Lattice Thermal Expansion Coefficients Research Articles

Strain accumulation and relaxation on crack formation in epitaxial AlN film on Si (111) substrate

The formation of cracks is often observed in the epitaxial growth of ultrawide-bandgap aluminum nitride (AlN) semiconductor films on economical and versatile silicon (Si) substrates due to the significant differences in in-plane lattice parameters and thermal expansion coefficients between the film and the substrate, which hampers the development of template, buffer layer, and device structure with a relatively thick AlN layer for devices. The present study aims to elucidate the conditions of crack formation through a simple but comprehensive estimation of strain energy accumulation and relaxation by lattice strain, misfit dislocation density, and crack formation. Strain energy in the epitaxial film from lattice and thermal mismatches is evaluated by an elastic strain equation tailored to the epitaxy of the hexagonal crystal structure. The effects of temperature, thickness, and dislocation density on the lattice and dislocation strain energies of the film are also considered. Finally, the comparison in the changes in the total strain energy and cleavage energy with decreasing temperature shows that cleavage energy is higher than strain energy if the film is thinner than 400 nm but becomes lower than the strain energy if the film is thicker than 400 nm during cooldown, suggesting the crack formation, which matches well with experimental observations.

Structural and thermal characterization of novel Ni(II)(L-threoninato)2(H2O)2 crystal

This study focuses on the synthesis and characterization of the novel Ni(II)(L-threoninato)2(H2O)2 crystal, a bivalent transition metal complex based on Ni(II) ions, based on crystalline structure and thermal properties. Through crystallographic analysis, we reveal that Ni(II)(L-threoninato)2(H2O)2 complex adopts an orthorhombic system with the C2221 space group. To gain insights into its thermal behavior, thermogravimetric analysis, and differential scanning calorimetry techniques were employed to estimate kinetic parameters related to its thermal degradation. Model-free analysis, employing Friedman, Ozawa-Flynn-Wall, and Kissinger-Akahira-Sunose methods, along with model-based analysis, were utilized to evaluate the activation energy and the logarithm of the pre-exponential factor. Furthermore, we elucidated the complex's crystalline structure thermal behavior using Powder X-ray Diffraction measurements as a function of temperature, at air and under vacuum. No phase transitions were detected at low temperatures only at high temperatures due to loss of water molecules. The measurements showed the crystal goes to a partially crystalline anhydrous phase with a certain long-range order in the [001] direction. Through Rietveld refinement the behavior of the lattice parameters and thermal expansion coefficients were obtained. This comprehensive study provides valuable knowledge about the Ni(II)(L-threoninato)2(H2O)2 crystals and lays the foundation for further exploration of similar complexes.

Effect of High Al Content AlGaN Buffer Layer on the Properties of GaN‐on‐Silicon Materials

The lattice parameter and thermal expansion coefficient mismatch between the silicon substrate and GaN lead to high tensile stress, which makes the GaN epitaxial layer prone to cracking. Effective compensation of tensile stress on GaN to prevent cracking is an important issue in GaN epitaxial growth. In this work, GaN‐on‐silicon materials with different AlGaN buffer layer structures are prepared by metal‐organic chemical vapor deposition (MOCVD). The GaN epitaxial material with a smooth and crack‐free surface is fabricated by inserting 7 AlGaN buffer layers. The crystal quality of GaN is characterized using high resolution X‐ray diffraction (HRXRD). The full‐width half maximum (FWHM) value of GaN(002) and GaN(102) crystal plane is 398 and 780 arcsec, respectively. The surface root mean square (RMS) roughness of GaN material is 0.31 nm, and the vertical breakdown voltage (BV) of the epitaxial wafer reaches 918 V. The results show that high Al content of the AlGaN buffer layer can effectively reduce the tensile stress and dislocation density of the GaN layer when the entire AlGaN layer thickness remains constant. Suppressing the generation of surface cracks and improving the crystal quality can improve the vertical breakdown voltage and two‐dimensional electron gas (2‐DEG) characteristics of GaN epitaxial wafers.

Effect of 10B enrichment on the thermal expansion of boron carbide

The effect of 10B enrichment on the lattice and bulk thermal expansion behaviour of boron carbide (B4C) has been investigated. The coefficients of lattice thermal expansion (LTE) of hot-pressed polycrystalline boron carbide (B4C) with natural 10B isotopic composition (N-B4C) as well as 50% and 90% 10B enriched B4C (50E-B4C, 90E-B4C) are measured in the temperature range of 298–1273 K using X-ray diffraction (XRD) method. The coefficients of bulk thermal expansion (BTE) was measured for the same specimens using dilatometry and compared with the measured LTE. A reasonable increase in both LTE and BTE of B4C has been observed with increase in 10B enrichment. The measured BTE is found to have higher values in comparison with the LTE, irrespective of 10B enrichment which is attributed to the intrinsic distorted structure of B4C as well as the presence of defects in the as-synthesized specimens.

(Invited) Integration of InP Heterojunction Bipolar Transistors on Silicon Substrates for 6G Networks

The fast expansion of the global datasphere creates a huge demand on the future wireless communication technologies. Although 5G, the 5th generation of the cellular network technology, is still being rolled out, the potential of 6G is already intensively explored. The vision of 6G implies not only massive connectivity and higher data rate (up to 100 Gb/s and more) in ultra-reliable low-latency communication, but machine-to-machine interaction will dominate over those involving humans. AI (artificial intelligence) and machine learning will be omnipresent.6G network will address a frequency spectrum beyond 100 GHz, which provides wider bandwidths and, therefore, potentially simpler modulation schemes and eventually lower power consumption. However, a clear disadvantage of these mm-waves or sub-THz frequencies are the atmospheric absorption and blockage effect by physical obstacles or rain. These challenges must be tackled by a sufficiently high transceiver device density per area together with advanced beam steering and AI empowered networks.The power amplifier (PA) is one of the most critical components of the RF front-end module. Here, InP based heterojunction bipolar transistors (HBT) clearly outperform PAs based on CMOS or SiGe BiCMOS concerning saturated output power, energy efficiency and breakdown voltage. However, the limiting downside of InP HBTs is the missing cost-efficient and scalable fabrication, which hampers its adoption in high-volume 6G applications. In this paper we review various InP HBT fabrication concepts with the aim of lowering the production costs and maintaining high device performance.The best HBT device performance is demonstrated for the lattice-matched deposition on native small-diameter (≤ 150 mm) InP substrates due to perfect III-V crystal quality. Although device fabrication on these wafers cannot profit from the advanced tool park available in 200/300 mm fabs, the main drawbacks are the high cost and brittleness of InP substrates. Realizing InP HBTs on Si substrates allows to benefit from their robustness, low cost, and the access to a mature device fabrication on large-diameter wafers.In wafer reconstitution, numerous diced InP wafer tiles, which already contain the grown HBT layer stack, are bonded to a Si carrier to populate the wafer surface. After planarization, the reconstructed Si wafer can be further fabricated in a CMOS compatible 200/300 mm fab. In transfer printing a comparable approach is followed while only the active device stack is transferred to the Si carrier. Transferring only a thin InP layer to the Si carrier, which serves as a starting surface for the HBT deposition, leads to a so-called engineered substrate. Compared to the previous transfer methods, the III-V device deposition is carried out after bonding. The key advantage of all these heterogenous integration concepts is the access to a CMOS compatible and scalable device fabrication while maintaining the high III-V crystal quality. Nevertheless, the HBT production costs can only be significantly reduced with a certain re-usability of the InP substrates.The monolithic deposition directly on Si substrates would be the most cost-efficient integration approach. Unfortunately, the mismatch in lattice constant and thermal expansion coefficient between Si and III-V materials leads to the formation of relaxation defects, which degrade the HBT device performance. The growth of thick strain relaxation buffers allows to reduce the defect density in the device layers but bears new challenges such as wafer warpage or III-V crack formation.Nano-ridge engineering (NRE) is based on selective area growth (SAG) applying trench-patterned Si wafers. The III-V deposition starts inside very narrow trenches to achieve efficient aspect ratio trapping (ART) of misfit defects. After the trench is filled with III-V material, the growth is continued to engineer a broad and pristine nano-ridge (NR) which holds the device stack. A second oxide pattern prevents sidewall deposition around the NRs during the deposition of the HBT heterolayers.In the first part of this paper, pros, and cons of the most common InP HBT integration scenarios as summarized in figure will be compared in detail. Among all concepts, NRE holds the potential of being the most cost-efficient integration approach due to SAG in line with very limited III-V material deposition. However, the key open question is what NR crystal quality, and hence, device performance can be realized. This question will be addressed in the second part of the paper, reporting about the latest developments at imec. Figure 1

Exploring the effects of substrate and substrate temperature on the properties of radio frequency magnetron sputtered ZnO thin films

The physical and optical properties of zinc oxide thin films strongly depend on the film deposition techniques. In this work, RF magnetron sputtering was used to deposit ZnO thin films on various substrates (glass, quartz, ITO-coated glass, FTO-coated glass, Si(100), Si(100)/ SiO2) to study the role of substrate and substrate temperature on the morphological, structural and optical properties of the film grown. A detailed discussion on various aspects like surface morphology, strain, photoluminescence, bandgap, surface roughness, and material composition has been done. Compressive stress was present in the films created at room temperature and differed for various substrates based on the lattice mismatch. The nature and temperature of the substrate are found to affect the surface topography of films. Substrate and substrate temperature influenced the emission characteristics of ZnO thin films. In oxygen-rich substrates, defect emissions differed. We found that quartz is a better substrate to fabricate ZnO at room temperature and both Si and Si/SiO2 are preferable at high temperatures due to the lower lattice mismatch and thermal expansion coefficient. Comparing the electrical characteristics of ZnO thin films grown on glass and quartz substrates, ZnO on glass exhibited lower resistivity and higher carrier concentration. The Triboelectric nanogenerator (TENG) combining ZnO and PDMS, was developed to operate in vertical contact-separation mode. The obtained open circuit voltage was 30 V and 33 V for ZnO grown over ITO and FTO respectively, and the slight increase is due to the rougher film's surface on FTO than ITO. The selection of substrate is crucial in many applications and our findings provide an overview of the suitability of substrate and substrate temperature for the fabrication of ZnO thin film-based optoelectronic devices.

Thermal and Photo‐Degradation Study of α‐FAPbI3‐Based Perovskite Using In Situ X‐Ray Diffraction

AbstractHybrid halide perovskite has established its credibility as high performance thin film photovoltaic technology. Perovskite based on formamidinium cation is at the core composition to top performances and stability. Herein, a depth study based on temperature‐controlled in situ X‐ray diffraction focusing on the photo‐active formamidinium lead iodide (α‐FAPbI3) is reported. In particular, the thermal stability of the latter and the degradation pathways under different experimental conditions are clarified. Based on this in situ technique, the lattice thermal expansion coefficient is reported that provides relevant information on possible mechanical stress created upon temperature cycling or damp heat test. The results support that α‐FAPbI3 degradation is substantially accelerated when temperature is combined to illumination and when it is interfaced with the extraction layers. In addition, by contrast to in darkness for which α‐FAPbI3 degrades directly into PbI2, the existence of a temperature gap under illumination involving an intermediate step with a non‐crystalline phase resulting from the perovskite degradation and contributing to the formation of PbI2 by‐product is revealed.

Correlations between the crystal structure and microwave dielectric properties in Ce2[Zr1-xMx]3(MoO4)9 (M = Mn1/3Nb2/3, Mn1/3Ta2/3) solid-solution ceramics

Ce2[Zr1-xMx]3(MoO4)9 (M = Mn1/3Nb2/3, Mn1/3Ta2/3;x = 0.02, 0.04, 0.06, 0.08 and 0.10) (abbreviated as CZ1-xNx and CZ1-xTx) ceramics were prepared through the solid-state reaction method. Effects of (Mn1/3Nb2/3)4+ and (Mn1/3Ta2/3)4+ ions on the sintering characteristics, crystal structures, microwave dielectric properties and infrared vibrational modes were studied in detail. X-ray diffraction (XRD) results reveal the formation of solid solutions for all components. Based on the chemical bond theory and Rietveld refinement, intrinsic structure parameters including the polarizability (P), the packing fraction (P.F.) and the octahedral distortion (Δocta.), and bond parameters including the lattice energy (U), bond energy (E) and thermal expansion coefficient (α) were calculated. Interestingly, the Ce–O bond plays a major role in the bond ionicity (fi), while Mo–O bond dominates the contributions in the lattice energy (U), bond energy (E) and thermal expansion coefficient (α). In addition, these parameters are used to explain the variations of the microwave dielectric properties of ceramics either changing the doping contents or replacing different ions at x = 0.06. Furthermore, far infrared (FIR) spectra uncover that the phonon modes provide the major polarization contribution of 68.59% in the CZ0.9T0.1 ceramic, implying that the main contribution to εr stems from the ionic polarization instead of the electronic polarization. Typically, the optimum microwave dielectric properties are achieved for the CZ0.9N0.1 and CZ0.9T0.1 ceramics with εr = 10.76, Q × f = 85,893 GHz (at 9.52 GHz), τf = −14.83 ppm °C−1 and εr = 10.72, Q × f = 87,355 GHz (at 9.81 GHz) and τf = −8.68 ppm °C−1, respectively. Notably, the CZ0.9T0.1 ceramic has a markedly increased Q × f while maintaining a good τf = −8.68 ppm °C−1 and a low sintering temperature of 700 °C.

Selective electrodeposition of indium microstructures on silicon and their conversion into InAs and InSb semiconductors

The idea of benefitting from the properties of III-V semiconductors and silicon on the same substrate has been occupying the minds of scientists for several years. Although the principle of III-V integration on a silicon-based platform is simple, it is often challenging to perform due to demanding requirements for sample preparation rising from a mismatch in physical properties between those semiconductor groups (e.g. different lattice constants and thermal expansion coefficients), high cost of device-grade materials formation and their post-processing. In this paper, we demonstrate the deposition of group-III metal and III-V semiconductors in microfabricated template structures on silicon as a strategy for heterogeneous device integration on Si. The metal (indium) is selectively electrodeposited in a 2-electrode galvanostatic configuration with the working electrode (WE) located in each template, resulting in well-defined In structures of high purity. The semiconductors InAs and InSb are obtained by vapour phase diffusion of the corresponding group-V element (As, Sb) into the liquified In confined in the template. We discuss in detail the morphological and structural characterization of the synthesized In, InAs and InSb crystals as well as chemical analysis through scanning electron microscopy (SEM), scanning transmission electron microscopy (TEM/STEM), and energy-dispersive X-ray spectroscopy (EDX). The proposed integration path combines the advantage of the mature top-down lithography technology to define device geometries and employs economic electrodeposition (ED) and vapour phase processes to directly integrate difficult-to-process materials on a silicon platform.Graphical abstract

Phases and Interfaces in the Cr–Fe–Si Ternary System: X-ray Diffraction and Electron Microscopy Study

The ternary Cr-Fe-Si system was investigated with X-ray diffraction, energy dispersive X-ray spectrometry, scanning and transmission electron microscopy, and electron diffraction. Samples melted at 1723 K were examined right after cooling or after annealing at 1073 K for 3 days to determine phases, grain sizes, and interphase interfaces. During annealing, a polymorphic transformation of the tetragonal α-FeSi2 to the orthorhombic β-FeSi2 phase occurs, while CrSi2 retains its hexagonal structure at high-temperature treatment. Thin layers of ε-FeSi with a cubic structure were observed and identified within the CrSi2 grains. Crystallographic orientation relationships are determined at the interphase interfaces. The contributions of lattice mismatch and thermal expansion coefficient misfit to deformation are discussed.

Design and Comparison of Diamond‐ and Sapphire‐Based NbN KIDs for Fusion Plasma Polarimetric Diagnostics

Design and characterization measurements performed on kinetic inductance detectors (KIDs) produced on sapphire and polycrystalline diamond substrates are presented. Designed as part of a nuclear fusion polarimetric diagnostic, the foreseen plasma‐probing frequency of the final devices is 1.3 THz with a maximum response time under 10 ms and cross‐polarization target accuracy of 1%. These detectors are based on superconducting micro‐resonators that undergo de‐tuning upon absorption of radiation. The main characteristics of the devices include polarization sensitiveness and lumped‐elements multi‐pixel configuration produced from photo‐lithographed niobium nitride (NbN) thin films. The direct current (DC) and microwave characterization measurements highlight large differences in the thin film quality, with the superconductor deposited on diamond showing reduced critical temperature, lower critical current density, and increased values of the kinetic inductance. This difference is likely due to the higher lattice constant and thermal expansion coefficient mismatch between film and substrate in the case of diamond, with the different surface finish quality of the crystalline samples available also playing a role. The devices on both substrates show a bolometric response to THz radiation that fulfills the requirement guidelines and represent a good starting point to optimize the design for the application at hand.

Hydrostatic pressure effect on the structural parameters of GaSb semiconducting material: Ab-initio calculations

Ab-initio calculations were performed to investigate the ground state and hydrostatic pressure effecton the structural properties of GaSb semiconducting material. The projected augmented wavepseudopotentials (PAW) approach in the framework of the density functional theory (DFT) asimplemented in the Quantum Espresso code was used. The exchange-correlation functional wasdescribed with the generalized gradient approximation (GGA). Utilizing the energy - volume (E-V)data, our values of the equilibrium lattice constant, the bulk modulus, and the pressure derivative ofthe bulk modulus of GaSb semiconductor obtained from the Birch–Murnaghan equation of statewere found 6.220 Å, 44.84 GPa and 4.22, respectively. Our obtained data agree well with theavailable experimental values and other theoretical data of the literature. In addition, the meltingpoint, the lattice thermal expansion coefficient and the microhardness of our material of interestwere also calculated and compared with the available experimental data of the literature.
 

Two-dimensional material templates for van der Waals epitaxy, remote epitaxy, and intercalation growth

Epitaxial growth, a crystallographically oriented growth induced by the chemical bonding between crystalline substrate and atomic building blocks, has been a key technique in the thin-film and heterostructure applications of semiconductors. However, the epitaxial growth technique is limited by different lattice mismatch and thermal expansion coefficients of dissimilar crystals. Two-dimensional (2D) materials with dangling bond-free van der Waals surfaces have been used as growth templates for the hetero-integration of highly mismatched materials. Moreover, the ultrathin nature of 2D materials also allows for remote epitaxial growth and confinement growth of quasi-2D materials via intercalation. Here, we review the hetero-dimensional growth on 2D substrates: van der Waals epitaxy (vdWE), quasi vdWE, and intercalation growth. We discuss the growth mechanism and fundamental challenges for vdWE on 2D substrates. We also examine emerging vdWE techniques that use epitaxial liftoff and confinement epitaxial growth in detail. Finally, we give a brief review of radiation effects in 2D materials and contrast the damage induced with their 3D counterparts.

Reduced Dislocation Density of an InP/GaAs Virtual Substrate Grown by Metalorganic Chemical Vapor Deposition

Integrating indium phosphide (InP) material on a gallium arsenide (GaAs) substrate to form an InP/GaAs virtual substrate has been an attractive research subject over the past decade. However, the epitaxial growth of InP on GaAs is challenging due to a large mismatch in the lattice constant and thermal expansion coefficient. This paper describes the successful hetero-epitaxy of InP on a GaAs substrate by metalorganic chemical vapor deposition. The hetero-epitaxy in this study utilized a hybrid growth method involving a thin indium gallium arsenide (InGaAs) linearly graded buffer, two-step InP growth, and a post-annealing process. Transmission electron microscopic observations showed that a traditional two-step InP/GaAs virtual substrate was smooth but had a high threading dislocation density (TDD) of 1.5 × 109 cm−2 near the InP surface. The high TDD value can be reduced to 2.3 × 108 cm−2 by growing the two-step InP on a thin InGaAs linearly graded buffer. The TDD of an InP/GaAs virtual substrate can be further improved to the value of 1.5 × 107 cm−2 by removing the low-temperature InP nucleation layer and carrying out a post-annealing process. A possible reason for the improvement in TDD may relate to a dislocation interaction such as the annihilation of mobile threading dislocations. Room-temperature photoluminescence spectra of InP/GaAs virtual substrates with different TDD values were compared in this study. The optical and micro-structural characterization results suggest that the proposed growth method may be feasible for making good-quality and relatively low-cost InP/GaAs virtual substrates for the integration of optoelectronic devices on them.

Electronic, magnetic, elastic, thermal and thermoelectric proprieties of Co2MnZ (Z=Al, Ge, Sn)

The full-potential linearized augmented plane-wave (FLAPW) method, which is based on density functional theory using the “WIEN2k” code. The exchange-correlation functional was taken within the generalized gradient approximation (GGA), has been used to study the structural, electronic, mechanical, magnetic, and thermal properties of Co2MnZ (Z = Al, Ge, Sn) (CMZ) alloy. When using the GGA + U approximation, all compounds depict half-metallic behavior due to spin polarization at the Fermi level.Get the Curie temperature value and the magnetic moment. We find that the ferromagnetic state is more stable than the antiferromagnetic and nonmagnetic states. The effects of heat on lattice parameters, compressive modulus, volume, heat capacity and thermal expansion coefficient at several pressures were investigated. The thermoelectric performance of these systems was examined.Calculations are performed using the BoltzTrap code, that relies on the semiclassical Boltzmann transport equation. The Seebeck coefficient, electrical conductivity, thermal conductivity, and figure of merit are calculated taking into account the upper and lower spins.

Realizing crack-free high-aluminum-mole-fraction AlGaN on patterned GaN beyond the critical layer thickness

Wide-bandgap III-nitride heterostructures are required for a variety of device applications. However, this alloy system has a large lattice constant and thermal expansion coefficient mismatch that limits the alloy composition and layer thickness for many heteroepitaxial device structures. Consequently, various methods have been devised to allow the heteroepitaxial growth of AlInGaN heterostructures to accommodate this inherent strain. In this work, we describe a non-planar-growth approach that enables the deposition of crack-free high-Al-mole-fraction AlxGa1−xN on patterned GaN/sapphire templates and bulk GaN substrates with large-area mesas. We have studied the effects of the patterned mesa width, the mesa etch depth, and the gap between the mesas on the heteroepitaxy of AlxGa1−xN superlattices with an average Al molar fraction 0.11 < x¯ < 0.21 and non-planar overgrowth growth thicknesses up to 3.5 μm. Similar to the planar growth approach, increasing the thickness and Al mole fraction of the AlxGa1−xN superlattices leads to surface cracking when exceeding the critical layer thickness. However, limiting the mesa dimension in one direction enables strain mitigation and drastically increases the critical layer thickness. Additionally, larger etch depths of the mesas increase the Al alloy composition and thickness for crack-free AlGaN heteroepitaxy whereas the gap in between the mesas seems to have no crucial influence. We demonstrate that the Al alloy composition and layer thicknesses of such heterostructures can be increased far beyond the critical layer thickness for planar growth and demonstrate the growth of a crack-free full AlxGa1−xN/GaN quantum-well laser heterostructure designed for operation at ∼370 nm.

X-ray Diffraction Techniques for Mineral Characterization: A Review for Engineers of the Fundamentals, Applications, and Research Directions

X-ray diffraction (XRD) is an important and widely used material characterization technique. With the recent development in material science technology and understanding, various new materials are being developed, which requires upgrading the existing analytical techniques such that emerging intricate problems can be solved. Although XRD is a well-established non-destructive technique, it still requires further improvements in its characterization capabilities, especially when dealing with complex mineral structures. The present review conducts comprehensive discussions on atomic crystal structure, XRD principle, its applications, uncertainty during XRD analysis, and required safety precautions. The future research directions, especially the use of artificial intelligence and machine learning tools, for improving the effectiveness and accuracy of the XRD technique, are discussed for mineral characterization. The topics covered include how XRD patterns can be utilized for a thorough understanding of the crystalline structure, size, and orientation, dislocation density, phase identification, quantification, and transformation, information about lattice parameters, residual stress, and strain, and thermal expansion coefficient of materials. All these important discussions on XRD analysis for mineral characterization are compiled in this comprehensive review, so that it can benefit specialists and engineers in the chemical, mining, iron, metallurgy, and steel industries.

Plasma-Assisted MOCVD Growth of Non-Polar GaN and AlGaN on Si(111) Substrates Utilizing GaN-AlN Buffer Layer

We report the growth of non-polar GaN and AlGaN films on Si(111) substrates by plasma-assisted metal-organic chemical vapor deposition (PA-MOCVD). Low-temperature growth of GaN or AlN was used as a buffer layer to overcome the lattice mismatch and thermal expansion coefficient between GaN and Si(111) and GaN’s poor wetting on Si(111). As grown, the buffer layer is amorphous, and it crystalizes during annealing to the growth temperature and then serves as a template for the growth of GaN or AlGaN. We used scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray diffraction (XRD) characterization to investigate the influence of the buffer layer on crystal structure, orientation, and the morphology of GaN. We found that the GaN buffer layer is superior to the AlN buffer layer. The thickness of the GaN buffer layer played a critical role in the crystal quality and plane orientation and in reducing the cracks during the growth of GaN/Si(111) layers. The optimum GaN buffer layer thickness is around 50 nm, and by using the optimized GaN buffer layer, we investigated the growth of AlGaN with varying Al compositions. The morphology of the AlGaN films is flat and homogenous, with less than 1 nm surface roughness, and has preferred orientation in a-axis.

Synthesis and characterization of Ca3-xLaxCo4-yCuyO9+δ cathodes for intermediate temperature solid oxide fuel cells

The calcium cobaltite Ca 3-x La x Co 4-y Cu y O 9+δ with x and y = 0 and 0.1 were synthesized and the electrical, thermal, and catalytic behaviors for the oxygen reduction reaction (ORR) for use as air electrodes in intermediate-temperature solid oxide fuel cells (IT-SOFCs) were evaluated. X‒ray diffraction confirms the Ca 3-x La x Co 4-y Cu y O 9+δ samples were crystallized in a monoclinic structure and scanning electron microscopic image shows lamella-like grain formation. Introduction of dopants decreases slightly the loss of lattice oxygen and thermal expansion co-efficient. The Ca 3-x La x Co 4-y Cu y O 9+δ samples exhibit good phase stability for long-term operation, thermal expansion, and chemical compatibility with the Ce 0.8 Gd 0.2 O 2-δ electrolyte. Among the studied samples, Ca 2.9 La 0.1 Co 4 O 9+δ shows a maximum conductivity of 176 Scm −1 at 800 °C. Although the doped samples exhibit a higher total electrical conductivity, an improved symmetrical cell performance is displayed by the undoped sample. Comparing the sintering temperatures, the composite cathode Ca 3 Co 4 O 9+δ + Ce 0.8 Gd 0.2 O 2-δ sintered at 800 °C exhibit the lowest area specific resistance of 0.154 Ω cm 2 at 800 °C in air. In the Ca 3-x La x Co 4-y Cu y O 9+δ + GDC composite cathodes, the charge-transfer process at high frequencies presents a major rate limiting step for the oxygen reduction reaction.

XFEM simulation of dislocation in SixGe1-x alloy under thermal loads

SixGe1-x is an alloy made of two semiconductor materials from group Ⅳ-A of the periodic table. The performance of semiconductors results in higher temperatures due to electric field formation and also they are exposed to elevated atmospheric temperatures. Here, the SixGe1-x domain with one free surface at the left is considered under thermal loads at different compositions when the edge dislocation is present in the domain. The Extended Finite Element Method (XFEM) is used for numerical modeling of the problem. The lattice constant, thermal conductivity, electrical conductivity and thermal expansion coefficient are functions of the amount of Si (xSi) in the alloy. The dislocation Burgers vector is also dependent on the Si concentration as its magnitude is defined in terms of the lattice constant. The XFEM formulation introduces the Volterra type edge dislocation through the enrichment functions. Firstly, the discrete heat equilibrium equation is solved and then considering the thermal strains, the discrete force equilibrium equation is solved. The two cases of the heat flux acting normal to the glide plane and along the glide plane of the dislocation are considered for different temperatures. The results indicate an increase in the Peach-Koehler force with the increase in ‘xSi’ greater than 0.2 for both the cases of the heat flux application at all temperatures. However, at the lower concentration of Si i.e. below 0.2, the Peach-Koehler force is affected differently.