  We report on photoluminescence in the 3-7 $\mu$m mid-wave infrared (MWIR)
range from sub-100 nm strained thin films of rocksalt PbSe(001) grown on
GaAs(001) substrates by molecular beam epitaxy. These bare films, grown
epitaxially at temperatures below 400 {\deg}C, luminesce brightly at room
temperature and have minority carrier lifetimes as long as 172 ns. The
relatively long lifetimes in PbSe thin films are achievable despite threading
dislocation densities exceeding $10^9$ $cm^{-2}$ arising from island growth on
the nearly 8% lattice- and crystal-structure-mismatched GaAs substrate. Using
quasi-continuous-wave and time-resolved photoluminescence, we show
Shockley-Read-Hall recombination is slow in our high dislocation density PbSe
films at room temperature, a hallmark of defect tolerance. Power-dependent
photoluminescence and high injection excess carrier lifetimes at room
temperature suggest that degenerate Auger recombination limits the efficiency
of our films, though the Auger recombination rates are significantly lower than
equivalent, III-V bulk materials and even a bit slower than expectations for
bulk PbSe. Consequently, the combined effects of defect tolerance and low Auger
recombination rates yield an estimated peak internal quantum efficiency of
roughly 30% at room temperature, unparalleled in the MWIR for a severely
lattice-mismatched thin film. We anticipate substantial opportunities for
improving performance by optimizing crystal growth as well as understanding
Auger processes in thin films. These results highlight the unique opportunity
to harness the unusual chemical bonding in PbSe and related IV-VI
semiconductors for heterogeneously integrated mid-infrared light sources
constrained by tight thermal budgets in new device designs.
