Terahertz spectroscopic characterization of Ge₂Sb₂Te₅ phase change materials for photonics applications

Ge-Sb-Te (GST) phase change materials exhibit a metal-to-insulator transition and therefore are expected to be useful for a variety of photonics applications in addition to their primary application in optical and electrical memory devices. Here, we demonstrate that the phase change causes drastic changes in the optical and dielectric properties of GST thin films in the terahertz (THz) frequency range. It is revealed that both the real and imaginary parts of the index of refraction as well as the dielectric function are enhanced with increasing annealing temperature. High contrast in the optical transmittance and THz conductivity were achieved by the phase change between the cubic and hexagonal phases while the phase change between amorphous and cubic phases was found to result in much smaller changes in these properties. The real part of the index of refraction and the dielectric function were found to be enhanced as the phase change progressed. The trend in the DC resistivity derived from the THz signals was found to be consistent with independent DC measurements. The observed annealing-dependent changes are thought to be governed by the free carrier absorption of THz waves. We propose that utilization of the nonvolatile and multi-level phase change nature of GST films is promising for a variety of THz opto-electronic applications such as THz modulation devices as well as plasmonic device applications with and without metastructures.