In situ monitoring of As-P exchange on Ge(100) surfaces in GaAs-rich CVD reactors for low-defect III-V multijunction solar cells

For high quality epitaxial III-V-on-Ge, utilized e.g. for highly-efficient III-V/Ge(100) multijunction solar cells, the Ge(100) substrate surface must be prepared with double-atomic steps in order to avoid anti-phase boundaries in the III-V buffer. Preparation of these surfaces was studied in detail under As- and GaAs-rich CVD reactor conditions. Nucleation of III-P buffers, however, should be carried out in P-rich ambience. Here, we study the interaction of P with vicinal Ge(100):As surfaces in realistic, GaAs-rich coated CVD reactors. We combine optical in situ spectroscopy with surface science techniques in ultra-high vacuum. We demonstrate that P-modified Ge(100):As surfaces remain prevalently (1×2) reconstructed and their surface structure depends on the molar flow of phosphorus precursor.