Chemical vapor deposition of amorphous hydrogenated silicon-germanium alloys (a-SiGe:H) was induced by photolysis of disilane and digermane mixtures with an ArF laser at 193 nm. The growth behavior on hydrogen-terminated and oxide-covered Si(111) surfaces was studied in real time in the monolayer region by spectroscopic ellipsometry (1.2-4.7 eV). One-, two-and three-layer models were employed to simulate the ellipsometric data. For the flat hydrogen-terminated silicon surface the slow formation of a well-defined monolayer is extracted from the analysis. On the native oxide-covered surface fast initial 3D growth was observed with coalescence at a larger film thickness. A three-layer model was needed to describe the growth of ultrathin films after coalescence, where a third layer, with a thickness in the nanometer range, takes into account the lower quality of the ultrathin film layer near the interface.