University of Münster

Technical specialities / equipement:

Methodology

• Micromechanical Exfoliation and Deterministic Transfer of van der Waals Heterostructures

Deterministic transfer of 2D crystals plays a key role in the fabrication of 2D heterostructures. It enables engineering of materials with new exciting electro-optical properties and paves the way for their integration into complex devices, the discovery of new phenomena and the exploration of novel applications.

The first step in the fabrication of heterostructures is the preparation of atomically thin crystals. The method used in our research group is micromechanical exfoliation and all-dry viscoelastic stamping [cite]. It provides the high-quality and rotational controlled van der Waals stacks. In more detail, bulk crystals of layered semiconductors are thinned down by an adhesive tape and afterwards placed on a piece of polymer.

 

With our home-built stacking device the desired atomically thin crystals are deterministically transferred to a substrate.

This procedure can be performed repeatedly to stack many crystals on top of each other. Vacuum annealing between stacking steps reduces interlayer residuals such as air bubbles or trapped water, resulting in larger and smoother interfaces and thus better devices.

• Fabrication of opto-/electronic circuitries

Electric contacts are required to access and manipulate the electronic properties of a material. Therefore, we fabricate gold contacts by lithography onto our substrates. Micrometer sized atomically thin flakes are stamped directly to the gold contacts or are connected by additional graphene flakes. In addition, we use field effect structures with hBN and graphene gate or solid electrolyte gate electrodes to apply electric field or to tune the charge carrier density in 2D materials. Wires soldered to the gold contact pads connect the flakes to the electrical measurement setups (DC & AC) such as source measurements units or lock-in amplifiers. All optical cryostats are equipped with wires and electrical feed throughs to enable simultaneous optical, electronic, or optoelectronic measurements down to millikelvin temperatures.

For the preparation we operate at the institute a maskless SmartPrint tool in addition to a PVD machine and for more advanced fabrication we have access to a full equipped state-of the art clean room facility at the Münster Nanofabrication Facility (MNF).

 

 

• Millikelvin magneto optics and transport

With our cryogen-free dilution refrigerator (Bluefors LD400) we are able to perform optical and transport measurements down to 10mK at magnetic fields up to 7T. The system is equipped with x-y-z piezo actuators and a low-temperature objective allowing lateral resolution of about 1 µm in the optical measurements.

 

Applying a magnetic field to a material can have a huge impact on its optical and electronic response. It is possible – especially at millikelvin temperatures, where the thermal broadening of the electronic bands is minimized – to probe the ground state and collective excitations of the system. The magnetic field perturbs the electronic band structure of the probed material.  For example, energy degeneracies in the electronic band structure are lifted due to an interaction of the magnetic field with the spin of the electrons located in certain bands. As another example, the quantum hall effect can be observed optically in photoluminescence and resonant inelastic light scattering measurements on a two-dimensional charge carrier system hosted in a GaAs quantum well with a tunable applied magnetic field. This can then be correlated to transport measurements under varying magnetic field.

• Resonant Inelastic light scattering

Inelastically scattered light on fundamental excitations such as phonons, plasmons or other collective electronic excitations involves exchange of energy such that the scattered light has lost or gained the energy of the excitation. By tuning the energy of the incoming or scattered light to a fundamental interband transition of the electronic bands, collective electronic excitations such as charge and spin density waves can be accessed and the intensity of phonon modes resonantly enhanced. The low-energy excitation spectra act like a fingerprint of a crystal structure, is sensitive to strain, defects, temperatures or unambiguously identifies correlated electronic phases and related phase transitions. Such low-lying collective excitations exist in low-dimensional structures such as two-dimensional charge carrier systems or the moiré superlattices formed by twisted TMDC homo- and hetero- bilayers.

 

 

For these experiments, our laboratory is equipped with an actively stabilized tunable near infrared Ti-Sa CW laser (Sirah Matisse 2) and visible solid-state lasers to excite the samples in resonance with their electronic/excitonic transitions. The scattered or emitted light from the sample is collected by efficient optics and is analyzed using a triple-stage spectrometer coupled to a liquid nitrogen cooled CCD camera (S&I trivista). The spectrometer allows us to measure the scattered light spectrum down to sub meV in energies.

 

 

• Sub-kelvin Imaging Spectroscopic Ellipsometry and transport

Ellipsometry is a non-destructive, optical method that measures the change in the polarization of collimated monochromatic light being reflected from thin films at a finite angle of incidence.) We perform Spectroscopic Imaging Ellipsometry (SIE) in the visible and near-infrared range with a lateral resolution with less than 2µm. Using an optical multilayer model and regression analysis, we can determine parameters such as optical constants, dielectric functions, and thicknesses of layered thin film systems.

 

Our samples can be cooled using an adiabatic demagnetization refrigerator (ADR, Kiutra S-Type) with optical access equipped with x-y-z and rotational piezo actuators. With this setup we perform ellipsometry at temperatures from 800 mK up to 300 K.

 

• Optical Characterization Labs

Our optical characterization labs combine various powerful light analysis tools. The Leica Microscope with epi-illumination is used during sample fabrication and for basic optical characterization during the fabrication process. In the Micro-Raman and -Photoluminescence setup optical properties can be studied using various laser sources (488 nm, 532 nm, 632 nm, 640 nm etc) in a temperature range from room temperature down 77 K using a liquid nitrogen continuous flow cryostat equipped with piezo actuators. In addition to the low noise spectrometer cameras, an intensified CCD (4 Picos – Stanford Computer Optics) enables time-resolved spectroscopy with a temporal accuracy down to 200 ps. The optical characterization is complemented by a µ-Raman and photoluminescence setu-up from Witec equipped with an Linkam 77K cell.   In a new setup the photocurrent induced by the various light sources can be measured using a light chopper and a lock-in amplifier, for example to characterize the quality of semiconductor metal junctions in the fabricated optoelectronic circuitries.