Apr 23, 2024
4:15pm - 4:30pm
Room 344, Level 3, Summit
Jerry Yang1,Lauren Hoang1,Tara Pena1,Zhepeng Zhang1,Andrew Mannix1,Eric Pop1
Stanford University1
Two-dimensional (2D) semiconductors have gained significant interest due to their atomically thin structure, theoretically pristine van der Waals interfaces, good carrier mobility, and potential utility for future opto-electronics. Among 2D semiconductors, tungsten disulfide, WS
2 is particularly interesting, as it can exhibit electronic ambipolarity at monolayer and bilayer thicknesses [1].
Raman spectroscopy is a fast, non-destructive characterization technique that enables rapid, large-area analysis of 2D semiconductors. Previous work has utilized Raman spectroscopy to quantify the strain and carrier density in monolayer MoS
2 [2], as well as evaluate the damage induced in MoS
2 from metal deposition [3]. However, these characterization techniques have not been experimentally examined in WS
2, which features different Raman signatures from MoS
2.
Here, we investigate the effects of dielectric environment and carrier density on the Raman spectrum of WS
2. We start with three samples of monolayer WS
2 grown by chemical vapor deposition, one on SiO
2/Si and the others on sapphire. We then transfer the two sapphire samples onto 125-µm thick polyethylene naphthalate (PEN) containing a patterned high-k/metal-gate stack as described in [4]. We encapsulate the as-grown SiO
2/Si sample and one PEN sample with a 1.5 nm Al seed layer + 10 nm Al
2O
3 [4], then the final PEN sample with 1 nm Si seed layer + 10 nm Al
2O
3 [5].
We find that, of the primary peaks in the WS
2 Raman spectrum, the 2LA(M) peak at ~350 cm
-1 is the most sensitive to encapsulation. The as-grown, unencapsulated WS
2 on SiO
2 sample exhibited a 2LA(M) peak position ~3.2 ± 0.3 cm
-1 higher than the transferred, unencapsulated WS
2 on Al
2O
3 back-gate sample. After encapsulation with the Al+Al
2O
3 layer, the 2LA(M) peak in the SiO
2 sample redshifts by an additional ~1.0 ± 0.4 cm
-1 while that of the Al
2O
3 sample redshifts by ~4.5 ± 0.5 cm
-1. We also find a 2× stronger red-shift for the 2LA(M) peak in the Si seed sample (~8.1 ± 0.8 cm
-1) than in the Al seed encapsulation sample. This indicates that the 2LA(M) peak may be useful as an optical marker for interfacial quality. In comparison, the first-order E’ and A’ peaks shift by less than 1.5 ± 0.5 cm
-1 across all samples.
We also perform
in situ Raman measurements as a function of carrier density in the same back-gated WS
2 structure. We find that the 2LA(M) peak red-shifts by 0.44 ± 0.06 cm
-1/V and the A’ peak red-shifts by 0.24 ± 0.03 cm
-1/V, while the E’ peak does not shift substantially. With a ~350 nF/cm
2 Al
2O
3 gate oxide capacitance, these shift rates correlate to ~0.19 ± 0.01 cm
-1 per 10
12 cm
-2 carriers for the 2LA(M) peak and ~0.1 ± 0.05 cm
-1 per 10
12 cm
-2 carriers for the A’ peak. This study is the first measurement of the carrier density-dependent Raman spectra of WS
2 without ionic liquid gating, which provides a better benchmark relative to the intrinsic carrier density of WS
2 compared with previous work [6].
Our results advance Raman spectroscopy for 2D materials in two ways: first, they extend previous studies on spectroscopic carrier density measurements to near-intrinsic carrier densities, and second, they showcase the utility of Raman spectroscopy for characterizing interactions between 2D materials and high-k dielectrics for industry-relevant integration. This work was supported in part by a NSF Graduate Fellowship (J.A.Y.), by the Stanford SystemX Alliance, and by the SRC-SUPREME Center.
[1] G. Lee
et al., ACS Appl. Mater. Interfaces, 12, 23127 (2020).
[2] A. Michail
et al., 2D Mater., 8, 015023 (2020).
[3] K. Schauble, E. Pop
et al., ACS Nano, 14, 14798 (2020).
[4] J. A. Yang, E. Pop
et al., arXiv:2309.10939 (2023).
[5] H. Zhang
et al., Chem. Mater., 29, 6772 (2017).
[6] T. Sohier
et al., Phys. Rev. X, 9, 031019 (2019).