Engineering the Palladium–WSe₂ Interface Chemistry for Field Effect Transistors with High-Performance Hole Contacts

Abstract

Palladium has been widely employed as a hole contact to WSe 2 and has enabled, at times, the highest WSe 2 transistor performance. However, there are orders of magnitude variation across the literature in Pd−WSe 2 contact resistance and I ON /I OFF ratios with no true understanding of how to consistently achieve high-performance contacts. In this work, WSe 2 transistors with impressive I ON /I OFF ratios of 10 6 and Pd−WSe 2 Schottky diodes with near-zero variability are demonstrated utilizing Ohmic-like Pd contacts through deliberate control of the interface chemistry. The increased concentration of a PdSe x intermetallic is correlated with an Ohmic band alignment and concomitant defect passivation, which further reduces the contact resistance, variability, and barrier height inhomogeneity. The lowest contact resistance occurs when a 60 min post-metallization anneal at 400 °C in forming gas (FG) is performed. X-ray photoelectron spectroscopy indicates this FG anneal produces 3× the concentration of PdSe x and an Ohmic band alignment, in contrast to that detected after annealing in ultrahigh vacuum, during which a 0.2 eV hole Schottky barrier forms. Raman spectroscopy and scanning transmission electron microscopy highlight the necessity of the fabrication step to achieve high-performance contacts as no PdSe x forms, and WSe 2 is unperturbed by room temperature Pd deposition. However, at least one WSe 2 layer is consumed by the necessary interface reactions that form PdSe x requiring strategic exploitation of a sacrificial WSe 2 layer during device fabrication. The interface chemistry and structural properties are correlated with Pd−WSe 2 diode and transistor performance, and the recommended processing steps are provided to enable reliable high-performance contact formation.

Atomically Controlled Tunable Doping in High‐Performance WSe2 Devices

“…It is known that the Fermi level of metal contact is pinned close to the mid-gap of WSe2. 33,34 Therefore, band movements modulated by scanning the back-gated voltage (VB) lead to either hole injection from the source to the valence band at negative VB or electron injection from the drain to the conduction band at positive VB. Consequently, transfer characteristics of a pristine WSe2 device exhibit a typical ambipolar behavior, as shown in Figure 1e.…”

Contacts for Molybdenum Disulfide: Interface Chemistry and Thermal Stability

“…More recent investigations published by McDonnell et al [18]. and Smyth et al [24,67,75]. demonstrate that the chamber pressure during contact deposition, a process parameter that is typically unreported in device papers, has a measurable impact on the chemistry of the interface.…”

“…Annealing the device after contact deposition is common practice in device processing and notable changes in device transport properties after annealing have been observed. [19,35,75,81,82] English et al report that post-deposition annealing reduces hysteresis and stabilizes electrical measurements for Au contacted FETs [19]. Baugher et al claim that vacuum annealing of devices with Ti-Au contacts eliminated all Schottky behavior [82].…”

“…In all of the examples mentioned, chemical characterization of the interface is lacking while the observed improvements are almost certainly correlated with changes in interface chemistry. Recently, Smyth et al [75] reported substantial improvement after annealing WSe 2 FETs contacted with Pd. They find that annealing Pd-WSe 2 in forming gas at 400 • C drives the formation of PdSe x which results in Ohmic band alignment.…”

Controlling the Pd Metal Contact Polarity to Trigonal Tellurium by Atomic Hydrogen‐Removal of the Native Tellurium Oxide

“…Recent reports have demonstrated the interface chemistry formed between metal contacts and transition metal dichalcogenides varies significantly depending on a variety of material properties and processing details, [ 31–36 ] which often affects the corresponding band alignment and contact performance. [ 36,37 ] In this letter, we demonstrate the removal of the native oxide on t‐Te films and coincident formation of an n ‐type t‐Te band alignment by atomic hydrogen (AH) exposure. We elucidate the effects of the native oxide on the interface chemistry and band alignment between Pd metal contacts and t‐Te.…”