Journal Papers (79)
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11:Title: Weber B; Hsueh YL; Watson TF; Li R; Hamilton AR; Hollenberg LCL; Rahman R; Simmons MY, 2018, 'Spin–orbit coupling in silicon for electrons bound to donors', npj Quantum Information, vol. 4Year : 2018
Publication Type: Journal Papers
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Abstract
Spin–orbit coupling (SOC) is fundamental to a wide range of phenomena in condensed matter, spanning from a renormalisation of the free-electron g-factor, to the formation of topological insulators, and Majorana Fermions. SOC has also profound implications in spin-based quantum information, where it is known to limit spin lifetimes (T1) in the inversion asymmetric semiconductors such as GaAs. However, for electrons in silicon—and in particular those bound to phosphorus donor qubits—SOC is usually regarded weak, allowing for spin lifetimes of minutes in the bulk. Surprisingly, however, in a nanoelectronic device donor spin lifetimes have only reached values of seconds. Here, we reconcile this difference by demonstrating that electric field induced SOC can dominate spin relaxation of donor-bound electrons. Eliminating this lifetime-limiting effect by careful alignment of an external vector magnetic field in an atomically engineered device, allows us to reach the bulk-limit of spin-relaxation times. Given the unexpected strength of SOC in the technologically relevant silicon platform, we anticipate that our results will stimulate future theoretical and experimental investigation of phenomena that rely on strong magnetoelectric coupling of atomically confined spins.
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12:Title: Ameen TA; Ilatikhameneh H; Tankasala A; Hsueh Y; Charles J; Fonseca J; Povolotskyi M; Kim JO; Krishna S; Allen MS; Allen JW; Rahman R; Klimeck G, 2018, 'Theoretical study of strain-dependent optical absorption in a doped self-assembled InAs/InGaAs/GaAs/AlGaAs quantum dot', Beilstein Journal of Nanotechnology, vol. 9, pp. 1075 - 1084Year : 2018
Publication Type: Journal Papers
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Abstract
A detailed theoretical study of the optical absorption in doped self-assembled quantum dots is presented. A rigorous atomistic strain model as well as a sophisticated 20-band tight-binding model are used to ensure accurate prediction of the single particle states in these devices. We also show that for doped quantum dots, many-particle configuration interaction is also critical to accurately capture the optical transitions of the system. The sophisticated models presented in this work reproduce the experimental results for both undoped and doped quantum dot systems. The effects of alloy mole fraction of the strain controlling layer and quantum dot dimensions are discussed. Increasing the mole fraction of the strain controlling layer leads to a lower energy gap and a larger absorption wavelength. Surprisingly, the absorption wavelength is highly sensitive to the changes in the diameter, but almost insensitive to the changes in dot height. This behavior is explained by a detailed sensitivity analysis of different factors affecting the optical transition energy.
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13:Title: Hile SJ; Fricke L; House MG; Peretz E; Chen CY; Wang Y; Broome M; Gorman SK; Keizer JG; Rahman R; Simmons MY, 2018, 'Addressable electron spin resonance using donors and donor molecules in silicon', Science Advances, vol. 4Year : 2018
Publication Type: Journal Papers
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Abstract
Phosphorus donor impurities in silicon are a promising candidate for solid-state quantum computing due to their exceptionally long coherence times and high fidelities. However, individual addressability of exchange coupled donors with separations ~15 nm is challenging. We show that by using atomic precision lithography, we can place a single P donor next to a 2P molecule 16 ± 1 nm apart and use their distinctive hyperfine coupling strengths to address qubits at vastly different resonance frequencies. In particular, the single donor yields two hyperfine peaks separated by 97 ± 2.5 MHz, in contrast to the donor molecule that exhibits three peaks separated by 262 ± 10 MHz. Atomistic tight-binding simulations confirm the large hyperfine interaction strength in the 2P molecule with an interdonor separation of ~0.7 nm, consistent with lithographic scanning tunneling microscopy images of the 2P site during device fabrication. We discuss the viability of using donor molecules for built-in addressability of electron spin qubits in silicon.
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14:Title: Salfi J; Voisin B; Tankasala A; Bocquel J; Usman M; Simmons MY; Hollenberg LCL; Rahman R; Rogge S, 2018, 'Valley Filtering in Spatial Maps of Coupling between Silicon Donors and Quantum Dots', Physical Review X, vol. 8Year : 2018
Publication Type: Journal Papers
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Abstract
Exchange coupling is a key ingredient for spin-based quantum technologies since it can be used to entangle spin qubits and create logical spin qubits. However, the influence of the electronic valley degree of freedom in silicon on exchange interactions is presently the subject of important open questions. Here we investigate the influence of valleys on exchange in a coupled donor–quantum-dot system, a basic building block of recently proposed schemes for robust quantum information processing. Using a scanning tunneling microscope tip to position the quantum dot with sub-nm precision, we find a near monotonic exchange characteristic where lattice-aperiodic modulations associated with valley degrees of freedom comprise less than 2% of exchange. From this we conclude that intravalley tunneling processes that preserve the onor’s ±x and ±y valley index are filtered out of the interaction with the ±z valley quantum dot, and that the ±x and ±y intervalley processes where the electron valley index changes are weak. Complemented by tight-binding calculations of exchange versus donor depth, the demonstrated electrostatic tunability of donor–quantum-dot exchange can be used to compensate the remaining intravalley ±z oscillations to realize uniform interactions in an array of highly coherent donor spins.
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15:Title: Chen CY; Ameen TA; Ilatikhameneh H; Rahman R; Klimeck G; Appenzeller J, 2018, 'Channel Thickness Optimization for Ultrathin and 2-D Chemically Doped TFETs', IEEE Transactions on Electron Devices, vol. 65, pp. 4614 - 4621Year : 2018
Publication Type: Journal Papers
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Abstract
The 2-D material-based TFETs are among the most promising candidates for low-power electronics applications since they offer ultimate gate control and high-current drives that are achievable through small tunneling distances (Λ) during the device operation. The ideal device is characterized by a minimized Λ. However, devices with the thinnest possible body do not necessarily provide the best performance. For example, reducing the channel thickness (T ch ) increases the depletion width in the source, which can be a significant part of the total Λ. Hence, it is important to determine the optimum T ch for each channel material individually. In this paper, we study the optimum T ch for three channel materials: WSe 2 , black phosphorus, and InAs using full-band self-consistent quantum transport simulations. To identify the ideal T ch for each material at a specific doping density, a new analytic model is proposed and benchmarked against the numerical simulations.
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16:Title: Sahasrabudhe H; Novakovic B; Nakamura J; Fallahi S; Povolotskyi M; Klimeck G; Rahman R; Manfra MJ, 2018, 'Optimization of edge state velocity in the integer quantum Hall regime', Physical Review B, vol. 97Year : 2018
Publication Type: Journal Papers
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Abstract
Observation of interference in the quantum Hall regime may be hampered by a small edge state velocity due to finite phase coherence time. Therefore designing two quantum point contact (QPCs) interferometers having a high edge state velocity is desirable. Here we present a new simulation method for designing heterostructures with high edge state velocity by realistically modeling edge states near QPCs in the integer quantum Hall effect (IQHE) regime. Using this simulation method, we also predict the filling factor at the center of QPCs and their conductance at different gate voltages. The 3D Schrödinger equation is split into 1D and 2D parts. Quasi-1D Schrödinger and Poisson equations are solved self-consistently in the IQHE regime to obtain the potential profile, and quantum transport is used to solve for the edge state wave functions. The velocity of edge states is found to be ⟨ E ⟩ / B , where ⟨ E ⟩ is the expectation value of the electric field for the edge state. Anisotropically etched trench gated heterostructures with double-sided delta doping have the highest edge state velocity among the structures considered.
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17:Title: Ilatikhameneh H; Ameen T; Chen F; Sahasrabudhe H; Klimeck G; Rahman R, 2018, 'Dramatic impact of dimensionality on the electrostatics of P-N junctions and its sensing and switching applications', IEEE Transactions on Nanotechnology, vol. 17, pp. 293 - 298Year : 2018
Publication Type: Journal Papers
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Abstract
Low-dimensional material systems provide a unique set of properties useful for solid-state devices. The building block of these devices is the p-n junction. In this paper, we present a dramatic difference in the electrostatics of p-n junctions in lower dimensional systems, as against the well understood three dimensional (3-D) systems. Reducing the dimensionality increases the depletion width significantly. We propose a novel method to derive analytic equations in 2-D and 1-D that considers the impact of neutral regions. The analytical results show an excellent match with both the experimental measurements and numerical simulations. The square root dependence of the depletion width on the ratio of dielectric constant and doping in 3-D changes to a linear and exponential dependence for 2-D and 1-D, respectively. This higher sensitivity of 1-D p-n junctions to its control parameters can be used toward new sensors. Utilizing the unconventional electrostatics of these low-dimensional junctions for sensing and switching applications has been discussed and a novel sensor is proposed
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18:Title: Ferdous R; Kawakami E; Scarlino P; Nowak MP; Ward DR; Savage DE; Lagally MG; Coppersmith SN; Friesen M; Eriksson MA; Vandersypen LMK; Rahman R, 2018, 'Valley dependent anisotropic spin splitting in silicon quantum dots', npj Quantum Information, vol. 4Year : 2018
Publication Type: Journal Papers
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Abstract
Spin qubits hosted in silicon (Si) quantum dots (QD) are attractive due to their exceptionally long coherence times and compatibility with the silicon transistor platform. To achieve electrical control of spins for qubit scalability, recent experiments have utilized gradient magnetic fields from integrated micro-magnets to produce an extrinsic coupling between spin and charge, thereby electrically driving electron spin resonance (ESR). However, spins in silicon QDs experience a complex interplay between spin, charge, and valley degrees of freedom, influenced by the atomic scale details of the confining interface. Here, we report experimental observation of a valley dependent anisotropic spin splitting in a Si QD with an integrated micro-magnet and an external magnetic field. We show by atomistic calculations that the spin-orbit interaction (SOI), which is often ignored in bulk silicon, plays a major role in the measured anisotropy. Moreover, inhomogeneities such as interface steps strongly affect the spin splittings and their valley dependence. This atomic-scale understanding of the intrinsic and extrinsic factors controlling the valley dependent spin properties is a key requirement for successful manipulation of quantum information in Si QDs.
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19:Title: Ilatikhameneh H; Ameen TA; Chen C; Klimeck G; Rahman R, 2018, 'Sensitivity Challenge of Steep Transistors', IEEE Transactions on Electron Devices, vol. 65, pp. 1633 - 1639Year : 2018
Publication Type: Journal Papers
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Abstract
Steep transistors are crucial in lowering power consumption of the ICs. However, the difficulties in achieving steepness beyond the Boltzmann limit experimentally have hindered the fundamental challenges in application of these devices in ICs. From a sensitivity perspective, an ideal switch should have a high sensitivity to the gate voltage and lower sensitivity to the device design parameters such as oxide and body thicknesses. In this paper, conventional tunnel-FET (TFET) and negative capacitance FET are shown to suffer from high sensitivity to device design parameters using full-band atomistic quantum transport simulations and analytical analysis. Although dielectric-engineered (DE)-TFETs based on 2-D materials show smaller sensitivity compared with the conventional TFETs, they have leakage issue. To mitigate this challenge, a novel DE-TFET design has been proposed and studied.
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20:Title: Ameen TA; Ilatikhameneh H; Huang JZ; Povolotskyi M; Rahman R; Klimeck G, 2017, 'Combination of Equilibrium and Nonequilibrium Carrier Statistics into an Atomistic Quantum Transport Model for Tunneling Heterojunctions', IEEE Transactions on Electron Devices, vol. 64, pp. 2512 - 2518Year : 2017
Publication Type: Journal Papers
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Abstract
Tunneling heterojunctions (THJs) have confined states close to the tunneling region, which significantly affect their transport properties. Accurate numerical modeling of THJs requires combining the nonequilibrium coherent quantum transport through the tunneling region as well as the quasi-equilibrium statistics arising from the strong scattering in the confined states. In this paper, a novel atomistic model is proposed to include both the effects: the strong scattering in the regions around THJ and the coherent tunneling. The new model matches reasonably well with experimental measurements of Nitride THJ and provides an efficient engineering tool for performance prediction and design of THJ-based devices
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