High-Temperature Complex Refractive Index of Phase Change Recording Medium of Ge₂Sb₂Te₅ Determined Using Phase Change Static Tester and Spectroscopic Ellipsometer

Abstract

We investigated the high-temperature optical properties of the optical phase change material Ge 2 Sb 2 Te 5 (GST) using an in situ ellipsometer equipped with a convetional heating chamber and using a spectroscopic ellipsometer combined with a phase change static tester. In addition, we compared the results for the high-temperature complex refractive indices of GST thin films determined using a spectroscopic ellipsometer after the samples were crystallized by the in situ and a phase-change random access memory (PRAM) methods. The ellipsometric constants and the RMS value of the surface roughness of the GST samples were measured by in situ ellipsometry and atomic force microscopy (AFM), respectively. Results showed wide variations at high-temperatures when the GST samples were crystallized by the in situ method. The complex refractive indices of GST thin films were consistent at mid-range temperatures up to 370 C for the two methods. In contrast, they showed a large variation at a higher temperature of $450 C for the in situ method and remained stable in the entire temperature range for the PRAM method. This indicates that the optical properties of GST thin film are affected by the film structure and protective layer for the in situ method but not for the PRAM method at high-temperatures. It is concluded that the combined use of the spectroscopic ellipsometer with the phase change static tester gives us more precise and consistent results not affected by either the film structure nor the protective material.