All pulications (111)
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11: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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12:Title: Shekhar P; Pendharker S; Sahasrabudhe H; Vick D; Malac M; Rahman R; Jacob Z, 2018, 'Extreme ultraviolet plasmonics and Cherenkov radiation in silicon', Optica, vol. 5, pp. 1590 - 1596Year : 2018
Publication Type: Journal Papers
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Abstract
Silicon is widely used as the material of choice for semiconductor and insulator applications in nanoelectronics, micro-electro-mechanical systems, solar cells, and on-chip photonics. In stark contrast, in this paper, we explore silicon’s metallic properties and show that it can support propagating surface plasmons, collective charge oscillations, in the extreme ultraviolet (EUV) energy regime not possible with other plasmonic materials such as aluminum, silver, or gold. This is fundamentally different from conventional approaches, where doping semiconductors is considered necessary to observe plasmonic behavior. We experimentally map the photonic band structure of EUV surface and bulk plasmons in silicon using momentum-resolved electron energy loss spectroscopy. Our experimental observations are validated by macroscopic electrodynamic electron energy loss theory simulations as well as quantum density functional theory calculations. As an example of exploiting these EUV plasmons for applications, we propose a tunable and broadband thresholdless Cherenkov radiation source in the EUV using silicon plasmonic metamaterials. Our work can pave the way for the field of EUV plasmonics.
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13: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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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: 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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16:Title: Ferdous R; Chan KW; Veldhorst M; Hwang JCC; Yang CH; Sahasrabudhe H; Klimeck G; Morello A; Dzurak AS; Rahman R, 2018, 'Interface-induced spin-orbit interaction in silicon quantum dots and prospects for scalability', Physical Review B, vol. 97Year : 2018
Publication Type: Journal Papers
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Abstract
We identify the presence of monatomic steps at the Si/SiGe or Si/SiO2 interface as a dominant source of variations in the dephasing time of silicon (Si) quantum dot (QD) spin qubits. First, using atomistic tight-binding calculations we show that the g-factors and their Stark shifts undergo variations due to these steps. We compare our theoretical predictions with experiments on QDs at a Si/SiO2 interface, in which we observe significant differences in Stark shifts between QDs in two different samples. We also experimentally observe variations in the g-factors of one-electron and three-electron spin qubits realized in three neighboring QDs on the same sample, at a level consistent with our calculations. The dephasing times of these qubits also vary, most likely due to their varying sensitivity to charge noise, resulting from different interface conditions. More importantly, from our calculations we show that by employing the anisotropic nature of the spin-orbit interaction (SOI) in a Si QD, we can minimize and control these variations. Ultimately, we predict that the dephasing times of the Si QD spin qubits will be anisotropic and can be improved by at least an order of magnitude, by aligning the external dc magnetic field towards specific crystal directions, given other decoherence mechanisms do not dominate over charge noise.
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17:Title: Chen F; Ilatikhameneh H; Tan Y; Klimeck G; Rahman R, 2018, 'Switching Mechanism and the Scalability of Vertical-TFETs', IEEE Transactions on Electron Devices, vol. 65, pp. 3065 - 3068Year : 2018
Publication Type: Journal Papers
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Abstract
In this brief, vertical tunnel field-effect transistors (v-TFETs) based on vertically stacked heretojunctions from 2-D transition metal dichalcogenide materials are studied by atomistic quantum transport simulations. The switching mechanism of a v-TFET is found to be different from previous predictions. As a consequence of this switching mechanism, the extension region where the materials are not stacked over is found to be critical for turning off the v-TFET. This extension region makes the scaling of v-TFETs challenging. In addition, due to the presence of both positive and negative charges inside the channel, v-TFETs also exhibit negative top gate capacitance. As a result, v-TFETs have good energy-delay products and are one of the promising candidates for low-power applications.
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18:Title: Tankasala A; Salfi J; Bocquel J; Voisin B; Usman M; Klimeck G; Simmons MY; Hollenberg LCL; Rogge S; Rahman R, 2018, 'Two-electron states of a group-V donor in silicon from atomistic full configuration interactions', Physical Review B, vol. 97Year : 2018
Publication Type: Journal Papers
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Abstract
Two-electron states bound to donors in silicon are important for both two-qubit gates and spin readout. We present a full configuration interaction technique in the atomistic tight-binding basis to capture multielectron exchange and correlation effects taking into account the full band structure of silicon and the atomic-scale granularity of a nanoscale device. Excited s -like states of A 1 symmetry are found to strongly influence the charging energy of a negative donor center. We apply the technique on subsurface dopants subjected to gate electric fields and show that bound triplet states appear in the spectrum as a result of decreased charging energy. The exchange energy, obtained for the two-electron states in various confinement regimes, may enable engineering electrical control of spins in donor-dot hybrid qubits.
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19: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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20: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
Topic:
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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