Defect-Dominated Doping and Contact Resistance in MoS₂

Achieving low resistance contacts is vital for the realization of nanoelectronic devices based on transition metal dichalcogenides. We find that intrinsic defects in MoS₂ dominate the metal/MoS₂ contact resistance and provide a low Schottky barrier independent of metal contact work function. Furthermore, we show that MoS₂ can exhibit both n-type and p-type conduction at different points on a same sample. We identify these regions independently by complementary characterization techniques and show how the Fermi level can shift by 1 eV over tens of nanometers in spatial resolution. We find that these variations in doping are defect-chemistry-related and are independent of contact metal. This raises questions on previous reports of metal-induced doping of MoS₂ since the same metal in contact with MoS₂ can exhibit both n- and p-type behavior. These results may provide a potential route for achieving low electron and hole Schottky barrier contacts with a single metal deposition

Contact Metal–MoS₂ Interfacial Reactions and Potential Implications on MoS₂‑Based Device Performance

Thin films of contact metals, specifically Au, Ir, Cr, and Sc, are deposited on exfoliated, bulk MoS₂ using electron beam deposition under two different reactor base pressures to determine the contact metal–MoS₂ interface chemistry and its dependence on the reactor ambient. The high work function metal Au does not react with MoS₂ regardless of reactor ambient. In contrast, interfacial reactions between MoS₂ and another high work function metal, Ir, are observed when it is deposited under both high vacuum (HV, ∼ 1 × 10^–6 mbar) and ultrahigh vacuum (UHV, ∼ 1 × 10^–9 mbar). Interfacial reactions occur between metals with low work functions (Cr, Sc) near the electron affinity of MoS₂ when the contact metal is deposited under UHV conditions. In addition, Sc is rapidly oxidized on the MoS₂ surface, whereas Cr is only partially oxidized when deposited under HV conditions. This indicates that deposition chamber ambient can affect the contact metal chemistry in addition to the chemistry present at the contact metal–MoS₂ interface. These results elucidate the true chemistry of some contact metal–MoS₂ interfaces and its dependence on the deposition ambient, and highlight the need to consider the chemical states present at the interface and their impact on contact resistance with MoS₂

Synthesis and Material Properties of Bi₂Se₃ Nanostructures Deposited by SILAR

Bi₂Se₃ was synthesized by a room-temperature deposition technique and successive ionic layer adsorption and reaction (SILAR) method with the aim to understand the formation, crystallinity, optical properties, and energy band structure of this material. The Bi₂Se₃ morphology was found to change from nanoparticles to that of a nanocluster network by increasing the SILAR deposition cycles. The crystalline structure of as-prepared Bi₂Se₃ determined from the grazing-incidence X-ray diffraction (GI-XRD) pattern was found to have a mixed of metastable orthorhombic and rhombohedral phases which was further confirmed from our analysis of the Raman spectra. The optical bandgap of Bi₂Se₃ varied from 1.58 to 1.05 eV for 15–90 cycles of deposition, in contrast to the semimetallic 0.3 eV bandgap exhibited by the pure rhombohedral phase. A schematic band diagram of Bi₂Se₃ prepared by 45 SILAR cycles was constructed for the mixed-phase Bi₂Se₃. The flat-band potential was determined to be at 0.46 V vs. RHE from Mott–Schottky analysis. Low-temperature annealing at 100 °C for 1 h resulted in the improvement of the rhombohedral phase fraction which was confirmed from analysis of GI-XRD pattern and pronounced E²_g and A²_1g bulk vibrational modes in the Raman spectrum. The absorption cutoff after annealing was found to be red-shifted combined with a sub-bandgap absorption above 0.78 eV. The post-annealing results indicated the onset of an early stage transition from semiconductor to semi-metallic properties for Bi₂Se₃

Realistic Metal–Graphene Contact Structures

The contact resistance of metal–graphene junctions has been actively explored and exhibited inconsistencies in reported values. The interpretation of these electrical data has been based exclusively on a <i>side</i>-contact model, that is, metal slabs sitting on a pristine graphene sheet. Using <i>in</i> <i>situ</i> X-ray photoelectron spectroscopy to study the wetting of metals on as-synthesized graphene on copper foil, we show that side-contact is sometimes a misleading picture. For instance, metals like Pd and Ti readily react with graphitic carbons, resulting in Pd- and Ti-carbides. Carbide formation is associated with C–C bond breaking in graphene, leading to an <i>end</i>-contact geometry between the metals and the periphery of the remaining graphene patches. This work validates the <i>spontaneous</i> formation of the metal–graphene end-contact during the metal deposition process as a result of the metal–graphene reaction instead of a simple carbon diffusion process

Hole Contacts on Transition Metal Dichalcogenides: Interface Chemistry and Band Alignments

MoO_<i>x</i> shows promising potential as an efficient hole injection layer for p-FETs based on transition metal dichalcogenides. A combination of experiment and theory is used to study the surface and interfacial chemistry, as well as the band alignments for MoO_<i>x</i>/MoS₂ and MoO_<i>x</i>/WSe₂ heterostructures, using photoelectron spectroscopy, scanning tunneling microscopy, and density functional theory. A Mo⁵⁺ rich interface region is identified and is proposed to explain the similar low hole Schottky barriers reported in a recent device study utilizing MoO_<i>x</i> contacts on MoS₂ and WSe₂

Al₂O₃ on Black Phosphorus by Atomic Layer Deposition: An <i>in Situ</i> Interface Study

<i>In situ</i> “half cycle” atomic layer deposition (ALD) of Al₂O₃ was carried out on black phosphorus (“black-P”) surfaces with modified phosphorus oxide concentrations. X-ray photoelectron spectroscopy is employed to investigate the interfacial chemistry and the nucleation of the Al₂O₃ on black-P surfaces. This work suggests that exposing a sample that is initially free of phosphorus oxide to the ALD precursors does not result in detectable oxidation. However, when the phosphorus oxide is formed on the surface prior to deposition, the black-P can react with both the surface adventitious oxygen contamination and the H₂O precursor at a deposition temperature of 200 °C. As a result, the concentration of the phosphorus oxide increases after both annealing and the atomic layer deposition process. The nucleation rate of Al₂O₃ on black-P is correlated with the amount of oxygen on samples prior to the deposition. The growth of Al₂O₃ follows a “substrate inhibited growth” behavior where an incubation period is required. <i>Ex situ</i> atomic force microscopy is also used to investigate the deposited Al₂O₃ morphologies on black-P where the Al₂O₃ tends to form islands on the exfoliated black-P samples. Therefore, surface functionalization may be needed to get a conformal coverage of Al₂O₃ on the phosphorus oxide free samples

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 <i>in situ</i> 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

HfO₂ on MoS₂ by Atomic Layer Deposition: Adsorption Mechanisms and Thickness Scalability

We report our investigation of the atomic layer deposition (ALD) of HfO₂ on the MoS₂ surface. In contrast to previous reports of conformal growth on MoS₂ flakes, we find that ALD on MoS₂ bulk material is not uniform. No covalent bonding between the HfO₂ and MoS₂ is detected. We highlight that individual precursors do not permanently adsorb on the clean MoS₂ surface but that organic and solvent residues can dramatically change ALD nucleation behavior. We then posit that prior reports of conformal ALD deposition on MoS₂ flakes that had been exposed to such organics and solvents likely rely on contamination-mediated nucleation. These results highlight that surface functionalization will be required before controllable and low defect density high-κ/MoS₂ interfaces will be realized. The band structure of the HfO₂/MoS₂ system is experimentally derived with valence and conduction band offsets found to be 2.67 and 2.09 eV, respectively

MoS₂ P‑type Transistors and Diodes Enabled by High Work Function MoO_<i>x</i> 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 (MoO_<i>x</i>, <i>x</i> < 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 MoO_<i>x</i> contacts. We also show drastic on-current improvement for p-type WSe₂ FETs with MoO_<i>x</i> 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 MoO_<i>x</i> Contact for Silicon Solar Cells

Using an ultrathin (∼15 nm in thickness) molybdenum oxide (MoO_<i>x</i>, <i>x</i> < 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 MoO_<i>x</i> is commonly considered to be a semiconductor with a band gap of 3.3 eV, from X-ray photoelectron spectroscopy we show that MoO_<i>x</i> 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 <i>dopant-free</i> 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