An improved method and apparatus for optimizing the electrical properties while crystallizing material is disclosed. In this invention, a material which is to be crystallized is formed on a substrate and subjected to a heat treatment to intentionally induce thermal stress while crystallizing the material. The heat treatment melts the material being crystallized and when the material solidifies, a built-in stress is retained which, in the case of n-doped Si on fused silica results in a tensile stress which produces an electron mobility in the film of 870 cm.sup.2 /volt-sec as compared to similarly fashioned unstressed n-doped Si on SiO.sub.2 coated Si which has an electron mobility of 500 cm.sup.2 /volt-sec.