Contact Engineering High-Performance n-Type MoTe2 Transistors.

Semiconducting MoTe2 is one of the few two-dimensional (2D) materials with a moderate band gap, similar to silicon. However, this material remains underexplored for 2D electronics due to ambient instability and predominantly p-type Fermi level pinning at contacts. Here, we demonstrate unipolar n-type MoTe2 transistors with the highest performance to date, including high saturation current (>400 µA/µm at 80 K and >200 µA/µm at 300 K) and relatively low contact resistance (1.2 to 2 kΩ·µm from 80 to 300 K), achieved with Ag contacts and AlOx encapsulation. We also investigate other contact metals (Sc, Ti, Cr, Au, Ni, Pt), extracting their Schottky barrier heights using an analytic subthreshold model. High-resolution X-ray photoelectron spectroscopy reveals that interfacial metal-Te compounds dominate the contact resistance. Among the metals studied, Sc has the lowest work function but is the most reactive, which we counter by inserting monolayer hexagonal boron nitride between MoTe2 and Sc. These metal-insulator-semiconductor (MIS) contacts partly depin the metal Fermi level and lead to the smallest Schottky barrier for electron injection. Overall, this work improves our understanding of n-type contacts to 2D materials, an important advance for low-power electronics.

Two-dimensional gallium nitride realized via graphene encapsulation.

The spectrum of two-dimensional (2D) and layered materials 'beyond graphene' offers a remarkable platform to study new phenomena in condensed matter physics. Among these materials, layered hexagonal boron nitride (hBN), with its wide bandgap energy (∼5.0-6.0 eV), has clearly established that 2D nitrides are key to advancing 2D devices. A gap, however, remains between the theoretical prediction of 2D nitrides 'beyond hBN' and experimental realization of such structures. Here we demonstrate the synthesis of 2D gallium nitride (GaN) via a migration-enhanced encapsulated growth (MEEG) technique utilizing epitaxial graphene. We theoretically predict and experimentally validate that the atomic structure of 2D GaN grown via MEEG is notably different from reported theory. Moreover, we establish that graphene plays a critical role in stabilizing the direct-bandgap (nearly 5.0 eV), 2D buckled structure. Our results provide a foundation for discovery and stabilization of 2D nitrides that are difficult to prepare via traditional synthesis.

Impurity and silicate formation dependence on O3 pulse time and the growth temperature in atomic-layer-deposited La2O3 thin films.

Atomic-layer-deposited La2O3 films were grown on Si with different O3 pulse times and growth temperatures. The interfacial reactions and impurity behaviors were observed using in situ X-ray photoelectron spectroscopy. Longer pulse time of O3 formed the solid SiO2 interfacial barrier layer, which suppressed La-silicate formation. Meanwhile, the carboxyl compound acting as an impurity phase was replaced with LaCO3 on increasing the O3 pulse time due to further oxidation and reaction of La. Higher growth temperatures enhanced La-silicate formation by mixed diffusion of Si and La2O3, during which most of the La2O3 phase was consumed at 400 °C. C and N impurities decreased with increasing growth temperature and completely disappear at 400 °C.

Theoretical Demonstration of the Ionic Barristor.

In this Letter, we use first-principles simulations to demonstrate the absence of Fermi-level pinning when graphene is in contact with transition metal dichalcogenides (TMDs). We find that formation of either an ohmic or Schottky contact is possible. Then we show that, due to the shallow density of states around its Fermi level, the work function of graphene can be tuned by ion adsorption. Finally we combine work function tuning of graphene and an ideal contact between graphene and TMDs to propose an ionic barristor design that can tune the work function of graphene with a much wider margin than current barristor designs, achieving a dynamic switching among p-type ohmic contact, Schottky contact, and n-type ohmic contact in one device.

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.

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.

Manganese Doping of Monolayer MoS2: The Substrate Is Critical.

Substitutional doping of transition metal dichalcogenides (TMDs) may provide routes to achieving tunable p-n junctions, bandgaps, chemical sensitivity, and magnetism in these materials. In this study, we demonstrate in situ doping of monolayer molybdenum disulfide (MoS2) with manganese (Mn) via vapor phase deposition techniques. Successful incorporation of Mn in MoS2 leads to modifications of the band structure as evidenced by photoluminescence and X-ray photoelectron spectroscopy, but this is heavily dependent on the choice of substrate. We show that inert substrates (i.e., graphene) permit the incorporation of several percent Mn in MoS2, while substrates with reactive surface terminations (i.e., SiO2 and sapphire) preclude Mn incorporation and merely lead to defective MoS2. The results presented here demonstrate that tailoring the substrate surface could be the most significant factor in substitutional doping of TMDs with non-TMD elements.

The unusual mechanism of partial Fermi level pinning at metal-MoS2 interfaces.

Density functional theory calculations are performed to unravel the nature of the contact between metal electrodes and monolayer MoS2. Schottky barriers are shown to be present for a variety of metals with the work functions spanning over 4.2-6.1 eV. Except for the p-type Schottky contact with platinum, the Fermi levels in all of the studied metal-MoS2 complexes are situated above the midgap of MoS2. The mechanism of the Fermi level pinning at metal-MoS2 contact is shown to be unique for metal-2D-semiconductor interfaces, remarkably different from the well-known Bardeen pinning effect, metal-induced gap states, and defect/disorder induced gap states, which are applicable to traditional metal-semiconductor junctions. At metal-MoS2 interfaces, the Fermi level is partially pinned as a result of two interface behaviors: first by a metal work function modification by interface dipole formation due to the charge redistribution, and second by the production of gap states mainly of Mo d-orbitals character by the weakened intralayer S-Mo bonding due to the interface metal-S interaction. This finding would provide guidance to develop approaches to form Ohmic contact to MoS2.

MoS2 P-type transistors and diodes enabled by high work function MoOx contacts.

The development of low-resistance source/drain contacts to transition-metal dichalcogenides (TMDCs) is crucial for the realization of high-performance logic components. In particular, efficient hole contacts are required for the fabrication of p-type transistors with MoS2, a model TMDC. Previous studies have shown that the Fermi level of elemental metals is pinned close to the conduction band of MoS2, thus resulting in large Schottky barrier heights for holes with limited hole injection from the contacts. Here, we show that substoichiometric molybdenum trioxide (MoOx, x < 3), a high work function material, acts as an efficient hole injection layer to MoS2 and WSe2. In particular, we demonstrate MoS2 p-type field-effect transistors and diodes by using MoOx contacts. We also show drastic on-current improvement for p-type WSe2 FETs with MoOx contacts over devices made with Pd contacts, which is the prototypical metal used for hole injection. The work presents an important advance in contact engineering of TMDCs and will enable future exploration of their performance limits and intrinsic transport properties.

Hole selective MoOx contact for silicon solar cells.

Using an ultrathin (∼ 15 nm in thickness) molybdenum oxide (MoOx, x < 3) layer as a transparent hole selective contact to n-type silicon, we demonstrate a room-temperature processed oxide/silicon solar cell with a power conversion efficiency of 14.3%. While MoOx is commonly considered to be a semiconductor with a band gap of 3.3 eV, from X-ray photoelectron spectroscopy we show that MoOx may be considered to behave as a high workfunction metal with a low density of states at the Fermi level originating from the tail of an oxygen vacancy derived defect band located inside the band gap. Specifically, in the absence of carbon contamination, we measure a work function potential of ∼ 6.6 eV, which is significantly higher than that of all elemental metals. Our results on the archetypical semiconductor silicon demonstrate the use of nm-thick transition metal oxides as a simple and versatile pathway for dopant-free contacts to inorganic semiconductors. This work has important implications toward enabling a novel class of junctionless devices with applications for solar cells, light-emitting diodes, photodetectors, and transistors.

Atomically thin heterostructures based on single-layer tungsten diselenide and graphene.

Heterogeneous engineering of two-dimensional layered materials, including metallic graphene and semiconducting transition metal dichalcogenides, presents an exciting opportunity to produce highly tunable electronic and optoelectronic systems. In order to engineer pristine layers and their interfaces, epitaxial growth of such heterostructures is required. We report the direct growth of crystalline, monolayer tungsten diselenide (WSe2) on epitaxial graphene (EG) grown from silicon carbide. Raman spectroscopy, photoluminescence, and scanning tunneling microscopy confirm high-quality WSe2 monolayers, whereas transmission electron microscopy shows an atomically sharp interface, and low energy electron diffraction confirms near perfect orientation between WSe2 and EG. Vertical transport measurements across the WSe2/EG heterostructure provides evidence that an additional barrier to carrier transport beyond the expected WSe2/EG band offset exists due to the interlayer gap, which is supported by theoretical local density of states (LDOS) calculations using self-consistent density functional theory (DFT) and nonequilibrium Green's function (NEGF).

Impact of intrinsic atomic defects on the electronic structure of MoS2 monolayers.

Monolayer MoS2 is a direct band gap semiconductor which has been recently investigated for low-power field effect transistors. The initial studies have shown promising performance, including a high on/off current ratio and carrier mobility with a high-κ gate dielectric. However, the performance of these devices strongly depends on the crystalline quality and defect morphology of the monolayers. In order to obtain a detailed understanding of the MoS2 electronic device properties, we examine possible defect structures and their impact on the MoS2 monolayer electronic properties, using density functional theory in combination with scanning tunneling microscopy to identify the nature of the most likely defects. Quantitative understanding based on a detailed knowledge of the atomic and electronic structures will facilitate the search of suitable defect passivation techniques. Our results show that S adatoms are the most energetically favorable type of defect and that S vacancies are energetically more favorable than Mo vacancies. This approach may be extended to other transition-metal dichalcogenides (TMDs), thus providing useful insights to optimize TMD-based electronic devices.

Mechanism of Fermi Level Pinning for Metal Contacts on Molybdenum Dichalcogenide.

The high contact resistance of transition metal dichalcogenide (TMD)-based devices is receiving considerable attention due to its limitation on electronic performance. The mechanism of Fermi level (EF) pinning, which causes the high contact resistance, is not thoroughly understood to date. In this study, the metal (Ni and Ag)/Mo-TMD surfaces and interfaces are characterized by X-ray photoelectron spectroscopy, atomic force microscopy, scanning tunneling microscopy and spectroscopy, and density functional theory systematically. Ni and Ag form covalent and van der Waals (vdW) interfaces on Mo-TMDs, respectively. Imperfections are detected on Mo-TMDs, which lead to electronic and spatial variations. Gap states appear after the adsorption of single and two metal atoms on Mo-TMDs. The combination of the interface reaction type (covalent or vdW), the imperfection variability of the TMD materials, and the gap states induced by contact metals with different weights are concluded to be the origins of EF pinning.

A density-functional theory study of tip electronic structures in scanning tunneling microscopy.

In this work, we report a detailed analysis of the atomic and electronic structures of transition metal scanning tunneling microscopy tips: Rh, Pd, W, Ir, and Pt pyramidal models, and transition metal (TM) atom tips supported on the W surface, by means of ab initio density-functional theory methods. The d electrons of the apex atoms of the TM tips (Rh, Pd, W, Ir, and Pt tetrahedral structures) show different behaviors near the Fermi level and, especially for the W tip, dz(2) states are shown to be predominant near the Fermi level. The electronic structures of larger pyramidal TM tip structures with a single apex atom are also reported. Their obtained density of states are thoroughly discussed in terms of the different d-electron occupations of the TM tips.

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.

Interface Chemistry and Band Alignment Study of Ni and Ag Contacts on MoS2.

High contact resistance of transition-metal dichalcogenide (TMD)-based devices is one of the bottlenecks that limit the application of TMDs in various domains. Contact resistance of TMD-based devices is strongly related to the interface chemistry and band alignment at the contact metal/TMD interfaces. To understand the metal/MoS2 interface chemistry and band alignment, Ni and Ag metal contacts are deposited on MoS2 bulk and chemical vapor deposition bilayer MoS2 (2L-MoS2) film samples under ultrahigh vacuum (∼3 × 10-11 mbar) and high vacuum (∼3 × 10-6 mbar) conditions. X-ray photoelectron spectroscopy is used to characterize the interface chemistry and band alignment of the metal/MoS2 stacks. Ni forms covalent contact on MoS2 bulk and 2L-MoS2 film by reducing MoS2 to form interfacial metal sulfides. In contrast, van der Waals gaps form at the Ag/MoS2 bulk and Ag/2L-MoS2 film interfaces, proved by the absence of an additional metal sulfide chemical state and the detection of Ag islands on the surface. Different from other metal/MoS2 systems studied in this work, Ag shows potential to form an Ohmic contact on MoS2 bulk regardless of the deposition ambient. Fermi levels (EF's) are pinned near the intrinsic EF of the 2L-MoS2 film with high defect density regardless of the work function of the metal, which highlights the impact of substrate defect density on the EF pinning effect and contact resistance.

Morphology-dependent fluorescence of europium-doped cerium oxide nanomaterials.

Europium-doped CeO2 nanomaterials have been investigated for a variety of sensing and biological applications, as doping enhances the catalytic activity of CeO2 and contributes visible fluorescence to the nanomaterial. However, scant evidence is available that directly compares Eu3+ fluorescence from multiple morphologies establishing useful correlation(s) between physical and optical trends in such structures. To address this shortcoming, Eu3+-doped CeO2 nanorods, nanowires, nanocubes, and annealed nanorods were synthesized and characterized, representing a range of crystalline defect sizes, defect concentrations, and surface moieties. Morphologies rich with oxygen defects and hydroxyl groups (assessed via X-ray photoelectron spectroscopy) quenched the Eu3+ fluorescence, while samples with larger crystalline domains and lower Ce3+ concentrations have relatively stronger emission intensities. Of the four morphologies, nanocubes exhibit the strongest emission, as each structure is monocrystalline with few oxygen defects and associated quenching sites. Furthermore, the Eu3+ hypersensitive transition is more responsive to the dopant concentration in the nanocubes, as defects induced by the dopant are not removed by thermal annealing.

Giant renormalization of dopant impurity levels in 2D semiconductor MoS2.

Substitutional doping in 2D semiconductor MoS2 was investigated by charge transition level (CTL) calculations for Nitrogen group (N, P, As, Sb) and Halogen group (F, Cl, Br, I) dopants at the S site of monolayer MoS2. Both n-type and p-type dopant levels are calculated to be deep mid-gap states (~1 eV from band edges) from DFT total energy-based CTL and separate DFT + GW calculations. The deep dopant levels result from the giant renormalization of hydrogen-like defect states by reduced dielectric screening in ultrathin 2D films. Theoretical analysis based on Keldysh formulation provides a consistent impurity binding energy of ~1 eV for dielectric thin films. These findings of intrinsic deep impurity levels in 2D semiconductors MoS2 may be applicable to diverse novel emerging device applications.

Atomic Layer Deposition of Layered Boron Nitride for Large-Area 2D Electronics.

Hexagonal boron nitride (h-BN) has been considered a promising dielectric for two-dimensional (2D) material-based electronics due to its atomically smooth and charge-free interface with an in-plane lattice constant similar to that of graphene. Here, we report atomic layer deposition of boron nitride (ALD-BN) using BCl3 and NH3 precursors directly on thermal SiO2 substrates at a relatively low temperature of 600 °C. The films were characterized by X-ray photoelectron spectroscopy, atomic force microscopy, and transmission electron microscopy wherein the uniform, atomically smooth, and nanocrystalline layered-BN thin film growth is observed. The growth rate is ∼0.042 nm/cycle at 600 °C, a temperature significantly lower than that of h-BN grown by chemical vapor deposition. The dielectric properties of the ALD-BN measured from Metal Oxide Semiconductor Capacitors are comparable with that of SiO2. Moreover, the ALD-BN exhibits a 2-fold increase in carrier mobility of graphene field effect transistors (G-FETs/ALD-BN/SiO2) due to the lower surface charge density and inert surface of ALD-BN in comparison to that of G-FETs fabricated on bare SiO2. Therefore, this work suggests that the transfer-free deposition of ALD-BN on SiO2 may be a promising candidate as a substrate for high performance graphene devices.