Sc has been employed as an electron contact to a number of two-dimensional (2D) materials (eg MoS 2, black phosphorous) and has enabled, at times, the lowest electron contact resistance. However, the extremely reactive nature of Sc leads to stringent processing requirements and metastable device performance with no true understanding of how to achieve consistent, high-performance Sc contacts. In this work, WSe 2 transistors with impressive subthreshold slope (109 mV dec− 1) and I ON/I OFF (10 6) are demonstrated without post-metallization processing by depositing Sc contacts in ultra-high vacuum (UHV) at room temperature (RT). The lowest electron Schottky barrier height (SBH) is achieved by mildly oxidizing the WSe 2 in situ before metallization, which minimizes subsequent reactions between Sc and WSe 2. Post metallization anneals in reducing environments (UHV, forming gas) degrade the I ON/I …