Lattice Thermal Expansion Coefficients Research Articles

Effect of compositionally graded AlGaN buffer layer grown by different functions of trimethylaluminum flow rates on the properties of GaN on Si (111) substrates

In the present paper, compositionally graded AlGaN buffer layer was employed to compensate the tensile stress and improve the crystalline quality of GaN grown on silicon substrate. During the growth of the graded buffer layer, the flow rate of trimethylaluminum (TMAl) was gradually decreased according to a series of functions, while the flow rate of trimethylgalliumm (TMGa) was increased with the same linear function. Therefore, these graded AlGaN layers showed different distributions of Al, which was confirmed by the high-resolution X-ray diffraction (HR-XRD) and the secondary ion mass spectroscopy (SIMS). A linear compositionally graded AlGaN layer was obtained by using the convex parabolic TMAl flow rate, which leads to a large compressive stress in GaN layer due to a linearly gradual change of lattice parameters and thermal expansion coefficient (TEC) from AlN seeding layer to GaN layer. Using the linear compositionally graded buffer layer, a 3-μm-thick crack-free GaN film with a low dislocation density was grown on silicon substrate. The dislocation evolution associated with stress was investigated by the cross-sectional micro-Raman scattering and transmission electron microscopy (TEM).

Integration of GaAs on Ge/Si towers by MOVPE

ABSTRACTWe report on the maskless integration of micron-sized GaAs crystals on patterned Si substrates by metal organic vapor phase epitaxy. In order to adapt the mismatch between the lattice parameter and thermal expansion coefficient of GaAs and Si, 2 μm tall Ge crystals were first grown as virtual substrate by low energy plasma enhanced chemical vapor deposition. We investigate the morphological evolution of the GaAs structures grown on top of the Ge crystals at the transition towards full pyramids with energetically stable {111} facets. A substantial release of strain is shown in GaAs crystals with a height of 2 μm and lateral sizes up to 15×15 μm2 by both X-ray diffraction and photoluminescence.

Quantification of Phase Transformation Kinetics under Thermomechanical Conditions Using Dilatometry Data

Using dilatometric technique, it is the purpose of present research to quantify the kinetics of austenite decomposition during cooling after isothermal hot compression. To reach this aim, based on lattice parameters and thermal expansion coefficients of different phases, a new model was proposed. In this model, the contributions of both isotropic and non-isotropic dilatations were considered. The model was applied to 22MnB5 steel to predict formation of bainite and martensite. Furthermore, the model was developed in order to determine the final fraction of deformation induced ferrite (DIF).

A high temperature X-ray diffraction study of the influence of MWCNTs on the thermal expansion of MWCNT/Ni composites

The inclusion of multiwall carbon nanotubes (MWCNTs) into metallic matrices is expected to improve their electrical and mechanical performance. This work analyses the lattice thermal expansion behavior of MWCNT-reinforced Ni composites in order to understand the influence of CNTs on their thermomechanical properties. The lattice thermal expansion coefficient (CTE) of the composites was estimated from X-ray diffraction measurements as a function of the temperature. The MWCNT-reinforced composites show higher lattice CTE (up to 12%) compared to pure Ni. This behavior is mainly produced by the expansion mismatch between the matrix and the reinforcements. We suggest three main mechanisms that explain the behavior of the CTE of metallic matrices in presence of MWCNT. These mechanisms correlate to the anchoring effect observed in previous macroscopic measurements of thermal expansion carried out on the same system.

Thermal expansion properties of stannosilicate molecular sieve with MFI type structure

An in situ high temperature X-ray diffraction study was carried out for investigating the thermal expansion properties of Si-MFI and SnSi-MFI molecular sieves. The thermal stability up to 973K and a negative lattice thermal expansion in anisotropic manner was exhibited by both the phases in the temperature range of 373–973K. The trend observed in contraction along the axes was as: a>c>b. The substitution of Sn4+ in MFI framework resulted in an expansion of unit cell volume and in an increase in the lattice thermal expansion coefficient in the temperature range 423–973K.

Crystal structure of new ternary compounds Al2R11Ge8 (R = Y, Gd, Tb, Dy, Ho, Er) and thermal properties of Al2Er11Ge8

The crystal structure of new ternary compounds Al2R11Ge8 (R=Y, Gd, Tb, Dy, Ho, Er) was studied using X-ray powder diffraction data by Rietveld method. Scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS) was used for sample composition examination. The compounds crystallized in the Ge10Ho11 structure type (space group I4/mmm). The phase was not found in the systems with R=La, Ce, Pr and Nd. The lattice thermal expansion study of the Al2Er11Ge8 compound was carried out using high-temperature powder X-ray diffraction technique from 293K to 743K, and the calculated lattice and volume thermal expansion coefficients were αa=0.88×10−5K−1, αc=1.22×10−5K−1, and αV=3.01×10−5K−1, respectively.

Elastic and thermo-physical properties of TiC, TiN, and their intermediate composition alloys using ab initio calculations

The equilibrium lattice parameters, elastic properties, material brittleness, heat capacities, and thermal expansion coefficients of TiC, TiN, and their intermediate composition alloys (Ti(C1−xNx), x=0.25, 0.5, and 0.75) were calculated using ab initio density functional theory (DFT) methods. We employed the Debye–Gruneisen model to calculate a finite temperature heat capacity and thermal expansion coefficient. The calculated elastic moduli and thermal expansion coefficients agreed well with the experimental data and with other DFT calculations. Accurate heat capacities of TiC, TiN, and their intermediate composition alloys were obtained by calculating not only the phonon contributions but also the electron contributions to the heat capacity. Our calculations indicated that the heat capacity differences between each composition originated mainly from the electronic contributions.

Role of Bond Strength on the Lattice Thermal Expansion and Oxide Ion Conductivity in Quaternary Pyrochlore Solid Solutions

Quaternary pyrochlore-type solid solutions, CaGdZrNb(1-x)Ta(x)O(7) (x = 0, 0.2, 0.4, 0.6, 0.8, 1), were prepared by a high-temperature ceramic route. The pyrochlore phases of the compounds were confirmed by powder X-ray diffraction (XRD), Raman spectroscopy, and transmission electron microscopy. The crystallographic parameters of the pyrochlore compounds were accurately determined by Rietveld analysis of the powder XRD data. The isovalent substitution of Ta in place of Nb at the B site can reveal the effect of chemical bonding on lattice thermal expansion and oxide ion conductivity because both Nb and Ta have the same ionic radius (0.64 Å). Lattice thermal expansion coefficients of the samples were calculated from high-temperature XRD measurements, and it was found that the thermal expansion coefficient decreases with substitution of Ta. Oxide ion conductivity measured by a two-probe method also shows the same trend with substitution of Ta, and this can be attributed to the high bond strength of the Ta-O bond compared to that of the Nb-O bond. Microstructural characterization using scanning electron microscopy proves that the size of the grains has a small effect on the oxide ion conductivity. Our studies established the role of chemical bonding in deciding the conductivity of pyrochlore oxides and confirmed that the 48f-48f mechanism of oxide ion conduction is dominant in pyrochlore oxides.

The atomistic study on the thermal expansion behaviors of nanowires

The thermal expansion coefficient of copper nanowire and bulk was investigated using molecular dynamics simulation in the temperature range up to 900 K. Both the size and crystal orientation effects on the thermal expansion coefficients of nanowires were studied. It is noticed that the linear thermal expansion is isotropic for single-crystalline bulk copper and the linear thermal expansion coefficient is exactly one third of the volumetric thermal expansion coefficient irrespective to the crystal orientations. The axial linear thermal expansion coefficient of an infinite long nanowire would be affected by both the surface and axial crystal orientation. Different temperature dependencies of lattice parameter and thermal expansion coefficient were found for nanowires with different width, and there is a transition from positive thermal expansion coefficient at low temperature to negative one at high temperature for the thinnest nanowires along [1 0 0] direction. However, the negative linear thermal expansion is not observed for nanowires along [1 1 0] and [1 1 1] direction. From the examination of the atomic configuration and radial distribution function for the thinnest [1 0 0] nanowires at various temperatures, it is concluded that the negative linear thermal expansion behavior is not caused by melting or the phase transformation.

The Research about the III-Nitride Compounds Epitaxially Grown on Si Substrate

The III-nitride compounds epitaxially grown on Si substrate have attracted more and more attentions and some progress have been achieved. Many methods have been tried to tackle the issue which caused by the large lattice mismatch and thermal expansion coefficient mismatches between silicon substrate and the III-nitride compounds. This paper presents buffer layer technology, selective area and lateral epitaxial over growth technology, and presents the researches about the III-nitride devices. Semi polar and non-polar GaN films grown on Si (such as Si(110), Si(112), Si(001) et al.) also have been instructed. At the end of this paper, the development trend of epitaxial technology has been discussed.

Epitaxial gallium nitride thin films grown on silicon substrates utilizing gallium nitride seed-layer formed by liquid source precursor

Gallium nitride (GaN) epitaxial thin films were deposited on Si substrates by a modified hydride vapor phase epitaxy (MHVPE) technique utilizing the GaN seed-layer formed from liquid source precursor. Tris N,N-dimethyldithiocarbamato gallium(III) (Ga(mDTC)3) powder was dissolved in chloroform (CHCl3) to prepare the liquid source precursor for seed-layer formation. The developed method was found to be suitable for the epitaxial growth of GaN on Si in spite of the large mismatch in lattice constants and thermal expansion coefficients, resulting in device-quality epitaxial films with fairly smooth surface morphology. The epitaxial GaN films obtained in this study had a hexagonal structure with (0002) preferred orientation with the FWHM value of 428.6 arcsec of the (0002) GaN XRD peak. Photoluminescence spectra of GaN films exhibited a strong and sharp peak at 3.41 eV with the FWHM value of 107meV.

Crystal structure, thermal expansion and magnetic properties of Pr 2Cu 0.8Ge 3 compound

A new ternary compound, Pr 2Cu 0.8Ge 3, was synthesized and its physical property investigated. The Rietveld method was used to refine the powder X-ray diffraction data and the results show that Pr 2Cu 0.8Ge 3 crystallized in an a-ThSi 2-type tetragonal structure with the space group I4 1/ amd (No. 141), a = 0.42001(1) nm, c = 1.44147(2) nm, Z = 2 and D calc = 7.322 g cm −3. Lattice thermal expansion study was carried out by high-temperature powder X-ray diffraction technique from room temperature (293 K) to 803 K, and the calculated lattice and volume thermal expansion coefficients obtained were 1.57 × 10 −5 K −1, 1.03 × 10 −5 K −1 and 4.18 × 10 −5 K −1 for α ¯ a , α ¯ c and α ¯ V , respectively. The magnetic susceptibility of Pr 2Cu 0.8Ge 3 was found to follow the Curie–Weiss law in the 20–300 K temperature range. The effective magnetic moment ( μ eff) and paramagnetic Curie temperature ( θ p) were estimated to be 3.56(3) μ B and 17.5(2) K, respectively.

Stability of High Temperature Chemical Vapor Deposited Silicon Based Structures on Metals for Solar Conversion

Highly crystallized silicon layers were grown on metal sheets at high temperature (950 degrees C) by thermal CVD from silane. An intermediate buffer layer was mandatory to prevent interdiffusion and silicide formation but also to compensate lattice parameters and thermal expansion coefficients mismatches between metal and silicon and ideally transfer some crystalline properties (grain size, texture) from the substrate to the silicon layer. After a thermodynamic study, aluminum nitride or titanium nitride diffusion barrier layers were selected and processed by CVD. The structure and the interfaces stabilities of these silicon/nitride/metal stacks were studied by field effect gun scanning and transmission electron microscopy, X-ray diffraction, Raman and energy dispersive X-ray spectroscopy. As a result, TiN deposited by CVD appears to be an efficient material as a buffer layer between steel and silicon.

Thermal expansion studies of silicalite-2 molecular sieves of MEL (ZSM-11) topology

Thermal expansion behaviour of siliceous polymorph of MEL type molecular sieves i.e. silicalite-2 is investigated by high temperature X-ray diffraction technique in the temperature range 298–773 K. The negative lattice thermal expansion coefficients, αa = −3.252 × 10−6 and αc = −6.436 × 10−6 K−1 in the temperature range 298–773 K for silicalite-2 samples were observed. The thermal expansion behavior of silicalite-2 is anisotropic, with the relative strength of contraction along ‘c’ axis is more than that along ‘a’ and ‘b’ axes. NTE seen over a temperature range 298−773 K could be associated with transverse vibrations of bridging oxygen atoms in the structure, which results in an apparent shortening of the Si–Si nonbonding distances.

Calculation of physical properties of the cubic perovskite-type oxide [formula omitted] using the PP-PW method based on the DFT theory

Various physical properties of the cubic perovskite-type oxide BiScO 3 have been investigated using the pseudo-potential plane-wave (PP-PW) method based on the density functional theory ( DFT). The computed equilibrium lattice parameters agree well with the available theoretical data. The elastic constants and their pressure dependence are predicted using the static finite strain technique. A set of isotropic elastic parameters and related properties, namely bulk and shear moduli, Young’s modulus, Poisson’s ratio, average sound velocity and Debye temperature are numerically estimated in the framework of the Voigt–Reuss–Hill approximation for BiScO 3 polycrystalline aggregate. The analysis of the site-projected l -decomposed density of states, charge transfer and charge density shows that bonding is predominantly of ionic nature. We distinguish hybridization between Sc-d states and O - p states in the valence bonding region. Through the quasi-harmonic Debye model, in which the phononic effects are considered, the thermal effect on the lattice constant, bulk modulus, heat capacities and thermal expansion coefficient is calculated.

Models for lattice thermal expansion and thermal conductivity for ternary (A NB 2+ NC 27− N) tetrahedral semiconductors

In this paper we have presented two expressions relating the lattice thermal expansion coefficient ( α L, 10 −6 K −1) and lattice thermal conductivity ( K in m W (cm K) −1) for A IIB IVC 2 V and A IB IIIC 2 VI semiconductor compounds with the product of ionic charge ( Z A Z B Z C), melting temperature ( T m) and nearest neighbour distance ( d in Å). The lattice thermal expansion coefficient and lattice thermal conductivity of these solids exhibits a linear relationship when plotted on a log–log scale against the melting temperature, but fall on different straight lines according to the product of ionic charge of the compounds. We have applied the modified relations on these solids and found a better agreement with experimental data as compared to the values evaluated by previous researchers so far. The results for lattice thermal expansion coefficient differ from experimental values by an amount up to 5.4% for CdSiP 2 and for lattice thermal conductivity the values differ form experiment by an amount up to 3.2% for AgInTe 2.

Strong Room Temperature 505 nm Emission from Hexagonal Crack Free InGaN Thin Film on Si(111) Grown by MBE

GaN growth on Si(111) substrate typically routes to the initial high dislocation density and cracks because of the large discrepancy of lattice constant and thermal expansion coefficient between GaN and Si. This article reports the use of plasma-assisted molecular beam epitaxy (MBE) to grow crack-free InGaN on Si(111) substrate using an optimized GaN/AlN buffer layer. X-ray diffraction revealed that monocrystalline InGaN was obtained. To assess the morphology of the sample, scanning electron microscopy (SEM) and atomic force microscopy (AFM) have been employed. A layer-cracking problem was not observed in our samples. The root–mean–square (rms) roughness value of the surfaces sample is 10.34 nm on a 10 μm × 10 μm scan area. The film is then characterized by photoluminescence (PL) spectroscopy. PL measurement exhibits a sharp and intense band edge emission of InGaN with yellow luminescence (YL), red luminescence (RL) and near infrared luminescence (NIRL) peaks. To date, no approaches have been reported that YL is observed in samples grown by MBE.

Influence of disorder-to-order transition on lattice thermal expansion and oxide ion conductivity in (CaxGd1−x)2(Zr1−xMx)2O7 pyrochlore solid solutions

The effect of simultaneous substitutions of Ca at A site and Nb or Ta at B site in pyrochlore-type solid solutions: (Ca(x)Gd(1-x))(2)(Zr(1-x)M(x))(2)O(7) (x = 0.1, 0.2, 0.3, 0.4, 0.5 and M = Nb or Ta) were studied by powder X-ray diffraction (XRD), FT NIR Raman spectroscopic techniques and transmission electron microscopy. The solid solutions were prepared by the conventional high-temperature ceramic route. The XRD results and Rietveld analysis revealed that the defect fluorite structure of Gd(2)Zr(2)O(7) progressively changed to a more ordered pyrochlore phase by simultaneous substitutions at A and B sites. Raman spectroscopy reveals the progressive ordering in the anion sublattice with simultaneous doping. High-resolution images and selected-area electron diffraction patterns obtained from TEM confirms the XRD and Raman spectroscopic results. High-temperature XRD studies show that the lattice expansion coefficient in these pyrochlore oxides is of the order of 10(-6) K(-1). Lattice thermal expansion coefficient increases with increase of disorder in pyrochlore oxides, and hence the variation of thermal expansion coefficient with composition is also a good indicator of disordering in pyrochlore-type oxides. The ionic conducting properties of the samples were characterised by impedance spectroscopy, and it was found that Nb-doped compositions show a considerable change in conductivity near the phase boundary of disordered pyrochlore and defect fluorite phases.

Large SiO 2-substituted GeO 2 single-crystals with the α-quartz structure

Colorless, transparent SiO 2-substituted GeO 2 single-crystals with the α-quartz-like structure have been grown by spontaneous nucleation using the slow-cooling method from K 2O–3MoO 3 flux. The cell parameter values as well as the semi-quantitative chemical analyses confirm the presence of Si-content in the as-grown crystals. Their thermal behavior was studied by means of differential scanning calorimetry and high-temperature X-ray diffraction. The high stability of the α-quartz-like structure as a function of temperature was confirmed up to 1000 °C. The structural parameter data have been used to estimate the lattice and volume thermal expansion coefficients from room temperature up to 1000 °C.

Thermal Contraction of Electrodeposited Bi/BiSb Superlattice Nanowires

The lattice parameter of Bi/BiSb superlattice nanowire (SLNW) has been measured using in situ high-temperature X-ray diffraction method. The single crystalline Bi/BiSb SLNW arrays with different bilayer thicknesses have been fabricated within the porous anodic alumina membranes (AAMs) by a charge-controlled pulse electrodeposition. Different temperature dependences of the lattice parameter and thermal expansion coefficient were found for the SLNWs. It was found that the thermal expansion coefficient of the SLNWs with a large bilayer thickness has weak temperature dependence, and the interface stress and defect are the main factors responsible for the thermal contraction of the SLNWs.