Journal Papers (79)
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61:Title: Neupane MR; Lake RK; Rahman R, 2012, 'Electronic states of Ge/Si nanocrystals with crescent-shaped Ge-cores', Journal of Applied Physics, vol. 112Year : 2012
Publication Type: Journal Papers
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Abstract
Ge/Si nanocrystals can serve as charge storage sites in a nanocrystal memory by providing a hole quantum-well in the Ge region. The electronic states of realistically shaped Ge/Si nanocrystals with crescent-shaped Ge-cores are calculated to determine the hole confinement energies, effective masses, barrier heights, and thermionic lifetimes. As the Ge crescent thickness increases from 1 nm to 3.5 nm, the hole confinement energy decreases from 0.52 to 0.28 eV, the barrier height to escape into the Si valence band increases from 0.25 to 0.51 eV, and the resulting thermionic hole lifetime increases from 10−9 to 10−5 s. The nanocrystals are modeled with an atomistic, 20-band sp3d5s* tight-binding model including spin-orbit coupling as implemented in NEMO3D. Geometry relaxation and strain are included using the valence-force-field model with Keating potentials.
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62:Title: Nielsen E; Rahman R; Muller RP, 2012, 'A many-electron tight binding method for the analysis of quantum dot systems', Journal of Applied Physics, vol. 112Year : 2012
Publication Type: Journal Papers
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Abstract
We present a method which computes many-electron energies and eigenfunctions by a full configuration interaction, which uses a basis of atomistic tight-binding wave functions. This approach captures electron correlation as well as atomistic effects, and is well suited to solid state quantum dot systems containing few electrons, where valley physics and disorder contribute significantly to device behavior. Results are reported for a two-electron silicon double quantum dot as an example.
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63: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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64: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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65:Title: Lansbergen G; Rahman R; Verduijn A; Tettamanzi G; Collaert N; Biesemans S; Klimeck G; Rogge S; Hollenberg L, 2011, 'Lifetime-enhanced transport in silicon due to spin and valley blockade', Physical Review Letters, vol. 107, pp. 136602-1 - 136602-5Year : 2011
Publication Type: Journal Papers
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Abstract
We report the observation of lifetime-enhanced transport (LET) based on perpendicular valleys in silicon by transport spectroscopy measurements of a two-electron system in a silicon transistor. The LET is manifested as a peculiar current step in the stability diagram due to a forbidden transition between an excited state and any of the lower energy states due to perpendicular valley (and spin) configurations, offering an additional current path. By employing a detailed temperature dependence study in combination with a rate equation model, we estimate the lifetime of this particular state to exceed 48 ns. The two-electron spin-valley configurations of all relevant confined quantum states in our device were obtained by a large-scale atomistic tight-binding simulation. The LET acts as a signature of the complicated valley physics in silicon: a feature that becomes increasingly important in silicon quantum devices.
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66:Title: Rahman R; Verduijn JA; Kharche N; Lansbergen G; Klimeck ; Hollenberg ; Rogge S, 2011, 'Engineered valley-orbit splittings in quantum-confined nanostructures in silicon', Physical Review B, vol. 83, pp. 195323-1 - 195323-5Year : 2011
Publication Type: Journal Papers
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Abstract
An important challenge in silicon quantum electronics in the few electron regime is the potentially small energy gap between the ground and excited orbital states in 3D quantum confined nanostructures due to the multiple valley degeneracies of the conduction band present in silicon. Understanding the “valley-orbit” (VO) gap is essential for silicon qubits, as a large VO gap prevents leakage of the qubit states into a higher dimensional Hilbert space. The VO gap varies considerably depending on quantum confinement, and can be engineered by external electric fields. In this work we investigate VO splitting experimentally and theoretically in a range of confinement regimes. We report measurements of the VO splitting in silicon quantum dot and donor devices through excited state transport spectroscopy. These results are underpinned by large-scale atomistic tight-binding calculations involving over 1 million atoms to compute VO splittings as functions of electric fields, donor depths, and surface disorder. The results provide a comprehensive picture of the range of VO splittings that can be achieved through quantum engineering
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67:Title: Rahman ; Lansbergen ; Verduijn A; Tettamanzi G; Park ; Collaert ; Biesemans ; Klimeck ; Hollenberg ; Rogge S, 2011, 'Electric field reduced charging energies and two-electron bound excited states of single donors in silicon', Physical Review B, vol. 84, pp. 115458-1 - 115458-7Year : 2011
Publication Type: Journal Papers
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Abstract
We present atomistic simulations of the D 0 to D − charging energies of a gated donor in silicon as a function of applied fields and donor depths and find good agreement with experimental measurements. A self-consistent field large-scale tight-binding method is used to compute the D − binding energies with a domain of over 1.4 million atoms, taking into account the full band structure of the host, applied fields, and interfaces. An applied field pulls the loosely bound D − electron toward the interface and reduces the charging energy significantly below the bulk values. This enables formation of bound excited D − states in these gated donors, in contrast to bulk donors. A detailed quantitative comparison of the charging energies with transport spectroscopy measurements with multiple samples of arsenic donors in ultrascaled metal-oxide-semiconductor transistors validates the model results and provides physical insights. We also report measured D − data showing the presence of bound D − excited states under applied fields.
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68: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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69: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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70: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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