T.J. Bullough

An atomistic calculation of extended planar defects in ionic crystals Application to stacking faults in the alkali halides

Abstract A method for calculating the energy and structure of extended planar defects in ionic crystals is described. The shell model is used and all the lattice sums are carried out over planes, periodic in two dimensions. The crystal block structure necessary for calculating defects on charged planes is described. The method is applied to a study of stacking faults on the (100), (110) and (111) planes of the lithium, sodium, potassium and rubidium halides with the rock-salt structure. Results are obtained that are qualitatively similar to those obtained earlier by Fontaine (1968). The (110) fault is particularly interesting since it is speculated that dislocations dissociate on this plane. Our calculated values are substantially higher than those reported by Fontaine for this fault, making dissociation less likely, but any calculation for an infinite stacking fault cannot rule out the possibility.

CBE growth of GaAs/GaAs, GaAs/Si and AlGaAs/GaAs using TEG, AsH3 and amine-alane precursors

The growth of high quality AlGaAs by CBE bas been limited by the high levels of carbon and oxygen contamination. The use of alane based precursors offers a significant reduction in such contamination. We report for the first time the CBE growth of AlxGa1-xAs from triethylgallium, dimethylethylamine-alane and arsine, and compare with. growth from triethylgallium, trimethylamine-alane and arsine. Some preliminary results of work on the CBE growth of GaAs on silicon will also be reported.

Nanometre hole formation in MgO using electron beams

Abstract Square cross-sectional holes with widths of about 1 nm can be formed in MgO crystals using circular cross-section high-current-density electron probes which are focused and oriented close to . High-resolution scanning transmission electron microscopy has been used to reveal, on a unit cell scale, the three-dimensional geometry of the hole structures as they grow. The holes develop from the electron exit face of the crystal, have a profile which is clearly related to that of the electron probe, and can be generated with incident electron energies as low as 40keV. Unlike other inorganic materials, there appears to be no lower current density threshold for hole formation and there is no excess Mg or 0 present in the MgO crystal after hole formation, suggesting that the MgO is desorbed as molecules or clusters. A model for the hole formation process is proposed.

S-shaped behaviour of the temperature-dependent energy band gap in dilute nitrides

We have investigated the temperature dependence of the band gap energy in GaInNAs, GaNAs and InGaAs quantum wells. In the structures containing nitrogen the well-known S-shaped characteristic was observed at low temperatures. We explain this anomalous temperature behaviour by strong carrier localization in potential fluctuations at low temperatures. In the nitrogen free samples, there was no S-shaped behaviour and the empirical Varshni dependence was followed.

Monitoring real-time CBE growth of GaAs and AlGaAs using dynamic optical reflectivity

Abstract Dynamic optical reflectivity (DOR) uses the interference oscillations arising from the multiple reflections, of a normally incident CW laser beam, between the surface of a growing film and the film-substrate interface. The oscillations have a period determined by the refractive index of the film and the laser wavelength. DOR measurements have been made, in real time, during the CBE growth of Al x Ga 1− x As layers on a GaAs(100) substrate. The results show that the growth rate and the aluminum composition x can be monitored.

Compositional variation in as-grown GaInNAs/GaAs quantum well structures

Abstract The variation of elemental composition in as-grown GaInNAs/GaAs quantum well structures has been investigated by energy dispersive X-ray analysis of cross-sections using a high resolution scanning transmission electron microscope. The formation of quaternary GaInNAs dot structures is indicated by low temperature photoluminescence measurements and by the correlation of indium and nitrogen distributions. The distributions of arsenic and nitrogen across the well structure suggest the presence of a continuous nitride-like layer formed at the surface of the GaAs buffer layer before the GaInNAs dots. The influence of this nitride-like interlayer on the mechanism of GaInNAs dot formation is discussed.

A simulation study of the initial phase of he precipitation in metals

Abstract The mutation of a helium filled vacancy by emitting interstitials is studied with computer simulation for the case of αFe and Mo. For Mo a comparative study was made with three sets of potentials. In all cases there is a tendency for the emitted interstitial to remain bound to the mutated complex. Mutation proceeds at lower helium fillings if the metal potential used simulates a less rigid lattice; in that case the calculated results sare also more in line with thermal helium desorption experiments.

Sputtering and the formation of nanometre voids and holes in aluminium in a scanning transmission electron microscope

Abstract Nanometre voids and holes have been produced in aluminium films up to 220nm thick by the stationary focused 100 keV high-current-density electron probe in a dedicated scanning transmission electron microscope. The electron energy is below the threshold for bulk displacements in aluminium but is sufficient to cause sputtering of aluminium atoms from the electron-exit surface. The sputtering leads to the formation of a pit with a diameter determined by the electron probe size at the electron-exit surface. As the pit aspect ratio increases, atoms are sputtered from the pit base onto the pit side walls where they experience a much reduced electron intensity, rather than being sputtered directly out of the pit. Eventually the pit seals to leave a void, separated from the end of the small pit that remains at the electron-exit surface. By repeatedly interrupting the irradiation so as to image a projection of the irradiated volume, it is shown that the void then moves from near the electron-exit surface ...

Beam equivalent pressure measurements in chemical beam epitaxy

Abstract We have made beam equivalent pressure measurements for a range of gaseous sources in a CBE system. For group III precursors introduced through a gas line with a given conductance, the beam pressure was found to be independent of the metalorganic species involved. These measurements lead us to question the validity of “corrected” BEP data in CBE.

XeCl excimer laser assisted CBE growth of GaAs

Abstract The effect of 12 ns, 308 nm (XeCl) excimer laser pulses on the CBE growth rate of GaAs, at temperatures below the maximum non-laser assisted growth rate, G max , has been studied as a function of laser fluence and repetition frequency. There is a threshold fluence for growth rate enhancement, above which the growth rate is dependent on repetition frequency, being restored to G max at 20 Hz. The growth rate in the laser spot is measured by dynamic optical reflectivity (DOR).