  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.
