Disorder-induced localization in crystalline phase-change materials

Abstract

Localization of charge carriers in crystalline solids has been the subject of numerous investigations over more than half a century. Materials that show a metal-insulator transition without a structural change are therefore of interest. Mechanisms leading to metal-insulator transition include electron correlation (Mott transition) or disorder (Anderson localization), but a clear distinction is difficult. Here we report on a metal-insulator transition on increasing annealing temperature for a group of crystalline phase-change materials, where the metal-insulator transition is due to strong disorder usually associated only with amorphous solids. With pronounced disorder but weak electron correlation, these phase-change materials form an unparalleled quantum state of matter. Their universal electronic behaviour seems to be at the origin of the remarkable reproducibility of the resistance switching that is crucial to their applications in non-volatile-memory devices. Controlling the degree of disorder in crystalline phase-change materials might enable multilevel resistance states in upcoming storage devices.

Interband characterization and electronic transport control of nanoscaledGeTe/Sb2Te3superlattices

“…In addition we show that it is possible to control the electronic transport properties of the CSL not only by an annealing treatment, as in Ref. [17], but also by means of a specific stack design. In fact, as we will see, the CSL conductivity depends on the layer sequence, giving a new design pathway for the control of the material properties.…”

“…Please note that the vacancy layers, when two-dimensionally extended, are labelled van der Waals gaps. [27] The similarity between the high-temperature-annealed GSTs [17] and the CSLs, besides the structure, is corroborated by the measured resistivity of the latter that decreases with increasing temperature, suggesting a metallic type conductivity [28]. Nonetheless, it is not obvious whether also the dielectric properties of CSLs are analogous to those of GST compounds [29][30][31][32].…”

“…In fact, by increasing the temperature, the system undergoes an insulator-metal transition (IMT) at 275 • C, due to disorder reduction. [17] Density functional theory calculations showed that the insulating state is caused by the localization of charge carriers in vacancy-rich areas, and the transition to the metallic state happens when vacancies reconfigure into ordered layers [18]. The presence of vacancy layers in the high temperature annealed phase of GSTs was proven by high-resolution TEM and electron diffraction [25].…”

“…Prior to investigate the properties of CSLs, we review the charge carrier transport [17,18] and the structural [19][20][21][22][23] properties of GST compounds. The crystal phase of GSTs is characterized by a strong dependence of the resistivity upon annealing treatment [17].…”

“…The crystal phase of GSTs is characterized by a strong dependence of the resistivity upon annealing treatment [17]. In fact, by increasing the temperature, the system undergoes an insulator-metal transition (IMT) at 275 • C, due to disorder reduction.…”