A back-illuminated silicon photodetector has a layer of Al2O3 deposited on a region of a silicon oxide surface that is left uncovered, while deposition is inhibited in another region by a contact shadow mask. The Al2O3 layer is an antireflection coating. In addition, the Al2O3 layer can also provide a chemically resistant separation layer between the silicon oxide surface and additional antireflection coating layers. In one embodiment, the silicon photodetector has a delta-doped layer near (within a few nanometers of) the silicon oxide surface. The Al2O3 layer is expected to provide similar antireflection properties and chemical protection for doped layers fabricated using other methods, such as MBE, ion implantation and CVD deposition.