Achieving high-quality In0.3Ga0.7As films on GaAs substrates by low-temperature molecular beam epitaxy

The effects of the thickness of the large-mismatched amorphous In0.6Ga0.4As buffer layer on In0.3Ga0.7As epi-films grown on a GaAs substrate have been systematically investigated. The In0.3Ga0.7As films with the In0.6Ga0.4As buffer layer of 0, 1, 2, and 4 nm thickness are grown by low-temperature molecular beam epitaxy (LT-MBE) and are characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). It is found that the degree of relaxation and the crystallinity of the as-grown In0.3Ga0.7As films are strongly affected by the thickness of the amorphous In0.6Ga0.4As buffer layer. The thinner In0.6Ga0.4As buffer layer is not enough to efficiently release the misfit strain between the In0.3Ga0.7As epilayer and the GaAs substrate, while the thicker In0.6Ga0.4As buffer layer is unfavorable to trap the dislocations and prevent them from extending into the In0.3Ga0.7As epi-films. We have demonstrated that the amorphous In0.6Ga0.4As buffer layer with a thickness of 2 nm can advantageously prevent threading and misfit dislocations from propagating into the subsequent In0.3Ga0.7As epilayer and increase the degree of relaxation of the as-grown In0.3Ga0.7As, ultimately leading to a high-quality In0.3Ga0.7As film. Our novel buffer layer technology has triggered a simple but effective approach to grow high-crystallinity In0.3Ga0.7As epitaxial film and is favorable for fabrication of GaAs-based high-efficiency four-junction solar cells.