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Contact resistance (R ) is a major limiting factor in the performance of graphene devices. R is sensitive to the quality of the interface and the composition of the contact, which are affected by the graphene transfer process and contact deposition conditions. In this work, a linear correlation is observed between the composition of Ti contacts, characterized by x-ray photoelectron spectroscopy, and the Ti/graphene contact resistance measured by the transfer length method. We find that contact composition is tunable via deposition rate and base pressure. Reactor base pressure is found to effect the resultant contact resistance. The effect of contact deposition conditions on thermal transport measured by time-domain thermoreflectance is also reported. Interfaces with higher oxide composition appear to result in a lower thermal boundary conductance. Possible origins of this thermal boundary conductance change with oxide composition are discussed.

We experimentally demonstrate the role of oxygen stoichiometry on the thermal boundary conductance across Au/TiO x /substrate interfaces. By evaporating two different sets of Au/TiO x / substrate samples under both high vacuum and ultrahigh vacuum conditions, we vary the oxygen composition in the TiO x layer from 0 x 2.85. We measure the thermal boundary conductance across the Au/TiO x /substrate interfaces with time-domain thermoreflectance and characterize the interfacial chemistry with x-ray photoemission spectroscopy. Under high vacuum conditions, we speculate that the environment provides a sufficient flux of oxidizing species to the sample surface such that one essentially co-deposits Ti and these oxidizing species. We show that slower deposition rates correspond to a higher oxygen content in the TiO x layer, which results in a lower thermal boundary conductance across the Au/TiO x /substrate interfacial region. Under the ultrahigh vacuum evaporation conditions, pure metallic Ti is deposited on the substrate surface. In the case of quartz substrates, the metallic Ti reacts with the substrate and getters oxygen, leading to a TiO x layer. Our results suggest that Ti layers with relatively low oxygen compositions are best suited to maximize the thermal boundary conductance.

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WSe2 has demonstrated potential for applications in thermoelectric energy conversion. Optimization of such devices requires control over interfacial thermal and electrical transport properties. Ti, TiOx, and Ti/TiOx contacts to the MBE-grown WSe2 are characterized by XPS and transport measurements. The deposition of Ti is found to result in W-Se bond scission yielding metallic W and Ti-Se chemical states. The deposition of Ti on WSe2 in the presence of a partial pressure of O2, which yields a TiOx overlayer, results in the formation of substoichiometric WSex (x < 2) as well as WOx. The thermal boundary conductance at Ti/WSe2 contacts is found to be reduced for greater WSe2 film thickness or when Au/TiOx interface is present at the contact. Electrical resistance of Au/Ti contacts is found to be higher than that of Au/TiOx contacts with no significant difference in the Seebeck coefficient between the two types of contact structures. This report documents the first experimental study of Ti/WSe2 interface chemistry and thermoelectric properties.

Using an in-vacuo deposition/characterization tool, we study the 2D materials synthesized by molecular beam epitaxy and the interfaces formed between layered materials and in-vacuo deposited metals. The metal-2D interface is probed with x-ray photoelectron spectroscopy. Full details of sample preparation and transition metal dichalcogenide synthesis are provided. Furthermore a detailed study of in-vacuo deposited Ti on graphene, shows that while there is clear evidence of Ti carbide formation, there is no conclusive evidence of reactions with the graphene layer. Instead the carbide forms from a combination of adventitious carbon on the graphene surface, and carbon added via the deposition process.

Thermal annealing of Ti contacts is commonly implemented in the fabrication of MoS2 devices; however, its effects on interface chemistry have not been previously reported in the literature. In this work, the thermal stability of titanium contacts deposited on geological bulk single crystals of MoS2 in ultrahigh vacuum (UHV) is investigated with X-ray photoelectron spectroscopy and scanning transmission electron microscopy (STEM). In the as-deposited condition, the reaction of Ti with MoS2 is observed resulting in a diffuse interface between the two materials that comprises metallic molybdenum and titanium sulfide compounds. Annealing Ti/MoS2 sequentially at 100, 300, and 600 °C for 30 min in UHV results in a gradual increase in the reaction products as measured by XPS. Accordingly, STEM reveals the formation of a new ordered phase and a Mo-rich layer at the interface following heating. Due to the high degree of reactivity, the Ti/MoS2 interface is not thermally stable even at a transistor operating temperature of 100 °C, while post-deposition annealing further enhances the interfacial reactions. These findings have important consequences for electrical transport properties, highlighting the importance of interface chemistry in the metal contact design and fabrication.

The deposition of a thin oxide layer at metal/semiconductor interfaces has been previously reported as a means of reducing contact resistance in 2D electronics. Using X-ray photoelectron spectroscopy with in-situ Ti deposition, we fabricate Au/Ti/TiO x /MoS 2 samples as well as Au/Ti/MoS 2 and Au/TiO x /MoS 2 for comparison. Elemental titanium reacts strongly with MoS 2 whereas no interface reactions are observed in the two types of samples containing TiO x /MoS 2 interfaces. Using time domain thermoreflectance for the measurement of thermal boundary conductance, we find that samples contacted with Ti and a thin TiO x layer at the interface (≤1.5 nm) exhibit the same behavior as samples contacted solely with pure Ti. The Au/TiO x /MoS 2 samples exhibit ~20% lower thermal boundary conductance, despite having the same MoS 2 interface chemistry as the samples with thin oxide at the Ti/MoS 2 interface. We identify the mechanism for this phenomenon, attributing it to the different interfaces with the top Au contact. Our work demonstrates that the use of thin interfacial oxide layers to reduce electrical contact resistance does not compromise heat flow in 2D electronic devices. We note that the thicknesses of the Ti and TiO x layers must be considered for optimal thermal transport.