92 research outputs found
Using photoelectron spectroscopy in the integration of 2D materials for advanced devices
No full textDue to copyright restrictions and/or publisher's policy full text access from Treasures at UT Dallas is limited to current UTD affiliates (use the provided Link to Article).The first commercial applications of two dimensional (2D) layered materials such as graphite and MoS₂ used their lubricant properties. Following the discovery of graphene and its potential applications in various fields, increased interest has focused on other 2D materials such as transition metal dichalcogenides (TMDs) offering tremendous opportunities in advanced optoelectronics and ultra-thin electronics. Using X-ray photoelectron spectroscopy (XPS), this review addresses the facets of the device fabrication and integration and correlates at the nanometer scale the device behavior to the TMD properties. Understanding and solving the integration challenges will make the TMD technology jump from the current phase of experimental proof of concept and laboratory research to a relevant prototype demonstration and production phase.National Science Foundation (NSF) award no. 1407765Erik Jonsson School of Engineering and Computer Scienc
High-κ Dielectric on ReS2: In-Situ Thermal Versus Plasma-Enhanced Atomic Layer Deposition of Al2O3
No full textWe report an excellent growth behavior of a high-κ dielectric on ReS2, a two-dimensional (2D) transition metal dichalcogenide (TMD). The atomic layer deposition (ALD) of an Al2O3 thin film on the UV-Ozone pretreated surface of ReS2 yields a pinhole free and conformal growth. In-situ half-cycle X-ray photoelectron spectroscopy (XPS) was used to monitor the interfacial chemistry and ex-situ atomic force microscopy (AFM) was used to evaluate the surface morphology. A significant enhancement in the uniformity of the Al2O3 thin film was deposited via plasma-enhanced atomic layer deposition (PEALD), while pinhole free Al2O3 was achieved using a UV-Ozone pretreatment. The ReS2 substrate stays intact during all different experiments and processes without any formation of the Re oxide. This work demonstrates that a combination of the ALD process and the formation of weak S–O bonds presents an effective route for a uniform and conformal high-κ dielectric for advanced devices based on 2D materials
Covalent nitrogen doping in molecular beam epitaxy-grown and bulk WSe2
No full textCovalent p-type doping of WSe2 thin films grown by molecular beam epitaxy and WSe2 exfoliated from bulk crystals is achieved via remote nitrogen plasma exposure. X-ray photoelectron and Raman spectroscopies indicate covalently bonded nitrogen in the WSe2 lattice as well as tunable nitrogen concentration with N2 plasma exposure time. Furthermore, nitrogen incorporation induces compressive strain on the WSe2 lattice after N2 plasma exposure. Finally, atomic force microscopy and scanning tunneling microscopy reveal that N2 plasma treatment needs to be carefully tuned to avoid any unwanted strain or surface damage
Enhancing Interconnect Reliability and Performance by Converting Tantalum to 2D Layered Tantalum Sulfide at Low Temperature
No full textSupplementary material is available on publisher's website. Use the DOI link below.Due to copyright restrictions and/or publisher's policy full text access from Treasures at UT Dallas is limited to current UTD affiliates (use the provided Link to Article).The interconnect half-pitch size will reach ≈20 nm in the coming sub-5 nm technology node. Meanwhile, the TaN/Ta (barrier/liner) bilayer stack has to be >4 nm to ensure acceptable liner and diffusion barrier properties. Since TaN/Ta occupy a significant portion of the interconnect cross-section and they are much more resistive than Cu, the effective conductance of an ultrascaled interconnect will be compromised by the thick bilayer. Therefore, 2D layered materials have been explored as diffusion barrier alternatives. However, many of the proposed 2D barriers are prepared at too high temperatures to be compatible with the back-end-of-line (BEOL) technology. In addition, as important as the diffusion barrier properties, the liner properties of 2D materials must be evaluated, which has not yet been pursued. Here, a 2D layered tantalum sulfide (TaSₓ) with ≈1.5 nm thickness is developed to replace the conventional TaN/Ta bilayer. The TaSx ultrathin film is industry-friendly, BEOL-compatible, and can be directly prepared on dielectrics. The results show superior barrier/liner properties of TaSₓ compared to the TaN/Ta bilayer. This single-stack material, serving as both a liner and a barrier, will enable continued scaling of interconnects beyond 5 nm node. ©2019 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimNIST through award number 70NANB17H041; NSF (Grant No. CCF‐1619062); Welch Foundation (Grant No. F‐1959‐20180324)Erik Jonsson School of Engineering and Computer Scienc
Complexe metallic alloys surfaces : structure, properties and nanostructuration
No full textCe travail a permis de déterminer les structures atomique et électronique de deux surfaces d'alliages métalliques complexes à l'aide d'une approche multi-techniques supportée par des calculs ab intio de structure électronique. Les surfaces de ces cristaux ont pu être corrélées à certains plans présents dans les modèles structuraux disponibles. La terminaison dominante de la surface (100) d'Al13Co4 est identifiée comme un plan corrugué incomplet du volume. La surface (010) de Al3(Mn, Pd) présente un nombre conséquent d'imperfections structurales. À l'exception de certains sites lacunaires, elle est identique au plan corrugué complet du massif. Dans une seconde étape, ces échantillons ont été utilisés comme substrat pour la croissance de films minces métalliques. Les atomes de Pb déposés sur ces deux surfaces suivent un mode de croissance pseudomorphique jusqu'à la formation de la monocouche. Dans le cas de l'Al13Co4, le coefficient de collage du Pb s'annule une fois cette monocouche formée. Sur la surface Al3(Mn, Pd), le croissance de couches additionnelles de Pb est observée. L'adsorption du Cu sur Al13Co4 mène de nouveau à un mode de croissance pseudomorphique jusqu'à la monocouche. Au-delà de ce dépôt, la phase ß-Al(Cu, Co) apparaît en surface. Pour des dépôts à des températures plus élevées, la phase- ß est suivie par la formation de la phase ?-Al4Cu9. Les phases ß et ? croissent suivant deux domaines (110) orientés l'un par rapport à l'autre avec un angle de 72°We report the investigation of pseudo-ten-fold surfaces on two complex metallic alloys considered as approximants to the decagonal quasicristal. The atomic and electronic structure of the both samples is investigated by means of a multi-technique approach supported by ab initio electronic structure calculations. The main termination of the (100) surface of Al13Co4 is attributed to an incomplete puckered layer. The (010) surface of Al3(Mn, Pd) exhibits an important amount of structural imperfections. With the exception of several vacancies, this surface is identical to the complete puckered layer. In a second stage, both surfaces have been used as templates for the growth of metallic thin films. On both surfaces, Pb adatoms adopt a pseudomorphic growth mode up to one monolayer. For the Al13Co4 surface, the sticking coefficient of Pb vanishes upon the completion of the monolayer. However, it remains sufficient for the growth of additional layers on the Al3(Mn, Pd) (010) surface. The adsorption of Cu on the Al13Co4 surface follows also a pseudomorphic growth mode up to one monolayer. The ß-Al(Cu, Co) phase appears for coverages greater than one monolayer. For higher temperature deposition, the ß-phase is followed by the formation of the ?-Al4Cu9 phase. Both ß and ? phases grow as two (110) domains rotated by 72° from each othe
Surfaces d'alliages métalliques complexes : structure, propriétés et nanostructuration
No full textWe report the investigation of pseudo-ten-fold surfaces on two complex metallic alloys considered as approximants to the decagonal quasicristal. The atomic and electronic structure of the both samples is investigated by means of a multi-technique approach supported by ab initio electronic structure calculations. The main termination of the (100) surface of Al13Co4 is attributed to an incomplete puckered layer. The (010) surface of Al3(Mn, Pd) exhibits an important amount of structural imperfections. With the exception of several vacancies, this surface is identical to the complete puckered layer. In a second stage, both surfaces have been used as templates for the growth of metallic thin films. On both surfaces, Pb adatoms adopt a pseudomorphic growth mode up to one monolayer. For the Al13Co4 surface, the sticking coefficient of Pb vanishes upon the completion of the monolayer. However, it remains sufficient for the growth of additional layers on the Al3(Mn, Pd) (010) surface. The adsorption of Cu on the Al13Co4 surface follows also a pseudomorphic growth mode up to one monolayer. The ß-Al(Cu, Co) phase appears for coverages greater than one monolayer. For higher temperature deposition, the b-phase is followed by the formation of the g-Al4Cu9 phase. Both b and g phases grow as two (110) domains rotated by 72° from each otherCe travail a permis de déterminer les structures atomique et électronique de deux surfaces d'alliages métalliques complexes à l'aide d'une approche multi-techniques supportée par des calculs ab intio de structure électronique. Les surfaces de ces cristaux ont pu être corrélées à certains plans présents dans les modèles structuraux disponibles. La terminaison dominante de la surface (100) d'Al13Co4 est identifiée comme un plan corrugué incomplet du volume. La surface (010) de Al3(Mn, Pd) présente un nombre conséquent d'imperfections structurales. À l'exception de certains sites lacunaires, elle est identique au plan corrugué complet du massif. Dans une seconde étape, ces échantillons ont été utilisés comme substrat pour la croissance de films minces métalliques. Les atomes de Pb déposés sur ces deux surfaces suivent un mode de croissance pseudomorphique jusqu'à la formation de la monocouche. Dans le cas de l'Al13Co4, le coefficient de collage du Pb s'annule une fois cette monocouche formée. Sur la surface Al3(Mn, Pd), le croissance de couches additionnelles de Pb est observée. L'adsorption du Cu sur Al13Co4 mène de nouveau à un mode de croissance pseudomorphique jusqu'à la monocouche. Au-delà de ce dépôt, la phase ß-Al(Cu, Co) apparaît en surface. Pour des dépôts à des températures plus élevées, la phase- ß est suivie par la formation de la phase g-Al4Cu9. Les phases b et g croissent suivant deux domaines (110) orientés l'un par rapport à l'autre avec un angle de 72
Tuning Electronic Transport in Epitaxial Graphene-Based Van Der Waals Heterostructures
No full textIncludes supplementary materialTwo-dimensional tungsten diselenide (WSe₂) has been used as a component in atomically thin photovoltaic devices, field effect transistors, and tunneling diodes in tandem with graphene. In some applications it is necessary to achieve efficient charge transport across the interface of layered WSe₂-graphene, a semiconductor to semimetal junction with a van der Waals (vdW) gap. In such cases, band alignment engineering is required to ensure a low-resistance, ohmic contact. In this work, we investigate the impact of graphene electronic properties on the transport at the WSe₂-graphene interface. Electrical transport measurements reveal a lower resistance between WSe₂ and fully hydrogenated epitaxial graphene (EGFH) compared to WSe₂ grown on partially hydrogenated epitaxial graphene (EGPH). Using low-energy electron microscopy and reflectivity on these samples, we extract the work function difference between the WSe₂ and graphene and employ a charge transfer model to determine the WSe₂ carrier density in both cases. The results indicate that WSe₂-EGFH displays ohmic behavior at small biases due to a large hole density in the WSe₂, whereas WSe₂-EGPH forms a Schottky barrier junction.
(Invited) Integration of 2D Materials for Advanced Devices: Challenges and Opportunities
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Investigation of MoS2 with Ambient Pressure X-ray Photoelectron Spectroscopy
No full textMolybdenum disulfide (MoS2) has potential applications as a low-cost catalyst for the hydrogen evolution reaction (HER). Defects on MoS2, such as edge sites and sulfur vacancies, are known to be the major active sites for HER. Controlling the formation of these defects allows for the enhancement in reactivity of MoS2 and other 2D materials. In this study, we have characterized the surface reactivity of bulk MoS2 samples using ambient pressure X-ray photoelectron spectroscopy (APXPS). Samples were exposed to 1 mbar of H2O vapor at temperatures ranging from 300-600 K. The APXPS Mo 3d, S 2p, and O 1s core levels for the as exfoliated surface showed no significant changes under all reaction temperatures due to the inert nature of the pristine MoS2 surface. To improve surface reactivity and activate the basal plane of MoS2 we have utilized Ar+ sputtering to form well controlled defect densities at the surface. The APXPS Mo 3d, S 2p, and O 1s core levels for the defective MoSx (x = 1.6, 1.2) surface showed the formation of MoO3 and MoOS at temperatures of 400 K and above. We have found that surface reactivity of MoS2 is strongly dependent on temperature and defect densities
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