| AVS 65th International Symposium & Exhibition | |
| 2D Materials Focus Topic | Monday Sessions |
| Session 2D+MI+NS-MoA |
| Session: | 2D Materials Characterization including Microscopy and Spectroscopy |
| Presenter: | Nhan Nguyen, National Institute of Standards and Technology |
| Authors: | Q. Zhang, Theiss Research & National Institute of Standards and Technology S. Zhang, Theiss Research & National Institute of Standards and Technology B. Sperling, National Institute of Standards and Technology N. Nguyen, National Institute of Standards and Technology |
| Correspondent: | Click to Email |
Two-dimensional (2-D) materials have brought new possibilities for the future electronic and optoelectronic applications [1], [2]. Electronic band alignment at the interface is one of the important parameters in many device designs. For instance, staggered band alignment is preferred to separate photon generated electron-hole pairs in optoelectronic and photovoltaic devices [3]. For the 2-D materials in the monolayer (ML) limit, it has been a challenge to accurately measure the electron affinity which determines how the bands align at the interface. In fact, most 2-D heterojunctions are designed using calculated or theoretically predicted band alignments [4]. In this work, we present an experimental measurement using internal photoemission spectroscopy (IPE) to determine the band offset of MX2 semiconductors (M = Mo, W; X = S, Se) in relative to an oxide barrier and suggest possible combination of the MX2 materials to be used for optoelectronic and photovoltaic applications. This IPE approach is seen as a unique method that can be applied to characterize other 2-D materials.
The IPE test structure is fabricated by exfoliating MX2 flakes on to the Al2O3/p+Si substrate and depositing Ti/Pt contacts on the flakes with large open areas for light absorption. By using gold film mediated exfoliation method [5], large area (> 75 x 75 μm2) ML MX2 flakes are obtained, confirmed by Raman spectrum and photoluminescence mapping. Photocurrents of the MX2-Al2O3-p+Si structure are measure with the incident photon energy swept from 2.0 eV to 5.5 eV and gate voltage VGS (applied to the Si back gate) stepped from -1.0 V to 1.6 V. The oxide flat band voltage (VFB) is extracted by the voltage where the photocurrent switches sign near and above photoemission thresholds. The band offsets at the MX2/Al2O3 and Al2O3/Si interfaces are extracted as the thresholds of the cube root of photoemission quantum yield (Y) being the ratio of the photocurrent over the incident light flux [6]. At gate bias below VFB, the band offset between Al2O3 and Si is measured and found to be 3.4 eV for all the 4 devices, which is also a well-established value. More importantly, the band offset at the MX2/Al2O3 interfaces combined with the known optical band gaps of ML MX2 suggest that MoS2/WS2 and MoSe2/WSe2 can possibly form the staggered heterojunction.
[1] G. Fiori, et al, Nat. Nanotech. 9, 768 (2014).
[2] F. Xia, et al, Nat. Photonics 8, 899 (2014).
[3] X. Hong, et al, Nature Nanotech. 9, 682 (2014)
[4] J. Kang, et al, Appl. Phys. Lett. 102, 012111 (2013)
[5] S. B. Desai, et al, Adv. Mater. 28, 4053 (2016).
[6] V. V. Afanasev and A. Stesmans, J. Appl. Phys. 102, 081301 (2007).