  While cadmium telluride (CdTe) thin films are being used in solar cell
prototyping for decades, the recent advent of two-dimensional (2D) materials
challenges the fundamental limit for thickness of conventional CdTe layers.
Here, we report our theoretical predictions on photocarrier dynamics in an
ultimately thin (about 1 nm) CdTe slab. It corresponds to a layer that is just
a single unit cell thick, when the bulk parent crystal in the zinc blende phase
is cleaved along the [110] facet. Using an \textit{ab-initio} method based on
density functional theory (DFT) and the Boltzmann equation in the relaxation
time approximation (RTA), we determine the thermalization time for charge
carriers excited to a certain energy for instance through laser irradiation.
Our calculations include contributions arising from all phonon branches in the
first Brillouin zone (BZ), thus capturing all relevant inter- and intraband
carrier transitions due to electron-phonon scattering. We find that the
photocarrier thermalization time is strongly reduced, by one order of magnitude
for holes and by three orders of magnitude for electrons, once the CdTe crystal
is thinned down from the bulk to a monolayer. Most surprisingly, the electron
thermalization time becomes independent of the electron excess energy up to
about 0.5~eV, when counted from the conduction band minimum (CBM). We relate
this peculiar behavior to the degenerate and nearly parabolic lowest conduction
band that yields a constant density of states (DOS) in the 2D limit. Our
findings may be useful for designing novel CdTe-based optoelectronic devices,
which employ nonequilibrium photoexcited carriers to improve the performance.
