  Virtual substrates based on thin Ge layers on Si by direct deposition have
achieved high quality recently. Their application to high efficiency III-V
solar cells is analyzed in this work. Replacing traditional Ge substrates with
Ge/Si virtual substrates in standard lattice-matched and upright metamorphic
GaInP/Ga(In)As/Ge solar cells is feasible according to our calculations using
realistic parameters of state-of-the-art Ge solar cells but with thin bases (<
5um). The first experimental steps are tackled by implementing Ge
single-junction and full GaInP/Ga(In)As/Ge triple-junction solar cells on
medium quality Ge/Si virtual substrates with 5um thick Ge layers. The results
show that the photocurrent in the Ge bottom cell is barely enough to achieve
current matching with the upper subcells, but the overall performance is poor
due to low voltages in the junctions. Moreover, observed cracks in the
triple-junction structure point to the need to reduce the thickness of the Ge +
III-V structure or using other advanced approaches to mitigate the thermal
expansion coefficient mismatch effects, such as using embedded porous silicon.
Next experimental work will pursue this objective and use more advanced Ge/Si
virtual substrates available with lower threading dislocation densities and
different Ge thicknesses.
