  Si hyperdoped with chalcogens (S, Se, Te) is well-known to possess unique
properties such as an insulator-to-metal transition and a room-temperature
sub-bandgap absorption. These properties are expected to be sensitive to a
post-synthesis thermal annealing, since hyperdoped Si is a thermodynamically
metastable material. Thermal stability of the as-fabricated hyperdoped Si is of
great importance for the device fabrication process involving
temperature-dependent steps like ohmic contact formation. Here, we report on
the thermal stability of the as-fabricated Te-hyperdoped Si subjected to
isochronal furnace anneals from 250 {\deg}C to 1200 {\deg}C. We demonstrate
that Te-hyperdoped Si exhibits thermal stability up to 400 {\deg}C with a
duration of 10 minutes that even helps to further improve the crystalline
quality, the electrical activation of Te dopants and the room-temperature
sub-band gap absorption. At higher temperatures, however, Te atoms are found to
move out from the substitutional sites with a migration energy of EM =
2.1+/-0.1 eV forming inactive clusters and precipitates that impair the
structural, electrical and optical properties. These results provide further
insight into the underlying physical state transformation of Te dopants in a
metastable compositional regime caused by post-synthesis thermal annealing as
well as pave the way for the fabrication of advanced hyperdoped Si-based
devices.
