We perform the growth of GaAs (111) epilayers on nominal Ge(111) wafers by molecular beam epitaxy (MBE). The polarity of GaAs is (111)A and homongeneous over the full area, as measured by transmission electron microscopy and high energy electron diffraction. This orientation conflicts with the common growth model for GaAs on Ge(111). Twinned domains are the main defects in our GaAs (111) epilayers. Using cathodoluminescence, we observe that some twin boundaries hold large number of non-radiative recombination centers. During growth, we find that only a narrow domain of As:Ga ratios lead to the growth of smooth and twin-free GaAs (111)A epilayers. At low As:Ga ratio, the surface is rough; while at high As:Ga ratio the epilayers present large densities of twinned domains.

We use high-resolution optical microscopy to characterize in situ the processes at play during the Al-induced crystallization of amorphous Ge. In addition to the well-established aluminum-induced layer exchange (ALILE) process, we demonstrate the existence of another crystallization mechanism with different kinetics and spatial extension using in situ monitoring. Further, ex situ characterizations show that both processes are active in our samples. The ALILE process is found to create a single Ge layer and 111-oriented crystallites in our growth conditions, while the other crystallization process yields a double Ge layer with mixed 111 and 100 orientations in the bottom layer, while the top layer remains amorphous. This work underlines the importance of in situ monitoring for the understanding and modeling of metal-induced crystallization.