Summary:

Phase change materials have been the basis of memory storage since their application to optical data storage in 1987. In the 2010s, phase change memories developed were 1000 times faster and more durable than NAND memories, and 10 times denser than DRAM memories. Phase-change memories based on chalcogenides offer the flexibility of faster speed, greater endurance or better thermal stability depending on the stoichiometry. Materials engineering of GeSbTe (GST) alloys has enabled stoichiometries with high temperature stability suitable for phase change memories (PCMs) embedded in automotive MCU applications. Alloys such as Ge-rich doped GeSbTe exhibit high temperature data retention due to the high crystallisation temperature. In PCMs, reversible switching between phases is initiated thermally. Studies indicate that less than 1 % of energy is used for the phase change, while most of the energy is lost through other heat dissipation pathways in the PCM cell. Knowledge of the thermal properties of these materials over the entire operating temperature range of the PCM cell is therefore crucial to improving memory performance. The flagship GST-225 alloy has been extensively characterised, but the current state of knowledge does not allow newly designed stoichiometries to be thermally characterised. Methods such as 3ω, thermo-reflectance and photo-thermal radiometry have been used to characterise GST thermally. These tools have certain drawbacks, such as the additional microfabrication of heating elements or transducers, high installation costs or complex data post-processing. Raman thermometry is an optical characterisation technique that does not require microfabrication and can offer the advantage of a simultaneous structural study. In this work, we studied particles such as GeTe, Ge-rich GeSbTe and N-doped GeSbTe using Raman thermometry. This was made possible by studying the temperature evolution of the vibrational modes present in GeSbTe-based alloys. We demonstrate for the first time the successful extraction of the temperature- and phase-dependent thermal properties of these materials at higher temperatures (~350°C) using Raman thermometry. The increased Ge content and additional N doping decreased the thermal conductivity, which is beneficial for PCM efficiency. The main contribution to thermal conductivity comes from phonons, with the electronic contribution being negligible. These results provide a better understanding of the behaviour of these materials at higher temperatures and the effect of nitrogen content. They demonstrate that Raman thermometry is a rich, quantitative and reliable thermal and structural characterisation technique for phase change materials.

Abstract:

Phase change materials have been the basis of memory storage since their application to optical data storage in 1987. In the 2010s, phase change memories developed were 1000 times faster and more durable than NAND memories, and 10 times denser than DRAM memories. Phase-change memories based on chalcogenides offer the flexibility of faster speed, greater endurance or better thermal stability depending on the stoichiometry. Materials engineering of GeSbTe (GST) alloys has enabled stoichiometries with high temperature stability suitable for phase change memories (PCMs) embedded in automotive MCU applications. Alloys such as Ge-rich doped GeSbTe exhibit high temperature data retention due to the high crystallisation temperature. In PCMs, reversible switching between phases is thermally initiated. Studies indicate that less than 1% of the energy is used for the phase change, while most of the energy is lost through other heat dissipation pathways in the PCM cell. Knowledge of the thermal properties of these materials over the entire operating temperature range of the PCM cell is therefore crucial to improving memory performance. The flagship GST-225 alloy has been extensively characterised, but the current state of knowledge does not allow newly designed stoichiometries to be thermally characterised. Methods such as 3ω, thermo-reflectance and photo-thermal radiometry have been used for the thermal characterisation of GST. These tools have certain drawbacks, such as the additional microfabrication of heating elements or transducers, high installation costs or complex data post-processing. Raman thermometry is an optical characterisation technique that does not require microfabrication and can offer the advantage of a simultaneous structural study. In this work, we studied particles such as GeTe, Ge-rich GeSbTe and N-doped GeSbTe using Raman thermometry. This was made possible by studying the temperature evolution of the vibrational modes present in GeSbTe-based alloys. We demonstrate for the first time the successful extraction of the temperature- and phase-dependent thermal properties of these materials at higher temperatures (~350°C) using Raman thermometry. The increased Ge content and additional N doping decreased the thermal conductivity, which is beneficial for PCM efficiency. The main contribution to thermal conductivity comes from phonons, with the electronic contribution being negligible. These results provide a better understanding of the behaviour of these materials at higher temperatures and the effect of nitrogen content. They demonstrate that Raman thermometry is a rich, quantitative and reliable thermal and structural characterisation technique for phase change materials.