strain in the layer of silicon application_number US 34661703 A description fig3 a u2013 3 i illustrate structures formed during fabrication of a strained silicon mosfet in accordance with an exemplary embodiment fig3 a shows a structure comprising a layer of silicon germanium 40 having an epitaxial layer of silicon 42 formed on its surface the silicon germanium layer 40 preferably has a composition si 1 x ge x where x is approximately 0 2 and is more generally in the range of 0 1 to 0 3 the silicon germanium layer 40 is typically grown on a silicon wafer silicon germanium may be grown for example by chemical vapor deposition using si 2 h 6 disilane and geh 4 germane as source gases with a substrate temperature of 600 to 900 degrees c a si 2 h 6 partial pressure of 30 mpa and a geh 4 partial pressure of 60 mpa sih 4 silane may be used in alternative processes growth of the silicon germanium material may be initiated using these ratios or alternatively the partial pressure of geh 4 may be gradually increased beginning from a lower pressure or zero pressure to form a gradient composition the thickness of the silicon germanium layer may be determined in accordance with the particular application the upper portion of the silicon germanium substrate 40 on which the strained silicon layer 42 is grown should have a uniform composition the silicon layer 42 is preferably grown by chemical vapor deposition cvd using si.