Electrical Activation of Indium Dopants in Germanium and Silicon-Germanium Alloys*

Electrical deactivation of indium dopants at high doping levels in germanium and silicon-germanium alloys is a major obstacle to using germanium to create high-performance semiconductor devices. We used X-ray absorption spectroscopy (EXAFS and XANES) to determine the local structure changes responsible for the decrease in the electrical activation of indium dopants observed in these semiconductors as doping levels increase. We then performed ab initio theoretical calculations (DFT and MD) to further study indium-vacancy complexes and indium clusters at a range of doping levels. With carbon co-doping, we were able to triple the electrically active fraction of indium at concentrations of up to ~0.7 at%.  Further experimental and theoretical analyses show that this significant improvement results from carbon-indium pair formation, relieving indium-induced strain in the germanium lattice and reducing the precipitation of indium.