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

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

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

Digermane Deposition on Si(100) and Ge(100): from Adsorption Mechanism to Epitaxial Growth

Controlled fabrication of nanometer-scale devices such as quantum dots and nanowires requires an understanding of the initial chemisorption mechanisms involved in epitaxial growth. Vapor phase epitaxy can provide controlled deposition when using precursors that are not reactive with the H-terminated surfaces at ambient temperatures. For instance, digermane (Ge₂H₆) has potential as such a precursor for Ge ALE on Si(100) surfaces at moderate temperatures; yet, its adsorption configuration and subsequent decomposition pathways are not well understood. In situ Fourier transform infrared spectroscopy and first principles calculations reveal that Ge₂H₆ chemisorbs through a β-hydride elimination mechanism, forming Ge₂H₅ and H on both Si(100)-(2 × 1) and Ge(100)-(2 × 1) surfaces, instead of the previously proposed Ge–Ge bond breaking mechanism, and subsequently decomposes into an ad-dimer. The resulting coverage of Ge after a saturation exposure is estimated to be about 0.3 monolayers. Interestingly, the decomposition of adsorbed Ge₂H₅ on Si(100) is faster than Si₂H₅ on Ge(100) at 173 K. The desorption temperature of hydrogen on Si(100) is shown to depend on the Ge coverage, falling from 698 K for ∼1/4 ML Ge on Si(100) to 573 K for a nearly full Ge coverage, consistent with H desorption on Ge(100). Furthermore, hydrogen is observed to migrate from Ge to Si, prior to desorption. This property opens the door for selective growth of Ge on patterned H-terminated Si surfaces