Table of contents

A series of InSb bars were doped with Sn donors by subjecting them to a thermal neutron flux. Reactor locations with different thermal/fast-neutron flux ratios were used, permitting some assessment of the type of damage emanating from fast-neutron exposure. Various figures of merit for the quality of the sample, such as the total electron concentration n, electrical mobility mu and the nuclear spin-lattice relaxation rate 1/T₁, are discussed for samples with reactor exposure times ranging up to several hours. The study attempts to quantify the advantages to be gained from sample irradiation in reactor locations with favourable thermal/fast-neutron flux ratios. The indications are that fast-neutron damage has a detrimental effect on the induced electron concentration.

An experimental system is described in which DLTS signals are analysed by fitting the transient to one or more exponentials. The quality of the measurements and the value of the technique is demonstrated by measurements of conductance DLTS on microwave GaAs MESFETS. The technique is shown to be rapid and very useful to determine the defect properties of commercial devices.

The photovoltage developed along the plane of the layers, due to illumination with a broad-band white-light source, was investigated in five different GaAs/AlGaAs MQW structures. Four of the samples were undoped, and the other was doped with silicon (n approximately 10¹⁷ cm^-3). In perfect material there should be no photovoltage, so an indication of material quality can be obtained from photovoltage measurements. Measurements of photovoltage were made either: (i) as a function of illumination intensity, with the temperature held constant, or (ii) as a function of temperature with the illumination intensity held constant. The temperature range was 1.3 to 300 K, and the illumination intensity could be varied over nine orders of magnitude. A theory of random doping fluctuations is used to explain the origin of the photovoltage in terms of n-i (or p-i) junctions. Barrier heights, in the range 20 to 200 meV, were obtained from the photovoltage data. This barrier height can be related to the compensation ratio and is, therefore, a measure of the electrical quality of the material.

The essential characteristics of NTC thermistor-type behaviour, besides the transition from a high resistance state to a permanent low resistance or memory state, have been analysed in the bulk modified glassy chalcogenide semiconductor Cu_0.05As_0.50Te_0.45. Experimental results have shown a strong decrease in electrical resistivity (which ranges from 10⁴ to 10⁵ Omega cm in AsTe glassy alloys and its value for this alloy is 7.9*10³ Omega cm at room temperature) and in the semiconductors electron mobility gap (which ranges from 0.4 to 0.5 eV for AsTe glassy semiconductors and is 0.37 eV for this alloy), due to the presence of the metallic element copper. Also, it has been shown that there are no marked non-ohmic effects in the range of voltages under study (up to 75 V for an interelectrode distance of 2.3 mm). Furthermore, the current-controlled negative differential resistance phenomenon has been justified in terms of a thermal model and confirmed by turnover voltage measurements (28.4 V for 24 degrees C and 16.9 V for 52 degrees C). Finally, several features of this glassy alloy, such as an easy synthesis, processing and low electrical resistivity values, make it appropriate for its use in the manufacture of NTC thermistors.

The -3/4 power law giving a relationship between carrier concentration and breakdown voltage, is studied by analytical procedures using a quasi-ionisation rate and connected to an equivalent multiplication region. It is found that the -3/4 power law originates from a fact that the ratio of the equivalent multiplication region width to the depletion region width is 1/8. Such a relationship was traced back to the effect of the electric field dependence of the quasi-ionisation rate in the general case. The power law is relatively independent of the material, the breakdown voltage and the carrier concentration.

The carrier recombination dynamics in MBE grown silicon films, doped with Sb or In, are investigated by the transient grating method under strong pumping conditions with 35 ps laser pulses. The temporal decay of the induced gratings gives carrier lifetimes of the order 10^-10-10^-8 s for doping concentrations in the range 10¹⁷-10²⁰ cm^-3. A complementary theoretical evaluation of the lifetimes shows the importance of the different kinds of band-to-band Auger recombination increases with increasing doping concentration for Sb-doped Si. For In-doped (10¹⁷-10¹⁸ cm^-3) Si and SRH recombination via in centres dominates. The measured lifetimes agree well with the theoretical values if it is assumed that, at the doping levels, internal stress increases the light absorption coefficient for lambda =0.53 mu m by a factor of up to 2 greater than the pure silicon value.

Previously published data of ultraviolet ellipsometric spectra of silicon-on-sapphire (SOS) is examined in terms of a silicon surface and native oxide with rough boundaries. This model is shown to be consistent with specular reflectance spectra, unlike previously proposed multilayer models. This questions the existence of amorphous silicon in the surface of SOS wafers.

The authors report the first growth of superlattices in the wurtzite CdS/CdSe system. The superlattices were grown by metal-organic chemical vapour deposition on GaAs (111) substrates. Transmission electron microscopy shows the superlattices to be of good quality with abrupt interfaces. The layers were purely hexagonal with no twinning. Low-temperature photoluminescence of the superlattices was studied for various layer thicknesses. All the structures showed a broad intense near-infrared emission which shifts to higher energy for the thinnest layers. In addition, thicker layers showed a narrower red emission at 690 nm. The photoluminescence spectra provide preliminary evidence that the structure is of type II.

It is proposed that superlattices can be used to 'engineer' certain scattering rates and hot-carrier transport properties with important implications for hot-electron devices. In particular, calculations indicate a possibility to engineer the superlattice band structure to achieve high built-in fields for higher mini-bands with zero built-in field for the lowest mini-band. By using such built-in fields it is estimated that the current gain ( beta ) for a hot-electron transistor employing a suitable superlattice for the base region can be increased by a large factor ( approximately=30) compared with conventional structures.

The authors report on a study of magnetotransport in a sheet of Si-donor impurities in MBE-grown GaAs. For densities in the 10¹² cm^-2 range both the quantum Hall effect and magnetic freeze out are shown to occur. They point out the possibility of tuning the onset of the magnetic freeze out with respect to a given magnetic filling factor by selecting different donor densities. It has been proved that the quantum Hall effect can be investigated in semiconductor materials without the need of a confining hetero-barrier.

The effects of surface pretreatment and a variety of metallisation techniques on the characteristics of Schottky diodes fabricated on Al_xIn_1-xAs grown by low-pressure and atmospheric-pressure MOCVD have been investigated. Barrier heights have been determined from I-V, C-V and I-T measurements and have been found to be in good agreement. Barrier heights of about 0.69 eV were measured for Al_0.48In_0.52As using Pt as the Schottky barrier contact.

A method for producing high-quality Schottky gates on a GaAs/AlGaAs heterojunction is described. At low temperatures the leakage current density through the gate is extremely low. This allows measurements of the quantised Hall effect as a function of the carrier density. Clear plateaus in the Hall resistance as a function of the gate voltage are observed at fixed and relatively low magnetic field.

Thin films of CuInSe₂ have been prepared from elemental stacked structures by annealing in vacuum. X-ray powder diffractometry and reflection electron diffraction show the films to be polycrystalline, possessing the characteristic chalcopyrite structure. Optical measurements indicate a direct gap material with E_g=1.01 eV. Both n- and p-type films have been prepared with resistivities in the range 1 to 10⁴ Omega cm. The potential application of this new preparation technique to large area solar cells is discussed.