TEM/STEM investigations of phase change materials for non-volatile memory applications

Phase change materials are very interesting for future information technology because of their possibility to encode information in the readable difference of physical properties between the crystalline and the amorphous phase. Phase change materials are the dominant non-volatile memory material used in rewritable optical memory. This includes the current state of the art Blue-ray disc. Mobile computer platforms like smart mobile phones, tablets and netbooks are in need of energy and space efficient memory. Optical or magnetic recording media do not meet these needs anymore. Phase change materials used as non-volatile electronic memories are promising candidates as competition for flash memory. Flash memory is the current state of the art electronic non-volatile memory. In addition, non-volatile electronic applications as competition to Dynamic Random Access Memory (DRAM) are possible because of the high switching speeds of phase change materials. The key to the successful application of phase change materials as electronic non-volatile memory is the understanding of their physical properties and especially their switching kinetics. In the present work, transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) were used in a systematic manner to investigate the properties of a variety of phase change materials. The crystal growth velocities of grains growing in 30 nm thick amorphous layers of the phase change materials Ag${}_4$In${}_3$Sb${}_{67}$Te${}_{26}$ (AIST) and GeTe were measured directly by TEM bright field imaging. Grains of the measured materials were grown in a matrix of the amorphous phase by ex situ heating. This is done for sputtered as deposited material and material molten by laser which is quenched to room temperature. Furthermore we investigated lamellas of as deposited and melt quenched AIST and GeTe by fluctuation electron microscopy (FEM). The comparison of growth velocity and FEM data reveals that increasing medium range order (MRO) leads to a decrease in growth velocity. This is also related to different glassy states of the phase change materials. The switching behaviour of electronic phase change material memory devices based on the material AIST was investigated by TEM bright field imaging. Cross section lamellas were prepared by a combined focused ion beam and scanning electron microscope procedure. The investigation of the amorphous state of a switched device showed the occurrence of the Thompson-Seebeck effect, leading to an inhomogeneous heat distribution and an inhomogeneous amorphization of the phase change material device. In further studies, the intermediates of the reaction path of a solvothermal synthesis of single crystalline hexagonal Sb2Te3 platelets were investigated. The chosen methods were STEM annular dark field (ADF) imaging and nano area electron diffraction (NAED). These platelets can be used as a model system for phase change materials. NAED has revealed surprising results on the structure and shape of the intermediate states during synthesis. The stable phases of the Ge, Sb and Te (GST) alloys have highly ordered layered structures similar to the structures of interfacial phase change materials (IPCM) but without an artificial super lattice design. Therefore the stable phases of GST are very interesting materials to understand the switching processes of IPCM or phase change materials of similar structure. Ge${}_1$Sb${}_2$Te${}_4$ (GST 124) in the stable phase was directly deposited on a Si (111) substrate by metal organic vapor phase epitaxy (MOVPE). In the present work GST 124 deposited by MOVPE was investigated by TEM and high resolution STEM (HRSTEM). The atomic structure of GST 124 in the stable phase was resolved and the high quality of the MOVPE deposited layers was demonstrated. Furthermore the adaption zone of the GST 124 to grow perfectly on the Si (111) substrate is imaged with atomic resolution.