L. Eaves

Field-Effect Tunneling Transistor Based on Vertical Graphene Heterostructures

Tunnel Barriers for Graphene Transistors Transistor operation for integrated circuits not only requires that the gate material has high-charge carrier mobility, but that there is also an effective way of creating a barrier to current flow so that the device can be switched off and not waste power. Graphene offers high carrier mobility, but the shape of its conduction and valence bands enables electron tunneling and makes it difficult to achieve low currents in an “off” state. Britnell et al. (p. 947, published online 2 February) have fabricated field-effect transistors in which a thin tunneling barrier created from a layered material—either hexagonal boron nitride or molybdenum disulfide—is sandwiched between graphene sheets. These devices exhibit on-off switching ratios of ≈50 and ≈10,000, respectively, at room temperature. Boron nitride or molybdenum disulfide layers sandwiched between graphene sheets act as tunneling barriers to minimize device leakage currents. An obstacle to the use of graphene as an alternative to silicon electronics has been the absence of an energy gap between its conduction and valence bands, which makes it difficult to achieve low power dissipation in the OFF state. We report a bipolar field-effect transistor that exploits the low density of states in graphene and its one-atomic-layer thickness. Our prototype devices are graphene heterostructures with atomically thin boron nitride or molybdenum disulfide acting as a vertical transport barrier. They exhibit room-temperature switching ratios of ≈50 and ≈10,000, respectively. Such devices have potential for high-frequency operation and large-scale integration.

Electron Tunneling through Ultrathin Boron Nitride Crystalline Barriers

We investigate the electronic properties of ultrathin hexagonal boron nitride (h-BN) crystalline layers with different conducting materials (graphite, graphene, and gold) on either side of the barrier layer. The tunnel current depends exponentially on the number of h-BN atomic layers, down to a monolayer thickness. Conductive atomic force microscopy scans across h-BN terraces of different thickness reveal a high level of uniformity in the tunnel current. Our results demonstrate that atomically thin h-BN acts as a defect-free dielectric with a high breakdown field. It offers great potential for applications in tunnel devices and in field-effect transistors with a high carrier density in the conducting channel.We investigate the electronic properties of heterostructures based on ultrathin hexagonal boron nitride (h-BN) crystalline layers sandwiched between two layers of graphene as well as other conducting materials (graphite, gold). The tunnel conductance depends exponentially on the number of h-BN atomic layers, down to a monolayer thickness. Exponential behaviour of I-V characteristics for graphene/BN/graphene and graphite/BN/graphite devices is determined mainly by the changes in the density of states with bias voltage in the electrodes. Conductive atomic force microscopy scans across h-BN terraces of different thickness reveal a high level of uniformity in the tunnel current. Our results demonstrate that atomically thin h-BN acts as a defect-free dielectric with a high breakdown field; it offers great potential for applications in tunnel devices and in field-effect transistors with a high carrier density in the conducting channel.

Resonant tunnelling and negative differential conductance in graphene transistors.

The chemical stability of graphene and other free-standing two-dimensional crystals means that they can be stacked in different combinations to produce a new class of functional materials, designed for specific device applications. Here we report resonant tunnelling of Dirac fermions through a boron nitride barrier, a few atomic layers thick, sandwiched between two graphene electrodes. The resonance occurs when the electronic spectra of the two electrodes are aligned. The resulting negative differential conductance in the device characteristics persists up to room temperature and is gate voltage-tuneable due to graphene’s unique Dirac-like spectrum. Although conventional resonant tunnelling devices comprising a quantum well sandwiched between two tunnel barriers are tens of nanometres thick, the tunnelling carriers in our devices cross only a few atomic layers, offering the prospect of ultra-fast transit times. This feature, combined with the multi-valued form of the device characteristics, has potential for applications in high-frequency and logic devices.

Twist-controlled resonant tunnelling in graphene/boron nitride/graphene heterostructures

Recent developments in the technology of van der Waals heterostructures made from two-dimensional atomic crystals have already led to the observation of new physical phenomena, such as the metal-insulator transition and Coulomb drag, and to the realization of functional devices, such as tunnel diodes, tunnel transistors and photovoltaic sensors. An unprecedented degree of control of the electronic properties is available not only by means of the selection of materials in the stack, but also through the additional fine-tuning achievable by adjusting the built-in strain and relative orientation of the component layers. Here we demonstrate how careful alignment of the crystallographic orientation of two graphene electrodes separated by a layer of hexagonal boron nitride in a transistor device can achieve resonant tunnelling with conservation of electron energy, momentum and, potentially, chirality. We show how the resonance peak and negative differential conductance in the device characteristics induce a tunable radiofrequency oscillatory current that has potential for future high-frequency technology.

High electron mobility, quantum Hall effect and anomalous optical response in atomically thin InSe

A decade of intense research on two-dimensional (2D) atomic crystals has revealed that their properties can differ greatly from those of the parent compound. These differences are governed by changes in the band structure due to quantum confinement and are most profound if the underlying lattice symmetry changes. Here we report a high-quality 2D electron gas in few-layer InSe encapsulated in hexagonal boron nitride under an inert atmosphere. Carrier mobilities are found to exceed 103 cm2 V-1 s-1 and 104 cm2 V-1 s-1 at room and liquid-helium temperatures, respectively, allowing the observation of the fully developed quantum Hall effect. The conduction electrons occupy a single 2D subband and have a small effective mass. Photoluminescence spectroscopy reveals that the bandgap increases by more than 0.5 eV with decreasing the thickness from bulk to bilayer InSe. The band-edge optical response vanishes in monolayer InSe, which is attributed to the monolayers mirror-plane symmetry. Encapsulated 2D InSe expands the family of graphene-like semiconductors and, in terms of quality, is competitive with atomically thin dichalcogenides and black phosphorus.

Tuning the Bandgap of Exfoliated InSe Nanosheets by Quantum Confinement

Strong quantization effects and tuneable near-infrared photoluminescence emission are reported in mechanically exfoliated crystals of γ-rhombohedral semiconducting InSe. The optical properties of InSe nanosheets differ qualitatively from those reported recently for exfoliated transition metal dichalcogenides and indicate a crossover from a direct to an indirect band gap semiconductor when the InSe flake thickness is reduced to a few nanometers.

High Broad‐Band Photoresponsivity of Mechanically Formed InSe–Graphene van der Waals Heterostructures

High broad‐band photoresponsivity of mechanically formed InSe–graphene van der Waals heterostructures is achieved by exploiting the broad‐band transparency of graphene, the direct bandgap of InSe, and the favorable band line up of InSe with graphene. The photoresponsivity exceeds that for other van der Waals heterostructures and the spectral response extends from the near‐infrared to the visible spectrum.

An investigation of the deep level photoluminescence spectra of InP(Mn), InP(Fe), and of undoped InP

Deep level photoluminescence bands related to Mn and Fe in InP are investigated. In InP(Mn) a strong broad band peaking at 1.15 eV and with phonon structure at 1.184, 1.145, and 1.107 eV is observed. A broad band peaking at 1.10 eV with weak phonon structure at 1.135, 1.098, and 1.062 eV is observed in InP(Fe). Several undoped InP crystals also reveal a band identical to that seen in InP(Mn) suggesting that Mn may be an inadvertent impurity. In these undoped samples and also in InP(Cr) the intensity of this band is greatly enhanced after thermal annealing (750 °C), suggesting that Mn may be diffusing to the surface. Mn has been previously reported to behave in this way in GaAs. Other unidentified deep level photoluminescence features in undoped material are also observed at 0.765 and 1.284 eV. Fe is detected as an inadvertent impurity in some undoped samples from studying the Fe2+(5T2−5E) internal transition.

Character of states near the Fermi level in (Ga,Mn)as : Impurity to valence band crossover

We discuss the character of states near the Fermi level in Mn doped GaAs, as revealed by a survey of dc transport and optical studies over a wide range of Mn concentrations. A thermally activated valence band contribution to dc transport, a mid-infrared peak at energy hbar omega approx 200 meV in the ac- conductivity, and the hot photoluminescence spectra indicate the presence of an impurity band in low doped ( 2% doping, no traces of Mn-related activated contribution can be identified in dc-transport, suggesting that the impurity band has merged with the valence band. No discrepancies with this perception are found when analyzing optical measurements in the high-doped GaAs:Mn. A higher energy (hbar omega approx 250 meV) mid-infrared feature which appears in the metallic samples is associated with inter-valence band transitions. Its red-shift with increased doping can be interpreted as a consequence of increased screening which narrows the localized-state valence-band tails and weakens higher energy transition amplitudes. Our examination of the dc and ac transport characteristics of GaAs:Mn is accompanied by comparisons with its shallow acceptor counterparts, confirming the disordered valence band picture of high-doped metallic GaAs:Mn material.

Electronic structure of self-assembled InAs quantum dots in GaAs matrix

Capacitance–voltage characteristics have been measured at various frequencies and temperatures for structures containing a sheet of self-assembled InAs quantum dots in both n-GaAs and p-GaAs matrices. Analysis of the capacitance–voltage characteristics shows that the deposition of 1.7 ML of InAs forms quantum dots with electron levels 80 meV below the bottom of the GaAs conduction band and two heavy-hole levels at 100 and 170 meV above the top of the GaAs valence band. The carrier energy levels agree very well with the recombination energies obtained from photoluminescence spectra.