April 22 - 26, 2024
Seattle, Washington
May 7 - 9, 2024 (Virtual)
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2024 MRS Spring Meeting & Exhibit
EL05.03.05

Effects of High-k Dielectric Encapsulation and Carrier Density on Raman Scattering in Synthetic Monolayer WS2

When and Where

Apr 23, 2024
4:15pm - 4:30pm
Room 344, Level 3, Summit

Presenter(s)

Co-Author(s)

Jerry Yang1,Lauren Hoang1,Tara Pena1,Zhepeng Zhang1,Andrew Mannix1,Eric Pop1

Stanford University1

Abstract

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, WS2 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 MoS2 [2], as well as evaluate the damage induced in MoS2 from metal deposition [3]. However, these characterization techniques have not been experimentally examined in WS2, which features different Raman signatures from MoS2.

Here, we investigate the effects of dielectric environment and carrier density on the Raman spectrum of WS2. We start with three samples of monolayer WS2 grown by chemical vapor deposition, one on SiO2/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 SiO2/Si sample and one PEN sample with a 1.5 nm Al seed layer + 10 nm Al2O3 [4], then the final PEN sample with 1 nm Si seed layer + 10 nm Al2O3 [5].

We find that, of the primary peaks in the WS2 Raman spectrum, the 2LA(M) peak at ~350 cm-1 is the most sensitive to encapsulation. The as-grown, unencapsulated WS2 on SiO2 sample exhibited a 2LA(M) peak position ~3.2 ± 0.3 cm-1 higher than the transferred, unencapsulated WS2 on Al2O3 back-gate sample. After encapsulation with the Al+Al2O3 layer, the 2LA(M) peak in the SiO2 sample redshifts by an additional ~1.0 ± 0.4 cm-1 while that of the Al2O3 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 WS2 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/cm2 Al2O3 gate oxide capacitance, these shift rates correlate to ~0.19 ± 0.01 cm-1 per 1012 cm-2 carriers for the 2LA(M) peak and ~0.1 ± 0.05 cm-1 per 1012 cm-2 carriers for the A’ peak. This study is the first measurement of the carrier density-dependent Raman spectra of WS2 without ionic liquid gating, which provides a better benchmark relative to the intrinsic carrier density of WS2 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).

Keywords

chemical vapor deposition (CVD) (chemical reaction) | optical properties | spectroscopy

Symposium Organizers

Silvija Gradecak, National University of Singapore
Lain-Jong Li, The University of Hong Kong
Iuliana Radu, TSMC Taiwan
John Sudijono, Applied Materials, Inc.

Symposium Support

Gold
Applied Materials

Session Chairs

Kevin O'Brien
Aaron Thean

In this Session