Self-Limiting Oxides on WSe₂ as Controlled Surface Acceptors and Low-Resistance Hole Contacts

Transition metal oxides show much promise as effective p-type contacts and dopants in electronics based on transition metal dichalcogenides. Here we report that atomically thin films of under-stoichiometric tungsten oxides (WO_<i>x</i> with <i>x</i> < 3) grown on tungsten diselenide (WSe₂) can be used as both controlled charge transfer dopants and low-barrier contacts for p-type WSe₂ transistors. Exposure of atomically thin WSe₂ transistors to ozone (O₃) at 100 °C results in self-limiting oxidation of the WSe₂ surfaces to conducting WO_<i>x</i> films. WO_<i>x</i>-covered WSe₂ is highly hole-doped due to surface electron transfer from the underlying WSe₂ to the high electron affinity WO_<i>x</i>. The dopant concentration can be reduced by suppressing the electron affinity of WO_<i>x</i> by air exposure, but exposure to O₃ at room temperature leads to the recovery of the electron affinity. Hence, surface transfer doping with WO_<i>x</i> is virtually controllable. Transistors based on WSe₂ covered with WO_<i>x</i> show only p-type conductions with orders of magnitude better on-current, on/off current ratio, and carrier mobility than without WO_<i>x</i>, suggesting that the surface WO_<i>x</i> serves as a p-type contact with a low hole Schottky barrier. Our findings point to a simple and effective strategy for creating p-type devices based on two-dimensional transition metal dichalcogenides with controlled dopant concentrations

Carrier Polarity Control in α‑MoTe₂ Schottky Junctions Based on Weak Fermi-Level Pinning

The polarity of the charge carriers injected through Schottky junctions of α-phase molybdenum ditelluride (α-MoTe₂) and various metals was characterized. We found that the Fermi-level pinning in the metal/α-MoTe₂ Schottky junction is so weak that the polarity of the carriers (electron or hole) injected from the junction can be controlled by the work function of the metals, in contrast to other transition metal dichalcogenides such as MoS₂. From the estimation of the Schottky barrier heights, we obtained p-type carrier (hole) injection from a Pt/α-MoTe₂ junction with a Schottky barrier height of 40 meV at the valence band edge. n-Type carrier (electron) injection from Ti/α-MoTe₂ and Ni/α-MoTe₂ junctions was also observed with Schottky barrier heights of 50 and 100 meV, respectively, at the conduction band edge. In addition, enhanced ambipolarity was demonstrated in a Pt–Ti hybrid contact with a unique structure specially designed for polarity-reversible transistors, in which Pt and Ti electrodes were placed in parallel for injecting both electrons and holes

Concerted Chemical-Mechanical Reaction in Catalyzed Growth of Confined Graphene Layers into Hexagonal Disks

Graphene and graphite synthesis of uniform films is becoming routine, so now efforts are turning to grow specific patterns or complex structures. More research is needed with regard to the practical aspects of the growth of graphene layers, especially as it relates to self-assembled structures. We used gallium-catalyzed thermal decomposition of silicon carbide to understand spatially confined growth. Growing graphene layers push on a large step in hard silicon carbide (SiC) with significant force, as observed by high-resolution transmission electron microscopy of film cross sections. Alternatively multiple graphitic layers can grow into disks embedded in silicon carbide if the crystal is heated above the transition to plastic deformation. Euler buckling appears to limit the size and deformation of the silicon carbide crystal to produce graphitic flakes with an oriented hexagonal shape. These results illustrate the effect of the mechanical force of growing graphene in confined spaces: the growing graphene can be redirected, graphene can deform the confining barrier, or growth of graphene can be limited. This also provides a route for fabrication of masses of homogeneous, hexagonal disks of graphite with dimensions that are tuned by directed self-assembly

Layer-by-Layer Oxidation Induced Electronic Properties in Transition-Metal Dichalcogenides

Recent progress in transition-metal dichalcogenides has opened up new possibilities for atomically thin nanomaterial based electronic device applications. Here we investigate atomic-scale self-assembled heterojunction modulated by layer-by-layer controlled oxidation in monolayer and few-layer dichalcogenide systems and their electronic properties within a first-principles framework. Pristine dichalcogenide systems exhibit semiconducting behavior. We observe reduction of the band gap for partial oxidation of the top layer. However, complete oxidation of the top layer makes the system metallic, owing to the charge transfer from the pristine to the oxidized layer, as observed in recent experiments. When the bottom layer gets partially oxidized with fully oxidized top layers, the system shows unprecedented semimetallic behavior. The appearance of valence band maximum and conduction band minimum at different k-points can introduce valley polarization. Therefore, our study shows controlled oxidation induced varying electronic properties in dichalcogenide based heterojunctions that can be exploited for advanced electronic, optoelectronic, and valleytronic device applications

Quantitative Raman Spectrum and Reliable Thickness Identification for Atomic Layers on Insulating Substrates

We demonstrate the possibility in quantifying the Raman intensities for both specimen and substrate layers in a common stacked experimental configuration and, consequently, propose a general and rapid thickness identification technique for atomic-scale layers on dielectric substrates. Unprecedentedly wide-range Raman data for atomically flat MoS₂ flakes are collected to compare with theoretical models. We reveal that all intensity features can be accurately captured when including optical interference effect. Surprisingly, we find that even freely suspended chalcogenide few-layer flakes have a stronger Raman response than that from the bulk phase. Importantly, despite the oscillating intensity of specimen spectrum <i>versus</i> thickness, the substrate weighted spectral intensity becomes monotonic. Combined with its sensitivity to specimen thickness, we suggest this quantity can be used to rapidly determine the accurate thickness for atomic layers

Electrostatically Reversible Polarity of Ambipolar α‑MoTe₂ Transistors

A doping-free transistor made of ambipolar α-phase molybdenum ditelluride (α-MoTe₂) is proposed in which the transistor polarity (p-type and n-type) is electrostatically controlled by dual top gates. The voltage signal in one of the gates determines the transistor polarity, while the other gate modulates the drain current. We demonstrate the transistor operation experimentally, with electrostatically controlled polarity of both p- and n-type in a single transistor

Thickness Scaling Effect on Interfacial Barrier and Electrical Contact to Two-Dimensional MoS₂ Layers

Understanding the interfacial electrical properties between metallic electrodes and low-dimensional semiconductors is essential for both fundamental science and practical applications. Here we report the observation of thickness reduction induced crossover of electrical contact at Au/MoS₂ interfaces. For MoS₂ thicker than 5 layers, the contact resistivity slightly decreases with reducing MoS₂ thickness. By contrast, the contact resistivity sharply increases with reducing MoS₂ thickness below 5 layers, mainly governed by the quantum confinement effect. We find that the interfacial potential barrier can be finely tailored from 0.3 to 0.6 eV by merely varying MoS₂ thickness. A full evolution diagram of energy level alignment is also drawn to elucidate the thickness scaling effect. The finding of tailoring interfacial properties with channel thickness represents a useful approach controlling the metal/semiconductor interfaces which may result in conceptually innovative functionalities

Conduction Tuning of Graphene Based on Defect-Induced Localization

The conduction properties of graphene were tuned by tailoring the lattice by using an accelerated helium ion beam to embed low-density defects in the lattice. The density of the embedded defects was estimated to be 2–3 orders of magnitude lower than that of carbon atoms, and they functionalized a graphene sheet in a more stable manner than chemical surface modifications can do. Current modulation through back gate biasing was demonstrated at room temperature with a current on–off ratio of 2 orders of magnitude, and the activation energy of the thermally activated transport regime was evaluated. The exponential dependence of the current on the length of the functionalized region in graphene suggested that conduction tuning is possible through strong localization of carriers at sites induced by a sparsely distributed random potential modulation

Self-Limiting Layer-by-Layer Oxidation of Atomically Thin WSe₂

Growth of a uniform oxide film with a tunable thickness on two-dimensional transition metal dichalcogenides is of great importance for electronic and optoelectronic applications. Here we demonstrate homogeneous surface oxidation of atomically thin WSe₂ with a self-limiting thickness from single- to trilayers. Exposure to ozone (O₃) below 100 °C leads to the lateral growth of tungsten oxide selectively along selenium zigzag-edge orientations on WSe₂. With further O₃ exposure, the oxide regions coalesce and oxidation terminates leaving a uniform thickness oxide film on top of unoxidized WSe₂. At higher temperatures, oxidation evolves in the layer-by-layer regime up to trilayers. The oxide films formed on WSe₂ are nearly atomically flat. Using photoluminescence and Raman spectroscopy, we find that the underlying single-layer WSe₂ is decoupled from the top oxide but hole-doped. Our findings offer a new strategy for creating atomically thin heterostructures of semiconductors and insulating oxides with potential for applications in electronic devices

Die Beanspruchung des Bergmanns beim Herrichten und Ausrauben eines Strebes Schlussbericht

TIB Hannover: FR 3073+a / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman