Condensed phase diagrams for the metal–W–S systems and their relevance for contacts to WS2

Abstract

Motivated by interest in contacts to the layered semiconductor WS 2 , this study investigates condensed phase equilibria in M-W-S systems (M = transition or post-transition metal) and relates the findings to earlier work on M-Mo-W systems. Thermodynamic data were collected or estimated for the binary phases bearing M, W, and/or S; and a literature search for existing ternary phases was performed.Condensed phase M-W-S isothermal phase diagrams were calculated where sufficient data was available.These phase diagrams reflected three general forms. The early transition metals and group 13 posttransition metals were mostly reactive with WS 2 , with the resulting diagrams being either metal sulfide dominant or ternary/solid solution dominant. The late transition metals were all WS 2 dominant, where the metal is in equilibrium with WS 2 . These findings are similar to those from our earlier study on the M-Mo-S phase diagrams and can be used to guide studies on contacts to WS 2 .

Origins of Fermi Level Pinning between Tungsten Dichalcogenides (WS2, WTe2) and Bulk Metal Contacts: Interface Chemistry and Band Alignment

“…In general, WTe 2 is the most reactive tungsten dichalcogenide followed by WSe 2 and WS 2 . This work shows that the interface chemistry at a metal−TMD interface can be roughly predicted by considering the thermodynamics of the metal− TMD system and provides experimental support for previous thermodynamic calculations of equilibrium phases in metal− tungsten dichalcogenide systems 58 (provided the metal is deposited under a pressure that is <1 × 10 −8 mbar). The abundance of background gases in an elastomer-sealed deposition tool (base pressure ∼1 × 10 −6 mbar) complicates the reaction thermodynamics during metallization, which should be considered when evaluating the performance of more reactive contact metals.…”

Reactivity of contact metals on monolayer WS2

“…The persistence of multiple Raman active modes even after metallization and annealing up to 300 ºC and mechanical exfoliation means that Cu remains unreactive with WS2 at room temperature and with annealing. Ternary phase diagrams calculated by Zeng et al show equilibrium between bulk WS2 and Cu at room temperature 23 , and x-ray diffraction studies have indicated no reactivity between bulk WS2 and Cu below 850 o C 33 , consistent with our findings for 1L WS2.…”

“…into three reactivity groups: metal-sulfide dominant (reactive), WS2 dominant (unreactive), and ternary/solid solution dominant (Figure 1), 23 similar to the study by Domask et al on metal/MoS2 systems 24 . In our current study, reactions between metals and WS2 can be correlated with changes in the Raman spectrum upon metallization and annealing.…”

“…We concluded that the 1L WS2 layer was no longer present and consumed in reaction with Ti. While a first-principles computational study utilizing the local density approximation (LDA) 28 and a phase-diagram calculation 23 predict a reaction between Ti and WS2, here we experimentally observed a room-temperature reaction between a continuous Ti film and 1L WS2.…”

“…The Raman spectrum corresponding to the as-deposited Al/WS2/sapphire sample shows peaks for the in-plane and out-of-plane optical vibrational modes, as well as the first-and second-order longitudinal acoustic modes of WS2 (Figure 4). Aluminum is not predicted to be in thermodynamic equilibrium with bulk WS2 at room temperature 23 , but Al has been found to be unreactive with MoS2 at room temperature 9,22 . Although the normalized intensity of the WS2 Raman-active peaks decreases upon metallization, we can observe the LA(M), LA(M) + TA(M), E′′(M), E′(M), 2LA(M) and E′(Γ) modes at 174, 313, 330, 346, 352, and 357 cm -1 , respectively, at or very near the original peak positions.…”