### Session Q33: Focus Session: Complex Oxide Thin Films -- Oxide/Semiconductor Interfaces and Defects

 Wednesday, March 17, 2010 11:15AM - 11:51AM Q33.00001: Interface chemistry between complex oxides and semiconductors: where chemistry and physics meet Invited Speaker: Chiara Marchiori Even though heavily based on semiconductors, microelectronics CMOS technology would not exist without the integration of thin oxide films which enable the exploitation of the semiconductor properties. Indeed, working principle of the metal-oxide-semiconductor field-effect transistor, the main building block of such a technology, is the modulation of charges at the oxide/semiconductor interface. The quality of this interface is of fundamental importance for device performance. For over four decades, SiO$_{2}$ was the gate dielectric of choice and device scaling meant improving performance while lowering production costs. However, as scaling is approaching fundamental limits, direct tunneling across the dielectric becomes unacceptable. At this point, the integration of more complex and higher dielectric constant oxides - high-K dielectrics''- with Si or even more complex semiconductors (Ge, III-V) is the key enabler of performance gain. I will review critical issues related to the oxide/semiconductor interfaces, starting with SiO$_{2}$/Si. Then, I will discuss how the level of complexity increases with the introduction of high-K dielectrics and other semiconductors in the stack. Among the issues to be addressed to fabricate high-performance devices, I will discuss the role played by: 1) interfacial chemistry and thermodynamical stability, 2) band alignment and surface band bending, 3) presence of defects at the interface and in the oxide bulk, 4) evolution of the gate stack properties upon post-deposition treatments. The impact of these parameters on electrical performance of devices will be discussed in detail. Finally, epitaxial oxide on Si will be explored as a promising approach for ultimate EOT scaling and the parameters governing the epitaxial growth of complex crystalline oxides on Si will be addressed. I will show that the development performed in this area might enable the integration of epitaxial oxides for monolithic integration, paving the way to technological developments that go beyond the simple Moore's scaling law Wednesday, March 17, 2010 11:51AM - 12:03PM Q33.00002: \textit{In-situ} XPS, STM and STS analyses of high k oxide/III-V interfaces Mao-Lin Huang , Yu-Shing Chang , Pen Chang , Han-Chin Chiu , Jyun-Yang Shen , Tsung-Da Lin , J. Raynien Kwo , Minghwei Hong , Tun-Wen Pi The new technology of high-k plus metal gate on high carrier mobility semiconductors like Ge and InGaAs hybrid with Si are urgently pursued for CMOS scaling beyond 15 nm node. The D$_{it}$ distributions of these high k/semiconductor interfaces are generally governed by a peak near the mid-gap, unlike SiO$_{2}$/Si showing a flat D$_{it}$ over the entire energy gap. To remove the mid-gap peak in further reducing D$_{it}$, \textit{in-situ} ALD growth of high k oxides such as Al$_{2}$O$_{3}$, HfO$_{2}$ on various reconstructed In$_{x}$Ga$_{1-x}$As(001) surfaces prepared by molecular beam epitaxiy have been conducted to tailor the interface on the atomic level. The mechanisms of interfacial traps at high-k/III-V heterostructures were investigated by using \textit{in-situ} x-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS), particularly in the initial stage of interfacial reactions. The surface species and chemical bonding configurations responsible for unpinned Fermi level are being determined. Wednesday, March 17, 2010 12:03PM - 12:15PM Q33.00003: In situ synchrotron measurements of surface compensation mechanisms in La$_{0.6}$Sr$_{0.4}$Co$_{0.2}$Fe$_{0.8}$O$_{3-\delta}$ thin films Tim Fister , Stephan Hruszkewycz , Dillon Fong , Jeffrey Eastman , Paul Fuoss , Hui Du , Paul Salvador With its desirable combination of thermal stability, catalytic activity, and electronic and ionic conductivity, La$_{0.6}$Sr$_{0.4}$Co$_{0.2}$Fe$_{0.8}$O$_{3-\delta}$ (LSCF) is rapidly becoming the standard cathode material for solid oxide fuel cells (SOFCs). Prior electrical measurements have isolated oxygen reduction at the cathode as a primary rate-limiting step in the performance of SOFCs. To better understand the nature of oxygen reduction at the high temperature, atmospheric oxygen partial pressure (pO2) conditions of a working SOFC, we study epitaxial LSCF thin films grown on (001)p-oriented NdGaO$_3$ and SrTiO$_3$ using in situ x-ray scattering and spectroscopy methods. We find that at sufficiently high temperatures, LSCF forms surface reconstructions at atmospheric pO2 levels. Using grazing-incidence spectroscopy methods, we also find that strontium segregates to the surface and that the minority B-site cation, cobalt, responds to changes in pO2 and temperature. We discuss the interplay between these changes in surface composition and structure and its implication on oxygen reduction in SOFCs. Wednesday, March 17, 2010 12:15PM - 12:27PM Q33.00004: First-principles study of charged vacancies in PbTiO3 Yanpeng Yao , Huaxiang Fu First-principles calculations within local density approximation (LDA) are performed to study the vacancy formation energies for charged vacancies in PbTiO$_3$. Within the constraint of thermodynamic limit, the chemical potential and Fermi energy dependence of the vacancy formation energy, as well as the transition energy levels, are studied. We found that: (i)the Formation energy of $V_{O}^{2+}$ is much lower than that of the neutral oxygen vacancy, irrespective of $\mu_O$; (ii) Under oxygen rich condition, the main defect is $V_{Pb}^{2-}$; (iii) When oxygen is deficient, either $V_{Pb}^{2-}$ or $V_{O}^{2+}$ can form, depending on the Fermi energy level; (iv) At certain condition, both $V_{Pb}^{2-}$ and $V_{O}^{2+}$ have similar formation energy, and can coexist. Wednesday, March 17, 2010 12:27PM - 12:39PM Q33.00005: Coupled effect of Lanthanum Defects and Oxygen Vacancies in Strontium Titanate Varadharajan Srinivasan , Elif Ertekin , Jeffrey Grossman Strontium Titanate, a high-dielectric constant paraelectric perovskite, exhibits multiple interesting properties, including an insulator-metal transition induced by electron doping and low temperature superconductivity. Electron doping can be achieved in a variety of manners, including (1) introduction of a large concentration of Lanthanum substitutional defects for Strontium and (2) the presence of Oxygen vacancies. We have explored, via total energy electronic structure methods based on Density Functional Theory, the coupled effect of substitutional Lanthanum defects and oxygen vacancies for a variety of configurations, and find that bound vacancy-Lanthanum pairs result in even larger increase in the carrier concentration than the individual defects alone. Strong electron localization around Oxygen atoms is observed in systems that are Oxygen deficient; these defect states lie near the Fermi level. When both Lanthanum defects and Oxygen vacancies are present, the localized defect states are pushed below the Fermi level, resulting in an even larger increase in carrier concentration. Wednesday, March 17, 2010 12:39PM - 12:51PM Q33.00006: Ionic-bombardment, an effective way to create memory effect on undoped SrTiO3 Yong Seung Kim , Heiko Gross , Namrata Bansal , Carlos Chaparro , Seongshik Oh Here we present bistable switching behaviors in undoped SrTiO3 single crystals treated by thermal annealing and Ar-ion-beam bombardment. So far, there have not been any reports of memory effect at a macroscopic scale for SrTiO3 having no cation dopants. We observed memory effects in both thermally annealed and ion-beam bombarded samples. However, the ion-beam bombarding provided significant better switching behaviors than the thermal annealing. We propose that this difference between the annealed and the ion-beam-bombarded sample in their switching performance is related to the concentration of oxygen vacancy clusters. Clustered vacancies are much less mobile than a single vacancy, and thus not an effective player for the switching event. Thermal annealing produces more clustered vacancies than ion-beam-bombardment. Creating vacancies while minimizing the clustering rate must be the key to a viable resistive memory device. We show that ionic bombardment is an effective method for such devices. Wednesday, March 17, 2010 12:51PM - 1:03PM Q33.00007: Ab-initio investigation of the octahedra rotation in strained Ruddlesden-Popper(RP) phases Jeehye Lee , Tomas Arias This talk presents the result of an ab-initio study of Ruddlesden-Popper(RP) phases for $Sr_{n+1}Ti_{n}O_{3n+1}$($n = 1, 2, 3, \infty$) compounds with the density functional theory(DFT). In particular, we present results for the relation between octahedral rotations and external strains. We find that rotational reconstructions are energetically favorable under nonzero in-plane strains with a critical strain that depends on both $n$ and rotational configurations. We also explore the relative stability of RP phases with different values of $n$ under epitaxial conditions. Wednesday, March 17, 2010 1:03PM - 1:15PM Q33.00008: Controllable modification of fractured Nb-doped SrTiO$_{3}$ surfaces Nathan P. Guisinger , TeYu Chien , Tiffany S. Santos , Matthias Bode , John W. Freeland Nano-scale surface modification of a fractured Nb-doped SrTiO$_{3}$(Nb:STO) surface is demonstrated in a controlled way using scanning tunneling microscopy. It is revealed with high-resolution images that by fracturing Nb:STO at low temperature, about 50 \% of SrO adatoms are randomly distributed on large ($>$500 nm) TiO$_{2}$ terminated terraces. By applying positive voltage pulses with appropriate bias and pulse duration, holes can be created with desired width and depth. By applying negative bias, the hole can be partially refilled from the transfer of adsorbates on the tip. The change of dI/dV contrast when creating/refilling the hole is consistent with the model of exposure/covering of the underlying TiO$_{2}$ layer by removal/deposition of SrO. This entire modification process can be explained by a surface atom manipulation mechanism. Wednesday, March 17, 2010 1:15PM - 1:27PM Q33.00009: Nanometer-scale striped surface terminations on fractured SrTiO$_{3}$ surfaces Jeffrey Guest , Nathan Guisinger , Tiffany Santos , Te-Yu Chien , Anand Bhattacharya , John Freeland , Matthias Bode Using cross-sectional scanning tunneling microscopy on in situ fractured SrTiO$_{3}$, one of the most commonly used substrates for the growth of complex oxide thin films and superlattices, atomically smooth terraces have been observed on (001) surfaces. Furthermore, it was discovered that fracturing this material at room temperature results in the formation of stripe patterned domains having characteristic widths (10 to 20 nm) of alternating surface terminations that extend over a long-range. Spatial characterization utilizing spectroscopy techniques revealed a strong contrast in the electronic structure of the two domains. Combining these results with topographic data, we are able to assign both TiO$_{2}$ and SrO terminations to their respective domains. The results of this experiment reveal that fracturing this material leads to reproducibly flat surfaces that can be characterized at the atomic-scale and suggests that this technique can be utilized for the study of technologically relevant complex oxide interfaces. Wednesday, March 17, 2010 1:27PM - 1:39PM Q33.00010: Probing complex oxide interfaces by cross-sectional scanning tunneling microscopy Teyu Chien , Jian Liu , Jacques Chakhalian , Nathan P. Guisinger , John W. Freeland Recently, complex oxide materials have drawn tremendous attention, especially the new physics emerging at interfaces in oxide heterostructures. With the advent of oxide thin film synthesis, high-quality epitaxial thin films with sharp interfaces between different types of complex oxides can nowadays be grown routinely. While there exist many tools that can provide a spatial picture of the structural and chemical environment in oxide heterostructures, probes which can gain insight into the properties of carriers at interfaces are limited. By utilizing cross-sectional scanning tunneling microscopy (CS-STM), we have been able to create atomically flat terrace on non-cleavable Nb-doped SrTiO$_{3}$ routinely. Here, we present our initial results on oxide heterostructures with CS-STM, which is a crucial tool to reveal new physics emerging at oxide interfaces. Wednesday, March 17, 2010 1:39PM - 1:51PM Q33.00011: Dislocation pileup in He implanted complex oxides Avishai Ofan , Lihua Zhang , Ophir Gaathon , Kenneth Evans-Lutterodt , Sasha Bakhru , Hassaram Bakhru , David Welch , Yimei Zhu , Richard M. Osgood, Jr. The origin of the anomalous high spatial selective etch rate in a 0.4 $\mu$m-wide buried deeply implant region in complex oxides is studied. LiNbO$_{3}$ samples, prepared with 10 $\mu$m implantation depth and 5x10$^{16}$ cm$^{-2}$ He$^{+}$ fluence, show wet-etch-rate enhancement of 10$^{4}$ occurs after implantation and a narrow window (175-275 $^{\circ}$C) of low-temperature post-implantation annealing. Examination using time-resolved optical microscopy, XRD, and TEM show that this enhancement arises from the more rapid etch-solution transport in the highly dense network of dislocations pileups formed in the implanted region after annealing. Full curing of the dislocations takes place at 350 $^{\circ}$C. A mechanism for the pileups build-up and disappearance is suggested. Wednesday, March 17, 2010 1:51PM - 2:03PM Q33.00012: Strain Screening by Mobile Oxygen Vacancies in Strontium Titanate Yongsam Kim , Timur Babakol , Ankit Disa , Joel Brock Small variations in the concentrations of oxygen vacancies in complex-oxide materials such as SrTiO$_{3}$ produce tremendous changes in physical properties such as the carrier density. Consequently, characterizing and controlling the oxygen vacancy concentration has become of interest both technologically and fundamentally. Recently, Freedman et al [1] calculated the elastic dipole tensor for several types of point defects in SrTiO$_{3}$ and showed that the appropriately averaged tensor for an oxygen vacancy is nearly traceless, providing a mechanism whereby highly mobile oxygen vacancies screen the elastic strain field. Here, we report detailed diffuse x-ray scattering measurements of bulk SrTiO$_{3}$ crystals prepared with controlled oxygen vacancy distributions under varying uniaxial stress states, demonstrating the traceless nature of the elastic dipole tensor, correlations between vacancies, and elastic strain screening. \\[4pt] [1] D.A. Freeman, et al, Phys. Rev. B \textbf{80}, 064108 (2009) Wednesday, March 17, 2010 2:03PM - 2:15PM Q33.00013: First Principles Study of the Electronic Properties of Biaxially Strained Strontium Titanate Robert Berger , Craig Fennie , Jeffrey Neaton Strontium titanate (STO) is one of a handful of materials that can be used to convert solar photons into hydrogen fuel. However, its band gap (3.1 eV) is not well matched to the solar spectrum, reducing its potential as an efficient photocatalyst. Through the growth of epitaxial thin films of STO on lattice mismatched substrates, experimentalists can now tune STO through a large structural parameter space and potentially optimize its electronic properties for photocatalysis. In this work, the effect of biaxial strain on the band structure of STO is explored using DFT and many-body perturbation theory within the GW approximation. In these calculations, in-plane lattice parameters are fixed, and all other structural parameters are relaxed. Changes in band gaps and band edge energies are investigated for strains within 5 percent of the cubic lattice parameter, and compared with experimental results. This work is supported by DOE through the Energy Materials Center, an EFRC at Cornell University.