A semiconductor device is disclosed comprising: a substrate having a surface comprising germanium; a layer of gallium on said surface; and a layer of gallium arsenide on the gallium covered surface. The semiconductor heterostructure of gallium arsenide on germanium is fabricated by the steps of: protecting by a shutter a surface comprising germanium in an environment having a partial pressure of arsenic less than 10−8torr; epitaxially growing a layer of gallium on the said surface immediately after exposure of said surface; and epitaxially growing a layer of gallium arsenide on the gallium covered surface.