Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session J40: Focus Session: Transport Properties of Nanostructures III: Semiconductors & Surfaces |
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Sponsoring Units: DMP DCP Chair: J. Crain, NIST Room: LACC 408A |
Tuesday, March 22, 2005 11:15AM - 11:27AM |
J40.00001: Charge transport in adiabatically driven ratchets - Waveform and phase dependence - Thorsten M\"{u}ller, Alida W\"{u}rtz, Axel Lorke, Dirk Reuter, Andreas D. Wieck We report on the adiabatic pumping of electrons in a driven lateral superlattice (LSL) with broken symmetry. The device is fabricated from a two-dimensional electron gas (2DEG), located $54.8\:\mathrm{nm}$ below the surface in a GaAs/Al$_x$Ga$_{1-x}$As heterostructure, grown by molecular beam epitaxy. The LSL is realized by two transducer gates, each comprising 75 stripes of 160 nm width and 1 $\mu$m period. The gates are interlaced off center by two thirds of the period and can thus induce a ratchet-like potential modulation in the 2DEG when appropriately biased. When the transducer is driven by two identical but phase-shifted ac signals, a lateral pumping current $I(\phi)$ results, which strongly depends on both the phase shift $\phi$ and the wave form $V(t)$. Surprisingly, we find that for different periodic signals, the phase dependence $I(\phi)$ closely resembles $V(t)$. A simple model of adiabatic pumping in 2DEGs is presented, which can reproduce our experimental findings. Possible applications for waveform sampling are discussed. [Preview Abstract] |
Tuesday, March 22, 2005 11:27AM - 11:39AM |
J40.00002: Cotunneling-mediated observation of excited states in the Coulomb blockade regime Roland Schleser, Elisabeth Ruh, Thomas Ihn, Klaus Ensslin, M. Tews, Daniela Pfannkuche, Danny Driscoll, Art Gossard We present finite bias transport measurements on a few-electron quantum dot. In the Coulomb blockade regime, we observe strong signatures of inelastic cotunneling which can directly be assigned to excited states observed in the non-blockaded regime. In addition, we observe structures related to sequential tunneling through the dot, occuring after it has been excited by an inelastic cotunneling process. [Preview Abstract] |
Tuesday, March 22, 2005 11:39AM - 11:51AM |
J40.00003: Imaging quantum interference patterns on a quantum point contact Carlo DaCunha, Nobuyuki Aoki, David Ferry Scanning gate microscopy (SGM) images have been taken inside the constriction of a quantum point contact (QPC) fabricated on an In$_{0.53}$Al$_{0.47}$As/In$_{0.53}$Ga$_{0.47}$As heterostructure. Shubnikov-de Haas measurements revealed the occupation of two sub-bands on this sample with carrier concentrations of 7.24x10$^{11}$cm$^{-2}$ and 2.42x10$^{11}$cm$^{-2}$, respectively. The images show the behavior of the wavefunction interference at different points of the transmission curve of the QPC. It is believed that these images correspond to different resonance peaks observed on the curve. Additional images have been taken at different temperatures indicating a reduction of the clear interference patterns, which is attributed to a decrease of the phase coherent area of the sample. [Preview Abstract] |
Tuesday, March 22, 2005 11:51AM - 12:27PM |
J40.00004: Scanning Probe Microscopy for Atomic-scale Silicon Device Fabrication Invited Speaker: Over the past three decades the driving force behind the expansion of the microelectronics industry has been the ability to pack ever more features onto a silicon chip, achieved by continually miniaturising the size of the individual components. However, after 2015 there is no known technological route to reduce device sizes below 10nm. In this talk we demonstrate a complete fabrication strategy towards atomic-scale device fabrication in silicon using a combination of scanning tunneling microscopy and high purity crystal growth. In particular we overcome one of the major obstacles to making functional semiconductor devices with an STM -- connecting macroscopic leads to the device once it is removed from the vacuum environment [1]. We demonstrate key steps of the fabrication process, including the ability to place individual phosphorus atoms in silicon at precise locations [2] and encapsulate them in epitaxial silicon with minimal diffusion and segregation of the dopants [3]. We present magnetoresistance data showing the cross-over from 2D to 1D transport in nano-scale quantum wires and arrays. Finally we discuss the implications of these results for the construction of more sophisticated atomic-scale devices in silicon such as a silicon based quantum computer. [1] F.J. Ruess, L. Oberbeck, M.Y. Simmons, K.E.J. Goh, A.R. Hamilton, T. Hallam, N.J. Curson and R.G. Clark, ``Fabrication of quantum wires using scanning probe microscopy'', Nano Letters 4, 1969 (2004). [2] S. R. Schofield, N. J. Curson, M. Y. Simmons, F. J. Ruess, T. Hallam, L. Oberbeck and R. G.Clark, ``Atomically precise placement of single dopants in silicon'', Physical Review Letters 91, 136104 (2003). [3] L. Oberbeck, N. J. Curson, T. Hallam, M. Y. Simmons and R.G. Clark, ``Measurement of phosphorus segregation in silicon at the atomic-scale using scanning tunneling microscopy'', Appl. Phys. Lett. 83, 1359 (2004). [Preview Abstract] |
Tuesday, March 22, 2005 12:27PM - 12:39PM |
J40.00005: Silicon nanoscale 2D donor devices fabricated by UHV-STM lithography J.S. Kline, S.J. Robinson, J.R. Tucker, T.-C. Shen, C. Yang, R.-R. Du, Y. Liu, X. Wang, T.P. Ma We developed a scheme to fabricate nanoscale electronic devices by patterning 2D shallow donors into single crystal silicon. The goal of this approach is to seamlessly integrate nano- and microelectronics. In this approach, we pattern the devices on H terminated Si(100)-2x1 surfaces via UHV-STM. Phosphine molecules selectively adsorb onto the patterned areas to define conduction pathways. Low temperature Si MBE is used to encapsulate the dopants in the Si lattice. Two-terminal electrical connection to the outside-world is provided by a template structure formed by conventional microfabrication. A third terminal used for gate modulation of the device is formed by silicon nitride jet vapor deposition and metallization. Low temperature electrical characterization of conducting wires show significant departure from Ohmic conduction for width $<$ 50nm. Electro and magnetotransport properties will be discussed. Tunnel junction and single electron transistor fabrication are currently underway. The low charged-defect density provided by complete encapsulation could allow the fabrication of a solid state quantum computer. [Preview Abstract] |
Tuesday, March 22, 2005 12:39PM - 12:51PM |
J40.00006: Single electron transistors at high temperature Mingting Kuo, Pei-Wen Li The tunneling current through a germanium (Ge) quantum dot (QD) embedded in SiO$_{2}$ matrix is studied theoretically. The energy levels and Coulomb interactions of electrons in a nanometer Ge QD are calculated using an effective mass model. In small Ge QDs, the effect of electron correlation is significant and hence, both the interlevel and intralevel Coulomb interactions are important in electron transport properties. The tunneling current of a Ge-QD single electron transistor (SET) is calculated using the Keldysh Green function method and two-level Anderson model. In addition to four peaks arising from the intralevel Coulomb interactions, extra differential conductance peaks are found due to the interlevel Coulomb interactions and the statistical nature of the open system. [Preview Abstract] |
Tuesday, March 22, 2005 12:51PM - 1:03PM |
J40.00007: Resistance Noise and Morphology in Percolating Films of Ag on Si(111) Daniel Dougherty, William Cullen, Ellen Williams Structure, conductance and noise in submonolayer Ag films on Si(111)-(7x7) near the onset of electrical conduction have been measured in situ in a UHV growth chamber. Noise measurements characteristic of the film morphology can be accomplished only when transport through the substrate is prevented. In this case, extremely low doping ($\sim $10$^{13}$ cm$^{-3})$ and low temperature measurements were used to accomplish this. Measurement of the exponent of the variation of the noise level with Ag coverage near the percolation threshold yields a value of 1.06 $\pm $ 0.09, inconsistent with lattice percolation at the interface with the Si substrate. The inverted random void model is consistent with this result, yielding the possibility that a broad distribution of degree overlap of adjacent Ag clusters is the origin of the noise. This model will be discussed in context of the detailed observations of film morphology near the percolation threshold using low-temperature STM and simultaneous conductivity measurements. Implications for the role of the Si substrate in electrical transport are also discussed. [Preview Abstract] |
Tuesday, March 22, 2005 1:03PM - 1:15PM |
J40.00008: Phonon Scattering by Molecular Dynamics: Temperature Dependence and Effect of Structural Disorder Christopher Kimmer, Edmund Webb III We use molecular dynamics to simulate individual phonon-grain boundary scattering events in Silicon as a function of temperature and grain-boundary disorder. The temperature dependence of the scattering is investigated by varying the lattice parameter and incident phonon's properties to match the equilibrium bulk crystal's lattice spacing and dispersion relation at the prescribed temperature. For a given twist angle, different grain boundary structures are formed by a simulated growth process wherein cooling from the melt permits different grains to grow towards one another and eventually impinge, resulting in a boundary. The effect of boundary disorder on the reflected and transmitted phonons is then characterized. The temperature dependence incorporated in this manner may be compared with existing scattering models in the continuum limit while the effects of disorder can be used to estimate uncertainties in scattering models for larger--scale methods such as direct-simulation Monte Carlo. [Preview Abstract] |
Tuesday, March 22, 2005 1:15PM - 1:27PM |
J40.00009: Heat transfer experiments on micro- and nanoscale: Interface and size effects Bernd Gotsmann, Martin Hinz, Mark A. Lantz, Urs Duerig, Othmar Marti, Johannes Windeln We present heat transfer experiments using a heated silicon cantilever/tip at or near contact with a variety of surfaces. Under ambient conditions cooling of the cantilever results from conduction through the air, through the cantilever beam and through the tip-surface contact, as well as from radiation cooling. By varying the ambient conditions, the sample material and the tip-sample distance we can quantify the various contributions. Under ambient conditions the heat transport is dominated by conduction through the air and the cantilever. At distances of a few times the mean free path of air molecules the heat transfer is accompanied by surprisingly a large momentum transfer. In vacuum, with conduction excluded, heat transport through the nm-sized tip-surface contact can be measured. The heat transport is found to depend decisively on the size of the tip, the size of the mechanical contact and the surface material. Radiative heat transport becomes significant under vacuum conditions. At small separations we observe a strong distance dependence due to near field effects. This deviation from Stefan-Boltzmann’s law also exhibits a strong material and temperature dependence. [Preview Abstract] |
Tuesday, March 22, 2005 1:27PM - 1:39PM |
J40.00010: Stochastic Evolution of Nano-Structures in the Continuum Step Model Masashi Degawa, Frence Szalma, Ellen Williams Stochastic Evolution of Nano-Structures in the Continuum Step Model * M. Degawa, F. Szalma and E.D. Williams, Department of Physics and MRSEC University of Maryland College Park MD, 20742 Technological demands of the fabrication of nano-structures and quantum dots provides renewed motivation for understanding the atomistic properties that control crystal shapes. With decreasing structure size, the issues of finite size and shape effects become non-negligible and also the increasing sensitivity to external perturbations, such as the substrate interface. We have previously shown that the effects of curvature, which cannot be neglected in nanoscale structures, yield a family of crystal shapes with constant surface chemical potentials. The member of this family that represents an absolute minimum in the total free energy follows the Pokrovsky-Talapov ECS (PT-ECS), which is also the result obtained in the limit of zero curvature. The remaining members of the family represent metastable states. Here, we extend the continuum results to include the discreteness of steps using the continuum step model. The metastable states now represent the structures, which are formed due to a barrier for the peeling (and also nucleation) of layers necessary for the evolution of the crystal shape. We relate the barrier height to the crystallite parameters, including volume and interface interactions, and discuss its consequences for the kinetics of shape evolution. *This work has been supported by the DOE-NNI and NSF-MRSEC. [Preview Abstract] |
Tuesday, March 22, 2005 1:39PM - 1:51PM |
J40.00011: Electrostatic interactions between atomic force microscope tip and mostly dielectric surface near glass transition point Sergei F. Lyuksyutov Behavior of thin dielectric near glass transition point is a mystery. A strong non-uniform electric field (10$^{9}$ Vm$^{-1})$ induced by a biased atomic force microscope tip creates nanoscopic mass transport resulting in nanostructure formation in a broad class of polymers near GTP. Similar trend under same experimental conditions is observed in iridovirus shell composed of proteins folded in capsomers. It is suspected that structural re-arrangement of polar amino acids is the reason. In all cases an AFM tip is a major player in the technique, we name, atomic force microscopy electrostatic nanolithography (AFMEN). This experimental technique produces very similar nanostructural changes in polymers, SAM, and biological cells. We attempt to describe this behavior. [Preview Abstract] |
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