Monolayer Hexagonal Boron Nitride Tunnel Barrier Contact for Low-Power Black Phosphorus Heterojunction Tunnel Field-Effect Transistors.

Transistor downscaling by Moore's law has facilitated drastic improvements in information technology, but this trend cannot continue because power consumption issues have pushed Moore's law to its limit. Tunnel field-effect transistors (TFETs) have been suggested to address these issues; however, so far they have not achieved the essential criteria for fast, low-power switches, i.e., an average subthreshold swing over four decades of current (SSave_4dec) below 60 mV/dec and a current of 1-10 µA/µm where the SS is 60 mV/dec (I60). Here, we report a black phosphorus (BP) heterojunction (HJ) TFET that exhibits a record high I60 of 19.5 µA/µm and subthermionic SSave_4dec of 37.6 mV/dec at 300 K, using a key material factor, monolayer hexagonal boron nitride tunnel barrier for the drain contact. This work, demonstrating the influence of the tunnel barrier contact on device performance, paves the way for the development of ultrafast low-power logic circuits beyond CMOS capabilities.

Ultrafast generation of fundamental and multiple-order phonon excitations in highly enriched (6,5) single-wall carbon nanotubes.

Using a macroscopic ensemble of highly enriched (6,5) single-wall carbon nanotubes, combined with high signal-to-noise ratio and time-dependent differential transmission spectroscopy, we have generated vibrational modes in an ultrawide spectral range (10-3000 cm(-1)). A total of 14 modes were clearly resolved and identified, including fundamental modes of A, E1, and E2 symmetries and their combinational modes involving two and three phonons. Through comparison with continuous wave Raman spectra as well as calculations based on an extended tight-binding model, we were able to identify all the observed peaks and determine the frequencies of the individual and combined modes. We provide a full summary of phonon frequencies for (6,5) nanotubes that can serve as a basic reference with which to refine our understanding of nanotube phonon spectra as well as a testbed for new theoretical models.

Disorder-induced magnetoresistance in a two-dimensional electron system.

We predict and demonstrate that a disorder-induced carrier density inhomogeneity causes magnetoresistance (MR) in a two-dimensional electron system. Our experiments on graphene show a quadratic MR persisting far from the charge neutrality point. Effective medium calculations show that for charged impurity disorder, the low-field MR is a universal function of the ratio of carrier density to fluctuations in carrier density, a power law when this ratio is large, in excellent agreement with experiment. The MR is generic and should occur in other materials with large carrier density inhomogeneity.

Topological insulator quantum dot with tunable barriers.

Thin (6-7 quintuple layer) topological insulator Bi(2)Se(3) quantum dot devices are demonstrated using ultrathin (2-4 quintuple layer) Bi(2)Se(3) regions to realize semiconducting barriers which may be tuned from ohmic to tunneling conduction via gate voltage. Transport spectroscopy shows Coulomb blockade with large charging energy >5 meV and additional features implying excited states.

Insulating behavior in ultrathin bismuth selenide field effect transistors.

Ultrathin (approximately three quintuple layer) field-effect transistors (FETs) of topological insulator Bi(2)Se(3) are prepared by mechanical exfoliation on 300 nm SiO(2)/Si susbtrates. Temperature- and gate-voltage-dependent conductance measurements show that ultrathin Bi(2)Se(3) FETs are n-type and have a clear OFF state at negative gate voltage, with activated temperature-dependent conductance and energy barriers up to 250 meV.

Dirac-source diode with sub-unity ideality factor.

An increase in power consumption necessitates a low-power circuit technology to extend Moore's law. Low-power transistors, such as tunnel field-effect transistors (TFETs), negative-capacitance field-effect transistors (NC-FETs), and Dirac-source field-effect transistors (DS-FETs), have been realised to break the thermionic limit of the subthreshold swing (SS). However, a low-power rectifier, able to overcome the thermionic limit of an ideality factor (η) of 1 at room temperature, has not been proposed yet. In this study, we have realised a DS diode based on graphene/MoS2/graphite van der Waals heterostructures, which exhibits a steep-slope characteristic curve, by exploiting the linear density of states (DOSs) of graphene. For the developed DS diode, we obtained η < 1 for more than four decades of drain current (ηave_4dec < 1) with a minimum value of 0.8, and a rectifying ratio exceeding 108. The realisation of a DS diode represents an additional step towards the development of low-power electronic circuits.

Analysis of Grain Boundary Dependent Memory Characteristics in Poly-Si One-Transistor Dynamic Random-Access Memory.

A capacitorless one-transistor dynamic random-access memory cell with a polysilicon body (poly-Si 1T-DRAM) has a cost-effective fabrication process and allows a three-dimensional stacked architecture that increases the integration density of memory cells. Also, since this device uses grain boundaries (GBs) as a storage region, it can be operated as a memory cell even in a thin body device. GBs are important to the memory characteristics of poly-Si 1T-DRAM because the amount of trapped charge in the GBs determines the memory's data state. In this paper, we report on a statistical analysis of the memory characteristics of poly-Si 1T-DRAM cells according to the number and location of GBs using TCAD simulation. As the number of GBs increases, the sensing margin and retention time of memory cells deteriorate due to increasing trapped electron charge. Also, "0" state current increases and memory performance degrades in cells where all GBs are adjacent to the source or drain junction side in a strong electric field. These results mean that in poly-Si 1T-DRAM design, the number and location of GBs in a channel should be considered for optimal memory performance.

Thickness-controlled black phosphorus tunnel field-effect transistor for low-power switches.

The continuous down-scaling of transistors has been the key to the successful development of current information technology. However, with Moore's law reaching its limits, the development of alternative transistor architectures is urgently needed1. Transistors require a switching voltage of at least 60 mV for each tenfold increase in current, that is, a subthreshold swing (SS) of 60 mV per decade (dec). Alternative tunnel field-effect transistors (TFETs) are widely studied to achieve a sub-thermionic SS and high I60 (the current where SS becomes 60 mV dec-1)2. Heterojunction (HJ) TFETs show promise for delivering a high I60, but experimental results do not meet theoretical expectations due to interface problems in the HJs constructed from different materials. Here, we report a natural HJ-TFET with spatially varying layer thickness in black phosphorus without interface problems. We have achieved record-low average SS values over 4-5 dec of current (SSave_4dec ~22.9 mV dec-1 and SSave_5dec ~26.0 mV dec-1) with record-high I60 (I60 = 0.65-1 µA µm-1), paving the way for application in low-power switches.

Gate-Tunable Reversible Rashba-Edelstein Effect in a Few-Layer Graphene/2H-TaS2 Heterostructure at Room Temperature.

We report the observation of current-induced spin polarization, the Rashba-Edelstein effect (REE), and its Onsager reciprocal phenomenon, the spin galvanic effect (SGE), in a few-layer graphene/2H-TaS2 heterostructure at room temperature. Spin-sensitive electrical measurements unveil full spin-polarization reversal by an applied gate voltage. The observed gate-tunable charge-to-spin conversion is explained by the ideal work function mismatch between 2H-TaS2 and graphene, which allows for a strong interface-induced Bychkov-Rashba interaction with a spin-gap reaching 70 meV, while keeping the Dirac nature of the spectrum intact across electron and hole sectors. The reversible electrical generation and control of the nonequilibrium spin polarization vector, not previously observed in a nonmagnetic material, are elegant manifestations of emergent two-dimensional Dirac Fermions with robust spin-helical structure. Our experimental findings, supported by first-principles relativistic electronic structure and transport calculations, demonstrate a route to design low-power spin-logic circuits from layered materials.

Structural insights into the monosaccharide specificity of Escherichia coli rhamnose mutarotase.

The crystal structure of Escherichia coli rhamnose mutarotase (YiiL) is completely different from the previously reported structures of the Lactococcus lactis galactose mutarotase and the Bacillus subtilis RbsD (pyranase). YiiL exists as a locally asymmetric dimer, which is stabilized by an intermolecular beta-sheet, various hydrophobic interactions, and a cation-pi interaction with a salt-bridge. The protein folds of YiiL are similar to those of a Streptomyces coelicolor mono-oxygenase and a hypothetical Arabidopsis thaliana protein At3g17210. By assaying the enzymatic activity of six active-site mutants and by comparing the crystal structure-derived active site conformations of YiiL, RbsD, and a galactose mutarotase, we were able to define the amino acid residues required for catalysis and suggest a possible catalytic mechanism for YiiL. Although the active-site amino acid residues of YiiL (His, Tyr, and Trp) differ greatly from those of galactose mutarotase (His, Glu, and Asp), their geometries, which determine the structures of the preferred monosaccharide substrates, are conserved. In addition, the in vivo function of YiiL was assessed by constructing a mutant E.coli strain that carries a yiiL deletion. The presence of the yiiL gene is critical for efficient cell growth only when concentrations of l-rhamnose are limited.

Kondo-like zero-bias conductance anomaly in a three-dimensional topological insulator nanowire.

Zero-bias anomalies in topological nanowires have recently captured significant attention, as they are possible signatures of Majorana modes. Yet there are many other possible origins of zero-bias peaks in nanowires--for example, weak localization, Andreev bound states, or the Kondo effect. Here, we discuss observations of differential-conductance peaks at zero-bias voltage in non-superconducting electronic transport through a 3D topological insulator (Bi(1.33)Sb(0.67))Se3 nanowire. The zero-bias conductance peaks show logarithmic temperature dependence and often linear splitting with magnetic fields, both of which are signatures of the Kondo effect in quantum dots. We characterize the zero-bias peaks and discuss their origin.

Solution structure and backbone dynamics of the XPC-binding domain of the human DNA repair protein hHR23B.

Human cells contain two homologs of the yeast RAD23 protein, hHR23A and hHR23B, which participate in the DNA repair process. hHR23B houses a domain (residues 277-332, called XPCB) that binds specifically and directly to the xeroderma pigmentosum group C protein (XPC) to initiate nucleotide excision repair (NER). This domain shares sequence homology with a heat shock chaperonin-binding motif that is also found in the stress-inducible yeast phosphoprotein STI1. We determined the solution structure of a protein fragment containing amino acids 275-342 of hHR23B (termed XPCB-hHR23B) and compared it with the previously reported solution structures of the corresponding domain of hHR23A. The periodic positioning of proline residues in XPCB-hHR23B produced kinked alpha helices and assisted in the formation of a compact domain. Although the overall structure of the XPCB domain was similar in both XPCB-hHR23B and XPCB-hHR23A, the N-terminal part (residues 275-283) of XPCB-hHR23B was more flexible than the corresponding part of hHR23A. We tried to infer the characteristics of this flexibility through (15)N-relaxation studies. The hydrophobic surface of XPCB-hHR23B, which results from the diverse distribution of N-terminal region, might give rise to the functional pleiotropy observed in vivo for hHR23B, but not for hHR23A.

Aharonov-Bohm oscillations in a quasi-ballistic three-dimensional topological insulator nanowire.

Aharonov-Bohm oscillations effectively demonstrate coherent, ballistic transport in mesoscopic rings and tubes. In three-dimensional topological insulator nanowires, they can be used to not only characterize surface states but also to test predictions of unique topological behaviour. Here we report measurements of Aharonov-Bohm oscillations in (Bi1.33Sb0.67)Se3 that demonstrate salient features of topological nanowires. By fabricating quasi-ballistic three-dimensional topological insulator nanowire devices that are gate-tunable through the Dirac point, we are able to observe alternations of conductance maxima and minima with gate voltage. Near the Dirac point, we observe conductance minima for zero magnetic flux through the nanowire and corresponding maxima (having magnitudes of almost a conductance quantum) at magnetic flux equal to half a flux quantum; this is consistent with the presence of a low-energy topological mode. The observation of this mode is a necessary step towards utilizing topological properties at the nanoscale in post-CMOS applications.

Symmetry protected Josephson supercurrents in three-dimensional topological insulators.

Coupling the surface state of a topological insulator to an s-wave superconductor is predicted to produce the long-sought Majorana quasiparticle excitations. However, superconductivity has not been measured in surface states when the bulk charge carriers are fully depleted, that is, in the true topological regime relevant for investigating Majorana modes. Here we report measurements of d.c. Josephson effects in topological insulator-superconductor junctions as the chemical potential is moved through the true topological regime characterized by the presence of only surface currents. We compare our results with three-dimensional quantum transport simulations, and determine the effects of bulk/surface mixing, disorder and magnetic field; in particular, we show that the supercurrent is largely carried by surface states, due to the inherent topology of the bands, and that it is robust against disorder. Our results thus clarify key open issues regarding the nature of supercurrents in topological insulators.