Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session L14: Insulators and Dielectrics: Properties and Defects |
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Sponsoring Units: DCMP Chair: Joshua Pomeroy, NIST Room: 273 |
Wednesday, March 15, 2017 11:15AM - 11:27AM |
L14.00001: The complexity of the CaF$_2$:Yb system: evidence that CaF$_2$:Yb2+ is not an impurity trapped exciton system Cameron MacKeen, Frank Bridges, Michael Kozina, Apurva Mehta, M. F. Reid, J.-P. R. Wells, Zoila Barandiarán Fluorite crystal structures doped with rare-earth elements exhibit an anomalous redshifted luminescence upon UV excitation, generally attributed to the relaxation of impurity trapped excitons (ITE). We find that the intensity of this luminescence decreases as the total concentration of Yb 2+ increases in unexposed samples, which is in conflict with the currently accepted ITE model. Further, using x-ray absorption spectroscopy and UV-vis studies of CaF$_2$:Yb, we find a large (but reversible) Yb valence reduction upon x-ray exposure at 200 K - from mostly 3+ to 2+. This valence reduction is stable for long time periods at low T < 50 K, but reverts to the initial state upon warming to 300 K. After reverting to the initial valence state of 3+ the anomalous luminescence does not reappear; only after annealing at 900 K do we again observe the anomalous emission below 150 K. To explore the mechanism at work, we employ extended x-ray fine-structure absorption spectroscopy (EXAFS) to probe local structure and its role in the anomalous luminescence. The x-ray and emission studies show that CaF$_2$:Yb is not described by the ITE model; the data appear more consistent with an intervalence charge transfer (IVCT) model. It is likely that many similar ITE systems have also been misidentified. [Preview Abstract] |
Wednesday, March 15, 2017 11:27AM - 11:39AM |
L14.00002: Chemical and Physical Phenomenon Underlying Novel Low Temperature Pyrochlore Nanoparticle Synthesis Madhab Pokhrel, Kareem Wahid, Yuanbing Mao A$_{\mathrm{2}}$B$_{\mathrm{2}}$O$_{\mathrm{7}}$ compounds have attracted considerable attention due to their potential applications in radiation-based environments. These compounds can crystallize in two forms, the ordered pyrochlore or disordered fluorite phase, which may alter their physical properties. While it is well known that A$_{\mathrm{2}}$B$_{\mathrm{2}}$O$_{\mathrm{7}}$ compounds can be produced by a variety of synthesis methods, the molten salt method is particularly attractive due to its low temperature of synthesis, low cost, and ease of fabrication. In this study, we demonstrate that homogenous co-precipitation coupled to molten salt synthesis leads to the formation of nano-sized pyrochlore phase RE$_{\mathrm{2}}$Hf$_{\mathrm{2}}$O$_{\mathrm{7}}$ (RE $=$ La, Pr) at temperatures (650$^{\circ}$ C) unobtainable by conventional solid-state synthesis of hydroxide precursors. This is compared with other RE$_{\mathrm{2}}$Hf$_{\mathrm{2}}$O$_{\mathrm{7}}$ compositions (RE $=$ Y, Gd, Er, Lu) that crystalize as disordered fluorites via both synthesis methods. A suite of characterization techniques, including X-ray diffraction, Raman spectroscopy, and scanning electron microscopy were employed to investigate the structural evolution of these nanoparticles. A mechanism has been proposed that elucidates the differences in low temperature synthesis of ordered pyrochlores when compared to disordered fluorite materials. [Preview Abstract] |
Wednesday, March 15, 2017 11:39AM - 11:51AM |
L14.00003: Comparison of Tunnel Barrier Thickness Determined by ARXPS and WKB Transport Fit Methods Zac Barcikowski, Y. X. Hong, J. M. Pomeroy Tunnel junction devices are fabricated with ultrathin AlOx tunnel barriers and characterized using in-situ angular resolved X-ray photoemission spectroscopy (ARXPS) and ex-situ transport measurements. In the ultrathin limit, accurate measurement of thin film thickness remains extremely difficult. We derive the thickness from ARXPS and WKB transport fitting results, which provide two indirect, yet fundamentally different methods of estimating the tunnel barrier thickness. We compare the results of both methods using several Co/AlOx/Co tunnel junctions with different tunnel barrier thicknesses. For example, data of a 1.1 nm Al film plasma oxidized for 7 s in 160 mtorr O$_{\mathrm{2}}$ yields a tunnel barrier thickness of 1.3 nm with WKB fitting and 1.5 nm with ARXPS. Tunnel barrier composition data and thickness (s) estimations obtained by ARXPS are compared with tunnel barrier height ($\Phi )$ and width (s) obtained by WKB transport fitting. [Preview Abstract] |
Wednesday, March 15, 2017 11:51AM - 12:03PM |
L14.00004: Anisotropic magnetoresistance and tunneling magnetoresistance of conducting filaments in NiO with different resistance states Diyang Zhao, Shuang Qiao, Yuxiang Luo, Aitian Chen, Pengfei Zhang, Ping Zheng, Zhong Sun, Minghua Guo, F. -K. Chiang, Jian Wu, Jianlin Luo, Jianqi Li, Yayu Wang, Yonggang Zhao Resistive switching (RS) effect in conductor/insulator/conductor thin-film stacks has attracted much attention due to its interesting physics and potentials for applications. NiO is one of the most representative systems and its RS effect has been generally explained by the formation and rupture of Ni related conducting filaments, which are very unique since they are formed by electric forming process. We study the MR behaviors in NiO RS films with different resistance states. Rich and interesting MR behaviors were observed, including the normal and anomalous anisotropic magnetoresistance (AMR) and tunneling magnetoresistance (TMR), etc., which provide new insights into the nature of the filaments and their evolution in the resistive switching process. First-principles calculation reveals the essential role of oxygen migration into the filaments during the RESET process and can account for the experimental results. Our work provides a new avenue for the exploration of the conducting filaments in RS materials, and is significant for understanding the RS mechanism as well as multifunctional device design. [Preview Abstract] |
Wednesday, March 15, 2017 12:03PM - 12:15PM |
L14.00005: Diffusion of Vacancies in 4H-SiC Rodrick Kuate Defo, Xingyu Zhang, David Bracher, Evelyn Hu, Efthimios Kaxiras Defect centers in silicon carbide (SiC) have emerged as strong contenders in the quest to realize quantum devices due to the material’s lower cost as compared to its counterpart diamond, and due to the microfabrication techniques now available and favorable optical emission wavelengths and spin properties. We investigate the stability and diffusion of silicon vacancies of varying charge states in 4H-SiC and under the influence of strain, using density-functional-theory, which should serve as an invaluable guide in controlling defect position within devices. [Preview Abstract] |
Wednesday, March 15, 2017 12:15PM - 12:27PM |
L14.00006: Photoactive defect centers in photorefractive Sn$_{\mathrm{2}}$P$_{\mathrm{2}}$S$_{\mathrm{6}}$. Sergey Basun, Larry Halliburton, Nancy Giles, Alexander Grabar, Dean Evans A variety of intrinsic and extrinsic defect centers were identified through photo-EPR studies of photorefractive Sn$_{\mathrm{2}}$P$_{\mathrm{2}}$S$_{\mathrm{6}}$ crystals: tin and sulfur vacancies, intrinsic small hole polarons (Sn$^{\mathrm{3+}}$ ions), dopant antimony ions on tin sites -- isolated (with no nearby perturbing defects) and with a charge-compensating tin vacancy at a nearest-neighbor tin site. All these centers are photo-rechargeable: their charge states alter under optical irradiation (for example, optical irradiation causes conversion of Sb$^{\mathrm{3+}}$ to Sb$^{\mathrm{2+}})$, so that they can serve as donors or traps of free charge carriers thus providing photo-refraction. The potential of the photo-rechargeable defect centers for photorefraction applications was evaluated through the measurements of the spectral range where photo-recharging occurs, the thermal stability of the photo-recharged states, the kinetic characters and activation energies of thermal relaxation (``general order'' kinetics with b$=$1.68 and activation energy of 0.28 eV for singly ionized tin vacancy; pure second order kinetics (b$=$2) or, in other words, hyperbolic decay 1/(1$+$t/$\tau )$ and activation energy 0.42 eV for isolated Sb$^{\mathrm{2+}}$ ions). [Preview Abstract] |
Wednesday, March 15, 2017 12:27PM - 12:39PM |
L14.00007: Absorption and Enhanced Photorefractive Beam Fanning in Sn2P2S6:Sb Dean Evans, Sergey Basun, Larry Halliburton, Alexander Grabar, Yaroslav Skrypka, Alexandr Shumelyuk, Serguey Odoulov In this work, we discuss new effects in ferroelectric/photorefractive Sn2P2S6:Sb (SPS:Sb) that are explored in great depth in order to obtain a fundamental understanding of the photo-induced absorption and enhanced photorefractive beam fanning dynamics, spanning ten orders of magnitude in time (ns to sec). Both temperature and intensity dependence studies are conducted to assist in identifying which processes are thermally driven vs. optically driven, and we compare undoped and Sb doped crystal to determine which of the processes are the result of intrinsic vs. extrinsic defects. Through the use of electron paramagnetic resonance (EPR) measurements, the observed optical processes are assigned to specific defects. Furthermore, both optical and photo-EPR results are used to determine the activation energies associated with the effects (i.e. photo-induced absorption decay and beam fanning build up and decay). Finally, models/energy level diagrams are determined for the photo-induced absorption and the beam fanning transient enhancement effects; these optical effects are assigned to the observed intrinsic and extrinsic defects. [Preview Abstract] |
Wednesday, March 15, 2017 12:39PM - 12:51PM |
L14.00008: Probing the oxygen vacancy related defect states in HfO$_{\mathrm{2}}$ gate dielectric using DLTS Arvind Kumar, Sandip Mondal, KSRK Rao Large numbers of theoretical works are present to understand the oxygen vacancy related traps (V$_{\mathrm{O}})$ in technologically relevant HfO$_{\mathrm{2}}$.$_{\mathrm{\thinspace }}$Most of these calculations are based on the atomistic models and typically ideal, hence, cannot provide the realistic device parameters. Here, we applied the Deep Level Transient Spectroscopy (DLTS): a sensitive tool; to study the deep level defects presents in high-k HfO$_{\mathrm{2}}$. DLTS signal is originating from the segregated oxygen vacancy at the interface, tunneling/hopping between bulk and interface traps and communication of interface defects with respective bands. We observed four prominent deep levels; the estimated activation energies are Ec$_{\mathrm{\thinspace }}$- 1.22, Ec - 1.36, Ec - 1.76 and Ec$_{\mathrm{\thinspace }}$- 2.03 eV, below the HfO$_{\mathrm{2}}$ conduction band. These trap states are related to V$_{\mathrm{O}}$ in different charge states as suggested theoretically. We also measured the capture cross-sections ($\sigma )$ of these states and found it is quite low (\textasciitilde 10$^{\mathrm{-18}}$ - 10$^{\mathrm{-21\thinspace }}$cm$^{\mathrm{2}})$, which indicate a negligible influence on the device performance. This study will be helpful to disclose the numerous fascinating facts witnessed in HfO$_{\mathrm{2}}$. [Preview Abstract] |
Wednesday, March 15, 2017 12:51PM - 1:03PM |
L14.00009: Effects of oxygen vacancy on the photoconductivity in BaSnO3 Jisung Park, Kookrin Char We have found the photoconductive behavior of BaSnO3, especially their magnitude and time dependence, is very sensitive to the oxygen vacancy concentration. We made epitaxial BaSnO3 film with BaHfO3 buffer layer by pulsed laser deposition. As we had reported before, MgO substrate with its large band gap size about 7.8 eV was used to exclude any photoconductance from the substrate. [1] BaHfO3 layer was used to reduce the threading dislocation density in BaSnO3 film. To control the oxygen vacancy concentration in the BaSnO3 film, we annealed the sample in Ar or O2 atmosphere with varying annealing conditions. After each annealing process, photoconductivity of BaSnO3 was measured during illumination of UV light. The result showed that the magnitude of photoconductivity of BaSnO3 increased after annealing at higher temperature in Ar atmosphere, while the changes in the dark current remains minimal. The result can be explained by a hole trap mechanism. Higher Fermi level due to the increased oxygen vacancy concentration can cause occupation of deep acceptor levels in dislocations of the BaSnO3 film. These occupied deep acceptor levels in turn trap photo-generated holes so that the recombination of electron-hole pair is deterred. [1] J. Park et al., Appl. Phys. Lett. 108, 092106 (2016) [Preview Abstract] |
Wednesday, March 15, 2017 1:03PM - 1:15PM |
L14.00010: Visualization of oxygen vacancy migration in calcium doped bismuth ferrite films Ji Soo Lim, Jin Hong Lee, Chan-Ho Yang Oxygen vacancies are inevitable defects in oxide systems and bring on unexpected functionalities. Configuration of oxygen vacancies is responsible for electronic conduction in resistive switching devices. Motion of oxygen vacancies within materials should be clarified to understand the substitution efficiency of external oxygen ions in solid oxide fuel cell. However, direct observation of oxygen vacancies in motion has been challenging and understanding of their dynamics is not entirely satisfactory. Ca-doped BiFeO$_{3}$ (BCFO) is a promising oxide material to investigate the motion of oxygen vacancies [1,2]. As-grown BCFO films spontaneously generate oxygen vacancies to stabilize the oxidation number of Fe$^{3+}$ ions. Application of an external electric field incites to migrate oxygen vacancies with a creation of conducting $p$-type region. We will present how real-time visualization can be utilized to understand the dynamical behavior of oxygen vacancies in an electric field. [1] C.-H. Yang, \textit{et al}. Nature Materials \textbf{8}, 485 (2009). [2] J. S. Lim, \textit{et al}. Physical Review B \textbf{94}, 035123 (2016) [Preview Abstract] |
Wednesday, March 15, 2017 1:15PM - 1:27PM |
L14.00011: Emission study on the gamma-ray irradiation effects on the ferroelectric Pb(Zr,Ti)O$_{\mathrm{3}}$ thin films Yunsang Lee, Junwhi Lim, Sun A Yang, S. D. Bu We investigated the photoluminescence of the gammy-ray irradiated Pb(Zr,Ti)O$_{\mathrm{3}}$ (PZT) thin films with the various total doses up to 1000 kGy. The PZT thin films were prepared on the Pt/Ti/SiO$_{\mathrm{2}}$/Si substrates by using a sol--gel method with a spin-coating process. It was found that the visible emission emerges near 550 nm with the gamma-ray irradiation. The intensity of the emission increased with the increasing dose amount. The spectral feature of the gamma-ray induced emission was quite narrow, which was distinguished from that formed by normal defects such as oxygen vacancy. We suggest that the gamma-ray irradiation should generate a specific type of defect state inside the PZT films, which could be detected by the low temperature photoluminescence spectroscopy. [Preview Abstract] |
Wednesday, March 15, 2017 1:27PM - 1:39PM |
L14.00012: Substitution sites for 3+ defects in LiNbO$_3$ Frank Bridges, Cameron MacKeen, Laszlo Kovacs The photorefractive response of LiNbO$_3$ can be suppressed by substitution of both 2+ and 3+ dopants, which is very important for harmonic generation applications. An issue that has not been completely resolved is the substitution site -- on Li with compensating Li vacancies, or self compensating clusters with the defect atom substituting on both Li and Nb sites. For Zn(2+) we have found that Zn primarily substitutes on the Li site with very little Zn on a Nb site. Here we consider the 3+ dopants, In and Er, and present EXAFS data for both defects. The signature for substitution on a Li site is a large peak near 2.9 {\AA} in an EXAFS r-space plot, while for Nb substitution the main In-Nb or Er-Nb peak would occur near 3.5 {\AA}. For In there is a large peak at 2.9 {\AA}, and most of the substitution appears to be on the Li site but the In-Nb distances are substantially increased (.1-.15 {\AA}) and the local environment about In is more distorted than around Zn. For Er, both congruent and stoichiometric crystals were studied; the EXAFS plots are nearly identical except for a range near 3 {\AA} where the amplitude is lower for the stoichiometric sample. Possible models for the local environment about each defect will be discussed. [Preview Abstract] |
Wednesday, March 15, 2017 1:39PM - 1:51PM |
L14.00013: Coupling between charge carriers and point defects in electric-field assisted sintering of ceramics C Stephen Hellberg, Noam Bernstein, Steve Erwin The densification that occurs during sintering of certain ceramics has been observed to occur more rapidly and at lower temperatures when a weak external electric field is applied. Also known as ``flash sintering'', the densification is accompanied by a sharp increase in the conductivity. We examine the coupling between the injected carriers and the formation of point defects. The analysis is based on density functional calculations of the defect formation energies in yttria-stabilized zirconia and Y$_2$O$_3$. [Preview Abstract] |
Wednesday, March 15, 2017 1:51PM - 2:03PM |
L14.00014: Novel Physical Model for DC Partial Discharge in Polymeric Insulators Allen Andersen, JR Dennison The physics of DC partial discharge (DCPD) continues to pose a challenge to researchers. We present a new physically-motivated model of DCPD in amorphous polymers based on our dual-defect model of dielectric breakdown. The dual-defect model is an extension of standard static mean field theories, such as the Crine model, that describe avalanche breakdown of charge carriers trapped on uniformly distributed defect sites. It assumes the presence of both high-energy chemical defects and low-energy thermally-recoverable physical defects. We present our measurements of breakdown and DCPD for several common polymeric materials in the context of this model. Improved understanding of DCPD and how it relates to eventual dielectric breakdown is critical to the fields of spacecraft charging, high voltage DC power distribution, high density capacitors, and microelectronics. [Preview Abstract] |
Wednesday, March 15, 2017 2:03PM - 2:15PM |
L14.00015: Barium Titanate Nanoparticles: Short-range Lattice Distortions with Long-range Cubic Order Richard C. Haskell, Chenyang Shi, Simon J.L. Billinge, Eric Puma, Sun Hwi Bang, Nathaniel J.H. Bean, Jean-Claude de Sugny, Robert G. Gambee, Adrian Hightower, Todd C. Monson Small barium titanate (BTO) nanoparticles ($\leq$ 100 nm) exhibit a distortion of the lattice unit cell as inferred from high-energy synchrotron X-ray diffraction patterns (XRD) analyzed using atomic pair distribution functions (PDFs). Fits to PDFs at temperatures of 20$^{\circ}$ to 220$^{\circ}$C suggest that Ti atom displacements from the center of the unit cell are comparable to or even greater than those in the bulk material and persist at temperatures well above 120$^{\circ}$C where the tetragonal to pseudo-cubic phase transition occurs in the bulk. Raman spectra acquired over a temperature range of 20$^{\circ}$ to 220$^{\circ}$C confirm that small BTO nanoparticles exhibit a distorted unit cell even above 120$^{\circ}$C. On the other hand, small BTO nanoparticles exhibit a long-range order consistent with a cubic lattice as recorded by laboratory XRD Bragg reflections at temperatures of 20$^{\circ}$ to 150$^{\circ}$C. We have reconciled these seemingly contradictory data sets by fitting the PDFs over their full range of 6 nm to reveal a long-range structure with a reduced lattice distortion that still manages to support tetragonal Raman lines but is sufficiently close to cubic to yield apparent Bragg peak singlets. [Preview Abstract] |
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