Optical Properties and Phase Transformation Kinetics of Ge-Sb-Te-(N) Alloy Thin Films Investigated by Ellipsometry

Abstract

ABSTRACTThe recent research works of optical properties and crystallization kinetics of phase change Ge-Sb-Te-(N) alloy by using ellipsometry are reviewed.The complex refractive index spectra of phase-change Ge2Sb2+xTe5thin films and those of the nitrogen Ge2Sb2Te5thin films have been determined.The crystallization behavior of amorphous Ge2Sb2Te5thin films investigated by in situ ellipsometry revealed that the crystallization process of Ge2Sb2Te5near 140°C is a two-step process. The kinetic exponent of the Johnson-Mehl-Avrami equation was about 4.4 for the first stage and 1.1 for the second stage. Ex situ study confirmed the cascaded crystallization behavior of phase-change Ge2Sb2Te5films.A passive type single wavelength ellipsometer adopting a DOAP (division-of-amplitude photopolarimeter) configuration with nanosecond time resolution is developed to monitor the phase transformation of Ge2Sb2Te5caused by a high power nanosecond laser pulse in real time. The two-step process - the fast nucleation-dominant stage followed by the slow anomalous grain growth stage is confirmed.Based on the recent analysis of the ellipsometric isotherm at moderately elevated temperature, we found for the first time that the fast nucleation dominant crystallization of Ge2Sb2Te5can be better explained by a modified JMA equation that illustrates the nucleation dominated process where the creation rate of a new nucleus is proportional to the density of preexisting nuclei and growth rate is negligible.