n+ doping of Ge by P or As implantation and flash-lamp annealing

The increasing interest in Ge as a high mobility substrate has led to numerous investigations on shallow junction formation by ion beam processing. It has been shown that p+ doping yields junctions with the required properties whereas the formation of n+ junctions is complicated by the high diffusivity and the low solubility of the dopants.
The present work is focused on the application of flash-lamp annealing to the formation of shallow n+ layers since this method has the potential of good dopant activation while dopant diffusion is suppressed. Shallow layers with high concentrations of P or As are formed by ion implantation. Then the samples are pre-heated at relatively low temperatures (e.g. 400 °C, for 240 s) in order to achieve the regrowth of the amorphous layer formed during implantation. Subsequently, flash-lamp annealing is performed at 800 and 900 °C, for 0.84, 3 and 20 ms. RBS/C is applied to monitor the regrowth of the amorphous layer. Selected samples are studied by XTEM in order to search for precipitates and end-of-range defects. The depth distributions of P and As are measured by SIMS. The sheet resistance of the shallow n+ layer is obtained by the variable probe spacing method. Spreading resistance profiling is applied to determine the carrier depth profile. Results on phosphorus show that dopant diffusion only occurs at 900 °C for 20 ms whereas the maximum activation is in the order of 4-6x1019 cm-3.