The combination of epitaxial strain and defect engineering facilitates the tuning of the transition temperature of BaTiO3 to >800 °C. Advances in thin-film deposition enable the utilization of both the electric and elastic dipoles of defects to extend the epitaxial strain to new levels, inducing unprecedented functionality and temperature stability in ferroelectrics.

We demonstrate a link between the growth process, the stoichiometry of LaAlO(3), and the interfacial electrical properties of LaAlO(3)/SrTiO(3) heterointerfaces. Varying the relative La:Al cation stoichiometry by a few atomic percent in films grown at 1×10(-3) Torr results in a 2 and 7 order-of-magnitude change in the 300 and 2 K sheet resistance, respectively, with highly conducting states occurring only in La-deficient or Al-excess films. Further reducing the growth pressure results in an increase of the carrier density and a dramatic change in mobility. We discuss the relative contributions of intrinsic and extrinsic effects in controlling the physical properties of this widely studied system.

We report dramatic variations in cation stoichiometry in SrTiO3 thin films grown via pulsed laser deposition and the implications of this nonstoichiometry for structural, dielectric, and thermal properties. The chemical composition of SrTiO3 thin films was characterized via X-ray photoelectron spectroscopy and Rutherford backscattering spectrometry. These studies reveal that deviations in laser fluence and deposition geometry can result in deviations of cation stoichiometry as large as a few percent. Additionally, X-ray diffraction was used to probe structural evolution and revealed an asymmetric strain relaxation mechanism in which films possessing Sr-excess undergo relaxation before those possessing Sr-deficiency. Furthermore, the dielectric constant decreases and the loss tangent increases with increasing nonstoichiometry with intriguing differences between Sr-excess and -deficiency. Thermal conductivity is also found to be sensitive to nonstoichiometry, with Sr-excess and -deficiency resulting in 65% and 35% reduction in thermal conductivity, respectively. These trends are explained by the expected defect structures.

Next-generation devices will rely on exotic functional properties not found in traditional systems. One class of materials of particular interest for applications are those possessing metal-to-insulator transitions (MITs). In this work, we probe the relationship between variations in the growth process, subsequent variations in cation stoichiometry, and the MIT in NdNiO3 thin films. Slight variations in the growth conditions, in particular the laser fluence, during pulsed-laser deposition growth of NdNiO3 produces films that are both single-phase and coherently strained to a range of substrates despite possessing as much as 15% Nd-excess. Subsequent study of the temperature-dependence of the electronic transport reveals dramatic changes in both the onset and magnitude of the resistivity change at the MIT with increasing cation nonstoichiometry giving rise to a decrease (and ultimately a suppression) of the transition and the magnitude of the resistivity change. From there, the electronic transport of nearly ideal NdNiO3 thin films are studied as a function of epitaxial strain, thickness, and orientation. Overall, transitioning from tensile to compressive strain results in a systematic reduction of the onset and magnitude of the resistivity change across the MIT, thinner films are found to possess sharper MITs with larger changes in the resistivity at the transition, and (001)-oriented films exhibit sharper and larger MITs as compared to (110)- and (111)-oriented films as a result of highly anisotropic in-plane transport in the latter.

Epitaxial VO/TiO thin film heterostructures were grown on (100) (m-cut) AlO substrates via pulsed laser deposition. We have demonstrated the ability to reduce the semiconductor-metal transition (SMT) temperature of VO to ∼44 °C while retaining a 4 order of magnitude SMT using the TiO buffer layer. A combination of electrical transport and X-ray diffraction reciprocal space mapping studies help examine the specific strain states of VO/TiO/AlO heterostructures as a function of TiO film growth temperatures. Atomic force microscopy and transmission electron microscopy analyses show that the columnar microstructure present in TiO buffer films is responsible for the partially strained VO film behavior and subsequently favorable transport characteristics with a lower SMT temperature. Such findings are of crucial importance for both the technological implementation of the VO system, where reduction of its SMT temperature is widely sought, as well as the broader complex oxide community, where greater understanding of the evolution of microstructure, strain, and functional properties is a high priority.

engineering can be used to tune the specifi c thermoelectric material properties.Detailed thermoelectric characterization of Na x CoO 2 thin fi lms has been hindered by the chemical instability in ambient conditions. [ 7,8 ] However, a recently developed method to obtain chemically stable, single-phase Na x CoO 2 thin fi lms by pulsed laser deposition due to the in situ deposition of an amorphous AlO x capping layer enables us to exploit the intrinsic properties of these thermoelectric thin fi lms. [ 8 ] Here, we show that by structural engineering in chemically stable Na x CoO 2 thin fi lms the thermoelectric properties can be controlled and enhanced as compared to bulk samples. By changing the single crystalline substrate material we can control the structural properties and as a consequence the electronic and thermal properties of the thermoelectric thin fi lms. Tuning of the grain size within the Na x CoO 2 thin fi lms signifi cantly infl uences the achievable Seebeck coeffi cient. We demonstrate that preservation of the crystallinity in these thin fi lms with enhanced Seebeck coeffi cient results in minimal reduction of the electrical conductivity and, therefore, leads to a doubling of the thermoelectric power factor at room temperature.Here, structural engineering is applied as a tool to obtain improved control over the thermoelectric properties of Na x CoO 2 thin fi lms, which is unique for epitaxial thin fi lms and cannot be obtained in single crystal or polycrystalline samples. To study this effect, Na x CoO 2 thin fi lms were grown by pulsed laser deposition (PLD) on various single crystal substrates. All Na x CoO 2 thin fi lms were deposited under the same conditions and have a thickness of 60 nm. Independent of the substrate material and structure, all thin fi lms showed a preferred growth orientation with the (00l) direction parallel to the surface normal.Previously it was shown that the crystallinity of Na x CoO 2 thin fi lms does not strongly depend on the deposition temperature, [ 8 ] and an optimum deposition temperature of 430 °C was determined. However, the effect of oxygen deposition pressure on the crystallinity was not systematically studied yet. Here, we observe a signifi cant decrease in crystallinity when the deposition pressure was reduced by one and two orders of magnitude from the previously reported value of 0.4 mbar, [ 8 ] resulting in an increased resistivity by a factor of fi ve, together with a strong reduction of the Seebeck coeffi cient. Based on these results we can conclude that, although the deposition pressure can clearly be used to tune the crystallinity of these Na x CoO 2 thin fi lms, it will not provide the required enhanced control over the thermoelectric properties. Therefore the optimized deposition parameters [ 8 ] are used, which result in a combination of the optimum crystallinity and thermoelectric properties. Furthermore, all thin fi lms have been cooled down after growth in 1 atm. of oxygen at a rate of 10 °C min -1 to optimize the oxidation level.

Epitaxial strain has been extensively used to control and induce new properties in complex oxide thin films. Understanding strain evolution and how to manipulate it are essential to the continued development of strain-induced effects in materials. The chemical complexity that underlies the diverse functionality of such oxide materials can have complex and unexpected effects on strain evolution and the breakdown of classic models of strain relaxation (e.g., misfit dislocation formation). We explore the connection between the growth process and the diverse and extensive range of point and volumetric defects that can be generated and accommodated in oxide systems and how this ultimately impacts strain evolution.Pulsed-laser deposition was used to produce thin films of the prototypical perovskite oxide SrTiO 3 with chemical compositions ranging from 4% Sr-deficiency to 4% Sr-excess. Small variations in film composition are found to give rise to three distinct modes of strain relaxation, critical thicknesses for relaxation that vary from 60 to 300 nm, and have a notable impact on interfacial intermixing. Atomicscale scanning transmission electron microscopy and spectroscopic studies provide information on defect structures, how the defect formations are connected to the epitaxial strain relaxation, and reveal that the presence of defect structures generally leads to a local chemical broadening of the interface.

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