Origin of defect intolerance in low-symmetry semiconductors

Defect tolerance has been usually observed in semiconductors with an empirical rule which shows (i) antibonding states in the valence band maximum (VBM) and (ii) bonding states in the conduction band minimum (CBM). However, the defects in some novel low-symmetry semiconductors do not follow this rule. For instance, experiments revealed the defects in low-symmetry semiconductor antimony selenide are intolerant, i.e., various deep levels and short carrier lifetime have been measured, although the Sb 5s-Se 4p antibonding and Sb 5p-Se 4p bonding states exist in the band edges. Through first-principles electronic structure calculations, we attribute this phenomenon into three progressive aspects: (i) inequivalent atomic sites complicates the defect transition levels; (ii) "defect-correlation" behavior promotes the formation of deep-level defects; (iii) structural metastability and phonon anharmonicity enhance the nonradiative recombination. These results suggest the defect tolerance in low-symmetry semiconductors should be reassessed.