Quality Improvement of Molecular Beam Epitaxy Grown Topological Insulator Thin Films and in situ Fabrication of Devices

Schleenvoigt, Michael

Master Thesis

FZJ-2018-01010

Quality Improvement of Molecular Beam Epitaxy Grown Topological Insulator Thin Films and in situ Fabrication of Devices

Schleenvoigt, M. (Corresponding author)FZJ*

2017

82 pp. (2017) = Masterarbeit, RWTH Aachen University, 2017

Please use a persistent id in citations: http://hdl.handle.net/2128/17019

Abstract: In 2005, Kane and Mele introduced topological insulators as a new material class in the vast field of solid state physics. Since then, research on the topic led to a plethora of discoveries, ranging from two-dimensional systems to many classes of three-dimensional topological insulators. Topological insulators exhibit promising features, useful for example for spintronic applications or quantum computing. However, due to large background doping, these features are often suppressed. Therefore, measures need to be taken in order to lower the doping, enhancing the topological characteristics.In this thesis, molecular beam epitaxy grown three-dimensional topological insulators of the tetradymite crystal class, consisting of bismuth, antimony, tellurium and selenium (V2VI3), are investigated. In the course of the work, three points of interest in a conventional sample are investigated with regard to the possibility to improve them. These are the interface to the substrate, the bulk and the interface to the environment.First, the interface of the grown film to the substrate has been examined. Two approaches have been pursued to influence the growth of the film. One has been the use of prepatterned samples to grow topological insulator films in fixed structures. The other has been changing the substrate material and growing thin layers of optimized films, fit to the new surfaces, as pseudo substrates for subsequent growths.Next, the focus has been set on the bulk of the film. In order to optimize the electronic qualities of the topological insulator, a quaternary material system has been grown via molecular beam epitaxy. The quaternary system promises less background doping and has not been grown by the means of molecular beam epitaxy before.Thirdly, the interface to ambient conditions has been investigated. To protect the surface of the film from contaminations during ex situ processes, a technique to fabricate devices in situ, the stencil lithography, has been refined. The masks used for the technique have been fabricated by etching onto structures patterned into silicon nitride.Lastly, to conclude the two previous chapters, Hall measurements have been performed on quaternary samples with in situ contacts grown via the stencil lithography method. The measurements have been performed at cryogenic temperatures and resulted in values comparable to literature.

Note: Masterarbeit, RWTH Aachen University, 2017

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