Time-resolved reflection and transmission studies of amorphous Ge-Te thin-film crystallization

Abstract

Measurements of the temperature and time dependence of visible diode laser transmission and reflection are combined with transmission electron microscopy (TEM) to study the crystallization of two 75 nm Ge-Te thin films. Near-stoichiometric Ge48Te52 transforms by the rapid growth of crystals through the film thickness followed by 2D growth in the film plane. Changes in film reflection and transmission are directly related to the volume fraction transformed. The optical measurements are interpreted in terms of classical Johnson-Mehl-Avrami kinetics. A Kissinger analysis gives an activation energy for crystallization of 1.7 eV. Isothermal measurements lead to an Avrami exponent of 4.5. The data are modeled using a numerical temperature-dependent expression developed by Greer [Acta Metall. 30, 171 (1982)]. Off-stoichiometric Ge54Te46 films show markedly different crystallization behavior. Transmission and reflection measurements indicate that the transformation proceeds by rapid growth of a crystalline layer at the free surface of the film followed by 1D growth of this layer through the film. The observation is confirmed by TEM imaging and diffraction. This work shows that reflection and transmission measurements can be an effective method for the study of crystallization kinetics of amorphous thin films, particularly when more traditional calorimetric methods cannot be employed.