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arXiv · 2412.08865

First-principles theory of direct-gap optical emission in hexagonal Ge and its enhancement via strain engineering

Christopher A. Broderick·Xie Zhang·Mark E. Turiansky·Chris G. Van de Walle
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Abstract

The emergence of hexagonal Ge (2H-Ge) as a candidate direct-gap group-IV semiconductor for Si photonics mandates rigorous understanding of its optoelectronic properties. Theoretical predictions of a "pseudo-direct" band gap, characterized by weak oscillator strength, contrast with a claimed high radiative recombination coefficient $B$ comparable to conventional (cubic) InAs. We compute $B$ in 2H-Ge from first principles and quantify its dependence on temperature, carrier density and strain. For unstrained 2H-Ge, our calculated spontaneous emission spectra corroborate that measured photoluminescence corresponds to direct-gap emission, but with $B$ being approximately three orders of magnitude lower than in InAs. We confirm a pseudo-direct- to direct-gap transition under $\sim 2$\% [0001] uniaxial tension, which can enhance $B$ by up to three orders of magnitude, making it comparable to that of InAs. Beyond quantifying strong enhancement of $B$ via strain engineering, our analysis suggests the dominance of additional, as-yet unquantified recombination mechanisms in this nascent material.

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Christopher A. Broderick, Xie Zhang, Mark E. Turiansky, Chris G. Van de Walle. 2024-12-12. First-principles theory of direct-gap optical emission in hexagonal Ge and its enhancement via strain engineering. https://arxiv.org/abs/2412.08865

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