In this study, a two-layer system consisting of a thin aluminum oxide (Al2O3) and a heavily boron-doped amorphous silicon (a-Si:B) is described. The double layer can be applied as a boron-dopant source in high-temperature diffusion as well as for surface passivation afterwards. The influence of different Al2O3 thicknesses and diffusion processes is investigated regarding the boron diffusion, surface passivation, and optical characteristics of this layer stack. Using a 5-nm thick Al2O3 film, a stable surface passivation as well as significant boron diffusion through the Al2O3 is possible. The best surface passivation is achieved for the annealing process with the highest thermal budget (950 °C for 60 min). Investigations by spectral ellipsometry prove the growth of silicon dioxide during the high-temperature diffusion process and indicate that the remaining boron-rich silicon layer is highly absorbing even for near-bandgap, near-infrared light. To improve future solar cells using this layer as the rear-side coating, the thickness of this layer has to be reduced.