92 research outputs found
Evidence for topological band inversion of the phase change material Ge2Sb2Te5
We present an angle-resolved photoemission study of a ternary phase change
material, namely Ge2Sb2Te5, epitaxially grown on Si(111) in the metastable
cubic phase. The observed upper bulk valence band shows a minimum at Gamma-bar
being 0.3 eV below the Fermi level E_F and a circular Fermi contour around
Gamma-bar with a dispersing diameter of 0.27-0.36 Anstroms^-1. This is in
agreement with density functional theory calculations of the Petrov stacking
sequence in the cubic phase which exhibits a topological surface state. The
topologically trivial cubic KH stacking shows a valence band maximum at Gamma
in line with all previous calculations of the hexagonal stable phase exhibiting
the valence band maximum at Gamma for a trivial Z_2 topological invariant nu_0
and away from Gamma for non-trivial nu_0. Scanning tunneling spectroscopy
exhibits a band gap of 0.4 eV around E_F
Hints for a general understanding of the epitaxial rules for van der Waals epitaxy from Ge-Sb-Te alloys
In this study, a generalized guideline is identified to predict the interaction between two-dimensional (2D) layered materials and substrate surfaces. Additionally, the van der Waals (vdW) heterostructures commensurability, the phase formation and the strain relaxation are identified during interface growth. To achieve such a general overview, the case of Ge-Sb-Te (GST) alloys on InAs(111) is studied. In this system, low-lattice mismatch conditions are fulfilled to avoid relaxation due to formation of misfit dislocations and allow to correctly identify vdW epitaxy. At the same time, the substrate can be efficiently prepared into self- and un-passivated surfaces to clarify the role of the surface interaction. Furthermore, the GST epilayer exhibits two different highly ordered 2D structures and a three-dimensional disordered structure, allowing to directly infer the nature of the epitaxy. This study opens the way for the design and mastering of vdW epitaxial growth of 2D heterostructures as well as hybrid 2D and non-layered materials
Interband characterization and electronic transport control of nanoscaled GeTe/SbTe superlattices
The extraordinary electronic and optical properties of the
crystal-to-amorphous transition in phase-change materials led to important
developments in memory applications. A promising outlook is offered by
nanoscaling such phase-change structures. Following this research line, we
study the interband optical transmission spectra of nanoscaled
GeTe/SbTe chalcogenide superlattice films. We determine, for films with
varying stacking sequence and growth methods, the density and scattering time
of the free electrons, and the characteristics of the valence-to-conduction
transition. It is found that the free electron density decreases with
increasing GeTe content, for sub-layer thickness below 3 nm. A simple
band model analysis suggests that GeTe and SbTe layers mix, forming a
standard GeSbTe alloy buffer layer. We show that it is possible to control the
electronic transport properties of the films by properly choosing the
deposition layer thickness and we derive a model for arbitrary film stacks
Interface formation of two- and three-dimensionally bonded materials in the case of GeTe-Sb2Te3 superlattices
GeTe–Sb2Te3 superlattices are nanostructured phase-change materials which are under intense investigation for non-volatile memory applications. They show superior properties compared to their bulk counterparts and significant efforts exist to explain the atomistic nature of their functionality. The present work sheds new light on the interface formation between GeTe and Sb2Te3, contradicting previously proposed models in the literature. For this purpose [GeTe(1 nm)–Sb2Te3(3 nm)]15 superlattices were grown on passivated Si(111) at 230 °C using molecular beam epitaxy and they have been characterized particularly with cross-sectional HAADF scanning transmission electron microscopy. Contrary to the previously proposed models, it is found that the ground state of the film actually consists of van der Waals bonded layers (i.e. a van der Waals heterostructure) of Sb2Te3 and rhombohedral GeSbTe. Moreover, it is shown by annealing the film at 400 °C, which reconfigures the superlattice into bulk rhombohedral GeSbTe, that this van der Waals layer is thermodynamically favored. These results are explained in terms of the bonding dimensionality of GeTe and Sb2Te3 and the strong tendency of these materials to intermix. The findings debate the previously proposed switching mechanisms of superlattice phase-change materials and give new insights in their possible memory application
Picosecond strain dynamics in GeSbTe monitored by time-resolved x-ray diffraction
Coherent phonons (CP) generated by laser pulses on the femtosecond scale have
been proposed as a means to achieve ultrafast, non-thermal switching in
phase-change materials such as GeSbTe(GST). Here we use
ultrafast optical pump pulses to induce coherent acoustic phonons and
stroboscopically measure the corresponding lattice distortions in GST using 100
ps x-ray pulses from the ESRF storage ring. A linear-chain model provides a
good description of the observed changes in the diffraction signal, however,
the magnitudes of the measured shifts are too large to be explained by thermal
effects alone implying the presence of transient non-equilibrium electron
heating in addition to temperature driven expansion. The information on the
movement of atoms during the excitation process can lead to greater insight
into the possibilities of using CP-induced phase-transitions in GST.Comment: 7 pages, 4 figures, Phys. Rev. B, in pres
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