Qingkai Qian1 Zhaofu Zhang1 Kevin J Chen1

1, The Hong Kong University of Science and Technology, Hong Kong, , Hong Kong

MoS2 and WSe2 are semiconductors with atomic-scale thickness and high mobility, which are favorable as channel materials for the next generation field effect transistors. To achieve high performance and good device stability, MoS2 and WSe2 have to be integrated with high quality high-k dielectric as the gate dielectric or passivation layer. However, the dangling-bond free surface of MoS2 and WSe2 has made it challenging to deposit high-k dielectric on them. Remote O2 (similarly UV-O3) or N2 plasma treatment [1-3] has been used as a surface functionalization technique to enhance the uniform dielectric deposition on MoS2 and WSe2, via O or N atom adsorptions on top of S and Se atoms. However, severe damages, such as oxidation or nitridation of MoS2 and WSe2, could be easily induced once the sample is overexposed. An in situ characterization technique (which can be easily integrated with the treatment and ALD chamber), becomes valuable to enable the real-time monitoring of the surface functionalization conditions. In this work, resonant Raman spectra of MoS2 and WSe2 with various O or N atom adsorption densities are studied by first-principles calculations for the first time, aiming to provide guidelines for Raman spectroscopy as a tool to monitor the O/N adsorptions for subsequent high-k dielectric integration.

Finite-difference method is used to calculate the Raman tensors in density functional theory (DFT) [4]. MoS2 and WSe2 supercells are adopted to study the influences of O/N adsorptions. The activations of both acoustic- and optical-phonon Raman scattering are analyzed based on the breaking of translational and reflection symmetries. Specifically, the low-coverage O/N adsorptions will act as perturbation to the periodic crystal lattice, breaking the translational symmetry and activating the acoustic-phonon Raman scattering. High-coverage adsorptions will further break the reflection symmetry in z direction, activating and intensifying the previously silent A2u and E1g Raman modes. High-coverage adsorptions also introduce new phonon modes associated with the local adatom oscillations with A1 and E symmetries. With increasing adsorptions, the previously active E2g1 peak shifts continuously, and the A1g mode is monotonously softened. To better identify the shift trends, Raman intensities are decomposed according to the phonon symmetries by adopting both (zxxz) and (zxyz) scattering geometries. The above observed A1g and E2g1 peak shifts, together with the activations of acoustic- and optical-phonon Raman scattering, can be used as valuable scalars to quantify the adsorption coverage density, and to realize robust surface functionalization of MoS2 and WSe2 for subsequent high-k dielectric integrations.

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[2] H. Zhu et al., ACS Appl. Mater. Inter. 8 (2016) 19119.
[3] Q. Qian et al., Nanotechnology 28 (2017) 175202.
[4] H. P. C. Miranda et al. Nano Lett. 17 (2017) 2381.