Microstructural analysis of RF-sputtered Ge-Bi-Te ternary chalcogenide alloy for phase change memory application

Abstract

Chalcogenide amorphous semiconductors have attracted much attention because of their interesting electrical properties [1]. They have two stable states, which are the low conductive amorphous state and the high conductive crystalline state, and one state can be converted to the other by applying electric pulses [2]. The Ge-Sb-Te (GST) system has been studied for phase change memory, because of fast crystallization and good data storage lifetime characteristics [3]. However, for the achievement of high density commercial memory devices, some problems will be solved like high current pulse required for the amorphization process, because sample must be heated to its melting point. Therefore, with the aim of finding possible alternative of generally used GST phase change memory material, ternary Ge-Bi-Te (GBT) system co-sputtered by different RF power is investigated using differential scanning calorimeter (DSC), x-ray diffractometer (XRD) and high resolution transmission electron microscopy (HRTEM). As the crystallization and melting temperatures of GBT are higher(260°) and lower(500°) than GST, sputtered GBT guarantees the reliability at room temperature and the reduction of writing current(reset current). Moreover, reset/set ratio measured by resistance change with increasing temperature indicates over 10 2 which satisfy the sensing margin of commercial use. In Fig. 1, we showed the XRD patterns of GBT annealed at 300° for 10 minutes as well as as-grown GBT. It shows that GBT crystallized into stable GeBi 2 Te 4 phase over 300°. Microstructures of crystallized GBT at each temperature are investigated by HR-TEM to confirm the phase appeared at RESET state. Through the bright field image in Fig. 2, we showed the crystallization starts between 200°a nd 300°. High resolution TEM image of 300° annealed GBT in Fig. 3 indicates that the sputtered GBT crystallized into GeBi 2 Te 4 single phase. In Fig. 3, <0001> direction growth single grain GBT with specific facet planes {1015}, {1017} and {0001} is shown. As the equilibrium shape of crystal is determined by the difference of surface energy based on broken bonds model [4], this grain will grow along with {0001},{1017} planes and {1015} plane will be disappeared.