WSe2-contact metal interface chemistry and band alignment under high vacuum and ultra high vacuum deposition conditions

Abstract

Contact metals (Au, Ir, and Cr) are deposited on bulk WSe2 under ultra-high vacuum (UHV, 1 × 10-9 mbar) and high vacuum (HV, 5 × 10-6 mbar) conditions and subsequently characterized with x-ray photoelectron spectroscopy (XPS) to elucidate the effects of reactor base pressure on resulting interface chemistry, contact chemistry, and band alignment. Au forms a van der Waals interface with WSe2 regardless of deposition chamber ambient. In contrast, Ir and Cr form a covalent interface by reducing WSe2 to form interfacial metal selenides. When Cr is deposited under HV conditions, significant oxygen incorporation is observed resulting in the thermodynamically favorable formation of tungsten oxyselenide and a substantial concentration of CrxOy. Regardless of contact metal, WOx (2.63 < x < 2.92) forms during deposition under HV conditions which may positively affect interface transport properties. Cr and Ir form unexpectedly large electron and hole Schottky barriers, respectively, when deposited under UHV conditions due to interfacial reactions that contribute to anomalous band alignment. These results reveal the true interface chemistry formed between metals and WSe2 under UHV and HV conditions and demonstrate the impact on the Fermi level position following contact formation on WSe2.