Forming Stable van der Waals Contacts between Metals and 2D Semiconductors

High-performing 2D electrical and optical devices can be realized by forming an ideal van der Waals (vdW) metal contact with weak interactions and stable interface states. However, the methods for applying metal contacts while avoiding damage from metal deposition present challenges in realizing a uniform, stable vdW interface. To overcome this problem, this study develops a method for forming vdW contacts using a sacrificial Se buffer layer. This study explores this method by investigating the difference in the Schottky barrier height between the vdW metal contact deposited using a buffer layer, a transferred metal contact, and a conventional directly deposited metal contact using rectification and photovoltaic characteristics of a Schottky diode structure with graphite. Evidently, the Se buffer layer method forms the most stable and ideal vdW contact while preventing Fermi-level pinning. A tungsten diselenide Schottky diode fabricated using these vdW contacts with Au and graphite as the top and bottom electrodes, respectively, exhibits excellent operation with an ideality factor of ≈1, an on/off ratio of > 107 , and coherent properties. Additionally, when using only the vdW Au contact, the electrical and optical properties of the device can be minutely modulated by changing the structure of the Schottky diode.

Reduced Fermi Level Pinning at Physisorptive Sites of Moire-MoS2/Metal Schottky Barriers

Weaker Fermi level pinning (FLP) at the Schottky barriers of 2D semiconductors is electrically desirable as this would allow a minimizing of contact resistances, which presently limit device performances. Existing contacts on MoS₂ have a strong FLP with a small pinning factor of only ∼0.1. Here, we show that Moire interfaces can stabilize physisorptive sites at the Schottky barriers with a much weaker interaction without significantly lengthening the bonds. This increases the pinning factor up to ∼0.37 and greatly reduces the n-type Schottky barrier height to ∼0.2 eV for certain metals such as In and Ag, which can have physisorptive sites. This then accounts for the low contact resistance of these metals as seen experimentally. Such physisorptive interfaces can be extended to similar systems to better control SBHs in highly scaled 2D devices.

Fabrication of Large-Area Molybdenum Disulfide Device Arrays Using Graphene/Ti Contacts

Two-dimensional (2D) molybdenum disulfide (MoS₂) is the most mature material in 2D material fields owing to its relatively high mobility and scalability. Such noticeable properties enable it to realize practical electronic and optoelectronic applications. However, contact engineering for large-area MoS₂ films has not yet been established, although contact property is directly associated to the device performance. Herein, we introduce graphene-interlayered Ti contacts (graphene/Ti) into large-area MoS₂ device arrays using a wet-transfer method. We achieve MoS₂ devices with superior electrical and photoelectrical properties using graphene/Ti contacts, with a field-effect mobility of 18.3 cm²/V∙s, on/off current ratio of 3 × 10⁷, responsivity of 850 A/W, and detectivity of 2 × 10¹² Jones. This outstanding performance is attributable to a reduction in the Schottky barrier height of the resultant devices, which arises from the decreased work function of graphene induced by the charge transfer from Ti. Our research offers a direction toward large-scale electronic and optoelectronic applications based on 2D materials.

Metal-Contact Improvement in a Multilayer WSe2 Transistor through Strong Hot Carrier Injection

Hot carrier injection (HCI), occurring when the horizontal electric field is strongly applied, usually affects the degradation of nanoelectronic devices. In addition, metal contacts play a significant role in nanoelectronic devices. In this study, Schottky contacts in multilayer tungsten diselenide (WSe₂) field-effect transistors (FETs) by hot carrier injection (HCI), occurring when a high drain voltage is applied, is investigated. A small number of hot carriers with high energy reduces the Schottky barrier height and improves the performance of FETs effectively rather than damaging the channel. Thermal annealing at the end of the fabrication process increases device performance by causing interfacial reactions of the source/drain electrodes. HCI causes a significant enhancement in the local asymmetry, especially in the subthreshold region. The subthreshold swing (SS) of the thermally annealed FETs is significantly improved from 9.66 to 0.562 V dec^-1 through the energy of HCI generated by a strong horizontal electric field. In addition, the contact resistances (R_SD), also called series resistances, extracted by a four-probe measurement and a Y-function method were also improved by decreasing to a 10th through the energy of HCI. To understand the asymmetrical characteristics of the channel after the stress, we performed electrical analysis, electrostatic force microscopy (EFM), and Raman spectroscopy.

Uncovering the Effects of Metal Contacts on Monolayer MoS2

Metal contacts are a key limiter to the electronic performance of two-dimensional (2D) semiconductor devices. Here, we present a comprehensive study of contact interfaces between seven metals (Y, Sc, Ag, Al, Ti, Au, Ni, with work functions from 3.1 to 5.2 eV) and monolayer MoS₂ grown by chemical vapor deposition. We evaporate thin metal films onto MoS₂ and study the interfaces by Raman spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, transmission electron microscopy, and electrical characterization. We uncover that (1) ultrathin oxidized Al dopes MoS₂ n-type (>2 × 10¹² cm^-2) without degrading its mobility, (2) Ag, Au, and Ni deposition causes varying levels of damage to MoS₂ (e.g. broadening Raman E' peak from <3 to >6 cm^-1), and (3) Ti, Sc, and Y react with MoS₂. Reactive metals must be avoided in contacts to monolayer MoS₂, but control studies reveal the reaction is mostly limited to the top layer of multilayer films. Finally, we find that (4) thin metals do not significantly strain MoS₂, as confirmed by X-ray diffraction. These are important findings for metal contacts to MoS₂ and broadly applicable to many other 2D semiconductors.

Defect-Assisted Contact Property Enhancement in a Molybdenum Disulfide Monolayer

Contact engineering for two-dimensional (2D) transition metal dichalcogenides (TMDCs) is crucial for realizing high-performance 2D TMDC devices, and most studies on contact properties of 2D TMDCs have mainly focused on Fermi level unpinning. Here, we investigated electrical and photoelectrical properties of chemical vapor deposition (CVD)-grown molybdenum disulfide (MoS₂) monolayer devices depending on metal contacts, Ti/Pt, Ti/Au, Ti, and Ag, and particularly demonstrated the essential role of defects in MoS₂ in contact properties. Remarkably, MoS₂ devices with Ag contacts show a field-effect mobility of 12.2 cm² V^-1 s^-1, an on/off current ratio of 7 × 10⁷, and a photoresponsivity of 1020 A W^-1, which are outstanding compared to similar devices with other metal contacts. These improvements are attributed to a reduced Schottky barrier height, thanks to the small work function of Ag and Ag-MoS₂ orbital hybridization at the interface, which facilitates efficient charge transfer between MoS₂ and Ag. Interestingly, X-ray photoelectron spectroscopic analysis reveals that Ag₂S was formed in our defect-containing CVD-grown MoS₂ monolayer, but such orbital hybridization is not observed in a nearly defect-free exfoliated MoS₂. This distinction shows that defects existing in MoS₂ enable Ag to effectively couple to MoS₂ and correspondingly enhance multiple electrical and photoelectrical properties.

Spatial Mapping of Thermal Boundary Conductance at Metal-Molybdenum Diselenide Interfaces

Improving the thermal transport across interfaces is a necessary consideration for micro- and nanoelectronic devices and necessitates accurate measurement of the thermal boundary conductance (TBC) and understanding of transport mechanisms. Two-dimensional transition-metal dichalcogenides (TMDs) have been studied extensively for their electrical properties, including the metal-TMD electrical contact resistance, but the thermal properties of these interfaces are significantly less explored irrespective of their high importance in their electronic devices. We isolate individual islands of MoSe₂ grown by chemical vapor deposition using photolithography and correlate the 2D variation of TBC with optical microscope images of the MoSe₂ islands. We measure the 2D spatial variation of the TBC at metal-MoSe₂-SiO₂ interfaces using a modified time-domain thermoreflectance (TDTR) technique, which requires much less time than full TDTR scans. The thermoreflectance signal at a single probe delay time is compared with a correlation curve, which enables us to estimate the change in the signal with respect to the TBC at the metal-MoSe₂-SiO₂ interface as opposed to recording the decay of the thermoreflectance signal over delay times of several nanoseconds. The results show a higher TBC across the Ti-MoSe₂-SiO₂ interface compared to Al-MoSe₂-SiO₂. An image-clustering method is developed to differentiate the TBC for different numbers of MoSe₂ layers, which reveals that the TBC in single-layer regions is higher than that in the bilayer. We perform traditional TDTR measurements over a range of delay times and verify that TBC is higher at the Ti-MoSe₂-SiO₂ interface compared to Al-MoSe₂-SiO₂, highlighting the importance of the choice of metal for heat dissipation at electrical contacts in TMD devices.

Thermoelectric Performance of 2D Tellurium with Accumulation Contacts

Tellurium (Te) is an intrinsically p-type-doped narrow-band gap semiconductor with an excellent electrical conductivity and low thermal conductivity. Bulk trigonal Te has been theoretically predicted and experimentally demonstrated to be an outstanding thermoelectric material with a high value of thermoelectric figure-of-merit ZT. In view of the recent progress in developing the synthesis route of 2D tellurium thin films as well as the growing trend of exploiting nanostructures as thermoelectric devices, here for the first time, we report the excellent thermoelectric performance of tellurium nanofilms, with a room-temperature power factor of 31.7 μW/cm K² and ZT value of 0.63. To further enhance the efficiency of harvesting thermoelectric power in nanofilm devices, thermoelectrical current mapping was performed with a laser as a heating source, and we found that high work function metals such as palladium can form rare accumulation-type metal-to-semiconductor contacts to Te, which allows thermoelectrically generated carriers to be collected more efficiently. High-performance thermoelectric Te devices have broad applications as energy harvesting devices or nanoscale Peltier coolers in microsystems.

Uncovering the Conduction Behavior of van der Waals Ambipolar Semiconductors

A long-standing puzzle about van der Waals semiconductors (vdWS) is regarding the origin(s) of the conduction behavior they exhibit. Of particular interest are those with ambipolar conduction, which may provide an alternative choice for practical applications when considering the difficulties of doping the ultrathin bodies of vdWS. Here, the conduction behavior of ambipolar vdWS is analytically and theoretically studied. Using numerical simulation, it is shown that ambipolar vdWS can be fully captured by a Schottky-barrier FET model. Based on this, it is found that the widely observed conduction polarity transition while changing the body thickness mainly comes from the tuning of band alignment at the metal/vdWS interfaces. This transition can be suppressed/inversed by introducing an inert hBN layer between the vdWS and the substrate. Through first-principles calculations, it is demonstrated that metal/vdWS/substrate interactions play a crucial role in tuning the Schottky-barrier heights, which finally determines the conduction behavior that ambipolar vdWS exhibit.

Post-Synthesis Modifications of Two-Dimensional MoSe2 or MoTe2 by Incorporation of Excess Metal Atoms into the Crystal Structure

Phase engineering has extensively been used to achieve metallization of two-dimensional (2D) semiconducting materials, as it should boost their catalytic properties or improve electrical contacts. In contrast, here we demonstrate compositional phase change by incorporation of excess metals into the crystal structure. We demonstrate post-synthesis restructuring of the semiconducting MoTe₂ or MoSe₂ host material by unexpected easy incorporation of excess Mo into their crystal planes, which causes local metallization. The amount of excess Mo can reach values as high as 10% in MoTe₂ thus creating a significantly altered material compared to its parent structure. The incorporation mechanism is explained by density functional theory in terms of the energy difference of Mo atoms incorporated in the line phases as compared to Mo ad-clusters. Angle resolved photoemission spectroscopy reveals that the incorporated excess Mo induces band gap states up to the Fermi level causing its pinning at these electronic states. The incorporation of excess transition metals in MoTe₂ and MoSe₂ is not limited to molybdenum, but other transition metals can also diffuse into the lattice, as demonstrated experimentally by Ti deposition. The mechanism of incorporation of transition metals in MoSe₂ and MoTe₂ is revealed, which should help to address the challenges in synthesizing defect-free single layer materials by, for example, molecular beam epitaxy. The easy incorporation of metal atoms into the crystal also indicates that the previously assumed picture of a sharp metal/2D-material interface may not be correct, and at least for MoSe₂ and MoTe₂, in-diffusion of metals from metal-contacts into the 2D material has to be considered. Most importantly though, the process of incorporation of transition metals with high concentrations into pristine 2D transition-metal dichalcogenides enables a pathway for their post-synthesis modifications and adding functionalities.

Various and Tunable Transport Properties of WSe2 Transistor Formed by Metal Contacts

The abundant electronic and optical properties of 2D materials that are just one-atom thick pave the way for many novel electronic applications. One important application is to explore the band-to-band tunneling in the heterojunction built by different 2D materials. Here, a gate-controlled WSe₂ transistor is constructed by using different work function metals to form the drain (Pt) and source (Cr) electrodes. The device can be gate-modulated to exhibit three modes of operation, i.e., the tunneling mode with remarkable negative differential resistance, the transition mode with a second electron tunneling phenomenon for backward bias, and finally the conventional diode mode with rectifying characteristics. In contrast to the heterojunctions built by different 2D materials, these devices show significantly enhanced tunneling current by two orders of magnitude, which may largely benefit from the clean interfaces. These results pave the way toward design of novel electronic devices using the modulation of metal work functions.

Ultrasensitive Phototransistors Based on Few-Layered HfS2

An ultrathin HfS2 -based ultrasensitive phototransistor is systematically studied. Au-contacted HfS2 phototransistors with ideal thickness ranging from 7 to 12 nm exhibit a high on/off ratio of ca. 10(7) , ultrahigh photoresponsivity over 890 A W(-1) , and photogain over 2300. Moreover, the response time is strongly dependent on the back-gate voltage and shows a reverse trend for Au and Cr metals.