Crystallization behavior of non-stoichiometric Ge–Bi–Te ternary phase change materials forPRAM application

Abstract

We investigated the properties of a Ge–Bi–Te ternary chalcogenide thin film which was deposited ona SiO2/Si substrate by varying RF-sputtering power on the GeTe and Bi target. The aimwas to search for an appropriate candidate for a new phase change memory.Various analyses are conducted in order to investigate the composition, phaseseparation, and crystallization behavior of the Ge–Bi–Te alloy. The XRD resultsof each annealed sample showed that the Ge–Bi–Te alloy crystallized intoGe2Bi2Te5,GeBi2Te4,GeBi4Te7 phase ataround 300 °C according to Ge content and expelled amorphous Ge crystallized as a single phase over400 °C.Combining these with the differential scanning calorimetry (DSC) results, we demonstrated thatTc and Tm 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 theGe phase crystal structure, were also confirmed with high-resolution transmission electronmicroscopy (HR-TEM) images and diffraction patterns. It is noted that some of theGe2Bi2Te5 grains show specific facetted planes such as {0113}, {0112}, and {0001}. Through successiveanalyses, we revealed the structural evolution of the Ge–Bi–Te alloy according to Gecontents and confirmed the potential of the Ge–Bi–Te alloy for phase-change random accessmemory (PRAM) applications.