Electro-thermal phenomena in phase change memory [electronic resource]
Keywords: Mechanical Engineering

Recent progress in materials science and the trends of nanoscale electronics have brought greater attention to the transport of heat and electricity in confined geometries. Research on developing metrology and understanding the electro-thermal phenomena can make a significant improvement in novel electronic devices such as phase change memory. Phase change memory is a particularly promising candidate for next-generation data storage because of its exceptional scalability and cycle endurance. The phase change memory devices store information through thermally-induced phase transitions of Ge2Sb2Te5 and related compounds. Because the temperature governs the phase change processes, thermal conduction in Ge2Sb2Te5 films strongly influences the device figures of merit including the programming time and the required energy. The present doctoral research develops innovative metrologies to characterize the thermal properties of Ge2Sb2Te5 films that are relevant for device operations and quantifies the importance of electro-thermal phenomena in phase change memory. Large temperature transients and rapid cycling of phase change memory pose unique challenges for thermal characterization of phase change materials. Here we develop experimental structures based on a micro-thermal stage that reproduces the heating time scales and the temperature excursions of phase change memory devices in the characterization samples. The measurement results show the thermal conductivity of Ge2Sb2Te5 films from room temperature to above 400 °C in amorphous, face-centered cubic, and hexagonal close-packed phases. Another key benefit of the micro-thermal stage is that a single structure enables simultaneous characterization of thermal and electrical properties using four-probe electrical-resistance thermometry with a programmable Ge2Sb2Te5 bridge. This work reports the in-plane electrical resistance and the out-of-plane thermal conductivity during repetitive cycling. We identify electron contribution to the thermal transport in Ge2Sb2Te5 films using the electrical properties and the Wiedmann Franz Law. The electrons are responsible for up to 70 % of the thermal transport in the hexagonal closed-packed phase, but phonons dominate the thermal transport in the amorphous and the face-centered cubic phases, which are consistent with the Einstein model for highly disordered materials. Phase change memory devices experience both large current densities and temperature excursions exceeding 600 °C, and these extreme conditions increase the relevance of thermoelectric transport and provide an ideal opportunity for studying their impact. This work develops a novel silicon-on-insulator experimental structure to measure the phase and temperature-dependent thermoelectric properties of Ge2Sb2Te5 films including the first data for films of thickness down to 25 nm. The Ge2Sb2Te5 films annealed at different temperatures contain varying fractions of the amorphous and crystalline phases, which strongly influence the thermoelectric properties. The data are consistent with modeling based on effective medium theory and suggest that careful consideration of phase purity is needed to account for thermoelectric transport. The simulations considering the thermoelectric heating show a Ge2Sb2Te5 peak temperature increase up to 44 % and a decrease in the programming power up to 30 %. The simulation results and the analysis discussed here provide physical insights into thermal phenomena and cell optimization opportunities.