A first-principles study of co-doping in lanthanum bromide

TL;DR

This study uses first-principles calculations to explain how co-doping LaBr3 with Sr, Ca, or Ba enhances scintillation performance by creating charge traps that reduce Auger quenching, without introducing new energy levels.

Contribution

It provides a detailed first-principles mechanism explaining how specific co-dopants improve LaBr3 scintillators, focusing on defect complexes and charge trapping effects.

Findings

Sr creates neutral complexes that trap electrons, reducing Auger quenching.

Only heavier alkaline earth metals like Sr, Ca, Ba have suitable solubilities and effects.

Alkaline elements have low solubility and can create scattering centers detrimental to mobility.

Abstract

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]