Remote Plasma Oxidation and Atomic Layer Etching of MoS₂

Exfoliated molybdenum disulfide (MoS₂) is shown to chemically oxidize in a layered manner upon exposure to a remote O₂ plasma. X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), and atomic force microscopy (AFM) are employed to characterize the surface chemistry, structure, and topography of the oxidation process and indicate that the oxidation mainly occurs on the topmost layer without altering the chemical composition of underlying layer. The formation of S–O bonds upon short, remote plasma exposure pins the surface Fermi level to the conduction band edge, while the MoO_<i>x</i> formation at high temperature modulates the Fermi level toward the valence band through band alignment. A uniform coverage of monolayer amorphous MoO₃ is obtained after 5 min or longer remote O₂ plasma exposure at 200 °C, and the MoO₃ can be completely removed by annealing at 500 °C, leaving a clean ordered MoS₂ lattice structure as verified by XPS, LEED, AFM, and scanning tunneling microscopy. This work shows that a remote O₂ plasma can be useful for both surface functionalization and a controlled thinning method for MoS₂ device fabrication processes

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

Atomic Layer Deposition of a High‑<i>k</i> Dielectric on MoS₂ Using Trimethylaluminum and Ozone

We present an Al₂O₃ dielectric layer on molybdenum disulfide (MoS₂), deposited using atomic layer deposition (ALD) with ozone/trimethylaluminum (TMA) and water/TMA as precursors. The results of atomic force microscopy and low-energy ion scattering spectroscopy show that using TMA and ozone as precursors leads to the formation of uniform Al₂O₃ layers, in contrast to the incomplete coverage we observe when using TMA/H₂O as precursors. Our Raman and X-ray photoelectron spectroscopy measurements indicate minimal variations in the MoS₂ structure after ozone treatment at 200 °C, suggesting its excellent chemical resistance to ozone

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

Partially Fluorinated Graphene: Structural and Electrical Characterization

Despite the number of existing studies that showcase the promising application of fluorinated graphene in nanoelectronics, the impact of the fluorine bonding nature on the relevant electrical behaviors of graphene devices, especially at low fluorine content, remains to be experimentally explored. Using CF₄ as the fluorinating agent, we studied the gradual structural evolution of chemical vapor deposition graphene fluorinated by CF₄ plasma at a working pressure of 700 mTorr using Raman and X-ray photoelectron spectroscopy (XPS). After 10 s of fluorination, our XPS analysis revealed a co-presence of covalently and ionically bonded fluorine components; the latter has been determined being a dominant contribution to the observation of two Dirac points in the relevant electrical measurement using graphene field effect transistor devices. Additionally, this ionic C–F component (ionic bonding characteristic charge sharing) is found to be present only at low fluorine content; continuous fluorination led to a complete transition to a covalently bonded C–F structure and a dramatic increase of graphene sheet resistance. Owing to the formation of these various C–F bonding components, our temperature-dependent Raman mapping studies show an inhomogeneous defluorination from annealing temperatures starting at ∼150 °C for low fluorine coverage, whereas fully fluorinated graphene is thermally stable up to ∼300 °C

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