  Defects play important roles in semiconductors for optoelectronic
applications. Common intuition is that defects with shallow levels act as
carrier providers and defects with deep levels are carrier killers. Here,
taking the Cu defects in CdTe as an example, we show that shallow defects can
play both roles. Using first-principles calculation methods combined with
thermodynamic simulations, we study the dialectic effects of Cu-related defects
on hole density and lifetime in bulk CdTe over a wide range of Cu incorporation
conditions. Because CuCd can form a relatively shallow acceptor in CdTe, we
find that increased Cu incorporation into CdTe indeed can help achieve high
hole density; however, too much Cu can cause significant non-radiative
recombination. We discuss two strategies to balance the contradictory effects
of Cu defects based on the calculated impact of Cd chemical potential, copper
defect concentrations, and incorporation temperature on lifetime and hole
density. The results indicates that to optimize the Cu doping in CdTe, it is
important to control the total amount of Cu incorporated into CdTe and optimize
the match between the Cu incorporation temperature and the Cd chemical
potential. These findings can help understand the roles of Cu defects in CdTe
and the potential complex defect behaviors of relatively shallow defect states
in semiconductors for optoelectronic applications.
