Compositional tuning in sputter-grown highly-oriented Bi-Te films and their optical and electronic structures.

Growth of Bi-Te films by helicon-wave magnetron sputtering is systematically explored using alloy targets. The film compositions obtained are found to strongly depend on both the sputtering and antenna-coil powers. The obtainable film compositions range from Bi55Te45 to Bi43Te57 when a Bi2Te3 alloy target is used, and from Bi42Te58 to Bi40Te60 (Bi2Te3) for a Te-rich Bi30Te70 target. All films show strong orientation of the van der Waals layers (00l planes) parallel to the substrate. The atomic level stacking of Bi2Te3 quintuple and Bi bi-layers has been directly observed by high resolution transmission electron microscopy. Band structure simulations reveal that Bi-rich Bi4Te3 bulk is a zero band gap semimetal with a Dirac cone at the Gamma point when spin-orbit coupling is included. Optical measurements also confirm that the material has a zero band gap. The tunability of the composition and the topological insulating properties of the layers will enable the use of these materials for future electronics applications on an industrial scale.

Sub-nanometre resolution of atomic motion during electronic excitation in phase-change materials.

Phase-change materials based on Ge-Sb-Te alloys are widely used in industrial applications such as nonvolatile memories, but reaction pathways for crystalline-to-amorphous phase-change on picosecond timescales remain unknown. Femtosecond laser excitation and an ultrashort x-ray probe is used to show the temporal separation of electronic and thermal effects in a long-lived (>100 ps) transient metastable state of Ge2Sb2Te5 with muted interatomic interaction induced by a weakening of resonant bonding. Due to a specific electronic state, the lattice undergoes a reversible nondestructive modification over a nanoscale region, remaining cold for 4 ps. An independent time-resolved x-ray absorption fine structure experiment confirms the existence of an intermediate state with disordered bonds. This newly unveiled effect allows the utilization of non-thermal ultra-fast pathways enabling artificial manipulation of the switching process, ultimately leading to a redefined speed limit, and improved energy efficiency and reliability of phase-change memory technologies.