Delta doping with silicon has been used to form a contact of metallic conductivity buried 1 mu m below the surface of a GaAs MBE-grown structure. The electrochemistry of the high-purity GaAs above the contact could then be investigated. The interfaces in the structure were located by photoelectrochemical etching; the spatial distribution of defects and also some information concerning their origin within the grown layers was obtained by photoelectrochemical imaging.