Keynote Speaker
Sebastian Schaper
Imaging Ellipsometry of anisotropic 2D-materials
Sebastian Schaper (né Funke)1, Jakob Henz1, Hendrik Lambers1, Pierre-Maurice Piel1, and Ursula Wurstbauer1
1University of Münster, Münster, Germany
To understand interactions of the material with its surrounding the knowledge of the dielectric function is crucial. The number of layers of a 2D-material can change the dielectric function or optical properties significantly [1]. The substrate influences the optical properties [2] and a stacking of different 2D-materials can formation of interlayer excitons [3]. We will utilize imaging ellipsometry to characterize the dielectric properties of 2D-materials with high lateral resolution.
Imaging ellipsometry combines ellipsometry and microscopy to allow ellipsometric measurements within microscopic images. Ellipsometry enables the determination of the optical properties and thickness simultaneously. The change of the state of polarization of light upon reflection from a sample surface is measured in the ellipsometric angles Δ and ψ, that describe the phase shift and phase amplitude ratio of p- to s-polarized light respectively.
Although ellipsometry allows the simultaneous determination of optical properties and thickness, a coupling of those parameters is observed for very thin layers (typically noted <10nm). This means thickness and optical properties can no longer be obtained unambiguously. We will show simulations and experiments to break this coupling and thus allow a simultaneous measurement of thickness and optical properties for thin layers to the limit of very few layers by using anisotropic substrates. This means the light-matter interaction is no longer independent from the relative direction of the incoming light to the sample. We will demonstrate this thickness dielectric function decoupling e.g. on insulating hexagonal Boron Nitride and will discuss implementation to other layered materials with and without in-plane anisotropy as e.g. the magnetic 2D material CrSBr.
To measure anisotropic optical properties, previously mentioned Δ and ψ measurements may not be sufficient any longer and we will introduce Mueller-matrix ellipsometry for a full characterization of anisotropic dielectric functions of various materials. We will also present measurement configuration of anisotropic materials, where Δ and ψ measurements can be used.
[1] Funke, S, B Miller, E Parzinger, P Thiesen, A W Holleitner, und U Wurstbauer. 2016. „Imaging Spectroscopic Ellipsometry of MoS 2“. Journal of Physics: Condensed Matter 28 (38): 385301. https://doi.org/10.1088/0953-8984/28/38/385301.
[2] Klein, J., A. Kerelsky, M. Lorke, M. Florian, F. Sigger, J. Kiemle, Mark C. Reuter, T. Taniguchi, K. Watanabe, und J. J. Finley. 2019. „Impact of substrate induced band tail states on the electronic and optical properties of MoS2“. Applied Physics Letters 115 (26). https://pubs.aip.org/aip/apl/article/115/26/261603/38043.
[3] Sigger, Florian, Hendrik Lambers, Katharina Nisi, Julian Klein, Nihit Saigal, Alexander W. Holleitner, und Ursula Wurstbauer. 2022. „Spectroscopic imaging ellipsometry of two-dimensional TMDC heterostructures“. Applied Physics Letters 121 (7): 071102. https://doi.org/10.1063/5.0109189.