Origins of Fermi Level Pinning between Tungsten Dichalcogenides (WS2, WTe2) and Bulk Metal Contacts: Interface Chemistry and Band Alignment

摘要

Thin metal films (Au, Ir, Cr, and Sc), deposited by an electron beam on bulk, exfoliated WS2 and WTe2 using two different reactor base pressures (high vacuum (HV) <2 X 10(-6) mbar; ultrahigh vacuum (UHV) <2 X 10(-9) mbar), are explored to study the effects of reactor ambient on the interface chemistry formed at room temperature between bulk metal contacts and tungsten dichalcogenides (TDCs). Au forms a van der Waals interface with WS2 and a covalent interface with WTe2, independent of reactor ambient. In contrast, an intermetallic is detected at the Ir-WS2 and Ir-WTe2 interfaces regardless of the reactor ambient. The low work function metals Cr and Sc, which are more reactive than their high work function counterparts (Au and Ir), completely reduce the TDC layer(s) in direct contact. Sc is completely oxidized in situ when deposited in an elastomer-sealed deposition tool (in HV). These results highlight that the interface between metals and TDCs is most often covalent, which contrasts the common misconception that a van der Waals gap is present. Furthermore, the band alignment between the four metals investigated here and bulk WS2 deviates significantly from that predicted by the Schottky-Mott rule. These results elucidate the true chemistry of select metal-TDC interfaces and highlight the rapid oxidation that manifests in situ in a HV metallization environment. Our work emphasizes the need to consider the true interface chemistry when engineering and modeling metal contacts to WS2 and WTe2.