Models of triplet self-trapped excitons in SiO2, HfO2, and HfSiO4

We predict by means of density functional theory simulations the structures of self-trapped triplet excitons in HfO2 and HfSiO4, and compare their properties to those of the self-trapped exciton in alpha quartz. The character of the hole and electron localization in excitons strongly depends on the geometrical structure and dielectric properties of these materials. In HfO2, the electron is localized predominantly on one Hf atom while the hole is localized on one or two oxygen atoms at the nearest or next-nearest-neighbor sites, depending on the crystal phase. We predict two exciton configurations in HfSiO4 with the excited electron localized either on a Hf or on a Si atom and the hole localized on the nearest-neighbor oxygen atoms. Excitation and luminescence energies are calculated for all triplet exciton states and compared with the available experimental data.