Metastability and anharmonicity enhance defect-assisted nonradiative recombination in low-symmetry semiconductors

Strong nonradiative recombination has been observed in quasi-one-dimensional antimony selenide, which runs counter to the simple intuition that claims high defect tolerance exists in semiconductors with antibonding state in the valence band and bonding state in the conduction band. Here we reveal such a defect intolerance actually stems from the richness of structural metastability and vibrational anharmonicity owing to the low-symmetry atomic structure. Taking the deep defect V$_{\rm Se}$ as a benchmark, we show the defect with its ground-state configuration alone does not act as a recombination center. Instead, we identify three different configurations with different formation energies, such richness of metastability offers a higher probability to accomplish a rapid recombination cycle. Another contributing factor is the anharmonicity in the potential energy surfaces that is caused by the large atomic relaxation, which elevates the total capture coefficient by 2-3 orders of magnitude compared with harmonic approximation. Therefore, the unique properties from both crystals and phonons in quasi-one-dimensional system enhance the nonradiative recombination, making the traditional intuition of defect tolerance invalid. These results highlight the importance of the correct identification of metastable defects and phonon anharmonicity in the nonradiative recombination in low-symmetry semiconductors.