Expanding the palette: Metamorphic strategies over multiple lattice constant ranges for extending the spectrum of accessible photovoltaic materials

Metamorphic grading strategies currently under investigation, covering the lattice constant ranges between GaP (Si) to GaAs and GaAs (Ge) to InP, for the expansion of the palette of accessible photovoltaic materials is presented. The development of a high-temperature GaP/Si virtual substrate process, enabling the heteroepitaxial integration of high-quality III-V materials on Si substrates, and a high-temperature GaAs_yP_1-y compositional grading growth regime to overcome the issue of poor dislocation dynamics, are discussed. To address the problem of intrinsic phase instabilities in the In_xGa_1-xAs and In_zGa_1-zP alloys when grading from GaAs to InP, different strategies for the avoidance of severe phase separation, including multi-component grades and linear vs. step grades, are described. All In_xGa_1-xAs- and In_zGa_1-zP-based graded buffers in the GaAs-to-InP range are found to exhibit unusual, multiple changes in tilt direction, which appear to have a convoluted composition/strain state dependence, and may play an important role in ultimate buffer design.