In addition to their wide-spread application in the re-writable optical memory markets, phase-change memory alloys are also poised to take a prominent role in future non-volatile memory applications due to their potential for low-energy usage and indefinite cyclability compared with their silicon-based flash memory counterparts. In contrast with their widespread use, however, the details of the crystalline to amorphous switching process utilized for memory storage remain an active research topic with many details still lacking. Considering the conflicting requirements for high-speed switching, yet long term data storage integrity, a deeper understanding of these materials is essential for insightful application development. We have used x-ray absorption fine structure spectroscopy (XAFS), a technique equally suitable for amorphous and crystalline phases to elaborate details in structural changes in the phase-change process for a variety of phase-change alloys in static measurements. As the kinetics of the switching process are the linchpin for optimizing switching characteristics, we have recently initialted dynamic measurements of light -induced structural changes in Ge-Sb-Te (GST) alloys. These measurements have been carried out synchronously using both femtosecond and nanosecond laser pump pulses in conjunction with 100~ps x-ray pulses generated by an electron storage ring. By synchronously triggering the laser with a variable sub-nanosecond delay, we have been able to use XAFS to probe details of the dynamics of the switching process. Preliminary results learned from this approach applied to GST alloys are presented.

A thermal modeling based on Finite Element Method (FEM) was used to simulate Phase Change Random Access Memory (PCRAM) cells. The factors affecting temperature distribution of a PCRAM cell structure such as geometry, device structure and the properties of the individual materials were investigated. The results of the analysis provided the fundamental design of a novel cell structure which has a better performance and reliability.

Phase change materials possess a unique combination of properties which include a pronounced property contrast between the amorphous and crystalline state, i.e. a high electrical and optical contrast. In particular the latter observation is indicative for a considerable structural difference between the amorphous and crystalline state. At the same time the crystallization of the amorphous state proceeds on a fast time scale. This raises the question how structure, properties and kinetics are related in phase change alloys. It will be demonstrated that only a small group of covalent semiconductors with octahedral-like coordination has the required property combination. This is related to their thermodynamic properties which govern the kinetics of crystallization.

In this paper, ultra-high memory density and high speed non-volatile phase change random access memory (PCRAM) was investigated by material engineering. The melting point, crystallization point and activation energy of crystallization of the Bismuth (Bi) doped Germanium-Antimony-Tellurium (GeSbTe) compound was measured using differential scanning calorimetry (DSC) and compared to other GeSbTe ternary compounds. It was observed that the melting temperature of Bi-doped GeSbTe was lower than that of GeSbTe. On the other hand, its activation barrier was found to be reduced, which in turn increased the speed of crystallization of Bi-doped GeSbTe. Bi-doped GeSbTe was then used as a phase change material in the fabrication of PCRAM devices. The properties of PCRAM fabricated using this material were then compared to those using GeSbTe, with emphasis on the programming current required. The results obtained revealed that lower programming current of up to 40% has been achieved for PCRAM with Bi-doped GeSbTe compared to those with other GeSbTe compounds. Bi-doped GeSbTe also has low RESET current and fast speed of crystallization with scaling, making it a suitable material for high speed, ultra-high density PCRAM fabrication in the future.