Conversion of p–n conduction type by spinodal decomposition in Zn-Sb-Bi phase-change alloys

Phase-change films with multiple resistance levels are promising for increasing the storage density in phase-change memory technology. Diffusion-dominated Zn2Sb3 films undergo transitions across three states, from high through intermediate to low resistance, upon annealing. The properties of the Zn2Sb3 material can be further optimized by doping with Bi. Based on scanning transmission electron microscopy combined with electrical transport measurements, at a particular Bi concentration, the conduction of Zn-Sb-Bi compounds changes from p- to n-type, originating from spinodal decomposition. Simultaneously, the change in the temperature coefficient of resistivity shows a metal-to-insulator transition. Further analysis of microstructure characteristics reveals that the distribution of the Bi-Sb phase may be the origin of the driving force for the p–n conduction and metal-to-insulator transitions and therefore may provide us with another way to improve multilevel data storage. Moreover, the Bi doping promotes the thermoelectric properties of the studied alloys, leading to higher values of the power factor compared to known reported structures. The present study sheds valuable light on the spinodal decomposition process caused by Bi doping, which can also occur in a wide variety of chalcogenide-based phase-change materials. In addition, the study provides a new strategy for realizing novel p–n heterostructures for multilevel data storage and thermoelectric applications

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

Insights into the local atomic arrangements of layered Ge-Sb-Te compounds are of particular importance from a fundamental point of view and for data storage applications. In this view, a detailed knowledge of the atomic structure in such alloys is central to understanding the functional properties both in the more commonly utilized amorphous–crystalline transition and in recently proposed interfacial phase change memory based on the transition between two crystalline structures. Aberration-corrected scanning transmission electron microscopy allows direct imaging of local arrangement in the crystalline lattice with atomic resolution. However, due to the non-trivial influence of thermal diffuse scattering on the high-angle scattering signal, a detailed examination of the image contrast requires comparison with theoretical image simulations. This work reveals the local atomic structure of trigonal Ge-Sb-Te thin films by using a combination of direct imaging of the atomic columns and theoretical image simulation approaches. The results show that the thin films are prone to the formation of stacking disorder with individual building blocks of the Ge2Sb2Te5, Ge1Sb2Te4 and Ge3Sb2Te6 crystal structures intercalated within randomly oriented grains. The comparison with image simulations based on various theoretical models reveals intermixed cation layers with pronounced local lattice distortions, exceeding those reported in literature

Conductive Tracks in Carbon Implanted Titania Nanotubes: Atomic-Scale Insights from Experimentally Based Ab Initio Molecular Dynamics Modeling

Ion implantation of titania nanotubes is a highly versatile approach for tailoring structural and electrical properties. While recently self-organized nanoscale compositional patterning has been reported, the atomistic foundations and impact on electronic structure are not established at this point. To study these aspects, ab initio molecular dynamic simulations based on atomic compositions in C implanted titania nanotubes according to elastic recoil detection analysis are employed. Consistent with experimental data, carbon accumulates in chainlike precipitates, which are favorable for enhancing conductivity, as revealed by density-functional theory electronic ground states calculations are demonstrated

Influence of substrate dimensionality on the growth mode of epitaxial 3D-bonded GeTe thin films: From 3D to 2D growth

The pseudo-binary line of Sb2Te3-GeTe contains alloys featuring different crystalline characteristics from two-dimensionally (2D-) bonded Sb2Te3 to three-dimensionally (3D-) bonded GeTe. Here, the growth scenario of 3D-bonded GeTe is investigated by depositing epitaxial GeTe thin films on Si(111) and Sb2Te3-buffered Si(111) substrates using pulsed laser deposition (PLD). GeTe thin films were grown in trigonal structure within a temperature window for epitaxial growth of 210–270 °C on unbuffered Si(111) substrates. An unconventional growth onset was characterized by the formation of a thin amorphous GeTe layer. Nonetheless, the as-grown film is found to be crystalline. Furthermore, by employing a 2D-bonded Sb2Te3 thin film as a seeding layer on Si(111), a 2D growth of GeTe is harnessed. The epitaxial window can substantially be extended especially towards lower temperatures down to 145 °C. Additionally, the surface quality is significantly improved. The inspection of the local structure of the epitaxial films reveals the presence of a superposition of twinned domains, which is assumed to be an intrinsic feature of such thin films. This work might open a way for an improvement of an epitaxy of a 3D-bonded material on a highly-mismatched substrate (e.g. Si (111)) by employing a 2D-bonded seeding layer (e.g. Sb2Te3)

Formation of BaTiO 3 thin films from (110) TiO 2 rutile single crystals and BaCO 3 by solid state reactions

Abstract The formation of BaTiO 3 thin films from (110) TiO 2 rutile single crystals and BaCO 3 was investigated experimentally by solid -solid and gassolid reactions in vacuum. X-ray diffraction revealed the formation of an intermediate Ba 2 TiO 4 phase before BaTiO 3 is formed. According to our calculations the formation of Ba 2 TiO 4 is associated with a maximum decrease in the Gibbs energy at a CO 2 pressure lower than 10 À 4 mbar. Reactions at 600 -900 -C showed different processes to occur in the solid -solid and gas -solid reactions. The observations are interpreted in terms of the different mass transport mechanisms involved. The results shed new light on the phase sequence during BaTiO 3 formation; in particular a dissociation of BaCO 3 prior to its participation in the reaction has become rather unlikely.

Coupled magnetic and structural transitions in La0.7Sr0.3MnO3 films on SrTiO3

The magnetic properties of three epitaxial La0.7Sr0.3MnO3 films of thickness 5, 15 and 40 nm grown on SrTiO3 (001) substrates were investigated. The structural transition of the SrTiO3 substrate induces a magnetic transition in the manganite films due to magnetoelastic coupling. Below the temperature of the structural transition additional steps in the magnetization reversal characteristics appear characterized by clearly defined coercive fields. These additional coercive fields depend on the cooling history of the sample and are related to the formation of structural domains in the La0.7Sr0.3MnO3 films induced by the substrate

Compositional Patterning in Carbon Implanted Titania Nanotubes

Ranging from novel solar cells to smart biosensors, titania nanotube arrays constitute a highly functional material for various applications. A promising route to modify material characteristics while preserving the amorphous nanotube structure is present when applying low-energy ion implantation. In this study, the interplay of phenomenological effects observed upon implantation of low fluences in the unique 3D structure is reported: sputtering versus readsorption and plastic flow, amorphization versus crystallization and compositional patterning. Patterning within the oxygen and carbon subsystem is revealed using transmission electron microscopy. By applying a Cahn–Hilliard approach within the framework of driven alloys, characteristic length scales are derived and it is demonstrated that compositional patterning is expected on free enthalpy grounds, as predicted by density functional theory based ab initio calculations. Hence, an attractive material with increased conductivity for advanced devices is provided. © 2021 The Authors. Advanced Functional Materials published by Wiley-VCH Gmb

Structural Transitions in Ge2Sb2Te5 Phase Change Memory Thin Films Induced by Nanosecond UV Optical Pulses

Ge-Sb-Te-based phase change memory alloys have recently attracted a lot of attention due to their promising applications in the fields of photonics, non-volatile data storage, and neuromorphic computing. Of particular interest is the understanding of the structural changes and underlying mechanisms induced by short optical pulses. This work reports on structural changes induced by single nanosecond UV laser pulses in amorphous and epitaxial Ge2Sb2Te5 (GST) thin films. The phase changes within the thin films are studied by a combined approach using X-ray diffraction and transmission electron microscopy. The results reveal different phase transitions such as crystalline-to-amorphous phase changes, interface assisted crystallization of the cubic GST phase and structural transformations within crystalline phases. In particular, it is found that crystalline interfaces serve as crystallization templates for epitaxial formation of metastable cubic GST phase upon phase transitions. By varying the laser fluence, GST thin films consisting of multiple phases and different amorphous to crystalline volume ratios can be achieved in this approach, offering a possibility of multilevel data storage and realization of memory devices with very low resistance drift. In addition, this work demonstrates amorphization and crystallization of GST thin films by using only one UV laser with one single pulse duration and one wavelength. Overall, the presented results offer new perspectives on switching pathways in Ge-Sb-Te-based materials and show the potential of epitaxial Ge-Sb-Te thin films for applications in advanced phase change memory concepts

Research Update: Van-der-Waals epitaxy of layered chalcogenide Sb2Te3 thin films grown by pulsed laser deposition

An attempt to deposit a high quality epitaxial thin film of a two-dimensionally bonded (layered) chalcogenide material with van-der-Waals (vdW) epitaxy is of strong interest for non-volatile memory application. In this paper, the epitaxial growth of an exemplary layered chalcogenide material, i.e., stoichiometric Sb2Te3 thin films, is reported. The films were produced on unreconstructed highly lattice-mismatched Si(111) substrates by pulsed laser deposition (PLD). The films were grown by vdW epitaxy in a two-dimensional mode. X-ray diffraction measurements and transmission electron microscopy revealed that the films possess a trigonal Sb2Te3 structure. The single atomic Sb/Te termination layer on the Si surface was formed initializing the thin film growth. This work demonstrates a straightforward method to deposit vdW-epitaxial layered chalcogenides and, at the same time, opens up the feasibility to fabricate chalcogenide vdW heterostructures by PLD