Xiaoye Qin

MoS2 functionalization for ultra-thin atomic layer deposited dielectrics

The effect of room temperature ultraviolet-ozone (UV-O3) exposure of MoS2 on the uniformity of subsequent atomic layer deposition of Al2O3 is investigated. It is found that a UV-O3 pre-treatment removes adsorbed carbon contamination from the MoS2 surface and also functionalizes the MoS2 surface through the formation of a weak sulfur-oxygen bond without any evidence of molybdenum-sulfur bond disruption. This is supported by first principles density functional theory calculations which show that oxygen bonded to a surface sulfur atom while the sulfur is simultaneously back-bonded to three molybdenum atoms is a thermodynamically favorable configuration. The adsorbed oxygen increases the reactivity of MoS2 surface and provides nucleation sites for atomic layer deposition of Al2O3. The enhanced nucleation is found to be dependent on the thin film deposition temperature.

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.

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 side-contact model, that is, metal slabs sitting on a pristine graphene sheet. Using in situ 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 end-contact geometry between the metals and the periphery of the remaining graphene patches. This work validates the spontaneous 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-k 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.

HfO2 on UV–O3 exposed transition metal dichalcogenides: interfacial reactions study

The surface chemistry of MoS2, WSe2 and MoSe2 upon ultraviolet (UV)?O3 exposure was studied in situ by x-ray photoelectron spectroscopy (XPS). Differences in reactivity of these transition metal dichalcogenides (TMDs) towards oxidation during UV?O3 were observed and correlated with density functional theory calculations. Also, sequential HfO2 depositions were performed by atomic layer deposition (ALD) while the interfacial reactions were monitored by XPS. It is found that the surface oxides generated on MoSe2 and WSe2 during UV?O3 exposure were reduced by the ALD process (?self-cleaning effect?). The effectiveness of the oxide reduction on these TMDs is discussed and correlated with the HfO2 film uniformity.

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 MoOx 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.

In situ atomic layer deposition half cycle study of Al2O3 growth on AlGaN

The atomic layer deposition (ALD) of Al2O3 on the native oxide and hydrofluoric acid treated Al0.25Ga0.75 N surface was studied using in situ X-ray photoelectron spectroscopy (XPS), after each individual “half cycle” of the ALD process. Initially, Al2O3, Ga2O3, and N-O states were detected on both surfaces at differing concentrations. During the course of the deposition process, the N-O bonds are seen to decrease to within XPS detection limits, as well as a small decrease in the Ga2O3 concentration. The Al2O3 growth rate initially is seen to be very low, indication of low reactivity between the trimethyl-aluminum molecule and the AlGaN surface.

Impact of N2 and forming gas plasma exposure on the growth and interfacial characteristics of Al2O3 on AlGaN

The interface and atomic layer deposition (ALD) of Al2O3 on the annealed, N2 plasma and forming gas (N2:H2) exposed Al0.25Ga0.75N surface was studied using in situ X-ray photoelectron spectroscopy and low energy ion scattering spectroscopy. Exposure of the Al0.25Ga0.75N surface to the plasma treatments is able to remove spurious carbon, and readily facilitate uniform ALD Al2O3 nucleation.

Indium diffusion through high-k dielectrics in high-k/InP stacks

Evidence of indium diffusion through high-k dielectric (Al2O3 and HfO2) films grown on InP (100) by atomic layer deposition is observed by angle resolved X-ray photoelectron spectroscopy and low energy ion scattering spectroscopy. The analysis establishes that In-out diffusion occurs and results in the formation of a POx rich interface.

A crystalline oxide passivation for Al2O3/AlGaN/GaN

In situ X-ray photoelectron spectroscopy and low energy electron diffraction are performed to study the formation of a crystalline oxide on the AlGaN surface. The oxidation of the AlGaN surface is prepared by annealing and remote N2 + O2 plasma pretreatments resulting in a stable crystalline oxide. The impact of the oxide on the interface state density is studied by capacitance voltage (C-V) measurements. It is found that a remote plasma exposure at 550 °C shows the smallest frequency dispersion. Crystalline oxide formation may provide a novel passivation method for high quality AlGaN/GaN devices.