Thickness Mapping and Layer Number Identification of Exfoliated van der Waals Materials by Fourier Imaging Micro-Ellipsometry

Abstract

As performance of van der Waals heterostructure devices is governed by the nanoscale thicknesses and homogeneity of their constituent mono-to few-layer flakes, accurate mapping of these properties with high lateral resolution becomes imperative. Spectroscopic ellipsometry is a promising optical technique for such atomically thin-film characterization due to its simplicity, noninvasive nature and high accuracy. However, the effective use of standard ellipsometry methods on exfoliated micron-scale flakes is inhibited by their tens-of-microns lateral resolution or slow data acquisition. In this work, we demonstrate a Fourier imaging spectroscopic micro-ellipsometry method with sub-5 μm lateral resolution and three orders-of-magnitude faster data acquisition than similar-resolution ellipsometers. Simultaneous recording of spectroscopic ellipsometry information at multiple angles results in a highly sensitive system, which is used for performing angstrom-level accurate and consistent thickness mapping on exfoliated mono-, bi-and trilayers of graphene, hexagonal boron nitride (hBN) and transition metal dichalcogenide (MoS 2 , WS 2 , MoSe 2 , WSe 2 ) flakes. The system can successfully identify highly transparent monolayer hBN, a challenging proposition for other characterization tools. The optical microscope integrated ellipsometer can also map minute thickness variations over a micron-scale flake, revealing its lateral inhomogeneity. The prospect of adding standard optical elements to augment generic optical imaging and spectroscopy setups with accurate in situ ellipsometric mapping capability presents potential opportunities for investigation of exfoliated 2D materials.

In-Column Backscattered Electron Microscopy for a Rapid Identification of the Number of Layers in MoS2 Nanosheets: Implications for Electronic and Optoelectronic Devices

“…The result shows that the height of one layer of MoS 2 is ∼0.78 nm, which is comparable to the theoretical thickness of 1L MoS 2 (0.67 nm). 28,30,54 However, it is overestimated when measuring the height between the MoS 2 layer and SiO 2 substrate (∼1.97 nm height between 1L MoS 2 and the SiO 2 substrate in the top right panel of Figure 5b), showing the limitations of using AFM. Such an overestimation is known to result from the presence of trapped air and/or adsorbed water molecules between MoS 2 and the SiO 2 substrate or the difference in AFM tip−sample interaction between MoS 2 and the SiO 2 substrate.…”

“…In the MoS 2 /SiO 2 sample, however, it should be interpreted from a different perspective, as the chemical composition of MoS 2 remains unchanged, regardless of the layer number. As the incident PE−sample interaction volume is always larger than the thickness of the MoS 2 layer up to 4L (2.68 nm) because of the atomic scale thickness of MoS 2 (0.67 nm for 1L MoS 2 54 ), the number of Mo and S atoms that interacts with the incident PEs is proportional to the MoS 2 layer number (i.e., the MoS 2 thickness). Following this mechanism, therefore, more BSEs are produced with increasing the MoS 2 layer number, and this creates a layer-number-sensitive positive contrast.…”