New Imaging Ellipsometry Papers: 2D Materials and Nanostructures
A round-up of scientific papers published about imaging ellipsometry, using the Accurion nanofilm_EP4.
Blue Scientific is the official distributor for Accurion in the UK and Nordic region (Finland, Denmark, Sweden, Iceland and Norway). For more information and quotes, please get in touch:
Imaging Spectroscopic Ellipsometry of MoS2
Spectroscopic imaging ellipsometry was used to study flakes of micromechanically exfoliated Molybdenum-disulphide (MoS2), approximately 10 µm x 50 µm. The optical properties, homogeneities and anisotropic behaviour of both mono- and multilayer flakes were characterised. Imaging ellipsometry is an ideal technique for characterising defects and wrinkles on flakes.
MoS2 is a 2D material with great potential; multilayers have an indirect bandgap and monolayers have a direct bandgap. 2D materials often exhibit superior material properties compared to multilayers.
The optical dispersion of MoS2 is dominated by its excitonic behaviour. This can be observed with a sapphire substrate; whereas with a common silicon dioxide or silicon substrate, there is optical contrast of mono- to multilayers but no excitonic behaviour.
Knife-edge illumination can be used to reduce interference from backside reflection. Lateral distribution of optical properties was observed towards the edges of the flakes. Raman was used as a complementary method to ensure that these variations were not caused by strain or lattice defects. A model is proposed, with anisotropic behaviour of the n and k for the out-of-plane component of MoS2. The critical point energy of the out-of-plane dispersion is in good agreement with the direct bandgap of a monolayer of MoS2.
Automated Search for Mono- and n-layers in 2D Materials
In 2D materials research it can still be difficult to locate small flakes of with monolayer thickness and distinguish between different layer numbers. Conventional methods such as optical microscopy require the use of special contrast enhancing substrates. If a particular substrate is required for functional reasons, additional transfer steps are required, which is time consuming and can cause cracking, wrinkling and contamination.
Spectroscopic imaging ellipsometry can be used to automatically search for regions of a specified layer thicknesses on any substrate.
Imaging ellipsometry combines the sensitivity of AFM with the speed of optical microscopy. First a scan of the whole sample is taken, with the ellipsometric angles Δ and Ψ. Each pixel is compared to a model with the specified layer thickness, to find pixels that match. Small flakes of only a few µm size can be located, with a defined thickness. For example, a 1 mm x 1 mm area can be analysed within 20 minutes to locate micron-sized flakes. Monolayers of graphene can be distinguished from tri-layers. A 10 µm x 10 µm of Molybdenum-disulphide can be located on an unknown substrate, while it is being characterised.
The paper includes:
- Localisation of mono- and tri-layers of graphene with extended optical investigation from the UV- to the NIR.
- Localizing a 10 μm flake of MoS2.
- Characterising a thin hBN layer with anisotropic dielectric function.
- Localising a thin hBN layer of a specified thickness and the characterised anisotropic dielectric function.
Spectroscopic Imaging Ellipsometry of Self-Assembled SiGe/Si Nanostructures
Spectroscopic imaging ellipsometry was used to characterise films containing self-assembled SiGe/Si in-plane nanowires, grown by molecular beam epitaxy. A strength of this technique is the high lateral ellipsometric resolution, enabling studies of individual nanowires.
The Δ and Ψ spectra for vertical and horizontal nanowires show strong in-plane anisotropic behaviour. Conventional ellipsometers would average vertical and horizontal nanowires, but with imaging ellipsometry they can be distinguished. The anisotropy can be explained by the presence of waveguide-like leaky resonant modes.
The search tool explained in the paper above was used to locate comparable regions. Horizontal and vertical nanowires can be differentiated clearly. Finding these comparable regions enables the statistics of a thousand single pixels to be recorded in a single measurement. The results show the potential of imaging ellipsometry for research into micro- and nano-structured semiconductors.
The paper covers:
- Investigating the optical response of SiGe/Si nanowires with imaging ellipsometry.
- Studying structural and optical properties with spectroscopic measurements.
- Identifying inhomogeneous Ge content of different film regions.
- The anisotropic response of nanowires is ascribed to waveguide-like resonances.
The instrument used in these papers is the Accurion nanofilm_EP4. Characterise surfaces with a lateral resolution as small as 1 micron. Resolve sample areas 1000 times smaller than most non-imaging ellipsometers, even those with micro spot spectroscopy.
Blue Scientific is the official distributor for Accurion in the UK and Scandinavia. For more information and quotes, please get in touch: