Temperature dependence of strain in Al0.22Ga0.78N/GaN heterostructures with and without Si3N4 passivation

The high-temperature characteristics of strain in Al_0.22Ga_0.78N layers, with and without a Si₃N₄ passivation layer, were investigated at temperatures from room temperature to 813 K by means of high-resolution X-ray diffraction. The results show that a small temperature-dependent strain relaxation occurs in the unpassivated Al_0.22Ga_0.78N layers when the temperature exceeds 523 K. After passivating, an additional tensile strain and an initial increase of the in-plane tensile strain with increasing temperature were observed in Al_0.22Ga_0.78N layers, and at higher temperatures the in-plane tensile strain only decreases slightly in the 100-nm-thick Al_0.22Ga_0.78N layer, but a pronounced temperature-dependent strain relaxation occurs in the 50-nm-thick one due to the fact that the thickness of the Al_0.22Ga_0.78N layer is close to the critical thickness, and hence the increase of tensile strain induced by passivation will result in partial strain relaxation via the formation of cracks or the glide motion and multiplication of dislocations.     

SiNx-Submikrometerschichten – Parameteroptimierung –

SiN_x-submicrometer coatings – Optimization of the film properties – The influence of the deposition conditions on the properties of SiN_x-coatings was investigated. The characterized SiN_x-coatings were deposited by the help of reactive magnetron sputtering. Gas pressure and film thickness were varied. Scanning electron microscopic views of the cross sections show a columnar structure varying with the deposition parameters. The different structures are comparable to the known structure zone models. There is a transition from dense structures to open columnar structures with increasing gas pressure. The Microstructure of coatings also changes with increasing film thickness. Especially deposition conditions promoting shadowing effects lead to a large growth of the column diameter with increasing thickness. The intrinsic stresses and the ultramicrohardness of the coatings change with changing gas pressure, too. Dense structures have high intrinsic stresses and a high hardness while coarse columnar structures have low intrinsic stresses and a low hardness. The influence of the deposition parameters on wear behaviour and adhesion of the SiN_x-coatings was investigated by cavitation tests. Dense coatings with high intrinsic stresses show adhesion failures, and coatings with lower stresses and coarse columnar structures fail because of their lower intrinsic stability. Thus, there is an optimum gas pressure, at which the best properties are reached. It can be shown that with decreasing film thickness adhesion increases.     

Carrier dynamics in coalescence overgrowth of GaN nanocolumns

Coalescence overgrowth of pattern-grown GaN nanocolumns on c-plane sapphire substrate with metal organic chemical vapor deposition is demonstrated. The subsequent coalescence overgrowth opens a possibility for dislocation reduction due to the lateral strain relaxation in columnar geometry. We present further growth optimization and innovative characterization of metal organic chemical vapor deposition layers, overgrown on the columnar structure with varying diameters of columns. Nano-imprint lithography was applied to open circular holes of 250, 300, 450, and 600 nm diameter on the SiO₂ layer, deposited on the GaN layer on the c-plane sapphire template. After the growth of ~ 1 μm high GaN nanocolumns, the further coalescence conditions led to an overgrown layer ~2 μm thickness. Photoelectrical and optical properties of the overgrown layers and a reference sample were investigated by time-resolved picosecond transient grating and time-integrated photoluminescence. We note a 3-4 fold increase in carrier lifetime in the overgrown epilayers when the diameter of columns increased from 250 to 450 nm. This feature is a clear indication of an ~4-fold reduced defect density.     

Strain and relaxation in Si-MBE structures studied by reciprocal space mapping using high resolution X-ray diffraction

High resolution X-ray diffraction measurements have been done on Si(001)-based structures grown by molecular beam epitaxy (MBE). By systematically varying the angle of incidence and the diffraction angle, the diffraction intensity data can be displayed in a two-dimensional X-ray diffraction intensity map that can be interpretted as a reciprocal space map of the reciprocal lattice points. The experimental technique is described and results for studies of the strain and the strain relaxation with high temperature annealing are described for the following material systems: strained Si/Si_1–xGe_x heterostructures, highly B-doped Si layers on Si and highly B-doped Si_1–xGe_x layers on Si. The strain relaxation in Si_1xGe_x layers occurs via generation of misfit dislocations creating a shift and a characteristic mosaic broadening of the layer peak in the reciprocal space maps. A summary of how the degree of relaxation, as measured from reciprocal space maps, depends on the annealing temperature and the layer thickness is given. The relaxation of the strain induced by B, for doping concentrations up to 5 × 10²⁰ cm^–3, is obtained by diffusion of B into the substrate. For a Si_0.82Ge_0.18 layer with partially compensated compressive strain due to a B concentration of 3 × 10²⁰cm^–3, the maps show a combination of strain relaxation via misfit dislocations and B diffusion into the substrate.     

Electric and Mechanical Switching of Ferroelectric and Resistive States in Semiconducting BaTiO3–δ Films on Silicon

Materials that can couple electrical and mechanical properties constitute a key element of smart actuators, energy harvesters, or many sensing devices. Within this class, functional oxides display specific mesoscale responses which often result in great sensitivity to small external stimuli. Here, a novel combination of molecular beam epitaxy and a water‐based chemical‐solution method is used for the design of mechanically controlled multilevel device integrated on silicon. In particular, the possibility of adding extra functionalities to a ferroelectric oxide heterostructure by n‐doping and nanostructuring a BaTiO₃ thin film on Si(001) is explored. It is found that the ferroelectric polarization can be reversed, and resistive switching can be measured, upon a mechanical load in epitaxial BaTiO_3?δ /La_0.7Sr_0.3MnO₃/SrTiO₃/Si columnar nanostructures. A flexoelectric effect is found, stemming from substantial strain gradients that can be created with moderate loads. Simultaneously, mechanical effects on the local conductivity can be used to modulate a nonvolatile resistive state of the BaTiO_3?δ heterostructure. As a result, three different configurations of the system become accessible on top of the usual voltage reversal of polarization and resistive states.     

Mechanical behavior of segmented oxide protective coatings

Mechanically and thermally induced fractures were examined in sputtered coatings consisting of an NiCrAlY underlayer, either a thin or a thick transition layer grading from NiCrAlY to ZrO₂, and an outer ZrO₂ layer. A pronounced columnar (fibrous) microstructure was obtained, although the columnar boundaries in the ZrO₂ layers and in the thick transition layers were much more open than in the NiCrAlY, effectively producing a more segmented structure. Some coatings also included a continuous fine-grained outer NiCrAlY sealing or close-out layer.For this complex metal and ceramic coating, a stress applied parallel to the layer plane always resulted in fracture perpendicular to the layer plane along open columnar boundaries. If the transition layer was thick, no fractures other than those at columnar boundaries were observed. If the transition layer was thin, fracture parallel to the layer plane occured in the ZrO₂-rich portion of this layer. When stress was perpendicular to the layer plane, fracture parallel to the layer plane occurred first at the outer ZrO₂(NiCrAlY) interface if an outer NiCrAlY sealing layer was present. Otherwise the ZrO₂-rich portion of the transition layer failed first, followed by fracture in the ZrO₂ layer.     

Electroluminescence of Si-SiO2-Si3N4 structures

Si-SiO₂-Si₃N₄ heterostructures obtained by depositing silicon nitride onto a silicon substrate oxidized in dry oxygen were studied by measuring electroluminescence (EL) in the electrolyte-insulator-semiconductor system. The EL spectra display the emission bands typical of a silicon oxide layer and an intense band at 2.7 eV characteristic of the radiative relaxation of excited silylene centers. Since these centers are typical of silicon oxynitride layers, it is concluded that such a layer is formed at the boundary.     

Structure and properties of Nb<Subscript>3</Subscript>Sn coatings produced by unsteady-current electrocodeposition of Nb and Sn

We have studied the effect of electric-current mode on the structure and characteristics of niobium stannide coatings produced by electrochemical coreduction of niobium and tin ions at the cathode in molten salts. The results demonstrate that single-phase Nb₃Sn coatings with a superconducting transition temperature T _c = 17.3–17.9 K can be obtained using unsteady-current deposition. The coatings produced in galvanostatic mode and by ac deposition at a frequency of 50 Hz have a columnar grain structure. Current-reversal deposition with pulse ratios above 7–9 results in a layered microstructure with layers parallel to the substrate surface, instead of the columnar microstructure, and ensures a considerably higher critical current.     

Texture Transfer Mechanism of Buffer Layer in Coated Conductors

We have studied the texture transfer process of buffer layers prepared by chemical solution deposition (CSD) methods on YSZ (00l) single crystal substrates and biaxial textured Ni?CW substrates. The structure, texture, and surface morphology of the buffer layers were investigated by X-ray diffraction (XRD), atomic force microscopy (AFM), and scanning electron microscopy (SEM). Our results show that the degree of texture and the surface morphology of the buffer layers vary with the changes of the substrate and the lattice mismatch of the top buffer layers with La₂Zr₂O₇ (LZO) after crystallization in argon?Chydrogen atmosphere. Moreover, the growth mode of multi-layer films and the type of the lattice strain have strong influence on the formation and the transfer of the bi-axial texture in multi-layer buffer architecture. It suggests that there exists a possible connection between the strain relaxation and the texture transfer in buffer layer fabricated by CSD methods. Information on the texture transfer of buffer layer is important for optimizing the buffer layer architecture in coated conductors.     

Linking Atomic Structure and Local Chemistry at Manganese‐Segregated Antiphase Boundaries in ZrO2–La2/3Sr1/3MnO3 Thin Films

This paper presents direct experimental evidence of Mn segregation at three types of antiphase boundaries in La_2/3Sr_1/3MnO₃ thin films doped with ZrO₂. The local atomic structure of these antiphase boundaries is investigated by high‐angle annular dark‐field and annular bright‐field imaging in a scanning transmission electron microscope. Chemical composition and cation valence are determined by electron energy‐loss spectroscopy. The evidence that strain relaxation and Zr_Mn substitution are driving forces for the formation of the antiphase boundaries is found in this study. Analysis of atomic structure, image contrast, Mn valence state, and Mn occupancy shows that the antiphase boundaries are charge‐neutral with minimized internal electric fields.     

Out‐of‐Plane Strain Induced in a Moiré Superstructure of Monolayer MoS2 and MoSe2 on Au(111)

Making contact of transition metal dichalcogenides (TMDCs) with a metal surface is essential for fabricating and designing electronic devices and catalytic systems. It also generates strain in the TMDCs that plays significant role in both electronic and phonon structures. Therefore, detailed understanding of mechanism of the strain generation is important to fully comprehend the modulation effect for the electronic and phonon properties. Here, MoS₂ and MoSe₂ monolayers are grown on Au surface by chemical vapor deposition and it is demonstrated that the contact with a crystalline Au(111) surface gives rise to only out‐of‐plane strain in both MoS₂ and MoSe₂ layers, whereas no strain generation is observed on polycrystalline Au or SiO₂/Si surfaces. Scanning tunneling microscopy analysis provides information regarding consequent specific adsorption sites between lower S (Se) atoms in the S? Mo? S (Se? Mo? Se) structure and Au atoms via unique moiré superstructure formation for MoS₂ and MoSe₂ layers on Au(111). This observation indicates that the specific adsorption sites give rise to out‐of‐plane strain in the TMDC layers. Furthermore, it also leads to effective modulation of the electronic structure of the MoS₂ or MoSe₂ layer.     

Fracture behaviour of an Al2O3–ZrO2 multi‐layered ceramic with residual stresses due to phase transformations*

The fracture behaviour of a ceramic multi‐layer designed with thin internal compressive layers and obtained by slip casting is studied. It consists of nine alternated Al₂O₃–5vol%tZrO₂ and Al₂O₃–30vol%mZrO₂ layers of 530 μm and 100 μm thickness, respectively. Mechanical characterization includes evaluation of Vickers Hardness, Young's modulus and fracture strength under four‐point bending. In addition, the residual stress magnitude and distribution in the laminate is determined both analytically, from calculations using the differential strain between layers and the elastic properties, and experimentally, using indentation techniques. The experimental findings in terms of mechanical strength and fractography show a subcritical growth of the natural flaws in the laminate before catastrophic failure occurs, owing to the relevant role of the thin Al₂O₃–30vol%mZrO₂ layers with compressive stresses inherent to the zirconia phase transformation. These layers are also responsible for the increase in toughness to levels of at least three times that of the reference Al₂O₃–5vol%tZrO₂ monolith.     

The microstructure of Nb3Sn in modified jelly roll superconducting composites

Transmission electron microscopy (TEM) has been used to characterize the microstructure of the Nb₃Sn layers developed during heat treatment of two superconducting wires, with and without 0.8 wt% titanium addition to the niobium, manufactured by the modified jelly roll (MJR) process. The composites in the as-received state are shown to contain pre-reacted layers formed during fabrication anneals, while heat treatments over the range 650 to 750° C yield a two-fold layer structure of columnar and equiaxed grains. Examples of both transverse and longitudinal TEM micrographs are given. The addition of 0.8 wt% titanium to the niobium before fabrication leads to coarsening of the equiaxed grains after identical reaction times. The results are discussed in terms of a recently proposed model for the development of microstructure in A15 multifilamentary composites.     

Growth of β-FeSi2 layers deposited from a molten salt

β-FeSi₂ layers have been successfully grown using a molten salt method for the first time. It was found that single phase and homogeneous β-FeSi₂ layers with a columnar domain structure can be grown on FeSi substrates. The layer thickness was demonstrated to be controllable by the growth temperature and time, and was diffusion controlled. It was shown that the layers were void- and crack-free compared to similar layers grown on Fe substrates: this difference is explained in terms of Fe diffusion. This vacuum-free simple growth technique is useful for the fabrication of large area semiconductor devices at low cost.     

Coexistence of Quasi-One- and -Two-Dimensional Electronic Structures in (EDO-TTF)2X (X = GaCl4, ReO4)

Crystal structures and physical properties of (EDO-TTF)₂X where EDO-TTF = ethylenedioxytetrathiafulvalene, X = GaCl₄ and ReO₄ were investigated. These isostructural salts belonged to triclinic system with space group P1, and two kinds of the donor layers coexisted in a crystal. EDO-TTF formed head-to-tail and head-to-head type columnar structures in each donor layer, for which quasi-one- and -two- dimensional Fermi surfaces were calculated, respectively.     

Interfacial microstructure of Si3N4/Si3N4 brazing joint with Cu–Zn–Ti filler alloy

In this study, Si₃N₄ ceramic was jointed by a brazing technique with a Cu–Zn–Ti filler alloy. The interfacial microstructure between Si₃N₄ ceramic and filler alloy in the Si₃N₄/Si₃N₄ joint was observed and analyzed by using electron-probe microanalysis, X-ray diffraction and transmission electron microscopy. The results indicate that there are two reaction layers at the ceramic/filler interface in the joint, which was obtained by brazing at a temperature and holding time of 1223 K and 15 min, respectively. The layer nearby the Si₃N₄ ceramic is a TiN layer with an average grain size of 100 nm, and the layer nearby the filler alloy is a Ti₅Si₃N_x layer with an average grain size of 1–2 μm. Thickness of the TiN and Ti₅Si₃N_x layers is about 1 μm and 10 μm, respectively. The formation mechanism of the reaction layers was discussed. A model showing the microstructure from Si₃N₄ ceramic to filler alloy in the Si₃N₄/Si₃N₄ joint was provided as: Si₃N₄ ceramic/TiN reaction layer/Ti₅Si₃N_x reaction layer/Cu–Zn solution.     

Trash into Treasure: δ‐FAPbI3 Polymorph Stabilized MAPbI3 Perovskite with Power Conversion Efficiency beyond 21%

Effective passivation and stabilization of both the inside and interface of a perovskite layer are crucial for perovskite solar cells (PSCs), in terms of efficiency, reproducibility, and stability. Here, the first formamidinium lead iodide (δ‐FAPbI₃) polymorph passivated and stabilized MAPbI₃ PSCs are reported. This novel MAPbI₃/δ‐FAPbI₃ structure is realized via treating a mixed organic cation MA _x FA_{1‐ x} PbI₃ perovskite film with methylamine (MA) gas. In addition to the morphology healing, MA gas can also induce the formation of δ‐FAPbI₃ phase within the perovskite film. The in situ formed 1D δ‐FAPbI₃ polymorph behaves like an organic scaffold that can passivate the trap state, tunnel contact, and restrict organic‐cation diffusion. As a result, the device efficiency is easily boosted to 21%. Furthermore, the stability of the MAPbI₃/δ‐FAPbI₃ film is also obviously improved. This δ‐FAPbI₃ phase passivation strategy opens up a new direction of perovskite structure modification for further improving stability without sacrificing efficiency.     

Einfluss unterschiedlicher Oberflächenzustände vor dem Plasmanitrieren auf Eigenschaften und Zerspanungsverhalten des Schnellarbeitsstahls S 6‐5‐2

Surface states and wear behavior of drills of ground, sandblasted and plasmanitrided samples and drills made of AISI M2 high speed steel In the present work the effect of different surface conditions on plasma nitriding response of AISI M2 high speed steel was investigated. The plasma nitriding of ground and sandblasted samples and drills was performed at temperatures of 400°C and 500°C for two gas mixtures: 5 vol.% N₂ and 76 vol.% N₂ in hydrogen. Surface layers were characterized before and after plasma nitriding concerning the microstructure, roughness, microhardness, chemical composition, phase composition and residual stress states. Machining tests were carried out with drills during which drilling forces and flank wear have been measured. A significant effect of the surface state prior to nitriding on residual stress states and the properties of the nitrided layer and untreated core has been observed. Thinner nitrided layers on ground and sandblasted samples were attributed to high compressive residual stress states and a stress affected diffusion of nitrogen and carbon. In the machining tests, sandblasted drills exhibited the best performance. Lower nitrogen concentrations in the gas atmosphere without the formation of a compound layer gave the lowest drill flank wear for sandblasted surfaces while higher nitrogen concentrations led to a reduction of drilling forces and torque.     

Low-frequency photodielectric phenomena in polycrystalline Pb<Subscript>3</Subscript>O<Subscript>4</Subscript> layers

Photopolarization phenomena in the polycrystalline layers of Pb₃O₄ with a binder have been studied. The samples exhibited a positive photocapacitive effect, an increase in dielectric losses under illumination, and a low-frequency dispersion of dielectric parameters. Possible mechanisms of relaxation processes in the dark and under illumination and the nature of relaxators are discussed.     

Epitaxial PbZrxTi1−xO3 Ferroelectric Bilayers with Giant Electromechanical Properties

Giant electromechanical response viaferroelastic domain switching is achieved in epitaxial (001) ferroelectric tetragonal (T) PbZr_0.3Ti_0.7O₃/rhombohedral (R) PbZr_0.55Ti_0.45O₃ bilayers, grown on La_0.67Sr_0.33MnO₃ buffered SrTiO₃ substrates. X‐ray diffraction and transmission electron microscopy show that the domain structure of the T films is tuned as a function of its thickness, from a fully a₁/a₂‐domains (30 nm thick T layer) to a three domain stress‐free c/a₁/c/a₂ polytwin state (100 nm thick T layer). A large switchable polarization is found up to 65 μC cm⁻². Quantitative piezoelectric force microscopy reveals enhanced piezoelectric coefficients, with d₃₃ coefficients ranging from 250 to 350 pm V⁻¹, which is up to seven times higher than the nominal PbZr_xTi_1−xO₃ thin film values. These are attributed to the motion of nanoscale ferroelastic domains. Fatigue testing proves that these domains are reversible and repeatable up to 10⁷ cycles. In‐situ X‐ray synchrotron measurements reveal that the ferroelastic domain switching is promoted by a pulsating strain effect imposed by the R layer. The study reports a fundamental understanding of the origin of giant piezoelectric coefficients in epitaxial ferroelectric bilayers.