MBE GaAs microcrystals on patterned Si : a defect study

Fast epitaxial growth on deeply patterned substrates is a very promising method to get epilayers of high structural quality, as layer cracking and wafer bowing can be avoided (C. V. Falub et al., Science 335, 1330 (2012). This approach is applied here to grow crack-free GaAs microcrystals on Si pillars which is a typical hetero system of interest in the field of the monolithic integration of III-V's on Si. The (001) offcut Si substrate was patterned into square pillars of various size by deep reactive ion etching (DRIE). 2 µm thick GaAs was then deposited by MBE at 580 °C at a rate of 0.5 ML/s with V/III=50. The defect analysis was carried out by TEM, HR-TEM, STEM-ADF (Annular Dark Field) and GPA (Geometrical Phase Analysis). The GaAs deposits were 2 µm high. The anti-phase domains were located only at the interface and their density was drastically limited by the substrate offcut. Threading dislocations (TDs) were present but were confined to the bottom 500 nm of the GaAs as they annihilated each another or bent back to the interface due to mutual interactions. They did not reach the GaAs microcrystal top that will serve as active region in devices. The TDs originated from misfit dislocations of both 60° and edge type. Twins were also present originating at the interface very likely at the offcut substrate steps. Some of them reached the topmost part of GaAs. As no dangling bond is associated with twins, they should not be electrically active in devices.