Stoichiometry determination of chalcogenide superlattices by means of X-ray diffraction and its limits

In this paper we explore the potential of stoichiometry determination for chalcogenide superlattices, promising candidates for next-generation phase-change memory, via X-ray diffraction. To this end, a set of epitaxial GeTe/Sb2Te3 superlattice samples with varying layer thicknesses is sputter-deposited. Kinematical scattering theory is employed to link the average composition with the diffraction features. The observed lattice constants of the superlattice reference unit cell follow Vegard's law, enabling a straight-forward and non-destructive stoichiometry determination.Comment: physica status solidi (RRL) - Rapid Research Letters (2019

Structure and composition of weakly-coupled superlattices - a diffraction perspective

Memory cells using phase change materials are developed as the next generation of non-volatile memory. They are based on the reversible switching between a crystalline and an amorphous state via optical or electrical pulses. Simpson et al. added a new facet to the scientific discussion, when they presented their interfacial phase change memory (iPCM) in 2011. The presented device showed in their experiments superior switching properties as compared to conventional phase change memories. The discovery of iPCM inspired many research projects in the following years. The core of the new memory type is a superlattice structure of GeTe and Sb2Te3, which is also called a chalcogenide superlattice (CSL). Since Simpson et al. claimed to have discovered a fundamentally new atomic switching mechanism, most research activities were focused on the verification and understanding of this new 'crystalline-to-crystalline' switching. First, this thesis represents a part of the research investigating the growth and structural properties of Sb2Te3, GeTe and their CSL system. The thin films are deposited using magnetron sputtering. For the analysis, a multitude of experimental techniques is employed: while the structural analysis makes extensive use of high-resolution X-ray diffraction, atom probe tomography is proven to add new insights to the nanoscopic picture of CSLs. Furthermore, a software tool is developed to simulate idealized diffraction experiments, especially of disordered samples, based on kinematical scattering theory. Using this approach, the structural fingerprints of chalcogenide superlattices which are typically found in X-ray diffraction spectra are discussed and linked to the primary structural motifs and to the intrinsic disorder of the sample.It is shown that also the chemical composition is encoded in diffraction patterns of superlattice samples. The identified characteristics are mapped to experimental data, which proves that Vegard's law can be applied, enabling a straightforward and non-destructive stoichiometry determination. Chemical analysis of thin films is often carried out using energy dispersive X-ray spectroscopy in electron microscopes or transmission electron microscopes. In contrast to these two methods, X-ray diffraction requires neither vacuum nor special sample preparation. The developed technique therefore offers a complementary method to determine structure and stoichiometry of the investigated superlattice sample.A second aspect of this thesis is intended to lay the basis to utilize superlattices beyond memory applications. In particular, the constituents that were used by Simpson et al. both belong to the class of incipient metals, which are discussed to be metavalently bonded. This means that they feature electron systems that are in between a covalently bonded and a fully delocalized metallic state. The property is only known among phase change materials and related chalcogenides. Recent experiments suggest that this situation cannot be maintained below a critical crystal size of few nanometers, which is an observation of scientific interest. However, the analysis of very thin films is challenging and requires specific methods. Epitaxial superlattices are promising candidates to make certain the fingerprints properties of metavalent bonding accessible to a broader range of experimental techniques via repetition of the thin structure which enhances the signal-to-noise-ratio. This however requires a precise process control and deeper understanding of the grown structures but opens paths to further investigate metavalently bonded materials. To mimic the thin structure in a superlattice, a suitable non-metavalent confinement material must be used as a spacer layer. For this thesis, the layered transition-metal dichalcogenide TiTe2 was chosen since it is not only covalently bonded but also immiscible with Sb2Te3. It is even more interesting as it was recently utilized to realize a stable multi-level phase change memory by acting as a diffusion barrier. In this thesis, the investigation of the growth and structural properties of the material system TiTe2/Sb2Te3 is presented alongside GeTe/Sb2Te3. It is shown that precisely controlled high quality textured TiTe2/Sb2Te3 superlattices can be deposited by magnetron sputtering at elevated temperatures. Thus, the system is proven to be a favorable platform for future investigations of confinement effects of metavalent bonding

Role of grain boundaries in Ge-Sb-Te based chalcogenide superlattices

Interfacial phase change memory devices based on a distinct nanoscale structure called superlattice have been shown to outperform conventional phase-change devices. This improvement has been attributed to the hetero-interfaces, which play an important role for the superior device characteristics. However, the impact of grain boundaries (GBs), usually present in large amounts in a standard sputter-deposited superlattice film, on the device performance has not yet been investigated.Therefore, in the present work, we investigate the structure and composition of superlattice films by high resolution x-ray diffraction (XRD) cross-linked with state-of-the art methods, such as correlative microscopy, i.e. a combination of high-resolution transmission electron microscopy and atom probe tomography to determine the structure and composition of GBs at the nanometer scale. Two types of GBs have been identified: high-angle grain boundaries (HAGBs) present in the upper part of a 340 nm-thick film and low-angle grain boundaries present in the first 40 nm of the bottom part of the film close to the substrate. We demonstrate that the strongest intermixing takes place at HAGBs, where heterogeneous nucleation of Ge2Sb2Te5 can be clearly determined. Yet, the Ge1Sb2Te4 phase could also be detected in the near vicinity of a low-angle grain boundary. Finally, a more realistic view of the intermixing phenomenon in Ge–Sb–Te based chalcogenide superlattices will be proposed. Moreover, we will discuss the implications of the presence of GBs on the bonding states and device performance

Impact of Pressure on the Resonant Bonding in Chalcogenides

Resonant bonding has been appreciated as an important feature in some chalcogenides. The establishment of resonant bonding can significantly delocalize the electrons and shrink the band gap, leading to low electrical resistivity and soft optical phonons. Many materials that exhibit this bonding mechanism have applications in phase-change memory and thermoelectric devices. Resonant bonding can be tuned by various means, including thermal excitations and changes in composition. In this work, we manipulate it by applying large hydrostatic-like pressure. Synchrotron X-ray diffraction and density functional theory reveal that the orthorhombic lattice of GeSe appears to become more symmetric and the Born effective charge has significantly increased at high pressure, indicating that resonant bonding has been established in this material. In contrast, the resonant bonding is partially weakened in PbSe at high pressure due to the discontinuity of chemical bonds along a certain lattice direction. By controlling resonant bonding in chalcogenides, we are able to modify the material properties and tailor them for various applications in extreme conditions