Impact of Synthesized MoS₂ Wafer-Scale Quality on Fermi Level Pinning in Vertical Schottky-Barrier Heterostructures

Abstract

Transition metal dichalcogenide (TMD)-based vertical Schottky heterostructures have recently shown promise as a next generation device for a variety of applications. In order for these devices to operate effectively, the interface between the TMD and metal contacts must be well-understood and optimized. In this work, the interface between synthesized MoS2 and gold or platinum metal contacts is explored as a function of MoS2 film quality to understand Fermi level pinning effects. Raman, X-ray photoelectron spectroscopy, and ultraviolet photoelectron spectroscopy are used to physically characterize both MoS2 and MoS2/metal interface. Metal/MoS2/metal purely vertical heterostructure cross-point devices were fabricated to explore the injection behavior across the Schottky barrier formed between MoS2 and the metal. The temperature dependence of the device behavior is used to understand injection mechanisms, and modeling is performed to verify the injection mechanisms across the interface barrier. By combining both physical characterization with electrical results and modeling, Fermi level pinning is investigated as a function of macroscopic MoS2 quality. Low-quality MoS2 was found to exhibit much stronger pinning than high-quality films, which is consistent with an observed increase in covalency of the metal/MoS2 interface. Additionally, MoS2 was found to pin gold much more strongly than platinum, which is consistent with an increased covalent interaction between MoS2 and gold. These results show that the synthesis temperature and, therefore, the quality of MoS2 dramatically impacts Fermi level pinning and the resultant current–voltage characteristics of Schottky barrier-mediated devices.

Substrate dependent resistive switching in amorphous-HfOx memristors: an experimental and computational investigation

“…Prior to spin coating the substrates with negative photoresist, NPR (NR-71 3000P), we cleaned the substrates with organic solvents and piranha solution, then dried with N 2 blower. As described in our recent paper, both the bottom and top electrodes (BE and TE) were patterned using a maskless aligner (Heidelberg Instruments MLA 150), 36 followed by the metal depositions using e-beam evaporation (20 nm and 5 nm Ti underneath the 70 nm BE and 150 nm TE respectively, deposited at the fixed deposition rates 1.0 Å/s). Both the BE and TE layers were deposited using 99.999% Au source at ultra-high vacuum (~3x10 -6 Torr) and the Ti layers were deposited without breaking vacuum using 99.95% Ti source pellets (Kurt j. Lesker).…”

Highly Efficient Experimental Approach to Evaluate Metal to 2D Semiconductor Interfaces in Vertical Diodes with Asymmetric Metal Contacts

“…Figure a,b illustrate our vertical metal–SCTMD–metal heterojunctions fabricated on a silicon nitride (SiN) membrane with a schematic (Figure a) and a corresponding cross-sectional diagram with two-probe measurement scheme (Figure b). The SiN membrane with a 2 μm diameter through-hole is used as the vertical heterojunction platform, on top of which mechanically exfoliated multilayer SCTMD flakes of WSe 2 , WS 2 , MoSe 2 , or MoS 2 are dry transferred. , We limit the SCTMDs to a thickness range of 10 nm ≤ t SC‑TMD ≤ 40 nm, at which FN tunneling and SE become the dominant transport mechanisms in the vertical charge flows. , After confirming the SCTMD layer thicknesses with an atomic force microscope (Supporting Information (SI) Figure 1), top and bottom metal–SCTMD contacts are fabricated by consecutive metal depositions with e-beam evaporation and sputtering (SI Figure 2). We utilize the same metals on both sides of the contacts to minimize asymmetric metal–SCTMD band alignments at the junctions. ,, We point out that our vertical devices form intrinsically clean metal–SCTMD interfaces since neither lithography nor chemical processes are required, allowing us to exploit material-choice flexibilities with varying metal film work functions and functionalities.…”

“…The SiN membrane with a 2 μm diameter through-hole is used as the vertical heterojunction platform, on top of which mechanically exfoliated multilayer SCTMD flakes of WSe 2 , WS 2 , MoSe 2 , or MoS 2 are dry transferred. 30,39 We limit the SCTMDs to a thickness range of 10 nm ≤ t SC-TMD ≤ 40 nm, at which FN tunneling and SE become the dominant transport mechanisms in the vertical charge flows. 40,42 After confirming the SCTMD layer thicknesses with an atomic force microscope (Supporting Information (SI) Figure 1), top and bottom metal−SCTMD contacts are fabricated by consecutive metal depositions with e-beam evaporation and sputtering (SI Figure 2).…”

“…34−38 In comparison, vertically assembled devices such as those with metal−SCTMD−metal junctions can be free from any environmental artifacts with far shorter channel lengths than those found in laterally formed devices. 39,40 Charge flows through optimally structured vertical junctions, that is, channels made of multilayer SCTMD films, can be solely determined by the charge carrier injections at the metal−SCTMD contacts and not by the SCTMD channels, which provides an ideal empirical platform for addressing the material properties at metal−SCTMD interfaces. 21,39−41 In this report, we demonstrate a simple but powerful experimental approach to address the energy band alignments of metal−SCTMD contacts that determine the SB heights and the majority carrier types across the junctions in the 2D semiconducting channels of WSe 2 , WS 2 , MoSe 2 , and MoS 2 .…”

Plasma Treatment of Ultrathin Layered Semiconductors for Electronic Device Applications

“…In Table , we collate the reported changes to the carrier mobility of plasma-treated TMDs, as compared with values obtained before processing. Due to Fermi level pinning at the 2D semiconductor/metal interface, the used electrode materials also need to be appropriately matched to alleviate mobility throttling related to the Schottky barrier height. ,,− Thus, for completeness, we also list the contact metals used in each study.…”