All pulications (111)

• 21:

  Title: Chen F; Ilatikhameneh H; Tan Y; Valencia D; Klimeck G; Rahman R, 2017, 'Transport in vertically stacked hetero-structures from 2D materials', in Journal of Physics: Conference Series
     

  Year : 2017

  Publication Type: Conference Proceeding

  Topic:

  Abstract

  In this work, the transport of tunnel field-effect transistor (TFET) based on vertically stacked hereto-structures from 2D transition metal dichalcogenide (TMD) materials is investigated by atomistic quantum transport simulations. WTe2-MoS2 combination was chosen due to the formation of a broken gap hetero-junction which is desirable for TFETs. There are two assumptions behind the MoS2-WTe2 hetero-junction tight binding (TB) model: 1) lattice registry. 2) The S − Te parameters being the average of the S − S and Te − Te parameters of bilayer MoS2 and WTe2. The computed TB bandstructure of the hetero-junction agrees well with the bandstructure obtained from density functional theory (DFT) in the energy range of interest for transport. NEGF (Non-Equilibrium Green's Function) equations within the tight binding description is then utilized for device transfer characteristic calculation. Results show 1) energy filtering is the switching mechanism; 2) the length of the extension region is critical for device to turn off; 3) MoS2-WTe2 interlayer TFET can achieve a large on-current of 1000µA/µm with VDD = 0.3V, which suggests interlayer TFET can solve the low ON current problem of TFETs and can be a promising candidate for low power applications.

  View
• 22:

  Title: Chen FW; Ilatikhameneh H; Ameen TA; Klimeck G; Rahman R, 2017, 'Thickness Engineered Tunnel Field-Effect Transistors Based on Phosphorene', IEEE Electron Device Letters, vol. 38, pp. 130 - 133
     

  Year : 2017

  Publication Type: Journal Papers

  Topic:

  Abstract

  Thickness engineered tunneling field-effect transistors (TE-TFET) as a high-performance ultra-scaled steep transistor is proposed. This device exploits a specific property of 2-D materials: layer thickness-dependent energy bandgaps (E g ). Unlike the conventional hetero-junction TFETs, TE-TFET uses spatially varying layer thickness to form a hetero-junction. This offers advantages by avoiding the lattice mismatch problems at the interface. Furthermore, it boosts the ON-current to 1280 μA/μm with 15-nm channel length. Providing higher ON currents, phosphorene TE-TFET outperforms the homojunction phosphorene and the TMD TFETs in terms of extrinsic energy-delay product. TE-TFET also scales well to 9 nm with constant field scaling E = V DD /L ch = 33$ mV/nm. In this letter, the operation principles of TE-TFET and its performance sensitivity to the design parameters are investigated through full-band atomistic quantum transport simulations.

  View
• 23:

  Title: Watson TF; Weber B; Hsueh YL; Hollenberg LCL; Rahman R; Simmons MY, 2017, 'Atomically engineered electron spin lifetimes of 30 s in silicon', Science Advances, vol. 3
     

  Year : 2017

  Publication Type: Journal Papers

  Topic:

  Abstract

  Scaling up to large arrays of donor-based spin qubits for quantum computation will require the ability to perform high-fidelity readout of multiple individual spin qubits. Recent experiments have shown that the limiting factor for high-fidelity readout of many qubits is the lifetime of the electron spin. We demonstrate the longest reported lifetimes (up to 30 s) of any electron spin qubit in a nanoelectronic device. By atomic-level engineering of the electron wave function within phosphorus atom quantum dots, we can minimize spin relaxation in agreement with recent theoretical predictions. These lifetimes allow us to demonstrate the sequential readout of two electron spin qubits with fidelities as high as 99.8%, which is above the surface code fault-tolerant threshold. This work paves the way for future experiments on multiqubit systems using donors in silicon.

  View
• 24:

  Title: Fay P; Li W; Digiovanni D; Cao L; Ilatikhameneh H; Chen F; Ameen T; Rahman R; Klimeck G; Lund C; Keller S; Islam SM; Chaney A; Cho Y; Jena D, 2017, 'III-N heterostructure devices for low-power logic', in China Semiconductor Technology International Conference 2017, CSTIC 2017
     

  Year : 2017

  Publication Type: Conference Proceeding

  Topic:

  Abstract

  Future generations of ultra-scaled logic may require alternative device technologies to transcend the limitations of Si CMOS; in particular, power dissipation constraints in aggressively-scaled, highly-integrated systems make device concepts capable of achieving switching slopes (SS) steeper than 60 mV/decade especially attractive. Tunneling field effect transistors (TFETs) are one such device technology alternative. While a great deal of research into TFETs based on Si, Ge, and narrow band gap III-Vs has been reported, these approaches each face significant challenges. An alternative approach based on the use of III-N wide band gap semiconductors in conjunction with polarization engineering offers potential advantages in terms of drain current density and switching slope. In this talk, the prospects for III-N based TFETs for logic will be discussed, including both simulation projections as well as experimental progress.

  View
• 25:

  Title: Zheng C; Zhang Q; Weber B; Ilatikhameneh H; Chen F; Sahasrabudhe H; Rahman R; Li S; Chen Z; Hellerstedt J; Zhang Y; Duan WH; Bao Q; Fuhrer MS, 2017, 'Direct Observation of 2D Electrostatics and Ohmic Contacts in Template-Grown Graphene/WS2 Heterostructures', ACS Nano, vol. 11, pp. 2785 - 2793
     

  Year : 2017

  Publication Type: Journal Papers

  Topic:

  Abstract

  Large-area two-dimensional (2D) heterojunctions are promising building blocks of 2D circuits. Understanding their intriguing electrostatics is pivotal but largely hindered by the lack of direct observations. Here graphene–WS2 heterojunctions are prepared over large areas using a seedless ambient-pressure chemical vapor deposition technique. Kelvin probe force microscopy, photoluminescence spectroscopy, and scanning tunneling microscopy characterize the doping in graphene–WS2 heterojunctions as-grown on sapphire and transferred to SiO2 with and without thermal annealing. Both p–n and n–n junctions are observed, and a flat-band condition (zero Schottky barrier height) is found for lightly n-doped WS2, promising low-resistance ohmic contacts. This indicates a more favorable band alignment for graphene–WS2 than has been predicted, likely explaining the low barriers observed in transport experiments on similar heterojunctions. Electrostatic modeling demonstrates that the large depletion width of the graphene–WS2 junction reflects the electrostatics of the one-dimensional junction between two-dimensional materials.

  View
• 26:

  Title: Ameen TA; Ilatikhameneh H; Huang JZ; Povolotskyi M; Rahman R; Klimeck G, 2017, 'Combination of Equilibrium and Nonequilibrium Carrier Statistics into an Atomistic Quantum Transport Model for Tunneling Heterojunctions', IEEE Transactions on Electron Devices, vol. 64, pp. 2512 - 2518
     

  Year : 2017

  Publication Type: Journal Papers

  Topic:

  Abstract

  Tunneling heterojunctions (THJs) have confined states close to the tunneling region, which significantly affect their transport properties. Accurate numerical modeling of THJs requires combining the nonequilibrium coherent quantum transport through the tunneling region as well as the quasi-equilibrium statistics arising from the strong scattering in the confined states. In this paper, a novel atomistic model is proposed to include both the effects: the strong scattering in the regions around THJ and the coherent tunneling. The new model matches reasonably well with experimental measurements of Nitride THJ and provides an efficient engineering tool for performance prediction and design of THJ-based devices

  View
• 27:

  Title: Nishat MRK; Tankasala A; Kharche N; Rahman R; Ahmed SS, 2017, 'Multiscale-multiphysics modeling of nonpolar InGaN LEDs', in 2017 IEEE 17th International Conference on Nanotechnology, NANO 2017, pp. 85 - 88
     

  Year : 2017

  Publication Type: Conference Proceeding

  Topic:

  Abstract

  In this work, we develop and employ a multiscale-multiphysics simulator (based on coupled VFF molecular mechanics, 10-band sp 3 s*-spin tight-binding formalism, many-body full configuration interaction, and a TCAD transport module) to study and compare the performance of realistically-sized multiple-quantum-well wurtzite InGaN LEDs in polar (c-plane) and nonpolar (a-plane) crystallographic directions. The a-plane device exhibited smaller yet non-vanishing internal fields and higher optical transition rate as compared to the c-plane counterpart.

  View
• 28:

  Title: Huang JZ; Long P; Povolotskyi M; Ilatikhameneh H; Ameen TA; Rahman R; Rodwell MJW; Klimeck G, 2017, 'A Multiscale Modeling of Triple-Heterojunction Tunneling FETs', IEEE Transactions on Electron Devices, vol. 64, pp. 2728 - 2735
     

  Year : 2017

  Publication Type: Journal Papers

  Topic:

  Abstract

  A high performance triple-heterojunction (3HJ) design has been previously proposed for tunneling FETs (TFETs). Compared with single HJ TFETs, the 3HJ TFETs have both shorter tunneling distance and two transmission resonances that significantly improve the ON-state current (I ON ). Coherent quantum transport simulation predicts that I ON = 460 μA/μm can be achieved at gate length Lg = 15 nm, supply voltage V DD = 0.3 V, and OFF-state current I OFF = 1 nA/μm. However, strong electron-phonon and electron-electron scattering in the heavily doped leads implies that the 3HJ devices operate far from the ideal coherent limit. In this paper, such scattering effects are assessed by a newly developed multiscale transport model, which combines the ballistic nonequilibrium Green's function method for the channel and the drift-diffusion scattering method for the leads. Simulation results show that the thermalizing scattering in the leads both degrades the 3HJ TFET's subthreshold swing through scattering-induced leakage and reduces the turn-ON current through the access resistance. Assuming bulk scattering rates and carrier mobilities, the I ON is dropped from 460 μA/μm down to 254 μA/μm, which is still much larger than the single HJ TFET case.

  View
• 29:

  Title: Weber B; Hsueh YL; Watson TF; Li R; Hamilton AR; Hollenberg LCL; Rahman R; Simmons MY, 2017, 'Electron spin relaxation of single phosphorus donors and donor clusters in atomically engineered silicon devices', in 2017 Silicon Nanoelectronics Workshop, SNW 2017, pp. 23 - 24
     

  Year : 2017

  Publication Type: Conference Proceeding

  Topic:

  Abstract

  We demonstrate the single-shot spin read-out of single donors and few-donor clusters, positioned with atomic precision by scanning tunneling microscopy (STM) in atomically engineered silicon devices [1-3]. In donor clusters, we measure spin lifetimes of up to half a minute, recorded at a read-out fidelity of up to 99.8% [2]. Importantly, measuring spin relaxations rates of electrons bound to a single P donor in orientation-dependent electric and magnetic fields, we identify a previously unreported spin relaxation pathway for donor-based qubits in silicon [1].

  View
• 30:

  Title: Usman M; Bocquel J; Salfi J; Voisin B; Tankasala A; Rahman R; Simmons MY; Rogge S; Hollenberg LCL, 2016, 'Spatial metrology of dopants in silicon with exact lattice site precision', Nature Nanotechnology, vol. 11, pp. 763 - 768
     

  Year : 2016

  Publication Type: Journal Papers

  Topic:

  Abstract

  Scaling of Si-based nanoelectronics has reached the regime where device function is affected not only by the presence of individual dopants, but also by their positions in the crystal. Determination of the precise dopant location is an unsolved problem in applications from channel doping in ultrascaled transistors to quantum information processing. Here, we establish a metrology combining low-temperature scanning tunnelling microscopy (STM) imaging and a comprehensive quantum treatment of the dopant–STM system to pinpoint the exact coordinates of the dopant in the Si crystal. The technique is underpinned by the observation that STM images contain atomic-sized features in ordered patterns that are highly sensitive to the STM tip orbital and the absolute dopant lattice site. The demonstrated ability to determine the locations of P and As dopants to 5 nm depths will provide critical information for the design and optimization of nanoscale devices for classical and quantum computing applications.

  View