Fabrication and characterization of phase change compound materials for nonvolatile PC-RAM device

Abstract

The phase change memory utilizes the discrete difference of the material properties in two different physical state of the material, which are the amorphous and the crystalline state. This historical background of this field goes way back to the 1920``s when a researcher named A. Waterman introduced "The electrical conductivity of moybdenite". In the 1950``s, T. Vengel had reported "Semiconducting properties of the phase change materials during the phase transition" and Standford. R. Ovshinsky, who can be described as the pioneer of the modem phase change memory device, proposed "Reversible electrical switching phenomena in disordered structure" in 1968. From then on, the phase change memory research has been most actively studied and the field has grown to effect the related industries. However, the phase change memory device was actually confined to the optical memory storage during the 1970``s and 80``s and the device was functioned by the external laser power which irradiated the material surface to melt and quench to obtain the desired crystal structure. In 1998, the "Amorphous Materials -The key to new devices" which Standford. R. Ovshinsky had reported in the IEEE Proc. Of CAS played an important role to set a new stage of the electrically functioned phase change memory device. The main difference between the optical and the electrical PC-RAM was that the external power source which acted to transform the crystalline structure of the phase change material has changed from the optical laser power to the electrical current. By utilizing the specific heat of the phase change material, the input current heats the material to reach the melting or the crystallization temperature. The core materials related to this field is the Ge-Sb-Te chalcogenides which show a very fast phase transition speed within less than a few tenth of nano-seconds while sustaining the large electrical resistivity difference. In this research, the bulk chalcogenide Ge-Sb-Te compounds wh...