Origin of contact polarity at metal-2D transition metal dichalcogenide interfaces
Keian Noori, Fengyuan Xuan, Su Ying Quek
Abstract
Abstract Using state-of-the-art ab initio GW many-body perturbation theory calculations, we show that monolayer MoS 2 on Au is a p -type contact, in contrast to the vast majority of theoretical predictions using density functional theory. The predominantly n -type behaviour observed experimentally for MoS 2 /Au junctions can be attributed to the presence of sulfur vacancies, which pin the Fermi level. GW calculations on WSe 2 /Au junctions likewise predict p -type contacts for pristine WSe 2 and n -type contacts for junctions with selenium vacancies. Experimentally, WSe 2 /metal junctions are predominantly p -type or ambipolar, with p -type junctions being observed for selenium-deficient WSe 2 , suggesting that selenium vacancies are not effective in pinning the Fermi level for WSe 2 /metal junctions. We rationalize these apparently contradictory results by noting that selenium vacancies in WSe 2 are readily passivated by oxygen atoms. Taken together, our state-of-the-art calculations clearly elucidate the relation between contact polarity and atomic structure. We show that non-local exchange and correlation effects are critical for determining the energy level alignment and even the contact polarity (in the case of MoS 2 on Au). We further reconcile a large body of experimental literature on TMDC/metal contact polarities by consideration of the defect chemistry.