Journal Papers (79)
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21:Title: Watson TF; Weber B; Hsueh YL; Hollenberg LCL; Rahman R; Simmons MY, 2017, 'Atomically engineered electron spin lifetimes of 30 s in silicon', Science Advances, vol. 3Year : 2017
Publication Type: Journal Papers
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Abstract
Scaling up to large arrays of donor-based spin qubits for quantum computation will require the ability to perform high-fidelity readout of multiple individual spin qubits. Recent experiments have shown that the limiting factor for high-fidelity readout of many qubits is the lifetime of the electron spin. We demonstrate the longest reported lifetimes (up to 30 s) of any electron spin qubit in a nanoelectronic device. By atomic-level engineering of the electron wave function within phosphorus atom quantum dots, we can minimize spin relaxation in agreement with recent theoretical predictions. These lifetimes allow us to demonstrate the sequential readout of two electron spin qubits with fidelities as high as 99.8%, which is above the surface code fault-tolerant threshold. This work paves the way for future experiments on multiqubit systems using donors in silicon.
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22:Title: Zheng C; Zhang Q; Weber B; Ilatikhameneh H; Chen F; Sahasrabudhe H; Rahman R; Li S; Chen Z; Hellerstedt J; Zhang Y; Duan WH; Bao Q; Fuhrer MS, 2017, 'Direct Observation of 2D Electrostatics and Ohmic Contacts in Template-Grown Graphene/WS2 Heterostructures', ACS Nano, vol. 11, pp. 2785 - 2793Year : 2017
Publication Type: Journal Papers
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Abstract
Large-area two-dimensional (2D) heterojunctions are promising building blocks of 2D circuits. Understanding their intriguing electrostatics is pivotal but largely hindered by the lack of direct observations. Here graphene–WS2 heterojunctions are prepared over large areas using a seedless ambient-pressure chemical vapor deposition technique. Kelvin probe force microscopy, photoluminescence spectroscopy, and scanning tunneling microscopy characterize the doping in graphene–WS2 heterojunctions as-grown on sapphire and transferred to SiO2 with and without thermal annealing. Both p–n and n–n junctions are observed, and a flat-band condition (zero Schottky barrier height) is found for lightly n-doped WS2, promising low-resistance ohmic contacts. This indicates a more favorable band alignment for graphene–WS2 than has been predicted, likely explaining the low barriers observed in transport experiments on similar heterojunctions. Electrostatic modeling demonstrates that the large depletion width of the graphene–WS2 junction reflects the electrostatics of the one-dimensional junction between two-dimensional materials.
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23: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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24:Title: Huang JZ; Long P; Povolotskyi M; Ilatikhameneh H; Ameen TA; Rahman R; Rodwell MJW; Klimeck G, 2017, 'A Multiscale Modeling of Triple-Heterojunction Tunneling FETs', IEEE Transactions on Electron Devices, vol. 64, pp. 2728 - 2735Year : 2017
Publication Type: Journal Papers
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Abstract
A high performance triple-heterojunction (3HJ) design has been previously proposed for tunneling FETs (TFETs). Compared with single HJ TFETs, the 3HJ TFETs have both shorter tunneling distance and two transmission resonances that significantly improve the ON-state current (I ON ). Coherent quantum transport simulation predicts that I ON = 460 μA/μm can be achieved at gate length Lg = 15 nm, supply voltage V DD = 0.3 V, and OFF-state current I OFF = 1 nA/μm. However, strong electron-phonon and electron-electron scattering in the heavily doped leads implies that the 3HJ devices operate far from the ideal coherent limit. In this paper, such scattering effects are assessed by a newly developed multiscale transport model, which combines the ballistic nonequilibrium Green's function method for the channel and the drift-diffusion scattering method for the leads. Simulation results show that the thermalizing scattering in the leads both degrades the 3HJ TFET's subthreshold swing through scattering-induced leakage and reduces the turn-ON current through the access resistance. Assuming bulk scattering rates and carrier mobilities, the I ON is dropped from 460 μA/μm down to 254 μA/μm, which is still much larger than the single HJ TFET case.
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25:Title: Ilatikhameneh H; Salazar RB; Klimeck G; Rahman R; Appenzeller J, 2016, 'From Fowler-Nordheim to Nonequilibrium Green's Function Modeling of Tunneling', IEEE Transactions on Electron Devices, vol. 63, pp. 2871 - 2878Year : 2016
Publication Type: Journal Papers
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Abstract
In this paper, an analytic model is proposed, which provides, in a continuous manner, the current–voltage ( $I$ – $V$ ) characteristic of high-performance tunneling FETs (TFETs) based on direct bandgap semiconductors. The model provides the closed-form expressions for $I$ – $V$ based on: 1) a modified version of the well-known Fowler–Nordheim (FN) formula (in the ON-state) and 2) an equation that describes the OFF-state performance while providing continuity at the ON/OFF threshold by means of a term introduced as the continuity factor. It is shown that the traditional approaches, such as FN, are accurate in TFETs only through correct evaluation of the total band bending distance and the tunneling effective mass. General expressions for these two key parameters are provided. Moreover, it is demonstrated that the tunneling effective mass captures both the ellipticity of evanescent states and the dual (electron/hole) behavior of the tunneling carriers, and it is further shown that such a concept is even applicable to semiconductors with nontrivial energy dispersion. Ultimately, it is found that the $I$ – $V$ characteristics obtained by using this model are in close agreement with the state-of-the-art quantum transport simulations both in the ON- and OFF-state, thus providing the validation of the analytic approach
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26:Title: Mohiyaddin FA; Kalra R; Laucht A; Rahman R; Klimeck G; Morello A, 2016, 'Transport of spin qubits with donor chains under realistic experimental conditions', Physical Review B, vol. 94Year : 2016
Publication Type: Journal Papers
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Abstract
The ability to transport quantum information across some distance can facilitate the design and operation of a quantum processor. One-dimensional spin chains provide a compact platform to realize scalable spin transport for a solid-state quantum computer. Here, we model odd-sized donor chains in silicon under a range of experimental nonidealities, including variability of donor position within the chain. We show that the tolerance against donor placement inaccuracies is greatly improved by operating the spin chain in a mode where the electrons are confined at the Si- SiO 2 interface. We then estimate the required time scales and exchange couplings, and the level of noise that can be tolerated to achieve high-fidelity transport. We also propose a protocol to calibrate and initialize the chain, thereby providing a complete guideline for realizing a functional donor chain and utilizing it for spin transport.
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27:Title: Ilatikhameneh H; Ameen T; Novakovic B; Tan Y; Klimeck G; Rahman R, 2016, 'Saving Moore's Law Down to 1 nm Channels with Anisotropic Effective Mass', Scientific Reports, vol. 6Year : 2016
Publication Type: Journal Papers
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Abstract
Scaling transistors’ dimensions has been the thrust for the semiconductor industry in the last four decades. However, scaling channel lengths beyond 10 nm has become exceptionally challenging due to the direct tunneling between source and drain which degrades gate control, switching functionality, and worsens power dissipation. Fortunately, the emergence of novel classes of materials with exotic properties in recent times has opened up new avenues in device design. Here, we show that by using channel materials with an anisotropic effective mass, the channel can be scaled down to 1 nm and still provide an excellent switching performance in phosphorene nanoribbon MOSFETs. To solve power consumption challenge besides dimension scaling in conventional transistors, a novel tunnel transistor is proposed which takes advantage of anisotropic mass in both ON- and OFF-state of the operation. Full-band atomistic quantum transport simulations of phosphorene nanoribbon MOSFETs and TFETs based on the new design have been performed as a proof.
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28:Title: Wang Y; Chen CY; Klimeck G; Simmons MY; Rahman R, 2016, 'Characterizing Si:P quantum dot qubits with spin resonance techniques', Scientific Reports, vol. 6Year : 2016
Publication Type: Journal Papers
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Abstract
Quantum dots patterned by atomically precise placement of phosphorus donors in single crystal silicon have long spin lifetimes, advantages in addressability, large exchange tunability, and are readily available few-electron systems. To be utilized as quantum bits, it is important to non-invasively characterise these donor quantum dots post fabrication and extract the number of bound electron and nuclear spins as well as their locations. Here, we propose a metrology technique based on electron spin resonance (ESR) measurements with the on-chip circuitry already needed for qubit manipulation to obtain atomic scale information about donor quantum dots and their spin configurations. Using atomistic tight-binding technique and Hartree self-consistent field approximation, we show that the ESR transition frequencies are directly related to the number of donors, electrons, and their locations through the electron-nuclear hyperfine interaction.
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29:Title: Ilatikhameneh H; Klimeck G; Appenzeller J; Rahman R, 2016, 'Design rules for high performance tunnel transistors from 2-D materials', IEEE Journal of the Electron Devices Society, vol. 4, pp. 260 - 265Year : 2016
Publication Type: Journal Papers
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Abstract
Tunneling field-effect transistors (TFETs) based on 2-D materials are promising steep sub-threshold swing devices due to their tight gate control. There are two major methods to create the tunnel junction in these 2-D TFETs: 1) electrical and 2) chemical doping. In this paper, design guidelines for both electrically and chemically doped 2-D TFETs are provided using full band atomistic quantum transport simulations in conjunction with analytic modeling. Moreover, several 2-D TFETs' performance boosters such as strain, source doping, and equivalent oxide thickness are studied. Later on, these performance boosters are analyzed within a novel figure-of-merit plot (i.e., constant ON-current plot).
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30:Title: Usman M; Bocquel J; Salfi J; Voisin B; Tankasala A; Rahman R; Simmons MY; Rogge S; Hollenberg LCL, 2016, 'Spatial metrology of dopants in silicon with exact lattice site precision', Nature Nanotechnology, vol. 11, pp. 763 - 768Year : 2016
Publication Type: Journal Papers
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Abstract
Scaling of Si-based nanoelectronics has reached the regime where device function is affected not only by the presence of individual 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.
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