A hydrogenated, silicon based semiconductor alloy has a defect density of less than 10.sup.16 cm.sup.-3. The alloy may comprise a hydrogenated silicon alloy or a hydrogenated silicon-germanium alloy. Hydrogen content of the alloy is generally less than 15%, and in some instances less than 11%. The tandem photovoltaic devices which incorporate the alloy exhibit low levels of photo degradation. In some instances, the material is fabricated by a high speed VHF deposition process.
STATEMENT OF GOVERNMENT INTEREST
 This invention was made, at least in part, under U.S. Government, Department of Energy, Contact No. DE-FC36-07G017053. The Government may have rights in this invention.