Physics of Amorphous-Silicon Alloys

Abstract

It has long been known that there are often two paths from the liquid to the solid state, depending on the rate of cooling. At temperatures above the melting point, T_m, a sufficient concentration of higher energy local configurations are accessible that the structure cannot withstand shear stresses and behaves like a fluid. If the temperature is reduced slowly, a first-order transition occurs at T_m to a state with lower energy and entropy that resists finite shear stresses and thus retains its shape. The structure then ordinarily exhibits a long-range periodicity which reflects the chemical nature of the constituent atoms, and the material is called a crystal. For simple materials, the crystal represents the absolute minimum energy arrangement, and thus is the stable phase at very low temperatures. However, for complex alloys, it often represents a compromise between the optimal chemical bonding and the strain energy. When such an alloy is cooled rapidly, a different series of processes can then take place. Since first-order phase transitions entail finite entropy changes, they require a finite time to occur.