ORCID Profile
0000-0001-9240-4245
Current Organisation
University of British Columbia
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Condensed Matter Physics | Electronic and Magnetic Properties of Condensed Matter; Superconductivity | Condensed Matter Modelling and Density Functional Theory | Quantum Chemistry | Nanoelectronics
Expanding Knowledge in the Physical Sciences | Expanding Knowledge in Engineering | Expanding Knowledge in the Chemical Sciences |
Publisher: American Chemical Society (ACS)
Date: 15-02-2011
DOI: 10.1021/NN1033967
Abstract: We report a novel method for probing the gate-voltage dependence of the surface potential of in idual semiconductor nanowires. The statistics of electronic occupation of a single defect on the surface of the nanowire, determined from a random telegraph signal, is used as a measure for the local potential. The method is demonstrated for the case of one or two switching defects in indium arsenide (InAs) nanowire field effect transistors at temperatures T=25-77 K. Comparison with a self-consistent model shows that surface potential variation is retarded in the conducting regime due to screening by surface states with density Dss≈10(12) cm(-2) eV(-1). Temperature-dependent dynamics of electron capture and emission producing the random telegraph signals are also analyzed, and multiphonon emission is identified as the process responsible for capture and emission of electrons from the surface traps. Two defects studied in detail had capture activation energies of EB≈50 meV and EB≈110 meV and cross sections of σ∞≈3×10(-19) cm2 and σ∞≈2×10(-17) cm2, respectively. A lattice relaxation energy of Sℏω=187±15 meV was found for the first defect.
Publisher: American Physical Society (APS)
Date: 02-05-2018
Publisher: The Optical Society
Date: 08-03-2011
DOI: 10.1364/OE.19.005464
Abstract: P-i-n junctions were fabricated along Si nanowires (SiNWs) via the conventional top-down approach using optical lithography. Each device comprises 500 identical SiNWs connected in parallel, and each SiNW has triangular cross-section with dimensions of ~6 nm (base) by ~8 nm (height). The photodiodes exhibit very good rectifying electrical characteristics with a low reverse bias current of ~0.2 fA per SiNW. The photocurrent spectral response exhibits three peaks between 400 nm to 700 nm, which arise due to local optical field enhancement associated with diffraction by the periodic SiNW array and interference in an air/SiO2/Si cavity.
Publisher: Springer Science and Business Media LLC
Date: 06-04-2014
DOI: 10.1038/NMAT3941
Abstract: Electron and nuclear spins of donor ensembles in isotopically pure silicon experience a vacuum-like environment, giving them extraordinary coherence. However, in contrast to a real vacuum, electrons in silicon occupy quantum superpositions of valleys in momentum space. Addressable single-qubit and two-qubit operations in silicon require that qubits are placed near interfaces, modifying the valley degrees of freedom associated with these quantum superpositions and strongly influencing qubit relaxation and exchange processes. Yet to date, spectroscopic measurements have only probed wavefunctions indirectly, preventing direct experimental access to valley population, donor position and environment. Here we directly probe the probability density of single quantum states of in idual subsurface donors, in real space and reciprocal space, using scanning tunnelling spectroscopy. We directly observe quantum mechanical valley interference patterns associated with linear superpositions of valleys in the donor ground state. The valley population is found to be within 5% of a bulk donor when 2.85 ± 0.45 nm from the interface, indicating that valley-perturbation-induced enhancement of spin relaxation will be negligible for depths greater than 3 nm. The observed valley interference will render two-qubit exchange gates sensitive to atomic-scale variations in positions of subsurface donors. Moreover, these results will also be of interest for emerging schemes proposing to encode information directly in valley polarization.
Publisher: IEEE
Date: 12-2010
Publisher: AIP Publishing
Date: 09-08-2010
DOI: 10.1063/1.3478555
Abstract: Single nanowire ZnTe photoconductors prepared by metal-organic chemical vapor deposition are presented. These photodetectors exhibit the highest reported visible responsivity of 360 A/W (at 530 nm) and gain of 8640 (at 3 V bias). The high gain reflects a long carrier lifetime (i.e., ∼1 μs) and the role of fast selective trapping of one carrier is presented to explain this. These results reveal that such single ZnTe nanowires are excellent candidates for applications requiring high performance visible nanoscale photoconductive detectors.
Publisher: AIP Publishing
Date: 22-07-2013
DOI: 10.1063/1.4816439
Abstract: Single phosphorus donors in silicon are promising candidates as qubits in the solid state. Here, we present low temperature scanning probe microscopy and spectroscopy measurements of in idual phosphorus dopants deliberately placed in p-type silicon ∼1 nm below the surface. The ability to image in idual dopants combined with scanning tunnelling spectroscopy allows us to directly study the transport mechanism through the donor. We show that for a single P donor, transport is dominated by a minority carrier recombination process with the surrounding p-type matrix. The understanding gained will underpin future studies of atomically precise mapping of donor-donor interactions in silicon.
Publisher: Springer Science and Business Media LLC
Date: 12-03-2008
Publisher: IEEE
Date: 2010
Publisher: American Physical Society (APS)
Date: 13-06-2013
Publisher: Springer Science and Business Media LLC
Date: 06-06-2016
Abstract: Scaling of Si-based nanoelectronics has reached the regime where device function is affected not only by the presence of in idual dopants, but also by their positions in the crystal. Determination of the precise dopant location is an unsolved problem in applications from channel doping in ultrascaled transistors to quantum information processing. Here, we establish a metrology combining low-temperature scanning tunnelling microscopy (STM) imaging and a comprehensive quantum treatment of the dopant-STM system to pinpoint the exact coordinates of the dopant in the Si crystal. The technique is underpinned by the observation that STM images contain atomic-sized features in ordered patterns that are highly sensitive to the STM tip orbital and the absolute dopant lattice site. The demonstrated ability to determine the locations of P and As dopants to 5 nm depths will provide critical information for the design and optimization of nanoscale devices for classical and quantum computing applications.
Publisher: IOP Publishing
Date: 04-06-2010
Publisher: AIP Publishing
Date: 11-04-2011
DOI: 10.1063/1.3579251
Abstract: Indium-arsenide–gallium-arsenide (InAs–GaAs) core-shell, wurtzite nanowires have been grown on GaAs (001) substrates. The core-shell geometries (core radii 11 to 26 nm, shell thickness & .5 nm) exceeded equilibrium critical values for strain relaxation via dislocations, apparent from transmission electron microscopy. Partial axial relaxation is detected in all nanowires increasing exponentially with size, while radial strain relaxation is & %, but undetected in nanowires with both smaller core radii & nm and shell thicknesses & nm. Electrical measurements on in idual core-shell nanowires show that the resulting dislocations are correlated with reduced electron field-effect mobility compared to bare InAs nanowires.
Publisher: IOP Publishing
Date: 20-09-2013
Publisher: AIP Publishing
Date: 25-12-2006
DOI: 10.1063/1.2424653
Abstract: Multilayer Ti∕Au contacts were fabricated on in idual, unintentionally doped zinc selenide nanowires with 80nm nominal diameter. Four-terminal contact structures were used to independently measure current-voltage characteristics of contacts and nanowires. Specific contact resistivity of Ti∕Au contacts is 0.024Ωcm2 and intrinsic resistivity of the nanowires is approximately 1Ωcm. The authors have also measured the spectral photocurrent responsivity of a ZnSe nanowire with 2.0V bias across Ti∕Au electrodes, which exhibits a turnon for wavelengths shorter than 470nm and reaches 22A∕W for optical excitation at 400nm.
Publisher: IOP Publishing
Date: 11-05-2016
DOI: 10.1088/0957-4484/27/24/244001
Abstract: The states of a boron acceptor near a Si/SiO2 interface, which bind two low-energy Kramers pairs, have exceptional properties for encoding quantum information and, with the aid of strain, both heavy hole and light hole-based spin qubits can be designed. Whereas a light-hole spin qubit was introduced recently (arXiv:1508.04259), here we present analytical and numerical results proving that a heavy-hole spin qubit can be reliably initialised, rotated and entangled by electrical means alone. This is due to strong Rashba-like spin-orbit interaction terms enabled by the interface inversion asymmetry. Single qubit rotations rely on electric-dipole spin resonance (EDSR), which is strongly enhanced by interface-induced spin-orbit terms. Entanglement can be accomplished by Coulomb exchange, coupling to a resonator, or spin-orbit induced dipole-dipole interactions. By analysing the qubit sensitivity to charge noise, we demonstrate that interface-induced spin-orbit terms are responsible for sweet spots in the dephasing time [Formula: see text] as a function of the top gate electric field, which are close to maxima in the EDSR strength, where the EDSR gate has high fidelity. We show that both qubits can be described using the same starting Hamiltonian, and by comparing their properties we show that the complex interplay of bulk and interface-induced spin-orbit terms allows a high degree of electrical control and makes acceptors potential candidates for scalable quantum computation in Si.
Publisher: Springer Science and Business Media LLC
Date: 20-07-2020
Publisher: American Physical Society (APS)
Date: 22-06-2012
Publisher: American Physical Society (APS)
Date: 14-06-2016
Publisher: AIP Publishing
Date: 02-07-2018
DOI: 10.1063/1.5036521
Abstract: Full electrical control of quantum bits could facilitate fast, low-power, scalable quantum computation. Although electric dipoles are highly attractive to couple spin qubits electrically over long distances, mechanisms identified to control two-qubit couplings do not permit single-qubit operations while two-qubit couplings are off. Here, we identify a mechanism to modulate electrical coupling of spin qubits which overcomes this drawback for hole spin qubits in acceptors which is based on the electrical tuning of the direction of the spin-dependent electric dipole by a gate. This allows the inter-qubit coupling to be turned off electrically by tuning to a “magic angle” of vanishing electric dipole-dipole interactions, while retaining the ability to manipulate the in idual qubits. This effect stems from the interplay of the Td symmetry of the acceptor state in the Si lattice with the magnetic field orientation and the spin-3/2 characteristic of hole systems. The magnetic field direction also allows us to greatly suppress spin relaxation by phonons that limit single qubit performance, while retaining sweet spots where the qubits are insensitive to charge noise. We propose suitable protocols to practically achieve full electrical tunability of entanglement and the isolation of the qubit.
Publisher: American Physical Society (APS)
Date: 29-05-0088
Publisher: IOP Publishing
Date: 06-04-2022
Abstract: Strain is extensively used to controllably tailor the electronic properties of materials. In the context of indirect band-gap semiconductors such as silicon, strain lifts the valley degeneracy of the six conduction band minima, and by extension the valley states of electrons bound to phosphorus donors. Here, single phosphorus atoms are embedded in an engineered thin layer of silicon strained to 0.8% and their wave function imaged using spatially resolved spectroscopy. A prevalence of the out-of-plane valleys is confirmed from the real-space images, and a combination of theoretical modelling tools is used to assess how this valley repopulation effect can yield isotropic exchange and tunnel interactions in the xy -plane relevant for atomically precise donor qubit devices. Finally, the residual presence of in-plane valleys is evidenced by a Fourier analysis of both experimental and theoretical images, and atomistic calculations highlight the importance of higher orbital excited states to obtain a precise relationship between valley population and strain. Controlling the valley degree of freedom in engineered strained epilayers provides a new competitive asset for the development of donor-based quantum technologies in silicon.
Publisher: The Electrochemical Society
Date: 16-04-2010
DOI: 10.1149/1.3367226
Abstract: II-VI nanowires offer a unique combination of properties for developing blue/UV optoelectronic devices. Key to realizing this potential is managing the nanowire growth process to ensure appropriate properties are realized. Taking ZnSe as an ex le, we report on the influence of growth conditions (namely, growth temperature and constituent flux ratio) on optimizing the luminescence response. It is shown that native defects ultimately dictate properties including steady state photoluminescence, ultra-fast optical response and even nonlinear behavior. We demonstrate how careful control over materials can enable the realization of a state of the art photo-detector based on such nanowires.
Publisher: American Chemical Society (ACS)
Date: 03-03-2014
DOI: 10.1021/NL4047015
Abstract: We demonstrate a single-hole transistor using an in idual acceptor dopant embedded in a silicon channel. Magneto-transport spectroscopy reveals that the ground state splits as a function of magnetic field into four states, which is unique for a single hole bound to an acceptor in a bulk semiconductor. The two lowest spin states are heavy (|m(j)| = 3/2) and light (|m(j)| = 1/2) hole-like, a two-level system that can be electrically driven and is characterized by a magnetic field dependent and long relaxation time, which are properties of interest for qubits. Although the bulklike spin splitting of a boron atom is preserved in our nanotransistor, the measured Landé g-factors, |g(hh)| = 0.81 ± 0.06 and |g(lh)| = 0.85 ± 0.21 for heavy and light holes respectively, are lower than the bulk value.
Publisher: AIP Publishing
Date: 15-01-2007
DOI: 10.1063/1.2431788
Abstract: The authors have performed variable-temperature electrical measurements on in idual single-crystalline, Mn-doped ZnO nanowires. Using a back-gated field-effect transistor structure fabricated with electron-beam lithography, they have established that nanowires exhibit n-type conduction. At a temperature of 225K, the field-effect mobility and free electron concentration are ≈35cm2V−1s−1 and ≈3.6×1017cm−3, respectively. Carrier concentration varies weakly with temperature down to 12K, signifying that the material is degenerate. Mobility decreases with decreasing temperature down to 12K, in a manner consistent with ionized impurity scattering in a degenerate semiconductor.
Publisher: IOP Publishing
Date: 22-01-2010
Publisher: American Physical Society (APS)
Date: 08-01-2016
Publisher: AIP Publishing
Date: 18-05-2015
DOI: 10.1063/1.4921640
Abstract: The energy spectrum of spin-orbit coupled states of in idual sub-surface boron acceptor dopants in silicon have been investigated using scanning tunneling spectroscopy at cryogenic temperatures. The spatially resolved tunnel spectra show two resonances, which we ascribe to the heavy- and light-hole Kramers doublets. This type of broken degeneracy has recently been argued to be advantageous for the lifetime of acceptor-based qubits [R. Ruskov and C. Tahan, Phys. Rev. B 88, 064308 (2013)]. The depth dependent energy splitting between the heavy- and light-hole Kramers doublets is consistent with tight binding calculations, and is in excess of 1 meV for all acceptors within the experimentally accessible depth range (& nm from the surface). These results will aid the development of tunable acceptor-based qubits in silicon with long coherence times and the possibility for electrical manipulation.
Publisher: American Association for the Advancement of Science (AAAS)
Date: 07-12-2018
Abstract: The controllable transition from charge to spin physics for a boron atom in silicon is an essential step to spin-orbit qubits.
Publisher: Springer Science and Business Media LLC
Date: 20-04-2016
DOI: 10.1038/NCOMMS11342
Abstract: In quantum simulation, many-body phenomena are probed in controllable quantum systems. Recently, simulation of Bose–Hubbard Hamiltonians using cold atoms revealed previously hidden local correlations. However, fermionic many-body Hubbard phenomena such as unconventional superconductivity and spin liquids are more difficult to simulate using cold atoms. To date the required single-site measurements and cooling remain problematic, while only ensemble measurements have been achieved. Here we simulate a two-site Hubbard Hamiltonian at low effective temperatures with single-site resolution using subsurface dopants in silicon. We measure quasi-particle tunnelling maps of spin-resolved states with atomic resolution, finding interference processes from which the entanglement entropy and Hubbard interactions are quantified. Entanglement, determined by spin and orbital degrees of freedom, increases with increasing valence bond length. We find separation-tunable Hubbard interaction strengths that are suitable for simulating strongly correlated phenomena in larger arrays of dopants, establishing dopants as a platform for quantum simulation of the Hubbard model.
Publisher: Springer Science and Business Media LLC
Date: 12-2010
Publisher: IOP Publishing
Date: 18-03-2015
DOI: 10.1088/0953-8984/27/15/154203
Abstract: The ability to control single dopants in solid-state devices has opened the way towards reliable quantum computation schemes. In this perspective it is essential to understand the impact of interfaces and electric fields, inherent to address coherent electronic manipulation, on the dopants atomic scale properties. This requires both fine energetic and spatial resolution of the energy spectrum and wave-function, respectively. Here we present an experiment fulfilling both conditions: we perform transport on single donors in silicon close to a vacuum interface using a scanning tunneling microscope (STM) in the single electron tunneling regime. The spatial degrees of freedom of the STM tip provide a versatility allowing a unique understanding of electrostatics. We obtain the absolute energy scale from the thermal broadening of the resonant peaks, allowing us to deduce the charging energies of the donors. Finally we use a rate equations model to derive the current in presence of an excited state, highlighting the benefits of the highly tunable vacuum tunnel rates which should be exploited in further experiments. This work provides a general framework to investigate dopant-based systems at the atomic scale.
Publisher: American Physical Society (APS)
Date: 26-01-2015
Publisher: IEEE
Date: 06-2014
Publisher: IOP Publishing
Date: 05-02-2021
Abstract: Quantum phenomena are typically observable at length and time scales smaller than those of our everyday experience, often involving in idual particles or excitations. The past few decades have seen a revolution in the ability to structure matter at the nanoscale, and experiments at the single particle level have become commonplace. This has opened wide new avenues for exploring and harnessing quantum mechanical effects in condensed matter. These quantum phenomena, in turn, have the potential to revolutionize the way we communicate, compute and probe the nanoscale world. Here, we review developments in key areas of quantum research in light of the nanotechnologies that enable them, with a view to what the future holds. Materials and devices with nanoscale features are used for quantum metrology and sensing, as building blocks for quantum computing, and as sources and detectors for quantum communication. They enable explorations of quantum behaviour and unconventional states in nano- and opto-mechanical systems, low-dimensional systems, molecular devices, nano-plasmonics, quantum electrodynamics, scanning tunnelling microscopy, and more. This rapidly expanding intersection of nanotechnology and quantum science/technology is mutually beneficial to both fields, laying claim to some of the most exciting scientific leaps of the last decade, with more on the horizon.
Publisher: IOP Publishing
Date: 18-03-2015
DOI: 10.1088/0953-8984/27/15/154207
Abstract: Atomistic tight-binding (TB) simulations are performed to calculate the Stark shift of the hyperfine coupling for a single arsenic (As) donor in silicon (Si). The role of the central-cell correction is studied by implementing both the static and the non-static dielectric screenings of the donor potential, and by including the effect of the lattice strain close to the donor site. The dielectric screening of the donor potential tunes the value of the quadratic Stark shift parameter (η2) from -1.3 × 10(-3) µm(2) V(-2) for the static dielectric screening to -1.72 × 10(-3) µm(2) V(-2) for the non-static dielectric screening. The effect of lattice strain, implemented by a 3.2% change in the As-Si nearest-neighbour bond length, further shifts the value of η2 to -1.87 × 10(-3) µm(2) V(-2), resulting in an excellent agreement of theory with the experimentally measured value of -1.9 ± 0.2 × 10(-3) µm(2) V(-2). Based on our direct comparison of the calculations with the experiment, we conclude that the previously ignored non-static dielectric screening of the donor potential and the lattice strain significantly influence the donor wave function charge density and thereby leads to a better agreement with the available experimental data sets.
Publisher: Springer Science and Business Media LLC
Date: 19-09-2010
Abstract: A single localized charge can quench the luminescence of a semiconductor nanowire, but relatively little is known about the effect of single charges on the conductance of the nanowire. In one-dimensional nanostructures embedded in a material with a low dielectric permittivity, the Coulomb interaction and excitonic binding energy are much larger than the corresponding values when embedded in a material with the same dielectric permittivity. The stronger Coulomb interaction is also predicted to limit the carrier mobility in nanowires. Here, we experimentally isolate and study the effect of in idual localized electrons on carrier transport in InAs nanowire field-effect transistors, and extract the equivalent charge sensitivity. In the low carrier density regime, the electrostatic potential produced by one electron can create an insulating weak link in an otherwise conducting nanowire field-effect transistor, modulating its conductance by as much as 4,200% at 31 K. The equivalent charge sensitivity, 4 × 10(-5) e Hz(-1/2) at 25 K and 6 × 10(-5) e Hz(-1/2) at 198 K, is orders of magnitude better than conventional field-effect transistors and nanoelectromechanical systems, and is just a factor of 20-30 away from the record sensitivity for state-of-the-art single-electron transistors operating below 4 K (ref. 8). This work demonstrates the feasibility of nanowire-based single-electron memories and illustrates a physical process of potential relevance for high performance chemical sensors. The charge-state-detection capability we demonstrate also makes the nanowire field-effect transistor a promising host system for impurities (which may be introduced intentionally or unintentionally) with potentially long spin lifetimes, because such transistors offer more sensitive spin-to-charge conversion readout than schemes based on conventional field-effect transistors.
Publisher: American Physical Society (APS)
Date: 03-09-2019
Publisher: Springer Science and Business Media LLC
Date: 07-2021
Publisher: Springer Science and Business Media LLC
Date: 26-06-2023
Publisher: American Chemical Society (ACS)
Date: 08-06-2020
Publisher: Royal Society of Chemistry (RSC)
Date: 2017
DOI: 10.1039/C7NR05081J
Abstract: High-precision physics modeling at the atomic scale indicates potential for direct observation of central-cell-effects in scanning tunnelling microscope images of single dopant wave functions.
Publisher: SPIE
Date: 04-11-2016
DOI: 10.1117/12.2231059
Publisher: American Physical Society (APS)
Date: 27-08-2018
Start Date: 2018
End Date: 2020
Funder: Australian Research Council
View Funded ActivityStart Date: 2016
End Date: 12-2018
Amount: $373,536.00
Funder: Australian Research Council
View Funded ActivityStart Date: 2018
End Date: 12-2020
Amount: $371,923.00
Funder: Australian Research Council
View Funded Activity