Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session C28: Dopants and Defects in Semiconductors II: OxidesFocus
|
Hide Abstracts |
Sponsoring Units: DMP FIAP DCOMP Chair: Zhigang Gui, University of Arkansas Room: 291 |
Monday, March 13, 2017 2:30PM - 3:06PM |
C28.00001: Strange conductivity of strontium titanate Invited Speaker: Matthew McCluskey Strontium titanate is a complex oxide with a range of interesting properties. Samples annealed at 1200C show persistent photoconductivity at room temperature. When irradiated with sub-gap light, the resistivity drops significantly. The increased conductivity persists for days with negligible decay. This unusual effect is attributed to the excitation of an electron from an acceptor defect into the conduction band. A large barrier for recapture prevents electrons from returning to the defect level. The defect is tentatively attributed to a titanium vacancy complex. Recent work suggests that optimized annealing conditions result in weakly p-type material (in the dark), consistent with the idea that acceptors are created. The measured room-temperature hole mobilities (100-1000 cm$^{\mathrm{2}}$/Vs) are surprisingly high. It is possible that this anomalous conductivity is due to a p-type surface layer rather than bulk doping. The results of Seebeck measurements, which discriminate between electron versus hole conduction by applying a thermal gradient, will be discussed. [Preview Abstract] |
Monday, March 13, 2017 3:06PM - 3:18PM |
C28.00002: Optimization of Aluminum-doped Zinc Oxide as a Transparent Conducting Oxide Tristan Cabrera, Stephen Hood, Hasitha Mahabaduge Transparent conducting oxide (TCO) thin films are extensively used in optoelectronics. This study focuses on the optimization of aluminum-doped zinc oxide (AZO) as a TCO for photovoltaic applications. AZO thin films were fabricated using RF magnetron sputtering in the argon environment and characterized using UV-Vis spectroscopy, X-ray diffraction, Hall measurements and four-point probe. The effects of substrate temperature, deposition pressure, and power on the properties of AZO were studied. Using a deposition pressure of 10 mTorr, deposition temperature of 250 $^{\circ}$ C, and 60 W RF power, optical transmittance of up to 90{\%} in the visible range was achieved along with a resistivity in the order of 10$^{\mathrm{-4}} \quad \Omega $ cm. The measured properties are among the most optimal reported for AZO thin films. [Preview Abstract] |
Monday, March 13, 2017 3:18PM - 3:30PM |
C28.00003: Theory of copper impurities in ZnO John Lyons, Audrius Alkauskas, Anderson Janotti, Chris G. Van de Walle Due to its connection to deep luminescence signals and its potential use as an acceptor dopant, copper has been one the most studied impurities in ZnO. From experiment, copper incorporating on the Zn site (CuZn) is known to lead to an acceptor level residing near the conduction band of ZnO, making CuZn an exceedingly deep acceptor. CuZn in ZnO has also long been linked with broad 2.4 eV green luminescence (GL) signals. In this work we explore the electrical and optical properties of Cu in ZnO using density functional theory (DFT). Due to the limitations of traditional forms of DFT, an accurate theoretical description of the electrical and optical properties of such deep centers has been difficult to achieve. Here we employ a screened hybrid density functional (HSE) to calculate the properties of Cu in ZnO. We determine the thermodynamic transition levels associated with CuZn in ZnO as well as the associated luminescence lineshapes [1] of characteristic optical transitions. We find that HSE-calculated optical transitions are in close agreement with experimental studies. [1] A. Alkauskas, J. L. Lyons, D. Steiauf, and C. G. Van de Walle, Phys. Rev. Lett. 109, 267401 (2012). [Preview Abstract] |
Monday, March 13, 2017 3:30PM - 3:42PM |
C28.00004: Scanning Probe Microscopy Investigation of H-irradiated ZnO and Co-doped ZnO thin films in dark and UV-light conditions D. D'Agostino, C. Di Giorgio, F. Bobba, A. Di Trolio, A. Amore Bonapasta, P. Alippi, A. Polimeni, A.M. Cucolo We studied the effect of hydrogen irradiation on ZnO and Co-doped ZnO thin films, discussing the induced multiferroicity and the change of the electronic response to the UV-lighting. Pure and Co-doped thin films were grown by pulsed laser deposition technique on Ag coated Si(p-type) substrate, giving rise to a ZnO/Ag/Si heterostructures, followed by post-growth hydrogenation at temperatures as high as 400 $^{\circ}$ C. In particular, the electronic properties have been probed by light-assisted Scanning Probe Microscopy experiments whereas the magnetic properties have been investigated by Hall transport and magnetization loop measurements. The effect of Co-doping on the electronic density of stases has been also considered. [Preview Abstract] |
Monday, March 13, 2017 3:42PM - 3:54PM |
C28.00005: Water Dissociation on the 0001 Hematite Surface Fabio Negreiros Ribeiro Hematite is one of the many types of iron oxide that is easily found in nature. It is most commonly used in catalysis and it is rarely present in its pristine form. The influence of charged defects in its properties is very important for the correct geometrical/electronic characterization in more realistic operative conditions. Its interaction with water is also interesting for both academic and industrial purposes. In this work we performed DFT+U calculations to study water dissociation on the $Fe_2O_3$(0001) surface using the CP2K software. We first determined the most stable configurations of a single water molecule adsorbed on the surface with and without oxygen/iron vacancies, considering also the charge state of each defect. The energy for the water molecule dissociation in $O^{2-}$ and two protons was evaluated for each case, and it was found that the surfaces presenting iron vacancies are the most efficient ones for such process. In a second step we performed a few ab-initio molecular dynamics simulations at room temperature with several water molecules to accurately determine the liquid water/hematite interface, highlighting how the defects and the charge state change this interface. [Preview Abstract] |
Monday, March 13, 2017 3:54PM - 4:06PM |
C28.00006: Observation of interacting polaronic gas behavior in Ta-doped TiO$_{2}$ thin films via terahertz time-domain spectroscopy Elbert Chia, Liang Cheng, James Lourembam, S. G. Wu, Mallikarjuna R. Motapothula, Tarapada Sarkar, Venky Venkatesan Using terahertz time-domain spectroscopy (THz-TDS), we obtained the complex optical conductivity [$\tilde{\sigma} (\omega)$] of Ta-doped TiO$_{2}$ thin films --- a transparent conducting oxide (TCO), in the frequency range 0.3--2.7~THz, temperature range 10--300~K and various Ta dopings. Our results reveal the existence of an interacting polaronic gas in these TCOs, and suggest that their large conductivity is caused by the combined effects of large carrier density \textit{and} small electron-phonon coupling constant due to Ta doping. [Preview Abstract] |
Monday, March 13, 2017 4:06PM - 4:18PM |
C28.00007: Polaron-mediated surface reconstruction in the reduced Rutile TiO$_2$ (110) surface. Michele Reticcioli, Martin Setvin, Xianfeng Hao, Ulrike Diebold, Cesare Franchini The role of polarons is of key importance for the understanding of the fundamental properties and functionalities of TiO$_2$. We use density functional theory with an on-site Coulomb interaction and molecular dynamics to study the formation and dynamics of small polarons in the reduced rutile (110) surface. We show that excess electrons donated by oxygen-vacancies (V$_{\rm{O}}$) form mobile small polarons that hop easily in subsurface and surface Ti-sites. The polaron formation becomes more favorable by increasing the V$_{\rm{O}}$ concentration level (up to ~20\%) due to the progressively lower energy cost needed to distort the lattice. However, at higher V$_{\rm{O}}$ concentration the shortening of the averaged polaron-polaron distance leads to an increased Coulomb repulsion among the trapped charges at the Ti-sites, which weakens this trend. This instability is overtaken by means of a structural $1\times2$ surface reconstruction, characterized by a distinctively more favorable polaron distribution. The calculations are validated by a direct comparison with experimental AFM and STM data. Our study identifies a fundamentally novel mechanism to drive surface reconstructions and resolves a long standing issue on the origin of the reconstruction in rutile (110) surface. [Preview Abstract] |
Monday, March 13, 2017 4:18PM - 4:30PM |
C28.00008: Probing photoactivity of single defects on TiO2(110) at atomic scale Chaoyu Guo, Xiangzhi Meng, Huixia Fu, Sheng Meng, Ying Jiang Titanium dioxide (TiO2) is one of the most widely used materials in photocatalysis. Although it is well accepted that the surface/near-surface defects play crucial roles in the photocatalytic£¬the atomic-scale information of photo-excited carrier dynamics is still lacking. Here, we addressed this issue using a laser-combined scanning tunneling microscope. Surface and subsurface oxygen vacancies of rutile TiO2(110) were created through sputtering and annealing, leading to the appearance of in-gap states. Upon the light illumination, those gap states exhibited energetic shift. Interestingly, the subsurface defects showed two distinct photo response: redshift and blueshift, while the surface defects only showed blueshift. Based on ab initio density functional theory calculations, the energy shift of the gap-states were ascribed to the photo-excited charge transfer between the gap states and valance or conduction band, which changed the oxygen vacancy's charge states. Further time-resolved experiments suggested that the lifetime of hot electrons/holes can be in the order of tens of nanoseconds. Our work highlights the importance of atomic environment in the photoactivity and may help to improve the photocatalytic efficiency by engineering the defect types properly. [Preview Abstract] |
Monday, March 13, 2017 4:30PM - 4:42PM |
C28.00009: Oxygen Vacancies in the Anatase (101) Surface: Formation Energies and Finite Size Effects Edward Tait, Caterina Ducati, Mike Payne, Nicholas Hine We present a study of oxygen vacancies in the anatase TiO$_{2}$ (101) surface. We take great care to minimise the impact of periodic boundary conditions on our results. We make use of the ONETEP\footnote{Skylaris C K, Haynes P D, Mostofi A A and Payne M C 2005 \emph{J. Chem. Phys} \textbf{122}} linear scaling DFT code to study defects in large (order 800 atom) simulation cells, to reduce long range strain interactions. We also make use of a scheme to correct for spurious periodic electrostatic interactions in the case of charged defects\footnote{Komsa, H-P and Pasquarello A 2013 \emph{Phys. Rev. Lett.} \textbf{110}:095505}. Finally we study the behaviour of defect formation energies as a function of distance from the surface, and demonstrate convergence to bulk formation energies with depth. We also present novel functionality for EELS calculations within LS-DFT, suitable for large nanomaterials systems. [Preview Abstract] |
Monday, March 13, 2017 4:42PM - 4:54PM |
C28.00010: Electronic and Optical Properties of Cr-N Co-doped TiO$_{\mathrm{2}}$ for Intermediate Band Solar Cells Katherine Inzani, Sverre Magnus Selbach A density functional theory (DFT) study is presented on the effect of Cr and N co-doping on the electronic structure and optical properties of TiO$_{\mathrm{2}}$. The wide-band gap and high photocatalytic response of anatase TiO$_{\mathrm{2}}$ place it as candidate host material for intermediate band solar cells. For this application, a suitable dopant and high doping level is required to introduce a mid-gap band, however defect control is still necessary for an efficient photovoltaic device. A proposed solution is non-compensated substitution of two dopants, which can improve the thermodynamic and kinetic solubilities whilst also providing passivation of recombination centers. DFT is used here to predict the effect of non-compensated Cr and N dopant concentrations on the electronic structure of TiO$_{\mathrm{2}}$, utilizing hybrid exchange correlation functionals to give accurate band gaps. The intermediate band character is further investigated by calculation of absorption spectra. Furthermore, the effect of dopant concentration on anatase and rutile phase stability is evaluated. The results presented are being used to guide experimental studies, advising effective dopant levels and giving structural predictions for material synthesis, and providing optical constants for input to solar cell modelling for cell design. [Preview Abstract] |
Monday, March 13, 2017 4:54PM - 5:06PM |
C28.00011: Determining the diffusivity of H$_{\mathrm{i}}^{\mathrm{+}}$ in In$_{\mathrm{2}}$O$_{\mathrm{3}}$ single crystals for over ten decades Michael Stavola, Philip Weiser, Ying Qin, Karla Villalta, W. Beall Fowler, Lynn Boatner Interstitial hydrogen (H$_{\mathrm{i}}^{\mathrm{+}})$ is an $n$-type dopant in In$_{\mathrm{2}}$O$_{\mathrm{3}}$ that has attracted attention for solar-cell applications [1]. An IR absorption line observed at 3306 cm$^{\mathrm{-1}}$ for In$_{\mathrm{2}}$O$_{\mathrm{3}}$ single crystals annealed in an H$_{\mathrm{2}}$ ambient has been assigned to the H$_{\mathrm{i}}^{\mathrm{+}}$ center [2]. Two types of experiments have been performed to determine the diffusivity of H$_{\mathrm{i}}^{\mathrm{+}}$ in In$_{\mathrm{2}}$O$_{\mathrm{3.}}$ At temperatures near 673 K, experiments have been performed to determine the diffusivity of H$_{\mathrm{i}}^{\mathrm{+}}$ from its indiffusion depth into In$_{\mathrm{2}}$O$_{\mathrm{3}}$. At 165 K, stress can be used to produce a preferential alignment of the H$_{\mathrm{i}}^{\mathrm{+}}$ center. With the help of theory, the kinetics with which this alignment can be produced yield the time constant for a single jump of the H$_{\mathrm{i}}^{\mathrm{+}}$ center and also the diffusivity of H$_{\mathrm{i}}^{\mathrm{+}}$ at 165 K [3]. These data determine the diffusivity for H$_{\mathrm{i}}^{\mathrm{+}}$ for over ten decades! [1] T. Koida \textit{et al.}, Jpn. J. Appl. Phys. \textbf{46}, L685 (2007). [2] W. Yin \textit{et al.}, Phys. Rev. B \textbf{91}, 075208 (2015). [3] P. Weiser \textit{et al.}, Appl. Phys. Lett., in press. [Preview Abstract] |
Monday, March 13, 2017 5:06PM - 5:18PM |
C28.00012: Vibrational properties of an OH center in $\beta \quad -$\textbf{Ga}$_{\mathbf{2}}$\textbf{O}$_{\mathbf{3}}$ W. B. Fowler, P. Weiser, M. Stavola Theoretical predictions suggest that hydrogen impurities play a crucial role in the electrical conductivity of $\beta -$Ga$_{2}$O$_{3}$ by acting as shallow donors and by passivating cation-vacancy acceptor complexes [1,2]. IR spectroscopy of $\beta -$Ga$_{2}$O$_{3}$ single crystals treated in an H$_{2}$ (D$_{2})$ ambient display a strong vibrational line at 3437 (2546) cm$^{-1}$ originating from an O-H(D) containing defect. This line is thermally stable at 1000$^{\circ}$ C, which suggests that it may arise from a V$_{Ga}$-H complex. Theoretical calculations using the CRYSTAL06 code [3] with hybridized DFT Hamiltonian and the polarization properties of the O-H vibrational line are used to determine possible microscopic structures for this defect and other possible defects. [1] J. B. Varley \textit{et al.}, Appl. Phys. Lett. \textbf{97}, 142016 (2010). [2] J. B. Varley \textit{et al.}, J. Phys.: Condens. Matter \textbf{23} (2011), 334212. [3] R. Dovesi \textit{et al.}, \textit{Crystal06 User's Manual} (University of Torino, Torino, 2006). [Preview Abstract] |
Monday, March 13, 2017 5:18PM - 5:30PM |
C28.00013: Abstract Withdrawn |
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