Optical Properties and Phase Transformation Kinetics of Ge-Sb-Te-(N) Alloy Thin Films Investigated by Ellipsometry
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Optical Properties and Phase Transformation Kinetics of Ge-Sb-Te-(N) Alloy Thin Films Investigated by Ellipsometry
Sang Youl Kim Department of Molecular Science and Technology, Ajou University Suwon, Korea 442-749 ABSTRACT The 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+xTe5 thin films and those of the nitrogen Ge2Sb2Te5 thin films have been determined. The crystallization behavior of amorphous Ge2Sb2Te5 thin films investigated by in situ ellipsometry revealed that the crystallization process of Ge2Sb2Te5 near 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 Ge2Sb2Te5 films. 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 Ge2Sb2Te5 caused 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 Ge2Sb2Te5 can 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.
INTRODUCTION The optical media based on the phase-change phenomena have attracted an extensive research interest with the advent of the optical data storage era. One of the commonly used recording material for the phase-change optical disk is Ge-Sb-Te alloys which show a reversible phase transformation between the crystalline phase and the amorphous one, and thus the corresponding reversible reflectivity change [1,2]. The stoichiometric compound of Ge2Sb2Te5 is well known for its key promising attributes including high-speed writing and erasing, adequate number of overwrite cycles, stable recorded marks, sufficiently high signal-to-noise ratio, and good recording sensitivity [3,4]. Modification of chemical composition achieved by addition of 2-3 atomic percent of nitrogen to Ge-Sb-Te thin films has been reported to enhance the dynamic properties of recording media and their overwrite cyclability [5-7]. The spectroscopic ellipsometry (SE) data were numerically inverted to yield the complex refractive index spectra of Ge-Sb-Te-(N) alloys. The reflectivity spectra calculated using these optical constants were compared with the measured spectra.
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A considerable interest has been focused on the study of crystallization behavior of Ge-SbTe alloys [8-12]. Reflectivity measurements alone were unable to explicit
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