All pulications (111)
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51:Title: Neupane MR; Rahman R; Lake RK, 2015, 'Effect of strain on the electronic and optical properties of Ge-Si dome shaped nanocrystals', Physical Chemistry Chemical Physics, vol. 17, pp. 2484 - 2493Year : 2015
Publication Type: Journal Papers
Topic:
Abstract
The effects of strain and confinement on the energy levels and emission spectra of dome-shaped, Ge-core–Si-shell nanocrystals (NCs) with diameters ranging from 5 to 45 nm are investigated with atomistic models. For NCs with base diameters ≥15 nm, the strain-induced increase in the energy gap is ∼100 meV. The increase in the energy gap is primarily the result of the downward shift in the occupied states confined in the Ge core. The fundamental energy gap varies from 960 meV to 550 meV as the NC diameter increases from 5 nm to 45 nm. Confinement and strain break the degeneracy of the lowest excited state and split it into two states separated by a few meV. For the smaller NCs, one of these states can be localized in the Si core and the other state can be in the Si cap. For diameters ≥20 nm, both of these states are localized in the Si cap. The electronic states are calculated using an atomistic sp3d5s* tight-binding model including spin–orbit coupling, and geometry relaxation is performed using a valence force field model.
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52:Title: Mol JA; Salfi J; Rahman R; Hsueh Y; Miwa JA; Klimeck G; Simmons MY; Rogge S, 2015, 'Interface-induced heavy-hole/light-hole splitting of acceptors in silicon', Applied Physics Letters, vol. 106Year : 2015
Publication Type: Journal Papers
Topic:
Abstract
The energy spectrum of spin-orbit coupled states of individual 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 (<2 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. This research was conducted by the Australian Research Council Centre of Excellence for Quantum Computation and Communication Technology (Project No. CE110001027) and the U.S. National Security Agency and the U.S. Army Research Office under Contract No. W911NF-08-1-0527. J.A.M. received funding from the Royal Society Newton International Fellowship scheme. M.Y.S. acknowledges an ARC Federation Fellowship. S.R. acknowledges an ARC Future Fellowship and FP7 MULTI. The authors are grateful to D. Culcer for helpful discussions.
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53:Title: Usman M; Hill CD; Rahman R; Klimeck G; Simmons MY; Rogge S; Hollenberg LCL, 2015, 'Strain and electric field control of hyperfine interactions for donor spin qubits in silicon', Physical Review B - Condensed Matter and Materials Physics, vol. 91Year : 2015
Publication Type: Journal Papers
Topic:
Abstract
Control of hyperfine interactions is a fundamental requirement for quantum computing architecture schemes based on shallow donors in silicon. However, at present, there is lacking an atomistic approach including critical effects of central-cell corrections and nonstatic screening of the donor potential capable of describing the hyperfine interaction in the presence of both strain and electric fields in realistically sized devices. We establish and apply a theoretical framework, based on atomistic tight-binding theory, to quantitatively determine the strain and electric-field-dependent hyperfine couplings of donors. Our method is scalable to millions of atoms, and yet captures the strain effects with an accuracy level of DFT method. Excellent agreement with the available experimental data sets allow reliable investigation of the design space of multiqubit architectures, based on both strain only as well as hybrid (strain + field) control of qubits. The benefits of strain are uncovered by demonstrating that a hybrid control of qubits based on (001) compressive strain and in-plane (100 or 010) fields results in higher gate fidelities and or faster gate operations, for all of the four donor species considered (P, As, Sb, and Bi). The comparison between different donor species in strained environments further highlights the trends of hyperfine shifts, providing predictions where no experimental data exists. While faster gate operations are realizable with in-plane fields for P, As, and Sb donors, only for the Bi donor, our calculations predict faster gate response in the presence of both in-plane and out-of-plane fields, truly benefiting from the proposed planar field control mechanism of the hyperfine interactions.
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54:Title: Tunnel field effect transistor having anisotropic effective mass”, H. Ilatikhameneh, T. Ameen, B. Novakovic, R. Rahman, G. Klimeck, Purdue University, IN, USA, 4/42016, US-patent application 62/317835Year : 2015
Publication Type: Patents
Topic: STM Electronics
Abstract
A tunnel field effect transistor (TFET) device includes a substrate, heavily doped source and drain regions disposed at opposite ends of a channel region forming a PiN or NiP structure, the channel region including a first substantially parallelogram portion having a first length defined along a longitudinal axis extending from the source region to the drain region and a second substantially parallelogram portion having a second length defined along the longitudinal axis larger than the first length, the TFET device having an effective channel length that is an average of the first and second lengths. The channel region includes a channel material with a first effective mass along a longitudinal axis extending from the source region to the drain region and a second effective mass along a lateral axis perpendicular to the longitudinal axis, the first effective mass being greater than the second effective mass.
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55:Title: Ilatikhameneh H; Klimeck G; Appenzeller J; Rahman R, 2015, 'Scaling Theory of Electrically Doped 2D Transistors', IEEE Electron Device Letters, vol. 36, pp. 726 - 728Year : 2015
Publication Type: Journal Papers
Topic:
Abstract
In this letter, it is shown that the existing scaling theories for chemically doped transistors cannot be applied to the novel class of electrically doped 2D transistors and the concept of equivalent oxide thickness (EOT) is not applicable anymore. Hence, a novel scaling theory is developed based on analytic solutions of the 2D Poisson equation. Full band atomistic quantum transport simulations verify the theory and show that the critical design parameters are the physical oxide thickness and distance between the gates. Accordingly, the most optimized electrically doped devices are those with the smallest spacing between the gates and the thinnest oxide, and not the smallest EOT.
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56:Title: Tosi G; Mohiyaddin FA; Tenberg S; Rahman R; Klimeck G; Morello A, 2015, 'Silicon quantum processor with robust long-distance qubit couplings', arxiv, vol. 8Year : 2015
Publication Type: Journal Papers
Topic:
Abstract
Practical quantum computers require a large network of highly coherent qubits, interconnected in a design robust against errors. Donor spins in silicon provide state-of-the-art coherence and quantum gate fidelities, in a platform adapted from industrial semiconductor processing. Here we present a scalable design for a silicon quantum processor that does not require precise donor placement and leaves ample space for the routing of interconnects and readout devices. We introduce the flip-flop qubit, a combination of the electron-nuclear spin states of a phosphorus donor that can be controlled by microwave electric fields. Two-qubit gates exploit a second-order electric dipole-dipole interaction, allowing selective coupling beyond the nearest-neighbor, at separations of hundreds of nanometers, while microwave resonators can extend the entanglement to macroscopic distances. We predict gate fidelities within fault-tolerance thresholds using realistic noise models. This design provides a realizable blueprint for scalable spin-based quantum computers in silicon.
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57:Title: Ilatikhameneh H; Ameen TA; Klimeck G; Appenzeller J; Rahman R, 2015, 'Dielectric Engineered Tunnel Field-Effect Transistor', IEEE Electron Device Letters, vol. 36, pp. 1097 - 1100Year : 2015
Publication Type: Journal Papers
Topic:
Abstract
The dielectric engineered tunnel field-effect transistor (DE-TFET) as a high-performance steep transistor is proposed. In this device, a combination of high-k and low-k dielectrics results in a high electric field at the tunnel junction. As a result, a record ON-current of ~1000 μA/μm and a subthreshold swing (SS) below 20 mV/decade are predicted for WTe 2 DE-TFET. The proposed TFET works based on a homojunction channel and electrically doped contacts both of which are immune to interface states, dopant fluctuations, and dopant states in the bandgap, which typically deteriorate the OFF-state performance and SS in the conventional TFETs.
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58:Title: Salazar RB; Ilatikhameneh H; Rahman R; Klimeck G; Appenzeller J, 2015, 'A predictive analytic model for high-performance tunneling field-effect transistors approaching non-equilibrium Green's function simulations', Journal of Applied Physics, vol. 118Year : 2015
Publication Type: Journal Papers
Topic:
Abstract
A new compact modeling approach is presented which describes the full current-voltage (I-V) characteristic of high-performance (aggressively scaled-down) tunneling field-effect-transistors (TFETs) based on homojunction direct-bandgap semiconductors. The model is based on an analytic description of two key features, which capture the main physical phenomena related to TFETs: (1) the potential profile from source to channel and (2) the elliptic curvature of the complex bands in the bandgap region. It is proposed to use 1D Poisson's equations in the source and the channel to describe the potential profile in homojunction TFETs. This allows to quantify the impact of source/drain doping on device performance, an aspect usually ignored in TFET modeling but highly relevant in ultra-scaled devices. The compact model is validated by comparison with state-of-the-art quantum transport simulations using a 3D full band atomistic approach based on non-equilibrium Green's functions. It is shown that the model reproduces with good accuracy the data obtained from the simulations in all regions of operation: the on/off states and the n/p branches of conduction. This approach allows calculation of energy-dependent band-to-band tunneling currents in TFETs, a feature that allows gaining deep insights into the underlying device physics. The simplicity and accuracy of the approach provide a powerful tool to explore in a quantitatively manner how a wide variety of parameters (material-, size-, and/or geometry-dependent) impact the TFET performance under any bias conditions. The proposed model presents thus a practical complement to computationally expensive simulations such as the 3D NEGF approach.
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59:Title: Ilatikhameneh H; Tan Y; Novakovic B; Klimeck G; Rahman R; Appenzeller J, 2015, 'Tunnel Field-Effect Transistors in 2-D Transition Metal Dichalcogenide Materials', IEEE Journal on Exploratory Solid-State Computational Devices and Circuits, vol. 1, pp. 12 - 18Year : 2015
Publication Type: Journal Papers
Topic:
Abstract
In this paper, the performance of tunnel field-effect transistors (TFETs) based on 2-D transition metal dichalcogenide (TMD) materials is investigated by atomistic quantum transport simulations. One of the major challenges of TFETs is their low ON-currents. 2-D material-based TFETs can have tight gate control and high electric fields at the tunnel junction, and can, in principle, generate high ON-currents along with a subthreshold swing (SS) smaller than 60 mV/decade. Our simulations reveal that high-performance TMD TFETs not only require good gate control, but also rely on the choice of the right channel material with optimum bandgap, effective mass, and source/drain doping level. Unlike previous works, a full-band atomistic tight-binding method is used self-consistently with 3-D Poisson equation to simulate ballistic quantum transport in these devices. The effect of the choice of the TMD material on the performance of the device and its transfer characteristics are discussed. Moreover, the criteria for high ON-currents are explained with a simple analytic model, showing the related fundamental factors. Finally, the SS and energy delay of these TFETs are compared with conventional CMOS devices.
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60:Title: Li W; Sharmin S; Ilatikhameneh H; Rahman R; Lu Y; Wang J; Yan X; Seabaugh A; Klimeck G; Jena D; Fay P, 2015, 'Polarization-Engineered III-Nitride Heterojunction Tunnel Field-Effect Transistors', IEEE Journal on Exploratory Solid-State Computational Devices and Circuits, vol. 1, pp. 28 - 34Year : 2015
Publication Type: Journal Papers
Topic:
Abstract
The concept and simulated device characteristics of tunneling field-effect transistors (TFETs) based on III-nitride heterojunctions are presented for the first time. Through polarization engineering, interband tunneling can become significant in III-nitride heterojunctions, leading to the potential for a viable TFET technology. Two prototype device designs, inline and sidewall-gated TFETs, are discussed. Polarization-assisted p-type doping is used in the source region to mitigate the effect of the deep Mg acceptor level in p-type GaN. Simulations indicate that TFETs based on III-nitride heterojunctions can be expected to achieve ON/OFF ratios of 106 or more, with switching slopes well below 60 mV/decade, ON-current densities approaching 100 μA/μm, and energy delay products as low as 67 aJ-ps/μm.
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