The evolution and annealing of pores in, and the crystallization of, vapor-deposited films of amorphous solid water have been studied by using variable-energy positron annihilation spectroscopy for temperatures in the range 50-150 K. Both positron and positronium annihilation provide insight to the nature of the grown-in pores and their evolution with temperature. Crystallization of the films was observed at just below 140 K, in agreement with earlier studies, with the topmost 80 nm undergoing a transition consistent with crystallization at 90-100 K.
Erbium-doped silicon-rich silicon oxide films deposited by plasma enhanced chemical vapor deposition suffer from compressive stress as deposited, which converts to a large tensile stress on annealing due to the release of hydrogen. Although the cracking that results from this stress can be avoided by patterning the films into ridges, significant stress remains along the ridge axis. Measurements of erbium photoluminescence sensitized by silicon nanoclusters in stressed and relaxed films suggest an important role for internal film stresses in promoting the phase separation of excess silicon into nanoclusters, which has previously been thought of as a thermally driven process.
The concentration of vacancy-type defects in a silicon-on-insulator substrate consisting of a 110nm silicon overlayer and 200nm buried oxide has been quantified using Variable Energy Positron Annihilation Spectroscopy following 300keV1.5x10 15 cm -2 Si + ion implantation and subsequent annealing at temperatures ranging from 300-700 o C. The preferential creation of vacancies (relative to interstitials) in the silicon overlayer leads to a net vacancy-type defect concentration after annealing.Assuming that the defects have a structure close to that of the divacancy we determine the concentration to range from 1.7x10 19 -5x10 18 cm -3 for annealing temperatures of 300 -700 o C. The measured concentration is in excellent agreement with that predicted via Monte-Carlo simulation. The impact of this net vacancy population on the diffusion and activation of phosphorus introduced by a 2keV implantation to a dose of 1x10 15 cm -2 has been observed. For samples which combine both Si + and P + implantation, post-implantation phosphorus diffusion is markedly decreased relative to that for P + implantation only. Further, a four-fold increase in the electrical activation of phosphorus after post-implantation annealing at 750 o C is observed when both implantation of Si + and P + is performed. We ascribe this affect to the reduction of phosphorus -interstitial clusters by the excess vacancy concentration beyond the amorphous/crystalline interface created by the P + implantation.
The development stages of a system for variable-energy spin-polarised positron beam spectroscopy are described. Methods for achieving maximum practical beam polarisation include a Na-22 source configuration with a low-Z backing to reduce positron backscattering, and positioning a 250μm beryllium foil in front of the source to absorb the slower and decelerate the faster beta positrons, the latter suffering significantly less depolarisation during moderation than the former. To switch the direction of spin polarisation of electrons in the target sample the direction of the positron guiding field was reversed, and strong rare earth magnets were placed behind the sample. Systematic problems associated with low beam intensity and electronic drifts are discussed. Results for single-crystal iron compare well with those expected from theory, but suggest a beam/target polarisation of ~ 5% of that expected. Prospects for future beam development and application are outlined.
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