| AVS 64th International Symposium & Exhibition | |
| 2D Materials Focus Topic | Wednesday Sessions |
| Session 2D-WeA |
| Session: | Properties and Characterization of 2D Materials |
| Presenter: | Jun Hong Park, Institute for Basic Science (IBS), Ewha Womans University, Republic of Korea |
| Authors: | J.H. Park, Institute for Basic Science (IBS), Ewha Womans University, Republic of Korea I.J. Kwak, University of California at San Diego A. Rai, University of Texas at Austin S.K. Banerjee, University of Texas at Austin A.C. Kummel, University of California at San Diego |
| Correspondent: | Click to Email |
Transition metal dichalcogenides (TMDs) have been extensively studied because of their unique electronic and optical properties. In addition, having the non-zero band gap, they can be directly integrated in logic devices. However, it is of crucial importance to tune the band gap and Fermi level positions of TMDs effectively to enhance the ON/OFF current ratio. In this study, the band gap of WSe2 was engineered using a (NH4)2S solution and a gaseous O3 treatment. Prior to the chemical treatments, the surface of MBE (molecular beam epitaxy) grown WSe2 was studied using STM (scanning tunneling ,microscopy). Large grains of WSe2 about 100 nm were observed. The band gap of monolayer (ML) WSe2 was about 2.1 eV in STS (scanning tunneling spectroscopy), consistent with previous STM studies. A bare WSe2 sample was emerged in ammonium sulfide solution (40% (NH4)2S(aq)) at 300K. Afterward, the sulfur treated (ST) WSe2 sample was transferred to a UHV scanning tunneling microscopy (STM) chamber to observe the effect of the treatment on the band gap. The STM imaging revealed that new electronic states were generated by ST across the entire WSe2 surface, with asymmetric bias dependence. In STS, band gap of the ST WSe2 was reduced to 1.2 eV, while Fermi level was pinned near the valence band. A WSe2 field effect transistor (FET) was fabricated to show the effect of ST on electrical properties. Electrical measurement showed that ST of monolayer WSe2 transistors increased ON states current by two order of magnitude in the p-branch, while ON state current in n -branch was increased by an order of magnitude which is consistent with the (NH4)2S decreasing the band gap and inducing p-type doping. The band gap was also tuned by ozone (O3) treatment at room temperature. A MBE grown WSe2 was inserted in a vacuum chamber. O3 gas was prepared by UV lamp excitation of a continuous O2 flow into the chamber at 300 K for 2 min. After dosing O3 gas for 2 min, the band gap of the WSe2 was about 1.3 eV, while Fermi level was near conduction band indication (n-type). As exposure time of O3 increased to 6 min, the band gap of ML WSe2 was decreased to 1.1eV and the formation of WOx was observed on the ML terraces. Therefore, O3 had the opposite behavior of (NH4)2S since O3 treatment maintained n-type doping while shrinking the band gap.