@article{long_creange_moballegh_dickey_2019, title={Electromigration-induced leakage current enhancement and its anisotropy in single crystal TiO 2}, volume={125}, ISSN={0021-8979 1089-7550}, url={http://dx.doi.org/10.1063/1.5090402}, DOI={10.1063/1.5090402}, abstractNote={Time-dependent increases in electrical conductivity in dielectrics exposed to a direct-current voltage bias can be detrimental to device reliability and lifetime, while the same phenomenon may be exploited for resistive switching functionality. The applied electric field induces the electromigration of charged point defects, resulting in spatially inhomogeneous stoichiometry and modifications in the electrode electrostatics. Herein, we monitor the time-dependent changes in leakage current in rutile TiO2 with platinum electrodes. Current–voltage measurements before and after the extended electrical biasing demonstrate that changes in the electrostatics of the cathode Schottky barrier are responsible for the time-dependent increases in leakage current. We also find that electromigration is accelerated along the ⟨001⟩ relative to the ⟨010⟩ crystallographic direction in contrast to what would be predicted by high-temperature diffusion kinetics. We find the observed anisotropic behavior to be consistent with a change in the defect charge state of the titanium interstitials and dominant transport path at lower temperatures. A comparison of optical color front migration kinetics to the temporal evolution of leakage current further supports the interpretation that ionic electromigration near the cathode interface is responsible for the increased leakage current in the electrically degraded state.}, number={18}, journal={Journal of Applied Physics}, publisher={AIP Publishing}, author={Long, Daniel and Creange, Nicole and Moballegh, Ali and Dickey, Elizabeth C.}, year={2019}, month={May}, pages={184101} }
 @article{baker_bowes_long_moballegh_harris_dickey_irving_2017, title={Defect mechanisms of coloration in Fe-doped SrTiO3 from first principles}, volume={110}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.4978861}, DOI={10.1063/1.4978861}, abstractNote={To understand the underlying defect mechanisms governing the coloration of Fe-doped SrTiO3 (Fe:STO), density functional theory calculations were used to determine defect formation energies and to interpret optical absorption spectra. A grand canonical defect equilibrium model was developed using the calculated formation energies, which enabled connection to annealing experiments. It was found that FeTi0 is stable in oxidizing conditions and leads to the optical absorption signatures in oxidized Fe:STO, consistent with experiment. Fe:STO was found to transition from brown to transparent as PO2 was reduced during annealing. The defect equilibrium model reproduces a consistent PO2 of this coloration transition. Most critical to reproducing the PO2 of the coloration transition was inclusion of a FeTi-VO first nearest neighbor complex, which was found to be strongly interacting. The coloration transition PO2 was found to be insensitive to the presence of minority background impurities, slightly sensitive to Fe content, and more sensitive to annealing temperature.}, number={12}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Baker, Jonathon N. and Bowes, Preston C. and Long, Daniel M. and Moballegh, Ali and Harris, Joshua S. and Dickey, Elizabeth C. and Irving, Douglas L.}, year={2017}, month={Mar}, pages={122903} }
 @article{donovan_long_moballegh_creange_dickey_hopkins_2017, title={Impact of intrinsic point defect concentration on thermal transport in titanium dioxide}, volume={127}, ISSN={1359-6454}, url={http://dx.doi.org/10.1016/j.actamat.2017.01.018}, DOI={10.1016/j.actamat.2017.01.018}, abstractNote={The thermal conductivity of functional oxide materials can be significantly impacted by variations in point defect concentration, especially at high concentrations where defect interactions can result in extended defects and secondary phase formation. In this work, we systematically study the impact of high point defect concentrations on thermal transport in rutile TiO2. Using atmospherically controlled annealing, we vary equilibrium point defect concentrations and measure the resulting thermal conductivity using time domain thermoreflectance. We verify our results with analytical modeling and find that it is not until very high defect concentrations (> 0.5 mol.%) that the phonon thermal conductivity is impacted. We vary the partial pressure of oxygen to low enough levels that sub-stoichiometric Magnéli phases form and find that these highly defective phases severely reduce the thermal conductivity and anisotropy from intrinsic levels.}, journal={Acta Materialia}, publisher={Elsevier BV}, author={Donovan, Brian F. and Long, Daniel M. and Moballegh, Ali and Creange, Nicole and Dickey, Elizabeth C. and Hopkins, Patrick E.}, year={2017}, month={Apr}, pages={491–497} }
 @article{wang_huang_bayer_moballegh_cao_klein_dickey_irving_randall_chen_et al._2016, title={Defect chemistry and resistance degradation in Fe-doped SrTiO3 single crystal}, volume={108}, ISSN={1359-6454}, url={http://dx.doi.org/10.1016/j.actamat.2016.02.022}, DOI={10.1016/j.actamat.2016.02.022}, abstractNote={Defect chemistry and transport in Fe-doped SrTiO3 single crystal are studied to understand its resistance degradation mechanism. The temporal evolution of electric conductivity under a voltage stress was obtained computationally by solving the transport equations for ionic and electronic defects coupled with the defect reaction equilibrium equations. The computational results are compared to the corresponding experimental measurement under similar conditions. It is shown that the local electron and hole concentrations are controlled by the local electronic defect equilibria rather than by their quasi-steady state diffusional transport. It is the electric field-induced migration of oxygen vacancies and the subsequent instantaneous reestablishment of the local defect equilibria that lead to the resistance degradation. The resistance degradation behavior and the defect distributions under a long-term voltage stress are strongly influenced by the sample-annealing oxygen partial pressure, degrading electric field, and temperature. The present study contributes to the understanding of resistance degradation mechanism and provides guidance to improve the lifetime and reliability of wide band-gap semiconducting capacitors.}, journal={Acta Materialia}, publisher={Elsevier BV}, author={Wang, Jian-Jun and Huang, Hou-Bing and Bayer, Thorsten J. M. and Moballegh, Ali and Cao, Ye and Klein, Andreas and Dickey, Elizabeth C. and Irving, Douglas L. and Randall, Clive A. and Chen, Long-Qing and et al.}, year={2016}, month={Apr}, pages={229–240} }
 @article{bayer_wang_carter_moballegh_baker_irving_dickey_chen_randall_2016, title={The relation of electrical conductivity profiles and modulus data using the example of STO:Fe single crystals: A path to improve the model of resistance degradation}, volume={117}, ISSN={1359-6454}, url={http://dx.doi.org/10.1016/j.actamat.2016.07.024}, DOI={10.1016/j.actamat.2016.07.024}, abstractNote={Resistance degradation in perovskites is characterized by an increase in current over time with applied electric field. This behavior can be simulated and spatially resolved conductivity profiles can be measured, but some inconsistencies remain. A new approach to address these problems is presented that utilizes time-resolved impedance spectroscopy with an applied DC voltage to provide new insight into the resistance degradation phenomenon. In particular, this method allows the in-situ acquisition of spatio-temporal variations in conductivity. In SrTiO3 a single bulk-dominated maximum of the imaginary part of the modulus M″ transitions to two maxima during degradation, reflecting the hole conductivity in the anode region and the electron conductivity in the cathode region. To clarify the influence of conductivity profiles on impedance data, the reversed route is presented by using simulated conductivity profiles to calculate impedance data. It will be emphasized that this methodology is not limited to the perovskite system considered here, but can be adapted to any kind of system characterized by a spatially varying conductivity.}, journal={Acta Materialia}, publisher={Elsevier BV}, author={Bayer, Thorsten J.M. and Wang, Jian-Jun and Carter, Jared J. and Moballegh, Ali and Baker, Jonathon and Irving, Douglas L. and Dickey, Elizabeth C. and Chen, Long-Qing and Randall, Clive A.}, year={2016}, month={Sep}, pages={252–261} }
 @article{moballegh_dickey_2015, title={Electric-field-induced point defect redistribution in single-crystal TiO2– and effects on electrical transport}, volume={86}, ISSN={1359-6454}, url={http://dx.doi.org/10.1016/j.actamat.2014.11.032}, DOI={10.1016/j.actamat.2014.11.032}, abstractNote={The spatial redistribution of non-stoichiometric point defects in rutile TiO2 is studied as a function of voltage and time. Single crystals are equilibrated initially to a well-defined stoichiometry with n-type conductivity and a carrier concentration on the order of 1018 cm−3. The crystals are subsequently electroded with Pt contacts that exhibit Schottky behavior. When subjected to an applied voltage of 15 V, a time-dependent increase and saturation in the leakage current is observed, which is associated with an accumulation of point defects and an attendant decrease in stoichiometry at the cathode electrode. This local change in stoichiometry degrades the Schottky barrier, leading to asymmetric electrodes and thus macroscopic rectifying behavior. Cathodoluminescence spectroscopy shows that Ti interstitials dominate the point defect redistribution process. Under larger applied voltages, of around 30 V, qualitatively different behavior is observed in which the resistivity increases as a function of time. This behavior is associated with condensation of point defects into a region of extended defects and Magnéli phases near the cathode, sufficient to increase the bulk stoichiometry and resistivity. These experiments demonstrate that a one-dimensional drift-diffusion process, as opposed to filamentary growth, dominates in these experimental conditions and that the Pt–TiO2–Pt system remains closed, with no significant oxygen transport across the Pt–TiO2 interfaces. We believe this is the first observation of a second higher-voltage regime in which the bulk stoichiometry and thus resistivity is increased as large concentrations of defects condense into metallic Magnéli phases in the near-electrode regions.}, journal={Acta Materialia}, publisher={Elsevier BV}, author={Moballegh, Ali and Dickey, Elizabeth C.}, year={2015}, month={Mar}, pages={352–360} }
 @article{rost_sachet_borman_moballegh_dickey_hou_jones_curtarolo_maria_2015, title={Entropy-stabilized oxides}, volume={6}, journal={Nature Communications}, author={Rost, C. M. and Sachet, E. and Borman, T. and Moballegh, A. and Dickey, E. C. and Hou, D. and Jones, J. L. and Curtarolo, S. and Maria, J. P.}, year={2015} }
 @article{moballegh_dickey_2014, title={Investigation of the Nature and Mechanism of Resistive Switching in TiO2-x}, volume={20}, ISSN={1431-9276 1435-8115}, url={http://dx.doi.org/10.1017/S1431927614011258}, DOI={10.1017/s1431927614011258}, abstractNote={The spatial redistribution of the charged point defects under direct-current (DC) biasing can have significant implications for electroceramic device performance and lifetime [1,2]. The transport behavior of point defects is regulated by the boundary conditions of the electrodes, which can block ion transfer across the interface to varying degrees. When the electrodes are impermeable to mass transport, there will be an accumulation of point defects in the near-electrode region that can lead to significant modifications in the local electronic carrier concentrations at the interface. Such defect redistribution is responsible for the long-term increases in leakage current in many capacitor devices. On the other hand, this same phenomena can lead to resistive switching behavior via modification of the interface Schottky barrier at the reverse-biased cathode [1-4].}, number={S3}, journal={Microscopy and Microanalysis}, publisher={Cambridge University Press (CUP)}, author={Moballegh, Ali and Dickey, Elizabeth C.}, year={2014}, month={Aug}, pages={1904–1905} }
 @article{moballegh_dickey_2013, title={Electric Field-Induced Point Defect Redistribution in TiO2}, volume={19}, ISSN={1431-9276 1435-8115}, url={http://dx.doi.org/10.1017/S1431927613011914}, DOI={10.1017/s1431927613011914}, abstractNote={Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.}, number={S2}, journal={Microscopy and Microanalysis}, publisher={Cambridge University Press (CUP)}, author={Moballegh, A. and Dickey, E.C.}, year={2013}, month={Aug}, pages={1984–1985} }
 @article{burch_harris_white_moballegh_li_maria_dickey_2012, title={In situ heating studies of flux grown barium titanate thin films}, volume={18}, url={https://app.dimensions.ai/details/publication/pub.1054927598}, DOI={10.1017/s1431927612009245}, abstractNote={Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.}, number={S2}, journal={Microscopy and Microanalysis}, author={Burch, M.J. and Harris, D. and White, R.M. and Moballegh, A. and Li, J. and Maria, J.P. and Dickey, Elizabeth C.}, year={2012}, pages={1478–1479} }
 @article{banoee_seif_nazari_jafari-fesharaki_shahverdi_moballegh_moghaddam_shahverd_2010, title={ZnO nanoparticles enhanced antibacterial activity of ciprofloxacin against Staphylococcus aureus and Escherichia coli}, volume={93B}, journal={Journal of Biomedical Materials Research. Part B, Applied Biomaterials}, author={Banoee, M. and Seif, S. and Nazari, Z.E. and Jafari-Fesharaki, P. and Shahverdi, H.R. and Moballegh, A. and Moghaddam, K.M. and Shahverd, A.R.}, year={2010}, pages={557–561} }
 @article{moballegh_shahverdi_aghababazadeh_mirhabibi_2007, title={ZnO nanoparticles obtained by mechanochemical technique and the optical properties}, volume={601}, journal={Surface Science}, author={Moballegh, A. and Shahverdi, H.R. and Aghababazadeh, R. and Mirhabibi, A.R.}, year={2007}, pages={2850–2854} }