Ge K-edge (innermost electron shell) EXAFS analysis of crystallized Ge47Te53 film, which shows presence of Ge quasi-crystals and GeTe crystals with non-negligible amount of Ge-site vacancies
High-end automobiles now come standard with “infotainment” systems that rival the information, entertainment and communications devices in your house and office. With consumers hungry for more security and energy efficiency in their cars, the demand for more computer memory has never been higher. Plus, expectations are growing for performance under increasingly aggressive environments, such as in space and/or missile applications, where the data must be retained for more than ten years at temperatures over 100 degrees Celsius.
Phase change memory (PCM) – which stores information using a special glass that can morph between crystalline and amorphous states – is one of the newest memory technologies under development. PCM is like flash memory, which retains stored information even when not powered, a characteristic called non-volatile memory. But PCM is faster.
“We have investigated the structural basis of phase change speed using germanium telluride as the model system,” said Himanshu Jain, a professor at Lehigh University in Pennsylvania, and director of the National Science Foundation’s International Materials Institute for New Functionality in Glass (IMI-NFG). Jain and colleagues determined the local atomic arrangements of each constituent element before and after the phase change event. The results indicate how the phase-switching speed and crystallization temperature strongly depend on the composition.
“Our findings will help fine-tune the compositional window and, hopefully, the film-processing conditions for better performance in more demanding applications,” said Jain.
At NSLS beamline X18B, the scientists used a technique called EXAFS (extended x-ray absorption fine structure) to characterize thermally deposited germanium-telluride films, systematically covering a much wider range of Ge-Te formulations than previously attempted. According to Jain, the comprehensive, quantitative structural information has given deeper insight of the phase-change process.
Next, the team is planning to use EXAFS to establish how atomic rearrangements are induced in PCMs by the application of electrical field. They are now performing new experiments that will obtain structure under in situ application of electrical conditions as phase transformation occurs.
In 2009, IMI-NFG and Brookhaven’s National Synchrotron Light Source organized the Workshop on Applications of Synchrotron Techniques in Glass Research. The structure of glass is more complex than corresponding crystalline solids, which makes its determination especially difficult. To characterize glass structure, routine laboratory methods are inadequate, and researchers turn instead to advanced synchrotron-based techniques.
“Synchrotron-based EXAFS is ideal for determining the structure of amorphous materials,” said Jain. “Since PCMs exists in both crystalline and amorphous states, there is no better method for understanding the evolution of atomic arrangement of glass to that of the corresponding crystalline state, and vice versa.”
He added that NSLS-II will open up a new dimension of measurements and understanding of the very fast changes that make PCM applications possible. “With the new facility, we should be able to monitor structural changes in real time down to picosecond level in a very small region of the sample,” said Jain. Now over 75 percent complete, NSLS-II will produce x-rays up to 10,000 times brighter than those at NSLS, providing unprecedented opportunity for materials research at the nanometer scale.
Jain and Byung-Ki Cheong, Korea Institute of Science and Technology; Yong Gyu Choi, Korea Aerospace University; and Andriy Kovalskiy, of both Lehigh University and Austin Peay State University in Tennessee, described their findings in the March 15, 2012, issue of Chemical Physics Letters.
Their work is supported by IMI-NFG and a grant from the Fundamental R&D Program for Core Technology of Materials, Ministry of Knowledge Economy, Republic of Korea. NSLS is a U.S. Department of Energy user facility.