Low-Temperature Selective Growth of Heavily Boron-Doped Germanium Source/Drain Layers for Advanced pMOS Devices

摘要

The peculiarities of heavily boron-doped germanium, selectively grown at lowtemperature by means of a cyclic deposition and etch chemical vapor depositionprocess, are investigated through the analysis of the structural and electricalmaterial properties. The incorporation of B in Ge can exceed 6×10~(20) cm~(-3), closeto a factor 100 above the solubility limit, without any significant degradation ofthe Ge:B crystalline quality, although high B-doping induces an unwantedcontraction of the Ge lattice. Micro-Hall effect measurements and the multiringcircular transmission line method are used to evaluate the active carrier concentrationsand resistivities of Ti/Ge:B contacts. Even though the resistivity of asgrownlayers saturates for chemical B concentrations approaching 1×10~(21) cm~(-3)and increases beyond that level, a contact resistivity below 3×10~(-9) Ω cm~2 is obtained for the highest active doping concentration, showing that a compromisemust be found to decrease the total contact resistance. Finally, first principlessimulations are used to understand dopant deactivation mechanisms in the Ge:Bsystem. In conclusion, the formation of boron-interstitial clusters is most likelythe cause for electrical performance degradation at high doping values.