Impact of Compositional Fluctuation on Dislocations in Metamorphic III-V Solar Cells Revealed by Cathodoluminescence Spectrum Imaging 
Kevin L Schulte, Ryan M France, Harvey L Guthrey, John F Geisz
National Renewable Energy Laboratory, Golden, CO, United States

Compositionally graded buffers (CGBs) are essential components of high-efficiency III-V inverted metamorphic solar cells. High-quality CGBs with low defect densities are a requirement for high device efficiencies. In this work we use cathodoluminescence spectrum imaging to investigate the effect of growth conditions on the material properties of AlyGa1-x-yInxAs CGBs with xIn = > 0.30. We find that significant compositional fluctuation exists in these materials, and that it correlates with the distribution of threading dislocations in the CGBs. Threading dislocations collect in regions with large compositional fluctuation, implying that concurrent strain fluctuations in these regions restrict dislocation motion. Thus, the compositional fluctuation leads to elevated threading dislocation densities that degrade CGB quality. We investigate the effectiveness of specific growth conditions at suppressing compositional fluctuation and limiting threading dislocation density, and find that CGBs grown at high temperatures with high Al-content exhibit the lowest defect densities. We also demonstrate that use of a high V/III ratio also suppresses compositional fluctuation and defect density. These materials insights lead directly to the development of metamorphic Ga1-xInxAs solar cells grown on AlyGa1-x-yInxAs CGBs with improved device efficiency.