Optical properties of highly n-doped germanium obtained by in situ doping and laser annealing.

High n-type doping in germanium is essential for many electronic and optoelectronic applications especially for high performance Ohmic contacts, lasing and mid-infrared plasmonics. We report on the combination of in situ doping and excimer laser annealing to improve the activation of phosphorous in germanium. An activated n-doping concentration of 8.8  ×  10¹⁹ cm⁻³ has been achieved starting from an incorporated phosphorous concentration of 1.1  ×  10²⁰ cm⁻³. Infrared reflectivity data fitted with a multi-layer Drude model indicate good uniformity over a 350 nm thick layer. Photoluminescence demonstrates clear bandgap narrowing and an increased ratio of direct to indirect bandgap emission confirming the high doping densities achieved. [ABSTRACT FROM AUTHOR]