Formation and optical properties of SiGe/Si quantum structures

Quantum structures consisting of Si and Ge were successfully formed by using gas source molecular beam epitaxy (GSMBE). The growth was strongly dependent on growth temperature and crystalline orientation. The SiGe/Si quantum wells grown by the method clearly showed size quantization effects when the well width as well as Ge content were changed and coupling of wells was well understood in terms of effective mass approximation. The luminescence peak energy, however, was found not to precisely follow the Ge content dependence, theoretically predicted above 40% of Ge. It was also followed by the deviation of Ge content dependence of activation energy of PL intensity. These properties were shown to come from island formation on SiGe layers. In the case of pure Ge wells, the critical thickness where the PL properties significantly changed was found to be 3.7 MLs above which quantum islands were formed on wet layers with 3 MLs thickness. When the Ge thickness was reduced to sub-monolayer, the formation of quantum wires was found, which was confirmed both in PL aspects as well as TEM observation. Band filling effect characteristic of SiGe/Si quantum wells diminished probably due to the change in the density of states and a new luminescence which was reasonably assigned to biexcitons was found to be formed in the wires. The formation of quantum structures in the Si/Ge system was promising to realize optoelectronic devices and light emitting diodes operating at room temperature was demonstrated.