Advanced X-ray Characterisation for the development of efficient high power transistors

Published : 4 January 2017

One of the objectives of the « Commissariat aux Energies Atomiques et Alternatives » (CEA) is to support scientific and technological research for renewable energies. As part of this objective, CEA-LETI is developing highly efficient power transistors and lighting (with LEDs) with several important industrial partners. These high performance components include thin layers of III-nitride semiconductor materials (GaN, AlGaN, InGaAlN), which are grown by epitaxial deposition on 200 mm diameter silicon wafers. The reliability and performance of these layers is closely linked to their microstructural characteristics, such as their crystalline quality, the strain in the layers and any composition gradients.

Currently, CEA-LETI is studying two different III-N hetero-structures for use in high power components. The first, AlGaN/GaN structures, are already being integrated into industrial devices, while the second, InGaAlN/GaN have the potential to produce even better performance, especially as they are lattice matched on GaN pseudo-substrates. Finally, it has been shown [1] that ultrathin layers of dichalcogenide materials (MoSe2, MoS2, etc) can be integrated into these components to minimize strain during growth, and to improve the performance of the devices.

It is necessary to have a deep understanding of the microstructure of these materials in order to fully master such complex systems. It is therefore the goal of this PhD to develop techniques to measure and analyze these complex hetero-structures using techniques based on X-rays. These developments will both ensure the quality of the GaN based structures currently being used at CEA-LETI, and increase the understanding of the innovative structures described above, leading to their improvement. The work will include:

– The determination of the microstructure of these materials (quaternary alloys of InGaAlN on GaN, interface layers influence on the quality of growth) and especially the measurement of strain fields in the materials with reciprocal space mapping using X-ray diffraction

– Analysis of segregation/diffusion at the interfaces or in the bulk of InGaAlN by combining X-ray diffraction, grazing incidence X-ray fluorescence (GIXRF), and X-ray photoelectron spectroscopy (XPS). In-situ annealing will also be implemented in these experiments.

For these studies, the student will have access to state of the art laboratory equipment (whether for characterization or growth of structures) and will also have the opportunity to work at the synchrotron. The analysis will also be complemented with other advanced characterization tools (Auger spectroscopy, Atom probe tomography, TEM etc) available on the nano-characterisation platform at CEA-LETI.

[1].Gupta, P.; Rahman, A. A.; Subramanian, S.; Gupta, S.; Thamizhavel, A.; Orlova, T.; Rouvimov, S.; Vishwanath, S.; Protasenko, V.; Laskar, M. R.; Xing, H. G.; Jena, D.; Bhattacharya, A. Scientific Reports 2016, 6, 23708.

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