High-performance monolayer MoS2nanosheet GAA transistor.

In this article, a 0.7 nm thick monolayer MoS2nanosheet gate-all-around field effect transistors (NS-GAAFETs) with conformal high-κmetal gate deposition are demonstrated. The device with 40 nm channel length exhibits a high on-state current density of ~410µAµm-1with a large on/off ratio of 6 × 108at drain voltage = 1 V. The extracted contact resistance is 0.48 ± 0.1 kΩµm in monolayer MoS2NS-GAAFETs, thereby showing the channel-dominated performance with the channel length scaling from 80 to 40 nm. The successful demonstration of device performance in this work verifies the integration potential of transition metal dichalcogenides for future logic transistor applications.

Reconfigurable submicrometer spin-wave majority gate with electrical transducers.

Spin waves are excitations in ferromagnetic media that have been proposed as information carriers in hybrid spintronic devices with much lower operation power than conventional charge-based electronics. Their wave nature can be exploited in majority gates by using interference for computation. However, a scalable spin-wave majority gate that can be cointegrated alongside conventional electronics is still lacking. Here, we demonstrate a submicrometer inline spin-wave majority gate with fan-out. Time-resolved imaging of the magnetization dynamics by scanning transmission x-ray microscopy illustrates the device operation. All-electrical spin-wave spectroscopy further demonstrates majority gates with submicrometer dimensions, reconfigurable input and output ports, and frequency-division multiplexing. Challenges for hybrid spintronic computing systems based on spin-wave majority gates are discussed.

Ferroelectric Control of Magnetism in Ultrathin HfO2\Co\Pt Layers.

The recent demonstration of ferroelectricity in ultrathin HfO2 has kickstarted a new wave of research into this material. HfO2 in the orthorhombic phase can be considered the first and only truly nanoscale ferroelectric material that is compatible with silicon-based nanoelectronics applications. In this article, we demonstrate the ferroelectric control of the magnetic properties of cobalt deposited on ultrathin aluminum-doped, atomic layer deposition-grown HfO2 (tHfO2 = 6.5 nm). The ferroelectric effect is shown to control the shape of the magnetic hysteresis, quantified here by the magnetic switching energy. Furthermore, the magnetic properties such as the remanence are modulated by up to 41%. We show that this modulation does not only correlate with the charge accumulation at the interface but also shows an additional component associated with the ferroelectric polarization switching. An in-depth analysis using first order reversal curves shows that the coercive and interaction field distributions of cobalt can be modulated up to, respectively, 5.8% and 10.5% with the ferroelectric polarization reversal.

Doping-Free Complementary Logic Gates Enabled by Two-Dimensional Polarity-Controllable Transistors.

Atomically thin two-dimensional (2D) materials belonging to transition metal dichalcogenides, due to their physical and electrical properties, are an exceptional vector for the exploration of next-generation semiconductor devices. Among them, due to the possibility of ambipolar conduction, tungsten diselenide (WSe2) provides a platform for the efficient implementation of polarity-controllable transistors. These transistors use an additional gate, named polarity gate, that, due to the electrostatic doping of the Schottky junctions, provides a device-level dynamic control of their polarity, that is, n- or p-type. Here, we experimentally demonstrate a complete doping-free standard cell library realized on WSe2 without the use of either chemical or physical doping. We show a functionally complete family of complementary logic gates (INV, NAND, NOR, 2-input XOR, 3-input XOR, and MAJ) and, due to the reconfigurable capabilities of the single devices, achieve the realization of highly expressive logic gates, such as exclusive-OR (XOR) and majority (MAJ), with fewer transistors than possible in conventional complementary metal-oxide-semiconductor logic. Our work shows a path to enable doping-free low-power electronics on 2D semiconductors, going beyond the concept of unipolar physically doped devices, while suggesting a road to achieve higher computational densities in two-dimensional electronics.

Microwave Characterization of Ba-Substituted PZT and ZnO Thin Films.

The microwave dielectric properties of (Ba0.1Pb0.9)(Zr0.52Ti0.48)O3 (BPZT) and ZnO thin films with thicknesses below were investigated. No significant dielectric relaxation was observed for both BPZT and ZnO up to 30 GHz. The intrinsic dielectric constant of BPZT was as high as 980 at 30 GHz. The absence of strong dielectric dispersion and loss peaks in the studied frequency range can be linked to the small grain diameters in these ultrathin films.

Electrically Driven Unidirectional Optical Nanoantennas.

Directional antennas revolutionized modern day telecommunication by enabling precise beaming of radio and microwave signals with minimal loss of energy. Similarly, directional optical nanoantennas are expected to pave the way toward on-chip wireless communication and information processing. Currently, on-chip integration of such antennas is hampered by their multielement design or the requirement of complicated excitation schemes. Here, we experimentally demonstrate electrical driving of in-plane tunneling nanoantennas to achieve broadband unidirectional emission of light. Far-field interference, as a result of the spectral overlap between the dipolar emission of the tunnel junction and the fundamental quadrupole-like resonance of the nanoantenna, gives rise to a directional radiation pattern. By tuning this overlap using the applied voltage, we record directivities as high as 5 dB. In addition to electrical tunability, we also demonstrate passive tunability of the directivity using the antenna geometry. These fully configurable electrically driven nanoantennas provide a simple way to direct optical energy on-chip using an extremely small device footprint.

Exchange-driven Magnetic Logic.

Direct exchange interaction allows spins to be magnetically ordered. Additionally, it can be an efficient manipulation pathway for low-powered spintronic logic devices. We present a novel logic scheme driven by exchange between two distinct regions in a composite magnetic layer containing a bistable canted magnetization configuration. By applying a magnetic field pulse to the input region, the magnetization state is propagated to the output via spin-to-spin interaction in which the output state is given by the magnetization orientation of the output region. The dependence of this scheme with input field conditions is extensively studied through a wide range of micromagnetic simulations. These results allow different logic operating modes to be extracted from the simulation results, and majority logic is successfully demonstrated.

Scaling trends and performance evaluation of 2-dimensional polarity-controllable FETs.

Two-dimensional semiconducting materials of the transition-metal-dichalcogenide family, such as MoS2 and WSe2, have been intensively investigated in the past few years, and are considered as viable candidates for next-generation electronic devices. In this paper, for the first time, we study scaling trends and evaluate the performances of polarity-controllable devices realized with undoped mono- and bi-layer 2D materials. Using ballistic self-consistent quantum simulations, it is shown that, with the suitable channel material, such polarity-controllable technology can scale down to 5 nm gate lengths, while showing performances comparable to the ones of unipolar, physically-doped 2D electronic devices.

Toward an Understanding of the Electric Field-Induced Electrostatic Doping in van der Waals Heterostructures: A First-Principles Study.

Since the discovery of graphene, a broad range of two-dimensional (2D) materials has captured the attention of the scientific communities. Materials, such as hexagonal boron nitride (hBN) and the transition metal dichalcogenides (TMDs) family, have shown promising semiconducting and insulating properties that are very appealing for the semiconductor industry. Recently, the possibility of taking advantage of the properties of 2D-based heterostructures has been investigated for low-power nanoelectronic applications. In this work, we aim at evaluating the relation between the nature of the materials used in such heterostructures and the amplitude of the layer-to-layer charge transfer induced by an external electric field, as is typically present in nanoelectronic gated devices. A broad range of combinations of TMDs, graphene, and hBN has been investigated using density functional theory. Our results show that the electric field induced charge transfer strongly depends on the nature of the 2D materials used in the van der Waals heterostructures and to a lesser extent on the relative orientation of the materials in the structure. Our findings contribute to the building of the fundamental understanding required to engineer electrostatically the doping of 2D materials and to establish the factors that drive the charge transfer mechanisms in electron tunneling-based devices. These are key ingredients for the development of 2D-based nanoelectronic devices.

Characterization of thin films of the solid electrolyte Li(x)Mg(1-2x)Al(2+x)O4 (x = 0, 0.05, 0.15, 0.25).

RF-sputtered thin films of spinel Li(x)Mg(1-2x)Al(2+x)O4 were investigated for use as solid electrolyte. The usage of this material can enable the fabrication of a lattice matched battery stack, which is predicted to lead to superior battery performance. Spinel Li(x)Mg(1-2x)Al(2+x)O4 thin films, with stoichiometry (x) ranging between 0 and 0.25, were formed after a crystallization anneal as shown by X-ray diffraction and transmission electron microscopy. The stoichiometry of the films was evaluated by elastic recoil detection and Rutherford backscattering and found to be slightly aluminum rich. The excellent electronic insulation properties were confirmed by both current-voltage measurements as well as by copper plating tests. The electrochemical stability window of the material was probed using cyclic voltammetry. Lithium plating and stripping was observed together with the formation of a Li-Pt alloy, indicating that Li-ions passed through the film. This observation contradicted with impedance measurements at open circuit potential, which showed no apparent Li-ion conductivity of the film. Impedance spectroscopy as a function of potential showed the occurrence of Li-ion intercalation into the Li(x)Mg(1-2x)Al(2+x)O4 layers. When incorporating Li-ions in the material the ionic conductivity can be increased by 3 orders of magnitude. Therefore it is anticipated that the response of Li(x)Mg(1-2x)Al(2+x)O4 is more adequate for a buffer layer than as the solid electrolyte.

High Cycling Stability and Extreme Rate Performance in Nanoscaled LiMn2O4 Thin Films.

Ultrathin LiMn2O4 electrode layers with average crystal size of ∼15 nm were fabricated by means of radio frequency sputtering. Cycling behavior and rate performance was evaluated by galvanostatic charge and discharge measurements. The thinnest films show the highest volumetric capacity and best cycling stability, retaining the initial capacity over 70 (dis)charging cycles when manganese dissolution is prevented. The increased stability for film thicknesses below 50 nm allows cycling in both the 4 and 3 V potential regions, resulting in a high volumetric capacity of 1.2 Ah/cm3. It is shown that the thinnest films can be charged to 75% of their full capacity within 18 s (200 C), the best rate performance reported for LiMn2O4. This is explained by the short diffusion lengths inherent to thin films and the absence of phase transformation.

Switching mechanism in two-terminal vanadium dioxide devices.

Two-terminal thin film VO2 devices show an abrupt decrease of resistance when the current or voltage applied exceeds a threshold value. This phenomenon is often described as a field-induced metal-insulator transition. We fabricate nano-scale devices with different electrode separations down to 100 nm and study how the dc switching voltage and current depend on device size and temperature. Our observations are consistent with a Joule heating mechanism governing the switching. Pulsed measurements show a switching time to the high resistance state of the order of one hundred nanoseconds, consistent with heat dissipation time. In spite of the Joule heating mechanism which is expected to induce device degradation, devices can be switched for more than 10(10) cycles making VO2 a promising material for nanoelectronic applications.

Quasi-particle properties from tunneling in the v = 5/2 fractional quantum Hall state.

Quasi-particles with fractional charge and statistics, as well as modified Coulomb interactions, exist in a two-dimensional electron system in the fractional quantum Hall (FQH) regime. Theoretical models of the FQH state at filling fraction v = 5/2 make the further prediction that the wave function can encode the interchange of two quasi-particles, making this state relevant for topological quantum computing. We show that bias-dependent tunneling across a narrow constriction at v = 5/2 exhibits temperature scaling and, from fits to the theoretical scaling form, extract values for the effective charge and the interaction parameter of the quasi-particles. Ranges of values obtained are consistent with those predicted by certain models of the 5/2 state.

Electrical control of spin relaxation in a quantum dot.

We demonstrate electrical control of the spin relaxation time T1 between Zeeman-split spin states of a single electron in a lateral quantum dot. We find that relaxation is mediated by the spin-orbit interaction, and by manipulating the orbital states of the dot using gate voltages we vary the relaxation rate W identical withT1(-1) by over an order of magnitude. The dependence of W on orbital confinement agrees with theoretical predictions, and from these data we extract the spin-orbit length. We also measure the dependence of W on the magnetic field and demonstrate that spin-orbit mediated coupling to phonons is the dominant relaxation mechanism down to 1 T, where T1 exceeds 1 s.

Energy-dependent tunneling in a quantum dot.

We present measurements of the rates for an electron to tunnel on and off a quantum dot, obtained using a quantum point contact charge sensor. The tunnel rates show exponential dependence on drain-source bias and plunger gate voltages. The tunneling process is shown to be elastic, and a model describing tunneling in terms of the dot energy relative to the height of the tunnel barrier quantitatively describes the measurements.