Strain-enhanced radiative recombination in lonsdaleite Ge

Growth of conventional semiconductor materials in metastable crystal phases presents rich new opportunities to engineer electronic band structure. Experimental demonstration of direct-gap-like optical emission from lonsdaleite Ge (2H-Ge) is attracting significant attention as a route to a direct-gap material for Si photonics applications [1]. The 2H-Ge direct band gap originates via back-folding of the L-point conduction band minimum of conventional, indirect-gap diamond-structured Ge (3C-Ge). This “pseudo-direct” band gap has theoretically predicted low oscillator strength [2], in stark contrast to a high experimentally inferred radiative recombination coefficient B comparable to that conventional 3C-InAs [1]. We report first-principles calculations of the B coefficient for 2H-Ge. We find that B in pristine 2H-Ge is at least three orders of magnitude lower than in 3C-InAs. To explain this discrepancy, we explore the impact of strain that could be unintentionally present in 2H-Ge nanowires. We show that [0001] uniaxial tensile stress can drive significant enhancement of B by inducing a pseudo-direct- to direct-gap transition. In the context of experiment, our computed radiative lifetimes suggest as-yet unquantified nonradiative processes that dominate carrier recombination in unstrained 2H-Ge.

[1] E. M. T. Fadaly et al., Nature 580, 205 (2020)

[2] C. Rödl et al., Phys. Rev. Materials 3, 034602 (2019)

This work is supported by the European Commission and by the US Department of Energy.