Accurate Extraction of Schottky Barrier Height and Universality of Fermi Level De‐Pinning of van der Waals Contacts

Abstract

Due to Fermi level pinning (FLP), metal‐semiconductor contact interfaces result in a Schottky barrier height (SBH), which is usually difficult to tune. This makes it challenging to efficiently inject both electrons and holes using the same metal—an essential requirement for several applications, including light‐emitting devices and complementary logic. Interestingly, modulating the SBH in the Schottky–Mott limit of de‐pinned van der Waals (vdW) contacts becomes possible. However, accurate extraction of the SBH is essential to exploit such contacts to their full potential. In this study a simple technique is proposed to accurately estimate the SBH at the vdW contact interfaces by circumventing several ambiguities associated with SBH extraction. Using this technique on several vdW contacts, including metallic 2H‐TaSe2, semi‐metallic graphene, and degenerately doped semiconducting SnSe2, it is demonstrated that vdW contacts exhibit a universal de‐pinned nature. Superior ambipolar carrier injection properties of vdW contacts are demonstrated (with Au contact as a reference) in two applications, namely, a) pulsed electroluminescence from monolayer WS2 using few‐layer graphene (FLG) contact, and b) efficient carrier injection to WS2 and WSe2 channels in both n‐type and p‐type field effect transistor modes using 2H‐TaSe2 contact.

Fermi Level Pinning Dependent 2D Semiconductor Devices: Challenges and Prospects

“…Weak interlayer interaction leads to mild orbital overlap and interface dipole between two neighbored 2D materials, which can further suppress FLP. [107] Le Quang et al have observed a rigid shift of 120 meV between the scanning tunneling spectra of TMD at contact interfaces to monolayer and bilayer graphene. The shift equals to their work function difference, indicating that Fermi level at the contact is mostly depinned.…”

Fermi‐Level Pinning‐Free WSe2 Transistors via 2D Van der Waals Metal Contacts and Their Circuits

“…This result indicates that Cl–SnSe 2 forms a nearly ideal interface with WSe 2 without pinning and thus largely follows the Schottky–Mott rule ( S = 1); this S value is comparable to the values obtained using the transferred metal, [ 8 ] 1T‐WTe 2 , [ 32 ] and degenerately n‐doped SnSe 2 . [ 18 ] Such a near‐ideal interface was further verified by the enhanced subthreshold swing (SS) value and minimal hysteresis, which are strongly related to the interfacial contamination (Figure S14, Supporting Information). [ 33,39,40 ] The schematics of the band diagrams are shown in Figure S15, Supporting Information, depicting the cases of evaporated metal and Cl–SnSe 2 contacts, respectively.…”

“…[ 8,14 ] However, the approach suffers from poor atmospheric contamination, and requires an inert‐gas processing environment, which hence increases the fabrication complexity. On the other hand, recently, the use of vdW 2D metals as contacts has been shown to provide atomically clean interfaces at metal–semiconductor junctions; [ 15–18 ] these clean interfaces from vdW 2D metals could potentially prevent Fermi‐level pinning, thus promising excellent control over the Schottky barrier and the resulting device polarity. Several attempts have been made to use 2D metals as vdW contacts on various 2D semiconductors, such as highly p‐doped WSe 2 , [ 15 ] NbSe 2 , [ 16 ] 1T‐TaS 2 , [ 17 ] and degenerately doped semiconducting SnSe 2 .…”

“…Several attempts have been made to use 2D metals as vdW contacts on various 2D semiconductors, such as highly p‐doped WSe 2 , [ 15 ] NbSe 2 , [ 16 ] 1T‐TaS 2 , [ 17 ] and degenerately doped semiconducting SnSe 2 . [ 18 ] Although all of these studies have shown improvement in contact properties (e.g., contact resistance, FET mobility) through the use of 2D metals, the underlying mechanism remains elusive; these improvements could have resulted from Fermi‐level depinning, doping effects, or interactions within the contact interface, yet such aspects have not been investigated, which limits further development. Furthermore, to obtain the p‐type transistors, a high‐work‐function vdW 2D metal that can completely eliminate the Fermi‐level pinning is strongly desired.…”

Trion-trion annihilation in monolayer WS$_2$

“…The p-doping side (V g < 0 V) shows three distinct features (Figure 1b and c), namely, neutral bright exciton (χ 0 ), positive trion (χ + ), and positively charged biexciton (β + ). Due to the relative positions of the Fermi levels between 1L-WS 2 and graphene, it is more difficult to inject holes than electrons [32]. Thus it is challenging to dope the 1L-WS 2 flake strongly ptype, as suggested by the sustained χ 0 peak intensity even at high negative V g (Figure 1b).…”