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The relationship between implantation damage and electrical activation has been investigated in GaAs implanted with 100-keV 30Si+ to doses of 5×1013/cm2 and 2×1014/cm2, using low and moderate beam currents at room temperature (RT) and at slightly elevated temperatures. For a given Si+ dose, the damage, measured by ion channeling immediately after implantation, was varied by more than a factor of 2 over the range of conditions studied. A strong negative correlation was established between this damage and the electrical activation obtained after high-temperature annealing, i.e., an increase in the initial damage led to a decrease in the sheet-carrier concentration. The results demonstrate a scheme for increasing the sheet-carrier concentration beyond that typically obtained for high-dose Si in GaAs, namely, by using a slightly elevated implant temperature (∼90 °C for a beam current of 1 μA/cm2). In addition, the initial damage is shown to consist of two components; one that is stable at RT and another that is not. The sheet-carrier concentration was found to be affected only by that component of the damage that is stable at RT, although both components were reduced by increasing the implant temperature or by reducing the dose rate.

Submonolayer phase transitions of parahydrogen and orthodeuterium on graphite have been investigated with positronium annihilation. It is shown that in contrast with conventional specific-heat measurements, quantitative assessments of gas in the mixed commensurate solid plus gas phases are possible, which will lead to a redrawing of the phase diagrams.The study of physisorbed monolayers on smooth graphite surfaces is now a mature field. The adsorption of a range of gases, from hydrogen to xenon, has been well investigated with a variety of methods, including isotherms, specific heats, neutron scattering, low-energy electron diffraction, x-ray diffraction, and others. The nature of the twodimensional ͑2D͒ phase transitions and the role of the underlying carbon substrate have been especially interesting. 1 H 2 and D 2 monolayers on graphite allow one to study ordering phenomena and phase transitions in quantum systems. In both cases at low coverages, a commensurate ()ϫ)) R30°phase forms due to the corrugation of the carbon surface potential, and at coverages near monolayer completion an incommensurate triangular 2D solid phase is created. Specific-heat measurements on submonolayers have allowed phase diagrams to be drawn for H 2 ͑Refs. 2 and 3͒ and D 2 ͑Ref. 4͒ that describe the transitions from 2D gas to commensurate solid plus gas. These measurements, however, based on broad peaks, are not very informative. They do indicate differences between H 2 and D 2 , which have been attributed to differing zero point motions of the quantum particles, but they cannot give quantitative descriptions of the mixed gas/solid phases.In this paper we report on measurements with a new method employing positronium ͑Ps͒ formation, which offers the prospect of detailed examination of these phase transitions.Positronium offers a unique means of studying the behavior of two-dimensional systems. It is well known that positrons injected into metals quickly thermalize by electronic and phononic excitations and may then diffuse to a surface to cause the emission of Ps by capturing a surface electron. 5 In the special case of graphite, Sferlazzo et al. 6 with a positron beam experiment observed that under normal conditions at room temperature little emission occurs. However, their 2-␥ angular correlation study revealed that large amounts of Ps were emitted at high temperatures. The lack at low temperatures was attributed to the band structure of graphite, 7 which showed that parallel momentum conservation would forbid surface electrons at the top of filled bands participating in Ps formation, whereas at high temperatures the abundance could be explained with a mechanism in which momentum conservation was satisfied by the emission and absorption of phonons. 6 The basal plane of graphite offers an ideally smooth substrate for the thermodynamic study of condensed gas mono-layers: their creation and their phase transitions. 8 The exfoliated form of graphite, grafoil, is a leaflike structure ͑of typical dimension, 100 nm͒ 8 with basal plane surfa...

A high repetition rate dye laser utilizing transverse flow and linear, close-coupled flashlamps has been built. Measurements and performance figures including average power, bandwidth, beam divergence, and solvent mixtures will be discussed.