| AVS 65th International Symposium & Exhibition | |
| 2D Materials Focus Topic | Wednesday Sessions |
| Session 2D+AM+EM+NS-WeM |
| Session: | Dopants, Defects, and Interfaces in 2D Materials |
| Presenter: | Daniil Marinov, IMEC, Belgium |
| Authors: | D. Marinov, IMEC, Belgium J. Ludwig, IMEC & KU Leuven, Belgium D. Chiappe, IMEC, Belgium E. Voronina, Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow State University T. Rakhimova, Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow State University J.-F. de Marneffe, IMEC, Belgium I. Asselberghs, IMEC, Belgium S. De Gendt, IMEC, KU Leuven, Belgium |
| Correspondent: | Click to Email |
Two-dimensional transition metal dichalcogenides (e.g. MoS2, WS2) are promising materials for a number of electronic and optoelectronic applications. Wafer-scale integration of these materials into sophisticated devices requires atomic-scale control of the processing steps such as deposition, etch, clean and doping. Reduction of the contact resistance is a major roadblock towards demonstration of high-performance devices. Significant Schottky barrier at the metal-MX2 interface as well as surface contamination (e.g. by polymer residues) are the main factors contributing to the high contact resistance in fabricated MX2 devices. In this study, a fully dry cleaning and doping technique is developed with a particular focus on contact engineering.
We demonstrate that a remote H2 plasma is efficient for removal of organic residues from MX2 surfaces. However, sulfur can be also stripped from the topmost layer by reactive H atoms. The main challenge is thus to precisely control the sulfur loss while maintaining the cleaning efficiency. At high substrate temperature, a 200 nm PMMA layer can be fully removed selectively to a single layer of WS2 without damaging the 2D material (as confirmed by photoluminescence measurements). At low substrate temperatures significant S-vacancy formation was observed. Surface temperature is therefore the key parameter for controlling the reactivity of H atoms on WS2.
Controllable formation of sulfur vacancies opens routes for substitutional doping. After H2 plasma strip, WS2 and MoS2 samples were exposed to a flow of molecular gases (Cl2, CO, OCS) without igniting the plasma. It is shown that Cl2 and OCS can react with H2 plasma treated MX2 forming stable surface groups. Ex-situ conductive AFM measurements confirm that molecular doping prevents the loss of conductivity (that is observed after H2 plasma alone). Moreover, OCS and Cl2 exposure enhances electrical current injection in the material through grain boundaries and edges. The latter effect is beneficial for contact resistance reduction on MX2.
To gain a deeper insight in the observed surface phenomena, DFT simulation of the interaction of atomic (H, Cl, F) and molecular (OCS, Cl2) species with MX2 surface was performed. S-vacancy creation by atomic hydrogen via formation of gas phase H2S was observed in simulations, in qualitative agreement with the experiments. Moreover, dissociative adsorption of Cl2 and OCS in S-vacancy sites is predicted by the DFT model.
Dr D. Marinov has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 752164.