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Kévin Guilloy

Strained germanium for light emission

Published on 5 July 2016
Thesis presented July 05, 2016

Despite the indirect nature of its bandgap, germanium is a promising candidate as a potential light source for silicon photonic, since the application of tensile strain reduces the energy difference between its direct and indirect bandgaps. However, the application of very large strains raises a number of issues, from a technological point of vue as well as for the determination of the material properties. After laying the theoretical foundations of this problem, two straining approaches are employed: the first one using nanowires grown by the Vapor-Liquid-Solid mechanism, the second using micro-structuration of germanium-on-insulator substrates.
For the first one, a study of the n-type doping of CVD-grown nanowires using 4-probes electrical measurements and EDX spectroscopy reveals that they reach a phosphorus atomic concentration of 7 1019 cm-3, these dopants being fully activated. A micro-fabrication process is then used to apply tensile strain to single nanowires, reaching 1.5 % uniaxial stretch measured by X-ray micro-diffraction. The strain measurement is correlated with a direct bandgap measurement by photocurrent spectroscopy, leading to a good agreement with theoretical predictions from the literature.
The last chapter describes the fabrication process of structures obtained by amplification of the residual stress of germanium layers on insulator. X-ray diffraction, coupled to Raman spectroscopy, reveals that the structures reach 4.9 % uniaxial stretch and 1.9 % biaxial stretch. The relation between Raman-shift and strain differs significantly from models published in the literature above a few percents of strain. Finally, the measurement of the direct transition with the light- and heavy-holes bands by electro-absorption spectroscopy shows that their strain dependence is not in complete agreement with the deformation potential theory above 2 % but in agreement with predictions from tight-binding simulations.

Spectroscopy, Optics, Strain, Photonics

On-line thesis.