We have studied the magnetic properties of self-organized ErAs aggregates embedded in a GaAs matrix using telegraph noise spectroscopy. The electrical transport through the samples is mesoscopic and exhibits universal conductance fluctuations and two-level fluctuations (TLF's). A statistical analysis of a TLF allows us to measure the extremely small magnetization of an isolated ErAs cluster. Rotating the sample in a magnetic field reveals the sixfold magnetic anisotropy expected for ErAs with a rocksalt structure. At high temperatures the switching is thermally activated, while at low temperatures it is the tunneling of the magnetization which dominates. The magnetic-field dependence of the crossover temperature between the thermally activated and tunneling regimes can be explained with a simple model.

Erbium-doped gallium arsenide has been grown by molecular beam epitaxy under varying growth conditions and analysed by secondary ion mass spectrometry. The concentration of erbium incorporated into the gallium arsenide lattice for a given effusion cell temperature has been found to vary considerably with the V : III (As : Ga) flux ratio. Higher levels of erbium incorporation occur when growth takes place close to stoichiometry rather than under arsenic-rich conditions. This behaviour has been observed for erbium concentrations between 1016 and 5 × 1019cm−3. SIMS data show the existence of an erbium-rich surface layer which, in the presence of unintentional impurities, is incorporated into the GaAs at an enhanced rate forming an unusual doping spike.

This paper reviews our recent experimental work on erbium incorporated into gallium arsenide during MBE growth. The electrical behaviour of the erbium is considered in relation to its interaction with intentionally added dopants (silicon, selenium and berylium) and the effect of the erbium on the deep state population is discussed. At higher concentrations the morphology of phase separation is considered and results are presented showing the controlled precipitation of erbium arsenide as quantum structures. The magnetic properties of these dots and wires are reported.

The growth of erbium doped GaAs by molecular beam epitaxy (MBE) can, depending on growth conditions, result in the precipitation of small spherical particles of erbium arsenide. It has been observed that by reducing the V:III (As:Ga) flux ratio to close to stoichiometry wire-like precipitates roughly aligned in the direction of growth are produced. The concentration of erbium incorporated into the GaAs lattice for a constant erbium flux is also affected by the As:Ga flux ratio with an increase in erbium doping being observed with decreasing As:Ga flux ratio. Electrical measurements have been performed on erbium doped GaAs samples co-doped with selenium, an n-type dopant. Measurements have revealed that when erbium is present, the electron concentration is reduced by an amount approximately equal to 70% of the erbium concentration. DLTS measurements have shown that although large concentrations of deep levels are present in erbium doped material, the measured concentration of these deep levels is not high enough to account for the observed reduction in electron concentration with erbium doping.

Telegraph noise, i.e., two-level fluctuations (TLF), in the magnetoresistance of Er-doped GaAs has been used to probe the magnetic moment of the small ErAs clusters formed during the molecular beam epitaxy growth process. At high temperatures the TLF are thermally activated but below 350 mK tunneling of the magnetization dominates.

Gallium arsenide doped with erbium has been grown by molecular beam epitaxy. At growth temperatures in the range 540–605 °C, and with arsenic to gallium flux ratios of 2 and more, the erbium forms uniform crystalline microprecipitates of ErAs when the concentration exceeds 7×1017 cm−3. The diameter can be varied in the range 11–21 Å by altering the growth temperature. Reducing the arsenic to gallium flux ratio to close to stoichiometry changes the growth mode to one yielding quantum wires aligned in the growth direction. Subtle changes in growth conditions lead to bifurcated structures, which we refer to as quantum trees.

MNOS (Metal-Nitride-Oxide-Silicon) memory devices commercially available today consist of transistor arrays where each device represents a memory bit. Typical devices have densities greater than 8 K bits and are generally manufactured on epitaxial based processes for isolation. The state of each bit is determined by its threshold voltage and is sensed by interpreting if the transistor is in the “off” or “on” condition. A new MNOS memory element is described where detection of junction tunnelling current is used as the sense mechanism. Substrate forms the “third” terminal and the element has the possibility of being the basis of a dense array. The technique can be developed in p or n channel and can be used as an add-on to volatile random access memories.

The effect of substrate orientation on tungsten silicide formation has been studied. It has been found that the silicide on Si(100) forms at low temperature, has fine grained surface morphology and has low sheet resistance as compared with that formed on Si(111). We attribute this to the difference in microstructure of the films, due mainly to the difference in the nucleation process for the two substrates.

Avalanche injection into a dielectric due to hot electrons produced by breakdown in a depletion region is investigated in a simplified manner. A closed form solution for the field distribution at breakdown is presented which provides an insight into the injection properties under varying physical conditions. The model presented describes the junction breakdown characteristics of a Metal-Nitride-Oxide-Silicon gated diode. This device is capable of being written and erased into various charge states thus varying the breakdown properties of the junction. This is analysed in detail where the major parameters are considered to be (i) junction “walk-out” distance (ii) depletion width (iii) applied junction bias (iv) applied gate bias.

The purpose of this paper is to design a low-pass filter by the Standard Fourier series method and to investigate analytically the effect of varying the sampling frequency for a given number of taps and cut-off frequency. It is shown that to achieve the sharpest cut-off one requires a sampling frequency as close to the Xyquist frequency as is permissible by aliasing and the filtering of higher harmonics of the transfer function.