MOVPE SiGeSn Development for the Next Generation Four Junction Solar Cells

The Multijunction (MJ) monolithic approach is very attractive for a competitive concentrating photovoltaic
(CPV) technology; it has been successfully applied for InGaP/GaInAs/Ge triple-junction structures but it is more difficult
to be exploited for manufacturing 4-junction solar cells, in particular when III-V and IV elements are both used. So far,
the integration of the 1 eV SiGeSn material in the lattice-matched InGaP/GaInAs/Ge triple-junction structure has
required the utilization of two different growth apparatus, nearly losing the economic advantage of the monolithic
architecture, owing to the related higher capital expenditure. The central technical challenge for realizing
InGaP/GaInAs/SiGeSn/Ge solar cell at low cost, with an industrial approach, lies in the growth of III-V and IV elements
in the same MOVPE equipment, by solving the “cross contamination” problem among the III-V elements and the IV
elements. In this contribution, for the first time, the results of the investigation concerning the growth of SiGe(Sn) and
III-V compounds in the same MOVPE growth chamber are presented. The epitaxial layers have been characterized by
XRD, SEM, TEM, EDX, SIMS and ECV profiling. It is eventually shown that by starting from a modification of the
MOVPE equipment and by setting up proper growth condition the contamination of III-V elements in IV based materials
can be drastically reduced from 1020 cm-3 to 2*1017 cm-3, while the contamination of IV elements in III-V compounds can
be reduced from 4-5*1017cm-3 to 6*1016 – 3*1014 cm-3 depending on the substrate used.