Shallow (<0.2 μm) n+ layers in Si with high conductivity (<40 Ω/⧠) have been formed by high-dose (2×1016 cm−2) As implants. Experimental observations of As distributions and carrier concentrations are successfully simulated by a computer program which accounts for both the concentration dependent diffusion and As clustering effects. Reduction of electrical carriers in high-dose As implanted Si during moderate temperature (∼800 ° C) heat treatments is readily explained by the kinetics of As clustering. Physical limitations on the conductivity which can be achieved by thermally annealed As implants in Si are also discussed.
MoSi2 and WSi2 films produced by As-ion implantation through the respective metallic films deposited on Si substrates were analyzed by backscattering and x-ray diffraction. The backscattering results indicate that As atoms are snowplowed into Si during the formation of the silicides. Crystallographic observations on similar samples both before and after various heat treatments provide evidence for the existence at low temperatures of the hexagonal phase of WSi2, presumably unreported up to now, which is similar to the corresponding phase of MoSi2. In the case of the W disilicide, however, the temperature for the transition, hexagonal to tetragonal, is so low that the low-temperature phase is unlikely to be obtained by the usual diffusion-controlled mechanisms.
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Fluorine distribution profiles for silicon implanted with BF+2 have been measured by SIMS as a function of anneal temperature and time. Anomalous migration of fluorine is observed in samples having amorphized layers after implantation. Outdiffusion of fluorine occurs during recrystallization of the amorphous layer, and fluorine collects in regions of residual damage during annealing. This gettering of fluorine by defects illustrates the residual damage below the amorphized layer in samples implanted at room temperature is more difficult to anneal out than that in samples implanted at lower temperture (∼−110 °C).
Electrical properties of recrystallized amorphous silicon layers, formed by BF+2 implants or Si++B+ implants, have been studied by differential resistivity and Hall-effect measurements. Electrical carrier distribution profiles show that boron atoms inside the amorphized Si layers can be fully activated during recrystallization at 550 °C. The mobility is also recovered. However, the tail of the B distribution, located inside a damaged region near the original amorphous-crystalline interface, remains inactive. This inactive tail has been observed for all samples implanted with BF+2. Only in a thicker amorphous layer, formed for example by Si+ predamage implants, can the entire B profile be activated. The etch rate of amorphous silicon in HF and the effect of fluorine on the recrystallization rate are also reported.
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