Performance of solar cells and other electronic devices, such as transistors, can be improved greatly if carrier mobility is increased. Si and Ge have Type-II bandgap alignment in cubically strained and relaxed layers. Quantum well and superlattice with Si, Ge, and SiGe have been good noble structures to build high electron mobility layer and high hole mobility layers.
However, the atomic lattice constant of Ge is bigger than that of Si and direct epitaxial growth generates a large density of misfit dislocations that decrease carrier mobility and shorten device lifetime. So, it required special buffer layers, such as superlattice or gradient indexed layers, to grow Ge on Si wafers or Si on Ge wafers. The growth of these buffer layers takes extra effort, such as a post-annealing process, to remove dislocations by dislocation gliding inside the buffer layer.
This innovation builds off NASA’s work in making high-quality crystalline SiGe thin films grown on sapphire substrates. In this case, a distinct layer structure is used to create quantum well structures to provide a very high-mobility pathway for both p-type and n-type charge carriers.
The primary intended application is for solar cells where the bandgap structure and charge carrier mobility combine to provide the potential for highly efficient solar cells. The layer structure enables back-side illumination such that the effective solar cell area for light capture is maximized. Conversion efficiency is expected to be on the order of 30% or greater.
The fabrication method for high-mobility layer structures of rhombohedrally aligned SiGe on a triagonal substrate utilizes C-plane (0001) sapphire, which has a triangle plane, and a Si (Ge) (C) (111) crystal or an alloy of group TV semiconductor (111) crystal grown on the sapphire.