) single crystals were synthesised by vapour transport reaction and characterised by means of X-ray diffraction and electron microscopy. Transmission spectra were used for the evaluation of the energy gap in the temperature range 10-340 K. The electrical behaviour of the material was examined. Measurements of the n-type electrical conductivity as a function of the reciprocal temperature and an I(U) characteristic led to the determination of trap parameters. Four energy levels were resolved in the range 0.29-1.0 eV below the conduction band, with concentrations varying from 10⁹ to 10²⁰ cm^-3. The electrical anisotropy was also investigated, and was found to be thermally activated.

Thermal oxide layers, of thickness in range 2-25 nm, were grown on n- and p-type GaP in dry oxygen. The dependence of the Mott-Schottky intercept with the V axis on the oxide layer thickness and the substrate doping concentration obeyed some specific laws, which led the authors to conclude that the space charge in the semiconductor depletion region governed the Mott-Schottky behaviour and that interfacial fixed charges and homogeneously distributed charges in the oxide layer were less important. To reconcile these experiments with the dependence of the Schottky barrier height, as obtained from J-V measurements, on the oxide layer thickness, a model based on the existence of pinholes in the oxide layer was developed as an alternative to former models relying on surface states.

n-GaAs of high peak mobility ( mu =250000 cm² V^-1 s^-1) has been grown in a molecular beam epitaxy (MBE) system with modified source-furnace geometry. High-resolution far-infrared magnetospectroscopy has been performed on this material. The photoconductive response at 2-4 K reveals a rich spectrum of excited state transitions. These are identified from theoretical models, and transitions from the ground impurity state up to the N=8 Landau level are noted. For the first time in MBE material, transitions involving D^- states are observed.

Picosecond photoconductivity in iron- and oxygen-doped InP has been measured using an original experimental arrangement. It is supposed that the carrier recombination in this material is caused by trapping at the donor-acceptor complexes.

The magneto-optical spectrum of the shallow donors in n-GaAs is studied using a far-infrared laser at photon energies greater than the binding energy of the donors. The transitions from the (1s) ground state into the Landau levels are not observed but, instead, series of transitions from the ground state into donor states associated with the individual Landau levels. The strongest series corresponds to the (000)-(0N0) transitions and the second strongest series to (000-0N2) transitions where N is the index of the Landau level involved (and where the high-field notation is employed). The corresponding low-field notation is 1s-2p₊, 1s-3d₊₂, 1s-4f₊₃, 1s-5g₊₄ etc. for the strongest series which can be followed up to the N=17 Landau level. Central cell structure due to the presence of several different donors can be observed on many of the lines associated with the N=1 and N=2 Landau levels. The 1s-3p₊ transition can be used to determine chemical shifts accurately for the donor species present, as this line avoids the overabsorption effects which can distort the stronger 1s-2p₊ line. A few weak lines remain unidentified.

The chemical preparation of GaSb(001) substrates was performed by Br₂-HCl-HNO₃-CH₃COOH solution. The removed layer thickness was evaluated as a function of the constituent concentrations and the etching time. The surface quality was controlled by SEM, RHEED and AES. With this treatment, excellent surface morphology of substrates and growth film is obtained.

The authors consider the resonant absorption of infrared radiation by a sharply defined layer of donor atoms in GaAs. It is shown that in all essential respects this density-tunable resonance is comparable with the signal obtained on quantum well heterostructures. This suggests that it is possible to create a detector structure without a heterojunction interface.

A publication by Spink and Thomas (see ibid., vol.3, p.1123 (1988)) re-analysing the authors' ellipsometric spectra of silicon-on-sapphire wafers with varying degrees of haze is shown to contain inaccuracies and to misrepresent the results. The angle of incidence variation of the ellipsometric parameters is shown to be consistent with their previously published dielectric models based on effective medium modelling, and cannot be explained by the scattering theory proposed by the re-analysis. Reflectance, scattering and ellipsometry are shown to be influenced by different scales of roughness and to contain complementary information.

The temperature dependence of the free-electron concentration for a weakly doped semiconductor is derived for a model assuming acceptor-like, D^- charge state and negative U character of the DX centre. A single activation energy is obtained, independent of background acceptor concentration. Experimental data for Al_xGa_1-xAs can be well reproduced within this model.

The authors have grown (Al,Ga)As/GaAs two-dimensional electron gas (2DEG) structures with lightly doped regions of (Al,Ga)As and superlattices in the undoped GaAs. Using this technique, they have obtained ultralow density (2*10¹⁰ cm^-2), high-mobility samples where phonon scattering at 4 K is the dominant factor in limiting the mobility; reducing the measurement temperature to below 1.5 K gives mobilities of up to 10⁷ cm² V^-1 s^-1.

A simple model for high-field transport and trapping of electrons in silicon dioxide is presented. The trapping and detrapping of electrons is represented by Auger recombination and the reverse transition, impact ionisation. The trapping rate as well as the average energy of ballistic electrons are calculated as functions of the electric field in the oxide by means of a relaxation-time approximation to the Boltzmann transport equation.

The authors report the first direct measurements of the conduction band electron effective mass in MBE grown Al_0.48In_0.52As on InP by the technique of optically detected cyclotron resonance (ODCR). The effective mass value derived is m*=0.10 m₀+or-0.01 m₀. A value for the carrier momentum relaxation time is also deduced, indicating a lattice-limited mobility for this material of the order of 10⁵ cm² V^-1 s^-1.

Donor bound exciton luminescence has been detected in silicon grown by molecule beam epitaxy at 550 degrees C and 750 degrees C with an intensity comparable with that observed in silicon grown from the melt with a similar electrically active donor concentration. Therefore, the strong suppression of luminescence from electrically active impurities reported in previous investigations is not an inherent property of MBE silicon grown at low temperatures and must be associated with the presence of other contaminants. The major inadvertent electrically active dopant is seen to be phosphorus in the material investigated. The relative strength of the bound exciton luminescence from the epitaxial layers and that arising in the substrate can be varied by heat treatment and deliberate contamination by transition metals. The latter treatment is also seen to result in the appearance of dislocation D-band luminescence which is not observed in the as-grown material, implying that D-band luminescence is associated with decorated as distinct from undecorated dislocations.

CU_xS thin films, deposited from chemical baths on glass substrates and air-annealed at 150 degrees C and then given a polymer protective coating, have been found to offer optical transmission and reflection characteristics near to ideal solar control characteristics. The transmission spectra in the visible region resembles the phototopic vision spectra, with an integrated transmittance of approximately=30%-providing a greenish-yellow illumination of the interior under daylight. The low integrated visible reflectance of approximately=9% prevents glare and the mild bluish-purple colour in reflected daylight improves cosmetic appearance. Due to the low sheet resistance of the films approximately=30 Omega Square Operator ^-1, the integrated infrared transmittance is very low, approximately=10% and the integrated infrared reflectance is acceptably high, 25%. The total optical and thermal transmittance into the interior through the Cu_xS coated glazing is in the 30-35% range, assuring substantial saving in the cooling costs when these glazings are employed in regions with a generally warm climate. Various possibilities in the commercial production of the coatings are suggested.