Phase Change Memories (PCM) are one of the best candidates for the next generation of non volatile memories. A great research effort is still needed in order to optimize the properties of phase change (PC) materials which are used in PCM devices. In particular, doping has been demonstrated to improve retention in devices. Moreover, a study of the effect of scaling and interface material on PC materials properties is still an open research field. In this context, the first part of the thesis is dedicated to investigate the local structure of C or N doped amorphous GeTe. The impact of doping is observed experimentally with the appearance of a new peak in the pair distribution function of doped GeTe, indicating the formation of a bond at a new distance that is absent in the undoped amorphous material. The presence of new environments involving carbon and nitrogen is confirmed through ab initio simulations. The subject of the second part of this thesis is the impact of confinement on Ge2Sb2Te5 (GST) crystallization mechanism. Nano-sized clusters of GST have been made by sputtering, deposited and then studied through X-ray diffraction using synchrotron radiation. The crystalline clusters experience a tensile strain that can be ascribed to the effect of the embedding Al2O3 matrix. Their crystallization temperature has been found to be only 25°C higher than the one observed for a thin film of GST of 10 nm deposited under the same conditions. This result is positive for the future development PCM because it indicates that the scaling effect on the crystallization temperature in phase change material can be small. The third and last part of the thesis is dedicated to the investigation of the interface material effect on the crystallization temperature of GeTe and GST thin films through reflectivity and X-ray diffraction measurements. In both GeTe and GST film 100 nm thick interfaced with Ta the crystallization temperature is higher than in the case of TiN or SiO2 interface. Such an interface effect on relatively thick films was never reported before. The results suggest that the SiO2/GeTe interface is energetically favourable for the nucleation and growth of grains with a preferred orientation and that nucleation and growth mechanisms are different for different interface materials.
Material studies for advanced phase change memories : doping, size reduction and interface effects