Efficient In Situ Doping of Strained Germanium Tin Epilayers at Unusually Low Temperature

Efficient p- and n-type in situ doping of compressively strained germanium tin (Ge_1-xSn_x) semiconductor epilayers, grown by chemical vapor deposition on a standard Si(001) substrate, is demonstrated. Materials characterization results reveal unusual impact of dopants manifesting via a pronounced reduction of Sn content in the epilayer, accompanied by an enhancement of the growth rate, due to increasing p-type doping concentration. Furthermore, the opposite behavior for n-type doping is observed, resulting in a less pronounced increase of Sn concentration and no effect on growth rate. Nevertheless, a very high density of electrically active holes up to ≈4 × 10²⁰ cm⁻³ is obtained in p-type doped Ge_1-xSn_x epilayer resulting in the lowest resistivity of 0.15 mΩ cm among all in situ doped epitaxial and strained group-IV semiconductors. Also, the metal-to-insulator transition in Ge_1-xSn_x is experimentally demonstrated for doping levels above 1 × 10¹⁷ cm⁻³, which is substantially lower than in any group-IV semiconductor, and theoretically predict it to be as low as ≈1 × 10¹⁷ cm⁻³. The findings enabled by the doping regime explored in this work can open novel prospects to engineer low resistivity contacts and charge current injection in applications covering next-generation transistors, qubits, diodes, electrically driven light sources, sensors and hybrid quantum devices.