@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{paisley_brumbach_shelton_allerman_atcitty_rost_ohlhausen_doyle_sitar_maria_et al._2018, title={Nitride surface chemistry influence on band offsets at epitaxial oxide/GaN interfaces}, volume={112}, ISSN={["1077-3118"]}, DOI={10.1063/1.5013605}, abstractNote={GaN surface and near-surface chemistry influence on band offsets of oxide overlayers is demonstrated through X-ray photoelectron spectroscopy measurements using epitaxial (111)-oriented MgO films on (0001)-oriented Ga-polar GaN as a case study. For identical cleaning and MgO growth conditions, GaN subsurface oxygen impurities influence the GaN bare surface band bending and the ultimate band offset to MgO heterolayers. As the GaN surface oxygen concentration increases from an atomic concentration of 0.9% to 3.4%, the valence band offset to MgO decreases from 1.68 eV to 1.29 eV, respectively. This study highlights the sensitivity of the oxide/nitride interface electronic structure to GaN epilayer preparation and growth conditions.}, number={9}, journal={APPLIED PHYSICS LETTERS}, author={Paisley, Elizabeth A. and Brumbach, Michael T. and Shelton, Christopher T. and Allerman, Andrew A. and Atcitty, Stanley and Rost, Christina M. and Ohlhausen, James A. and Doyle, Barney L. and Sitar, Zlatko and Maria, Jon-Paul and et al.}, year={2018}, month={Feb} }
 @article{runnerstrom_kelley_sachet_shelton_maria_2017, title={Epsilon-near-Zero Modes and Surface Plasmon Resonance in Fluorine-Doped Cadmium Oxide Thin Films}, volume={4}, ISSN={["2330-4022"]}, DOI={10.1021/acsphotonics.7b00429}, abstractNote={In this report we demonstrate fluorine-doped CdO as a model infrared plasmonic material by virtue of its tunable carrier density, high mobility, and intense extreme-subwavelength plasmon–polariton coupling. Carrier concentrations ranging from 1019 to 1020 cm–3, with electron mobility values as high as 473 cm2/V·s, are readily achieved in epitaxial CdO films over a thickness range spanning 50 to 500 nm. Carrier concentration is achieved by reactive sputtering in an Ar/O2 atmosphere with trace quantities of CF4. Infrared reflectometry measurements demonstrate the possibility of near-perfect plasmonic absorption through the entire mid-IR spectral range. A companion set of reflectivity simulations are used to predict, understand, and optimize the epsilon-near-zero plasmonic modes. In the context of other transparent conductors, CdO exhibits substantially higher electron mobility values and thus sharp and tunable absorption features. This highlights the utility of high-mobility transparent conducting oxides as...}, number={8}, journal={ACS PHOTONICS}, author={Runnerstrom, Evan L. and Kelley, Kyle P. and Sachet, Edward and Shelton, Christopher T. and Maria, Jon-Paul}, year={2017}, month={Aug}, pages={1885–1892} }
 @article{kelley_sachet_shelton_maria_2017, title={High mobility yttrium doped cadmium oxide thin films}, volume={5}, ISSN={["2166-532X"]}, DOI={10.1063/1.4993799}, abstractNote={Donor doped CdO thin films on c-plane sapphire are prepared by reactive co-sputtering from Cd-metal and Y-metal targets which are driven using pulsed-dc and RF power respectively. Intrinsic CdO exhibits a carrier density of 1.8 × 1019 cm−3 and a mobility of 330 cm2 V−1 s−1. By increasing the Y-flux, carrier density values can be increased smoothly and reproducibly to a maximum value of 3.3 × 1020 cm−3. Mobility increases with Y flux, and exhibits a broad plateau between approximately 5 × 1019 cm−3 and 2 × 1020 cm−3. Higher carrier concentrations produce a sharp drop in mobility. The increase in mobility is attributed to a reduction of intrinsic donors (i.e., oxygen vacancies) with increasing carrier density while the ultimate decrease in mobility results from a combination of factors including cadmium vacancies, reduced crystal quality, and smaller crystallite sizes, all of which accompany carrier density values greater than the mid 1020 cm−3 range. This work demonstrates that CdO thin films can be prepared by magnetron sputtering with transport properties and crystal quality that are comparable to those grown using molecular beam epitaxy.}, number={7}, journal={APL MATERIALS}, author={Kelley, Kyle P. and Sachet, Edward and Shelton, Christopher T. and Maria, Jon-Paul}, year={2017}, month={Jul} }
 @article{shelton_bryan_paisley_sachet_ihlefeld_lavrik_collazo_sitar_maria_2017, title={Step-free GaN surfaces grown by confined-area metal-organic vapor phase epitaxy}, volume={5}, ISSN={["2166-532X"]}, DOI={10.1063/1.4993840}, abstractNote={A two-step homoepitaxial growth process producing step-free surfaces on low dislocation density, Ga-polar ammonothermal GaN single crystals is described. Growth is conducted under very low supersaturation conditions where adatom incorporation occurs predominantly at step edges, and lateral growth is strongly preferred. The achievable step-free area is limited by the substrate dislocation density. For ammonothermal crystals with an average dislocation density of ∼1 × 104 cm−2, step-free mesas up to 200 × 200 μm2 in size are achieved. These remarkable surfaces create a unique opportunity to study the effect of steps on the properties and performance of semiconductor heterostructures.}, number={9}, journal={APL MATERIALS}, author={Shelton, Christopher T. and Bryan, Isaac and Paisley, Elizabeth A. and Sachet, Edward and Ihlefeld, Jon F. and Lavrik, Nick and Collazo, Ramon and Sitar, Zlatko and Maria, Jon-Paul}, year={2017}, month={Sep} }
 @article{meyer_cheaito_paisley_shelton_braun_maria_ihlefeld_hopkins_2016, title={Crystalline coherence length effects on the thermal conductivity of MgO thin films}, volume={51}, ISSN={["1573-4803"]}, DOI={10.1007/s10853-016-0261-5}, number={23}, journal={JOURNAL OF MATERIALS SCIENCE}, author={Meyer, Kelsey E. and Cheaito, Ramez and Paisley, Elizabeth and Shelton, Christopher T. and Braun, Jeffrey L. and Maria, Jon-Paul and Ihlefeld, Jon F. and Hopkins, Patrick E.}, year={2016}, month={Dec}, pages={10408–10417} }
 @article{bryan_bryan_mita_rice_hussey_shelton_tweedie_maria_collazo_sitar_2016, title={The role of surface kinetics on composition and quality of AlGaN}, volume={451}, ISSN={["1873-5002"]}, DOI={10.1016/j.jcrysgro.2016.06.055}, abstractNote={Metal–polar, Al-rich AlGaN films were grown on both single crystalline AlN and sapphire substrates. The role of surface morphology and surface kinetics on AlGaN composition is presented. With the reduced dislocation density of the films grown on AlN substrates, atomically smooth bilayer stepped surfaces are achieved with RMS roughness of less than 50 pm for a 5×5 µm2 AFM scan area. By controlling the surface supersaturation through adjusting the growth rate, a transition from 2D nucleation to step flow was observed. The critical misorientation angle for step-bunching in nominal Al0.70Ga0.30N grown with a growth rate of 600 nm/h on AlN substrates was found to be 0.4°. The composition of bilayer stepped AlGaN was strongly dependent on substrate misorientation angle, where a compositional variation by a factor of two for a change in misorientation angle from 0.05 to 0.40° was observed; this is explained by the different surface diffusion lengths of Ga and Al. Step-bunching resulted in strong compositional inhomogeneity as observed by photoluminescence and scanning transmission electron microscopy studies.}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Bryan, Isaac and Bryan, Zachary and Mita, Seiji and Rice, Anthony and Hussey, Lindsay and Shelton, Christopher and Tweedie, James and Maria, Jon-Paul and Collazo, Ramon and Sitar, Zlatko}, year={2016}, month={Oct}, pages={65–71} }
 @article{sachet_shelton_harris_gaddy_irving_curtarolo_donovan_hopkins_sharma_sharma_et al._2015, title={Dysprosium-doped cadmium oxide as a gateway material for mid-infrared plasmonics}, volume={14}, ISSN={1476-1122 1476-4660}, url={http://dx.doi.org/10.1038/NMAT4203}, DOI={10.1038/nmat4203}, abstractNote={The interest in plasmonic technologies surrounds many emergent optoelectronic applications, such as plasmon lasers, transistors, sensors and information storage. Although plasmonic materials for ultraviolet-visible and near-infrared wavelengths have been found, the mid-infrared range remains a challenge to address: few known systems can achieve subwavelength optical confinement with low loss in this range. With a combination of experiments and ab initio modelling, here we demonstrate an extreme peak of electron mobility in Dy-doped CdO that is achieved through accurate 'defect equilibrium engineering'. In so doing, we create a tunable plasmon host that satisfies the criteria for mid-infrared spectrum plasmonics, and overcomes the losses seen in conventional plasmonic materials. In particular, extrinsic doping pins the CdO Fermi level above the conduction band minimum and it increases the formation energy of native oxygen vacancies, thus reducing their populations by several orders of magnitude. The substitutional lattice strain induced by Dy doping is sufficiently small, allowing mobility values around 500 cm(2) V(-1) s(-1) for carrier densities above 10(20) cm(-3). Our work shows that CdO:Dy is a model system for intrinsic and extrinsic manipulation of defects affecting electrical, optical and thermal properties, that oxide conductors are ideal candidates for plasmonic devices and that the defect engineering approach for property optimization is generally applicable to other conducting metal oxides.}, number={4}, journal={Nature Materials}, publisher={Springer Science and Business Media LLC}, author={Sachet, Edward and Shelton, Christopher T. and Harris, Joshua S. and Gaddy, Benjamin E. and Irving, Douglas L. and Curtarolo, Stefano and Donovan, Brian F. and Hopkins, Patrick E. and Sharma, Peter A. and Sharma, Ana Lima and et al.}, year={2015}, month={Feb}, pages={414–420} }
 @article{shelton_sachet_paisley_hoffmann_rajan_collazo_sitar_maria_2014, title={Polarity characterization by anomalous x-ray dispersion of ZnO films and GaN lateral polar structures}, volume={115}, ISSN={["1089-7550"]}, DOI={10.1063/1.4863120}, abstractNote={We demonstrate the use of anomalous x-ray scattering of constituent cations at their absorption edge, in a conventional Bragg-Brentano diffractometer, to measure absolutely and quantitatively the polar orientation and polarity fraction of unipolar and mixed polar wurtzitic crystals. In one set of experiments, the gradual transition between c+ and c− polarity of epitaxial ZnO films on sapphire as a function of MgO buffer layer thickness is monitored quantitatively, while in a second experiment, we map the polarity of a lateral polar homojunction in GaN. The dispersion measurements are compared with piezoforce microscopy images, and we demonstrate how x-ray dispersion and scanning probe methods can provide complementary information that can discriminate between polarity fractions at a material surface and polarity fractions averaged over the film bulk.}, number={4}, journal={JOURNAL OF APPLIED PHYSICS}, author={Shelton, Christopher T. and Sachet, Edward and Paisley, Elizabeth A. and Hoffmann, Marc P. and Rajan, Joseph and Collazo, Ramon and Sitar, Zlatko and Maria, Jon-Paul}, year={2014}, month={Jan} }
 @article{kirchhofer_diercks_gorman_ihlefeld_kotula_shelton_brennecka_2014, title={Quantifying Compositional Homogeneity in Pb(Zr,Ti)O-3 Using Atom Probe Tomography}, volume={97}, ISSN={["1551-2916"]}, DOI={10.1111/jace.13135}, abstractNote={Atom probe tomography (APT) is a powerful materials characterization technique capable of ppm chemical resolution and near atomic scale spatial resolution. However, owing to a number of factors, the technique has not been widely applied to insulating materials and even less to complex oxides. In this study, we outline the methodology necessary to obtain high‐quality results on a technologically relevant complex oxide Pb(Zr,Ti)O3 (or PZT) using laser‐assisted APT on both bulk and thin film specimens. We show how, with optimized and well‐controlled conditions, APT complements conventional techniques such as STEM‐EDS. The correlative information can be used to obtain the nanoscale 3‐D chemical information and investigate the nanoscale distribution of cations. Using nearest‐neighbor cluster analysis routines, 5–10 nm segregation of B‐site cations was detected in bulk sintered PZT 53/47 from chemically prepared powders. No statistically significant segregation of B‐site cations was observed in thin film specimens. This work opens new avenues toward understanding the process‐structure properties in complex materials at length scales heretofore unachievable.}, number={9}, journal={JOURNAL OF THE AMERICAN CERAMIC SOCIETY}, author={Kirchhofer, Rita and Diercks, David R. and Gorman, Brian P. and Ihlefeld, Jon F. and Kotula, Paul G. and Shelton, Christopher T. and Brennecka, Geoff L.}, year={2014}, month={Sep}, pages={2677–2697} }
 @article{paisley_gaddy_lebeau_shelton_biegalski_christen_losego_mita_collazo_sitar_et al._2014, title={Smooth cubic commensurate oxides on gallium nitride}, volume={115}, ISSN={["1089-7550"]}, DOI={10.1063/1.4861172}, abstractNote={Smooth, commensurate alloys of ⟨111⟩-oriented Mg0.52Ca0.48O (MCO) thin films are demonstrated on Ga-polar, c+ [0001]-oriented GaN by surfactant-assisted molecular beam epitaxy and pulsed laser deposition. These are unique examples of coherent cubic oxide|nitride interfaces with structural and morphological perfection. Metal-insulator-semiconductor capacitor structures were fabricated on n-type GaN. A comparison of leakage current density for conventional and surfactant-assisted growth reveals a nearly 100× reduction in leakage current density for the surfactant-assisted samples. HAADF-STEM images of the MCO|GaN interface show commensurate alignment of atomic planes with minimal defects due to lattice mismatch. STEM and DFT calculations show that GaN c/2 steps create incoherent boundaries in MCO over layers which manifest as two in-plane rotations and determine consequently the density of structural defects in otherwise coherent MCO. This new understanding of interfacial steps between HCP and FCC crystals identifies the steps needed to create globally defect-free heterostructures.}, number={6}, journal={JOURNAL OF APPLIED PHYSICS}, author={Paisley, Elizabeth A. and Gaddy, Benjamin E. and LeBeau, James M. and Shelton, Christopher T. and Biegalski, Michael D. and Christen, Hans M. and Losego, Mark D. and Mita, Seiji and Collazo, Ramon and Sitar, Zlatko and et al.}, year={2014}, month={Feb} }
 @article{rao_prater_wu_shelton_maria_narayan_2013, title={Interface Magnetism in Epitaxial BiFeO3-La0.7Sr0.3MnO3 Heterostructures Integrated on Si(100)}, volume={13}, ISSN={["1530-6992"]}, DOI={10.1021/nl4023435}, abstractNote={We report on the heteroepitaxial growth of ferroelectric (FE)-antiferromagnetic (AFM) BiFeO3 (BFO) on ferromagnetic La0.7Sr0.3MnO3 (LSMO), integrated on Si(100) using pulsed laser deposition via the domain matching epitaxy paradigm. The BFO/LSMO films were epitaxially grown on Si(100) by introducing epitaxial layers of SrTiO3/MgO/TiN. X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, X-ray photo absorption spectroscopy, and atomic force microscopy were employed to fully characterize the samples. Furthermore, we have investigated the magnetic behavior of this five layer heterostructure, in which a d(5) system (Fe(3+)) manifested in FE-AFM BFO is epitaxially conjoined at the interface to a multivalent transition metal ion such as Mn(3+)/Mn(4+) in LSMO. The temperature- and magnetic field-dependent magnetization measurements reveal an unexpected enhancement in magnetic moment and improved magnetic hysteresis squareness originating from the BFO/LSMO interface. We observe a stronger temperature dependence of HEB when the polarity of field cooling is negative as compared to positive field cooling. We believe such an enhancement in magnetic moment and magnetic coupling is likely directly related to an electronic orbital reconstruction at the interface and complex interplay between orbital and spin degrees of freedom, similar to what has previously been reported in the literature. Future work will involve the linearly polarized X-ray absorption measurements to prove this hypothesis. This work represents a starting step toward the realization of magneto-electronic devices integrated with Si(100).}, number={12}, journal={NANO LETTERS}, author={Rao, S. S. and Prater, J. T. and Wu, Fan and Shelton, C. T. and Maria, J. -P. and Narayan, J.}, year={2013}, month={Dec}, pages={5814–5821} }
 @article{shelton_kotula_brennecka_lam_meyer_maria_gibbons_ihlefeld_2012, title={Chemically Homogeneous Complex Oxide Thin Films Via Improved Substrate Metallization}, volume={22}, ISSN={["1616-301X"]}, DOI={10.1002/adfm.201103077}, abstractNote={Abstract}, number={11}, journal={ADVANCED FUNCTIONAL MATERIALS}, author={Shelton, Christopher T. and Kotula, Paul G. and Brennecka, Geoff L. and Lam, Peter G. and Meyer, Kelsey E. and Maria, Jon-Paul and Gibbons, Brady J. and Ihlefeld, Jon F.}, year={2012}, month={Jun}, pages={2295–2302} }
 @article{paisley_shelton_mita_collazo_christen_sitar_biegalski_maria_2012, title={Surfactant assisted growth of MgO films on GaN}, volume={101}, ISSN={["0003-6951"]}, DOI={10.1063/1.4748886}, abstractNote={Thin epitaxial films of 〈111〉 oriented MgO on [0001]-oriented GaN were grown by molecular beam epitaxy and pulsed laser deposition using the assistance of a vapor phase surfactant. In both cases, surfactant incorporation enabled layer-by-layer growth and a smooth terminal surface by stabilizing the {111} rocksalt facet. Metal-insulator-semiconductor capacitor structures were fabricated on n-type GaN. A comparison of leakage current density for conventional and surfactant-assisted growth reveals a nearly 100× reduction in leakage current density for the surfactant-assisted samples. These data verify numerous predictions regarding the role of H-termination in regulating the habit of rocksalt crystals.}, number={9}, journal={APPLIED PHYSICS LETTERS}, author={Paisley, E. A. and Shelton, T. C. and Mita, S. and Collazo, R. and Christen, H. M. and Sitar, Z. and Biegalski, M. D. and Maria, J. -P.}, year={2012}, month={Aug} }