  Co-doping of Ce-doped LaBr$_3$ with Ba, Ca, or Sr improves the energy
resolution that can be achieved by radiation detectors based on these
materials. Here, we present a mechanism that rationalizes of this enhancement
that on the basis of first principles electronic structure calculations and
point defect thermodynamics. It is shown that incorporation of Sr creates
neutral $V_\text{Br}$-Sr$_\text{La}$ complexes that can temporarily trap
electrons. As a result, Auger quenching of free carriers is reduced, allowing
for a more linear, albeit slower, scintillation light yield response.
Experimental Stokes shifts can be related to different
Ce$_\text{La}$-Sr$_\text{La}$-$V_\text{Br}$ triple complex configurations.
Co-doping with other alkaline as well as alkaline earth metals is considered as
well. Alkaline elements are found to have extremely small solubilities on the
order of 0.1 ppm and below at 1000 K. Among the alkaline earth metals the
lighter dopant atoms prefer interstitial-like positions and create strong
scattering centers, which has a detrimental impact on carrier mobilities. Only
the heavier alkaline earth elements combine matching ionic radii with
sufficiently high solubilities. This provides a rationale for the experimental
finding that improved scintillator performance is exclusively achieved using
Sr, Ca, or Ba. The present mechanism demonstrates that co-doping of wide gap
materials can provide an efficient means for managing charge carrier
populations under out-of-equilibrium conditions. In the present case dopants
are introduced that manipulate not only the concentrations but the electronic
properties of intrinsic defects without introducing additional gap levels. This
leads to the availability of shallow electron traps that can temporarily
localize charge carriers, effectively deactivating carrier-carrier
recombination channels. The principles of this ... [continued]
