Silicon-germanium for photonic applications

Abstract

Germanium and silicon-germanium have become crucial materials in the silicon photonics field, enabling devices such as high speed photodetectors and high speed modulators to be realised.

In order to fabricate efficient and cost effective silicon photonic devices, high quality epitaxial germanium and silicon-germanium growth on silicon, or silicon-on-insulator, is of the utmost importance.

In this project, localised single crystal, defect free silicon-germanium on insulator islands have been grown using a rapid melt growth technique. Tailored tree-like structures have been used to modify the cooling rate of the structures during re-growth from the liquid phase. The resulting silicon-germanium composition profiles have been characterised using Raman spectroscopy.

Using these tailored tree-like structures, uniform composition silicon- germanium strips have been grown, which is the first time this has been demonstrated using a rapid melt growth technique. Additionally, the ability to locally tune the composition of adjacent silicon-germanium strips has been shown. This enables the possibility of growing a whole range of uniform composition strips, using only a single growth step and a single anneal step, for, amongst others, wavelength division multiplexing applications.

Epitaxial growth of germanium on silicon by plasma enhanced chemical vapour deposition has also been studied. Single crystal layers with a defect density of approximately 3.3x10⁸ cm^-2 and root mean square surface roughness of 3.5 nm have been demonstrated. It has also been shown that the defect density, surface roughness and crystallinity are all improved with a two minutes, 600 °C anneal.

This material has been used to fabricate 12.5 Gbit/s, 0.1 A/W waveguide integrated, zero bias photodetectors for 1550 nm silicon photonics applications, and also, germanium-on-silicon waveguides for mid-infrared silicon photonics applications.

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