BeforeHand aims at realizing chalcogenide heterostructures, aiming to provide a processing/storage, all-in-one device, to be embedded in automotive IoT smart devices.
Why using chalcogenide multilayers?
The basic idea is to create multilayers of chalcogenides of different alloy and compound combinations. The PCM cell takes advantage of the merits of the different sorts of materials used for the stack. To be more concrete, let us consider to choose a material with the ability to enhance the write speed (reduce the latency); then it must have a high crystallization speed. However, such materials often suffer from poor thermal stability (low retention). The dilemma between speed and stability is in practice solved by sacrificing the speed to assure the cell thermal stability. In recent time, research on superlattice structures of alternating Sb2Te3 and GeTe or GST revealed not only higher speed than GST, but also a better thermal stability than Sb2Te3 films is expected, although an experimental verification is missing. This shows that combining layers of materials with different physical properties might help to achieve the best material trade-off.
Here we aim at designing unusual active storage/processing layers through functionalization study of proper multilayer heterostructures. Alternated materials will be combined with classical planar and novel geometries, such as core-shell nanowires (NWs). Self-assembled NWs will be used as models to explore innovative cell configurations. NWs help to better understand the complex relationships between structure, interfaces and properties in such PCM multilayers.
The thermal properties measurement of phase change materials (PCM) is a crucial step for their implementation in phase change memory (PCRAM). Indeed, knowing both the thermal properties, as a function of the temperature, and the crystalline state will allow the calculation of the electrical power required for the phase change using an electro-thermal simulation model. In addition, it allows also to design the memory cell in order to avoid the thermal cross-talk effects with neighboring cells. The measurement of PCM thermal conductivity must be performed over the entire temperature range including the amorphous-crystalline phase transition and up to the melting temperature. Within the BeforeHand project we implemented two photothermal methods where the investigated PCM alloys are deposited as thin films on a Si substrate.
The first method is the Modulated PhotherThermal Radiometry (MPTR) that leads to measure the thermal resistance of the film, which includes both the intrinsic thermal conductivity of the film and the thermal resistance at the interface between the film and dielectric and metallic surrounding layers. This configuration is consistent with the PCRAM cell, by reproducing the contact of the PCM with the dielectrics and the metal electrode.
The second method is called the Periodic Pulse photoThermal Radiometry (PPTR) that leads to measure the thermal diffusivity of the PCM layer. All those thermal properties are measured in the 20°C-500°C allowing the different phase changes, from amorphous to different crystalline structures, to occur. As an original result we measured the thermal conductivity of a Ge rich – Ge2Sb2Te5 alloy that is reported in the figure where the phase change occurs at about 350°C.