Epitaxial growth of nanostructures

We study the epitaxial growth of nanostructures by molecular beam epitaxy (MBE) with a special focus on the van der Waals epitaxy of 2D materials (group-III-monochalcogenides/nitrides). MBE is well known for its high purity material and scalability, superior control of both thickness and doping profile and sharp interfaces. By correlating in situ reflection high energy electron diffraction and line-of-sight quadrupole mass spectrometry we obtain fundamental information on the growth processes, which are governed by thermodynamics (energetics) and kinetics (diffusion and nucleation). In addition, it allows to realize more complex structures where different 2D materials are stacked on top of one another or in the same plane, namely vertical and lateral heterostructures, respectively. The last ones are very intriguing and disclose a realm of new nanomaterials. Finally, we aim at understanding the critical role of the substrate surface on the lattice dynamics and the interplay between the symmetries of the substrate and the epitaxial layer. These topics are explored in close collaboration with Dr. Gregor Koblmüller from SNQS and the Chair led by Prof. Ian D. Sharp.

Relevant publications:

•  Two-dimensional single crystal monoclinic gallium telluride on silicon substrate via transformation of epitaxial hexagonal phase

•         Hints for a general understanding of the epitaxial rules for van der Waals epitaxy from Ge-Sb-Te alloys
•        Large-area van der Waals epitaxy and magnetic characterization of Fe3GeTe2 films on graphene
•        Modulation of van der Waals and classical epitaxy induced by strain at the step edges
  

•         Interplay between structural and thermoelectric properties in epitaxial Sb2+xTe3 alloys
  
•         Independent wavelength and density control of uniform GaAs/AlGaAs quantum dots grown by infilling self-assembled nanoholes
  
•         Controlling the formation of quantum dot pairs using nanohole templates

In-situ spectroscopy of van der Waals materials

We are interested in the optical and electronic properties of few layers 2D materials “beyond graphene”. When heterostructures are formed by stacking 2D materials, unique physical features emerge such as interlayer excitons, where the electron and hole are located in different layers, or dipolar excitons as strongly interacting many-body states, opening up the study of a new exciting field named Moiré flat band physics. For these purposes, the control of the surface and interface quality, grain orientation, layer thickness, substrate screening and band alignment are essential. This is finally possible with our all-UHV 2D-MBE-analytical cluster. Ultrapure few layer 2D materials and heterostructures with different alignment and hybrid configurations can be realized by MBE and the optical, electronic and excitonic properties are investigated by in-situ Raman and photoluminescence spectroscopies. Importantly, pristine information of air-reactive materials (for example, group-III-monochalcogenides) can be now unveiled, above all, the symmetry, chemical and excitonic structures, electronic gap, carrier recombination and lifetime. These topics are explored in close collaboration with Dr. Gregor Koblmüller, Dr. Andreas Stier and Dr. Nathan Wilson from SNQS.

Relevant publications:

•         Evolution of low-frequency vibrational modes in ultra-thin GeSbTe films

•        Laser induced structural change in chalcogenide based superlattices
•         Strain-induced active tuning of the coherent tunneling in quantum dot molecules
•         Universal recovery of the bright-exciton level-degeneracy in quantum dots without structural symmetry
  
  

Defect states in van der Waals materials

Defects or traps play a crucial role in device engineering for determining the suitability of a specific material in terms of performance and reliability. Our goal is to shed light on the main trap characteristics by performing standard deep level transient spectroscopy (DLTS) and optical DLTS (ODLTS and DLOS) of the MBE grown layered materials. DLTS is an electrical technique for monitoring deep levels that exist in the depletion region of single junctions or semiconductor devices by performing a temperature dependence capacitive transient. Interestingly, the capacitance in 2D materials, where defects are abundant and of different type, is very large due to quantum confinement and screening effects. Thus, we are able to obtain several fundamental parameters such as nature of majority/minority carrier trap, emission rates and activation energy, capture cross-sections, amount and concentration profile. Information about carrier dynamics will then be given via ODLTS/DLOS by exciting the auto-trap states with optical pulses and will open up the study of new functionalities by means of defect engineering. These topics are explored in close collaboration with Prof. Hubert Ebert from LMU within the e-conversion Cluster of Excellence.

Van der Waals materials for optoelectronics and energy conversion

Van der Waals materials have proven to be an attractive playground for applied materials, owing to their unique properties in flexible and transparent electronics and optoelectronics that go beyond the capability of conventional thin films and their applications in sustainable energy conversion. In our group, we investigate several means for widely tuning the 2D materials bandgaps such as control of layer numbers, alloying, substrate engineering, formation of heterostructures. In addition, due to the large amount of strain they can endure and the flat band structures, we employ ex-situ strain and electrical field engineering to enhance the material functionalities and correlation effects. We are especially interested in layered group-III monochalcogenides since they are efficient photoabsorbers and show high photoresponsivity with fast response time. Strikingly, they can be used for photocatalytic solar water splitting due to their band gaps in the visible, the vicinity of the band edges to the water redox potentials, large carrier mobilities and small exciton binding energies. Both these applications can be pursued by the exploitation of the high-quality material produced in our 2D-MBE-Analytical cluster. In addition, we focus on atomically thin group-III 2D-nitrides as an emerging class of 2D-materials for light generation and light-induced energy harvesting applications.The final goal is to optimize devices with multiple functions at the material scale in order to create superior systems.

Relevant publications:

•         Strain-induced active tuning of the coherent tunneling in quantum dot molecules
•         Universal recovery of the bright-exciton level-degeneracy in quantum dots without structural symmetry
•         Modulation of van der Waals and classical epitaxy induced by strain at the step edges