Rapid Detection of Defects in Monolayer 2D Semiconductors by Polarized Raman Spectroscopy

An outstanding challenge in integrating two-dimensional (2D) transition metal dichalcogenides (TMDs) into future optoelectronics is optimizing high-quality and large-scale growths of these atomically thin materials. Several methods exist for wafer- and chip-scale growths of 2D-TMDs, such as chemical vapor deposition (CVD) and atomic layer deposition (ALD). Still, we see a large degree of variability from growth to growth due to intrinsic and extrinsic factors like defects and humidity [1]. Optimizing the rapid characterization methods used to discern material quality before fabrication is essential to improve the yield of devices produced during research and development, saving time and money.<br/> <br/>Raman spectroscopy is a non-destructive and standard characterization technique, to examine various material features such as thickness [2], doping [3], strain [4], and disorder [5, 6]. However, the effect of doping and disorder on the Raman signatures still remains to be examined in some TMD systems (e.g., WSe₂). In this work, we propose using cross-polarized Raman spectra [7] to isolate the in-plane E’ mode from the out-of-plane A₁’ mode with a 532 nm laser to properly examine defect-induced Raman features in two monolayer TMD materials of broad interest. Based on the phonon dispersion curves, we focus on MoS₂ and WSe₂, because they have the most suitable optical phonon energy and dispersion for this method.<br/> <br/>The isolation of the E’ mode by cross-polarization is especially effective in the case of WSe₂, which shows a strong degeneracy of the E’ and A₁’ modes around 250 cm^-1, making analysis a unique challenge. Due to the nature of defects interrupting the vibration of atoms in-plane, the transverse and longitudinal modes are the most sensitive to changes in defect density. For this reason, we study the amplitude changes of the in-plane E’ mode in relation to the second order M point phonon modes that appear with increasing disorder.<br/> <br/>After the data are collected, the spectra are fit with a Gaussian and Lorentzian spectral blend to extract the peak height, position, and full width at half-maximum (FWHM). The ratio between the left shoulder second-order phonon modes and the first-order E’ mode is used as a figure of merit to estimate the expected defect density and the majority carrier mobility in the 2D material. Our approach can estimate defect densities down to the order of 10¹¹ cm^-2 using TMD samples procured by different synthesis methods. To compare the method’s validity, the same samples are also characterized by conductive atomic force microscopy (c-AFM) and electrical measurements of mobility.<br/> <br/>This rapid, non-destructive Raman characterization method can be integrated into commercial growth systems to accelerate TMD research and development for practical applications. Future work is also planned to expand these efforts to other TMDs. The work was supported by an NSF Graduate Fellowship (C.A.N.), an NSF Ascend Fellowship (T.P.), by the SRC SUPREME Center and the Stanford SystemX Alliance.<br/> <br/>[1] K. K. H. Smithe<i> et al.</i>, <i>ACS Nano, </i><b>11</b>, 8456 (2017).<br/>[2] Y. Zhao<i> et al.</i>, <i>Nano Lett., </i><b>13</b>, 1007 (2013).<br/>[3] A. Michail<i> et al.</i>, <i>Appl. Phys. Lett., </i><b>108</b>, 173102 (2016).<br/>[4] I. M. Datye<i> et al.</i>, <i>Nano Lett., </i><b>22</b>, 8052 (2022).<br/>[5] W. Shi<i> et al.</i>, <i>Chin. Phys. Lett., </i><b>33</b>, 057801 (2016).<br/>[6] W. Shi<i> et al.</i>, <i>2D Mater., </i><b>3</b>, 025016 (2016).<br/>[7] J. Kim<i> et al.</i>, <i>J. Phys.: Condens. </i><i>Matter, </i><b>32</b>, 343001 (2020).