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

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]Comment: 13 pages, 10 figures, accepted for publication in the Physical Review

Electronic stopping and proton dynamics in InP, GaP, and In0.5_{0.5}Ga0.5_{0.5}P from first principles

The phosphide-based III-V semiconductors InP, GaP, and In$_{0.5}$Ga$_{0.5}$P are promising materials for solar panels in outer space and radioisotope batteries, for which lifetime is a major issue. In order to understand high radiation tolerance of these materials and improve it further, it is necessary to describe the early stages of radiation damage on fast time and short length scales. In particular, the influence of atomic ordering, as observed e.g. in In$_{0.5}$Ga$_{0.5}$P, on electronic stopping is unknown.We use real-time time-dependent density functional theory and the adiabatic local density approximation to simulate electronic stopping of protons in InP, GaP, and the CuAu-I ordered phase of In$_{0.5}$Ga$_{0.5}$P across a large kinetic energy range.These results are compared to SRIM and we investigate the dependence on the channel of the projectile through the target.We show that stopping can be enhanced or reduced in In$_{0.5}$Ga$_{0.5}$P and explain this using the electron-density distribution. By comparing Ehrenfest and Born-Oppenheimer molecular dynamics, we illustrate the intricate dynamics of a proton on a channeling trajectory

Exciting Imperfection: Real-structure effects in magnesium-, cadmium-, and zinc-oxide

Computational condensed-matter physics, as a branch of modern solid-state physics, comprises the field of ab-initio calculations. Nowadays, challenging parameter-free studies can be carried out that deal with the many-body aspects due to the involved electron-electron interaction. Theoretical-spectroscopy techniques provide insight into electronic excitations, paving the way towards computer-aided materials design, e.g., for photovoltaics. In this thesis the transparent conductive oxides MgO, ZnO, and CdO are investigated; they are important materials for transparent-oxide electronics. Initially, a hybrid functional is used to model exchange and correlation. Subsequently, quasiparticle energies are calculated using Hedin's GW approximation of the electronic self energy. This leads to band structures, densities of states, spin-orbit splittings, effective band masses, and natural band discontinuities. Solving a Bethe-Salpeter equation for the optical polarization function yields the complex dielectric function including excitonic and local-field effects. All underlying atomic geometries result from density-functional theory based on local or semi-local approximations to exchange and correlation. Furthermore, this thesis deals with imperfections that affect the electronic and optical properties. The influence of uniaxial and biaxial strain on the band structure of ZnO is explored. Iso- and heterostructural alloys are investigated, taking the sample-preparation conditions into account via a cluster-expansion method. Exciton binding energies for the defect-related optical-absorption peaks are calculated for the oxygen vacancy in MgO. Effects due to free electrons in heavily doped ZnO are incorporated into the description of the optical properties. This allows to study the Mahan exciton as well as the inter-conduction-band absorption in this system and explains why a Mott transition of the exciton is hard to observe. The agreement with experimental results is reassuring

Effect of dynamical screening in the Bethe-Salpeter framework: Excitons in crystalline naphthalene

Solving the Bethe-Salpeter equation (BSE) for the optical polarization functions is a first principles means to model optical properties of materials including excitonic effects. One almost ubiquitously used approximation neglects the frequency dependence of the screened electron-hole interaction. This is commonly justified by the large difference in magnitude of electronic plasma frequency and exciton binding energy. We incorporated dynamical effects into the screening of the electron-hole interaction in the BSE using two different approximations as well as exact diagonalization of the exciton Hamiltonian. We compare these approaches for a naphthalene organic crystal, for which the difference between exciton binding energy and plasma frequency is only about a factor of ten. Our results show that in this case, corrections due to dynamical screening are about 15\,\% of the exciton binding energy. We analyze the effect of screening dynamics on optical absorption across the visible spectral range and use our data to establish an \emph{effective} screening model as a computationally efficient approach to approximate dynamical effects in complex materials in the future.Comment: 11 pages main text, 5 figures main text, 9 pages supplemental, 6 figures supplementa