Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session U20: High-K Dielectrics |
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Sponsoring Units: DMP Chair: Rodney McKee, ORNL Room: LACC 407 |
Thursday, March 24, 2005 8:00AM - 8:12AM |
U20.00001: Theory of structural and dielectric properties of amorphous high-K dielectrics David Vanderbilt, Davide Ceresoli Hafnia (HfO$_2$) and zirconia (ZrO$_2$) are leading candidates for replacing SiO$_2$ as the gate insulator in CMOS technology. Amorphous versions of these materials ($a$-HfO$_2$ and $a$-ZrO$_2$) can be grown as metastable phases on top of a silicon buffer; while they tend to recrystallize during subsequent annealing steps, they would otherwise be of considerable interest because of the promise they hold for improved uniformity and electrical passivity. In this work, we report our studies of $a$-HfO$_2$ and $a$-ZrO$_2$ by first-principles density-functional methods. We construct realistic amorphous models by two different techniques: (i) a ``melt-and-quench'' molecular dynamics approach, and (ii) an ``activation-relaxation technique'' (ART). In both cases, the structural, vibrational, and dielectric properties of the resulting models are analyzed in detail. The overall average dielectric constant is computed and found to be comparable to that of the monoclinic phase. Although the ART technique (as implemented by us in the SIESTA package) allows a dramatic saving in computational time, it yields results of similar quality with respect to ab-initio MD simulations done with the VASP code. These techniques show promise for future modeling of high-K dielectric ultrathin films and interfaces [Preview Abstract] |
Thursday, March 24, 2005 8:12AM - 8:24AM |
U20.00002: Dielectric properties of high-k oxides: Theory and experiment for Lu2O3 Emiliano Bonera, Giovanna Scarel, Marco Fanciulli, Pietro Delugas, Vincenzo Fiorentini The quest for a dielectric for the replacement of silica in Si-based devices has focused the attention of the scientific community on the class of high dielectric constant ($\kappa )$ insulators. In this work we unfold the physical origin of $\kappa $ and its direct connection with lattice dynamics and polarizability in these materials, analyzing the specific case of Lu$_{2}$O$_{3}$ in its ground-state bixbyite structure with a combined experimental and theoretical analysis. The vibrational dielectric function of Lu$_{2}$O$_{3}$ thin films grown by atomic-layer deposition was studied by infrared transmission and reflection-absorption spectroscopies, selectively accessing transverse and longitudinal optical frequencies. The static dielectric constant was extracted analyzing the infrared response. We also present first-principles density-functional linear-response calculations, which are in close agreement with experiment, and provide insight into the microscopic nature of vibrational spectra and dielectric properties. [Preview Abstract] |
Thursday, March 24, 2005 8:24AM - 8:36AM |
U20.00003: Novel Chalcogenide Buffer Layer for Oxide Heteroepitaxy on Si(001) D. Schmidt, T. Ohta, Q. Yu, F. S. Ohuchi, M. A. Olmstead We have developed a novel chalcogenide buffer layer for heteroepitaxial growth of oxides on silicon and applied it to growth of anatase TiO$_2$. Anatase, nearly lattice-matched to Si(001), is of interest for both spintronic and high-K dielectric applications: it can be made ferromagnetic at room temperature by doping with Co, and has a very large dielectric constant. Through use of a sub-nanometer buffer layer, Ga$_2$Se$_3$ grown on As terminated Si(100), we have been able to grow anatase nanocrystals on Si(001) without any detectable substrate oxidation or silicide formation. The As termination prevents silicon-selenide formation, and the gallium selenide prevents substrate oxidation. The cubic structure of Ga$_2$Se$_3$ offers a stable face for TiO$_2$(001) growth. In addition, the Ga$_2$Se$_3$ layer, with a lattice constant between Si and TiO$_2$, has a structure with inherent vacancies that can absorb strain, acting as a strain relief layer for the TiO$_2$ on Si. [Preview Abstract] |
Thursday, March 24, 2005 8:36AM - 8:48AM |
U20.00004: THz Optical Response from Coupled Ferroelectric/LO Phonon Mode in BaTiO3/Si(100) Muneaki Hase, Masahiro Kitajima, Venu Vaithyanathan, Darrell G. Schlom, Jeremy Levy, Hrvoje Petek The large second-order polarizabilities of epitaxially grown ferroelectrics can be used to generate THz-bandwidth electric fields in semiconductors. Epitaxial films of BaTiO$_{3 }$have been grown on Si(100) using oxide-MBE. X-ray diffraction measurements indicate a high degree of structural perfection and an out-of-plane polarization direction. The coupling between BaTiO$_{3}$ and Si is investigated using an optical pump-probe transient reflectivity experiment that uses 10 fs pulses of light centered around $\lambda $=400 nm. The pump pulse excites the LO phonon mode of the Si substrate directly (15.5 THz) as well as the lower frequency soft modes (1-4 THz) of the ferroelectric film. The observed Fano lineshape, obtained by Fourier transforming the time-domain data, suggests a strong coupling between the LO phonon and ferroelectric modes. The photon energy dependence of the observed electro-optic response suggests that the electro-optic response is generated by the LO phonon and is mixed strongly with the ferroelectric response. The high bandwidth coupling of optical and electronic degrees of freedom is promising for silicon-based quantum computing applications. This work was supported by the NSF, MEXT, NIMS, and DARPA QuIST. [Preview Abstract] |
Thursday, March 24, 2005 8:48AM - 9:00AM |
U20.00005: Valence band offsets and interface structure of HfxSi1-xO2 films on Si(111) from photoemission spectroscopy John Rowe, Les Fleming, Gerry Lucovsky, Marc Ulrich We have used synchrotron radiation to perform high resolution soft x-ray photoemission spectroscopy measurements on Si(111)/Hf$_{x}$Si$_{1-x}$O$_{2}$ films. Our samples included both thick ($\sim $ 75 {\AA}) and thin ($\sim $ 10 {\AA}) silicate films. All samples were grown by remote plasma enhanced chemical vapor deposition (RPECVD) at a temperature of 300 $^{\circ}$C using hafnium tert-butoxide and silane in a helium carrier gas. Si 2$p$ and Hf 4$f$ core levels were studied along with valence band spectra using photon energies of 64 and 150 eV. Measurements of the valence band edge were also unexpectedly high for the thick silicates, which may be attributable to bound charge in the form of defects in the films. The band offset parameter was estimated for the thin film compositions based on the Si $2p_{3/2}$ substrate peak position and its known binding energy with respect to the Si valence band edge (98.75 eV). Offsets for the thin film silicates ranged from 3.31 -- 3.9 eV, with an average value of 3.42 $\pm $ 0.03 for x $>$ 0.2. Core-level binding energies exhibit a nonlinear dependence with alloy composition in the thick silicate films, while the thin films show a linear dependence. A decrease in binding energy for both the Si 2$p_{3/2}$ [Si$^{4+}$] and Hf 4$f_{7/2}$ was observed as the composition changed from SiO$_{2}$ to HfO$_{2}$. [Preview Abstract] |
Thursday, March 24, 2005 9:00AM - 9:12AM |
U20.00006: Infrared absorption spectra at interfaces from first principles: Origin of LO and TO red shifts in ultrathin oxide films on silicon Feliciano Giustino, Alfredo Pasquarello The interpretation of the vibrational spectra of thin SiO$_2$ films on Si(100) has recently been challenged by the observation of a pronounced thickness-dependent red shift of the asymmetric oxygen stretching mode in Si--O--Si intertetrahedral bridges. The origin of this red shift has variably been ascribed to compressive strain of the interfacial oxide, to void incorporation, or to the presence of substoichiometric silica. We here clarify the origin of this red shift by using a density functional approach. For this purpose, we first introduce a formalism to calculate both the longitudinal and the transverse infrared absorption spectra of thin films on transparent substrates. Then, we extend our formulation in order to spatially map the absorption intensity across interfaces. When applying this method to a realistic model of the Si(100)-SiO$_2$ interface, we find that that the red shift arises from a softening of the stretching modes in the substoichiometric interfacial layer. The cumulative effect of an interfacial layer with reduced vibrational frequencies and of a stoichiometric oxide with bulk-like modes is shown to consistently explain the thickness-dependent shift observed in experiments. [Preview Abstract] |
Thursday, March 24, 2005 9:12AM - 9:24AM |
U20.00007: Thickness measurement of interfacial layer between HfO$_2$ film and Si substrate by Fourier analysis of x-ray reflectivity Y. J. Park, J.-S. Lee, B. H. Seung, S. Ji, K.-B. Lee, H. S. Hwang Thickness of interfacial layers between Si-substrates and HfO$_2 $ films have been estimated by Fourier analysis of x-ray reflectivity. It is demonstrated that enhancement of the signals corresponding to the positions of low-density-contrast interfaces can be achieved through careful data processing in Fourier analysis. Details of the data processing procedures and comparison between results of the analysis and TEM measurements are presented. [Preview Abstract] |
Thursday, March 24, 2005 9:24AM - 9:36AM |
U20.00008: Thin single crystal Sc2O3 Films on Si (111) with very sharp interface M. Hong, H. Y. Lee, A. R. Kortan, J. Kwo, P. Chang, Y. L. Huang, C. P. Chen We report the MBE growth and single-crystal synchrotron x-ray characterization of very high quality Sc$_{2}$O$_{3}$ films grown on Si (111). The Sc$_{2}$O$_{3 }$films of 3.5 and 18 nm thickness were deposited by e-beam evaporation on Si in a multi-chamber MBE/UHV system. Streaky RHEED patterns with a 4x4 reconstruction along the in-plane axes of Si were observed after an oxide growth $\sim $1 nm in thickness. X-ray diffraction results find that Sc$_{2}$O$_{3}$ films grow epitaxially in Bixbyite structure with the axis orientations aligned with those of Si. The 3.5 nm film yields a mosaic scan width of 0.044$^{\circ}$ (158 arc-sec.) for the Sc$_{2}$O$_{3}$ (444) peak, which is remarkably sharp, and suggests a defect free structure for the epi-layer. Since the bulk lattice constants of Si and Sc$_{2}$O$_{3}$ are mismatched by 9.2{\%}, the observed perfection in the film structure is very unusual. Electrical measurements of the Sc$_{2}$O$_{3}$ film exhibit low-leakage currents and cross-section TEM studies on Sc$_{2}$O$_{3}$/Si show an extremely sharp interface. [Preview Abstract] |
Thursday, March 24, 2005 9:36AM - 9:48AM |
U20.00009: Electronic properties and band alignments of Hf-based gate dielectrics on silicon R. Puthenkovilakam, J. Choi, J.P. Chang Ultra thin Hf-based dielectrics are being considered for possibly replacing SiO$_2$ gate oxide in silicon based metal- oxide-semiconductor (MOS) transistors. In this work we investigated the electronic structure and band alignments of the Hf-based gate dielectrics on silicon, which dictate the device performance of transistors in the sub-90nm devices. The electronic structure of HfO$_2$/Si interface showed dangling bond states at the interface due to the reduced coordination of Hf caused by the intrinsic bond constraints at the HfO$_2$/Si interface. However, our calculations indicate that these dangling bonds could be passivated by hydrogen or oxygen, which can appropriately change the coordination numbers at the interface, thereby removing the dangling bond states. We also considered the interface of HfSiO$_4$/Si and found that there are no dangling bond states at the interface, making HfSiO$_4$ a promising interfacial layer to improve the interface quality. Band offsets at the interfaces with Si were calculated using density functional theory, and it showed that the band offsets vary depending upon the interface stoichiometry. Band alignments were also determined experimentally using XPS and were in excellent agreement with the theoretical results. Incorporation of nitrogen into the HfO$_2$ network resulted in notable changes in the valence band structure of the material and the corresponding band alignments with silicon and is found to depend on the nitrogen concentration in the bulk of HfO$_2$ as well as at the interface. [Preview Abstract] |
Thursday, March 24, 2005 9:48AM - 10:00AM |
U20.00010: High-K MISFET channel mobility from magnetoresistance R.T. Bate, W.W. Chance, P. Kumar, W.P. Kirk Carrier trapping in the gate insulator or at the interface with the silicon can depress the effective channel mobility of high-K MISFETs below the drift mobility. This reduction in effective mobility can be distinguished from true mobility reduction due to carrier scattering by using the Hall effect to measure the channel carrier density [1]. However, channel Hall measurements have traditionally required nonstandard multidrain devices, which must be included in the test chip design. We propose measuring the reduction in drain current of conventional transistors by a magnetic field to determine the Hall coefficient. This method, which requires no multidrain devices, could become a routine diagnostic procedure. It is based on a theorem concerning the magnetoresistance of a rectangular plate with perfectly conducting end contacts [2], which has apparently not been tested experimentally, at least on MOSFET's. The validity of the method can be determined by comparison with channel carrier density determined in other ways, including split capacitance on MOSFETS, conventional Hall effect, and Corbino magnetoresistance on MISFETs. Progress toward these goals is described. [1] N.S Saks and A.K Agarwal, \textit{Appl. Phys. Letters} \textbf{77 }(20), 3281 -- 3283 (2000); R. T. Bate and W. P. Kirk, \textit{Bull. Am. Phys, Soc}. March, 2004, Abstract S6.011 [2] H. H. Jenson and H. Smith, \textit{J. Phys. C: Solid State}, \textbf{5,} 2867-2880, (1972) [Preview Abstract] |
Thursday, March 24, 2005 10:00AM - 10:12AM |
U20.00011: First-principles investigation of oxygen diffusion in compressively strained high-density silicon oxide T. Akiyama, H. Kageshima, M. Uematsu, T. Ito Oxidation of Si nanostructures is a key process in the fabrication of future Si-nano devices such as single electron transistors. It is well known that the oxidation is strongly affected by the initial structural size and shape, called pattern dependent oxidation. This characteristic can be intuitively understood by the oxidation retardation by the oxidation-induced strain: The diffusion of oxygen in the oxide is suppressed by the strained high-density oxide region generated in the nanostructures. However, the understanding of this phenomenon on atomic scale still remains unknown. In this work, we investigate microscopic mechanisms of oxygen diffusion in the strained high-density $\alpha$-quartz based on first-principles total-energy calculations. The calculations show that both incorporation of O$_2$ molecules into the oxide and its migration are significant factors of oxygen diffusion in the high-density $\alpha$-quartz. The calculated activation energy increases by 1.2 eV with a 10\% increase of density, indicating that the diffusion of oxygen can be suppressed by the high-density region. [Preview Abstract] |
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U20.00012: Size effect in self-trapped exciton photoluminescence from SiO2-based nanoscale materials Yuri Glinka Direct evidence for a size effect in self-trapped exciton (STE) photoluminescence (PL) from SiO$_{2}$-based nanoscale materials as compared to bulk type-III fused silica is obtained. The PL was induced by the two-photon absorption of focused 6.4 eV ArF laser light with intensity $\sim $10$^{6}$ Wcm$^{-2}$ and measured in a time-resolved detection mode. Two models are applied to examine the blue shift of the STEPL band with decreasing size of nanometer-sized silica fragments. The first model is based on the quantum confinement effect on Mott-Wannier-type excitons developed for semiconductor nanoscale materials, which is completely unusable in the case of wide-band-gap nanoscale materials. The second model takes into account the laser heating of carriers due to collisions with the boundary of nanometer-sized silica fragments in the presence of an intense laser field. The laser heating of carriers in excess of the activation energy for the exciton self-trapping give rise to the extremely hot STE's. As a result, the blue shift of the STEPL band is originated from the activation of hot (phonon-assisted) electronic transitions. [Preview Abstract] |
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U20.00013: Dielectric function of annealed sub-stoichiometric silicon oxide layers Emanuele Rimini, Corrado Spinella, Corrado Bongiorno, Giuseppe Nicotra We present an analytical methodology, based on electron energy loss spectroscopy and energy filtered transmission electron microscopy, which allow us to quantify the clustered silicon concentration and the complex dielectric function of annealed sub-stoichiometric silicon oxide layers, deposited by plasma enhanced chemical vapor deposition (PECVD). To put in evidence the Si clusters, formed as a consequence of the high temperature anneal and embedded in a SiO$_{2}$ host, we used energy selected bright field imaging with an energy loss tuned to the value of the Si bulk plasmon. The dielectric function of the sample is determined by fitting the experimental spectrum using the theoretical description proposed by Barrera and Fuchs(1). The model solves the problem of calculating the energy loss rate for electrons passing through a system of spherical particles of equal radii, located at random in a host material. The dielectric function of the host was preliminarily determined by a Kramers-Kronig analysis of reference pure PECVD oxide spectrum. (1) R.G. Barrera and R. Fuchs, Phys. Rev. B52, 3256 (1995) [Preview Abstract] |
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