Cheap virtual germanium substrates by CSVT deposition on porous silicon

Epitaxial deposition of high-quality GaAs layers on Si substrates is very appealing in view of the lower price and weight of Si compared to GaAs and Ge, but the large lattice mismatch (4%) between both materials causes stresses and dislocations in the deposited layers. An elegant method to reduce these problems in the III-V layers is the realization of a Si wafer with a Ge cap layer, a so-called "virtual" Ge substrate. We present the realization of virtual Ge substrates by combination of two existing and cheap technologies: porosification of the Si substrate, and subsequent Ge deposition using CSVT (Close Spaced Vapor Transport). Direct CSVT deposition of Ge on porous Si substrates results in the growth of polycrystalline Ge; however, we have demonstrated that if a thin monocrystalline Ge seeding layer is first deposited on the porous Si substrate (by e.g. PECVD), subsequent CSVT deposition of Ge results in a thick mono-crystalline Ge layer of epitaxial quality equal to or better than the seeding layer (XRD FWHM = 100 arcsec).