SiGe Quantum Cascade Structures: Physics, Growth and Technology

The demonstration of a semiconductor laser based on intersubband transitions in an InGaAs/InP cascade structure, has stimulated studies to transfer this approach to Si/SiGe. The feature, that intersubband transitions are by nature direct, made it appealing to use this concept to create a laser in a material with an indirect bandgap like Si. A laser in Si would allow the monolithic integration of optical active devices with mature Si electronics. Intersubband electroluminescence in SiGe layers was obtained first in pseudomorphically deposited structures. However, these structures were not suitable to obtain a laser for numerous reasons, in particular the number of cascades is limited due to the huge strain accumulated in these structures. Consequently, efforts were concentrated to strain compensated Si/SiGe quantum cascade (QC) structures grown on relaxed SiGe buffer layers. It should be noted that due to the large effective tunneling mass of electrons in the strain compensated structures, designs for intersubband transitions of hole states appear to be most promising for a SiGe cascade laser, despite the complexity of the valence band. Here, we summarize our research efforts to improve the performance of Si/SiGe quantum cascade structures by novel designs, advanced growth facilities as well as unique contact and waveguide layer sequences to minimize losses and to increase the gain