All pulications (111)

• 41:

  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 - 2878
     

  Year : 2016

  Publication Type: Journal Papers

  Topic:

  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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• 42:

  Title: Long P; Povolotskyi M; Huang JZ; Ilatikhameneh H; Ameen T; Rahman R; Kubis T; Klimeck G; Rodwell MJW, 2016, 'Extremely high simulated ballistic currents in triple-heterojunction tunnel transistors', in Device Research Conference - Conference Digest, DRC
     

  Year : 2016

  Publication Type: Conference Proceeding

  Topic:

  Abstract

  Future VLSI devices will require low CV dd 2 /2 switching energy, large on-currents (I on ), and small off-currents (I off ). Low switching energy requires a low supply voltage V dd , yet reducing V dd typically increases /off and reduces the I on /I off ratio. Though tunnel FETs (TFETs) have steep subthreshold swings and can operate at a low V dd , yet their I on is limited by low tunneling probability. Even with a GaSb/InAs heterojunction (HJ), given a 2nm-thick-channel (001)-confined TFET, [100] transport, and assuming V dd =0.3V and I oFF =10 -3 A/m, the peak tunneling probability is <;3 % (fig. 1 a) and I on is only 24 A/m (fig. 1b) [1]. This low I on will result in large CV dd /I delay and slow logic operation. Techniques to increase /on include graded AlSb/AlGaSb source HJs [2,3] and tunneling resonant states [4]. We had previously shown that tunneling probability is increased using (11 0) confinement and channel heterojunctions [1], the latter increasing the junction built-in potential and junction field, hence reducing the tunneling distance. Here we propose a triple heterojunction TFET combining these techniques. The triple-HJ design further thins the tunnel barrier to 1.2 nm, and creates two closely aligned resonant states 57meV apart. The tunneling probability is very high, >50% over a 120meV range, and the ballistic I on is extremely high, 800A/m at 30nm Lg and 475 A/m at 15nm Lg, both with I off =10 -3 A/m and V dd =0.3 V. Compared to a (001) GaSb/InAs TFET, the triple-HJ design increases the ballistic /on by 26:1 at 30nm L g and 19:1 at 15nm L g . The designs may, however, suffer from increased phonon-assisted tunneling.

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• 43:

  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. 94
     

  Year : 2016

  Publication Type: Journal Papers

  Topic:

  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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• 44:

  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. 6
     

  Year : 2016

  Publication Type: Journal Papers

  Topic:

  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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• 45:

  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. 6
     

  Year : 2016

  Publication Type: Journal Papers

  Topic:

  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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• 46:

  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 - 265
     

  Year : 2016

  Publication Type: Journal Papers

  Topic:

  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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• 47:

  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. 91
     

  Year : 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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• 48:

  Title: Usman M; Rahman R; Salfi J; Bocquel J; Voisin B; Rogge S; Klimeck G; Hollenberg LLC, 2015, 'Donor hyperfine Stark shift and the role of central-cell corrections in tight-binding theory', Journal of Physics Condensed Matter, vol. 27
     

  Year : 2015

  Publication Type: Journal Papers

  Topic:

  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 µm2 V−2 for the static dielectric screening to −1.72 × 10−3 µm2 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 µm2 V−2, resulting in an excellent agreement of theory with the experimentally measured value of −1.9 ± 0.2 × 10−3 µm2 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.

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• 49:

  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 62/317835
     

  Year : 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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• 50:

  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 - 728
     

  Year : 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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