of gas within the stagnant zone is substantially the same as the mixture which surrounds the fixture epitaxial deposition onto the substrate surface 30 occurs from the mixture within the stagnant zone creating a layer of uniform thickness the deposition rate has been observed to be substantially similar to what it would be using conventional fixturing additionally the illustrated configuration permits one to rapidly change the composition of the gas mixture without detrimentally affecting the layer being epitaxially formed for example if one wishes to change the concentration of n or p type impurity or if one wishes to deposit a second semiconductor layer the reactant gas flow 24 can rapidly be changed to a new mixture the zone 28 will remain substantially stagnant despite the turbulence of the flow 24 surrounding the fixture thus for example a sharp transition n n layer or a sharp transition gaas gainp layer might be controllably formed using the present invention still another advantage of the present invention is the reduced particulate contamination that occurs in the epitaxial film although the internal portions of conventional deposition reactors are often rigorously cleaned between depositions there frequently remains some degree of particulate contamination which can detrimentally affect an epitaxial deposition since the substrate 26 is contained.