All pulications (111)
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91:Title: Rahman R; Park SH; Klimeck G; Hollenberg LCL, 2011, 'Stark tuning of the charge states of a two-donor molecule in silicon', Nanotechnology, vol. 22Year : 2011
Publication Type: Journal Papers
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Abstract
A singly ionized two-donor molecule in silicon is an interesting test-bed system for implementing a quantum bit using charge degrees of freedom at the atomic limit of device fabrication. The operating principles of such a device are based on wavefunction symmetries defined by charge localizations and energy gaps in the spectrum. The Stark-shifted electronic structure of a two-donor phosphorus molecule is investigated using a multi-million-atom tight-binding framework. The effects of surface (S) and barrier (B) gates are analyzed for various voltage regimes. It is found that gate control is smooth for any donor separation, although at certain donor orientations the S and B gates may alter in functionality. Effects such as interface ionization, saturation of the lowest energy gap, and sensitivity to donor and gate placements are also investigated. Excited molecular states of P2 + are found to impose limits on the allowed donor separations and operating gate voltages for coherent operation. This work therefore outlines and analyzes the various issues that are of importance in the design and control of such donor molecular systems.
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92:Title: Rahman R; Verduijn J; Kharche N; Lansbergen GP; Klimeck G; Hollenberg LCL; Rogge S, 2011, 'Erratum: Engineered valley-orbit splittings in quantum-confined nanostructures in silicon (Physical Review B - Condensed Matter and Materials Physics (2011) 83 (195323))', Physical Review B - Condensed Matter and Materials Physics, vol. 83Year : 2011
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93:Title: Neupane MR; Lake RK; Rahman R, 2011, 'Core size dependence of the confinement energies, barrier heights, and hole lifetimes in Ge-core/Si-shell nanocrystals', Journal of Applied Physics, vol. 110Year : 2011
Publication Type: Journal Papers
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Abstract
The effect of the Ge core size on the confinement energies, barrier heights, and hole lifetimes in spherical Ge/Si core-shell nanocrystals is studied using an atomistic, tight-binding model with an sp3 d5 s∗ basis including spin-orbit coupling. Nanocrystal diameters range from 11 nm to 17.5 nm with Ge core diameters ranging from 1 nm to 7.5 nm. With a Ge core diameter of ~4 nm, and a Si shell thickness of ≥3 nm, the thermionic barrier presented by the Si shell increases the hole lifetime by a factor of ~2×108 compared to the hole lifetime in an all-Si nanocrystal in SiO2. A retention lifetime of 10 years is obtained with a 3 nm Ge core and a 3 nm Si shell with a 3 nm SiO2 tunnel oxide.
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94:Title: Tettamanzi G; Lansbergen G; Verduijn J; Rahman R; Paul A; Lee SH; Collaert N; Biesemans S; Klimeck G; Rogge S, 2010, 'Innovative characterization techniques for ultra-scaled FinFETs', in Proceedings of the 10th IEEE International Conference on Nanotechnology, IEEE, Korea, pp. 25 - 30, presented at 10th IEEE International Conference on Nanotechnology joint Symposium with Nano Korea 2010, Korea, 17 August 2010 - 20 August 2010Year : 2010
Publication Type: Conference Proceeding
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Abstract
This paper describes the first single impurity metrology study and the first experimental study of the behavior of the active cross-section area in function of gate voltages (V G ) for undoped FinFETs. From one side we show how we can identify chemical species, electric field and position for a donor present in the channel of a doped FinFET. From another side, for the undoped devices, we propose a mechanism of inversion of the bands from flat band to band bending in the interface regions respectively, all as a function of V G . By doing these we have confirmed the possibility that a combination of low temperature measurements and TB simulation techniques can be used to investigate transport in nano-scale FET devices. We have furthermore also given some answers to the fundamental technological question on how to obtain the best FinFET geometry for electronic functionalities.
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95:Title: Rahman R; Muller RP; Levy JE; Carroll MS; Klimeck G; Greentree AD; Hollenberg LCL, 2010, 'Coherent electron transport by adiabatic passage in an imperfect donor chain', Physical Review B - Condensed Matter and Materials Physics, vol. 82Year : 2010
Publication Type: Journal Papers
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Abstract
Coherent tunneling adiabatic passage (CTAP) has been proposed as a long-range physical quantum bits (qubit) transport mechanism in solid-state quantum computing architectures. Although the mechanism can be implemented in either a chain of quantum dots or donors, a one-dimensional chain of donors in Si is of particular interest due to the natural confining potential of donors that can, in principle, help reduce the gate densities in solid-state quantum computing architectures. Using detailed atomistic modeling, we investigate CTAP in a more realistic triple donor system in the presence of inevitable fabrication imperfections. In particular, we investigate how an adiabatic pathway for CTAP is affected by donor misplacements and propose schemes to correct for such errors. We also investigate the sensitivity of the adiabatic path to gate voltage fluctuations. The tight-binding based atomistic treatment of straggle used here may benefit understanding of other donor nanostructures, such as donor-based charge and spin qubits. Finally, we derive an effective 3 × 3 model of CTAP that accurately resembles the voltage tuned lowest energy states of the multimillion atom tight-binding simulations and provides a translation between intensive atomistic Hamiltonians and simplified effective Hamiltonians while retaining the relevant atomic-scale information. This method can help characterize multidonor experimental structures quickly and accurately even in the presence of imperfections, overcoming some of the numeric intractabilities of finding optimal eigenstates for nonideal donor placements.
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96:Title: Lansbergen GP; Rahman R; Caro J; Collaert N; Biesemans S; Klimeck G; Hollenberg LCL; Rogge S, 2009, '+Level spectrum of single gated as donors', in Caldas MJ; Studart N (eds.), AIP Conference Proceedings, AMER INST PHYSICS, Rio de Janeiro, BRAZIL, pp. 93 - 94, presented at 29th International Conference on Physics of Semiconductors, Rio de Janeiro, BRAZIL, 27 July - 01 August 2008Year : 2009
Publication Type: Conference Proceeding
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Abstract
We study the electrical transport through single As donors incorporated in the channel of a FinFET, i.e. a donor in a three‐terminal geometry. By means of spectroscopic measurements in conjuction with a NEMO‐3D model, we can identify the excited states and associate them with either the donors Coulomb potential, a triangular well at the interface or a hybridized combination of the two. The correspondence between the transport measurements, the theoretical model and the local environment provides an atomic understanding of actual gated donors in a nanostructure.
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97:Title: Shaikh Ahmed, Neerav Kharche, Rajib Rahman, Muhammad Usman, Sunhee Lee, Hoon Ryu, Hansang Bae, Steve Clark, Benjamin Haley, Maxim Naumov, Faisal Saied, Marek Korkusinski, Rick Kennel, Michael McLennan, Timothy B. Boykin, and Gerhard Klimeck.Year : 2009
Publication Type: Book Chapters
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Abstract
The rapid progress in nanofabrication technologies has led to the emergence of new classes of nanodevices and structures. At the atomic scale of novel nanostructured semiconductors the distinction between new device and new material is blurred and device physics and material science meet. The quantum mechanical effects in the electronic states of the device and the granular, atomistic representation of the underlying material become important. The variety of geometries, materials, and doping configurations in semiconductor devices at the nanoscale suggests that a general nanoelectronic modeling tool is needed. The Nanoelectronic Modeling tool (NEMO 3-D) has been developed to address these needs. Based on the atomistic valence-force field (VFF) method and a variety of nearest-neighbor tight-binding models, NEMO 3-D enables the computation of strain for over 64 million atoms and of electronic structure for over 52 million atoms, corresponding to volumes of (110nmx110nmx110nm) and (101nmx101nmx101nm), respectively. This article discusses the theoretical models, essential algorithmic and computational components, and optimization methods that have been used in the development and the deployment of NEMO 3-D. Also, successful applications of NEMO 3-D are demonstrated in the atomistic calculation of single-particle electronic states of (1) self-assembled quantum dots including long-range strain and piezoelectricity; (2) stacked quantum dots ; (3) Phosphorus impurities in Silicon used in quantum computation; (4) Si on SiGe quantum wells (QWs); and (5) SiGe nanowires.
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98:Title: Rahman R; Lansbergen G; Park SH; Verduijn J; Klimeck G; Rogge S; Hollenberg L, 2009, 'Orbital Stark effect and quantum confinement transition of donors in silicon', Physical Review - Section B - Condensed Matter, vol. 80, pp. 165314-1 - 165314-10Year : 2009
Publication Type: Journal Papers
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Abstract
Adiabatic shuttling of single impurity bound electrons to gate-induced surface states in semiconductors has attracted much attention in recent times, mostly in the context of solid-state quantum computer architecture. A recent transport spectroscopy experiment for the first time was able to probe the Stark shifted spectrum of a single donor in silicon buried close to a gate. Here, we present the full theoretical model involving large-scale quantum mechanical simulations that was used to compute the Stark shifted donor states in order to interpret the experimental data. Use of atomistic tight-binding technique on a domain of over a million atoms helped not only to incorporate the full band structure of the host, but also to treat realistic device geometries and donor models, and to use a large enough basis set to capture any number of donor states. The method yields a quantitative description of the symmetry transition that the donor electron undergoes from a three-dimensional Coulomb confined state to a two-dimensional (2D) surface state as the electric field is ramped up adiabatically. In the intermediate field regime, the electron resides in a superposition between the atomic donor states and the 2D surface states. In addition to determining the effect of field and donor depth on the electronic structure, the model also provides a basis to distinguish between a phosphorus and an arsenic donor based on their Stark signature. The method also captures valley-orbit splitting in both the donor well and the interface well, a quantity critical to silicon qubits. The work concludes with a detailed analysis of the effects of screening on the donor spectrum.
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99:Title: Rahman R; Park SH; Boykin TB; Klimeck G; Rogge S; Hollenberg L, 2009, 'Gate-induced g-factor control and dimensional transition for donors in multivalley semiconductors', Physical Review - Section B - Condensed Matter, vol. 80, pp. 155301-1 - 155301-5Year : 2009
Publication Type: Journal Papers
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Abstract
The dependence of the g factors of semiconductor donors on applied electric and magnetic fields is of immense importance in spin-based quantum computation and in semiconductor spintronics. The donor g -factor Stark shift is sensitive to the orientation of the electric and magnetic fields and is strongly influenced by the band-structure and spin-orbit interactions of the host. Using a multimillion atom tight-binding framework, the spin-orbit Stark parameters are computed for donors in multivalley semiconductors, silicon, and germanium. Comparison with limited experimental data shows good agreement for a donor in silicon. Results for gate-induced transition from three-dimensional to two-dimensional wave-function confinement show that the corresponding g -factor shift in Si is experimentally observable, and at modest B field, O(1 T) can exceed the Stark shift of the hyperfine interaction.
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100:Title: Rahman R; Park SH; Cole JH; Greentree AD; Muller RP; Klimeck G; Hollenberg LCL, 2009, 'Atomistic simulations of adiabatic coherent electron transport in triple donor systems', Physical Review B - Condensed Matter and Materials Physics, vol. 80Year : 2009
Publication Type: Journal Papers
Topic:
Abstract
A solid-state analog of stimulated Raman adiabatic passage can be implemented in a triple-well solid-state system to coherently transport an electron across the wells with exponentially suppressed occupation in the central well at any point of time. Termed coherent-tunneling adiabatic passage (CTAP), this method provides a robust way to transfer quantum information encoded in the electronic spin across a chain of quantum dots or donors. Using large-scale atomistic tight-binding simulations involving over 3.5 × 10 6 atoms, we verify the existence of a CTAP pathway in a realistic solid-state system: gated triple donors in silicon. Realistic gate profiles from commercial tools were combined with tight-binding methods to simulate gate control of the donor to donor tunnel barriers in the presence of crosstalk. As CTAP is an adiabatic protocol, it can be analyzed by solving the time-independent problem at various stages of the pulse justifying the use of time-independent tight-binding methods to this problem. This work also involves the first atomistic treatment to translate the three-state-based quantum-optics type of modeling into a solid-state description beyond the ideal localization assumption. Our results show that a three-donor CTAP transfer, with interdonor spacing of 15 nm can occur on time scales greater than 23 ps, well within experimentally accessible regimes. The method not only provides a tool to guide future CTAP experiments but also illuminates the possibility of system engineering to enhance control and transfer times.
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