Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session T14: Focus Session: Dopants and Defects in TiO2, SrTiO3, and Other Oxides |
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Sponsoring Units: DMP FIAP Chair: Kookrin Char, Seoul National University Room: 008A |
Thursday, March 5, 2015 11:15AM - 11:51AM |
T14.00001: Structural and electronic defects in SrTiO$_{3}$ and TiO$_{2}$ Invited Speaker: Anderson Janotti Control of defects and charge carriers is key to the development of metal oxide semiconductors as electronic materials. As in any semiconductor, one of the main challenges is to control the conductivity by doping as well as to reduce the concentration of native defects and unintentional impurities that act as compensation centers or sources of deep level luminescence, thus minimizing their deleterious effects. Experiments indicate that defects such as oxygen vacancies easily form and strongly affect the electronic properties of many oxide semiconductors. In SrTiO$_{3}$, it has been proposed that oxygen vacancies simultaneously act as a shallow donor that contribute to $n$-type conductivity, and as a deep center that causes luminescence well below the band-gap energy. This seemingly paradoxical behavior has remained unresolved. In rutile TiO$_{2}$, it has been proposed that excess electrons, either from oxygen vacancies or donor impurities, form small polarons. Although the formation of small polarons explains prominent features observed in the optical absorption spectra, charge localization seem incompatible with the high electron mobilities determined in Hall measurements of single crystals. In this talk we will discuss results of first-principles calculations for native point defects and impurities in SrTiO$_{3}$ and TiO$_{2}$, as they are prototypes of a large class of transition-metal oxide semiconductors. We will address the impact of defects and small-polaron formation on the electrical and optical properties in each material, providing a framework for interpreting similar phenomena in other complex oxides. [Preview Abstract] |
Thursday, March 5, 2015 11:51AM - 12:03PM |
T14.00002: Oxygen vacancy and hole conduction in ``leaky'' amorphous TiO2 from first-principles calculations Hieu Pham, Lin-Wang Wang In the last decade, titanium dioxide (TiO$_{\mathrm{2}})$ has been one of the most studied materials due to its low cost, lightweight, eco-friendliness and long-term stability to be used in energy applications. Specifically, it was found recently that amorphous TiO$_{\mathrm{2}}$ could be used as a protection layer for photo-induced water splitting. While protecting the light-absorbing photoanodes from corrosion, it can conduct hole carriers, perhaps through some defect levels. Nevertheless, the exact mechanism for such hole conductivity is not clearly understood. In this work, an amorphous TiO$_{\mathrm{2}}$ model is obtained from molecular dynamics employing the melt-and-quench technique. The electronic properties, polaronic states and hole conduction mechanism in amorphous structure were investigated by means of density functional theory. The formation energy of oxygen vacancy was found to reduce significantly (by a few eV) upon the amorphization. Our theoretical study suggested that the oxygen vacancies and their defect states provide hopping channels which are comparable with experimental observations and could be responsible for the hole conduction in the ``leaky'' TiO$_{\mathrm{2}}$ recently discovered for the photochemical water-splitting applications. [Preview Abstract] |
Thursday, March 5, 2015 12:03PM - 12:15PM |
T14.00003: Negative $U$ behavior of TiO${}_{2-x}$ Magn\'eli and Corundum phases Antonio Claudio Padilha, Alexandre Rocha, Hannes Raebiger, Gustavo Dalpian The isolated oxygen vacancy is known to be a negative $U$ defect in rutile TiO${}_2$. This effect manifests itself by a double donor level close to the conduction band. As oxygen is further removed, TiO${}_2$ no longer remains in the rutile structure and the Magn\'eli phases Ti${}_n$O${}_{2n-1}$ (4 $\leq n \leq$37) are obtained. Those structures are characterized by ordered planes of oxygen vacancies (so-called shear planes) between rutile-like layers. Further removal of oxygen leads to the formation of Ti${}_3$O${}_5$ and the corundum phase Ti${}_2$O${}_3$. In this work, we calculated using DFT+U the formation energy of several of these systems and show that these systems also have a double donor transition per oxygen vacancy. This means that these compounds as such also have negative $U$ behavior, even though the vacancies are not isolated and the donor level is a broad delocalized impurity band close to the bottom of the conduction band. The relaxation energy for the doubly ionized system is larger than that of the singly ionized one, rendering the latter unstable. [Preview Abstract] |
Thursday, March 5, 2015 12:15PM - 12:27PM |
T14.00004: Termination-specific study of oxygen vacancy transition levels on SrTiO$_{3}$(001) surfaces by scanning tunneling spectroscopy Wattaka Sitaputra, Nikhil Sivadas, Marek Skowronski, Di Xiao, Randall Feenstra We have studied the surface electronic structure of oxygen vacancies on SrTiO$_{3}$(001) surfaces using scanning tunneling spectroscopy and DFT calculations with local spin density approximation (LSDA$+$U). With high dynamic range measurements, a mid-gap level associated with the surface oxygen vacancies was observed for SrO-terminated surfaces. TiO$_{2}$-terminated surfaces, on the other hand, did not exhibit observable mid-gap states (this lack of signal is believed to be due to the nature of defect wavefunction involved, as well as possibly involving transport limitations in the STS measurements). Both vacuum-cleaved and MBE-grown surface have been studied. For the former, the Fermi level is pinned near mid-gap owing to disorder-induced surface states. The amount of surface disorder can be controlled in the case of epitaxially grown surfaces. Rougher MBE-grown surfaces were found to exhibit similar spectral characteristics to the cleaved surfaces, while a shift of the Fermi level toward the conduction band was observed for flatter grown surfaces. Notably, with a decreasing number of disorder-induced surface states, the Fermi level is found to be pinned within the observed band of oxygen vacancy levels. [Preview Abstract] |
Thursday, March 5, 2015 12:27PM - 12:39PM |
T14.00005: Magnetism and metal-insulator transition in oxygen deficient SrTiO3 Alejandro Lopez-Bezanilla, P. Ganesh, Peter Littlewood We report new findings in the electronic structure and magnetism of oxygen vacancies in SrTiO3. By means of first-principles calculations we show that the appearance of magnetism in oxygen-deficient SrTiO3 is not determined solely by the presence of a single oxygen vacancy but by the density of free carriers and the relative proximity of the vacant sites. While an isolated vacancy behaves as a non-magnetic double donor, manipulation of the doping conditions allows the stability of a single donor state with emergent local moments. Strong local lattice distortions enhance the binding of this state. Consequently we find that the free-carrier density and strain are fundamental components to obtaining trapped spin-polarized electrons in oxygen-deficient SrTiO3, which may have important implications in the design of switchable magneto-optic devices. [Preview Abstract] |
Thursday, March 5, 2015 12:39PM - 12:51PM |
T14.00006: The effect of oxygen vacancies and strain on the optical energy gap of strained SrTiO$_{3-\delta}$ thin films Nathan Steinle, Barry Koehne, Ryan Cottier, Daniel Currie, Nikoleta Theodoropoulou SrTiO$_{3-\delta}$ (STO) films were grown on single crystal SrTiO$_{3}$ and p-Si (001) substrates using molecular beam epitaxy (MBE). The single-phase STO/Si films were of high crystalline quality as verified by x-ray diffraction (XRD) and atomic force microscopy (AFM) with an rms roughness of less than 0.5 nm. Oxygen vacancies were introduced by controlling the oxygen pressure (P(O$_{2})$: 10$^{-8}$ to 10$^{-7}$ torr) during growth. The lattice mismatch of STO on Si causes a 1.7{\%} bi-axial, compressive strain. The oxygen vacancies cause a tensile strain because of the different Ti$^{3+}$ and Ti$^{4+}$ ionic radii. This agrees with our XRD measurements that show a decrease of the out of plane lattice constant as either the thickness or P(O$_{2})$ during growth increase. We used a Variable Angle Spectroscopic Ellipsometer M-2000 by Woolam and the VASE software to measure and model the optical properties of the films and substrates using Tauc-Lorentz and Gaussian oscillators for the STO layer. Our results show that the direct energy bandgap of STO at around 3.8 eV increases as either the thickness or P(O$_{2})$ decrease, in agreement with theoretical calculations. Additionally, absorption is observed in the 1.5-2 eV region for the films with increased Oxygen vacancies. [Preview Abstract] |
Thursday, March 5, 2015 12:51PM - 1:03PM |
T14.00007: Niobium doped strontium titanate: Effect of oxygen ambient on the doping mechanism Chun-Fu Chang, Q.Y. Chen, P.V. Wadekar, O. Lozano, M.S. Wong, W.C. Hsieh, Y.S. Wang, Y.T. Lin, H.H. Liu, C.W. Chang, H.C. Huang, H.H. Liao, W.K. Chu, H.W. Seo Double doping in oxides, a scenario where free electrons are created to anion doping as well as oxygen vacancies is currently under investigation for perovskite such as La:SrTiO3-$\delta $ in hopes of attaining materials with high mobility for transparent oxide electronics. In this report, we have investigated this phenomenon in another prominent conducting perovskite -- Nb:SrTiO3. Nb doped SrTiO3 thin films were grown under different oxygen pressures to control the oxygen dopant effect. The chemical quantification was done by proton-induced X-ray emission (PIXE) and X-ray photoelectron spectroscopy (XPS) techniques, while structural quantification was done by X-ray diffraction (XRD). Electrical measurements show that conductivity is strongly dependent on the oxygen partial pressures rather than Nb doping which is contradictory to the expectations. This discrepancy arises because of formation of multivalent Nb due to the processing conditions. The knowledge of interplay between the doping themselves and with the inherent atomic defects is essential to understand and ultimately tune the electrical and optical properties for transparent electronic applications. [Preview Abstract] |
Thursday, March 5, 2015 1:03PM - 1:15PM |
T14.00008: Small Polarons and Point Defects in Barium Cerate Michael Swift, Anderson Janotti, Chris G. Van de Walle Barium cerate (BaCeO$_{3})$ is a well-known proton-conducting material. In applications, it is frequently doped (for instance with yttrium) to increase stability and promote hydrogen uptake. However, the microscopic mechanisms of ionic conductivity and the effects of doping and native defects are still not fully understood. Many of the obstacles to the theoretical study of this material stem from the nature of the conduction band, which is made up of cerium 4$f$ states. These states present a challenge to first-principles techniques based on density functional theory within the standard approximations for exchange and correlation. Using a hybrid functional, we investigate the effects of hydrogen impurities and native defects on the electrical and optical properties of BaCeO$_{3}$. We discuss the tendency of excess electrons or holes to localize in the form of small polarons. We also explore the interactions of polarons with hydrogen impurities and oxygen vacancies, and their impact on luminescence properties. [Preview Abstract] |
Thursday, March 5, 2015 1:15PM - 1:27PM |
T14.00009: Use of nonpolar BaHfO$_{3}$ gate oxide for field effect on the high mobility BaSnO$_{3}$ Chulkwon Park, Useong Kim, Young Mo Kim, Chanjong Ju, Kookrin Char Recently, BaSnO$_{3}$ (BSO) has attracted attentions as a transparent conducting oxide and/or a transparent oxide semiconductor due to its novel properties: the excellent oxygen stability even at high temperature and the high electrical mobility at room temperature. We fabricated field effect transistors using La-doped BSO as the semiconducting channel on undoped BSO buffer layers on SrTiO$_{3}$ substrates. A non-polar perovskite BaHfO$_{3}$ was used as the gate insulator, and 4{\%} La-doped BSO as the source, the drain, and the gate electrodes grown by pulsed laser deposition. We have measured the optical and the dielectric properties of the epitaxial BaHfO$_{3}$ gate oxide layer, namely the optical band gap, the dielectric constant, and the breakdown field. Using such BaHfO$_{3}$ gate oxide, we observed carrier modulation in the active layer by field effect. In this presentation, we will report on the performance of such field effect transistors: the output and the transfer characteristics, the field effect mobility, the $I_{on}$/$I_{off}$ ratio, and the subthreshold swing. [Preview Abstract] |
Thursday, March 5, 2015 1:27PM - 1:39PM |
T14.00010: Field effect transistors based on BaSnO$_{3}$ with AlO$_{\mathrm{x}}$ and HfO$_{\mathrm{x}}$ gate oxides Young Mo Kim, Chulkwon Park, Useong Kim, Kookrin Char La-doped BaSnO$_{3}$ (BLSO) is a transparent perovskite oxide semiconductor with high electron mobility and excellent oxygen stability. We fabricated n-type field effect transistors (FETs) on undoped BaSnO$_{3}$ (BSO) buffer layers on SrTiO$_{3}$ (STO) substrates using BLSO as the semiconducting channels and amorphous AlO$_{\mathrm{x}}$ and HfO$_{\mathrm{x}}$ as the gate insulators. BSO buffer layers and BLSO channels were grown by pulsed laser deposition, while the AlO$_{\mathrm{x}}$ and HfO$_{\mathrm{x}}$ gate insulators were grown by atomic layer deposition (ALD). Sn:In$_{2}$O$_{\mathrm{3}}$ (ITO) was used as the source, the drain and the gate electrodes. At room temperature, we achieved the field effect mobility value of 17.8 cm$^{2}$/Vs for the AlO$_{\mathrm{x}}$ FET and 19.2 cm$^{2}$/Vs for the HfO$_{\mathrm{x}}$ FET. The subthreshold swing was measured to be 3.2 V/dec for the AlO$_{\mathrm{x}}$ FET and 1.2 V/dec for the HfO$_{\mathrm{x}}$ FET. [Preview Abstract] |
Thursday, March 5, 2015 1:39PM - 1:51PM |
T14.00011: P-type Semiconducting Behavior of BaSn$_{1-x}$Ru$_{x}$O$_{3}$ system Hyukwoo Kwon, Juyeon Shin, Kookrin Char BaSnO$_{3}$ is a promising transparent perovskite oxide semiconductor due to its high mobility and chemical stability. Exploiting such properties, we have applied BaSnO$_{3}$ to the field effect, the 2-dimensional electron gas, and the pn-junction devices. In spite of the success of the K-doped BaSnO$_{3}$ as a p-type doped, its carrier density at room temperature is rather small due to its high activation energy of about 0.5 eV. In continuation of our previous study on SrSn$_{1-x}$Ru$_{x}$O$_{3}$ system, we studied the p-type semiconducting behavior of BaSn$_{1-x}$Ru$_{x}$O$_{3}$ system. We have epitaxially grown the BaSn$_{1-x}$Ru$_{x}$O$_{3}$ (0$\le $x$\le $0.12) thin films by pulsed laser deposition. X-ray diffraction measurements show that the films maintain a single phase over the entire doping range and the lattice constants of the system decrease monotonously as the doping increases. Transport measurements show that the films are semiconducting and their resistivities dramatically decrease as the Ru doping increases. Hall measurement data show that the charge carriers are p-type and its corresponding mobility values vary from 0.3 $\sim$ 0.04 cm$^{2}$/V$\cdot$s, depending on the doping rate. The hole carrier densities, measured to be 10$^{17}$ $\sim$ 10$^{19}$ /cm$^{3}$, are larger than those of K-doped BaSnO$_{3}$. Using BaSn$_{1-x}$Ru$_{x}$O$_{3}$ and Ba$_{1-x}$La$_{x}$SnO$_{3}$ as p-type and n-type semiconductors, we will fabricate pn-junctions and report its performance. [Preview Abstract] |
Thursday, March 5, 2015 1:51PM - 2:03PM |
T14.00012: Effect of a Ru doped SnO$_{2-x}$ buffer layer on thin-film transistors based on SnO$_{2-x}$ channel layer Hyosik Mun, Hyeonseok Yang, Kookrin Char We report on studies of transparent thin-film transistor (TFT) devices based on SnO$_{2-x}$ thin film. SnO$_{2-x}$ thin films were prepared by pulsed layer deposition with and without Ru-doped SnO$_{2-x}$ buffer layer on r-plane sapphire substrates to investigate the effect of a Ru-doped SnO$_{2-x}$ buffer layer on the electrical properties of SnO$_{2-x}$ channel layer. The Ru-doped SnO$_{2-x}$ buffer layer was found to be very crystalline and insulating at the same time. Using such Ru-doped SnO$_{2-x}$ buffer layer made it possible for the SnO$_{2-x}$ channel layer to have both low carrier density and high mobility, probably due to reduction of the threading dislocation density. AlO$_{x}$ gate insulator layer was deposited by atomic layer deposition and ITO was used as the source, the drain, and the gate electrodes. We will compare the TFT performances with or without the Ru-doped SnO$_{2-x}$ buffer layer and discuss how such buffer layer enables the necessary device parameters for TFT. [Preview Abstract] |
Thursday, March 5, 2015 2:03PM - 2:15PM |
T14.00013: Aliovalently-Doped Garnets for Li Battery Solid State Electrolytes Derek K. Schwanz, Ernesto E. Marinero We report on a new family of fast ionic conductivity electrolytes based on the garnet LiLaZrO. Partial substitution of Zr by aliovalent atomic species through solid state solution synthesis results in ionic conductivities 2 orders of magnitude larger than the tetragonal phase of LiLaZrO and comparable to that of its cubic phase. The synthesis temperature is 400C lower than that required for the cubic stabilization of LiLaZrO. Ongoing impovements on microstructure and film density as well as optimization of the garnet stoichiometry are expected to yield ionic conductivities surpassing the highest values reported to-date on cubic doped LiLaZrMO (Ta, Al, W, Nb) [Preview Abstract] |
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