Strain relaxation in III-V solar cells grown on germanium substrates

The materials properties of III-V tandem solar cells grown on Ge were examined using triple axis x-ray scattering techniques. First, the 5 mu m GaAs buffer layer was found to be relaxed by approximate to 85 % with respect to the underlying Ge substrate. The extent of relaxation did not change with attice direction and a tilt on the order of 60 arcsec exists at the interface. Based on first order comparison of the coefficients of thermal expansion between the two materials, the GaAs layer is early fully relaxed at the growth temperature of about 700 degrees C and becomes strained during cooling. Second, Al0.63Ga0.37As and In(x)Gal(1-x)P (0.48 < X-In < 0.53) layers were determined to be pseudomorphic with respect to the GaAs buffer layer and maintain the same miscut direction as the substrate. Analysis of these layers also shows that the standard interpretations used to determine attice rotations and tilts for both strained and relaxed layers may be incorrect under certain circumstances.