The ability to tailor the performance of functional materials, such as semiconductors, via careful manipulation of defects has led to extraordinary advances in microelectronics. Functional metal oxides are no exception – protonic-defect-conducting oxides find use in solid oxide fuel cells (SOFCs) and oxygen-deficient high-temperature superconductors are poised for power transmission and magnetic imaging applications. Similarly, the advantageous functional responses in ferroelectric materials that make them attractive for use in microelectromechanical systems (MEMS), logic elements, and environmental energy harvesting, are derived from interactions of defects with other defects (such as domain walls) and with the lattice. Chemical doping has traditionally been employed to study the effects of defects in functional materials, but complications arising from compositional heterogeneity often make interpretation of results difficult. Alternatively, irradiation is a versatile means of evaluating defect interactions while avoiding the complexities of doping. Here, a generalized phenomenological model is developed to quantify defect interactions and compare material performance in functional oxides as a function of radiation dose. The model is demonstrated with historical data from literature on ferroelectrics, and expanded to functional materials for SOFCs, mixed ionic-electronic conductors (MIECs), He-ion implantation, and superconductors. Experimental data is used to study microstructural effects on defect interactions in ferroelectrics.
This work investigates the role of microstructure on radiation-induced changes to the functional response of ferroelectric thin films. Chemical solution-deposited lead zirconate titanate thin films with columnar and equiaxed grain morphologies are exposed to a range of gamma radiation doses up to 10 Mrad and the resulting trends in functional response degradation are quantified using a previously developed phenomenological model. The observed trends of global degradation as well as local rates of defect saturation suggest strong coupling between ferroelectric thin film microstructure and material radiation hardness. Radiation-induced degradation of domain wall motion is thought to be the major contributor to the reduction in ferroelectric response. Lower rates of defect saturation are noted in samples with columnar grains, due to increased grain boundary density offering more sites to act as defect sinks, thus reducing the interaction of defects with functional material volume within the grain interior. Response trends for measurements at low electric field show substantial degradation of polarization and piezoelectric properties (up to 80% reduction in remanent piezoelectric response), while such effects are largely diminished at increased electric fields, indicating that the defects created/activated are primarily of low pinning energy. The correlation of film microstructure to radiation-induced changes to the functional response of ferroelectric thin films can be leveraged to tune and tailor the eventual properties of devices relying on these materials.
This work investigates the role of Mn-doping of ferroelectric lead zirconate titanate (PZT) thin films exposed to a range of ionizing radiation doses. PZT thin films were fabricated with both undoped and 4% Mn-doped compositions, and the functional response was compared both before and after exposure to gamma radiation doses up to 10 Mrad. A phenomenological model was applied to quantify defect interactions and compare trends in the degradation of the functional response. Mn-doped PZT samples demonstrate reduced magnitude of functional response in non-irradiated samples but exhibit vastly superior radiation tolerance of dielectric and ferroelectric properties across the range of gamma doses studied here. Strong MnZr/Ti″−VO·· defect dipoles pin domain walls, resulting in a lower initial functional response and mitigating the deleterious effects of irradiation on extrinsic contributions to the said response. Piezoelectric response trends as a function of radiation dose are highly nonlinear. The results of this work can be leveraged to engineer next-generation radiation-tolerant ferroelectric materials for applications where high levels of functional response stability are required, especially at elevated ionizing radiation dose.
This work investigates the role of crystallization interfaces and chemical heterogeneity in the radiation tolerance of chemical solution-deposited lead zirconate titanate (PZT) thin films. Two sets of PZT thin films were fabricated with crystallization performed at (i) every deposited layer or (ii) every three layers. The films were exposed to a range of 60Co gamma radiation doses, between 0.2 and 20 Mrad, and their functional response was compared before and after irradiation. The observed trends indicate enhancements of dielectric, ferroelectric, and piezoelectric responses at low radiation doses and degradation of the same at higher doses. Response enhancements are expected to result from low-dose (≤2 Mrad), ionizing radiation-induced charging of internal interfaces—an effect that results in neutralization of pre-existing internal bias in the samples. At higher radiation doses (>2 Mrad), accumulation and self-ordering of radiation-modified, mobile, oxygen vacancy-related defects contribute to degradation of dielectric, ferroelectric, and piezoelectric properties, exacerbated in the samples with more crystallization layers, potentially due to increased defect accumulation at these internal interfaces. These results suggest that the interaction between radiation and crystallization interfaces is multifaceted—the effects of ionization, domain wall motion, point defect mobility, and microstructure are considered.
Transition metal carbide field emitters for field-emitter array devices and high current applications
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