The Si wafer is crystalline and can provide a template for this "crystal growth." However, in air, the surface of the wafer oxidizes to grow a thin disordered SiO₂ film. To remove this film, the wafer is heated up to 800 °Centigrade. At this temperature the oxide evaporates (and in our 10^-12 atmosphere, there are few air atoms around to regrow it). The wafer is then cooled to about 550°C, just enough heat to allow the arriving Si and Ge atoms to wander across the surface to find a proper crystalline position.