Dr. DI Gregor Langer
Phd thesis: "Fabrication and Characterisation of Si Ge Microdisks"
Within the last years computers became faster and more powerful. In the meantime the limiting factor for the calculation power is partially the communication-speed with the memory. The problem hereby is that the internal data transfer is purely electric up to now. For this reason intense research is performed in order to realise silicon-based wave-guides. In order to be able to offer computers at a low price also in the future, it is necessary that the light source on chip can be realised by means of silicon-technology.
It was possible within the last decade to realise a silicon and a germanium laser. However, both approaches are so far not suited for miniaturisation and therefore it is not possible to put them on a chip. Within the last decade it was further realised that if one structures SOI samples with or without Ge structures in them into micro-disks, one can observe silicon based luminescence even at room temperature. It was demonstrated that one can shift the luminescence to around 1500nm with the help of Ge-islands. This wavelength would be well suited for silicon wave-guides.
In the present thesis the investigations of such silicon micro-disks, were systematically continued. Also investigations on unstructured SOI material were performed and compared with the results from the disks. In the present thesis not only the luminescence at room temperature but also the luminescence at cryogenic temperatures was recorded, in order to understand the physical mechanisms which lead to room temperature luminescence.
In order to address the single physical effects it was first necessary to understand the already known physical mechanisms of SiGe luminescence. Therefore the existing literature was studied and some Matlab routines were written, which allowed to simulate SiGe luminescence as a function of temperature. The most important line-width broadening mechanisms were included within these fit-routines. Finally also the influence of the disks (only Fabry-Perot modes) and the silica pedestal were included.
The luminescence measurements, which were performed within the scope of this thesis, were fit with the routines. From the fits a number of important physical parameters and insights were obtained.
- The charge carrier densities in the SiGe quantum-wells were determined. Inside unstructured SOI material as well as inside disks.
- We saw that the TO phonon life-times within the SiGe quantum-wells decreased with increasing charge carrier density.
- From the form of the SiGe luminescence we were able to determine the temperature of the SOI material. But due to the whispering gallery modes this method was rather inaccurate for the disks.
- As the refractive index does change with temperature, one can determine the disk temperature above 50K on hand of the mode position of the whispering gallery modes. Silicon and germanium are both indirect semiconductors. Therefore it is possible that electron-hole plasmas might form. Free charge carriers do also have an influence on the refractive index. It was therefore necessary to estimate the influence of the free charge carriers theoretically. It was found that the influence of the charge carrier densities measured within the present thesis was below the measurement accuracy of our spectrometer.
- With increasing temperature the SiGe luminescence decreases. The binding energies of the charge carriers to the quantum-wells are determined via the Boltzmann statistics in the literature. In the present thesis the binding energies were determined via Fermi-Dirac statistics.
- Additionally the effect of mode splitting, which is based on the lifting of the mode degeneracy, is discussed.
- Part of the experimental work within this thesis was the fabrication of the disks. It was realised that the etching recipe developed for pure silicon, did not work for Ge-containing silicon layers, as the etching rates were very much different. Therefore some time was spent on developing an etching recipe on the ICP/RIE for Ge containing silicon disks.