Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session Y37a: Dielectric and Ferroelectric Oxides VIII |
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Sponsoring Units: DMP Chair: Alexei Gruverman, University of Nebraska, Lincoln Room: 383 |
Friday, March 17, 2017 11:15AM - 11:27AM |
Y37a.00001: Model for Charge Injection with Electron Beams into Highly Disordered Insulating Materials JR Dennison The Walden-Wintle model for charge injection and transport through highly disordered insulating materials has been extended to include charge injection with a charged particle beam. The original model is applicable to charge injection into a dielectric material from a pair of electrodes in a parallel-plate geometry. It provides a versatile approach to predict the time-dependent current at a rear grounded electrode and the incident surface voltage, as the injection current density evolves over time with the development of a space charge barrier near the injection electrode. The Walden-Wintle model has been applied to many standard cases including Fowler-Nordheim injection, Schottky injection, space charge limited injection, and various tunneling mechanisms. The present model modifies the approach to include electrode-less charge injection via a charged particle beam, along with concomitant effects for the injection current, surface voltage, and electron emission as a charge is built up in the insulator. The approach is equally valid for near-surface injection and for bulk injection of both non-penetrating and penetrating radiation. The results are based on our dynamic emission model for electron emission yields dependent on accumulating charge in both the positive and negative charging regimes. [Preview Abstract] |
Friday, March 17, 2017 11:27AM - 11:39AM |
Y37a.00002: Tip-based mechanical force effects on ferroelastic twin structures in epitaxial WO$_{3}$ thin films Shinhee Yun, Chang-Su Woo, Gi-Yeop Kim, Pankaj Sharma, Jin-Hong Lee, Kanghyun Chu, Jong Hyun Song, Sung-Yoon Chung, Jan Seidel, Si-Young Choi, Chan-Ho Yang A-site vacant perovskite WO$_{3}$ structure varies diversely depending on the temperature due to the typical phonon modes softening, which seems to be greatly related with empty A-sites. In addition, this empty structure allows rotations of WO$_{3}$ octahedra and distortions of linear W-O-W links instantaneously by external mechanical force. In this context, reorientations of single crystal WO$_{3}$ twin domains by pressure have been reported. In this presentation, we report on changes in domain structure and instantaneous piezo-response signal changes in epitaxial WO$_{3}$ thin films induced by tip-based shear stresses. For the experiments, we synthesized high quality epitaxial WO$_{3}$ thin films on YAlO$_{3\, }$substrates and characterized its crystal structure. The WO$_{3}$ films have ferroelastic twin structures consisting of four fold monoclinc building blocks which are subject to cooperative mosaic rotations. Ferroelastic twin structures have hierarchical domains (fine-, macro-, and super-macro-domains) following power law between film thickness and domain width. We also found that the domain structure can be changed by an injection of electron beam. [Preview Abstract] |
Friday, March 17, 2017 11:39AM - 11:51AM |
Y37a.00003: Fabrication of graded index single crystal (GRISC) in glass Keith Veenhuizen, Sean McAnany, Daniel Nolan, Bruce Aitken, Volkmar Dierolf, Himanshu Jain Spatially selective femtosecond laser induced crystallization of glass offers a compelling opportunity to expand integrated optics into dense, 3D geometries. Compared to polycrystalline lines, laser induced single crystal waveguides possess lower scattering losses, but they still suffer from losses at the crystal-glass interface due to the step-index profile and non-uniform interface. To correct this problem, we attempted to control the crystal growth dynamics and create waveguides with better transmission. We tuned the laser scanning speed and power to control lithium niobate crystal growth in 35Li2O-35Nb2O5-30SiO2 glass, such that nucleation and growth occur upon heating and ahead of the laser focus. This growth mode was verified via electron backscatter diffraction measurements of crystal cross-sections, which exhibited a symmetric, gradually varying lattice misorientation with respect to the c-axis orientation in the center. Theoretical simulations predict that such misorientation would decrease the refractive index of the crystal line from the center moving outwards. This graded refractive index single crystal waveguide in glass would improve transmission by a reduction in scattering at the crystal-glass interface due to tighter optical confinement in the crystal core. [Preview Abstract] |
Friday, March 17, 2017 11:51AM - 12:03PM |
Y37a.00004: New metastable (Hf,Zr)O$_2$ phases: are the traditional structure prediction methods good enough? Sergey Barabash The recent discovery of ferroelectric and antiferroelectric behavior in (Hf,Zr)O$_2$-based films[1] has further highlighted the paramount industrial importance of stabilizing metastable phases in (Hf,Zr)O$_2$, already sought for the outstanding dielectric properties. Potentially, together with the phases that are also observed at high temperatures or in strongly “doped'' materials, yet-unknown phases may also be stabilized. In fact, prior theoretical studies[2,3] have predicted several such possible new phases. Here we demonstrate that there is, in fact, a much larger number of potential metastable (Hf,Zr)O$_2$ phases, including several yet-unknown phases exhibiting ferroelectric properties. Many of the phases predicted here have quite low formation energies, yet were missed in previous studies. We discuss the challenges encountered by the earlier structure searches in (Hf,Zr)O$_2$, and offer an alternative methodology more appropriate to predict possible metastable structures in such systems. [1] M.H. Park et al., Adv.Mater. {\bf{27}}, 1811 (2015), and references therein. [2] T.D. Huan et al., Phys. Rev.B {\bf{90}}, 064111 (2014). [3] Q.Zeng et al., Acta Cryst. C {\bf{70}}, 76 (2014). [Preview Abstract] |
Friday, March 17, 2017 12:03PM - 12:15PM |
Y37a.00005: Investigation of local and integral polarization switching behavior of ultrathin HfO$_{2-}$based films Pratyush Buragohain, Ohheum Bak, Anna Chernikova, Andrei Zenkevich, Uwe Schroeder, Terence Mittman, Franz Fengler, Haidong Lu, Alexei Gruverman The discovery of ferroelectricity in hafnium oxide (HfO$_{2})$ based thin films represents an important step forward in ferroelectric based memory devices as they have several advantages over conventional perovskite based ferroelectrics like easy integration with existing Silicon technology, ease of fabrication and large band gap. Here, we report Piezoresponse Force Microscopy (PFM) investigation of the ferroelectric switching behavior in ultrathin Hf$_{0.5}$Zr$_{0.5}$O$_{2}$ (HZO) structures with the HZO thickness range from 30 nm down to 3 nm and sub-\textmu m lateral dimensions. A `Positive Up Negative Down' (PUND) technique was used to investigate the switching dynamics down on a 10-ns time scale. We also measured the electroresistance effect and the R$_{OFF}$/R$_{ON}$ was found to be in the range of up to 15. The obtained results make ultrathin HfO$_{2}$-based films an attractive candidate for application in ferroelectric tunnel junctions as non-volatile, low-power memory devices. [Preview Abstract] |
Friday, March 17, 2017 12:15PM - 12:27PM |
Y37a.00006: Effect of oxygen vacancies and strain on the phonon spectrum of HfO$_{\mathrm{2}}$ Lingyuan Gao, Eilam Yalon, Annabel Chew, Eric Pop, Alex Demkov The effect of strain and oxygen deficiency on the Raman spectrum of monoclinic HfO$_{\mathrm{2}}$ is investigated theoretically using first-principle calculations. In-plane compressive strain is found to blue shift the phonon frequencies, while tensile strain does the opposite. The simulations are compared to and good agreement is found with experimental results of Raman frequencies greater than 110 cm$^{\mathrm{-1}}$. Several Raman modes measured below 110 cm$^{\mathrm{-1}}$ and previously assigned to HfO$_{\mathrm{2}}$ cannot be assigned to HfO$_{\mathrm{2}}$. However, localized vibrational modes introduced by threefold-coordinated oxygen (O$_{\mathrm{3}})$ vacancies are identified at 96.4 cm$^{\mathrm{-1}}$ These results are important for a deeper understanding of vibrational modes in HfO$_{\mathrm{2}}$, which has technological applications in transistors, and particularly in resistive random-access memory (RRAM) whose operation relies on oxygen-deficient HfO$_{\mathrm{x}}$ [Preview Abstract] |
Friday, March 17, 2017 12:27PM - 12:39PM |
Y37a.00007: Stability of metallic single-electron devices with plasma oxidized, cobalt confined AlO$_{\mathrm{x}}$ tunnel barriers Y. X. Hong, Zac Barcikowski, A. N. Ramanayaka, Roy Murray, M. D. Stewart, Jr., Neil M. Zimmerman, J. M. Pomeroy Single-electron transistors (SETs) are fabricated by double-angle deposition in order to measure the electrical stability in the plasma oxidized, cobalt confined aluminum oxide (AlO$_{\mathrm{x}})$ barriers, as an estimate of the two-level fluctuator density compared with thermal oxidation. The electrical stability of metal-based SETs and superconducting devices suffers from oxide-induced high defect densities and long-term charge offset drift. These devices are typically made with thermal AlO$_{\mathrm{x}}$. Our tunnel junctions have two physical differences from those of thermal oxides: 1) plasma oxidation is shown to be more uniform and stoichiometric for AlO$_{\mathrm{x}}$ than thermal oxidation; and 2) high oxygen content is confined within the insulating regions by using a Co/AlO$_{\mathrm{x}}$/Co structure to provide a barrier against oxygen diffusion. In our prior work with large-area tunnel junctions, these AlO$_{\mathrm{x}}$ barriers sandwiched between cobalt layers exhibit better long-term resistance stability. In this work, we are developing SETs by double-angle deposition as a path toward low-capacitance and small-area Co/AlO$_{\mathrm{x}}$/Co tunnel junctions. We expect better charge offset stability on these devices than typical thermally oxidized devices with unconfined oxygen. [Preview Abstract] |
Friday, March 17, 2017 12:39PM - 12:51PM |
Y37a.00008: Lattice Thermal Conductivity in $\varepsilon$-Ga$_2$O$_3$ Michael Mehl, N. Neepa, V. D. Wheeler, D. J. Meyer $\beta$-Ga$_2$O$_3$ has seen increased popularity as a substrate and device material because of its large band gap and theoretical breakdown field, but it suffers from low thermal conductivity ($\kappa$). The question arises whether other polytypes of Ga$_2$O$_3$ might have higher thermal conductivity along with an ultra-wide band gap. One potential phase is is $\varepsilon$-Ga$_2$O$_3$, which has a large band gap (4.9 eV) and a wurtzite-like crystal structure. Unfortunately, the $\varepsilon$ phase is difficult to model from first principles, as several of the Ga Wyckoff positions are only partially occupied. In this talk we examine several structures which approximate $\varepsilon$-Ga$_2$O$_3$. For these structures we then calculate the lattice contribution to the bulk thermal conductivity tensor $\kappa$ by computing second- and third-order force constants. We compare our results with experimental and theoretical data for $\beta$-Ga$_2$O$_3$. [Preview Abstract] |
Friday, March 17, 2017 12:51PM - 1:03PM |
Y37a.00009: Magnetochromic Sensing in Iron Oxide Nanoparticles Kenneth O'Neal, Jonathan Patete, Peng Chen, Ruhani Nanavati, Brian Holinsworth, Jacqueline Smith, Carlos Marques, Jack Simonson, Meigan Aronson, Steve McGill, Stanislaus Wong, Janice Musfeldt We combine optical and magneto-optical spectroscopies with complementary vibrational and magnetic property measurements to reveal finite length scale effects in nanoscale $\alpha $-Fe$_{\mathrm{2}}$O$_{\mathrm{3}}$. Analysis of the d-to-d on-site excitations uncovers enhanced color contrast at particle sizes below approximately 75 nm due to size-induced changes in spin-charge coupling that are suppressed again below the super-paramagnetic limit. These findings provide a general strategy for amplifying magnetochromism in $\alpha $ -Fe$_{\mathrm{2}}$O$_{\mathrm{3}}$ and other iron-containing nano-materials that may be useful for advanced sensing applications. [Preview Abstract] |
Friday, March 17, 2017 1:03PM - 1:15PM |
Y37a.00010: Atomistic-Scale Mechanism of Electrical Dipole Switching in Phosphosilicate Glass Zhi Song, Xiao Shen Non-volatile memories based on the switching of electrical polarization in ferroelectric materials are among the candidates for future information storage. However, the fabrication of devices using typical ferroelectric materials requires exotic substrates and thus is incompatible with standard CMOS technology. Certain glassy materials such as phosphosilicate glass (PSG) are known to exhibit switchable electrical polarization similar to ferroelectric materials, while can be fabricated from thermal SiO2 on a standard wafer. However, the underlying mechanism of the switching of polarization in PSG is yet to be understood. Here we present results from first-principles calculations unveiling the atomistic-scale mechanism of electrical dipole switching in PSG. The results also provide guidance to enhance the switching speed and retention time. [Preview Abstract] |
Friday, March 17, 2017 1:15PM - 1:27PM |
Y37a.00011: Influence of electro-chemo-mechanical coupling on the metal-insulator transition in VO2 Janakiraman Balachandran, Panchapakesan Ganesh, Jaron Krogel, Ho Nyung Lee, Paul Kent, Olle Heinonen VO$_{2\, }$is a strongly correlated electronic system that exhibits polymorphic phases, with different electronic and optical properties. It's a classic example where fine electro-chemo-mechanical coupling between the electronic, spin, lattice and orbital degrees of freedom can be exploited to give rise to new emergent functionalities. The ability to grow thin-films of these materials and control its phases by applying external fields has made this a reality. In this talk we will present computational studies aimed at understanding phase-transitions in VO$_{2}$ induced by external mechanical fields. Our studies use wide range of beyond-DFT methods for correlated electronic structures, such as DFT$+$U, hybrid-DFT etc. and we will compare our results with those from quantum monte carlo (QMC) methods, and also relate them to experimental work. Our findings provide a basis for understanding influence of electro-chemo-mechanical coupling on metal-insulator transition in strongly correlated electronic oxides such as vanadium dioxide. * This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division and also as part of the Computational Materials Sciences Program. [Preview Abstract] |
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