Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session V14: Focus Session: Electronic and Atomic Structure of Interfaces and Gate Stacks I |
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Sponsoring Units: FIAP Chair: Alex Demkov, University of Texas-Austin Room: LACC 403B |
Thursday, March 24, 2005 11:15AM - 11:51AM |
V14.00001: Current Schottky Barrier Concepts Invited Speaker: The formation of Schottky barrier height at metal- semiconductor interfaces has been a subject of intense investigation and heated debate for over three decades. Early studies largely concentrated on the explanation of the experimentally observed ``Fermi level pinning'' phenomenon, the apparent lack of a strong dependence of the barrier height on the metal work function. Theories proposed early on typically relied on some empirical assumptions/mechanisms to explain the weak dependence of the barrier height on the metal, the most notable of which was the assumption that the distribution of the interface states was independent of the metal. This assumption has subsequently been shown by numerous ab initio calculations to be without basis. Experimental results contrary to the notion of ``pinning'' were also observed. For example, on well-controlled interfaces a sharp dependence of the barrier height on the interface atomic structure was experimentally established, as was a pervasive inhomogeneity of barrier height at polycrystalline interfaces. Recently, a simple analysis, by molecular chemical methods, reveals that the dipoles from bond polarization are in excellent agreement with the observed strength of the Fermi level pinning on different semiconductors. A host of seemingly contradictory experimental observations in the field of Schottky barrier heights thus seems reconcilable within one coherent explanation. In this presentation, a brief account of Schottky concepts, both old and new, will be given. The basis for the recent theoretical chemical analysis, as originally intended for molecular studies, and the steps that were taken for application to solid metal-semiconductor interfaces will be examined. Also will be discussed are further modifications of this scheme for applications to other material interfaces of current interest, such as the energy level alignment at organic interfaces and the band offsets at oxide interfaces. A particular emphasis is placed on the consistency that should be achieved between the condition assumed for the components of the interface and the method adopted to estimate the charge transfer. For the latter, a brief examination of existing concepts of group electronegativity will also be given. [Preview Abstract] |
Thursday, March 24, 2005 11:51AM - 12:03PM |
V14.00002: Interface band alignment in high-k gate stacks Bersch Eric, P. Hartlieb, S. Sayan, R. Bartynski, E. Garfunkel In order to successfully implement alternate high-K dielectric materials into MOS structures, the interface properties of MOS gate stacks must be better understood. Dipoles that may form at the metal/dielectric and dielectric/semiconductor interfaces make the band offsets difficult to predict. We have measured the conduction and valence band densities of states for a variety MOS stacks using \textit{in situ} using inverse photoemission (IPE) and photoemission spectroscopy (PES), respectively. Results obtained from clean and metallized (with Ru or Al) HfO$_{2}$/Si, SiO$_{2}$/Si and mixed silicate films will be presented. IPE indicates a shift of the conduction band minimum (CBM) to higher energy ($i.e$. away from $E_{F})$ with increasing SiO$_{2}$. The effect of metallization on the location of band edges depends upon the metal species. The addition of N to the dielectrics shifts the CBM in a way that is thickness dependent. Possible mechanisms for these observed effects will be discussed. [Preview Abstract] |
Thursday, March 24, 2005 12:03PM - 12:15PM |
V14.00003: Surface States and rectification at a Metal high-k Dielectric Contact Alex Demkov The properties of metal-to-insulator junctions are often discussed in terms of the Fermi level pinning by the interface states. An alternative point of view is based on the picture of polarized chemical bonds at the metal-to-insulator interface. We consider theoretically the case of molybdenum on the (111) surface of the tetragonal polymorph of hafnia, and trace the formation of the Schottky barrier from the Newens-Anderson chemisorption limit to a one nm thick layer of the (110) oriented metal. The role of the surface band of hafnia in the pinning of the Fermi level is discussed, including the analysis of relative roles of the evanescent and chemical interface states. We critically compare the predictions of the metal induced gaps states (MIGS) model with the results of direct density functional calculations. [Preview Abstract] |
Thursday, March 24, 2005 12:15PM - 12:51PM |
V14.00004: Metal screening for CMOS application through vacuum and interface work function ab-initio calculations: benefits and limitations Invited Speaker: Future reduction of transistor dimensions in line with historical trends cannot be achieved with the current SiO$_{2}$/polysilicon technology due to limited effective oxide thickness (EOT) scalability and excessive power consumption caused by high gate leakage current. Among the proposed solutions, the high permissivity dielectric (high-K)/metal combination seems to be a promising route. While considerable progress has been made towards identifying a favorable high-K dielectric, with HfO$_{2}$ and its silicates and nitrides as the leading candidates, n- and p-type metal gates with appropriate work functions still lack. A rough, ``first order'' metal gate screening can be performed with considerable confidence through measurement and calculation of metal vacuum work functions (WFs). However, charge exchange at metal/dielectric interfaces cause the metal effective WF on a particular dielectric to differ from its vacuum value, sometimes by as much as 1 eV [1]. For that reason, metal effective WF \textit{ab-initio} calculations using interface models are of primary importance if theory is to be used as a guide for correctly identifying metal gates. In this talk I will discuss the role of interface states on the pinning of metal Fermi levels and show results for model HfO$_{2}$/Si and Al$_{2}$O$_{3}$/Si interfaces that correctly reproduce experimental data with polysilicon as the gate metal [2]. Next I will describe results of theoretical metal screening for polysilicon replacement. We have found that while vacuum WF calculations can be quite accurate, hence useful as a predictive tool, metal/dielectric interface calculations are severely limited in accuracy by the lack of experimental information on the atomistic structure of the interfaces and possibly by an unexpected and still unclear drawback of density functional theory (DFT) within the local density approximation (LDA) [3]. Improvements based on empirical scaling of the DFT/LDA calculated metal/dielectric valence band offset and on bulk GW calculations of the dielectric valence band edge shift with respect to DFT/LDA results will be discussed. Finally I will describe experimental difficulties encountered when trying to modulate metal WFs through interface engineering and how simulations have contributed to understand why that may be so. [1] J. K. Schaeffer, L. R. C. Fonseca, S. B. Samavedam, Y. Liang, P. J. Tobin, and B. E. White, Appl. Phys. Lett. \textbf{85}, 1826 (2004). [2] C.C. Hobbs \textit{et al.}, \textit{IEEE Trans. Elec. Dev.} \textbf{51}, 971/978 (2004). [3] L. R. C. Fonseca and A. A. Knizhnik, unpublished. [Preview Abstract] |
Thursday, March 24, 2005 12:51PM - 1:03PM |
V14.00005: Properties of interfaces between metals and binary oxides. Matias Nunez, Marco Buongiorno Nardelli Metallic gate contacts are fundamental components of MOSFET architectures, and understanding their physical properties at a fundamental level is of great importance for the engineering of advanced electronic devices. In this poster we will present preliminary results of a comprehensive ab initio study of the structural and electronic properties of interfaces between metals and high-k dielectrics, mostly crystalline binary oxides. Our primary interest is in the characterization of the influence of lattice matching and chemical composition at the interface on the Schottky barrier formation and properties. In particular we will frame our results in a broad perspective that embraces, at its ends, the Bardeen and Schottky views of the band alignment problem. [Preview Abstract] |
Thursday, March 24, 2005 1:03PM - 1:15PM |
V14.00006: Bloch States of the Interface Phase Curt Billman, Fred Walker, Marco Buongiorno-Nardelli, Rodney McKee Electrical conductance and capacitance measurements of metal oxide semiconductor devices have revealed the existence of Bloch states that are associated with the interface phase that is formed between commensurate Ba$_{.72}$Sr $_{.28}$O and Si(001). Conductance measurements of the interface states show loss peaks that can be modeled using a single relaxation time in contrast to the distribution of relaxation times required to reproduce conductance measurements of interface traps at the amorphous SiO$_{2}$ / Si interface. The measured magnitude, which is on the order of 4x10$^{13}$ eV$^{-1}$ cm$^{-2}$ and the character of the interface phase Bloch state (IPBS) density is consistent with local density approximation (LDA) calculations using a physical structure based on a one mono-atomic layer silicide. Research sponsored jointly by the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy at Oak Ridge National Laboratory under contract DE-AC05-00OR22725 with UT-Battelle, LLC and at the University of Tennessee under contract DE-FG02-01ER45937. Calculations have been performed on CCS supercomputers at Oak Ridge National Laboratory. [Preview Abstract] |
Thursday, March 24, 2005 1:15PM - 1:51PM |
V14.00007: Intrinsic limitations for gate stack applications of complex high-k oxides in advanced Si devices: band edge states Invited Speaker: Valence and conduction band edge electronic states in high-k oxide dielectrics have been studied by X-ray absorption spectroscopy (XAS), ultra-violet photoemission spectroscopy (UPS), and vacuum ultra- violet spectroscopic ellipsometry (VUVSE) and photoconductivity (PC). These studies confirm results of \textit{numerous} theoretical studies which have demonstrated that valence and conduction band electronic states are comprised of transition metal/rare earth (TM/RE) atom d-states mixed with O-atom 2p states. Electronic states at the top of the valence band and bottom of the conduction band have a $\pi $-bonding symmetry, while those deeper in the valence band and higher in the conduction band have a $\sigma $-bonding symmetry. XAS studies of \textit{empty} TM/RE d-states by transitions from deep TM/RE p-states are combined with studies of conduction band edge states by transitions from the O-atom 1s state to provide qualitative and quantitative insights into electronic structure at the conduction band edge. This approach was first applied to HfO$_{2}$ and TiO$_{2}$, and then to the \textit{complex/binary oxides}: i) Zr$_{x}$Ti$_{1-x}$O$_{4}$, with x = 0.67 and 0.33, LaAlO$_{3}$, and LaScO$_{3}$. Thin films of these oxides are nano-crystalline as-deposited and/or after an anneal in an inert ambient at 500 to 1000\r{ }C. Analysis of the XAS spectra indicate that d- state degeneracies are completely removed for Hf in HfO$_{2}$, Ti in TiO$_{2}$ and the Zr titanates, La in LaAlO$_{3}$, and Sc in LaScO$_{3}$. This removal indicates a distorted local bonding arrangement for these TM/RE atoms, or equivalently \textit{Jahn-Teller term splittings }that increase the total binding energy. More importantly, the term split states identified in XAS spectra are directly correlated with d-state features at the conduction band edge by VUVSE and PC. These localized $\pi $-bonded states limit performance and reliability in scaled Si devices, and are associated with \textit{asymmetric} bias voltage dependent electron transport and trapping. [Preview Abstract] |
Thursday, March 24, 2005 1:51PM - 2:03PM |
V14.00008: Optical properties and metrology of the high-k/Si interface Stefan Zollner, Yong Liang, David Theodore, Z. Yu, Dina Triyoso, Jay Curless, Clarence Tracy Since future CMOS devices require equivalent gate oxide thicknesses on the order of 10-15 {\AA}, control and measurements of the interfacial layer at the Si/metal oxide interface are important. We deposited typical 35 to 150 {\AA} thick HfO$_{2}$ layers on Si for gate stack applications and measured these layers using variable-angle vacuum-UV spectroscopic ellipsometry (0.74 to 9.5 eV, rotating analyzer instrument with Berek waveplate compensator) and x-ray reflectivity (XRR: $\omega <$20000'', $\Delta \omega $=20'', dynamic range: 7 orders of magnitude) on commercial instruments. All our spectra (even for annealed films) can be fitted using a single layer (HfO$_{2}$ on Si), apparently containing no information about the amorphous interfacial layer, which is expected to be 5-10 {\AA} thick. We conclude that ellipsometry and XRR measurements on tool sets similar to ours cannot determine the thickness of the SiO$_{2}$ interfacial layer at the Si/high-k metal oxide interface. We also show that significant variations of the optical constants of high-k materials are possible, depending on growth conditions. [Preview Abstract] |
Thursday, March 24, 2005 2:03PM - 2:15PM |
V14.00009: Enhanced permittivity of the interfacial oxide in gate insulator stacks on silicon Alfredo Pasquarello, Feliciano Giustino While transition metal oxides are currently under consideration for the scaling of the equivalent oxide thickness in MOS devices, a thin SiO$_2$ layer often forms at the channel interface during the deposition process, affecting the gate capacitance. Within a density functional approach, we investigate the dielectric permittivity and estimate the equivalent oxide thickness of this interfacial layer. For this purpose, we consider a realistic model of the Si-SiO$_2$ interface which takes into account the amorphous nature of the oxide. Our calculations indicate that the static permittivity of the 0.5 nm thick interfacial layer is about 50\% larger than that of bulk silica. As a consequence, the equivalent thickness of the substoichiometric layer is smaller than the corresponding physical thickness by 0.2--0.3 nm. By spatially mapping the frequency-dependent dielectric response, we show that the enhanced permittivity of the interfacial layer originates from the larger electronic contribution of the Si--Si bonds and from the softening of the infrared active modes in the substoichiometric oxide. [Preview Abstract] |
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