Optical characterisation of III-V alloys grown on Si by spectroscopic ellipsometry

Increased interest in novel substrates for III-V material growth has led to the use of alloy fractions other than those lattice-matched to traditional GaAs and Ge substrates. The optical constants of these materials are generally not available in the literature. We characterise the unconventional alloy fractions used in a GaAsP/SiGe tandem on Si by spectroscopic ellipsometry on specially prepared samples, with consideration of transparent and opaque spectral regions during analysis. Complex indices of refraction for GaAs.84P.16,Ga.59In.41P, and Al.65In.35P are determined. These values allow improvement of the solar cell and will enable future optical modelling and design of this and other devices and materials.