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 for the realization of 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. Ultimately, we aim to elucidate the crucial role of the substrate surface in lattice dynamics and the interplay between the symmetries of the substrate and the epitaxial layer. These topics are explored in close collaboration with the Chair led by Prof. Ian D. Sharp.

Relevant publications:

• Van der waals epitaxy of 2D gallium telluride on graphene: growth dynamics and principal component analysis
• 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 a few-layer 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, which are strongly interacting many-body states, opening up a new and exciting field named Moiré flat band physics. For these purposes, controlling surface and interface quality, grain orientation, layer thickness, substrate screening, and band alignment is essential. This is now possible with our all-UHV 2D-MBE analytical cluster. Ultrapure few-layer 2D materials and heterostructures with different alignments and hybrid configurations can be realized by MBE, and their optical, electronic, and excitonic properties are investigated using in-situ Raman and photoluminescence spectroscopies. Importantly, pristine information on air-reactive materials (for example, group-III monochalcogenides) can now be unveiled, above all, the symmetry, chemical, and excitonic structures, electronic gap, carrier recombination, and lifetime. These topics are explored in close collaboration with Dr. Elena Blundo, 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-dependent capacitive transient. Interestingly, the capacitance in 2D materials, where defects are abundant and of different types, is very large due to quantum confinement and screening effects. Thus, we are able to obtain several fundamental parameters, such as the nature of majority/minority carrier traps, emission rates and activation energies, capture cross-sections, amounts, and concentration profiles. Information about carrier dynamics will then be provided via ODLTS/DLOS by exciting the auto-trap states with optical pulses, thereby opening up the study of new functionalities through 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 surpass the capabilities of conventional thin films, as well as their applications in sustainable energy conversion. In our group, we investigate several means for widely tuning the band gaps of 2D materials, including control of layer numbers, alloying, substrate engineering, and the 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, as they are efficient photoabsorbers and exhibit high photoresponsivity with a fast response time. Notably, they can be utilized for photocatalytic solar water splitting due to their band gaps in the visible range, the proximity of their band edges to the water redox potentials, large carrier mobilities, and small exciton binding energies. Both of these applications can be pursued by exploiting 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 ultimate goal is to optimize devices with multiple functions at the material level, thereby creating 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