Contact Resistance Engineering in WS2-Based FET with MoS2 Under-Contact Interlayer: A Statistical Approach

One of the primary factors hindering the development of 2D material-based devices is the difficulty of overcoming fabrication processes, which pose a challenge in achieving low-resistance contacts. Widely used metal deposition methods lead to unfavorable Fermi level pinning effect (FLP), which prevents control over the Schottky barrier height at the metal/2D material junction. We propose to harness the FLP effect to lower contact resistance in field-effect transistors (FETs) by using an additional 2D interlayer at the conducting channel and metallic contact interface (under-contact interlayer). To do so, we developed a new approach using the gold-assisted transfer method, which enables the fabrication of heterostructures consisting of TMDs monolayers with complex shapes, prepatterned using e-beam lithography, with lateral dimensions even down to 100 nm. We designed and demonstrated tungsten disulfide (WS2) monolayer-based devices in which the molybdenum disulfide (MoS2) monolayer is placed only in the contact area of the FET, creating an Au/MoS2/WS2 junction, which effectively reduces contact resistance by over 60% and improves the Ion/Ioff ratio 10 times in comparison to WS2-based devices without MoS2 under-contact interlayer. The enhancement in the device operation arises from the FLP effect occurring only at the interface between the metal and the first layer of the MoS2/WS2 heterostructure. This results in favorable band alignment, which enhances the current flow through the junction. To ensure the reproducibility of our devices, we systematically analyzed 160 FET devices fabricated with under-contact interlayer and without it. Statistical analysis shows a consistent improvement in the operation of the device and reveals the impact of contact resistance on key FET performance indicators.

determined by fitting Lorentzian functions to the data.The difference in peak positions was 18 cm -1 and 59 cm -1 , respectively, for the MoS2 and WS2 spectra, confirming that these are monolayers 1,2 .In the MoS2/WS2 heterostructure spectrum, the peaks corresponding to the component monolayers are marked with arrows.c) Photoluminescence spectra measured in the areas marked by dots in Figure S2a.MoS2 spectrum exhibits A and B exciton peaks, and the WS2 spectrum only exhibits A exciton peak, which is typical for the monolayer form of those materials 3,4 .In the MoS2/WS2 heterostructure photoluminescence spectrum, the peaks corresponding to the component monolayers are marked with arrows.All spectra were acquired with a 532 nm laser.

Supporting Information 2. XPS analysis of MoS2 and WS2 monolayers fabricated with goldassisted exfoliation
The XPS analysis was performed on four samples: a monolayer of MoS2 prepared without an annealing step, a monolayer of MoS2 prepared with an annealing step, and, similarly, on two samples with WS2 monolayer.For all samples, the SiO2/Si substrate was used.The presence of the Au 4f doublet line in the measured spectra was below the detection limit estimated in this case for 0.01% of atom concentration in the surface region.The estimation was carried out using the Si 2p line (close to 103 eV) and noise measured at 86 eV, where gold 4f peaks should occur.The Au 4f 7/2 and Au 4f 5/2 were incorporated (see insets in Figure S4), so that their intensity exceeded the level of the measured background.The surface area of these hypothetical peaks is more than 3000 times smaller than the surface area of the visible Si 2p line (for all investigated samples).Including in the calculation the relative sensitivity factor value for Au:Si, which is 17.1:0.8,and the fact that the measured atomic concentration of silicon in the surface layer was approx.
30%, which gives us the mentioned 0.01% concentration of maximal hypothetical gold atoms.

Supporting Information 3. Extraction of contact resistance and sheet resistance using the TLM method
On Figure S5a we present Rt distribution for each of 160 devices calculated for carrier concentration equal to 2×10 12 cm -2 with Rt mean values.In order to extract Rc for both type of devices we follow specific steps listed below: 1.
Determine the total resistance of each device in a single TLM set for various carrier concentrations.

2.
Fit equation S1 to the data for a single TLM set for each carrier concentration separately.

3.
Extract Rc from the intercept of the fitted equation.

5.
Calculate the mean value and standard deviation of Rc for each carrier concentration.
The showcase of all Rc and Rsh values extracted from measurements of 13 TLM structures with MoS2 UCI and 13 TLM structures without UCI is presented in Figures S5b and S5c Rt was calculated using Vds of 1V (at which transfer characteristics were measured) divided by Ids data from transfer characteristics at linear operation regime for different overdrive voltages (Vg-Vth).
Carrier concentration ns was calculated from overdrive voltage: Cox is a gate oxide capacitance calculated from the equation S3: εr is SiO2 relative permittivity (3.9) and dSiO2 is SiO2 thickness (285 nm).

Figure S2
Figure S2 Raman and photoluminescence measurements of MoS2/WS2 heterostructure and component

Figure S4
Figure S4 The XPS spectra of a) the MoS2 sample prepared without annealing, b) the MoS2 sample . Values were calculated based on the linear fitting of equation S1 to total resistance as a function of various channel lengths for each TLM set separately:   () = 2  +  ℎ   (S 1)

Figure
Figure S5 a) Rt distribution of 160 devices calculated for ns= 2×10 12 cm -2 with mean values