The charge injection mechanism from contact electrodes into two-dimensional (2D) dichalcogenides is an essential topic for exploiting electronics based on 2D channels, but remains not well understood. Here, low-work function metal ytterbium (Yb) was employed as contacts for tungsten disulfide (WS2) to understand the realistic injection mechanism. The contact properties in WS2 with variable temperature (T) and channel thickness (t(ch)) were synergetically characterized. It is found that the Yb/WS2 interfaces exhibit a strong pinning effect between energy levels and a low contact resistance (R-C) value down to 5 k omega mu m. Cryogenic electrical measurements reveal that R-C exhibits weakly positive dependence on T until 77 K as well as a weakly negative correlation with t(ch). In contrast to the non-negligible R-C values extracted, an unexpectedly low effective thermal injection barrier of 36 meV is estimated, indicating the presence of significant tunneling injection in the subthreshold regime and the inapplicability of the pure thermionic emission model to estimate the height of the injection barrier.
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