as the silicon concentration increases the film first forms a two phase mixture of gold and liquid gold silicon the composition of the latter is determined by the choice of the deposition temperature with further increase of silicon the liquid phase region marked 12 is reached and the remaining gold is dissolved with still further increase of the amount of silicon the second liquidus phase boundary marked 13 is reached and subsequent deposition of silicon atoms results in a solid phase of silicon in equilibrium with the silicon gold liquid if the substrate surface is suitably chosen for example mgo crystals the solid silicon nucleates heterogeneously onto the surface the choice of the temperature of deposition is determined by balancing two considerations quality in terms of defects of the epitaxial film too low a temperature or too rapid a growth rate of the film at that temperature can introduce defects versus too high a temperature when chemical interaction or mechanical integrity of the substrate limit the usefulness of the material we have started with vapor deposition of the metallic film and added silicon to it to traverse the phase diagram from point marked 11 in the figure however the metallic element and silicon can be co evaporated to reach any concentration between the points marked 12 and 13 in the figure and subsequently silicon added to reach the desired thickness before cooling to room temperature when the desired thickness of the silicon film is obtained the substrate with the film is cooled to room temperature even though the amount of gold required to catalyze a silicon film is small it can be further reduced by etching the gold away for example by using iodine etch available commercially this gold can be recycled the following.