Technological applications of novel metastable materials are frequently inhibited by abundant defects residing in these materials. Using first-principles methods we investigate the point defect thermodynamics and phase segregation in the technologically-important metastable alloy GaAsBi. Our calculations predict defect energy levels in good agreement with abundant previous experiments and clarify the defect structures giving rise to these levels. We find that vacancies in some charge states become metastable or unstable with respect to antisite formation, and this instability is a general characteristic of zincblende semiconductors with small ionicity. The dominant point defects degrading electronical and optical performances are predicted to be As$_G$$_a$, Bi$_G$$_a$, Bi$_G$$_a$+Bi$_A$$_s$, As$_G$$_a$+Bi$_A$$_s$, V$_G$$_a$ and V$_G$$_a$+Bi$_A$$_s$, of which the first-four and second-two defects are minority-electron and minority-hole traps, respectively. V$_G$$_a$ is also found to play a critical role in controlling the metastable Bi supersaturation through mediating Bi diffusion and clustering. To reduce the influences of these deleterious defects, we suggest shifting the growth away from As-rich condition and/or using hydrogen passivation to reduce the minority-carrier traps. We expect this work to aid in the applications of GaAsBi to novel electronic and optoelectronic devices, and shine a light on controlling the deleterious defects in other metastable materials.
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