Journal Papers (79)
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71: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
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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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72: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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73: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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74:Title: Lansbergen G; Rahman R; Wellard C; Caro J; Collaert N; Biesemans S; Klimeck G; Hollenberg L; Rogge S, 2008, 'Atomistic understanding of a single gated dopant atom in a MOSFET', Materials Research Society Symposium Proceedings, vol. 1067, pp. 12 - 17Year : 2008
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75:Title: Lansbergen GP; Rahman R; Wellard CJ; Caro J; Woo I; Colleart N; Biesemans S; Klimeck G; Hollenberg LCL; Rogge S, 2008, 'Level spectrum of a single gated arsenic donor in a three terminal geometry', Materials Research Society Symposium Proceedings, vol. 1117, pp. 1 - 6Year : 2008
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76:Title: Naumov M; Lee S; Haley B; Bae H; Clark S; Rahman R; Ryu H; Saied F; Klimeck G, 2008, 'Eigenvalue solvers for atomistic simulations of electronic structures with NEMO-3D', Journal of Computational Electronics, vol. 7, pp. 297 - 300Year : 2008
Publication Type: Journal Papers
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Abstract
The atomistic simulations of electronic structures, using a tight binding model with millions of atoms, require solution of very large sparse Hermitian eigenvalue problems. To obtain the eigenpairs of interest in the interior of the spectrum, we must take advantage of the most efficient parallel numerical algorithms. Several methods have been developed and implemented in Nanoelectronic Modeling software package NEMO-3D, including (P)ARPACK, (Block) Lanczos and Tracemin. In this paper, the performance and tradeoffs of these algorithms for realistic models are discussed. The effectiveness of code optimization techniques such as SSE2 vectorization is also presented.
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77:Title: Lansbergen G; Rahman R; Wellard CJ; Woo I; Caro J; Collaert N; Biesemans S; Klimeck G; Hollenberg L; Rogge S, 2008, 'Gate-induced quantum-confinement transition of a single dopant atom in a silicon FinFET', Nature Physics, vol. 4, pp. 656 - 661Year : 2008
Publication Type: Journal Papers
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Abstract
The ability to build structures with atomic precision is one of the defining features of nanotechnology. Achieving true atomic-level functionality, however, requires the ability to control the wavefunctions of individual atoms. Here, we investigate an approach that could enable just that. By collecting and analysing transport spectra of a single donor atom in the channel of a silicon FinFET, we present experimental evidence for the emergence of a new type of hybrid molecule system. Our experiments and simulations suggest that the transistor’s gate potential can be used to control the degree of hybridization of a single electron donor state between the nuclear potential of its donor atom and a nearby quantum well. Moreover, our theoretical analysis enables us to determine the species of donor (arsenic) implanted into each device as well as the degree of confinement imposed by the gate.
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78:Title: Klimeck G; Ahmed SS; Bae H; Clark S; Haley B; Lee S; Naumov M; Ryu H; Saied F; Prada M; Korkusinski M; Boykin TB; Rahman R, 2007, 'Atomistic simulation of realistically sized nanodevices using NEMO 3-D - Part I: Models and benchmarks', IEEE Transactions on Electron Devices, vol. 54, pp. 2079 - 2089Year : 2007
Publication Type: Journal Papers
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Abstract
Device physics and material science meet at the atomic scale of novel nanostructured semiconductors, and the distinction between new device or new material is blurred. Not only the quantum-mechanical effects in the electronic states of the device but also the granular atomistic representation of the underlying material are important. Approaches based on a continuum representation of the underlying material typically used by device engineers and physicists become invalid. Ab initio methods used by material scientists typically do not represent the band gaps and masses precisely enough for device design, or they do not scale to realistically large device sizes. The plethora of geometry, material, and doping configurations in semiconductor devices at the nanoscale suggests that a general nanoelectronic modeling tool is needed. The 3-D NanoElectronic MOdeling (NEMO 3-D) tool has been developed to address these needs. Based on the atomistic valence force field and a variety of nearest neighbor tight-binding models (e.g., s, sp 3 s*, and sp 3 d 5 s*), NEMO 3-D enables the computation of strain and electronic structure for more than 64 and 52 million atoms, corresponding to volumes of (110 nm) 3 and (101 nm) 3 , respectively. The physical problem may involve very large scale computations, and NEMO 3-D has been designed and optimized to be scalable from single central processing units to large numbers of processors on commodity clusters and supercomputers. NEMO 3-D has been released with an open-source license in 2003 and is continually developed by the Network for Computational Nanotechnology (NCN). A web-based online interactive version for educational purposes is freely available on the NCN portal ( http://www.nanoHUB.org). In this paper, theoretical models and essential algorithmic and computational components that have been used in the development and successful deployment of NEMO 3-D are discussed.
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79:Title: Rahman R; Wellard CJ; Bradbury FR; Prada M; Cole JH; Klimeck G; Hollenberg LCL, 2007, 'High precision quantum control of single donor spins in silicon', Physical Review Letters, vol. 99Year : 2007
Publication Type: Journal Papers
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Abstract
The Stark shift of the hyperfine coupling constant is investigated for a P donor in Si far below the ionization regime in the presence of interfaces using tight-binding and band minima basis approaches and compared to the recent precision measurements. In contrast with previous effective mass-based results, the quadratic Stark coefficient obtained from both theories agrees closely with the experiments. It is also shown that there is a significant linear Stark effect for an impurity near the interface, whereas, far from the interface, the quadratic Stark effect dominates. This work represents the most sensitive and precise comparison between theory and experiment for single donor spin control. Such precise control of single donor spin states is required particularly in quantum computing applications of single donor electronics, which forms the driving motivation of this work.
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