Origins of Fermi Level Pinning for Ni and Ag Metal Contacts on Tungsten Dichalcogenides.

Tungsten transition metal dichalcogenides (W-TMDs) are intriguing due to their properties and potential for application in next-generation electronic devices. However, strong Fermi level (EF) pinning manifests at the metal/W-TMD interfaces, which could tremendously restrain the carrier injection into the channel. In this work, we illustrate the origins of EF pinning for Ni and Ag contacts on W-TMDs by considering interface chemistry, band alignment, impurities, and imperfections of W-TMDs, contact metal adsorption mechanism, and the resultant electronic structure. We conclude that the origins of EF pinning at a covalent contact metal/W-TMD interface, such as Ni/W-TMDs, can be attributed to defects, impurities, and interface reaction products. In contrast, for a van der Waals contact metal/TMD system such as Ag/W-TMDs, the primary factor responsible for EF pinning is the electronic modification of the TMDs resulting from the defects and impurities with the minor impact of metal-induced gap states. The potential strategies for carefully engineering the metal deposition approach are also discussed. This work unveils the origins of EF pinning at metal/TMD interfaces experimentally and theoretically and provides guidance on further enhancing and improving the device performance.

Nb2O5, LiNbO3, and (Na, K)NbO3 Thin Films from High-Concentration Aqueous Nb-Polyoxometalates.

Synthesizing functional materials from water contributes to a sustainable energy future. On the atomic level, water drives complex metal hydrolysis/condensation/speciation, acid-base, ion pairing, and solvation reactions that ultimately direct material assembly pathways. Here, we demonstrate the importance of Nb-polyoxometalate (Nb-POM) speciation in enabling deposition of Nb2O5, LiNbO3, and (Na, K)NbO3 (KNN) from high-concentration solutions, up to 2.5 M Nb for Nb2O5 and ∼1 M Nb for LiNbO3 and KNN. Deposition of KNN from 1 M Nb concentration represents a potentially important advancment in lead-free piezoelectrics, an application that requires thick films. Solution characterization via small-angle X-ray scattering and Raman spectroscopy described the speciation for all precursor solutions as the [HxNb24O72](x-24) POM, as did total pair distribution function analyses of X-ray scattering of amorphous gels prior to conversion to oxides. The tendency of the Nb24-POM to form extended networks without crystallization leads to conformal and well-adhered films. The films were characterized by X-ray diffraction, atomic force microscopy, scanning electron microscopy, ellipsometry, and X-ray photoelectron spectroscopy. As a strategy to convert aqueous deposition solutions from {Nb10}-POMs to {Nb24}-POMs, we devised a general procedure to produce doped Nb2O5 thin films including Ca, Ag, and Cu doping.

Contribution of the Sub-Surface to Electrocatalytic Activity in Atomically Precise La0.7 Sr0.3 MnO3 Heterostructures.

Electrocatalytic reactions are known to take place at the catalyst/electrolyte interface. Whereas recent studies of size-dependent activity in nanoparticles and thickness-dependent activity of thin films imply that the sub-surface layers of a catalyst can contribute to the catalytic activity as well, most of these studies consider actual modification of the surfaces. In this study, the role of catalytically active sub-surface layers was investigated by employing atomic-scale thickness control of the La0.7 Sr0.3 MnO3 (LSMO) films and heterostructures, without altering the catalyst/electrolyte interface. The activity toward the oxygen evolution reaction (OER) shows a non-monotonic thickness dependence in the LSMO films and a continuous screening effect in LSMO/SrRuO3 heterostructures. The observation leads to the definition of an "electrochemically-relevant depth" on the order of 10 unit cells. This study on the electrocatalytic activity of epitaxial heterostructures provides new insight in designing efficient electrocatalytic nanomaterials and core-shell architectures.

Enhanced Visible-Light-Driven Hydrogen Production through MOF/MOF Heterojunctions.

A strategy for enhancing the photocatalytic performance of MOF-based systems (MOF: metal-organic framework) is developed through the construction of MOF/MOF heterojunctions. The combination of MIL-167 with MIL-125-NH2 leads to the formation of MIL-167/MIL-125-NH2 heterojunctions with improved optoelectronic properties and efficient charge separation. MIL-167/MIL-125-NH2 outperforms its single components MIL-167 and MIL-125-NH2, in terms of photocatalytic H2 production (455 versus 0.8 and 51.2 µmol h-1 g-1, respectively), under visible-light irradiation, without the use of any cocatalysts. This is attributed to the appropriate band alignment of these MOFs, the enhanced visible-light absorption, and long charge separation within MIL-167/MIL-125-NH2. Our findings contribute to the discovery of novel MOF-based photocatalytic systems that can harvest solar energy and exhibit high catalytic activities in the absence of cocatalysts.

Surface chemistry of 2-propanol and O2 mixtures on SnO2(110) studied with ambient-pressure x-ray photoelectron spectroscopy.

Tin dioxide (SnO2) has various applications due to its unique surface and electronic properties. These properties are strongly influenced by Sn oxidation states and associated defect chemistries. Recently, the oxidation of volatile organic compounds (VOCs) into less harmful molecules has been demonstrated using SnO2 catalysts. A common VOC, 2-propanol (isopropyl alcohol, IPA), has been used as a model compound to better understand SnO2 reaction kinetics. We have used ambient-pressure x-ray photoelectron spectroscopy (AP-XPS) to characterize the surface chemistry of IPA and O2 mixtures on stoichiometric, unreconstructed SnO2(110)-(1 × 1) surfaces. AP-XPS experiments were performed for IPA pressures ≤3 mbar, various IPA/O2 ratios, and several reaction temperatures. These measurements allowed us to determine the chemical states of adsorbed species on SnO2(110)-(1 × 1) under numerous experimental conditions. We found that both the IPA/O2 ratio and sample temperature strongly influence reaction chemistries. AP-XPS valence-band spectra indicate that the surface was partially reduced from Sn4+ to Sn2+ during reactions with IPA. In situ mass spectrometry and gas-phase AP-XPS results indicate that the main reaction product was acetone under these conditions. For O2 and IPA mixtures, the reaction kinetics substantially increased and the surface remained solely Sn4+. We believe that O2 replenished surface oxygen vacancies and that SnO2 bridging and in-plane oxygen are likely the active oxygen species. Moreover, addition of O2 to the reaction results in a reduction in formation of acetone and an increase in formation of CO2 and H2O. Based on these studies, we have developed a reaction model that describes the catalytic oxidation of IPA on stoichiometric SnO2(110)-(1 × 1) surfaces.

Impact of Etch Processes on the Chemistry and Surface States of the Topological Insulator Bi2Se3.

The unique properties of topological insulators such as Bi2Se3 are intriguing for their potential implementation in novel device architectures for low power and defect-tolerant logic and memory devices. Recent improvements in the synthesis of Bi2Se3 have positioned researchers to fabricate new devices to probe the limits of these materials. The fabrication of such devices, of course, requires etching of the topological insulator, in addition to other materials including gate oxides and contacts which may impact the topologically protected surface states. In this paper, we study the impact of He+ sputtering and inductively coupled plasma Cl2 and SF6 reactive etch chemistries on the physical, chemical, and electronic properties of Bi2Se3. Chemical analysis by X-ray photoelectron spectroscopy tracks changes in the surface chemistry and Fermi level, showing preferential removal of Se that results in vacancy-induced n-type doping. Chlorine-based chemistry successfully etches Bi2Se3 but with residual Se-Se bonding and interstitial Cl species remaining after the etch. The Se vacancies and residuals can be removed with postetch anneals in a Se environment, repairing Bi2Se3 nearly to the as-grown condition. Critically, in each of these cases, angle-resolved photoemission spectroscopy (ARPES) reveals that the topologically protected surface states remain even after inducing significant surface disorder and chemical changes, demonstrating that topological insulators are quite promising for defect-tolerant electronics. Changes to the ARPES intensity and momentum broadening of the surface states are discussed. Fluorine-based etching aggressively reacts with the film resulting in a relatively thick insulating film of thermodynamically favored BiF3 on the surface, prohibiting the use of SF6-based etching in Bi2Se3 processing.

Enhancing Interconnect Reliability and Performance by Converting Tantalum to 2D Layered Tantalum Sulfide at Low Temperature.

The interconnect half-pitch size will reach ≈20 nm in the coming sub-5 nm technology node. Meanwhile, the TaN/Ta (barrier/liner) bilayer stack has to be >4 nm to ensure acceptable liner and diffusion barrier properties. Since TaN/Ta occupy a significant portion of the interconnect cross-section and they are much more resistive than Cu, the effective conductance of an ultrascaled interconnect will be compromised by the thick bilayer. Therefore, 2D layered materials have been explored as diffusion barrier alternatives. However, many of the proposed 2D barriers are prepared at too high temperatures to be compatible with the back-end-of-line (BEOL) technology. In addition, as important as the diffusion barrier properties, the liner properties of 2D materials must be evaluated, which has not yet been pursued. Here, a 2D layered tantalum sulfide (TaSx ) with ≈1.5 nm thickness is developed to replace the conventional TaN/Ta bilayer. The TaSx ultrathin film is industry-friendly, BEOL-compatible, and can be directly prepared on dielectrics. The results show superior barrier/liner properties of TaSx compared to the TaN/Ta bilayer. This single-stack material, serving as both a liner and a barrier, will enable continued scaling of interconnects beyond 5 nm node.

High-κ Dielectric on ReS2: In-Situ Thermal Versus Plasma-Enhanced Atomic Layer Deposition of Al2O3.

We report an excellent growth behavior of a high-κ dielectric on ReS2, a two-dimensional (2D) transition metal dichalcogenide (TMD). The atomic layer deposition (ALD) of an Al2O3 thin film on the UV-Ozone pretreated surface of ReS2 yields a pinhole free and conformal growth. In-situ half-cycle X-ray photoelectron spectroscopy (XPS) was used to monitor the interfacial chemistry and ex-situ atomic force microscopy (AFM) was used to evaluate the surface morphology. A significant enhancement in the uniformity of the Al2O3 thin film was deposited via plasma-enhanced atomic layer deposition (PEALD), while pinhole free Al2O3 was achieved using a UV-Ozone pretreatment. The ReS2 substrate stays intact during all different experiments and processes without any formation of the Re oxide. This work demonstrates that a combination of the ALD process and the formation of weak S⁻O bonds presents an effective route for a uniform and conformal high-κ dielectric for advanced devices based on 2D materials.

Dislocation driven spiral and non-spiral growth in layered chalcogenides.

Two-dimensional materials have shown great promise for implementation in next-generation devices. However, controlling the film thickness during epitaxial growth remains elusive and must be fully understood before wide scale industrial application. Currently, uncontrolled multilayer growth is frequently observed, and not only does this growth mode contradict theoretical expectations, but it also breaks the inversion symmetry of the bulk crystal. In this work, a multiscale theoretical investigation aided by experimental evidence is carried out to identify the mechanism of such an unconventional, yet widely observed multilayer growth in the epitaxy of layered materials. This work reveals the subtle mechanistic similarities between multilayer concentric growth and spiral growth. Using the combination of experimental demonstration and simulations, this work presents an extended analysis of the driving forces behind this non-ideal growth mode, and the conditions that promote the formation of these defects. Our study shows that multilayer growth can be a result of both chalcogen deficiency and chalcogen excess: the former causes metal clustering as nucleation defects, and the latter generates in-domain step edges facilitating multilayer growth. Based on this fundamental understanding, our findings provide guidelines for the narrow window of growth conditions which enables large-area, layer-by-layer growth.

High-Mobility Helical Tellurium Field-Effect Transistors Enabled by Transfer-Free, Low-Temperature Direct Growth.

The transfer-free direct growth of high-performance materials and devices can enable transformative new technologies. Here, room-temperature field-effect hole mobilities as high as 707 cm2 V-1 s-1 are reported, achieved using transfer-free, low-temperature (≤120 °C) direct growth of helical tellurium (Te) nanostructure devices on SiO2 /Si. The Te nanostructures exhibit significantly higher device performance than other low-temperature grown semiconductors, and it is demonstrated that through careful control of the growth process, high-performance Te can be grown on other technologically relevant substrates including flexible plastics like polyethylene terephthalate and graphene in addition to amorphous oxides like SiO2 /Si and HfO2 . The morphology of the Te films can be tailored by the growth temperature, and different carrier scattering mechanisms are identified for films with different morphologies. The transfer-free direct growth of high-mobility Te devices can enable major technological breakthroughs, as the low-temperature growth and fabrication is compatible with the severe thermal budget constraints of emerging applications. For example, vertical integration of novel devices atop a silicon complementary metal oxide semiconductor platform (thermal budget <450 °C) has been theoretically shown to provide a 10× systems level performance improvement, while flexible and wearable electronics (thermal budget <200 °C) can revolutionize defense and medical applications.

Fermi Level Manipulation through Native Doping in the Topological Insulator Bi2Se3.

The topologically protected surface states of three-dimensional (3D) topological insulators have the potential to be transformative for high-performance logic and memory devices by exploiting their specific properties such as spin-polarized current transport and defect tolerance due to suppressed backscattering. However, topological insulator based devices have been underwhelming to date primarily due to the presence of parasitic issues. An important example is the challenge of suppressing bulk conduction in Bi2Se3 and achieving Fermi levels ( EF) that reside in between the bulk valence and conduction bands so that the topologically protected surface states dominate the transport. The overwhelming majority of the Bi2Se3 studies in the literature report strongly n-type materials with EF in the bulk conduction band due to the presence of a high concentration of selenium vacancies. In contrast, here we report the growth of near-intrinsic Bi2Se3 with a minimal Se vacancy concentration providing a Fermi level near midgap with no extrinsic counter-doping required. We also demonstrate the crucial ability to tune EF from below midgap into the upper half of the gap near the conduction band edge by controlling the Se vacancy concentration using post-growth anneals. Additionally, we demonstrate the ability to maintain this Fermi level control following the careful, low-temperature removal of a protective Se cap, which allows samples to be transported in air for device fabrication. Thus, we provide detailed guidance for EF control that will finally enable researchers to fabricate high-performance devices that take advantage of transport through the topologically protected surface states of Bi2Se3.

Realizing Large-Scale, Electronic-Grade Two-Dimensional Semiconductors.

Atomically thin transition metal dichalcogenides (TMDs) are of interest for next-generation electronics and optoelectronics. Here, we demonstrate device-ready synthetic tungsten diselenide (WSe2) via metal-organic chemical vapor deposition and provide key insights into the phenomena that control the properties of large-area, epitaxial TMDs. When epitaxy is achieved, the sapphire surface reconstructs, leading to strong 2D/3D (i.e., TMD/substrate) interactions that impact carrier transport. Furthermore, we demonstrate that substrate step edges are a major source of carrier doping and scattering. Even with 2D/3D coupling, transistors utilizing transfer-free epitaxial WSe2/sapphire exhibit ambipolar behavior with excellent on/off ratios (∼107), high current density (1-10 µA·µm-1), and good field-effect transistor mobility (∼30 cm2·V-1·s-1) at room temperature. This work establishes that realization of electronic-grade epitaxial TMDs must consider the impact of the TMD precursors, substrate, and the 2D/3D interface as leading factors in electronic performance.

Defects and Surface Structural Stability of MoTe2 Under Vacuum Annealing.

Understanding the structural stability of transition-metal dichalcogenides is necessary to avoid surface/interface degradation. In this work, the structural stability of 2H-MoTe2 with thermal treatments up to 500 °C is studied using scanning tunneling microscopy and scanning transmission electron microscopy. On the exfoliated sample surface at room temperature, atomic subsurface donors originating from excess Te atoms are observed and presented as nanometer-sized, electronically-induced protrusions superimposed with the hexagonal lattice structure of MoTe2. Under a thermal treatment as low as 200 °C, the surface decomposition-induced cluster defects and Te vacancies are readily detected and increase in extent with the increasing temperature. Driven by Te vacancies and thermal energy, intense 60° inversion domain boundaries form resulting in a "wagon wheel" morphology after 400 °C annealing for 15 min. Scanning tunneling spectroscopy identified the electronic states at the domain boundaries and the domain centers. To prevent extensive Te loss at higher temperatures, where Mo6Te6 nanowire formation and substantial desorption-induced etching effects will take place simultaneously, surface and edge passivation with a monolayer graphene coverage on MoTe2 is tested. With this passivation strategy, the structural stability of MoTe2 is greatly enhanced up to 500 °C without apparent structural defects.

Schottky Barrier Height of Pd/MoS2 Contact by Large Area Photoemission Spectroscopy.

MoS2, as a model transition metal dichalcogenide, is viewed as a potential channel material in future nanoelectronic and optoelectronic devices. Minimizing the contact resistance of the metal/MoS2 junction is critical to realizing the potential of MoS2-based devices. In this work, the Schottky barrier height (SBH) and the band structure of high work function Pd metal on MoS2 have been studied by in situ X-ray photoelectron spectroscopy (XPS). The analytical spot diameter of the XPS spectrometer is about 400 µm, and the XPS signal is proportional to the detection area, so the influence of defect-mediated parallel conduction paths on the SBH does not affect the measurement. The charge redistribution by Pd on MoS2 is detected by XPS characterization, which gives insight into metal contact physics to MoS2 and suggests that interface engineering is necessary to lower the contact resistance for the future generation electronic applications.

New Mo6 Te6 Sub-Nanometer-Diameter Nanowire Phase from 2H-MoTe2.

A novel phase transition, from multilayered 2H-MoTe2 to a parallel bundle of sub-nanometer-diameter metallic Mo6 Te6 nanowires (NWs) driven by catalyzer-free thermal-activation (400-500 °C) under vacuum, is demonstrated. The NWs form along the 〈11-20〉 2H-MoTe2 crystallographic directions with lengths in the micrometer range. The metallic NWs can act as an efficient hole injection layer on top of 2H-MoTe2 due to favorable band-alignment. In particular, an atomically sharp MoTe2 /Mo6 Te6 interface and van der Waals gap with the 2H layers are preserved. The work highlights an alternative pathway for forming a new transition metal dichalcogenide phase and will enable future exploration of its intrinsic transportation properties.

Electronic properties of MoS2/MoOx interfaces: Implications in Tunnel Field Effect Transistors and Hole Contacts.

In an electronic device based on two dimensional (2D) transitional metal dichalcogenides (TMDs), finding a low resistance metal contact is critical in order to achieve the desired performance. However, due to the unusual Fermi level pinning in metal/2D TMD interface, the performance is limited. Here, we investigate the electronic properties of TMDs and transition metal oxide (TMO) interfaces (MoS2/MoO3) using density functional theory (DFT). Our results demonstrate that, due to the large work function of MoO3 and the relative band alignment with MoS2, together with small energy gap, the MoS2/MoO3 interface is a good candidate for a tunnel field effect (TFET)-type device. Moreover, if the interface is not stoichiometric because of the presence of oxygen vacancies in MoO3, the heterostructure is more suitable for p-type (hole) contacts, exhibiting an Ohmic electrical behavior as experimentally demonstrated for different TMO/TMD interfaces. Our results reveal that the defect state induced by an oxygen vacancy in the MoO3 aligns with the valance band of MoS2, showing an insignificant impact on the band gap of the TMD. This result highlights the role of oxygen vacancies in oxides on facilitating appropriate contacts at the MoS2 and MoOx (x < 3) interface, which consistently explains the available experimental observations.

Surface Analysis of WSe2 Crystals: Spatial and Electronic Variability.

Layered semiconductor compounds represent alternative electronic materials beyond graphene. WSe2 is one of the two-dimensional materials with wide potential in opto- and nanoelectronics and is often used to construct novel three-dimensional architectures with new functionalities. Here, we report the topography and the electronic property of the WSe2 characterized by means of scanning tunneling microscopy and spectroscopy (STM and STS), X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma mass spectrometry. The STM images reveal the presence of atomic-size imperfections and a variation in the electronic structure caused by the presence of defects and impurities below the detection limit of XPS. Both STS and photoemission reveal a spatial variation in the Fermi level position. The analysis of the core levels indicates the presence of different doping levels. The presence of a large concentration of defects and impurities has a strong impact on the electronic properties of the WSe2 surface. Our findings demonstrate that the growth of controllable and high quality two-dimensional materials at nanometer scale is one of the most challenging tasks that requires further attention.

Covalent Nitrogen Doping and Compressive Strain in MoS2 by Remote N2 Plasma Exposure.

Controllable doping of two-dimensional materials is highly desired for ideal device performance in both hetero- and p-n homojunctions. Herein, we propose an effective strategy for doping of MoS2 with nitrogen through a remote N2 plasma surface treatment. By monitoring the surface chemistry of MoS2 upon N2 plasma exposure using in situ X-ray photoelectron spectroscopy, we identified the presence of covalently bonded nitrogen in MoS2, where substitution of the chalcogen sulfur by nitrogen is determined as the doping mechanism. Furthermore, the electrical characterization demonstrates that p-type doping of MoS2 is achieved by nitrogen doping, which is in agreement with theoretical predictions. Notably, we found that the presence of nitrogen can induce compressive strain in the MoS2 structure, which represents the first evidence of strain induced by substitutional doping in a transition metal dichalcogenide material. Finally, our first principle calculations support the experimental demonstration of such strain, and a correlation between nitrogen doping concentration and compressive strain in MoS2 is elucidated.

Tuning electronic transport in epitaxial graphene-based van der Waals heterostructures.

Two-dimensional tungsten diselenide (WSe2) has been used as a component in atomically thin photovoltaic devices, field effect transistors, and tunneling diodes in tandem with graphene. In some applications it is necessary to achieve efficient charge transport across the interface of layered WSe2-graphene, a semiconductor to semimetal junction with a van der Waals (vdW) gap. In such cases, band alignment engineering is required to ensure a low-resistance, ohmic contact. In this work, we investigate the impact of graphene electronic properties on the transport at the WSe2-graphene interface. Electrical transport measurements reveal a lower resistance between WSe2 and fully hydrogenated epitaxial graphene (EG(FH)) compared to WSe2 grown on partially hydrogenated epitaxial graphene (EGPH). Using low-energy electron microscopy and reflectivity on these samples, we extract the work function difference between the WSe2 and graphene and employ a charge transfer model to determine the WSe2 carrier density in both cases. The results indicate that WSe2-EG(FH) displays ohmic behavior at small biases due to a large hole density in the WSe2, whereas WSe2-EG(PH) forms a Schottky barrier junction.

Recombination Kinetics and Effects of Superacid Treatment in Sulfur- and Selenium-Based Transition Metal Dichalcogenides.

Optoelectronic devices based on two-dimensional (2D) materials have shown tremendous promise over the past few years; however, there are still numerous challenges that need to be overcome to enable their application in devices. These include improving their poor photoluminescence (PL) quantum yield (QY) as well as better understanding of exciton-based recombination kinetics. Recently, we developed a chemical treatment technique using an organic superacid, bis(trifluoromethane)sulfonimide (TFSI), which was shown to improve the quantum yield in MoS2 from less than 1% to over 95%. Here, we perform detailed steady-state and transient optical characterization on some of the most heavily studied direct bandgap 2D materials, specifically WS2, MoS2, WSe2, and MoSe2, over a large pump dynamic range to study the recombination mechanisms present in these materials. We then explore the effects of TFSI treatment on the PL QY and recombination kinetics for each case. Our results suggest that sulfur-based 2D materials are amenable to repair/passivation by TFSI, while the mechanism is thus far ineffective on selenium based systems. We also show that biexcitonic recombination is the dominant nonradiative pathway in these materials and that the kinetics for TFSI treated MoS2 and WS2 can be described using a simple two parameter model.