Revisiting the Local Structure in Ge-Sb-Te based Chalcogenide Superlattices

Abstract

The technological success of phase-change materials in the field of data storage and functional systems stems from their distinctive electronic and structural peculiarities on the nanoscale. Recently, superlattice structures have been demonstrated to dramatically improve the optical and electrical performances of these chalcogenide based phase-change materials. In this perspective, unravelling the atomistic structure that originates the improvements in switching time and switching energy is paramount in order to design nanoscale structures with even enhanced functional properties. This study reveals a high-resolution atomistic insight of the [GeTe/Sb 2 Te 3 ] interfacial structure by means of Extended X-Ray Absorption Fine Structure spectroscopy and Transmission Electron Microscopy. Based on our results we propose a consistent novel structure for this kind of chalcogenide superlattices.The need for fast and efficient management of information stimulates research on materials that can be switched on nanometer length scales and sub-nanosecond time scales. Phase-Change materials (PCMs) possess a unique property portfolio, which is ideally suited for memory device applications [1][2][3][4][5][6] . A PCM is identified by its ability of switching rapidly and reversibly between a crystalline and an amorphous state, where the amorphous state is obtained by melting the crystalline state followed by rapid quenching. These two states significantly differ in their properties, such as the optical reflectivity as well as the electrical conductivity. The phase transformation is in general triggered by thermal heating, or by either electrical and optical pulses of different time duration and amplitude. The large contrast in reflectivity between these two states lays at the base of already working PCM-based optical rewritable media devices-like DVDs or Blu-Ray Disc-where information is encoded as amorphous marks in a crystalline background. The contrast in resistivity could be exploited in the next generation of electronic solid-state memories based on PCMs, which might replace the current leading storage technologies, namely FLASH and magnetic disks. Furthermore, these materials could be employed in displays or data visualization applications by combining both their optical and electronic property modulations 7 . Hence, a lot of interest and effort is currently devoted to uncover the complex physical origin of the high contrast between the two phases [8][9][10]

Interband characterization and electronic transport control of nanoscaledGeTe/Sb2Te3superlattices

“…Transmission experiments are performed on films with thickness below 100 nm, to guarantee high film quality, and at normal incidence, to simplify the interpretation of the results. Our analysis supports recent results showing that the GeTe layers are not isolated but intercalated within a Sb 2 Te 3 layer [14,26]. We explain how intermixing modifies the conductivity of the film.…”

“…One might consequently expect that the as grown structure is characterized by well separated layers of pure GeTe and Sb 2 Te 3 . Recent TEM and EXAFS experiments [14,26] have shown that the structure is indeed layered, but GeTe is not actually pure since it is intercalated into a Sb 2 Te 3 quintuple layer. Please note that the vacancy layers, when two-dimensionally extended, are labelled van der Waals gaps.…”

Textured Sb2Te3 films and GeTe/Sb2Te3 superlattices grown on amorphous substrates by molecular beam epitaxy

“…Furthermore, it is recognized that this material class offers great prospects when heterostructures of these materials, so called van der Waals (vdW) heterostructures, are created. 5 A practical example of such vdW heterostructures are GeTe/Sb 2 Te 3 superlattices (SL) [6][7][8] that are of particular interest due to their improved switching characteristics as phase change memory elements. 9 Furthermore, record high thermoelectric figures of merits have been realized in Sb 2 Te 3 /Bi 2 Te 3 superlattices.…”

Local atomic arrangements and lattice distortions in layered Ge-Sb-Te crystal structures

“…However, ordering of Ge cations in GST326 in the middle of building block was assumed. Although different GST compounds (stable and metastable) are very close in terms of structure174548495051, it is not clear why the cation layers in the GST124 structure should not possess intermixing. As an explanation, it was proposed that the Te, Sb and Ge atoms in the (0001) plane have to satisfy the 3Ge-Te-3Sb rule, in which Te atoms are surrounded by three Ge and three Sb atoms located in opposite corners of the octahedron structures45.…”

“…In particular, theoretical simulations showed that iPCMs can be a 3-D topological insulator15 or a Dirac semimetal11. Investigations of atomic structure in such technologically relevant iPCM revealed that various layered GST crystal structures can be formed during iPCMs production161718. Consequently, the knowledge of the proper local atomic arrangement in layered GST alloys is of paramount importance in order to understand the switching mechanism of iPCMs and their material properties.…”