Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Q41: Focus Session: Complex Oxide Thin-Film Growth |
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Sponsoring Units: DMP DCOMP Chair: Stephen Hellberg, Naval Research Laboratory Room: Mile High Ballroom 3C |
Wednesday, March 5, 2014 2:30PM - 3:06PM |
Q41.00001: Effect of Growth Induced (Non)Stoichiometry on Interfacial Conductance in LaAlO$_{3}$/SrTiO$_{3}$ Invited Speaker: Lane Martin There has been great interest in the emergence of novel phenomena at the heterointerface between SrTiO$_{3}$ and LaAlO$_{3}$ including the observation of two dimensional conductivity, superconductivity, magnetism, and more. Despite extensive work in the field there is still a debate about the mechanism for and how to deterministically control these exotic states of matter. In this talk we will explore the implications of variations in cation stoichiometry for the crystal structure, dielectric, thermal, and electronic properties of such materials. We will demonstrate a strong link between the growth process, the stoichiometry of the LaAlO$_{3}$, and the resulting interfacial electrical properties. Varying the La-cation stoichiometry by a few atomic percent in films grown at 1 x 10$^{-3}$ Torr results in a 2 and 7 order-of-magnitude change in the 300K and 2K sheet resistance, respectively, with highly conducting states occurring only in La-deficient films. Further reducing the growth pressure results in an increase of the sheet carrier density and a dramatic change in the carrier mobility. We will highlight the relative contributions of \textit{intrinsic} and \textit{extrinsic} effects in controlling the properties of these heterointerfaces and how these factors are controlled by the growth process. We will explore the effect of the growth process on the evolution from 2D to 3D conductance via high magnetic field transport measurements of the Shubnikov-de Haas effect and will explore the thickness evolution of the conducting interface. Overall we will demonstrate a strong link between the growth process, the stoichiometry of the resulting materials, the desired properties of the system, and the implications for understanding the physics and how to engineer these and other materials including nickelates, ferroelectrics, and more. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q41.00002: Hybrid functional calculations for the electronic properties of LaAlO$_3$ Jimmy-Xuan Shen, Andr\`{e} Schleife, Anderson Janotti, Chris G. Van de Walle The perovskite LaAlO$_3$ is a wide-band-gap (5.5--6.5 eV) insulator of high interest in both applied and fundamental research. On the applied side, LaAlO$_3$ has been considered a candidate to replace SiO$_2$ as a gate dielectric in field-effect transistors due to its high dielectric constant (23--25) and near-perfect lattice matching with Si ($<1$\%). On the fundamental science side, two-dimensional electron gases (2DEGs) with high carrier density have been observed at the interface of LaAlO$_3$ and SrTiO$_3$. The origin of the 2DEG and the dependency of its density on the thickness of the LaAlO$_3$ top layer have been highly debated. Interestingly, many of the basic electronic properties of LaAlO$_3$ are still poorly understood. For example, measurements of the band gap vary by as much as 1 eV. Here, we report the electronic structure of LaAlO$_3$ using density functional calculations with a hybrid functional. We compute the electronic structure and the dielectric function of bulk LaAlO$_3$ in the rhombohedral and cubic phases, and compare the optical properties with experiments. [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q41.00003: Bandgap engineering of SrTiO$_3$ via Al-substitution Agham Posadas, Chungwei Lin, Stefan Zollner, Alex Demkov Epitaxial SrTiO$_3$ is was originally envisioned as a replacement gate dielectric for scaled CMOS technology because of its very high dielectric constant of $\sim$300 at room temperature. However, one critical issue that prevented this technology to be developed is the zero conduction band offset with Si making it unsuitable for use as a gate insulator. We have epitaxially grown Al-substituted SrTiO$_3$ on Si using molecular beam epitaxy, replacing 10-20\% of the Ti atoms with Al. We observe a 0.3 eV increase in the band gap by both spectroscopic ellipsometry and electron energy loss spectroscopy. Capacitor structures show a dramatic decrease in leakage current by six orders of magnitude. This approach may allow SrTiO$_3$ to become useful as a gate dielectric on silicon. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q41.00004: Structural and electrical properties of strained La-doped SrTiO$_{3}$ films Miri Choi, Agham B. Posadas, Heidi Seinige, Andrew Kellock, Martin M. Frank, Maxim Tsoi, Alexander A. Demkov Structural and electrical properties of La-doped thin (10nm and 20nm) SrTiO$_{3}$ films are investigated. Films with three different La doping concentrations (5, 15 and 25{\%}) are grown by molecular beam epitaxy. Epitaxially strained La-doped SrTiO$_{3}$ films were grown on four different substrates, LaAlO$_{3}$(100), (LaAlO$_{3})_{0.3}$(Sr$_{2}$AlTaO$_{6})_{0.7}$(100), SrTiO$_{3}$(100) and DyScO$_{3}$(110), in order to have different strain conditions. We compare the structural properties of La-doped SrTiO$_{3}$ films using X-ray diffraction as a function of strain and La dopant concentrations. We also determine the electrical properties of strained La-doped films using Hall measurements, electrical resistivity and mobility at room temperature. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q41.00005: Electronic and optical properties of La,Cr co-doped SrTiO$_{3}$ epitaxial thin films Ryan Comes, Hongliang Zhang, Yingge Du, Robert Colby, Mark Bowden, Timothy Droubay, Scott Chambers SrTiO$_{3}$ (STO) is a wide-gap semiconductor well suited for photocatalytic H$_{2}$ production due to the alignment of its band edges with the half-cell energies of the H$_{2}$O redox reactions. However, the wide optical gap of STO (3.3 eV) makes the material an inefficient light absorber in the visible spectrum, preventing formation of electron-hole pairs needed for photocatalysis. Cr dopants on the Ti site have been shown to reduce the optical bandgap if the Cr ion is in the 3$+$ state. However, charge conservation in Cr-doped STO dictates that a Cr ion must either be in the 4$+$ oxidation state or compensate the 3$+$ state via an oxygen vacancy. In each case, defect electronic states occur, reducing the efficiency of electron-hole pair formation. To compensate this, others have shown that doping STO with La and Cr ions in equal quantities can promote the formation of the Cr$^{3+}$oxidation$^{\, }$state. In this work, we examine the electronic and optical properties of La,Cr-doped STO films grown using oxide molecular beam epitaxy. Films were characterized via \textit{in situ} x-ray photoelectron spectroscopy to measure valence band and core level energies, confirming that most Cr ions are in the 3$+$ state. Optical absorption measurements show that the optical bandgap is reduced by 0.8 eV from that of undoped STO. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q41.00006: Ionic Liquid Gating of SrTiO3 Nanowires Terence Bretz-Sullivan, Allen Goldman In recent years, ionic liquid (IL) field effect gating of complex oxides has revealed novel electronic phases in electronic density regimes not easily attainable by chemical doping or by solid gate dielectric field effect tuning. Specifically, ionic liquid gated Strontium Titanate (STO) serves as an ideal system to study due to its relevance to the LaAlO3/SrTiO3 hetero-interface. Nevertheless, IL gating of nanoscale regions of STO has not been extensively explored. In this talk, the results of IL gated nanowires of STO will be discussed. Nanowires, patterned by electron beam lithography, are defined by a narrow channel of width 100nm in the resist PMMA on top of single crystal STO substrates. The IL is confined to this channel and thus by applying a gate voltage will accumulate electrons at the IL/STO interface, i.e. at the channel floor, by the formation on an electric double layer. Non-linear current-voltage characteristics have been observed using a two-terminal geometry over a set of gate voltages and a temperature range of 2K-35K. These characteristics exhibit behavior similar to Coulomb Blockade physics; however, the possibility of other phenomena has not been ruled out. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q41.00007: Momentum-resolved view of mixed 2D and nonbulklike 3D electronic structure of the surface state on SrTiO$_{3}$ (001) N.C. Plumb, M. Salluzzo, E. Razzoli, M. Mansson, J. Krempasky, C.E. Matt, T. Schmitt, M. Shi, J. Mesot, L. Patthey, M. Radovic The recent discovery of a metallic surface state on SrTiO$_{3}$ may open a route to simplified low-dimensional oxide-based conductors, as well as give new insights into interfacial phenomena in heterostructures such as LaAlO$_{3}$/SrTiO$_{3}$. Our recent angle-resolved photoemission spectroscopy (ARPES) study demonstrates that not only quasi-2D but also non-bulklike 3D Fermi surface components make up the surface state. Like their more 2D counterparts, the size and character of the 3D components are fixed with respect to a broad range of sample preparations. As seen in previous studies, the surface state can be ``prepared'' by photon irradiation under UHV conditions. An extremely high fraction of the surface valence states are affected by this process, especially in relation to the stability of oxygen core level intensity during the same exposure, which points to a key role of electronic/structural changes that spread over the surface as the metal emerges. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q41.00008: Implication of structural changes to ferroelectrity in epitaxially grown PbVO3 thin films Seol Hee Oh, Hye-Jin Jin, Ran Hee Shin, William Jo, Yu-Seong Seo, Jai-Seok Ahn PbVO3 (PVO) having a perovskite-type tetragonal structure is an intriguing polar magnetic material because its structural distortion would be linked to enhanced ferroelectric polarization and considered as a candidate of multiferroic materials. However, ferroelectricity of PVO is not experimentally demonstrated yet as a form of thin-films partly due to its large tetragonality (c/a$=$1.23) compared with proper ferroelectric PbTiO3 (c/a $=$ 1.06) [1]. Polarization of PVO is generated by lone pair of Pb2$+$ ions and estimated to $\sim$ 152 $\mu $C/cm2 [3]. We used laser ablation to synthesize epitaxial PVO thin films on LaAlO3 (001) substrates under argon ambient from a stable Pb2V2O7 sintered target. X-ray diffraction was used to investigate the phase formation and texture of the films. Only under an optimized condition with no oxygen partial pressure, epitaxial growth of the PVO films was possible. We confirmed four-fold symmetry of in-plane alignment of the films on PbVO3 [001] // LaAlO3 [001], but the a- and c- lattice constants of the PVO films show changes due to the compressive stress from the substrates. In addition, surface morphology of the films displays drastic changes in accordance with the growth conditions. Some elongated grains are related to the Pb2V2O7 pyrochlore structure. The relation between structural deformation and ferroelectricity in the PVO films was examined by local measurement of piezoresponse force microscopy. [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q41.00009: PZT Films Fabricated by Metal Organic Decomposition Method Vladimir Sobolev, Valeriy Ishchuk High quality lead zirconate titanate films have been fabricated on different substrates by metal organic decomposition method and their ferroelectric properties have been investigated. Main attention was paid to studies of the influence of the buffer layer with conditional composition Pb$_{1.3}$(Zr$_{0.5}$Ti$_{0.5})$O$_{3}$ on the properties of Pb(Zr$_{0.5}$Ti$_{0.5})$O$_{3}$ films fabricated on the polycrystalline titanium and platinum substrates. It is found that in the films on the Pt substrate (with or without the buffer layer) the dependencies of the remanent polarization and the coercivity field on the number of switching cycles do not manifest fatigue up to 10$^{9}$ cycles. The remanent polarization dependencies for films on the Ti substrate with the buffer layer containing an excess of PbO demonstrate an fundamentally new feature that consists of a remanent polarization increase after 10$^{8}$ switching cycles. The increase of remanent polarization is about 50{\%} when the number of cycles approaches 10$^{10}$, while the increase of the coercivity field is small. A monotonic increase of dielectric losses has been observed in all cases. [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q41.00010: Ferroelectric properties in PbZrO$_{3}$/BaZrO$_{3}$ superlattices: an \textit{ab-initio} study Nabil Al-Aqtash, Ahmad Alsaad, Renat Sabirianov Properties of tetragonal (BaZrO$_{3})_{1}$/(PbZrO$_{3})_{\mathrm{n}}$ ferroelectric superlattices with n $=$ 1--3 are calculated from first principles within the density functional theory. We show that an antiferroelectric PbZrO$_{3}$ displays ferroelectric behavior if deposited on a paraelectric substrate (BaZrO$_{3})$. We have performed total energy calculations to investigate the origins of the ferroelectricity and analyze the polarization of BaZrO$_{3}$/(PbZrO$_{3})_{\mathrm{n}}$ superlattices as function of PbZrO$_{3}$ thickness. The densities of states (DOS) show that there is a strong hybridization between Zr/Pb and O atoms which play important role in stabilizing the ferroelectric ground state in the superlattices. Our calculations show that the polarization and tetragonality ($c/a$ ratio) are reduced in the BaZrO$_{3}$/PbZrO$_{3}$ superlattices with respect to bulk tetragonal PbZrO$_{3}$. Moreover, the tetragonality and polarizations of superlattices increase with increasing the fraction of PbZrO$_{3}$ in the superlattices. The estimated polarization of the (BaZrO$_{3})_{1}$/(PbZrO$_{3})_{1}$ superlattice is (38.46 $\mu $C/cm$^{2})$, while it is (56.82$\mu $C/cm$^{2})$ for the (BaZrO$_{3})_{1}$/(PbZrO$_{3})_{3}$ superlattice and (74.22$\mu $C/cm$^{2})$ for bulk tetragonal PbZrO$_{3}$ These ferroelectric superlattices have good lattice matching with shape-memory NiMnIn Heusler alloys and could be very useful as a ferroelectric substrate to systems. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q41.00011: Ferroelectric properties of BaTiO$_{3}$/PbZr$_{0.2}$Ti$_{0.8}$O3 bilayer thin film Pavel Salev, Chun Yang, Alexei Grigoriev The thin film ferroelectric BaTiO$_{3}$/PbZr$_{0.2}$Ti$_{0.8}$O$_{3}$ bilayer was epitaxially grown on SrRuO$_{3}$/SrTiO$_{3}$ substrate by RF sputtering. Electrical measurements of polarization switching revealed two different switching regimes -- a small ferroelectric hysteresis loop at low applied voltage and a larger loop at a high voltage. The measured dielectric permittivity corresponds to weak electrostatic coupling between two layers according to Landau-Ginsburg-Devonshire theory. This weak coupling may allow for independent polarization states to exist in individual layers. This can lead to stable head-to-head and tail-to-tail polarization domain configurations, which would explain the two switching regimes observed in electrical measurements. The compensation of polarization gradient across the interface can be explained by the enhancement of interface charge carrier density due to strong bending of electron energy bands. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q41.00012: Combinatorial studies in Ba$_{0.45}$Sr$_{0.55}$TiO$_{3}$ thin films for microwave components by radio frequency magnetron sputtering Fikadu Alema, Aaron Reinholz, Konstantin Pokhodnya The optimization of dielectric properties of ferroelectric thin films for microwave applications can be limited due to the time and resources consumption of the corresponding device fabrication and testing for each doping level. We report the use of a combinatorial technique to achieve the optimal doping level of Ba$_{0.45}$Sr$_{0.55}$TiO$_{3}$ (BST) thin film with three dopants, Mg, Nb and lanthanide (Ln) metal. The process uses two R.F. magnetron sputtering BST sources doped with few at. {\%} of Mg$^{\mathrm{II}}$/Nb$^{\mathrm{V}}$ in charge compensating concentration and Ln$^{\mathrm{IV}}$, respectively. The guns were shifted and tilted each by 30$^{\circ}$ in opposite directions to realize the dopants gradient across a static wafer. The film is reactively co-sputtered on the static 4'' platinized Al$_{2}$O$_{3}$ wafer. The film crystallinity and phase purity were analyzed and correlated to its dielectric properties measured on 2432 MIM capacitors that are of lithographically fabricated using Pt top electrode. After electrical testing, the wafer was diced into 22 16x16 mm$^{2}$ samples, and the elemental analysis of each piece was performed. The correlation between the composition and dielectric properties was established and the optimal dopant concentrations for obtaining maximum tunability of 75{\%} and minimum loss of 0.02 were determined. [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q41.00013: Interplay of the strain and microstructure in ferroelectric epitaxial CaTiO3 Films Qian He, Qiao Liang, Michael Biegalski, Albina Borisevich CaTiO3 (CTO) was predicted to become ferroelectric under lattice strain. However, other factors such as oxygen octahedral tilts or microstructural details can play a role. In this work, two 20 nm CTO films were grown on LSAT and NGO by PLD. They both show ferroelectricity, with Tc near 140 K on LSAT and near 70 K on NGO, and the remnant polarization at 10K of 5 and 2 $\mu $C/cm, respectively. This is surprising given that the strain of CTO on both substrates is similar. AC-STEM shows two major differences in microstructure between two CTO films: Firstly, the first few nm of CTO on NGO show perfect epitaxial growth, and after that grains start to develop, but the c-axis of CTO remains aligned with the c-axis of NGO, suggesting the presence of 180$^{\circ}$ grain boundaries only. However for CTO/LSAT, grains begin to develop at the interface and their c-axes have two possible orientations, resulting in both 180$^{\circ}$ and 90$^{\circ}$ grain boundaries. These grain boundaries are either dislocations or ferroelastic twins. Secondly, the octahedral tilt behavior at the film/substrate interface is different: CTO/LSAT has a 5-6 unit cell transition region from the untilted LSAT to the tilted CTO, which is not the case in CTO/NGO. The connection between the microstructure, substrate strain and connections to the ferroelectric properties will be discussed in detail. [Preview Abstract] |
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