Research On Phase Change Materials

Phase change memory is a potential candidate of next-generation non-volatile memory for low-voltage, low-power, high speed and high density application. It has called broad attention from industry and academy.As the key functional material of the Phase change memory application, phase change material interests researchers around the world for the memory application and the delicacy of itself. This dissertation would focus on the widely applied phase change materials Ge₂Sb₂Te₅ and Sb₂Te₃ under support of National 863 project.The work of this dissertation could be divided into fundamental and application research. In the first part, after discussion on the thermodynamics of amorphous/crystalline states and phase change process, amorphous Ge₂Sb₂Te₅ and Sb₂Te₃ thin films are deposited on Si/SiO₂ substrates by RF magnetron sputtering and are crystallized by rapid thermal processing. As the knowledge of lattice structure plays a key role in understanding the phase change mechanism, the focus of attention in the phase change materials research, the structure of the phase change thin films are investigated by both XRD and Raman methods. For the lattice dynamics of crystalline materials, first-principles study identifies the observed Raman active modes of HCP Ge₂Sb₂Te₅ for the first time with confirmation of the atom stacking of the phase. This work forms the foundation of phase change mechanism research, and the lattice dynamics method could be extended to the investigation of innovative materials.From the application point of view, the selecting rules of phase change materials are put forward according to the requirements of phase change memory. The crystalline temperature of Sb₂Te₃ is increased by Ag-doping for the purpose of improving the stability of its amorphous state at room temperature. The crystalline mechanism and electric properties of Ag- Sb₂Te₃ material are then discussed together with its device application. In addition to the work above, a behavior model referring to the electrical switch of amorphous phase change material is set up with proved accuracy. This model could be applied in the transient analysis of the electrical behavior of the memory device.The last section of the dissertation deals with the delamination of W/Sb₂Te₃ film stack. The strain of the W film and the interface adhesion are investigated and a solution of applying TiN adhesion layer is then put forward.