P-type and n-type regions are defined in the first surface of a substrate upon which is formed an epitaxial layer of preferably Si—Ge material, preferably capped by Si material. During epitaxy formation, dopant in the defined regions diffuses down to form p-type and n-type junctions in the Si material, and diffuses up to form p-type and n-type junctions in the Si—Ge epitaxial material. Si junctions are buried beneath the surface and are surface recombination velocity effects are reduced. Photon energy striking the second substrate surface generates electron-hole pairs that experience the high bandgap of the Si materials and the low bandgap of the Si—Ge epitaxy. The tandem structure absorbs photon energy from about 0.6 eV to about 3.5 eV and exhibits high conversion efficiency.