Enhanced Interlayer Charge Injection Efficiency in 2D Multilayer ReS2 via Vertical Double-Side Contacts.

Two-dimensional (2D) van der Waals (vdW) layered materials have provided novel opportunities to explore interesting physical properties such as thickness-dependent bandgap, moiré excitons, superconductivity, and superfluidity. However, the presence of interlayer resistance along the thickness and Schottky barrier in metal-to-2D vdW semiconducting materials causes a limited interlayer charge injection efficiency, perturbing various intrinsic properties of 2D vdW multilayers. Herein, we report a simple but powerful contact electrode design to enhance interlayer carrier injection efficiency along the thickness by constructing vertical double-side contact (VDC) electrodes. A 2-fold extended contact area of VDC not only strongly limits an interlayer resistance contribution to the field-effect mobility and current density at the metal-to-2D semiconductor interface but also significantly suppresses both current transfer length (≤1 µm) and specific contact resistivity (≤1 mΩ·cm2), manifesting clear benefits of VDC in comparison with those in conventional top-contact and bottom-contact configurations. Our layout for contact electrode configuration may suggest an advanced electronic device platform for high-performing 2D optoelectronic devices.

Negative Differential Interlayer Resistance in WSe2 Multilayers via Conducting Channel Migration with Vertical Double-Side Contacts.

The inherent interlayer resistance in two-dimensional (2D) van der Waals (vdW) multilayers is expected to significantly influence the carrier density profile along the thickness, provoking spatial modification and separation of the conducting channel inside the multilayers, in conjunction with the thickness-dependent carrier mobility. However, the effect of the interlayer resistance on the variation in the carrier density profile and its direction along the thickness under different electrostatic bias conditions has been elusive. Here, we reveal the presence of a negative differential interlayer resistance (NDIR) in WSe2 multilayers by considering various contact electrode configurations: (i) bottom contact, (ii) top contact, and (iii) vertical double-side contact (VDC). The contact-structure-dependent shape modification of the transconductance clearly manifests the redistribution of carrier density and indicates the direction of the conducting channel migration along the thickness. Furthermore, the distinct characteristic of the electrically tunable NDIR in 2D WSe2 multilayers is revealed by the observed discrepancy between the top- and bottom-channel resistances determined by four-probe measurements with VDC. Our results provide an optimized device layout and further insights into the distinct carrier transport mechanism in 2D vdW multilayers.