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.

The electrical switching behavior of GeTe-based phase change memory devices is characterized by time resolved experiments. SET pulses with a duration of less than 16 ns are shown to crystallize the material. Depending on the resistance of the RESET state, the minimum SET pulse duration can even be reduced down to 1 ns. This finding is attributed to the increasing impact of crystal growth upon decreasing switchable volume. Using GeTe or materials with similar crystal growth velocities, hence promises nonvolatile phase change memories with dynamic random access memorylike switching speeds.

Phase change materials, in which a material is reversibly switched between an amorphous and crystalline state with corresponding contrast in optical and electronic transport properties, are excellent nonvolatile storage media. Rewritable digital versatile disks (DVDs) and Blu-ray discs are based on such materials in which the optical contrast between the amorphous and crystalline phases enables data storage. Additionally, the large change in electronic transport properties with resistivity contrast of up to six orders of magnitude on crystallization and fast switching speed is at the heart of a new class of nonvolatile data storage devices with electronic read/write operation and potential for future miniaturization. The amorphous state is characterized by saturated covalent bonds, whereas the crystalline phase forms resonant bonds. This bonding mechanism can account for the high electronic polarizabilities that characterize crystalline phase change materials. Interestingly, the relevant electronic states also govern the charge transport in the crystalline phase, leading to unique transport properties including a high degree of electronic localization, in those phase change materials, which are characterized by a high degree of disorder.

Thin films of phase change materials (PCM) based on ternary GeÀSbÀTe (GST) alloys are of practical relevance because of their use in optical data storage devices like CDs, DVDs, and Bluray disks. 1 The crystallization and amorphization of PCMs leads to pronounced changes of the electronic and optical properties, which can be used to store, erase, and read information. Different chemical bonding appears as demanding criterion for the property changes upon phase change. 2 Only a subset of GSTs are assumed to exhibit the required properties for memory applications, 3 and most studies were performed on phases nGeTe•mSb 2 Te 3 , e.g., Ge 2 Sb 2 Te 5 ("GST225" with n = 2; m = 1). Upon heating, thin films of GST225 first transform to a metastable NaCl-type structure (space group: Fm3m) characterized by structural disordering of Ge/Sb atoms and intrinsic structural vacancies (20% per formula unit) on joint crystallographic sites. Depending on the measurement technique and film thickness, different crystallization temperatures (T c1 ) were reported ranging from 140 to 175 °C. 4À8 For instance, in situ X-ray scattering experiments on GST225 films and nanoparticles 9 indicate no significant difference of T c1 (∼160 °C). Moreover, the speed of crystallization critically depends on film thickness, 10 and after repeated cycling, phase segregation was reported for GST225. 11 At more elevated temperatures (T c2 ∼ 300 °C), a second and

Scanning tunneling microscopy and spectroscopy have been employed to reveal the evolution of the band gap and the Fermi level as a function of the annealing temperature for Ge1Sb2Te4, a promising material for phase change memory applications. The band gap decreases continuously from 0.65 eV in the amorphous phase via 0.3 eV in the metastable crystalline phase to zero gap in the stable crystalline phase. The Fermi level moves from the center of the gap in the amorphous phase close to the valence band within the crystalline phases. Moreover, the metastable phase has been imaged with atomic resolution, presumably showing the Te lattice at negative sample bias and the Ge/Sb/vacancy lattice at positive bias.