AlScN: A III-V semiconductor based ferroelectric

Ferroelectric switching is unambigiously demonstrated for the first time in a III-V semiconductor based material: AlScN -- A discovery which could help to satisfy the urgent demand for thin film ferroelectrics with high performance and good technological compatibility with generic semiconductor technology which arises from a multitude of memory, micro/nano-actuator and emerging applications based on controlling electrical polarization. The appearance of ferroelectricity in AlScN can be related to the continuous distortion of the original wurtzite-type crystal structure towards a layered-hexagonal structure with increasing Sc content and tensile strain, which is expected to be extendable to other III-nitride based solid solutions. Coercive fields which are systematically adjustable by more than 3 MV/cm, high remnant polarizations in excess of 100 \mu C/cm$^2$ which constitute the first experimental estimate of the previously inaccessible spontaneous polarization in a III-nitride based material, an almost ideally square-like hysteresis resulting in excellent piezoelectric linearity over a wide strain interval from -0.3% to +0.4% as well as a paraelectric transition temperature in excess of 600{\deg}C are confirmed. This intriguing combination of properties is to our knowledge as of now unprecedented in the field of polycrystalline ferroelectric thin films and promises to significantly advance the commencing integration of ferroelectric functionality to micro- and nanotechnology, while at the same time providing substantial insight to one of the central open questions of the III-nitride semiconductors - that of their actual spontaneous polarization

Electrical properties of an amorphous zirconium oxide thin film and structure formation during crystallization

Metastable amorphous oxides with a strong oxygen deficiency often show surprising phenomena upon relaxation into thermodynamically stable phases. For example, Nagarajan et al. found a new type of chemically driven insulator-metal transition in highly non-stoichiometric gallium oxide films (GaOx).[1] Here, an internal solid-state disproportionation reaction leads to the growth Ga2O3 nuclei in the initially insulating GaOx matrix which thereby attains metal-like conductivity. Moreover, it has been recently shown that such films can act as memristive switches. [2]. Highly non-stoichiometric titania (TiO1.6) films show a similar disproportionation reaction upon heating but as the phase diagram for this material is more complex, various phases can be found during the relaxation [3]. Please click Additional Files below to see the full abstract

Enhancing Reliability of Studies on Single Filament Memristive Switching via an Unconventional cAFM Approach

Memristive devices are highly promising for implementing neuromorphic functionalities in future electronic hardware, and direct insights into memristive phenomena on the nanoscale are of fundamental importance to reaching this. Conductive atomic force microscopy (cAFM) has proven to be an essential tool for probing memristive action locally on the nanoscale, but the significance of the acquired data frequently suffers from the nonlocality associated with the thermal drift of the tip in ambient conditions. Furthermore, comparative studies of different configurations of filamentary devices have proven to be difficult, because of an immanent variability of the filament properties between different devices. Herein, these problems are addressed by constraining the memristive action directly at the apex of the probe through functionalization of a cAFM tip with an archetypical memristive stack, which is comprised of Ag/Si3N4. The design of such functionalized cantilevers (entitled here as "memtips") allowed the capture of the long-term intrinsic current response, identifying temporal correlations between switching events, and observing emerging spiking dynamics directly at the nanoscale. Utilization of an identical memtip for measurements on different counter electrodes made it possible to directly compare the impact of different device configurations on the switching behavior of the same filament. Such an analytical approach in ambient conditions will pave the way towards a deeper understanding of filamentary switching phenomena on the nanoscale

Multi‐Method Characterization of the High‐Entropy Spinel Oxide Mn0.2_{0.2}Co0.2_{0.2}Ni0.2_{0.2}Cu0.2_{0.2}Zn0.2_{0.2}Fe2_{2}O4_{4}: Entropy Evidence, Microstructure, and Magnetic Properties

The novel spinel Cu0.2Co0.2Mn0.2Ni0.2Zn0.2Fe2O4 comprising six transition metal cations was successfully prepared by a solution-combustion method followed by distinct thermal treatments. The entropic stabilization of this hexa-metallic material is demonstrated using in situ high temperature powder X-ray diffraction (PXRD) and directed removal of some of the constituting elements. Thorough evaluation of the PXRD data yields sizes of coherently scattering domains in the nanometre-range. Transmission electron microscopy based methods support this finding and indicate a homogeneous distribution of the elements in the samples. The combination of 57Fe Mössbauer spectroscopy with X-ray absorption near edge spectroscopy allowed determination of the cation occupancy on the tetrahedral and octahedral sites in the cubic spinel structure. Magnetic studies show long-range magnetic exchange interactions which are of ferri- or ferromagnetic nature with an exceptionally high saturation magnetization in the range of 92–108 emu g−1 at low temperature, but also an anomaly in the hysteresis of a sample calcined at 500 °C

Silicon nanocrystal synthesis with the atmospheric plasma source HelixJet

The HelixJet, a plasma source operating under atmospheric pressure with RF power, was used for the synthesis of silicon nanoparticles (Si-NPs) in the context of relevance in nanomedicine, sensor technology, and nanotechnology. The HelixJet was operated with a variety of He/Ar/H2/SiH4 gas mixtures to characterize the Si-NPs in regard to their size, crystallinity, structure, and photoluminescence. Distinct varieties of nanomaterials in the size range from 3 nm to over 100 nm were synthesized depending on the operation parameters of the HelixJet. Admixture of H2 alongside high RF powers led to the formation of crystalline nanoparticles with a strong photoluminescence intensity, where the photoluminescence properties as well as the nanocrystal synthesis yield were tunable by adjustment of the synthesis parameters. Post-synthesis in-flight annealing allowed the formation of large crystalline nanoparticles. In addition, the experiments conducted in this study resulted in a design improvement of the HelixJet plasma source that extends the stability of the operating range. Furthermore, the added spatial separation of the He/H2 and He/Ar/SiH4 streams (SiH4 injection on-axis) minimizes material deposition within the HelixJet and enables continuous long-term operation

Al1−xScxN Thin Films at High Temperatures: Sc-Dependent Instability and Anomalous Thermal Expansion

Ferroelectric thin films of wurtzite-type aluminum scandium nitride (Al1-xScxN) are promising candidates for non-volatile memory applications and high-temperature sensors due to their outstanding functional and thermal stability exceeding most other ferroelectric thin film materials. In this work, the thermal expansion along with the temperature stability and its interrelated effects have been investigated for Al1-xScxN thin films on sapphire Al2O3(0001) with Sc concentrations x (x = 0, 0.09, 0.23, 0.32, 0.40) using in situ X-ray diffraction analyses up to 1100 °C. The selected Al1-xScxN thin films were grown with epitaxial and fiber textured microstructures of high crystal quality, dependent on the choice of growth template, e.g., epitaxial on Al2O3(0001) and fiber texture on Mo(110)/AlN(0001)/Si(100). The presented studies expose an anomalous regime of thermal expansion at high temperatures >~600 °C, which is described as an isotropic expansion of a and c lattice parameters during annealing. The collected high-temperature data suggest differentiation of the observed thermal expansion behavior into defect-coupled intrinsic and oxygen-impurity-coupled extrinsic contributions. In our hypothesis, intrinsic effects are denoted to the thermal activation, migration and curing of defect structures in the material, whereas extrinsic effects describe the interaction of available oxygen species with these activated defect structures. Their interaction is the dominant process at high temperatures >800 °C resulting in the stabilization of larger modifications of the unit cell parameters than under exclusion of oxygen. The described phenomena are relevant for manufacturing and operation of new Al1-xScxN-based devices, e.g., in the fields of high-temperature resistant memory or power el. appl

First evidence for the forging of gold in an Early Bronze Age Site of Central Europe (2200–1800 BCE)

Evidence of gold processing in the fortified site of Bruszczewo (Poland) is the first testimony of the production of gold artefacts in a domestic Early Bronze Age site of Central Europe. This paper highlights the potential of macrolithic tool ensembles as a key element for the recognition of metallurgical work processes. Moreover, it presents an optimised methodological approach to tackle the application of stone tools in metallurgical production, based on technological characterisation, use-wear analysis, portable X-ray fluorescence, transmission electron microscopy and energy dispersive X-ray spectroscopy. Finally, the absence of gold sources in Central Europe raises the question about the origin of the metal, constituting an especially striking issue, as gold was a raw material of restricted access. As Bruszczewo was one of the few enclosed Early Bronze Age sites north of the Central European Mountain Range, the patterning of metal processing (including gold) sheds light on the mode of the production of metal artefacts, apparently restricted to central sites of power, which controlled the communication trails.47Journal of Archaeological Science: Report

High‐Pressure Sintering of Rhombohedral Cr2S3 Using TZM Tools

The influence of sintering parameters on the physical properties and the chemical structure of rhombohedral Cr2S3 (rh‐Cr2S3) is investigated using high pressures and high temperatures. The densification of the powder is performed by applying the high‐pressure field‐assisted sintering technique/spark plasma sintering. Using a titanium–zirconium–molybdenum (TZM) alloy as sintering tool, it is possible to increase the magnitude of the applied pressure to several hundred MPa at temperatures as high as 1223 K. A relative density of up to 99.9% is achieved at a sintering temperature of 1223 K and a pressure of 395 MPa. The presence of phase‐pure rh‐Cr2S3 is proven by X‐ray diffraction analysis and transmission electron microscopy. The Seebeck coefficients of the self‐doped samples change drastically with the sintering temperatures ranging between −650 and −350 μV K−1. The densities and the thermal conductivities of the sintered samples increase with increasing sintering temperatures. The electrical conductivity is largely increased compared with the thermal conductivity potentially due to the current‐assisted high‐pressure sinterin

Modulation of Electrical Conductivity and Lattice Distortions in Burolk HVPE-Gwn GaN

The nature of self-organized three-dimensional structured architectures with spatially modulated electrical conductivity emerging in the process of hydride vapor phase epitaxial growth of single crystalline n-GaN wafers is revealed by photoelectrochemical etching. The amplitude of the carrier concentration modulation throughout the sample is derived from photoluminescence analysis and the localized heterogeneous piezoelectric response is demonstrated. The formation of such architectures is rationalized based on the generation of V-shaped pits and their subsequent overgrowth in variable direction. Detailed structure analysis with respect to X-ray diffraction and transmission electron microscopy gives striking evidence for inelastic strain to manifest in distortions of the P63mc wurtzite-type structure. The deviation from hexagonal symmetry by angular distortions of the β angle between the basal plane and c-axis is found to be of around 1°. It is concluded that the lattice distortions are generated by the misfit strains originating during crystal growth, which are slightly relaxed upon photoelectrochemical etching