Superlattice-like Structures with Nitrogen-doped Germanium-Antimony-Telluride for Phase Change Random Access Memory.

摘要

Phase Change Random Access Memory (PCRAM) is regarded as one of the leading Nonvolatile memory (NVM) technology candidates to replace FLASH as device feature sizes are reduced due to its high endurance, excellent retention and most importantly scalability. Ge2Sb2Te5 (GST) is the most common phase change material used in prototypical PCRAM devices due to its good speed and stability. Despite these attractive memory attributes, the programming current of GST based PCRAM is relatively high and the crystallization time is relatively long as such these issues compromise the memory density and slows the write/erase speed respectively.;In general, PCRAM performance, in particular programming current and speed are usually enhanced by altering the material properties of the phase change material but to do so often require modification of GST's stoichiometry usually through trial and error. Because the properties of phase change materials are strongly dependent on the film thickness, interface and composition, phase change superlattice-like (SLL) structures composed of two or more phase change materials alternatively stacked on top of each other offers a systematic way of improving the PCRAM performance.;This work investigates the potential of reducing the programming current and crystallization time through the use of SLL structures based on nitrogen-doped GST (N-GST) in PCRAM devices. To reduce the programming current of PCRAM devices, we proposed and fabricated a lattice matched N-GST/GST SLL structure and implemented this in PCRAM devices. The N-GST/GST SLL PCRAM showed a ∼37% reduction in RESET current and this significantly improves PCRAM's memory density. The lattice matching material combination possibly reduced the stress within the SLL structure enabling the N-GST/GST SLL PCRAM to achieve improved switching endurances of 108 cycles.;The influence of the interfaces and layer thickness on the crystallization of N-GST/GST SLL structures was investigated using transient crystallization pulses. The crystallization and nucleation times of the N-GST/GST SLL PCRAM were found to be reduced by ∼37.5% and ∼46.4% respectively when compared to GST based PCRAM. These improvements are most likely due to the N-GST/GST interfaces which promote heterogeneous nucleation.;Through nano-scale thermal engineering, a low thermal conductivity electrode using N-GST was developed. By alternatively stacking thin films of N-GST and TiW, a low thermal conductivity SLL electrode was created and implemented in PCRAM devices. Devices with SLL electrodes displayed ∼10% reduction in RESET current and ∼34% reduction in power compared to PCRAM devices with conventional TiW electrodes possibly due to the improved thermal confinement and heating efficiency.