Abstract:

Two-dimensional transition metal disulfides (TMDs) have recently attracted significant research attention due to their rich physical and chemical properties. Graphene has also been studied intensively due to its high electron mobility of ~200000 cm²·V⁻¹·s⁻¹. Since there is no band gap, it is difficult for a graphene-based device to achieve high current on/off ratio. For TMDs, such as MoS₂, MoSe₂, WSe₂, and WS₂, the band gaps of these materials can be adjusted according to the number of layers. Since TMD has the advantage of suppressing source-drain tunneling current in an ultra-short transistor and offering superior immunity to short-channel effects, it is also attractive for use as a channel material in Si complementary metal oxide semiconductor (CMOS) devices larger than 22 nm. Among them, MoS₂ in single-layer and multi-layer films have been intensively researched for many years. MoS₂-based field effect transistors (FETs) with excellent electrical properties have been reported. WS₂ has lower in-plane electronic mass than MoS₂, MoSe₂, and MoTe₂, and therefore has potential for higher carrier mobility or higher output current for WS₂-based FETs. Experimental research on WS₂ is limited compared to MoS₂, and more work is needed to further exploit the full potential of WS₂-based FETs. Therefore, the electron-phonon interaction and vibration properties of WS₂ used in nano-electronic applications and FETs must be investigated. To this end, mono-layer (1L), few-layer (FL), and bulk WS₂ films were prepared using mechanical exfoliation from a WS₂ crystal. 3M scotch-tape was used for transferring the WS₂ films. Detailed temperature-dependent Raman study on 1L, FL, and bulk WS₂ films has been conducted using a 514-nm excitation laser. Raman spectroscopy, as an effective and non-destructive approach for phonon vibration study, has been used to evaluate TMDs. The Raman spectra reveal much useful information on the test sample in terms of peak position and spectral shape change. With the film thickness increasing to bulk, the A_1g(Γ) and E_2g¹(Γ) modes show blue-shift and red-shift, respectively, with respect to 1L WS₂. Moreover, when the dominant Raman vibration modes swaps between E_2g¹(Γ) and A_1g(Γ), the "cross-over" temperature was identified for 1L, FL, and bulk WS₂ films. WS₂ shows smaller frequency change Δ between the E_2g¹(Γ) and A_1g(Γ) modes than MoS₂, with varying film thickness. The temperature coefficient of the Raman peak position was one magnitude lower for WS₂ than MoS₂, implying that WS₂ has better thermal stability than MoS₂. The results of this systematic study provide a physical guidance for WS₂-based device design.

Key words: WS₂, Film thickness, Raman, Photoluminescence, Vibration mode