$\mathrm{Ge}$ with a quasi-direct band gap can be realized by strain engineering, alloying with $\mathrm{Sn}$, or ultrahigh n-type doping. In this work, we use all three approaches together to fabricate direct-band-gap $\mathrm{Ge}\text{−}\mathrm{Sn}$ alloys. The heavily doped n-type $\mathrm{Ge}\text{−}\mathrm{Sn}$ is realized with CMOS-compatible nonequilibrium material processing. $\mathrm{P}$ is used to form highly doped n-type $\mathrm{Ge}\text{−}\mathrm{Sn}$ layers and to modify the lattice parameter of $\mathrm{P}$-doped $\mathrm{Ge}\text{−}\mathrm{Sn}$ alloys. The strain engineering in heavily-$\mathrm{P}$-doped $\mathrm{Ge}\text{−}\mathrm{Sn}$ films is confirmed by x-ray diffraction and micro Raman spectroscopy. The change of the band gap in $\mathrm{P}$-doped $\mathrm{Ge}\text{−}\mathrm{Sn}$ alloy as a function of $\mathrm{P}$ concentration is theoretically predicted by density functional theory and experimentally verified by near-infrared spectroscopic ellipsometry. According to the shift of the absorption edge, it is shown that for an electron concentration greater than 1 × 10 cm the band-gap renormalization is partially compensated by the Burstein-Moss effect. These results indicate that $\mathrm{Ge}$-based materials have high potential for use in near-infrared optoelectronic devices, fully compatible with CMOS technology.

• Received 6 July 2018
• Revised 11 October 2018

DOI:https://doi.org/10.1103/PhysRevApplied.10.064055