Low temperature growth GaAs on GE by chemical beam epitaxy

Hetero-epitaxy of GaAs films on Ge were grown by the chemical beam epitaxy (CBE) technique for super-high-efficiency multi-junction solar cells. However, the growth of III-V on Ge remains a key challenge due to anti-phase domains (APDs) and the inter-diffusion at the interface, resulting in the deterioration of the solar cell efficiency. In order to overcome these problems, we introduced the CBE technique, which was carried out under a higher vacuum (~10^-2 Pa) and at relatively lower temperature (360~460°C) as compared with the growth of GaAs on Ge by the typical metalorganic vapor phase epitaxy (MOVPE). The effects of the low growth temperature, the source flow ratio of [V/III] and pre-annealing temperature on crystal qualities and optical properties of GaAs films on Ge were investigated. The results showed that GaAs films on Ge had a good structural quality at growth temperature from 400 to 460°C with the [V/III] of flow ratio from 50 to 70 by a high resolution x-ray diffraction. The pre-annealing temperature and time were confirmed to play an important role in the modification of Ge substrate surface. The low-temperature (4.5 K) photoluminescence spectra showed the emission peak around 1.51 eV, corresponding to an exciton bound-to-acceptor, in sample grown at 400°C and the full width at half maximum of this peak was evaluated 10 meV by Gaussian fitting. An emission peak of 1.27 eV caused by Ge diffusion into GaAs film was observed for the sample grown at 460°C. Such a critical temperature that activates the Ge diffusion seems relatively low in CBE system as compared with that reported in MOVPE system.