@article{radue_runnerstrom_kelley_rost_donovan_grimley_lebeau_maria_hopkins_2019, title={Charge confinement and thermal transport processes in modulation-doped epitaxial crystals lacking lattice interfaces}, volume={3}, ISSN={["2475-9953"]}, DOI={10.1103/PhysRevMaterials.3.032201}, abstractNote={Elizabeth Radue,1 Evan L. Runnerstrom,2,3 Kyle P. Kelley,2,3 Christina M. Rost,1 Brian F. Donovan,4 Everett D. Grimley,2 James M. LeBeau,2 Jon-Paul Maria,2,3 and Patrick E. Hopkins1,5,6,* 1Department of Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, Virginia 22904, USA 2Department of Materials Science, North Carolina State University, Raleigh, North Carolina 27695, USA 3Department of Materials Science and Engineering, Pennsylvania State University, State College, Pennsylvania 16801, USA 4Department of Physics, United States Naval Academy, Annapolis, Maryland 21402, USA 5Department of Materials Science and Engineering, University of Virginia, Charlottesville, Virginia 22904, USA 6Department of Physics, University of Virginia, Charlottesville, Virginia 22904, USA}, number={3}, journal={PHYSICAL REVIEW MATERIALS}, author={Radue, Elizabeth and Runnerstrom, Evan L. and Kelley, Kyle P. and Rost, Christina M. and Donovan, Brian F. and Grimley, Everett D. and LeBeau, James M. and Maria, Jon-Paul and Hopkins, Patrick E.}, year={2019}, month={Mar} }
 @article{kelley_runnerstrom_sachet_shelton_grimley_klump_lebeau_sitar_suen_padilla_et al._2019, title={Multiple Epsilon-Near-Zero Resonances in Multilayered Cadmium Oxide: Designing Metamaterial-Like Optical Properties in Monolithic Materials}, volume={6}, ISSN={["2330-4022"]}, DOI={10.1021/acsphotonics.9b00367}, abstractNote={In this Letter, we demonstrate a new class of infrared nanophotonic materials based on monolithic, multilayered doped cadmium oxide (CdO) thin films, where each CdO layer is individually tuned to support a separate epsilon-near-zero (ENZ) resonance. Infrared reflectivity measurements reveal that the optical response of the multilayered stack combines multiple discrete absorption events, each associated with an individual ENZ plasmonic polaritonic mode. Structural and chemical characterization confirm that the multilayers are homoepitaxial and monolithic, with internal interfaces defined by discrete steps in dopant density and carrier concentration. Structurally, the layers are indistinguishable as they differ from their neighbors by only ∼1 in 10000 constituent atoms. The optoelectronic property contrast, however, is pronounced, as each layer maintains an independent electron concentration, as corroborated by secondary ion mass spectroscopy and numerical solutions to Poisson’s equation. It is this electro...}, number={5}, journal={ACS PHOTONICS}, author={Kelley, Kyle P. and Runnerstrom, Evan L. and Sachet, Edward and Shelton, Christopher T. and Grimley, Everett D. and Klump, Andrew and LeBeau, James M. and Sitar, Zlatko and Suen, Jonathan Y. and Padilla, Willie J. and et al.}, year={2019}, month={May}, pages={1139–1145} }
 @article{matveyev_mikheev_negrov_zarubin_kumar_grimley_lebeau_gloskovskii_tsymbal_zenkevich_2019, title={Polarization-dependent electric potential distribution across nanoscale ferroelectric Hf0.5Zr0.5O2 in functional memory capacitors}, volume={11}, ISSN={["2040-3372"]}, DOI={10.1039/c9nr05904k}, abstractNote={Using standing-waves in HAXPES technique, we reveal non-linear electrostatic potential profile across nanoscale ferroelectric (FE) HfZrO4 layer in memory capacitors for both polarization directions, implying the drift of non-FE charges at interfaces.}, number={42}, journal={NANOSCALE}, author={Matveyev, Yury and Mikheev, Vitalii and Negrov, Dmitry and Zarubin, Sergei and Kumar, Abinash and Grimley, Everett D. and LeBeau, James M. and Gloskovskii, Andrei and Tsymbal, Evgeny Y. and Zenkevich, Andrei}, year={2019}, month={Nov}, pages={19814–19822} }
 @article{fengler_nigon_muralt_grimley_sang_sessi_hentschel_lebeau_mikolajick_schroeder_2018, title={Analysis of Performance Instabilities of Hafnia-Based Ferroelectrics Using Modulus Spectroscopy and Thermally Stimulated Depolarization Currents}, volume={4}, ISSN={["2199-160X"]}, DOI={10.1002/aelm.201700547}, abstractNote={Abstract}, number={3}, journal={ADVANCED ELECTRONIC MATERIALS}, author={Fengler, Franz P. G. and Nigon, Robin and Muralt, Paul and Grimley, Everett D. and Sang, Xiahan and Sessi, Violetta and Hentschel, Rico and LeBeau, James M. and Mikolajick, Thomas and Schroeder, Uwe}, year={2018}, month={Mar} }
 @article{grimley_wynn_kelley_sachet_dean_freeman_maria_lebeau_2018, title={Complexities of atomic structure at CdO/MgO and CdO/Al2O3 interfaces}, volume={124}, ISSN={["1089-7550"]}, DOI={10.1063/1.5053752}, abstractNote={We report the interface structures of CdO thin films on 001-MgO and 0001-Al2O3 substrates. Using aberration corrected scanning transmission electron microscopy, we show that epitaxial growth of (001)-CdO∥(001)-MgO occurs with a lattice misfit greater than 10%. A high density of interface misfit dislocations is found to form. In combination with molecular dynamics simulations, we show that dislocation strain fields form and overlap in very thin heterostructures of CdO and MgO (<3 nm). On the c-Al2O3 substrate, we find that CdO grows with a surface normal of 025. We show that three rotation variants form due to the symmetry of the sapphire surface. These results contribute insights into the epitaxial growth of these rock-salt oxides.}, number={20}, journal={JOURNAL OF APPLIED PHYSICS}, author={Grimley, Everett D. and Wynn, Alex P. and Kelley, Kyle P. and Sachet, Edward and Dean, Julian S. and Freeman, Colin L. and Maria, Jon-Paul and LeBeau, James M.}, year={2018}, month={Nov} }
 @article{dycus_mirrielees_grimley_kirste_mita_sitar_collazo_irving_lebeau_2018, title={Structure of Ultrathin Native Oxides on III-Nitride Surfaces}, volume={10}, ISSN={["1944-8252"]}, DOI={10.1021/acsami.8b00845}, abstractNote={When pristine material surfaces are exposed to air, highly reactive broken bonds can promote the formation of surface oxides with structures and properties differing greatly from bulk. Determination of the oxide structure is often elusive through the use of indirect diffraction methods or techniques that probe only the outermost layer. As a result, surface oxides forming on widely used materials, such as group III-nitrides, have not been unambiguously resolved, even though critical properties can depend sensitively on their presence. In this study, aberration corrected scanning transmission electron microscopy reveals directly, and with depth dependence, the structure of ultrathin native oxides that form on AlN and GaN surfaces. Through atomic resolution imaging and spectroscopy, we show that the oxide layers are comprised of tetrahedra-octahedra cation-oxygen units, in an arrangement similar to bulk θ-Al2O3 and β-Ga2O3. By applying density functional theory, we show that the observed structures are more stable than previously proposed surface oxide models. We place the impact of these observations in the context of key III-nitride growth, device issues, and the recent discovery of two-dimensional nitrides.}, number={13}, journal={ACS APPLIED MATERIALS & INTERFACES}, author={Dycus, J. Houston and Mirrielees, Kelsey J. and Grimley, Everett D. and Kirste, Ronny and Mita, Seiji and Sitar, Zlatko and Collazo, Ramon and Irving, Douglas L. and LeBeau, James M.}, year={2018}, month={Apr}, pages={10607–10611} }
 @article{park_schenk_fancher_grimley_zhou_richter_lebeau_jones_mikolajick_schroeder_2017, title={A comprehensive study on the structural evolution of HfO2 thin films doped with various dopants}, volume={5}, ISSN={["2050-7534"]}, DOI={10.1039/c7tc01200d}, abstractNote={Quantitative phase analysis is first performed on doped Hafnia films to elucidate the structural origin of unexpected ferroelectricity.}, number={19}, journal={JOURNAL OF MATERIALS CHEMISTRY C}, author={Park, M. H. and Schenk, T. and Fancher, C. M. and Grimley, E. D. and Zhou, C. and Richter, C. and LeBeau, J. M. and Jones, J. L. and Mikolajick, T. and Schroeder, U.}, year={2017}, month={May}, pages={4677–4690} }
 @article{moghadam_xiao_ahmadi-majlan_grimley_bowden_ong_chambers_lebeau_hong_sushko_et al._2017, title={An Ultrathin Single Crystalline Relaxor Ferroelectric Integrated on a High Mobility Semiconductor}, volume={17}, ISSN={["1530-6992"]}, DOI={10.1021/acs.nanolett.7b02947}, abstractNote={The epitaxial growth of multifunctional oxides on semiconductors has opened a pathway to introduce new functionalities to semiconductor device technologies. In particular, the integration of gate materials that enable nonvolatile or hysteretic functionality in field-effect transistors could lead to device technologies that consume less power or allow for novel modalities in computing. Here we present electrical characterization of ultrathin single crystalline SrZrxTi1-xO3 (x = 0.7) films epitaxially grown on a high mobility semiconductor, Ge. Epitaxial films of SrZrxTi1-xO3 exhibit relaxor behavior, characterized by a hysteretic polarization that can modulate the surface potential of Ge. We find that gate layers as thin as 5 nm corresponding to an equivalent-oxide thickness of just 1.0 nm exhibit a ∼2 V hysteretic window in the capacitance-voltage characteristics. The development of hysteretic metal-oxide-semiconductor capacitors with nanoscale gate thicknesses opens new vistas for nanoelectronic devices.}, number={10}, journal={NANO LETTERS}, author={Moghadam, Reza M. and Xiao, Zhiyong and Ahmadi-Majlan, Kamyar and Grimley, Everett D. and Bowden, Mark and Ong, Phuong-Vu and Chambers, Scott A. and Lebeau, James M. and Hong, Xia and Sushko, Peter V. and et al.}, year={2017}, month={Oct}, pages={6248–6257} }
 @article{richter_schenk_park_tscharntke_grimley_lebeau_zhou_fancher_jones_mikolajick_et al._2017, title={Si doped hafnium oxide-a "fragile" ferroelectric system}, volume={3}, number={10}, journal={Advanced Electronic Materials}, author={Richter, C. and Schenk, T. and Park, M. H. and Tscharntke, F. A. and Grimley, E. D. and LeBeau, J. M. and Zhou, C. Z. and Fancher, C. M. and Jones, J. L. and Mikolajick, T. and et al.}, year={2017} }
 @article{lim_ahmadi-majlan_grimley_du_bowden_moghadam_lebeau_chambers_ngai_2017, title={Structural and electrical properties of single crystalline SrZrO3 epitaxially grown on Ge (001)}, volume={122}, ISSN={["1089-7550"]}, DOI={10.1063/1.5000142}, abstractNote={We present structural and electrical characterization of SrZrO3 that has been epitaxially grown on Ge(001) by oxide molecular beam epitaxy. Single crystalline SrZrO3 can be nucleated on Ge via deposition at low temperatures followed by annealing at 550 °C in ultra-high vacuum. Photoemission spectroscopy measurements reveal that SrZrO3 exhibits a type-I band arrangement with respect to Ge, with conduction and valence band offsets of 1.4 eV and 3.66 eV, respectively. Capacitance-voltage and current-voltage measurements on 4 nm thick films reveal low leakage current densities and an unpinned Fermi level at the interface that allows modulation of the surface potential of Ge. Ultra-thin films of epitaxial SrZrO3 can thus be explored as a potential gate dielectric for Ge.}, number={8}, journal={JOURNAL OF APPLIED PHYSICS}, author={Lim, Z. H. and Ahmadi-Majlan, K. and Grimley, E. D. and Du, Y. and Bowden, M. and Moghadam, R. and LeBeau, J. M. and Chambers, S. A. and Ngai, J. H.}, year={2017}, month={Aug} }
 @inproceedings{schroeder_pesic_schenk_mulaosmanovic_slesazeck_ocker_richter_yurchuk_khullar_muller_et al._2016, title={Impact of field cycling on HfO2 based non-volatile memory devices}, booktitle={2016 46th european solid-state device research conference (essderc)}, author={Schroeder, U. and Pesic, M. and Schenk, T. and Mulaosmanovic, H. and Slesazeck, S. and Ocker, J. and Richter, C. and Yurchuk, E. and Khullar, K. and Muller, J. and et al.}, year={2016}, pages={364–368} }
 @article{xu_bowes_grimley_irving_lebeau_2016, title={In-situ real-space imaging of single crystal surface reconstructions via electron microscopy}, volume={109}, ISSN={["1077-3118"]}, url={http://dx.doi.org/10.1063/1.4967978}, DOI={10.1063/1.4967978}, abstractNote={Here, we report a high temperature in-situ atomic resolution scanning transmission electron microscopy (STEM) study of single crystal surface structure dynamics. With the approach, we gain direct insight into a double layer reconstruction that occurs on the polar SrTiO3 (110) surface. We find that structural details of this reconstruction can be directly attributed to charge redistribution and the thermal mismatch between the surface and the bulk material. Periodic surface defects, similar to dislocations, are found, which act to relieve stress as the temperature is lowered. Combining STEM observations, electron energy loss spectroscopy, and density functional theory, we highlight the combined role of lattice misfit and charge compensation to determine the structure and chemistry of the observed polar surface reconstruction.}, number={20}, journal={APPLIED PHYSICS LETTERS}, author={Xu, Weizong and Bowes, Preston C. and Grimley, Everett D. and Irving, Douglas L. and LeBeau, James M.}, year={2016}, month={Nov} }
 @article{pesic_fengler_larcher_padovani_schenk_grimley_sang_lebeau_slesazeck_schroeder_et al._2016, title={Physical Mechanisms behind the Field-Cycling Behavior of HfO2-Based Ferroelectric Capacitors}, volume={26}, ISSN={["1616-3028"]}, DOI={10.1002/adfm.201600590}, abstractNote={Novel hafnium oxide (HfO2)‐based ferroelectrics reveal full scalability and complementary metal oxide semiconductor integratability compared to perovskite‐based ferroelectrics that are currently used in nonvolatile ferroelectric random access memories (FeRAMs). Within the lifetime of the device, two main regimes of wake‐up and fatigue can be identified. Up to now, the mechanisms behind these two device stages have not been revealed. Thus, the main scope of this study is an identification of the root cause for the increase of the remnant polarization during the wake‐up phase and subsequent polarization degradation with further cycling. Combining the comprehensive ferroelectric switching current experiments, Preisach density analysis, and transmission electron microscopy (TEM) study with compact and Technology Computer Aided Design (TCAD) modeling, it has been found out that during the wake‐up of the device no new defects are generated but the existing defects redistribute within the device. Furthermore, vacancy diffusion has been identified as the main cause for the phase transformation and consequent increase of the remnant polarization. Utilizing trap density spectroscopy for examining defect evolution with cycling of the device together with modeling of the degradation results in an understanding of the main mechanisms behind the evolution of the ferroelectric response.}, number={25}, journal={ADVANCED FUNCTIONAL MATERIALS}, author={Pesic, Milan and Fengler, Franz Paul Gustav and Larcher, Luca and Padovani, Andrea and Schenk, Tony and Grimley, Everett D. and Sang, Xiahan and LeBeau, James M. and Slesazeck, Stefan and Schroeder, Uwe and et al.}, year={2016}, month={Jul}, pages={4601–4612} }
 @article{grimley_schenk_sang_pesic_schroeder_mikolajick_lebeau_2016, title={Structural Changes Underlying Field-Cycling Phenomena in Ferroelectric HfO2 Thin Films}, volume={2}, ISSN={["2199-160X"]}, DOI={10.1002/aelm.201600173}, abstractNote={Since 2011, ferroelectric HfO2 has attracted growing interest in both fundamental and application oriented groups. In this material, noteworthy wake‐up and fatigue effects alter the shape of the polarization hysteresis loop during field cycling. Such changes are problematic for application of HfO2 to ferroelectric memories, which require stable polarization hystereses. Herein, electrical and structural techniques are implemented to unveil how cyclic switching changes nanoscale film structure, which modifies the polarization hysteresis. Impedance spectroscopy and scanning transmission electron microscopy identify regions with different dielectric and conductive properties in films at different cycling stages, enabling development of a structural model to explain the wake‐up and fatigue phenomena. The wake‐up regime arises due to changes in bulk and interfacial structuring: the bulk undergoes a phase transformation from monoclinic to orthorhombic grains, and the interfaces show changes in and diminishment of a nonuniform, defect rich, tetragonal HfO2 layer near the electrodes. The evolution of these aspects of structuring contributes to the increase in Pr and the opening of the constricted P–V hysteresis that are known to occur with wake‐up. The onset of the fatigue regime is correlated to an increasing concentration of bulk defects, which are proposed to pin domain walls.}, number={9}, journal={ADVANCED ELECTRONIC MATERIALS}, author={Grimley, Everett D. and Schenk, Tony and Sang, Xiahan and Pesic, Milan and Schroeder, Uwe and Mikolajick, Thomas and LeBeau, James M.}, year={2016}, month={Sep} }
 @article{haislmaier_grimley_biegalski_lebeau_trolier-mckinstry_gopalan_engel-herbert_2016, title={Unleashing Strain Induced Ferroelectricity in Complex Oxide Thin Films via Precise Stoichiometry Control}, volume={26}, ISSN={["1616-3028"]}, DOI={10.1002/adfm.201602767}, abstractNote={Strain tuning has emerged as a powerful means to enhance properties and to induce otherwise unattainable phenomena in complex oxide films. However, by employing strain alone, the predicted properties sometimes fail to emerge. In this work, the critical role of precise stoichiometry control for realizing strain‐induced ferroelectricity in CaTiO3 films is demonstrated. An adsorption controlled growth window is discovered for CaTiO3 films grown by hybrid molecular beam epitaxy, which ensures an excellent control over the Ti:Ca atomic percent ratio of <0.8% in the films. Superior ferroelectric and dielectric properties are found for films grown inside the stoichiometric growth window, yielding maximum polarization, dielectric constant, and paraelectric‐to‐ferroelectric transition temperatures. Outside this growth window, properties are severely deteriorated and ultimately suppressed by defects in the films. This study exemplifies the important role of precise compositional control for achieving strain‐induced properties. Untangling the effects of strain and stoichiometry on functional properties will accelerate both fundamental discoveries yet to be made in the vast materials design space of strained complex oxide films, as well as utilization of strain‐stabilized phenomena in future devices.}, number={40}, journal={ADVANCED FUNCTIONAL MATERIALS}, author={Haislmaier, Ryan C. and Grimley, Everett D. and Biegalski, Michael D. and LeBeau, James M. and Trolier-McKinstry, Susan and Gopalan, Venkatraman and Engel-Herbert, Roman}, year={2016}, month={Oct}, pages={7271–7279} }
 @article{sang_grimley_niu_irving_lebeau_2015, title={Direct observation of charge mediated lattice distortions in complex oxide solid solutions}, volume={106}, ISSN={["1077-3118"]}, DOI={10.1063/1.4908124}, abstractNote={Using aberration corrected scanning transmission electron microscopy combined with advanced imaging methods, we directly observe atom column specific, picometer-scale displacements induced by local chemistry in a complex oxide solid solution. Displacements predicted from density functional theory were found to correlate with the observed experimental trends. Further analysis of bonding and charge distribution was used to clarify the mechanisms responsible for the detected structural behavior. By extending the experimental electron microscopy measurements to previously inaccessible length scales, we identified correlated atomic displacements linked to bond differences within the complex oxide structure.}, number={6}, journal={APPLIED PHYSICS LETTERS}, author={Sang, Xiahan and Grimley, Everett D. and Niu, Changning and Irving, Douglas L. and LeBeau, James M.}, year={2015}, month={Feb} }
 @article{eaton_moyer_alipour_grimley_brahlek_lebeau_engel-herbert_2015, title={Growth of SrVO3 thin films by hybrid molecular beam epitaxy}, volume={33}, ISSN={["1520-8559"]}, DOI={10.1116/1.4927439}, abstractNote={The authors report the growth of stoichiometric SrVO3 thin films on (LaAlO3)0.3(Sr2AlTaO6)0.7 (001) substrates using hybrid molecular beam epitaxy. This growth approach employs a conventional effusion cell to supply elemental A-site Sr and the metalorganic precursor vanadium oxytriisopropoxide (VTIP) to supply vanadium. Oxygen is supplied in its molecular form through a gas inlet. An optimal VTIP:Sr flux ratio has been identified using reflection high-energy electron-diffraction, x-ray diffraction, atomic force microscopy, and scanning transmission electron microscopy, demonstrating stoichiometric SrVO3 films with atomically flat surface morphology. Away from the optimal VTIP:Sr flux, characteristic changes in the crystalline structure and surface morphology of the films were found, enabling identification of the type of nonstoichiometry. For optimal VTIP:Sr flux ratios, high quality SrVO3 thin films were obtained with smallest deviation of the lattice parameter from the ideal value and with atomically smooth surfaces, indicative of the good cation stoichiometry achieved by this growth technique.}, number={6}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY A}, author={Eaton, Craig and Moyer, Jarrett A. and Alipour, Hamideh M. and Grimley, Everett D. and Brahlek, Matthew and LeBeau, James M. and Engel-Herbert, Roman}, year={2015}, month={Nov} }
 @article{sang_grimley_schenk_schroeder_lebeau_2015, title={On the structural origins of ferroelectricity in HfO2 thin films}, volume={106}, ISSN={["1077-3118"]}, DOI={10.1063/1.4919135}, abstractNote={Here, we present a structural study on the origin of ferroelectricity in Gd doped HfO2 thin films. We apply aberration corrected high-angle annular dark-field scanning transmission electron microscopy to directly determine the underlying lattice type using projected atom positions and measured lattice parameters. Furthermore, we apply nanoscale electron diffraction methods to visualize the crystal symmetry elements. Combined, the experimental results provide unambiguous evidence for the existence of a non-centrosymmetric orthorhombic phase that can support spontaneous polarization, resolving the origin of ferroelectricity in HfO2 thin films.}, number={16}, journal={APPLIED PHYSICS LETTERS}, author={Sang, Xiahan and Grimley, Everett D. and Schenk, Tony and Schroeder, Uwe and LeBeau, James M.}, year={2015}, month={Apr} }
 @article{lomenzo_zhao_takmeel_moghaddam_nishida_nelson_fancher_grimley_sang_lebeau_et al._2014, title={Ferroelectric phenomena in Si-doped HfO2 thin films with TiN and Ir electrodes}, volume={32}, number={3}, journal={Journal of Vacuum Science & Technology. B, Microelectronics and Nanometer Structures}, author={Lomenzo, P. D. and Zhao, P. and Takmeel, Q. and Moghaddam, S. and Nishida, T. and Nelson, M. and Fancher, C. M. and Grimley, E. D. and Sang, X. H. and LeBeau, J. M. and et al.}, year={2014} }