118 MRS BULLETIN • VOLUME 37 • FEBRUARY 2012 • www.mrs.org/bulletin © 2012 Materials Research Society Introduction Many materials can be fabricated in both amorphous and crys- talline forms, but few of them possess the unique combination of properties such as fast crystallization, large optical and elec- trical contrasts between the phases, and high stability of the amorphous phase, which make them useful materials for data storage. Ovshinsky 1 proposed the concept of using chalcogen- ides as data storage materials in the 1960s, but the technologi- cal success of rewritable optical storage media based on phase change materials was enabled only by the breakthrough discovery of fast crystallizing materials by Yamada and co-workers 2 in the late 1980s. These new phase change materials were based on compositions along the pseudo-binary line between GeTe and Sb 2 Te 3 , most notably the composition Ge 2 Sb 2 Te 5 (GST), and are still the most commonly used materials today. Phase change optical data storage has been a great success, and rewritable Blu-ray disks based on it already are the third generation of optical media. This progress has triggered a renewed interest in the development of solid-state memory devices employing phase change materials. This article reviews the relationship between the composi- tion, structure, optical and electrical, and physical properties of phase change materials. The latest developments in phase change memory (PCM) technology are discussed as well. Structural properties of phase change materials In the last decades, a number of compounds have been identi- fied that possess the characteristic properties of technologically useful phase change materials. In recent years, the main focus in the search for superior phase change alloys was the optimization of materials for electronic data storage, such as materials with very high data transfer rates that can compete with dynamic random access memories 3 or the attempt to find materials with elevated operation temperatures for embedded memories or automotive applications. The search for such novel or improved phase change materials would be alleviated if design principles were available to develop new phase change materials. In an attempt to explain the characteristic properties of phase change alloys, research has focused on the determination of the atomic arrangements in phase change materials. 4 The crystalline phase is often characterized by an octahedral-like atomic arrange- ment, where each atom has, to a first approximation, six nearest neighbors. The frequent appearance of this structural motif is a consequence of the unique bonding that prevails in crystalline phase change materials. 5 For an octahedrally coordinated atom, six covalent bonds are established, but there are not enough electrons available to saturate these bonds. Therefore, resonant bonds form, similar to the case of benzene. 5 Resonant bonding is characterized by large Born effective charges Z T , high optical Phase change materials Simone Raoux, Daniele Ielmini, Matthias Wuttig, and Ilya Karpov Phase change materials can be switched rapidly and repeatedly between amorphous and crystalline phases, which differ distinctly in their optical and electrical properties. This combination of properties is utilized to store information in rewritable optical storage media and in emerging phase change memory technology. This article describes the physical properties of phase change materials such as Ge 2 Sb 2 Te 5 and relates these properties to speciic structural and bonding characteristics. Electrical conduction and switching, which are relevant for phase change memory operation, are explained from a physical perspective. Phase change memory device integration and technology development are discussed, including aspects of access device selection and integration. Simone Raoux, IBM T.J. Watson Research Center; simone_raoux@almaden.ibm.com Daniele Ielmini, Dipartimento di Elettronica e Informazione and IUNET, Politecnico di Milano; ielmini@elet.polimi.it Matthias Wuttig, I. Physikalisches Institut (IA), RWTH Aachen University; wuttig@physik.rwth-aachen.de Ilya Karpov, Intel Corporation; ilya.v.karpov@intel.com DOI: 10.1557/mrs.2012.357