Band-Gap and Band-Edge Engineering of Multicomponent Garnet Scintillators from First Principles

Abstract

Complex doping schemes in RE3Al5O12 (RE=rare earth element) garnet compounds have recently led to pronounced improvements in scintillator performance. Specifically, by admixing lutetium and yttrium aluminate garnets with gallium and gadolinium, the band-gap was altered in a manner that facilitated the removal of deleterious electron trapping associated with cation antisite defects. Here, we expand upon this initial work to systematically investigate the effect of substitutional admixing on the energy levels of band edges. Density functional theory was used to survey potential admixing candidates that modify either the conduction band minimum (CBM) or valence band maximum (VBM). We considered two sets of compositions based on Lu3B5O12 where B = Al, Ga, In, As, and Sb; and RE3Al5O12, where RE = Lu, Gd, Dy, and Er. We found that admixing with various RE cations does not appreciably effect the band gap or band edges. In contrast, substituting Al with cations of dissimilar ionic radii has a profound impact on the band structure. We further show that certain dopants can be used to selectively modify only the CBM or the VBM. Specifically, Ga and In decrease the band gap by lowering the CBM, while As and Sb decrease the band gap by raising the VBM. These results demonstrate a powerful approach to quickly screen the impact of dopants on the electronic structure of scintillator compounds, identifying those dopants which alter the band edges in very specific ways to eliminate both electron and hole traps responsible for performance limitations. This approach should be broadly applicable for the optimization of electronic and optical performance for a wide range of compounds by tuning the VBM and CBM.

Chemical manipulation of hydrogen induced high p-type and n-type conductivity in Ga2O3

“…Instead of averaging the potential to perform the alignment correction, we simply shifted the density of states such that the deepest state in the material aligned across different structures, which has been shown to give similar corrections. 26 In any case, the magnitude of this correction was no greater than 0.1 eV. A VGa was created by removing a tetrahedrally-coordinated Ga ion from the cell, as this vacancy structure has been identified as being more favorable.…”

Excitation Transfer Engineering in Ce‐Doped Oxide Crystalline Scintillators by Codoping with Alkali‐Earth Ions

“…Codoping of Ce‐doped gadolinium gallium aluminum garnet Gd 3 Al 2 Ga 3 O 12 (GAGG) single crystals with the divalent cation Mg 2+ is highly promising for applications of this scintillator in the new generation of PET (positron emission tomography) scanners . This scintillator is a product of purposeful engineering of the band gap and the energy position of the activator levels in the gap . The crystal exhibits a high light yield of up to ≈70 000 phot/MeV, has luminescence decay time shorter than 100 ns, and its emission band peaks at ≈520 nm which perfectly matches the sensitivity spectrum of conventional Silicon Photomultipliers (SiPMs).…”

Garnet Scintillators of Superior Timing Characteristics: Material, Engineering by Liquid Phase Epitaxy

“…Most recent members of the garnet scintillators family are (Lu,Y,Gd) 3 (Ga,Al) 5 O 12 :Ce multicomponent garnets . Negative influence of electron traps is diminished in them by adopting the so‐called “band‐gap engineering strategy,” which allows the downward shift of conduction band bottom and Ce 3+ 5d 1 excited state by optimizing the matrix composition.…”

“…Negative influence of electron traps is diminished in them by adopting the so‐called “band‐gap engineering strategy,” which allows the downward shift of conduction band bottom and Ce 3+ 5d 1 excited state by optimizing the matrix composition. Consequently, influence of electron traps and Ce 3+ excited state ionization were minimized …”