Relaxed germanium films on off-oriented silicon wafers as substrates for epitaxial III-V solar cells

Off-oriented germanium wafers are the common substrates for high-efficiency multi-junction III-V photovoltaic cells for space applications and terrestrial concentrator photovoltaic systems. A miscut of 6° with respect to the [001]-orientation provides a Ge surface formed solely by double atomic steps which is mandatory to avoid anti-phase boundaries in subsequent III-V growth. This project is dedicated to the replacement of Ge wafers with epitaxial Ge films grown on miscut Si wafers. This solution could be cost-efficient because Si wafers are much cheaper than Ge wafers. Further cost reduction potential is at hand due to the larger wafer sizes available for Si. The growth of smooth relaxed Ge films with low defect densities on on-axis Si substrates can be accomplished by controlling the strain relaxation during epitaxy using surfactants. Thus, the application of surfactant-mediated epitaxy to 6° off-oriented Si(001) substrates could provide cheap Ge substrates for epitaxial III-V films.