Oxidation and Etching of Layered Molybdenum Disulfide at Elevated Temperatures

Undergraduate #133
Discipline: Technology and Engineering
Subcategory: Materials Science

Frances Kwok - Pennsylvania State University
Co-Author(s): Timothy N. Walter and Suzanne E. Mohney, Pennsylvania State University, University Park, PA

The unique electronic properties of molybdenum disulfide (MoS2), a member of layered transition metal dichalcogenides (TMDs), make it desirable for nanoelectronics, especially because monolayer MoS2 is flexible up to 11% without fracture.1 Flexible devices are less prone to shattering than current rigid ones. It is also earth-abundant in the form of the bulk material, molybdenite. Current commercial transistors use semiconductors like silicon, germanium, and gallium arsenide2, but layered TMDs offer unique advantages because of their properties change based on the number of layers3. Oxidation at elevated temperatures (250 °C-400 °C) is explored as an alternative method of etching the surfaces and edges of layered MoS2. Etching with oxygen gas may compete with plasma or wet etchants in device processing because of its availability as an abundant element and convenience in setting up this process. The goal is to determine effective etching conditions for MoS2 using oxygen gas. These findings can then be applied to other layered TMDs.

MoS2 flakes are mechanically exfoliated onto silicon dioxide on silicon (SiO2/Si) substrates and vary in thickness based on the number of layers. Samples are treated under different annealing conditions by varying the temperature, time, and oxidizing agent. Control samples are annealed in inert environments. To observe oxidation effects, samples are imaged via optical microscopy, field-emission scanning electron microscopy, and atomic force microscopy before and after each treatment. Auger electron spectroscopy and Raman spectroscopy determine the composition of the oxidized samples. High temperature oxidation results in layer and edge oxidation as well as triangular oxide islands or pits depending on the condition applied.
Findings show that anisotropic etching of edges and of layers occurs in environments where water is present. This project will continue with control of the environments that the samples are treated in, such as manipulating gas ratios, in order to optimize the effectiveness of etching by oxidation. From there, the control of the etch rate can be used in device processing.
1. S. Bertolazzi, J. Brivio, A. Kis, ACS Nano, 2011, 5, 9703-9709.
2. A.C. Ferrari et al., Nanoscale, 2015, 7, 4598-4810.
3. R. Ganatra and Q. Zhang, ACS Nano, 2014, 8, 4074-4099.

Funder Acknowledgement(s): Penn State’s EFRI 2-DARE: 2D Crystals Formed by Activated Atomic Layer Deposition REM is sponsored by the National Science Foundation (EFMA 1433378).

Faculty Advisor: Suzanne E. Mohney,