Low temperature dopant activation of BF2 implanted silicon

Boron activation and carrier mobility were measured after low temperature furnace heat treatments, in silicon layers implanted with BF2+ ions at 60 keV and at fluence in the 1 - 5 x 10(15) ions cm-2 range. These quantities were correlated with boron and fluorine chemical depth profiles obtained with secondary ion mass spectrometry (SIMS), and with the lattice defects revealed by transmission electron microscopy (TEM). High dopant activation, well above the extrapolated boron solid solubility, was found for all the fluences investigated after a thermal treatment of 20 min at 600-degrees-C. In the high fluence implanted samples, the solid phase epitaxial regrowth of the amorphous layer induces a severe fluorine redistribution which causes the formation of a defective band at the sample surface containing microtwins and small precipitates; a decrease in both the activated dopant concentration and carrier mobility was found in this region. The comparison with dopant activation data obtained in samples diffused at higher temperature (from 900 to 1000-degrees-C) shows that twins are electrically active only when they are decorated by isolated impurities and/or in presence of very small precipitates.