  The family of three-dimensional topological insulators opens new avenues to
discover novel photophysics and to develop novel types of photodetectors. ZrTe5
has been shown to be a Dirac semimetal possessing unique topological electronic
and optical properties. Here we present spatially-resolved photocurrent
measurements on devices made of nanoplatelets of ZrTe5, demonstrating the
photothermoelectric origin of the photoresponse. Due to the high electrical
conductivity and good Seebeck coefficient, we obtain noise-equivalent powers as
low as 42 pW/Hz1/2 at room temperature for visible light illumination at zero
bias. We also show that these devices suffer from significant ambient
reactivity such as the formation of a Te-rich surface region driven by Zr
oxidation, as well as severe reactions with the metal contacts. This reactivity
results in significant stresses in the devices, leading to unusual geometries
that are useful for gaining insight into the photocurrent mechanisms. Our
results indicate that both the large photothermoelectric response and
reactivity must be considered when designing or interpreting photocurrent
measurements in these systems.
