FLUENCE DEPENDENT CONCENTRATION OF LOW-ENERGY GA IMPLANTED IN SI

Using an ion beam produced in a liquid-metal ion source, 7 keV Ga was implanted in silicon with fluences ranging from 2 X 10(13) to 1 X 10(17) cm-2. These implantation profiles were analyzed quantitatively by secondary-ion mass spectrometry. The derived Ga concentrations were compared with a theoretical retention model which assumes a Gaussian implant distribution and accounts for the concurrent specimen erosion by sputtering. Furthermore, dynamic computer simulations employing the binary collision approximation were carried out to model the implantation process. All three data sets yield a linear increase of the Ga concentration with fluence to about 1 x 10(16) cm-2 and, beyond a fluence of 3 x 10(16) cm-2, a saturation value. In this regime the experimentally determined Ga peak concentration is lower by a factor of 3 than the corresponding values obtained from the model and the simulations. This observation indicates that Ga segregates to the surface for very high fluences.