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P-type doping of MoS2has proved to be a significant bottleneck in the realization of fundamental devices such as p-njunction diodes and p-type transistors due to its intrinsic n-type behavior. We report a CMOS compatible, controllable and area selective phosphorus plasma immersion ion implantation (PIII) process for p-type doping of MoS2. Physical characterization using SIMS, AFM, XRD and Raman techniques was used to identify process conditions with reduced lattice defects as well as low surface damage and etching, 4X lower than previous plasma based doping reports for MoS2. A wide range of nondegenerate to degenerate p-type doping is demonstrated in MoS2field effect transistors exhibiting dominant hole transport. Nearly ideal and air stable, lateral homogeneous p-njunction diodes with a gate-tunable rectification ratio as high as 2 × 104are demonstrated using area selective doping. Comparison of XPS data from unimplanted and implanted MoS2layers shows a shift of 0.67 eV toward lower binding energies for Mo and S peaks indicating p-type doping. First-principles calculations using density functional theory techniques confirm p-type doping due to charge transfer originating from substitutional as well as physisorbed phosphorus in top few layers of MoS2. Pre-existing sulfur vacancies are shown to enhance the doping level significantly.

We demonstrate a low and constant effective Schottky barrier height (ΦB∼ 40 meV) irrespective of the metal work function by introducing an ultrathin TiO2ALD interfacial layer between various metals (Ti, Ni, Au, and Pd) and MoS2. Transmission line method devices with and without the contact TiO2interfacial layer on the same MoS2flake demonstrate reduced (24×) contact resistance (RC) in the presence of TiO2. The insertion of TiO2at the source-drain contact interface results in significant improvement in the on-current and field effect mobility (up to 10×). The reduction in RCand ΦBhas been explained through interfacial doping of MoS2and validated by first-principles calculations, which indicate metallic behavior of the TiO2-MoS2interface. Consistent with DFT results of interfacial doping, X-ray photoelectron spectroscopy (XPS) data also exhibit a 0.5 eV shift toward higher binding energies for Mo 3d and S 2p peaks in the presence of TiO2, indicating Fermi level movement toward the conduction band (n-type doping). Ultraviolet photoelectron spectroscopy (UPS) further corroborates the interfacial doping model, as MoS2flakes capped with ultrathin TiO2exhibit a reduction of 0.3 eV in the effective work function. Finally, a systematic comparison of the impact of selective doping with the TiO2layer under the source-drain metal relative to that on top of the MoS2channel shows a larger benefit for transistor performance from the reduction in source-drain contact resistance.