@article{gaddy_paisley_maria_irving_2014, title={Overcoming the polarization catastrophe in the rocksalt oxides MgO and CaO}, volume={90}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.90.125403}, abstractNote={Interfaces between dissimilar polar materials may provide a pathway to new device functionality, including high carrier mobility layers at the interface. The development of these materials has proven challenging, in part because of the high energy cost of forming polar surfaces. Our density functional theory calculations explore the mechanisms by which a real material satisfies the electrostatic criteria for stability imposed by a polar surface. The consequences of polarity are studied by comparing the formation energies, charge distribution, and electronic structure of a number of low-index surfaces of rocksalt MgO and CaO. These surfaces are explored both in their bare, undecorated form as well as with surface reconstructions and adsorbed foreign species. Our ground-state surface energies are extended to relevant environmental conditions by use of ab initio thermodynamics. We find that the high energy of bare polar surfaces is the result of the significant charge redistribution that arises to compensate the polarity and pushes electronic states into the forbidden band gap. Other mechanisms of polarity compensation (reconstruction or foreign species adsorption) are therefore seen more frequently. We explain the experimental observations of surface roughness during growth in the [111] direction. In typical epitaxial growth conditions, there is preferential formation of an octopolar reconstruction of the {111} surface, which exposes {001}-type nanofacets. The low energy of the {001} surface likely causes these facets to grow, leading to a rough surface morphology. Our results indicate that when water vapor is present during growth, a smooth, polar surface can be stabilized by the formation of a hydroxyl layer.}, number={12}, journal={PHYSICAL REVIEW B}, author={Gaddy, Benjamin E. and Paisley, Elizabeth A. and Maria, Jon-Paul and Irving, Douglas L.}, year={2014}, month={Sep} }
 @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{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{paisley_craft_losego_lu_gruverman_collazo_sitar_maria_2013, title={Epitaxial PbxZr1-xTiO3 on GaN}, volume={113}, ISSN={["1089-7550"]}, DOI={10.1063/1.4792599}, abstractNote={Epitaxial integration of PbxZr1−xTiO3 (PZT) (111) with GaN (0002) presents the possibility of polarity coupling across a functional-oxide/nitride heteropolar interface. This work describes the synthesis and characterization of such thin film heterostructures by magnetron sputtering, with specific attention given to process optimization. Using x-ray diffraction and electrical characterization, the growth of epitaxial PZT (∼250 nm) on GaN and PZT on MgO/GaN stacks was verified. A two-stage growth process was developed for epitaxial PZT with a deposition temperature of 300 °C and an ex-situ anneal at 650 °C, which was effective in mitigating interfacial reactions and promoting phase-pure perovskite growth. Electrical analysis of interdigital capacitors revealed a nonlinear and hysteretic dielectric response consistent with ferroelectric PZT. Piezoresponse force microscopy (PFM) characterization shows clear evidence of ferroelectric switching, and PFM hysteresis loop analysis shows minimal evidence for direct polarity coupling, but suggests that band offsets which accompany the oxide-nitride heterostructures influence switching.}, number={7}, journal={JOURNAL OF APPLIED PHYSICS}, author={Paisley, E. A. and Craft, H. S. and Losego, M. D. and Lu, H. and Gruverman, A. and Collazo, R. and Sitar, Z. and Maria, J. -P.}, year={2013}, month={Feb} }
 @article{paisley_craft_losego_lu_gruverman_collazo_sitar_maria_2013, title={Epitaxial lead zirconate titanate on gallium nitride (vol 113, 074107, 2013)}, volume={114}, number={23}, journal={Journal of Applied Physics}, author={Paisley, E. A. and Craft, H. S. and Losego, M. D. and Lu, H. and Gruverman, A. and Collazo, R. and Sitar, Z. and Maria, J. P.}, year={2013} }
 @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} }
 @article{paisley_losego_gaddy_tweedie_collazo_sitar_irving_maria_2011, title={Surfactant-enabled epitaxy through control of growth mode with chemical boundary conditions}, volume={2}, ISSN={["2041-1723"]}, DOI={10.1038/ncomms1470}, abstractNote={Property coupling at interfaces between active materials is a rich source of functionality, if defect densities are low, interfaces are smooth and the microstructure is featureless. Conventional synthesis techniques generally fail to achieve this when materials have highly dissimilar structure, symmetry and bond type-precisely when the potential for property engineering is most pronounced. Here we present a general synthesis methodology, involving systematic control of the chemical boundary conditions in situ, by which the crystal habit, and thus growth mode, can be actively engineered. In so doing, we establish the capability for layer-by-layer deposition in systems that otherwise default to island formation and grainy morphology. This technique is demonstrated via atomically smooth {111} calcium oxide films on (0001) gallium nitride. The operative surfactant-based mechanism is verified by temperature-dependent predictions from ab initio thermodynamic calculations. Calcium oxide films with smooth morphology exhibit a three order of magnitude enhancement of insulation resistance.}, journal={NATURE COMMUNICATIONS}, author={Paisley, Elizabeth A. and Losego, Mark. D. and Gaddy, Benjamin E. and Tweedie, James S. and Collazo, Ramon and Sitar, Zlatko and Irving, Douglas L. and Maria, Jon-Paul}, year={2011}, month={Sep} }
 @article{losego_craft_paisley_mita_collazo_sitar_maria_2010, title={Critical examination of growth rate for magnesium oxide (MgO) thin films deposited by molecular beam epitaxy with a molecular oxygen flux}, volume={25}, ISSN={["0884-2914"]}, DOI={10.1557/jmr.2010.0096}, abstractNote={The authors report a study of molecular beam deposition of MgO films on amorphous SiO2 and (0001) GaN surfaces over a large range of temperatures (25–400 °C) and molecular oxygen growth pressures (10−7–10−4 Torr). This study provides insight into the growth behavior of an oxide with volatile metal constituents. Unlike other materials containing volatile constituents (e.g., GaAs, PbTiO3), all components of MgO become volatile at normal epitaxial growth temperatures (≥250 °C). Consequently, defining which species is the adsorption controller becomes ambiguous. Different growth regimes are delineated by the critical substrate temperature for Mg re-evaporation and the Mg:O flux ratio. These regimes have impact on phase purity, quartz crystal microbalance calibration, and film microstructure. The universal decay in deposition rate above growth 10−5 Torr O2 is also considered. By introducing a third flux of inert argon gas, rate reduction is attributed to increased molecular scattering and not oxidation of the metal source.}, number={4}, journal={JOURNAL OF MATERIALS RESEARCH}, author={Losego, Mark D. and Craft, H. Spalding and Paisley, Elizabeth A. and Mita, Seiji and Collazo, Ramon and Sitar, Zlatko and Maria, Jon-Paul}, year={2010}, month={Apr}, pages={670–679} }
 @article{paisley_losego_aygun_craft_maria_2008, title={Barrier layer mechanism engineering in calcium copper titanate thin film capacitors through microstructure control}, volume={104}, ISSN={["1089-7550"]}, DOI={10.1063/1.3033166}, abstractNote={A peak permittivity greater than 10 000 has been achieved for calcium copper titanate (CCT) thin films by engineering a thin film microstructure that maximizes space charge contributions to polarizability. This permittivity is an order of magnitude greater than previous polycrystalline thin film efforts. This unique microstructure control is accomplished using a chemical solution deposition process flow that produces highly dense parallel layers ∼100 nm in thickness. We observe a thickness dependent permittivity where the entire film thickness constitutes the conducting region of a barrier layer capacitor despite the presence of multiple grain boundaries within that thickness. The model predictions are in good agreement with experimental data and are consistent with existing literature reports. These trends in permittivity with dielectric thickness raise new questions regarding the nature of barrier layers in CCT—and specifically, these results suggest that grain boundaries may not always participate as high resistance interlayers.}, number={11}, journal={JOURNAL OF APPLIED PHYSICS}, author={Paisley, E. A. and Losego, M. D. and Aygun, S. M. and Craft, H. S. and Maria, J. -P.}, year={2008}, month={Dec} }