Strain-enhanced optical gain of hexagonal Ge nanowire

The absence of an efficient light source compatible with silicon complementary metal-oxide-semiconductor technology remains a pivotal bottleneck in integrated photonics. Recently, the hexagonal diamond phase of germanium (2H-Ge) has emerged as a promising alternative in light of the direct nature of its bandgap, yet its light emission efficiency falls behind that of III–V semiconductors. Here, by performing theoretical calculations using an atomistic semi-empirical pseudopotential method, we systematically investigate the electronic structure and interband optical gain of 0001-oriented 2H-Ge nanowires (NWs). We show that quantum confinement in pure 2H-Ge NWs enables diameter-tunable bandgaps across the infrared spectrum, but retains a pseudodirect character with weak near-band-edge optical transitions at the Brillouin zone center. Interestingly, we demonstrate that a moderate uniaxial tensile strain can induce a conduction band inversion, which dramatically enhances the optical gain by over two orders of magnitude and switches the dominant polarization of the emission. We illustrate such an enhancement by correlating the gain characteristics with the energy ordering of the active conduction band states. Our results thus provide essential theoretical guidance and optimization strategies to realize high-performance, polarized light emitters based on 2H-Ge NWs for integrated photonic applications.