Influences of two-step growth and off-angle Ge substrate on crystalline quality of GaAs buffer layers grown by MOVPE

GaAs buffer layers were grown on off-angle Ge substrates with a two-step growth process by metalorganic vapor phase epitaxy in order to prepare a high quality buffer layer for the InGaAsN p-i-n solar-cell structure. In our contribution, we present results from the optimized two-step growth of GaAs buffer layers, namely with a low-temperature step at 470 C and a high-temperature step at 580 C, combined with the use of Ge substrates misoriented by 4 and 6 towards [1 1 0]. HRXRD results showed that by using the off-angle substrates, the FWHM of (0 0 4) rocking curves was decreased to 6.7 sec, which is about 4 times the FWHM of the GaAs commercial substrate. A decreased FWHM indicates a narrower distribution of crystal orientation. Furthermore, a smooth surface with a RMS roughness of 0.7 nm was clearly observed by AFM. Cross-sectional dark-field TEM images showed the GaAs buffer layer with 20-30 nm diamond-shaped anti-phase domains (APDs) at the GaAs/Ge interface, followed by APDs-free GaAs regions on the 6 off-angle Ge substrate. However, anti-phase boundaries were generated along the [0 0 1] direction for the buffer layer on the on-axis substrate. Our results demonstrated that the use of 6 off-angle Ge substrate and a two-step growth process allow the suppression of APDs in the GaAs buffer layer.