| AVS 63rd International Symposium & Exhibition | |
| 2D Materials Focus Topic | Friday Sessions |
| Session 2D+NS-FrM |
| Session: | 2D Materials: Device Physics and Applications |
| Presenter: | Morgann Berg, Sandia National Laboratories |
| Authors: | M. Berg, Sandia National Laboratories K. Keyshar, Rice University I. Bilgin, Northeastern University, Los Alamos National Laboratory F. Liu, Northeastern University, Los Alamos National Laboratory H. Yamaguchi, Los Alamos National Laboratories R. Vajtai, Rice University C. Chan, Sandia National Laboratories G. Gupta, Los Alamos National Laboratories S. Kar, Northeastern University P. Ajayan, Rice University T. Ohta, Sandia National Laboratories A. Mohite, Los Alamos National Laboratories |
| Correspondent: | Click to Email |
Tailoring band alignment layer-by-layer using heterojunctions of two-dimensional (2D) semiconductors is an attractive prospect for producing next-generation electronic and optoelectronic devices that are ultra-thin, flexible, and efficient. 2D layers of transition metal dichalcogenides (TMDs) in laboratory devices have already demonstrated properties favorable for electronic and optoelectronic applications. Despite these strides, a systematic understanding of how band alignment evolves from monolayer to multilayer for MoS2, a model TMD system, is still missing owing to the lack of a suitable experimental approach. Quantitative determination of the electronic band alignment necessitates that measurements be performed in a controlled environment (such as vacuum) using a substrate that interacts minimally with the overlying TMDs (preferably insulating) to suppress the electronic influence of supporting substrates and prevent chemical modification of TMDs due to adsorbates (primarily water).
Here we report on the local band alignment of monolayer, bilayer, and tri-layer MoS2 on a 285-nm-thick SiO2 substrate, measured using a new approach to probe the occupied electronic states based on photoemission electron microscopy with deep ultraviolet excitation. The spatially-resolved, simultaneous measurements of the vacuum level and the valence band edge at the Brillouin zone center show that the addition of layers to monolayer MoS2 increases the relative work function, and pushes the valence band edge toward the vacuum level. We also find n-type doping of few-layer MoS2 and type-I band alignment across monolayer-to-bilayer and bilayer-to-trilayer lateral junctions. Our results differ from some earlier reports based on Kelvin probe and scanning photocurrent microscopies [Sci. Reports, 5, 10990 (2015), Nano Lett., 15, 2278 (2015)], and highlight the strong influence of environmental effects on the band alignment in MoS2 homojunctions. We are now applying this exciting new metrology to systematically examine the ionization energies of a series of TMDs. The results will provide fundamental information necessary to assess the band alignments of TMD heterojunction devices, and to validate or refine existing theoretical predictions [APL, 103, 053513 (2013), J. Phys. Chem. C, 119, 13169 (2015)].
This work was performed at CINT (DE-AC04-94AL85000), and is supported by Sandia LDRD, US DOE EERE SunShot Initiative BRIDGE (DE-FOA-0000654 CPS25859), and Army Research Office MURI (W911NF-11-1-0362). SNL is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Co., for the US DOE NNSA (DE-AC04-94AL85000).