Dr. DI Stefan Zerlauth
"MBE Growth and Characterization of Silicon-based Heterostructures"
In this thesis silicon-based heterostructures (Si1-xGex/Si1-yCy) have been grown by molecular beam epitaxy (MBE) and have been investigated on their structural, optical and electronic properties.
After the acceptance by the manufacturer RIBER in Paris the installation of the MBE machine marked the beginning of this work. After the machine was put into operation, the first emphasis was set on the development of standard processes for the reproducible growth of heterostructures and the improvement of the MBE chamber. To keep the flux of the sources constant, an additional control unit has been installed, which allows the production of superlattices with an exact constant period. To improve the crystalline quality of the layers, the chamber has been furnished with silicon plates inside and the hot parts have been covered with pyrolytic graphite. An additional turbomolecular pump improved the vacuum conditions during growth considerably. These arrangements make it possible to grow crystalline Si1-xGex layers of high quality at low temperatures (350°C), which show a well pronounced, undisturbed photoluminescence. This criterion is fulfilled only by two other silicon MBE machines in the world.
A further emphasis has been put on the investigation of the influence of carbon on the surrounding silicon matrix. In order to clarify this question, 500 samples have been grown and characterized with different measurements. It has been found out that carbon builds an alloy crystal (Si1-yCy) with silicon. The strain within these layers is very large, so only a few percent of carbon can be added to get layers of a high crystalline quality. The substitutional incorporation of carbon is very sensitive to the substrate temperature and the carbon flux. By substituting one percent silicon by carbon, the layers get rough and the crystalline quality decreases. This roughness can be suppresed kinetically, when the growth temperature is between 400°C and 550°C.
After the optimization of the growth parameters Si1-yCy structures could be grown, of which the photoluminescence lines show a full width half maximum value, that has not been achieved in literature yet. This criterion is very sensitive to crystalline disturbances and therefore a proof, that high quality Si1-yCy layers can be grown by MBE.
The modulation doped samples which have been grown in Linz so far cannot follow the reported record mobility samples, but their mobilities excel the best MOSFET(=metal oxide semiconductor field effect transistor) structures. Modulation doped Si1-yCy channels show enhanced electron mobilities and have not been surpassed by samples grown from other groups. It will be necessary to start further experiments concerning this topic to find out whether carbon can fulfill the high expectations set to it. A further aspect worth examining is whether the electronic transport in Si1-yCy layers is negatively influenced by large local strain-fields. This could be achieved by using the obtained knowledge of the growth parameters for a completely substitutional carbon incorporation.