Phase-change materials: The view from the liquid phase and the metallicity parameter
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oduction Switching behavior of phase-change materials and the relevance of liquid states For more than half a century, mixtures of elements in the nonmetal section of the periodic table have been famous for their ability to form glasses (e.g., As-Se) from their liquid states, often with great stability, low acoustic losses, and high corrosion resistance. They have found many applications, particularly in infrared optics (e.g., night vision) and semiconductor electronics.1 However, as the compositions tested include heavier elements (e.g., Se replaced by Te and As replaced by Sb), the glass-forming ability diminishes, seemingly as the bandgap decreases. While As2Se3 is extremely difficult to crystallize on laboratory time scales, Bi2Te3 is metallic and difficult to vitrify even by sputtering. In between the two is As2Te3, which undergoes a semiconductor-to-metal transition when heated above its melting point, Tm.2 In 1968, Ovshinsky3 discovered that he could use the ability of Te-containing alloys to generate solid phases of greatly differing electronic conductivities depending on whether they were vitreous or crystalline, to make switching devices and memory devices. Apart from some excellent fundamental
studies in Ovshinsky’s laboratory, however, development was slow, with a resurgence of interest only in the late 1980s after Yamada et al.4 published a study on the three-component Ge-Sb-Te system that showed special behavior in fast phase switching and property contrasts along what we will call the “Yamada line,” (the composition line joining GeTe to Sb2Te3). The Yamada line,5 comprising the most popular phase-change materials (PCMs), features several closely related crystalline compounds with simple structures and remarkably fast crystallization kinetics. These crystalline compounds have been studied in detail by Wuttig, who in 2005, highlighted the storage potential of devices containing them.6 A follow-up 2007 paper on the subject by Wuttig and Yamada in collaboration,7 has been greatly cited. Recently, this interest has been translated into manufactured devices, and even more recently, attention has been directed to the liquid states of PCMs, in order to understand their special crystallization properties.8–12 In this article, we review these liquid-state issues and then take the development one step further by introducing and exploiting a new material parameter, the “metallicity.” Using this parameter in our plots allows us to strengthen the claim that it is the existence of a metal-to-semiconductor transition
Shuai Wei, Institute of Physics, RWTH Aachen University, Germany; [email protected] Pierre Lucas, Department of Materials Science and Engineering, The University of Arizona, USA; [email protected] C. Austen Angell, School of Molecular Sciences, Arizona State University, USA; [email protected] doi:10.1557/mrs.2019.207
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