Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session V25: Focus Session: Dopants and Defects in Semiconductors - Oxides, general |
Hide Abstracts |
Sponsoring Units: DMP Chair: Evan Glaser, Naval Research Laboratory Room: D135 |
Thursday, March 18, 2010 8:00AM - 8:12AM |
V25.00001: Reduction and desorption of native oxides on GaAs and Ge during atomic layer deposition of Al$_{2}$O$_{3}$ Hang Dong Lee, Tian Feng, Lei Yu, Daniel Mastrogiovanni, Alan Wan, Torgny Gustafsson, Eric Garfunkel The reduction of native oxides and control of interfacial defects is of central importance if materials such as GaAs and Ge are to be used for post-Si CMOS. We and others have earlier observed that under certain conditions such oxides are removed during atomic layer deposition (ALD) growth. We find, using medium energy ion scattering spectroscopy (MEIS) and XPS, that after just a single ALD half cycle (exposure to a trimethylaluminum (TMA) precursor) $\sim $65{\%} of the native oxide is removed and a 5 {\AA} oxygen-rich aluminum oxide is formed. The source for the oxygen in the aluminum oxide is therefore the native oxide. For Ge substrates, one single TMA pulse removes a substantial amount of the native oxides and a 3{\AA} Al$_{2}$O$_{3}$ film is grown. The native oxides are also completely removed after $>$ 450 \r{ }C preheating of the substrate. The mechanisms for these processes will be discussed. [Preview Abstract] |
Thursday, March 18, 2010 8:12AM - 8:24AM |
V25.00002: Switching and retention characterization of low current TiOx memristive devices Feng Miao, Joshua J. Yang, Julien Borghetti, Matthew D. Pickett, Gilberto Medeiros-Ribeiro, R. Stanley Williams To tap into the promising prospects of TiOx based memristive devices for non-volatile memory applications, low current and low power operation are required. Here we report on switching and retention characterization of 50nm x 50nm cross bar devices. Resistances at room temperature in the OFF state reached values of 0.5TOhm with the switching current of sub-5 $\mu $A and an OFF/ON ratio of about 100 in steady state. Some basic switching characterizations, including speed, endurance and performance at chip operating temperature (85$^{\circ}$C), are studied. Furthermore, upon setting the device to an ON state, the device resistance exhibits a transient phenomenon, which is quite peculiar, including stochastic jumps on the resistance. By comparing with thermally activated conduction and switching behaviors, possible mechanisms of such transient phenomenon will be discussed. By studying the temperature dependence, we also compare the low current switching behavior with virgin state volatile switching and high current stable switching behaviors, which have been observed on the same devices. The different switching mechanisms coexisting in such devices will be discussed, in the context of low current memristors for low power, non-volatile memory applications. [Preview Abstract] |
Thursday, March 18, 2010 8:24AM - 8:36AM |
V25.00003: Nitrogen-doped TiO$_2$ for visible light photocatalysis Joel Varley, Anderson Janotti, Chris Van de Walle Using first-principles calculations we investigate the effects of nitrogen doping on the photocatalytic properties of rutile TiO$_2$. We find that N can be incorporated into the bulk with appreciable solubility in the form of a substitutional or N$_2$ split-interstitial defect. Substitutional N gives rise to a deep-acceptor state, while the split-interstitial behaves as a donor. We confirm that substitutional N on the O site is found to effectively lower the absorption of light to the visible-range by approximately 0.6 eV with respect to the bandgap of 3.1 eV, in good agreement with recent experiments. Our results indicate that the N$_2$ split-interstitial defects do not to contribute to the observed onset of visible-light absorption yet can potentially play a role in the pinning of the Fermi Level. We conclude that N is a good dopant choice for TiO$_2$, increasing the versatility of TiO$_2$ for use as a photocatalyst for sub-UV illumination. This work was supported by the NSF MRSEC Program under Grant No. DMR05-20415. [Preview Abstract] |
Thursday, March 18, 2010 8:36AM - 8:48AM |
V25.00004: DFT-based first-principle calculation of Nb- and W-doped anatase TiO$_2$: Its stoichiometry and the formation of impurity complexes Hideyuki Kamisaka, Takahiro Suenaga, Hisao Nakamura, Koichi Yamashita The structure and electronic states of Nb-doped anatase TiO$_2$ (TNO), a novel transparent conducting oxide, were calculated from the first-principle. Emphasis was put on the importance of non-stoichiometry of the system, because the high electronic conductivity emerges only in samples fabricated under a strong reductive atmosphere, and O$_2$ gas annealing deteriorates the conductivity. The non-stoichiometric system was modeled with periodic unit cells containing several combinations of Nb dopants, oxygen vacancies, and interstitial oxygen atom. From the result, experimental measurements of Nb 3d XPS signal and the effect of O$_2$ gas annealing were rationalized. Some complex structures exhibit strong energy stabilization. These structures might be a constitutive part of the electronic pathway. Similar calculations were performed for W-doped anatase TiO$_2$, and the difference from TNO was discussed. [Preview Abstract] |
Thursday, March 18, 2010 8:48AM - 9:00AM |
V25.00005: Structure and defect studies of In$_{2}$O$_{3}$:Zn,Zr for higher stability TCO Aditi Herwadkar, Kwiseon Kim The defects structures among the transparent conducting oxides (TCO) plays a major role in determining stability of the oxide over a temperature range and in tuning electrical and optical properties for the different TCO applications In$_{2}$O$_{3}$ crystallizes in the cubic bixbyite structure. The structure can be derived from the related fluorite structure by removing one fourth of the anions and allowing for small shifts of the ionic positions. In$_{2}$O$_{3}$ has two non-equivalent six-fold coordinated cation sites. For one of the sites, the cation is bounded by two structural vacancy along the body diagonal and for the other non-equivalent site the vacancies lie along the face diagonal. These vacancies are actually empty oxygen vacancy positions. Indium is in +3 charge state. ZnO on the other hand crystallizes to form wurtzite structure with four-fold coordination for Zn and is in +2 charge state where as the crystal structure of ZrO is rulite with Zr in +4 charge state and is four fold coordinated. Co-doping of Zn and Zr with each substituting the In atom satisfies the octet rule and is lower in energy then the individual substitutions with overall neutrality. The formation enthalpy as a function of pair (Zn, Zr) shows a minimum at experimental composition of In$_{2}$(Zn,Zr)$_{3}$O$_{24}$. We in this work present the electronic structure optimization and study the defect states in this material. [Preview Abstract] |
Thursday, March 18, 2010 9:00AM - 9:12AM |
V25.00006: Defect States in the Wide Gap Semiconductor Ga2O3 Tracy Lovejoy, S. Zheng, E.N. Yitamben, A. Pakhomov, F.S. Ohuchi, M.A. Olmstead, E.G. Villora, K. Shimamura, Y. Yamashita, H. Yoshikawa, S. Ueda, I. Pis, K. Kobayashi, S. Vaithiyalingam Ga2O3 is a transparent wide gap semiconducting oxide with potential applications as a transparent conductive oxide (TCO). The mechanism for conductivity in this material is under debate. The long established picture involves conduction by oxygen vacancy defect states, but a recent paper [Appl. Phys. Lett. 92 202120 (2008)] shows the conductivity can be intentionally controlled over three orders of magnitude by silicon doping on the order of typical Si impurity levels in Ga2O3 source materials. In light of this, the actual role of oxygen vacancies is unclear. We illuminate this issue by measuring the conductivity in single crystals of beta-Ga2O3 as a function of annealing in vacuum. Contrary to expectation, high vacuum annealing causes an order of magnitude decrease in conductivity, but only along the open channel direction in the crystal. As neither Si impurity nor oxygen vacancy concentration is expected to decrease with annealing in vacuum, this data suggests a new model is necessary. Preliminary hard and conventional x-ray photoemission, and Rutherford backscattering results indicate that high vacuum annealing decreases the concentration of gallium interstitials, which may be the origin of the conductivity phenomenon in this material. [Preview Abstract] |
Thursday, March 18, 2010 9:12AM - 9:24AM |
V25.00007: Vibrational Lifetimes and Decay Mechanisms of O-H Stretch Modes in Metal Oxide Proton Conductors Gunter Luepke, Erik Spahr, Lanlin Wen, Michael Stavola, Lynn Boatner, Leonard Feldman, Norman Tolk In proton conducting metal oxides, measurements of the O-H and O-D vibrational lifetimes show that proton transfer processes successfully compete with multi-phonon decay, in contrast to conventional semiconductors, such as Si or Ge, where the latter dominates. In perovskite KTaO$_{3}$, the O-H stretch mode decays via a phonon-assisted proton-tunneling process involving the O-Ta-O bending motion. In rutile TiO$_{2}$, the local oscillatory motion of the proton quickly couples to a wag-mode-assisted classical transfer process along the c-channel with a jump rate of $>$1 THz. The transfer rates in these oxides are 7 to 9 orders of magnitude larger than that due to thermally activated diffusion at room temperature which is significant for renewable energy applications of proton conducting oxides. [Preview Abstract] |
Thursday, March 18, 2010 9:24AM - 9:36AM |
V25.00008: Defect physics of the kesterite thin-film solar cell absorber Cu$_2$ZnSnS$_4$ Shiyou Chen, X.G. Gong, Aron Walsh, Su-Huai Wei Using first-principles calculations, we studied the thermodynamic stability and intrinsic defects of the quaternary Cu$_2$ZnSnS$_4$ (CZTS) semiconductor, which are crucial for further improvement of its solar cell efficiency. We find that (i) The chemical potential region that CZTS can form stoichiometrically is very small. Therefore, it will be very difficult to obtain high quality CZTS samples; (ii) The p-type defects have much lower formation energy than n-type defects, and the dominant acceptor is Cu$_{\mathrm{Zn}}$, however, the associated acceptor level is relatively deep; (iii) The formation of the self-compensated defect pair [Cu$_{\mathrm{Zn}}$+Zn$_{\mathrm{Cu}}$] will not lead to strong carrier separation, and thus will not contribute to the same beneficial effect observed in CuInSe$_2$; (iv) We predict that to avoid the issues in (ii) and (iii), non-equilibrium techniques and Cu-poor/Zn-rich conditions should be used to grow the CZTS sample without ZnS secondary phase, so V$_{\mathrm{Cu}}$ and Zn$_{\mathrm{Cu}}$ become the dominant defects. [Preview Abstract] |
Thursday, March 18, 2010 9:36AM - 9:48AM |
V25.00009: Predictive modeling of dopant control in semiconductors for spintronics and solar energy applications using non-compensated n-p codoping W. G. Zhu, Z. Y. Zhang We apply a recently established non-compensated n-p codoping approach (W. G. Zhu et al, Phys. Rev. Lett. in press) for the design of oxide-based diluted magnetic semiconductors with enhanced Curie temperatures for spintronics and carbon-based two-dimensional nanostructures with appropriate band gaps for solar energy applications. Using first-principles calculations within density functional theory, we demonstrate that the magnetic dopant concentration can be greatly enhanced in ZnO with the use of this approach. The resulting magnetic property will be discussed. We also apply this approach to tune the band gap of graphene-based nanostrucures to around 2 eV for potential solar energy applications. [Preview Abstract] |
Thursday, March 18, 2010 9:48AM - 10:00AM |
V25.00010: ABSTRACT WITHDRAWN |
Thursday, March 18, 2010 10:00AM - 10:12AM |
V25.00011: Electronic and defect structure of CuSCN John Jaffe, Tiffany Kaspar, Timothy Droubay We calculate the band structure, bonding characteristics and basic native defect configurations of hexagonal copper thiocyanate, $\beta $-CuSCN, for the first time. $\beta $-CuSCN is predicted to be an indirect-gap semiconductor with an unusual orbital character: While the highest valence bands have the expected character of Cu-3d levels hybridized with sulfur 3p states, the conduction band minimum (at the K point of the hexagonal Brillouin zone) has mostly cyanide antibonding character. This quasi-molecular character results in some unusual properties, including electron effective masses that are comparable to or even larger than the hole effective masses. However, optical absorption measurements on polycrystalline films do not support the indirect nature of the lowest transitions, though they also do not clearly contradict it. The dominant p-type character of this material is explained in terms of copper vacancies, possibly augmented by CN unit vacancies, which are expected to be acceptors. By contrast, a vacancy of a complete SCN unit would be a donor, but is not expected to occur at significant concentrations in this material. [Preview Abstract] |
Thursday, March 18, 2010 10:12AM - 10:24AM |
V25.00012: First-principles study of defects in TlBr Mao-Hua Du TlBr is a promising radiation detection material due to its high gamma-ray stopping efficiency, high resistivity (that reduces dark current and noise), large enough band gap of 2.68 eV (suitable for room temperature applications), and long electron carrier lifetime (for efficient collection of the radiation-generated carriers). The defect properties obtained from density functional calculations will be presented to discuss their roles in carrier trapping and recombination (which affects the carrier lifetime) and carrier compensation (which affects the resistivity). [Preview Abstract] |
Thursday, March 18, 2010 10:24AM - 10:36AM |
V25.00013: Defect complexes in semiconductors and insulators Hannes Raebiger The interaction of isolated defects and impurities (concentration upto $\sim$10$^{18}$ cm$^{-3}$) is usually rationalized as that of point charges in a dielectric medium, but as defect concentrations are in the order of atomic percent ($\sim$10$^{21}$ cm$^{-3}$), the statistical probability for two or more defects to sit on neighboring sites, forming a cluster or complex, becomes significant [1]. The formation of such clusters changes the local chemical environment, which in turn affects the electronic (and optical and magnetic) properties of the constituent defects non-trivially. To understand these changes, I study a variety of bound defect complexes in wide-gap semiconductors, composed of both deep and shallow defects, focusing on the shifting of the gap levels caused by defect--defect chemical interactions. First the electronic structure is calculated from first principles calculations, and then I will outline a simple theory that describes the level shifts due to cluster formation qualitatively and semi-quantitatively in terms local atomic shielding constants derived from local charge self-regulation [2].\\[4pt] [1] R. Behringer, J. Chem. Phys. {\bf 29}, 537 (1958).\\[0pt] [2] H. Raebiger, S. Lany, and A. Zunger, Nature {\bf 453}, 763 (2008). [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700