A study on device characteristics of non-volatile memory with Ge doped Sb-Te phase change materials

Abstract

Phase change random access memory (PCRAM) using $Ge_2Sb_2Te_5$ as the storage material has many advantages compared to conventional non-volatile flash memory in terms of scalability, writing endurance and speed. However, there are still some technical issues for its use as a high-performance non-volatile memory. SET speed enhancement is one of them. Improved SET speed is needed not only for a wider range of application but also for multi-level cell (MLC) operation since verification process must be included. Another important issue is the RESET current ($I_{RESET}$) reduction. With technology node shrinking down, maximum current accessible from a switching element (transistor) tends to reduce more rapidly than the required operating current for a memory element (resistor). Thus, various methods of reducing $I_{RESET}$ have been proposed such as increase in resistivity of the bottom electrode and addition of some light elements in GST for the increased joule heating and decrease in the programming volume size for the decreased RESET power. However, these methods lead to increase SET resistance ($R_SET$), producing in turn a smaller sensing margin and slower sensing speed of the devices. In order to achieve fast SET together with low $R_{SET}$ and low $I_{RESET}$ for PCRAM device, a new phase change material (PCM) shall be explored. A PCM featuring high crystallization speed, low melting temperature ($T_m$), high crystallization temperature ($T_c$), proper electrical resistivity ($ρ_{cryst.}) and low thermal conductivity ($Λ_{cryst.}$) is preferable for a high speed, high density PCRAM device. Ge doped Sb-Te (Ge-ST) is one of the promising candidates, which have been utilized as the optical recording material of Blu-ray disk and recently in a novel PCRAM device named phase change line memory. In this study, film composition, apparent characteristics of amorphous to crystalline phase changes, electrical resistivity, thermal conductivity, crystal to microstructure...