Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session B43: Focus Session: Materials for Quantum Information Processing I |
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Sponsoring Units: DMP Chair: Steve Lyon, Princeton University Room: Colorado Convention Center 506 |
Monday, March 5, 2007 11:15AM - 11:27AM |
B43.00001: Sputtered Gold as an Effective Schottky Gate for Strained Si/SiGe Nanostructures Gavin Scott, Ming Xiao, Ed Croke, Eli Yablonovitch, HongWen Jiang Metallization of Schottky surface gates by sputtering Au on strained Si/SiGe heterojunctions enables the depletion of the two dimensional electron gas (2DEG) at a relatively small voltage while maintaining an extremely low level of leakage current. A fabrication process has been developed to enable the formation of sub-micron Au electrodes sputtered onto Si/SiGe without the need of a wetting layer. [Preview Abstract] |
Monday, March 5, 2007 11:27AM - 11:39AM |
B43.00002: Phosphorus Donors in Highly Strained Silicon M. S. Brandt, H. Huebl, A. R. Stegner, M. Stutzmann, G. Vogg, F. Bensch, E. Rauls, U. Gerstmann Donors in strained Si layers have been proposed for quantum computing applications. The lifting of the six-fold valley degeneracy, characteristic for unstrained Si, leads to a suppression of the Kohn-Luttinger oscillations in strained layers which would otherwise limit the exchange interaction of neighboring qubits. Via electrically detected magnetic resonance, we have determined the hyperfine interaction of phosphorus donors in fully strained Si thin films grown on virtual Si$_1$$_-$$_x$Ge$_x$ substrates with $x\leq0.3$, extending the regime investigated earlier by a factor of 20 to higher strains. For highly strained epilayers, hyperfine interactions as low as 0.8 mT are observed [1], significantly below the limit predicted by valley repopulation. Within a Green's function approach, density functional theory shows that the additional reduction is caused by the volume increase of the unit cell and a relaxation of the Si ligands of the donor. [1] H. Huebl et al., Phys. Rev. Lett. \textbf{97}, 166402 (2006). [Preview Abstract] |
Monday, March 5, 2007 11:39AM - 11:51AM |
B43.00003: Process integration and electron spin coherence of donor atom implants in silicon T. Schenkel, A. Persaud, A. M. Tyryshkin, S. A. Lyon, J. Bokor, C. C. Lo, R. deSousa, I. Chakarov We implanted low doses (2 to 4 x $10^{11}_ cm^{-2})$ of P, Sb, and Bi ions into isotopically enriched silicon (28-Si) and characterized diffusion, electrical activation and electron spin coherence after rapid thermal annealing. Phosphorus and bismuth both exhibit enhanced segregation to an imperfect Si/SiO2 interface, while dopant movement is suppressed for antimony ions. Pulsed electron spin resonance shows that spin echo decay is sensitive to the dopant depths, and the interface quality. At 5.2 K, a spin de-coherence time, T2, of 0.3 ms is found for Sb profiles peaking 50 nm below a Si/SiO2 interface, increasing to 0.75 ms when the surface is passivated with hydrogen. These measurements provide benchmark data for the development of devices in which quantum information is encoded in donor electron spins [1]. [1] T. Schenkel, et al., Appl. Phys. Lett. 88, 112101 (2006). [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:03PM |
B43.00004: Double donors in Si quantum computer architecture Maria J. Calderon, Belita Koiller, Sankar Das Sarma We discuss the possibility of performing single spin measurements in Si-based quantum computers through electric field control of electrons bound to double donors near a barrier interface[1]. In particular, we investigate the feasibility of shuttling donor-bound electrons between the double donor impurity in the bulk and the Si/SiO$_2$ interface by tuning an external electric field. We find that both the required electric fields and the tunneling times involved are probably too large for practical implementations. We also investigate operations with double donors in their first excited state: In this case ionization fields are smaller and tunneling times are faster, as required in spin-to-charge conversion measurements. This work is supported by LPS and NSA. \newline \newline [1] M.J. Calderon, B. Koiller, and S. Das Sarma, cond- mat/0610089. [Preview Abstract] |
Monday, March 5, 2007 12:03PM - 12:15PM |
B43.00005: Electric-field driven donor-based charge qubits in semiconductors Belita Koiller, Xuedong Hu, Sankar Das Sarma We theoretically investigate donor-based charge qubit operation driven by external electric fields [1]. We consider initially a single electron bound to a shallow-donor pair in GaAs: This system allows the basic physics of the problem to be presented. In the case of Si, heteropolar configurations such as P-Sb$^+$ pairs are also considered. For both homopolar and heteropolar pairs, the multivalley conduction band structure of Si leads to short-period oscillations of the tunnel-coupling strength as a function of the relative position of the donors. However, for any fixed donor configuration, the response of the bound electron to a uniform electric field in Si is qualitatively very similar to the GaAs case, with no valley quantum interference-related effects, leading to the conclusion that valley interference does not prevent the coherent manipulation of donor-based charge qubits by external electric fields. We also discuss the effect of perturbations due to additional distant donors. [1] B. Koiller, X. Hu, and S. Das Sarma, Phys. Rev. B 73, 045319 (2006) [Preview Abstract] |
Monday, March 5, 2007 12:15PM - 12:27PM |
B43.00006: Suppressing anisotropic hyperfine induced electron spin echo modulations in Si:P Wayne Witzel, Xuedong Hu, Sankar Das Sarma In previous work,\footnote{W.M. Witzel, {\it et al.}, Phys. Rev. B {\bf 72}, 161306(R) (2005).} our theory of spectral diffusion (SD) agrees well with electron spin echo decay measurements\footnote{A.M. Tyryshkin, {\it et al.}, cond-mat/0512705.} in Si:P. In addition to SD decay, these experiments show strong electron spin echo envelope modulations (ESEEM) that significantly reduce spin coherence at short echo times relative to SD decay. Strong demands imposed by fault tolerant quantum computing require suppression (or exploitation) of this effect in order to realize spin-based quantum computation in Si:P systems. It is known that these modulations, caused by anisotropic hyperfine interactions with $^{29}$Si nuclei, can be suppressed via isotopic purification, or by applying a strong, $\sim 10$~T,\footnote{S. Saikin and L. Fedichkin, Phys. Rev. B {\bf 67}, 161302(R) (2003).} magnetic field. Our insights lead to an alternative approach that eliminates predominant modulations at modest magnetic fields ($\sim 1$~T) with little need for isotopic purification. Our calculations are in remarkable agreement with experiment, showing good theoretical understanding of refocused electron spin coherence in Si:P systems. [Preview Abstract] |
Monday, March 5, 2007 12:27PM - 1:03PM |
B43.00007: Electron transport through STM-patterned dopants in silicon Invited Speaker: The recent adaptation of scanning probe systems for nanoscale device fabrication has opened the door to creating electronic devices in silicon with single atom precision. Using a combination of STM lithography and molecular beam epitaxy we show how we can pattern planar, highly doped P layers in silicon down to the atomic-scale and electrically contact them outside the microscope environment. Having developed this technology we demonstrate conduction through silicon nanowires with widths down to 8nm that still exhibit ohmic conduction with resistivities as low as 3x10$^{-6}\Omega $m. We present a study to determine what ultimately limits conduction in these systems as well as studies of tunnel gaps as charge detectors, ordered dopant arrays and transport through silicon dots. We will present an overview of devices that have been made with this technology and highlight some of the challenges to achieving truly atomically precise devices such as a silicon based quantum computer. [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:15PM |
B43.00008: Electron Spin Resonance on Arrays of Etched Quantum Dots in $^{28}$Si/SiGe Jianhua He, A. M. Tyryshkin, S. A. Lyon, D. E. Savage, M. A. Eriksson Relaxation times of 2-dimensional electrons in Si quantum wells (QW) in Si/SiGe heterostructures are found to be shorter than the extremely long relaxation of 3-dimensionally confined donor- bound electrons in Si. Confining the electrons into quantum dots (QD) could suppress the Dyakonov-Perel spin relaxation due to fluctuating Rashba fields, and thus lead to long relaxation times for electrons in QDs. We have developed a reliable, low- defect density process to pattern large area 2D arrays (several cm$^2$) of nominally 100nm dots with a 200nm pitch in a CVD grown $^{28}$Si/SiGe quantum well. The process involves nanoimprinting, reactive ion etching and wet etching. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) imaging has been used to characterize the etch depth (various depths up to 60nm) and uniformity. After etching we find an electron spin resonance signal with a g-factor of 1.9998, which is considerably shifted from that of the unetched QW (2.0003). This line exhibits anisotropies of its g-factor and linewidth that are similar to those of 2D electrons, as might be expected for large, flat QDs. This new line is also broader (420 mG) than that from the unetched QW (90 mG) which could result from an inhomogeneity in dot sizes. [Preview Abstract] |
Monday, March 5, 2007 1:15PM - 1:27PM |
B43.00009: Electron Spin Resonance of Electrons in a Large-Area Silicon MOSFET Shyam Shankar, A. M. Tyryshkin, Sushobhan Avasthi, S. A. Lyon Spins of electrons in two-dimensional (2D) semiconductor heterostructures are considered as qubit candidates for quantum information processing. Electron spin resonance (ESR) of silicon MOSFETs can be useful in characterizing electrons in 2D structures, but previous attempts have been inconclusive. To have sufficient signal for ESR measurements, a large area n- channel silicon FET with a 100nm thick oxide was made using standard processing techniques. Two ESR signals were seen at temperatures below 20K with a gate bias above the threshold voltage of 0.9V. A weak signal with a linewidth of 1G, at g=1.9988(1) may be similar to one seen by Wallace and Silsbee (PRB 1991). A stronger signal is found at g=2.0000(1) with a linewidth of 400mG. This signal shows a noticeable increase in g- factor from 1.9999 at 1V to 2.0000 at 1.7V gate bias and a corresponding decrease in linewidth from 500mG to 400mG. A small g-factor and linewidth change is also seen when the FET is rotated with respect to the applied magnetic field. The signal intensity shows non-Curie temperature behavior below 10K. Such a signal, possibly from conduction electrons or electrons in shallow traps, has not been reported before and is being further investigated. [Preview Abstract] |
Monday, March 5, 2007 1:27PM - 1:39PM |
B43.00010: Electrically Detected Magnetic Resonance of Shallow Donors in Accumulation Layer MOSFETs Cheuk Chi Lo, J. Bokor, T. Schenkel, R. de Sousa, Jianhua He, G. Sabouret, S. Shankar, F. R. Bradbury, A. M. Tyryshkin, S. A. Lyon The ability to read out the spin state of a single donor-bound electron is an essential, but not yet demonstrated, capability for building a quantum computer processor using the spins of electrons bound to Si donors as the qubits. We present measurements of the spins of ensembles of shallow donors embedded in the channel of a MOSFET. Our approach is based on spin-dependent transport arising from the fact that the scattering cross-section of conduction electrons with donor electrons depends on whether the two electrons form a spin singlet or triplet. Our measurements are done on accumulation layer MOSFETs doped with phosphorus or antimony. In continuous wave EDMR experiments, with large area devices (up to 100x100 microns) the measured signals, $\Delta$R/R of the channel, is of the order 10$^{-6}$. The EDMR signal is a function of the field modulation frequency, with pronounced passage effects observed above 1kHz. This result implies that the spin relaxation time, T$_1$, of the donors in the FET channel may be quite long, in excess of 1ms at 5K. A long T$_1$, of this magnitude, suggests that it may be possible to scale the devices to submicron dimensions and read out the state of an individual donor electron spin. [Preview Abstract] |
Monday, March 5, 2007 1:39PM - 1:51PM |
B43.00011: Large Tunable Valley Splitting in a Si/SiGe Quantum Point Contact Lisa McGuire, K.A. Slinker, Mark Friesen, Srijit Goswami, O.J. Chu, Robert Joynt, S.N. Coppersmith, Mark A. Eriksson Quantum dots formed in Si/SiGe two-dimensional electron gases are of interest for use in semiconductor quantum computing due to their potentially long spin lifetimes. However, silicon has a near degeneracy in orbital states due to the presence of multiple valley minima. If the splitting between valley states is smaller than the spin splitting, decoherence rates will be enhanced, complicating qubit operation. Here we present measurements of the valley spitting in a quantum point contact (QPC). The valley splitting is shown to be large, of order 0.5 - 2 meV, over the entire measured range in gate voltage and magnetic field. These results are in contrast with numerous past measurements of valley splitting in laterally unconfined Si/SiGe systems, such as Hall bars. We discuss a physical mechanism, based on interference between states on neighboring atomic steps at the quantum well interface, that explains the discrepancy between the experiments on laterally confined and laterally unconfined systems. We find, in all cases, the magnitude of the valley splitting is suppressed due to atomic steps, but this suppression is substantially lifted in laterally confined nanostructures like QPCs. Work supported by ARO, NSA, and NSF. [Preview Abstract] |
Monday, March 5, 2007 1:51PM - 2:03PM |
B43.00012: Multiscale analytic calculation of valley splitting in silicon quantum wells. Sucismita Chutia, Susan Coppersmith, Mark Friesen Valley splitting in Si/SiGe quantum wells is a central issue for Si based quantum dot quantum computers. The valley coupling arises due to the mixing of states at a sharp quantum well interface. The effective mass theory provides an essential tool for studying the valley splitting in various geometries. However, the magnitude of the splitting must still be determined microscopically, e.g., from atomistic theories. Here, we develop a multiscale theory that bridges the effective mass and atomistic approaches. Since the valley coupling occurs within just a few atomic layers of the interface, we splice a tight binding treatment of the interface into an effective mass treatment of the extended wavefunction. This multiscale theory yields analytical solutions for the valley splitting with no adjustable parameters.~ [Preview Abstract] |
Monday, March 5, 2007 2:03PM - 2:15PM |
B43.00013: Enhancement mode single electron transistor in pure silicon Binhui Hu, C.H. Yang, G.M. Jones, M.J. Yang Solid state implementations of lateral qubits offer the advantage of being scalable and can be easily integrated by existing main stream IC technologies. In addition, the two Zeeman states of an electron spin in a quantum dot (QD) provide a promising candidate for a qubit. Spins in lateral QDs in the GaAs/AlGaAs single electron transistors (SETs) have been intensively investigated. In contrast, Si provides a number of advantages, including long spin coherence time, large g-factor, and small spin-orbit coupling effect. We have demonstrated Si SET in the few electron regime.* In this talk, we will report the isolation of a single electron in a Si QD using a fabrication technique that incorporates the standard Al/SiO2/Si system with an enhancement mode SET structure. Our SET is built in highly resistive Si substrates with bilayer gates. The high purity Si minimizes the potential disorder from impurities. The top gate induces 2D electrons, and several side gates help define the tunneling barriers, fine tune the shape of the QD, and control the number of electrons in it. We will discuss the operating principle, computer simulation, and low temperature transport data. *APPLIED PHYSICS LETTERS 89, 073106 (2006) [Preview Abstract] |
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