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InGaN/GaN quantum well (QW) heterostructures are key elements of active regions in devices, such as lasers and light emitting diodes. A current bottleneck towards highly-efficient devices operating across the optical spectrum, relates to the quantum efficiency drop (green-gap) following the increase of the indium content to reach the green part of the spectrum.

Due to the presence of a miscibility gap in the InGaN phase diagram and because of the high lattice mismatch of about 11% between GaN and InN, the fabrication of GaN/InGaN heterostructures is a very challenging task. Therefore, active research continues to improve the growth and efficiency of InGaN LEDs.

Our group fabricates GaN/InGaN heterostructures using a state-of-the-art MBE facility. We have bachelor and master theses available in this area.

Lighting accounts for 15–22% of electricity consumption, depending upon the country, and solid-state lighting has the potential to provide enormous energy savings across the globe. The Nobel prize in Physics 2014 was assigned for the invention of efficient blue light-emitting diodes (LEDs) which has enabled bright and energy-saving white light sources.

LEDs are based on a pn-junction diode in which the active region is usually a semiconductor heterostructure whose carriers are excited by the applied voltage. Electron-hole pair radiative recombination causes the emission of a photon, with a frequency related to the band gap energy of the semiconductor. The blue-LEDs are based on GaN/InGaN heterostructure pn-diodes.

Our group fabricates novel GaN/InGaN heterostructures using a state-of-the-art MBE facility. To assess the potential of the novel MBE structures pn-diodes need to be fabricated and their electroluminescence to be characterised. The more technological part of the work (lithography, dry-etching, metal contacts) will be realised in the state-of-the-art clean room facility of the physics department.

We have bachelor and master theses available in this area.

The discovery of the extraordinary properties of graphene has also sparked a great deal of interest in other 2D materials. Transition metal dichalcogenides (TDMs) represent a prominent example. In contrast to graphene, a monolayer of molybdenum disulfide (MoS2), for example, represents a semiconductor with a direct band gap. This makes this material highly interesting for potential applications in the field of light-emitting diodes and digital circuits.

Modern epitaxial techniques can be used to successfully fabricate such systems. In our group, we aim to fabricate transition metal dichalcogenides using molecular beam epitaxy (MBE). For this purpose, an ultra-high vacuum facility is being built in our laboratory, which will have a total of two growth chambers and one analysis chamber. One of the growth chambers is already operational, the second chamber is currently under construction and will offer significantly more possibilities in terms of material combination and growth control once completed.

In this area, a master thesis is preferably to be assigned, which deals with the growth and characterization of 2D materials. In parallel, the construction of the second MBE chamber will be supported together with the permanent staff of the group.

When a direct semiconductor is optically excited, it relaxes back to its ground state by emitting photons, among other things. From this so-called photoluminescence spectrum (PL), important conclusions can be drawn about the electronic structure of the material. If this versatile investigation method is combined with a microscope-like setup, semiconductors can be analyzed with nanostructures in a spatially resolved manner (µ-PL).

In our group we have a self-developed micro-photoluminescence setup, which allows temperature-dependent PL measurements in the range of 5 K to 500 K. The excitation is done by a HeCd-emitter. For excitation we use a HeCd laser, which can also be used to excite semiconductor materials with a band gap outside the visible spectrum.

In the field of optical characterization there are many opportunities for bachelor and master theses. In addition to the characterization of 2D materials and GaN/InGaN heterostructures, further development of the equipment is also planned. On the one hand, the optimization of the spatial resolution and on the other hand the further development of the measurement and analysis software are in the foreground. In addition, our micro-photoluminescence setup will also be available in the future for the Master's research practical course. The didactic development of a corresponding experiment for the advanced lab-course ("Master F-Praktikum") could be a very appealing project, especially for students with the profile of teaching.