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(1 7) cm-2. These implantation profiles were analyzed quantitatively by s econdary-ion mass spectrometry. The derived Ga concentrations were com pared with a theoretical retention model which assumes a Gaussian impl ant distribution and accounts for the concurrent specimen erosion by s puttering. Furthermore, dynamic computer simulations employing the bin ary collision approximation were carried out to model the implantation process. All three data sets yield a linear increase of the Ga concen tration 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 experimentall y 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.