Crystallography of TiSi2 (C54) epitaxy on (111)Si and (001)Si surfaces

Following the success in understanding the textures in TiSi₂ (C49) epitaxy on (001)_Si surface using the edge-to-edge matching model that was originally developed for predicting the crystallographic features of diffusion-controlled phase transformations in solids, the present work applies this model to understand the in-plane texture in TiSi₂ (C54) thin films on Si single crystal surfaces and to explain why its epitaxial growth is more favoured on (111)_Si than on (001)_Si. Based on the actual atomic spacing along the matching directions across the interface between the thin films and the substrate, the model predicts most of the experimentally observed orientation relationships (ORs) and that the preferred order among the different systems is C49/(001)_Si system > C54/(111)_Si system > C49/(111)_Si system ≅ C54/(001)_Si system. The model has strong potential to be used to develop new thin film materials.     

High pressure/low temperature sintering of nanocrystalline alumina

Bulknanocrystalline α-Al₂O₃samples with a relative density >98% and a grain size < 50 nm have been produced by high pressure/low temperature sintering, using a toroidal-type high pressure apparatus. Nanocrystalline (n-) alumina powder with metastable γphase was used as the starting material. During sintering, the γphase transforms to αphase. The transformation temperature decreases from ~1075 °C at ambient pressure to about 460 °C at 8 GPa. Grain growth is limited by the low sintering temperature, and a multiplicity of nucleation events in the parent γ phase at very high pressure creates a nanoscale α grain size. The average grain size of the α-Al₂O₃ increases from 18 nm in the original powder to only about 49 nm in the sintered compact (98.2% dense). In addition, we found that high pressure could increase the nucleation rate while reducing the growth rate of the transformed α phase so that its grain size decreased with sintering pressure under the same sintering temperature and time. Due to its high surface area, n-Al₂O₃ powder readily absorbs chemical species from the environment. Alumina hydrates, formed by the reaction of Al₂O₃ with chemisorbed OH⁻ species during sintering, had a profound influence on sintering and phase transformation behaviors of n-Al₂O₃. To control grain size of the transformed α phase, it is essential to eliminate the hydrates before sintering.     

Enhanced formation of low-resistivity TiSi2 contacts for deep submicron devices

Low resistivity C54-TiSi₂ is currently the most commonly used silicide for metal contacts in ultralarge scale integrated circuits devices. In the present paper, we review recent results of investigations on the effects of stress and high temperature sputtering on the formation of C54-TiSi₂. Enhanced formation of C54-TiSi₂ on (001)Si by tensile stress and high temperature sputtering is correlated to the growth of thicker amorphous interlayer at the Ti/(001)Si interface. The enhanced transformation is attributed to the presence of higher density of silicide crystallites, which serve as the nucleation sites for the C49-TiSi₂, in the amorphous layer. As a result, the average grain size of C49-TiSi₂ is smaller which leads to lower C49- to C54-TiSi₂ transformation temperature.     

Growth of Fe silicides on Si(111) surfaces from bcc-Fe to fine-polycrystal and β-FeSi2 phases: Structure and magnetism

We studied the structure and magnetism of the bcc-Fe(111) phase to the fine-polycrystal and β-FeSi₂ phases in solid phase epitaxial growth of iron silicides on Si(111) surfaces. In the polycrystal phase displaying Debye rings in reflection high-energy electron diffraction, we found randomly-oriented bcc-Fe fine-crystals to be responsible for the phase, and obtained ferromagnetic hysteresis but the magnetization was small. In the polycrystal phase, other irons and silicides were sometimes included: epitaxially-grown bcc-Fe crystals with preferential orientation in different directions, and β-FeSi₂(101) or (110) layers. We show a growth scheme with annealing for structure and magnetism of these phases.     

Investigation of stress behaviors and mechanism of void formation in sputtered TiSix films

We have investigated the stress behaviors and a mechanism of void formation in TiSi_x films during annealing. TiSi_x thin films were prepared by DC magnetron sputtering using a TiSi_2.1 target in the substrate temperature range of 200–500 °C. The as-deposited TiSi_x films at low substrate temperature (<300 °C) have an amorphous structure with low stress of 1×10⁸ dynes/cm². When the substrate temperature increases to 500 °C, the as-deposited TiSi_x film has a mixture of C49 and C54 TiSi₂ phase with stress of 8×10⁹ dynes/cm². No void was observed in the as-deposited TiSi_x film. Amorphous TiSi_x film transforms to C54 TiSi₂ phase with a random orientation of (311) and (040) after annealing at 750 °C. The C49 and C54 TiSi₂ mixture phase transforms to (040) preferred C54 TiSi₂ phase after annealing over 650 °C. By increasing substrate temperature, the transformation temperature for C54 TiSi₂ can be reduced, resulting in relieved stress of TiSi₂ film. The easy nucleation of the C54 phase was attributed to an avoidance of amorphous TiSi_x phase. We found that amorphous TiSi_x→C54 TiSi₂ transformation caused higher tensile stress of 2×10¹⁰ dynes/cm², resulting in more voids in the films, than C49→C54 transformation. It was observed that void formation was increased with thermal treatment. The high tensile stress caused by volume decreases in the silicide must be relieved to retard voids and cracks during C54 TiSi₂ formation.     

Study of the influence of Nb content and sintering temperature on TiO2 sensing films

Nb-doped TiO₂ nanopowders have been synthesized with a wide range of Nb contents (0-10 at.%) and of calcination temperatures (600-900 °C). The materials have been structurally characterized by means of X-ray diffraction, Raman spectroscopy and transmission electron microscopy. The niobium introduction retards the anatase-to-rutile transformation and hinders grain growth mechanisms. The electrical behavior studied for CO and ethanol gases showed that the response to ethanol is slightly diminished by the incorporation of Nb atoms and not affected by the structure modification. However, the response to CO is modified by structure changes such as the anatase/rutile transformation, grain size and Nb segregation. For the samples treated at 700 °C, the best response to CO is achieved for 4 at.% of Nb.     

Synthesis of neodymium-doped yttrium aluminum garnet (YAG) nanocrystalline powders leading to transparent ceramics

We report on the synthesis of Nd:YAG nanoparticles by the coprecipitation technique and the optimum conditions for the processing of transparent ceramics. The powders prepared by the coprecipitation technique display significantly less agglomeration of crystallites, indicating higher sinterability. The crystallite size dependence on the calcinations temperature suggests the optimum temperature of 1100 °C, at which phase purity of the YAG nanopowder with the highest sinterability can be obtained. We have demonstrated that the optimum temperature for the vacuum-sintering is about 1785 °C for the uniaxially pressed samples to obtain transparent ceramic with uniform particle size of about 5-7 μm, and above this temperature, enormously large grain growth occurs, and facilitates large pores formation in the intergrain region that makes the ceramic fragile at the grain boundary.     

Effect of cooling rate on grain size of ferrite in a carbon steel

Abstract

Ferrite grain refinement by accelerated cooling has been studied in a carbon steel. The size of ferrite grains d_α formed by continuous cooling transformation from polygonal austenite has been measured as a function of cooling rate and austenite grain size d_γ. In the cooling rate range studied (q= 0·05–5 K s^?1), d_α was found to be proportional to q^?0·26d_γ^0·46. The mechanism of grain refinement by accelerated cooling is discussed, and it is shown that this occurs in the transformation where the ratio of nucleation to growth rate increases with a decrease in temperature. The austenite grain size dependence of ferrite grain size is shown to become progressively large as the nucleation mode changes from homogeneous to grain surface to edge to corner. A theoretical estimation of ferrite grain size formed by continuous cooling transformation was attempted on the basis of nucleation and growth rates. In the alloy studied, ferrite grain size was theoretically estimated to be proportional to q^?0·17d_γ^0·33. This was in close agreement with the dependence obtained in the present experiment.

MST/466     

Characterization of the prior particle boundaries in a powder metallurgy Ti_2AlNb alloy

Ti_2 AlNb-based alloy powder metallurgy(PM) compacts were prepared via hot isostatic pressing(HIP)under relatively low temperature(920 and 980℃) and at certain pressure(130 MPa).The microstructure,composition and orientation of B2,α_2 and O phases in the compacts were characterized and analyzed with an aim to investigate the effect of unsuitable HIPping parameters on the appearance of prior particle boundary(PPB),which seriously affects the mechanical properties of the alloy.The results show that more α_2 phase is the characteristics of the PPB in Ti_2AlNb-based alloy when HIPped at relatively low temperature.Increasing HIPping temperature to the upper part of the two-phase region can effectively inhibit the formation of PPB.Electron backscatter diffraction measurements show the specific orientation relationship between phases,which helps us understand the origin of a2 and O phase and the corresponding transformation path.The HIPping at a higher temperature can weaken the micro-texture intensity of the α_2 and O phase due to the increase of misorientation in B2 phase.The α_2 phase at cell wall keeps the Burgers orientation relationship(BOR) with the grain on one side,and does not satisfy the BOR with the other.It is found that some O phase variants inside the cell HIPped at 980℃ can only maintain α_2-O OR with α_2 owing to the α_2→O phase transformation forming the O phase,while these O variants deviate from B2-O OR with B2 phase.     

Preparation of novel calcia-stabilized TZP ceramics

The preparation of tetragonal zirconia polycrystal (TZP) ceramics stabilized by the addition of calcia is presented. These novel ceramics were obtained by means of a fast-firing treatment applied to compacts of nanocrystalline powders with a composition of ZrO₂ — 4 mol% CaO. Powders were synthesized by a nitrate-citrate gel-combustion process and they exhibited the tetragonal phase at room temperature due to their small crystallite size (12–13 nm). These powders were compacted by uniaxial pressing and fired at 1400–1500 °C for 3–5 min, obtaining fine-grained, dense ceramics, which also retained the metastable tetragonal phase. Longer heat treatments increased the average grain size leading to large amounts of the monoclinic phase. It was also found that the critical grain size for the tetragonal-to-monoclinic transformation is about 150 nm for this composition. This small value explains the necessity of a fast-firing treatment of the samples, allowing densification but avoiding a significant grain growth.     

Epitaxial growth of Cr2O3 thin film on Al2O3 (0001) substrate by radio frequency magnetron sputtering combined with rapid-thermal annealing

We prepared epitaxially grown Cr₂O₃ thin films on Al₂O₃ (0001) substrate by using radio frequency magnetron sputtering at room temperature followed by a rapid-thermal annealing process. It is shown that the phase and the crystallinity of as-grown samples depend on the flow ratios of the working gas (Ar/O₂). X-ray diffraction (XRD) results show that oxygen-rich CrO₃ phase having preferred orientation is formed when sputter-grown at room temperature under the working gas (Ar/O₂) flow ratios of 9:1 and 7:3. XRD Φ-scanning results exhibit that annealing causes the oxygen-rich CrO₃ phase to transform into epitaxial Cr₂O₃ phase, which is confirmed by X-ray photoemission spectroscopy. The samples grown at the gas flow ratio of 9:1 exhibit very smooth surfaces with root mean square roughness of 0.1-0.3 nm.     

Synthesis,structure and electrical studies of praseodymium doped barium zirconium titanate

Praseodymium (Pr) doped barium zirconium titanate with nominal composition (Ba_1−xPr_x)(Zr_0.52Ti_0.48)O₃ (x = 0.1 and 0.2) were synthesized using solid state reaction method. X-ray analysis conform the formation of cubic phase Pr-doped barium zirconium titanate along with minor pyrochloric phase. The increase in grain size after primary investigation reveals the influence of Pr ions on the domain structure and its microstructure. In order to correlate the effect of the chemical composition with the conduction mechanism, different AC electrical parameters have been addressed. The frequency dependant tangent loss of the sample was less for both the ceramics. The temperature dependence results show that the dielectric parameters and resistivity increases as Pr-content in the ceramic increases; this is attributed to the grain size and dipole dynamics. Complex impedance (Z*) plots show frequency dependent behavior as the response for the grain resistance mechanisms. This mechanism has been represented by an equivalent circuit. The temperature dependence of the electrical conductivity and Seebeck coefficient showed n-type non-degenerated semiconductor in the measured temperature range. The temperature dependent conductivity measurement suggests a novel negative temperature coefficient of resistance behavior of the samples. Furthermore, the frequency dependent conductivity plot shows increasing behavior.     

Grain coarsening mechanism of Cu thin films by rapid annealing

Cu thin films have been produced by an electroplating method using nominal 9N anode and nominal 6N CuSO₄·5H₂O electrolyte. Film samples were heat-treated by two procedures: conventional isothermal annealing in hydrogen atmosphere (abbreviated as H₂ annealing) and rapid thermal annealing with an infrared lamp (abbreviated as RTA). After heat treatment, the average grain diameters and the grain orientation distributions were examined by electron backscattering pattern analysis. The RTA samples (400 °C for 5 min) have a larger average grain diameter, more uniform grain distribution and higher ratio of (111) orientation than H₂ annealed samples (400 °C for 30 min). This means that RTA can produce films with coarser and more uniformly distributed grains than H₂ annealing within a short time, i.e. only a few minutes. To clarify the grain coarsening mechanism, grain growth by RTA was simulated using the phase field method. The simulated grain diameter reaches its maximum at a heating rate which is the same order as that in the actual RTA experiment. The maximum grain diameter is larger than that obtained by H₂ annealing with the same annealing time at the isothermal stage as in RTA. The distribution of the misorientation was analyzed which led to a proposed grain growth model for the RTA method.     

Titanium dioxide thin films for high temperature gas sensors

Titanium dioxide (TiO₂) thin film gas sensors were fabricated via the sol-gel method from a starting solution of titanium isopropoxide dissolved in methoxyethanol. Spin coating was used to deposit the sol on electroded aluminum oxide (Al₂O₃) substrates forming a film 1 μm thick. The influence of crystallization temperature and operating temperature on crystalline phase, grain size, electronic conduction activation energy, and gas sensing response toward carbon monoxide (CO) and methane (CH₄) was studied. Pure anatase phase was found with crystallization temperatures up to 800 °C, however, rutile began to form by 900 °C. Grain size increased with increasing calcination temperature. Activation energy was dependent on crystallite size and phase. Sensing response toward CO and CH₄ was dependent on both calcination and operating temperatures. Films crystallized at 650 °C and operated at 450 °C showed the best selectivity toward CO.     

Controlled grain orientation at the surface of Bi2Sr2CaCu2Ox superconducting glass-ceramics

Bi₂Sr₂CaCu₂O_x glass with Cu⁺/(Cu⁺ + Cu²⁺) = 0.76 was prepared by using a conventional melt-quenching method, and crystalline phases and grain orientations at the surface of superconducting glass-ceramics obtained by annealing at various atmospheres were examined. The grain orientation of the Bi₂Sr₂CaCu₂O_x phase (low-T _c phase) at the surface was severely affected by oxygen partial pressure in annealing. The favourable grain orientation, in which the plate-like grains of the low-T _c phase are oriented parallel to the surface plane, was first established in the samples obtained through a newly developed two-step annealing method: first annealing at 780°C in oxygen and second annealing at above 750°C in nitrogen. It was concluded that the favourable grain orientation of the low-T _c phase at the surface occurred due to the formation of a liquid phase in nitrogen.     

Preparation and Characterization of Nanometer Grained High Temperature Superconducting High-Quality Epitaxial Bi-2223 Thin Films Grown by DC Sputtering

Bi₂Sr₂Ca₂Cu₃O_10+?? HTSC epitaxial thin films with thickness in the order of 6.0?nm were prepared onto (100) aligned SrTiO₃ single-crystal substrates by DC sputtering from stoichiometric targets. As-grown samples were characterized by X-ray diffraction, AC-susceptibility and scanning electron microscopy. X-ray diffraction patterns show that all obtained superconducting thin films were c-axis oriented with a Bi-2223 phase. All reflections (except the substrate ones) can be assigned to the (00l) reflections of the film material (h=k=0, l??0), indicating that the films were grown preferentially with the c-axis normal to the film plane. In order to investigate the crystal quality of these Bi-2223 films, the rocking curves of the (0012) peaks were explored by ??-scans. The rocking curve of the (0012) reflection had a full width at half maximum (FWHM) of 0.30??. This demonstrates that our prepared Bi-2223 thin films have good crystalline quality and high degree of c-axis orientation. The grain size has well known important effects in the magnetic, optical, and electrical properties of metals and alloys. High temperature superconducting thin films, obtained in this work, have nanometer grain size. The mean size of the grains of the samples were determined by X-ray diffraction (XRD) and found to be in the order of 34.8?nm. The superconducting transitions temperature of several Bi-2223 samples is about 103?K. Surface morphology of the films and chemical composition were studied using scanning electron microscopy (SEM) and energy dispersive X-ray microanalysis (EDX).     

Polymeric citrate precursor route to the synthesis of nano-sized barium lead titanates

Monophasic oxides of the formula, Ba_1−xPb_xTiO₃ (0≤x≤1.0) were obtained at temperatures close to 500 °C from polymeric precursors formed using citric acid and ethylene glycol. These oxides were found to be tetragonal for all values of x, the distortion increasing with Pb content. Differential scanning calorimetry (DSC) shows that the decomposition of the precursor to the oxides varies with composition and the decomposition temperature is highest (530 °C) for the x=0.5 composition. X-ray line broadening studies of 900 °C sintered samples show grain size of 50-54 nm in all compositions. TEM studies show agglomerated grains of the size, 40-60 nm. The dielectric constant (ε) decreases with lead-doping having a value 70 for PbTiO₃. The dielectric loss (D) for different compositions varied between 0.005 and 0.01 at 100 kHz. The dielectric constant and loss show excellent frequency stability.     

Improvement of mechanical properties of cast pure titanium by repeated heat treatment

ABSTRACT

Pure titanium components fabricated by casting have a coarse grain microstructure. To improve the mechanical strength of pure titanium components by refining the grain size, the cast samples were repeatedly heat-treated. During the heat treatment, the titanium samples were repeatedly heated above the alpha-to-beta (α–β) transition temperature and cooled to room temperature to undergo phase transformation. The heating cycle was performed 1, 3, 5, and 7 times. As the number of heating cycles increased, the grain size decreased. The tensile strength was 267.9?MPa in the as-cast sample and improved to 343.4?MPa after 7 heat-treatment cycles owing to the grain size refinement, while the elongation was maintained during the heat treatment.

This paper is part of a thematic issue on Titanium.     

Microstructure and lattice bending in polycrystalline laser-crystallized silicon thin films for photovoltaic applications

Grain size, grain boundary population, orientation distribution and lattice defects of polycrystalline silicon thin films are investigated by electron backscatter diffraction (EBSD). The silicon thin films are produced by a combination of diode laser melt-mediated crystallization of an amorphous silicon seed layer and epitaxial thickening of the seed layer by solid phase epitaxy (SPE). The combined laser-SPE process delivers grains exceeding several 10 μm of width and far larger than 100 μm in length. Strong lattice rotations between 10 and 50° from one side of the grain to the other are observed within the larger grains of the film. The misorientation axes are well aligned with the direction of movement of the laser. The intragranular misorientation is associated both with geometrically necessary dislocations and low angle boundaries, which can serve as recombination centres for electron-hole pairs. Since the lateral grain size is up to two orders of magnitude larger than the film thickness, the high dislocation density could become an important factor reducing the solar cell performance.     

Deposition of microcrystalline silicon in an integrated distributed electron cyclotron resonance PECVD reactor

The deposition of μc-Si in a low pressure high density plasma reactor is studied. Films were deposited either from pure silane or from the mixture of SiH₄ and H₂ onto glass substrates and deposition kinetics followed with kinetic phase modulated ellipsometry Growth rates of up to 0.8 nm/s were achieved with good quality material. Crystalline fraction shows a strong dependence on process pressure and exceeds 80% for samples grown at optimal conditions. It is found that hydrogen dilution is not needed for integrated distributed electron cyclotron resonance (IDECR) discharge to produce crystallized material. The grain size measured with X-ray diffraction was found to be between 10 and 15 nm and of single (111) orientation. Both ellipsometric data and Raman analysis show a strong dependence of crystallinity or hydrogen residence time in the reactor.