All pulications (111)

• 1:

  Title: Federico Mazzola, Chin-Yi Chen, Rajib Rahman, Xie-Gang Zhu, Craig M. Polley, Thiagarajan Balasubramanian, Phil D. C. King, Philip Hofmann, Jill A. Miwa & Justin W. Wells npj Quantum Materials volume 5, Article number: 34 (2020) The sub-band structure of atomically sharp dopant profiles in silicon
     

  Year : 2020

  Publication Type:

  Topic:

  Abstract

  The downscaling of silicon-based structures and proto-devices has now reached the single-atom scale, representing an important milestone for the development of a silicon-based quantum computer. One especially notable platform for atomic-scale device fabrication is the so-called Si:P δ-layer, consisting of an ultra-dense and sharp layer of dopants within a semiconductor host. Whilst several alternatives exist, it is on the Si:P platform that many quantum proto-devices have been successfully demonstrated. Motivated by this, both calculations and experiments have been dedicated to understanding the electronic structure of the Si:P δ-layer platform. In this work, we use high-resolution angle-resolved photoemission spectroscopy to reveal the structure of the electronic states which exist because of the high dopant density of the Si:P δ-layer. In contrast to published theoretical work, we resolve three distinct bands, the most occupied of which shows a large anisotropy and significant deviation from simple parabolic behaviour. We investigate the possible origins of this fine structure, and conclude that it is primarily a consequence of the dielectric constant being large (ca. double that of bulk Si). Incorporating this factor into tight-binding calculations leads to a major revision of band structure; specifically, the existence of a third band, the separation of the bands, and the departure from purely parabolic behaviour. This new understanding of the band structure has important implications for quantum proto-devices which are built on the Si:P δ-layer platform.

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• 2:

  Title: Sengupta P; Khandekar C; Van Mechelen T; Rahman R; Jacob Z, 2020, 'Electron g -factor engineering for nonreciprocal spin photonics', Physical Review B, vol. 101
     

  Year : 2020

  Publication Type: Journal Papers

  Topic:

  Abstract

  We study the interplay of electron and photon spin in nonreciprocal materials. Traditionally, the primary mechanism to design nonreciprocal photonic devices has been magnetic fields in conjunction with magnetic oxides, such as iron garnets. In this work, we present an alternative paradigm that allows tunability and reconfigurability of the nonreciprocity through spintronic approaches. The proposed design uses the high spin-orbit coupling (SOC) of a narrow-band-gap semiconductor (InSb) with ferromagnetic dopants. A combination of the intrinsic SOC and a gate-applied electric field gives rise to a strong external Rashba spin-orbit coupling (RSOC) in a magnetically doped InSb film. The RSOC which is gate alterable is shown to adjust the magnetic permeability tensor via the electron g factor of the medium. We use electronic band structure calculations ( k ⋅ p theory) to show that the gate-adjustable RSOC manifest itself in the nonreciprocal coefficient of photon fields via shifts in the Kerr and Faraday rotations. In addition, we show that photon spin properties of dipolar emitters placed in the vicinity of a nonreciprocal electromagnetic environment are distinct from reciprocal counterparts. The Purcell factor ( F p ) of a spin-polarized emitter (right-handed circular dipole) is significantly enhanced due to a larger g factor while a left-handed dipole remains essentially unaffected. Our search for novel nonreciprocal material platforms can lead to electron-spin-controlled reconfigurable photonic devices

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• 3:

  Title: Valley interference and spin exchange at the atomic scale in silicon
     

  Author : B. Voisin, J. Bocquel, A. Tankasala, M. Usman, J. Salfi, R. Rahman, M. Y. Simmons, L. C. L. Hollenberg & S. Rogge

  Journal : Nature Communications

  Year : 2020

  Publication Type: Journal Papers

  Topic: Semiconductor Quantum Computing

  Abstract

  Tunneling is a fundamental quantum process with no classical equivalent, which can compete with Coulomb interactions to give rise to complex phenomena. Phosphorus dopants in silicon can be placed with atomic precision to address the different regimes arising from this competition. However, they exploit wavefunctions relying on crystal band symmetries, which tunneling interactions are inherently sensitive to. Here we directly image lattice-aperiodic valley interference between coupled atoms in silicon using scanning tunneling microscopy. Our atomistic analysis unveils the role of envelope anisotropy, valley interference and dopant placement on the Heisenberg spin exchange interaction. We find that the exchange can become immune to valley interference by engineering in-plane dopant placement along specific crystallographic directions. A vacuum-like behaviour is recovered, where the exchange is maximised to the overlap between the donor orbitals, and pair-to-pair variations limited to a factor of less than 10 considering the accuracy in dopant positioning. This robustness remains over a large range of distances, from the strongly Coulomb interacting regime relevant for high-fidelity quantum computation to strongly coupled donor arrays of interest for quantum simulation in silicon.

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• 4:

  Title: Nakamura J; Fallahi S; Sahasrabudhe H; Rahman R; Liang S; Gardner GC; Manfra MJ, 2019, 'Aharonov–Bohm interference of fractional quantum Hall edge modes', Nature Physics, vol. 15, pp. 563 - 569
     

  Year : 2019

  Publication Type: Journal Papers

  Topic:

  Abstract

  The braiding statistics of certain fractional quantum Hall states can be probed via interferometry of their edge states. Practical difficulties—including loss of phase coherence—make this a challenging task. We demonstrate the operation of a small Fabry–Perot interferometer in which highly coherent Aharonov–Bohm oscillations are observed in the integer and fractional quantum Hall regimes. Careful design of the heterostructure suppresses Coulomb effects and promotes strong phase coherence. We characterize the coherency of edge-mode interference by the energy scale for thermal damping and determine the velocities of the inner and outer edge modes independently via selective backscattering of edge modes originating in the N = 0, 1, 2 Landau levels. We also observe clear Aharonov–Bohm oscillations at fractional filling factors ν = 2/3 and ν = 1/3, which indicates that our device architecture provides a platform for measurement of anyonic braiding statistics.

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• 5:

  Title: Ameen TA; Ilatikhameneh H; Fay P; Seabaugh A; Rahman R; Klimeck G, 2019, 'Alloy Engineered Nitride Tunneling Field-Effect Transistor: A Solution for the Challenge of Heterojunction TFETs', IEEE Transactions on Electron Devices, vol. 66, pp. 736 - 742
     

  Year : 2019

  Publication Type: Journal Papers

  Topic:

  Abstract

  Being fundamentally limited to a current-voltage steepness of 60mV/dec, MOSFETs struggle to operate below 0.6 V. Further reduction in V DD and, consequently, power consumption can be achieved with novel devices, such as tunneling transistors (TFETs) that can overcome this limitation. TFETs, however, face challenges with low ON-current leading to slow performance. TFETs made from III-nitride heterostructures are quite promising in this regard. The lattice mismatch induces a piezoelectric polarization field in a nitride heterojunction that can boost the ON-current. However, it is shown here that the carrier thermalization at the heterointerface degrades the subthreshold characteristics. Therefore, a good design should minimize the number of confined quantum well (QW) states at the heterointerface so as not to degrade the subthreshold characteristics while maintaining the lattice mismatch induced polarization to boost the ON-current. We show here that an InAlN QW on an InGaN substrate alloy engineered TFET design is promising to fulfill these requirements. Proper engineering of the alloy mole fractions and the width of the well can eliminate (or at least minimize) the undesired thermalization effects and, at the same time, provide a lattice mismatch to induce a piezoelectric field for boosting the ON-current. We have used a suitable atomistic quantum transport model to simulate these devices. The model accounts for the different mechanisms that are involved, and captures realistic scattering thermalization effects. This model has been benchmarked in our earlier work with experimental measurements of nitride tunneling heterojunction diodes and is used here to optimize the alloy engineered nitride TFET.

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• 6:

  Title: Wu P; Ameen T; Zhang H; Bendersky LA; Ilatikhameneh H; Klimeck G; Rahman R; Davydov AV; Appenzeller J, 2019, 'Complementary Black Phosphorus Tunneling Field-Effect Transistors', ACS Nano, vol. 13, pp. 377 - 385
     

  Year : 2019

  Publication Type: Journal Papers

  Topic:

  Abstract

  Band-to-band tunneling field-effect transistors (TFETs) have emerged as promising candidates for low-power integration circuits beyond conventional metal-oxide-semiconductor field-effect transistors (MOSFETs) and have been demonstrated to overcome the thermionic limit, which results intrinsically in sub-threshold swings of at least 60 mV/dec at room temperature. Here, we demonstrate complementary TFETs based on few-layer black phosphorus, in which multiple top gates create electrostatic doping in the source and drain regions. By electrically tuning the doping types and levels in the source and drain regions, the device can be reconfigured to allow for TFET or MOSFET operation and can be tuned to be n-type or p-type. Owing to the proper choice of materials and careful engineering of device structures, record-high current densities have been achieved in 2D TFETs. Full-band atomistic quantum transport simulations of the fabricated devices agree quantitatively with the current–voltage measurements, which gives credibility to the promising simulation results of ultrascaled phosphorene TFETs. Using atomistic simulations, we project substantial improvements in the performance of the fabricated TFETs when channel thicknesses and oxide thicknesses are scaled down.

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• 7:

  Title: Pang CS; Chen CY; Ameen T; Zhang S; Ilatikhameneh H; Rahman R; Klimeck G; Chen Z, 2019, 'WSe2 Homojunction Devices: Electrostatically Configurable as Diodes, MOSFETs, and Tunnel FETs for Reconfigurable Computing', Small, vol. 15
     

  Year : 2019

  Publication Type: Journal Papers

  Topic:

  Abstract

  In this paper, electrostatically configurable 2D tungsten diselenide (WSe2) electronic devices are demonstrated. Utilizing a novel triple‐gate design, a WSe2 device is able to operate as a tunneling field‐effect transistor (TFET), a metal–oxide–semiconductor field‐effect transistor (MOSFET) as well as a diode, by electrostatically tuning the channel doping to the desired profile. The implementation of scaled gate dielectric and gate electrode spacing enables higher band‐to‐band tunneling transmission with the best observed subthreshold swing (SS) among all reported homojunction TFETs on 2D materials. Self‐consistent full‐band atomistic quantum transport simulations quantitatively agree with electrical measurements of both the MOSFET and TFET and suggest that scaling gate oxide below 3 nm is necessary to achieve sub‐60 mV dec−1 SS, while further improvement can be obtained by optimizing the spacers. Diode operation is also demonstrated with the best ideality factor of 1.5, owing to the enhanced electrostatic control compared to previous reports. This research sheds light on the potential of utilizing electrostatic doping scheme for low‐power electronics and opens a path toward novel designs of field programmable mixed analog/digital circuitry for reconfigurable computing.

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• 8:

  Title: Ferdous R; Kawakami E; Scarlino P; Nowak MP; Ward DR; Savage DE; Lagally MG; Coppersmith SN; Friesen M; Eriksson MA; Vandersypen LMK; Rahman R, 2018, 'Valley dependent anisotropic spin splitting in silicon quantum dots', npj Quantum Information, vol. 4
     

  Year : 2018

  Publication Type: Journal Papers

  Topic:

  Abstract

  Spin qubits hosted in silicon (Si) quantum dots (QD) are attractive due to their exceptionally long coherence times and compatibility with the silicon transistor platform. To achieve electrical control of spins for qubit scalability, recent experiments have utilized gradient magnetic fields from integrated micro-magnets to produce an extrinsic coupling between spin and charge, thereby electrically driving electron spin resonance (ESR). However, spins in silicon QDs experience a complex interplay between spin, charge, and valley degrees of freedom, influenced by the atomic scale details of the confining interface. Here, we report experimental observation of a valley dependent anisotropic spin splitting in a Si QD with an integrated micro-magnet and an external magnetic field. We show by atomistic calculations that the spin-orbit interaction (SOI), which is often ignored in bulk silicon, plays a major role in the measured anisotropy. Moreover, inhomogeneities such as interface steps strongly affect the spin splittings and their valley dependence. This atomic-scale understanding of the intrinsic and extrinsic factors controlling the valley dependent spin properties is a key requirement for successful manipulation of quantum information in Si QDs.

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• 9:

  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. 8
     

  Year : 2018

  Publication Type: Journal Papers

  Topic:

  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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• 10:

  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 - 4621
     

  Year : 2018

  Publication Type: Journal Papers

  Topic:

  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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