Dr. DI Daniel Gruber
"Spintronics applications of SiGe heterostructures"
The present work is about the design, the growth and the processing technology for modulation-doped Si/Si1-xGex heterostructures for spintronics applications, with the emphasis on a possible future quantum computer in the Si1-xGex material system.
Based on a previous proposal, a complete sample layer structure, consisting of a double quantum well and a n-type doped hetero-backgate layer, was developed with the help of self-consistent band structure simulations. The devised structure allows for tuning of the g-factor of electrons in a two-dimensional electron gas in a fashion, that selective spin manipulation by means of pulsed electron spin resonance is possible.
The devised structure was implemented with the molecular beam epitaxy method. Especially the parameters for Sb doping were optimised, which also involved the installation of a new Sb doping source.
Next, a new process was developed in order to be able to create samples, on which the influence of electrical fields, applied by gates both on top and beneath the double quantum wells, could be studied by magneto-transport, conventional and electrically detected electron spin resonance experiments.
Magneto-transport experiments performed on the processed samples showed, that the backgate structure worked as expected and the measured behaviour was in good agreement to the simulations. However, the anticipated effect of the shift of the electron wave function on the magneto-transport properties was not observed, which presumably was due to the poor quality of the grown layers. Possible improvements were suggested.
Finally, an existing electron spin resonance apparatus was modified for magneto-transport measurements in the microwave resonator. This allowed for the electrical detection of the spin resonance signal by means of the measurement of resonant changes in sample conductivity, with an improved sensitivity compared to the conventional method. The function of the setup was demonstrated with a Si channel Si/SiGe 2DEG structure. Because of the problems mentioned above, no measurements on the devised sample structure were possible.
"Substitutional Carbon in Si/SiGeC Heterostructures"
Carbon doping of silicon or silicon germanium (Si1-xGex)has attracted great interest, because of the possibility of band gap engineering and suppression of dopant diffusion. However, the introduction of carbon doping gives rise to concern regarding the problem of interstitial carbon, which can negatively influence the electrical device characteristics, or regarding silicon carbide (SiC) precipitation under thermal treatment.
This thesis is divided into two parts: The first part deals with experiments made mainly with Fourier Transform Infrared Spectroscopy (FTIR) on Si1-yCy layers grown by Molecular Beam Epitaxy (MBE).With this technique one is able to probe the local surroundings of the carbon atoms in the silicon crystal, making it possible to determine, in which modification the carbon is present. Optical absorption from substitutional carbon, coherent and incoherent SiC precipitates and interstitial carbon has been measured in this work. Spectroscopy on thin layers is difficult, because a silicon reference spectrum has to be subtracted, which is very sensitive to differences between the thicknesses of sample and reference layer. To overcome this problem, a method for correction of thickness has been developed. The sensitivity of the method has been demonstrated with quantitative measurements of substitutional carbon and SiC.
The FTIR technique has also been used to study the precipitation of carbon in Si1-yCy under thermal treatment. The experiments show that IR spectroscopy is the method of choice for this problem, making it possible to distinguish between two forms of SiC precipitates: one which is coherently bound to the Si crystal, and the relaxed form. During annealing experiments on Si1-yCy layers, both forms can be seen and their behavior can be studied. It could also be shown that SiC precipitates exist even in as-grown samples, with a concentration depending on the carbon source used during growth.
The second part of this thesis treats Si1-x-yGexCy heterobipolar transistors (HBTs). Carbon doping of the base makes it possible to suppress Transient Enhanced Diffusion (TED) of boron, which is a common problem in SiGe device processing. A HBT structure with a Si1-x-yGexCy base was grown by MBE and experiments done with the Secondary Ion Mass Spectroscopy (SIMS) technique show, that diffusion of boron is completely suppressed in the structure. Devices have been processed, which show that, although the performance is not yet optimum, the amount of carbon used in the structure seems to have no negative influence on the electrical characteristics.