Optical constants of germanium antimony telluride (GST) in amorphous, crystalline, and intermediate states

The optical constants of germanium antimony telluride (GST), measured by spectroscopic ellipsometry (SE), for the spectral range of 350-30,000 nm are presented. Thin films of GST with composition Ge₂Sb₂Te₅ are prepared by sputtering. As-deposited samples are amorphous, and when heated above the phase transition temperature near 150 °C, films undergo an amorphous to face-centered cubic crystalline phase transition. The optical constants and thicknesses of amorphous and crystalline GST films are determined from multi-angle SE measurements, applying a general oscillator model in both cases. Then, in order to evaluate the optical constants at intermediate states throughout the phase transition, GST films are heated in situ on a temperature stage, and single-angle SE measurements are carried out at discrete temperature steps in a range from 120–158 °C. It is shown that ellipsometric data for partially crystallized states can be fit by treating the GST as an effective medium consisting of its amorphous and crystalline states. Its optical constants, fractional crystallinity, and thickness can be determined at intermediate crystallization states throughout the phase transition. As a practical demonstration of the usefulness of this method, samples are held at fixed temperatures near the transition temperature, and SE is performed periodically. The fraction of crystallinity is determined as a function of time, and an activation energy for the amorphous to crystalline phase transition is determined.