Phase Change Materials: Challenges on the Path to a Universal Storage Device

Abstract

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.

Disorder‐Induced Localization in Crystalline Pseudo‐Binary GeTe–Sb2Te3 Alloys between Ge3Sb2Te6 and GeTe

“…[ 9,14,15 ] As was already mentioned, the meta-stable cubic structures of the GeSbTe compounds between Ge 1 Sb 4 Te 7 and Ge 3 Sb 2 Te 6 may be regarded as cubic modifi cations of the rhombohedral GeTe structure, where the cation sublattices are fi lled by germanium, antimony, and vacancies. [ 2,9,25 ] Against this background, it is not surprising, that the peak patterns in Figure 3 fi t to rhombohedral unit cells. The tiny hump which appears in the measurement of GeTe at 27.3° results from minor inclusions of crystalline germanium, a consequence of the aforementioned germanium segregation in this particular composition.…”

“…Table 1 presents a summary of the properties of the most common compositions in the (GeTe) x -(Sb 2 Te 3 ) 1-x system. While the compounds between Ge 1 Sb 4 Te 7 and Ge 3 Sb 2 Te 6 display the aforementioned irreversible cubic-to-hexagonal transition on heating, [ 9 ] Ge 8 Sb 2 Te 11 and GeTe show a reversible structural transition from a rhombohedrally distorted low-temperature phase to a cubic high-temperature phase. [ 14,15 ] As was already pointed out by Siegrist et al, [ 6 ] the compositions in the range between Ge 1 Sb 2 Te 4 and Ge 3 Sb 2 Te 6 follow a generic (Ge 1 Sb 2 Te 4 -like) behavior: In all these compositions, the conductivity of the crystalline state increases by at least two orders of magnitude on annealing while the temperature coeffi cient of the resistivity changes from non-metallic (d ρ /d T < 0) to metallic (d ρ /d T > 0) behavior.…”

Transport properties of cubic crystalline Ge2Sb2Te5: A potential low-temperature thermoelectric material

“…5,7 In Fig. 2 and 3, we present the averaged reflectivity and optical absorption coefficients for c-GST, respectively.…”

Resonant bonding leads to low lattice thermal conductivity

“…For example, PbTe, Bi 2 Te 3 and Bi 1 À x Sb x have been the best thermoelectric materials in the intermediate, room and lowtemperature ranges, respectively [1][2][3][4] . Alloys of GeTe and Sb 2 Te 3 (GST) have been the most popular materials for optical storage technologies such as compact disc and phase-change random access memory 5,6 . These materials all have low thermal conductivity, which is crucial for thermoelectric and phasechange memory applications.…”