  Defects play an important role in tailoring the optoelectronic properties of
materials. Here we demonstrate that sulphur vacancies are able to engineer
sub-band photoresponse into the short-wave infrared range due to formation of
in-gap states in Bi2S3 single crystals supported by density functional (DF)
calculations. Sulfurization and subsequent refill of the vacancies results in
faster response but limits the spectral range to the near infrared as
determined by the bandgap of Bi2S3. A facile chemical treatment is then
explored to accelerate the speed of sulphur deficient (SD)-based detectors on
the order of 10 ms without sacrificing its spectral coverage into the infrared,
while holding a high D* close to 10^15 Jones in the visible-near infrared range
and 10^12 Jones at 1.6 um. This work also provides new insights into the role
sulphur vacancies play on the electronic structure and, as a result, into
sub-bandgap photoresponse enabling ultrasensitive, fast and broadband
photodetectors.
