Thin group IV semiconductor structures are provided comprising a thin Si substrate and a second region formed directly on the Si substrate, where the second region comprises either (i) a Ge1.sub.-xSn.sub.x layer; or (ii) a Ge layer having a threading dislocation density of less than about 10.sup.5/cm.sup.2, and the effective bandgap of the second region is less than the effective bandgap of the Si substrate. Further, methods for preparing the thin group IV semiconductor structures are provided. Such structures are useful, for example, as components of solar cells.
STATEMENT OF GOVERNMENT FUNDING
 The invention described herein was made in part with government support under grant number DEFG3608GO18003, awarded by the Department of Energy; and grant number FA9560-60-01-0442, awarded by the United States Air Force Multidisciplinary University Research Initiative Program. The United States Government has certain rights in the invention.