Influence of space charge on the conductivity of nanocrystalline SrTiO3

Abstract

A grand canonical multiscale space-charge model has been developed to study and predict the electrical properties of polycrystalline perovskites with complex defect chemistries. This model combines accurate data from hybrid exchange-correlation functional density functional theory calculations (defect formation energies, resultant grand canonical calculations of defect concentrations, and ionization states) with finite-element simulation of the electric field and its coupling to defect redistribution and reionization throughout the grain. This model was used to simulate the evolution of the oxygen partial pressure-dependent conductivity of polycrystalline acceptor-doped strontium titanate as the grain size decreases, and the results were compared to previous experiments. These results demonstrate that as the grain size is reduced from the microscale to nanoscale, the experimentally observed disappearance of ionic conductivity and forward shift of the oxygen partial pressure of the n–p crossover are successfully reproduced and explained by the model. Mechanistically, the changes to conductivity stem from the charge transfer from the grain boundary core into the grain interior, forming a space-charge layer near the grain boundary core that perturbs the local defect chemistry. The impact of the grain size on the electrical conductivity and the underlying defect chemistry across the grain are discussed. In addition to the findings herein, the model itself enables exploration of the electrical response of polycrystalline semiconductor systems with complex defect chemistries, which is critical to the design of future electronic components.

Understanding Alkali Contamination in Colloidal Nanomaterials to Unlock Grain Boundary Impurity Engineering

“…In oxide polycrystals, the influence of the space charge due to metal excess at the interfaces is well-studied. The dangling bonds of the excess metal in oxides will lead to bond electrons at the interface and will fill the electronic states near the conduction band, which lowers the material intrinsic band gap. − GBE successfully allowed control over the mechanical and chemical properties of bulk metallic alloys ( e.g ., steel and Mg alloy) and inorganic compounds ( e.g. , CIGS).…”

Room temperature deposition of freestanding BaTiO3 films: temperature-induced irreversible structural and chemical relaxation

“…For this reason, the observed macroscopic properties are likely related to the defect rich grain boundaries that are created from fracture surfaces during the deposition process. At elevated temperatures, it is expected that both defect reorganization and oxygen reincorporation at the grain boundaries are possible, in addition to the formation of a grain boundary space-charge layer, [57] although the relative contributions remain unclear.…”

“…On the other hand, due to the AD process itself, where fractured particle surfaces are randomly recombined and consolidated in a rapid, dynamic process, cationic and anionic vacancies along the grain boundary are expected [73]. Here, a redistribution of defects at the grain boundary or a reincorporation of oxygen from the surrounding atmosphere can also result in a significant decrease in the apparent unit cell volume [57,74]. Such significant chemical strains are well-known in oxides.…”

“…Through the deposition process, it is understood that both cationic and anionic defects are formed at the grain boundaries due to dynamic fracture and random reorganization of newly created nanoparticles. This is expected to lead to three primary types of changes in the defect concentration during annealing: (i) Local reorganization of defects at the grain boundary through diffusion processes, (ii) Reincorporation of oxygen to fill oxygen vacancies, and (iii) Electron transfer creating a space charge layer [57]. Other mechanisms, such as recrystallization, grain boundary rotation, and densification/grain growth, are also expected to influence the defect concentrations, although their individual contributions remain unclear and require additional more detailed local microscopy investigations.…”

“…Importantly, this increase in remanent thermal expansion was found to become larger with an increase in oxygen vacancy concentration through doping. Considering the potential occurrence of a space charge effect, creating an electron-rich barrier layer at the grain boundaries, [57] it was shown by Watanabe et al [100] that the presence of space charges along the grain boundary effectively blocks oxygen diffusion into the grains. Therefore, the space charge effect must be either negligible, which may be attributable to the grain size of AD films reducing the space charge effect in comparable size regions, [57] or there is a different conduction path, such as lowdensity amorphous regions as reported by Cao et al…”