Understanding the Electro-thermal and Phase-transformation Processes in Phase-change Materials for Data Storage Applicat
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Understanding the Electro-thermal and Phase-transformation Processes in Phase-change Materials for Data Storage Applications C. D. Wright, M. Armand, M. M. Aziz, S. Senkader, and W. Yu School of Engineering, Computer Science and Mathematics, University Of Exeter, Exeter EX4 4QF, UK ABSTRACT Attempts at the practical utilization of Sb-Te based alloys beyond optical data storage have been made recently by employing these materials in both scanning probe type memories, and in electrical memory devices - namely Phase-Change Random Access Memory (PC-RAM). We have developed models to simulate the electrical, thermal, and phase-change characteristics of this important class of material. In this paper we describe the physical basis of our models and present simulation results for different memory configurations and operating conditions. INTRODUCTION During the last decade there has been great interest in chalcogenide amorphous materials for applications in optical data storage [1, 2]. The most prominent and widespread use of such materials is in the rewritable phase-change optical memory disks such as CD-RW and the various rewritable DVD formats. In the past, utilization of these materials in non-volatile electrical memory devices was also reported [3] and, more recently, large scale integration of this novel electrical memory has been successfully demonstrated [4]. The use of phase-change materials for scanning probe based memories, similar in concept to the well-known IBM Millipede system, has also gained attention recently. Both optical and electrical data storage rely on the reversible phase transformation of chalcogenide based alloys, like GeSbTe (GST) or AgInSbTe, between amorphous and (poly)crystalline states. In optical disk memories such phase-transformations are of course brought about by laser heating, whereas in electrical memory devices (often called phase-change random access memory, PC-RAM, or Ovonic Universal Memory, OUM), material is transformed between the two states electrically via Joule heating. Data is written or erased by resistive heating caused by a pulse of electrical current. The amorphous phase is obtained by a current pulse sufficiently large to achieve melting. Subsequently a rapid quench allows the material to solidify in the amorphous phase. Similarly, crystallization is initiated by a pulse heating of the device to sufficiently high temperatures. The state of memory cell is then read out non-destructively by measuring the cell’s electrical resistance, the crystalline phase having a low resistivity in comparison to the amorphous phase; the difference in resistivity between the two phases can be more than threeorders-of-magnitude [5]. Yet another possibility is the use of phase-change materials in a scanning probe based storage system. The feasibility of a high density, high data-rate, scanned probe memory has already been demonstrated by IBM Zurich group via their Millipede system, where a 2-D array of around 1000 heated AFM-type tips write to and read from a polymer medium with bit
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