In the Spectroscopy working group, the focus of the work is on material characterisation using optical spectroscopy, essentially photoluminescence spectroscopy (PL). However, it is also possible to investigate electroluminescence.

In PL, the sample to be analysed is irradiated with light (usually a laser) and the light generated, the luminescence, is measured spectrally. This can be used to obtain information on the band gap energy in semiconductors and their nanocrystals as well as information on the content of impurities/dopants and crystal lattice defects, as long as these allow radiative recombination transitions. By investigating temperature-dependent changes in the luminescence intensity, the influence of non-radiative defects on the charge carrier recombination behaviour can also be investigated. Time-resolved luminescence transients can be used to determine an effective charge carrier lifetime, which, when compared between different samples or at different temperatures, allows further conclusions to be drawn about relevant recombination channels in the material.

Semiconductors with a large bandgap are important for power, high-frequency and optoelectronics. Their high-quality production is still challenging and contactless optical characterisation options can make an important contribution to improving manufacturing processes, but also efficiently control subsequent processing steps of various semiconductor components through the information obtained.

Gallium nitride (GaN), aluminium nitride (AlN) and other compound semiconductors based on them are particularly suitable for the production of light emitters in the blue and ultraviolet spectral range due to their direct and large band gap. Due to their high charge carrier mobility and high breakdown field strength, they are also used in high-performance and high-frequency electronic components.

Point defects (native defects, impurities and dopants) in the semiconductor material have a strong influence on the optical and electronic properties of the components. Both thick crystals (on the order of millimetres thick) and thin layers (on the order of micrometres or less) with various dopants are examined at the photoluminescence (PL) measuring station. These often cause deep defects in the band gap, through which charge carriers can recombine radiatively or non-radiatively, which influences the charge carrier lifetime and thus the functionality of the subsequent device. Temperature-dependent PL measurements are used to analyse the defects involved as well as their activation energies and recombination mechanisms. At low temperatures, transition metal impurities in particular can be identified at characteristic internal transitions in the defect centres.

Currently, there is a strong focus on investigating the properties of individual quantum systems, such as defect centres in semiconductors, with regard to their suitability for generating, processing, transmitting or storing quantum information, i.e. as quantum bits - qubits. Here we are working on the characterisation of point defects in semiconductors with a large band gap, in particular diamond and AlN. However, the development of nanodiamond layers for the use of known qubit centres, e.g. the nitrogen-vacancy (NV) centre in diamond, for optical magnetic field sensor applications, also forms part of our work.

How long charge carriers, which have been introduced into a material or component via optical or electrical excitation, remain available as freely movable charge carriers is of fundamental importance for a variety of applications. For some, such as high-frequency components, this charge carrier lifetime should be as short as possible; for others, such as solar cells, it should be as long as possible. Using time-resolved photoluminescence (TRPL) and microwave-detected photoconductivity (MDP or µ-PCD), this material parameter can be determined in a wide variety of semiconductor material systems. In particular, MDP can also be used to determine photoconductivity, which bridges the gap between optical and contact-based electrical conductivity measurement methods.

For a brief insight into the WIDE-MDP project (Microwave-detected photoconductivity on wide-bandgap semiconductors for the next generation of power and optoelectronics), watch our Video.

We are also working on the development of contactless conductivity measurement using terahertz spectroscopy as part of a joint project with several other partners from research institutes and industry.

In the extraction of raw materials, but also in the recycling process, it is essential to be able to recognise different elements or substances quickly and specifically without contact in order to be able to sort and process them efficiently.

Some chemical elements allow internal radiative transitions, e.g. some transition or rare earth metals, which often enable very clear identification, even if they are embedded in different mineral environments. Using spatially resolved photoluminescence, correlations of local rare earth concentrations with the rock structures are determined. However, fundamental investigations into the spectrally selective excitation of rare earth luminescence and temperature-dependent quenching processes are also being carried out.

For example, over a period of three years, the EIT RawMaterials - an integrated spectroscopy sensor system for laser-induced fluorescence and hyperspectral imaging was developed over three years in the joint project InSPECtor. The novel sensor system enables efficient non-contact and non-destructive mapping of geological samples with regard to rare earth distribution with high spatial resolution. A video gives you a brief insight into the project.

As a result, the joint project "RAMSES-4-CE - Raman, Absorption and eMission Spectroscopy in an intEgrated Sensor for the Circular Economy", this system was expanded to include a Raman sensor that can reliably recognise many materials that occur in recycling material flows in particular. The efficient data processing of such a multi-sensor system using machine learning was also at the forefront here.

The idea of spectroscopic material recognition in the recycling sector is being continued in the DIGISORT project, in which various optical methods are used to differentiate between the classes of material produced after mechanical shredding of the Li-ion batteries to be recycled. Among other things, high-speed RGB cameras and object recognition using machine learning are used here.