Crystallization behavior of non-stoichiometric Ge-Bi-Te ternary phase change materials for PRAM application

Abstract

We investigated the properties of a Ge-Bi-Te ternary chalcogenide thin film which was deposited on a SiO2/Si substrate by varying RF-sputtering power on the GeTe and Bi target. The aim was to search for an appropriate candidate for a new phase change memory. Various analyses are conducted in order to investigate the composition, phase separation, and crystallization behavior of the Ge-Bi-Te alloy. The XRD results of each annealed sample showed that the Ge-Bi-Te alloy crystallized into Ge2Bi2Te5, GeBi2Te4, GeBi4Te7 phase at around 300 degrees C according to Ge content and expelled amorphous Ge crystallized as a single phase over 400 degrees C. Combining these with the differential scanning calorimetry (DSC) results, we demonstrated that T-c and T-m of the Ge-Bi-Te alloy are respectively higher and lower than those of conventional Ge-Sb-Te (GST) films. All the phases, including not only various Ge-Bi-Te ternary phases but also the Ge phase crystal structure, were also confirmed with high-resolution transmission electron microscopy (HR-TEM) images and diffraction patterns. It is noted that some of the Ge2Bi2Te5 grains show specific facetted planes such as {0113}, {0112}, and {0001}. Through successive analyses, we revealed the structural evolution of the Ge-Bi-Te alloy according to Ge contents and confirmed the potential of the Ge-Bi-Te alloy for phase-change random access memory (PRAM) applications.