An Atomic-Scale Look at Point Defects in 2D Semiconductors

Monday, October 14, 2019 - 2:30pm

Point defects in semiconductors are of great technological significance in semiconductor industry and have been proposed more recently as a room temperature qubit platform. In two-dimensional semiconductors such as transition metal dichalcogenide (TMD) monolayers, defects have an even larger impact on material properties, but offer exciting possibilities as atomic quantum systems. In this talk, I will present a comprehensive study of point defects in monolayer WS2, and other TMDs. The atomic, electronic, vibronic and optical properties of single defects were probed using a combination of high-resolution scanning probe microscopy techniques. We identified isoelectronic chalcogen and transition metal substitutions as the dominant defects based on their unique electronic fingerprint [1,2]. Most importantly, we found four defining effects that govern the in-gap defect states in TMDs: Crystal-field splitting, spin-orbit coupling, electron-phonon interaction and charge localization. Specifically, we observed large spin-orbit splitting at sulfur vacancies [3] and chromium substituting tungsten [2], local strain at substitutional defects [2], charge localization and associated hydrogenic bound states [4] and vibronic excitation of quasi-local modes [2]. Moreover, we demonstrate electron-induced luminescence at individual point defects. Luminescence maps and bias-dependent optical spectra suggest that the in-gap defect states are final states in an inelastic electron tunneling process, which is mediated by the tip plasmon [5]. The atomic-scale characterization of atomic, electronic and optical defect properties will guide future efforts of targeted defect engineering and doping of TMDs. 
[1] S. Barja et al., Nat. Commun. 10, 3382 (2019)
[2] B. Schuler et al., ACS Nano, 13, 10520 (2019)
[3] B. Schuler et al., Phys. Rev. Lett. 123, 076801 (2019)
[4] M. Aghajanian et al., submitted
[5] B. Schuler et al., submitted

3 Le Conte Hall
Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, USA