Theory of electron transport in direct-gap Ge_1-xSn_x group-IV alloys

Incorporation of Sn in Ge to form the Ge_1-xSn_x alloy has been predicted and experimentally confirmed to drive an indirect- to direct-gap transition [1]. This signals significant potential for applications in electronic and photonic devices compatible with a Si platform, stimulating significant ongoing effort to develop post-CMOS devices [2]. We combine atomistic electronic structure calculations with explicit numerical solution of the Boltzmann transport equation (BTE) to quantify the evolution of electron transport in semiconducting Ge_1-xSn_x. Using alloy supercell calculations we evaluate intra- and inter-valley alloy scattering rates, demonstrating that Sn-induced hybridization of the direct (Γ_6c) and indirect (L_6c) conduction band edge states of Ge mediates strong Sn-induced inter-valley scattering. At zero applied electric field, we predict a strong increase in electron mobility as the alloy becomes direct-gap for Sn composition x > 8%. Solution of the BTE in the presence of a driving electric field demonstrates increased sensitivity of the mobility to applied electric field in the direct-gap regime. Finally, we summarize atomistic calculations of the Sn composition dependent alloy band-to-band tunneling rate and its implications for applications in tunneling field-effect transistors.

[1] O. Moutanabbir et al., Appl. Phys. Lett. 118, 110502 (2021)

[2] J. Doherty et al., Chem. Mater. 32, 4383 (2020)

This work is supported by Science Foundation Ireland and by the European Commission.