Scanning tunneling spectroscopy investigation of van der Waals based metal/semiconductor interfaces

We study van der Waals (vdW) based heterostructures, combining graphene and few-layer semiconducting Transition Metal Dichalcogenides (TMDs) films, by using cryogenic scanning tunneling microscopy (STM) in ultra-high vacuum.... [ view full abstract ]

We study van der Waals (vdW) based heterostructures, combining graphene and few-layer semiconducting Transition Metal Dichalcogenides (TMDs) films, by using cryogenic scanning tunneling microscopy (STM) in ultra-high vacuum. The fabrication of vdW structures is done either by exfoliating and transferring WSe₂ flakes on epitaxial graphene (Gr) on SiC(0001), or by growing by molecular beam epitaxy (MBE) monolayer (1L) either MoSe₂ or WSe₂ flakes on similar substrates (Fig. 1a). We then locally probe the local density of states of the TMD flakes using scanning tunneling spectroscopy (STS). STS provides a direct access to the band onsets and the electronic band-gap of the TMD (Fig. 1b) [1,2].

A noteworthy result is presented in Fig.1b : The dI/dV spectra that we measure on 1L-MoSe₂ are strongly impacted by the number of graphene layers below the TMD flake. As shown in the figure, the spectra are rigidly shifted towards negative biases by 130 mV when going from bilayer (BLG) to single-layer graphene (SLG). This shift is observed either for 1L-MoSe₂ or 1L-WSe₂ flakes, and is still present with the same order of magnitude for3L-WSe₂. Additionally, this effect does not depend on the TMD preparation method. As it will be detailed in the contribution [3], we interpret our experimental data within the framework of semiconductor/metal (SC/M) interfaces, where graphene (either SLG or BLG) acts as the metal part of the junction (this is justified because SLG or BLG on SiC are both heavily n-doped). We will show that all our measurements reflect an unconventional behavior of the SC/M junction (namely the absence of Fermi level pinning), which was recently predicted for junctions made of two-dimensional materials coupled with vdW interactions [4]

1. M. Ugeda et al.,Nat.Mater.,13,(2014), 1091.

2. A. Bradley, et al., Nano.Lett.,15,(2015),2594.

3. T. Le Quang, et al., submitted to2D Mater.

4. Y. Liu et al., Sci.Adv.,2,(2016),e1600069.