Characteristic crystal growth from amorphous WO3 film by vacuum heating

Selective growth of WO₂, W and WO_3−x crystals from amorphous WO₃ film by vacuum heating at 400–900°C was clarified. The grown WO_3−x crystals were incommensurate structure based on crystallographic share structure. The growth process of WO₂ crystal in the amorphous film was directly observed at high temperature in the electron microscope. The growth front of the WO₂ crystal consumes WO₃ microcrystallites with various orientations. The growth speed of the WO₂ depended on WO₃ microcrystallites orientation. The origin of the wavy growth front of WO₂ was due to an orientation dependence of the WO₃ microcrystallites.     

Crystallization of amorphous Ni35Zr65 and Fe40Ni40P14B6 under proton irradiation

Two amorphous alloys, Ni₃₅Zr₆₅ and Fe₄₀Ni₄₀P₁₄B₆, were irradiated using 400 keV protons at several temperatures below the crystallization temperature, T_x, to peak doses in the neighborhood of 3.5 to 4.5 dpa. Irradiation at 250°C resulted in the crystallization of both alloys, which were examined by transmission electron microscopy of samples electrolytically polished to various distances from the irradiated surface to study the effect of dose. Samples masked from the proton beam remained amorphous during irradiation. In the Ni₃₅Zr₆₅ alloy crystallization of the equilibrium phases propagated throughout the entire sample, while the in the Fe₄₀Ni₄₀P₁₄B₆ alloy crystallization was observed only in those parts of the samples lying within the proton range. Neither alloy crystallized during irradiation at 100°C. In both these alloys the amorphous phase is therefore evidently stable at irradiation temperatures below approximately 0.6 T_x. An examination of the literature on irradiation damage of binary alloys and intermetallic compounds suggests that there is a tendency for initially amorphous alloys to remain amorphous at irradiation temperatures, T_irr < 0.3 T_L, where T_L (≈T_x) is the “melting” temperature (either a eutectic, peritectic or congruent melting temperature). Also, these same alloys, even when they are initially crystalline, transform to the amorphous state during irradiation at T < 0.3 T_L. Some other crystalline alloys have also been shown to transform to the amorphous state at T_irr < 0.3 T_L even though they have never been prepared in this condition by rapid quenching techniques. The temperature 0.3 T_L appears to be a lower limit, however, since the crystalline to amorphous transformation occurs in many of these alloys at temperatures greater than 0.3 T_L. It is suggested, by analogy with results on void formation in irradiated metals, that this low temperature limit is related to the low mobility of vacancies in these materials, although the mechanism of crystallization, or conversely amorphization, is not fully understood.     

Structure and thermal stability of MOCVD ZrO2 films on Si (1 0 0)

The structure and thermal stability of ZrO₂ films grown on Si (1 0 0) substrates by metalorganic chemical vapor deposition have been studied by high-resolution transmission electron microscopy, selected area electron diffraction and X-ray energy dispersive spectroscopy. As-deposited films consist of tetragonal ZrO₂ nanocrystallites and an amorphous Zr silicate interfacial layer. After annealing at 850°C, some monoclinic phase is formed, and the grain size is increased. Annealing a 6 nm thick film at 850°C in O₂ revealed that the growth of the interfacial layer is at the expense of the ZrO₂ layer. In a 3.0 nm thick Zr silicate interfacial layer, there is a 0.9 nm Zr-free SiO₂ region right above the Si substrate. These observations suggest that oxygen reacted with the Si substrate to grow SiO₂, and SiO₂ reacted with ZrO₂ to form a Zr silicate interfacial layer during the deposition and annealing. Oxygen diffusion through the tetragonal ZrO₂ phase was found to be relatively easier than through the monoclinic phase.     

Correlation between interfacial structure and c-axis-orientation of LiNbO3 films grown on Si and SiO2 by electron cyclotron resonance plasma sputtering

Thin films of crystalline lithium niobate (LN) grown on Si(1 0 0) and SiO₂ substrates by electron cyclotron resonance plasma sputtering exhibit distinct interfacial structures that strongly affect the orientation of respective films. Growth at 460–600 °C on the Si(1 0 0) surface produced columnar domains of LiNbO₃ with well-oriented c-axes, i.e., normal to the surface. When the SiO₂ substrate was similarly exposed to plasma at temperatures above 500 °C, however, increased diffusion of Li and Nb atoms into the SiO₂ film was seen and this led to an LN–SiO₂ alloy interface in which crystal-axis orientations were randomized. This problem was solved by solid-phase crystallization of the deposited film of amorphous LN; the degree of c-axis orientation was then immune to the choice of substrate material.     

Ag diffusion in amorphous As50Se50 films studied by XPS

X-ray photoelectron spectroscopy and depth profile analysis were used to investigate the X-ray-induced silver photodiffusion into an amorphous As₅₀Se₅₀ thin film. At the initial stages of irradiation an induction period was observed while core level spectra analysis revealed the existence of a mixed As–Se–Ag interlayer between the metal and the chalcogenide matrix. It was found that during the induction period this interlayer is enriched in silver and the existing As–Se–Ag intermediate species are transformed to Ag–Se–Ag that form the metal source for the effective silver photodiffusion. With further irradiation photodiffusion proceeds by the disruption of Ag–Se bonds and the recombination of As atoms with Se to stable As–Se units. Ultimately, silver concentration reaches a plateau when the diffusion stops. A separated Ag₂Se phase on the film’s surface is identified at this stage. Depth profile analysis shows that silver has been homogenously diffused into the chalcogenide matrix and the Ag₂Se phase exists only at the top surface layers probably in the form of quasi-crystalline clusters that prohibit further Ag diffusion.     

Crystallization kinetics of (Ni0.75Fe0.25)78Si10B12 amorphous alloy

Amorphous ribbon specimen of (Ni_0.75Fe_0.25)₇₈Si₁₀B₁₂ has been prepared by a single roller melt-spinning technique in the air atmosphere. The crystallization kinetics of the alloy has been investigated using different thermal analysis by means of continuous heating and isothermal heating. The activation energy of the alloy has been calculated by using Kissinger plot method and Ozawa plot method based on differential thermal analysis data, respectively. The products of crystallization have been analyzed by X-ray diffraction. A single phase of γ-(Fe, Ni) solid solution with grain size of about 10.3 and 18.5 nm precipitates in the amorphous matrix after annealing at temperatures 715 and 745 K, respectively. The crystallized phases are γ-(Fe, Ni) solid solution, Fe₂Si, Ni₂Si, Fe₃B and unidentified phase after annealing at 765 K. The details of nucleation and growth during the isothermal crystallization are expatiated in terms of local Avrami exponent and local activation energy.     

Preparation and characterization of fiber-textured SrAl2O4:Eu films grown using a homo-buffer layer

This paper describes the preparation and characterization of fiber-textured SrAl₂O₄:Eu films on a quartz glass substrate using a homo-buffer layer. The effect of the buffer layer on the crystallinity and adhesion was investigated by cross-section transmission electron microscopy and X-ray diffraction (XRD). The results show that the prepared film was not only well crystallized but also highly textured. The preferred orientation of this textured film was confirmed to be (0 3 1) by pole figure measurement. In addition, this film exhibits excellent optical transparency, with an average transmittance of more than 80% in the visible range.     

Electron microscopy studies of epitaxial MgB2 superconducting thin films grown by in situ reactive evaporation

The morphology and chemistry of epitaxial MgB₂ thin films grown using reactive Mg evaporation on different substrates have been characterized by transmission electron microscopy methods. For polycrystalline alumina and sapphire substrates with different surface planes, an MgO transition layer was found at the interface region. No such layer was present for films grown on MgO and 4-H SiC substrates, and none of the MgB₂ films had any detectable oxygen incorporation nor MgO inclusions. High-resolution electron microscopy revealed that the growth orientation of the MgB₂ thin films was closely related to the substrate orientation and the nature of the intermediary layer. Electrical measurements showed that very low resistivities (several μΩ cm at 300 K) and high superconducting transition temperatures (38 to 40 K) could be achieved. The correlation of electrical properties with film microstructure is briefly discussed.     

Growth and characterization of KY(WO4)2 crystals

Single-crystal growth of KY(WO₄)₂ (KYW) by top-seeded solution growth technique has been investigated. The effects of seed orientation, temperature gradient experienced by the growing crystal and rate of crystallization on crystal quality are reported. The best results are obtained when the growth is seeded along the 0 1 0 direction. Minute deviations from this growth direction are found to be detrimental to crystal perfection. The differential thermal analysis shows that the amount of super-cooling required for dissolution and crystallization of KYW in the flux is only 5° and this promotes an easy formation of tiny crystallites in the solution. Consequently, the crystal rotation and the solution cooling rates are found to have pronounced effects on the growth of KYW crystal.     

Complex crystallization mode of amorphous/nanocrystalline composite Al86Ni2Co5.8Gd5.7Si0.5

Kinetics of the first crystallization stage of Al₈₆Ni₂Co_5.8Gd_5.7Si_0.5 amorphous alloy and structure of the partially crystallized specimens were investigated by measurements of electrical resistance, X-ray diffraction and transmission electron microscopy. The presence of Al nanocrystals and eutectic colonies consisted of mutually oriented crystals of Al and metastable phase was found. The transient nucleation and slowing-down diffusion-limited growth and the interface-controlled growth of the quenched-in nuclei were identified as mechanisms of formation of the Al nanocrystals and eutectic colonies, respectively.     

Surface and interfacial structure of epitaxial SrTiO3 thin films on (0 0 1) Si grown by molecular beam epitaxy

Effects of relaxation of interfacial misfit strain and non-stoichiometry on surface morphology and surface and interfacial structures of epitaxial SrTiO₃ (STO) thin films on (0 0 1) Si during initial growth by molecular beam epitaxy (MBE) were investigated. In situ reflection high-energy electron diffraction (RHEED) in combination with X-ray diffraction (XRD), atomic force microscopy (AFM), X-ray photoelectron spectrometry (XPS) and transmission electron microscopy (TEM) techniques were employed. Relaxation of the interfacial misfit strain between STO and Si as measured by in situ RHEED indicates initial growth is not pseudomorphic, and the interfacial misfit strain is relaxed during and immediately after the first monolayer (ML) deposition. The interfacial strain up to 15 ML results from thermal mismatch strain rather than lattice mismatch strain. Stoichiometry of STO affects not only surface morphology but interfacial structure. We have identified a nanoscale Sr₄Ti₃O₁₀ second phase at the STO/Si interface in a Sr-rich film.     

Three-dimensional Bi2Se3 nanopattern films self-assembled with ultrathin nanosheets on the surface of Se nanotubes

Regular three-dimensional (3D) rose-like Bi₂Se₃ nanopattern film is fabricated through a simple chemical route. This nanopattern film is self-assembled with ultrathin Bi₂Se₃ nanosheets having thicknesses of less than 8 nm. The Bi₂Se₃ nanosheets were formed on the surface of Se nanotubes, and the Se nanotubes were used as both the Se resource and the substrate to support the growth of the Bi₂Se₃ nanopattern film. Since several length scales are involved in this confined 3D structure, which are developed at different time, during the formation of the micro-structure, a careful control of these length scales is expected to provide new opportunities for engineering the boundary scattering of phonons of different wavelengths and developing new thermoelectric materials with novel properties.     

Laser-induced structural transformation of AsxSe1-x thin amorphous films

The changes in the optical and holographic recording properties of amorphous As_xSe_1-x films with 0 ≤ x ≤ 0.2 under laser irradiation have been measured. The dependence of the transmissivity and diffraction efficiency on the irradiation energy density shows two qualitatively different regions. Below an energy density threshold, E_th, only small changes in the local structure of the system can be detected. In this low-E region, the transmissivity varies reversibly with exposure. Qualitative explanation of the observed behavior may be based on associating such with alteration of deep defect states. Above E_th, the changes were attributed to crystallization transformation. The Raman spectra reveal the progressive transformation of the system from amorphous into the crystalline phase under laser irradiation.     

Growth of epitaxial TmFeCuO4 thin films by pulsed laser deposition

Epitaxial thin films of TmFeCuO₄ with a two-dimensional triangular lattice structure were successfully grown on yttria-stabilized-zirconia substrates by pulsed laser deposition and ex situ annealing in air. The films as-deposited below 500 °C showed no TmFeCuO₄ phase and the subsequent annealing resulted in the decomposition of film components. On the other hand, as-grown films deposited at 800 °C showed an amorphous nature. Thermal annealing converted the amorphous films into highly (0 0 1)-oriented epitaxial films. The results of scanning electron microscopic analysis suggest that the crystal growth process during thermal annealing is dominated by the regrowth of non-uniformly shaped islands to the distinct uniform islands of hexagonal base.     

Improved growth of PbI2 single crystals

The phase equilibrium and the crystallization process of lead iodide (PbI₂) melt have been primarily investigated according to the lead–iodine phase diagram. It is found that the iodine evaporation and the segregated lead deposition are the two important factors that affect the PbI₂ crystal quality. The new method of Pulling U-type quartz growth ampoule has been made to impede the decomposition of PbI₂ and the vaporization and condensation of iodine. An orange and translucent PbI₂ single crystal of large size was obtained by the improved growth method, i.e. U-type ampoule pulling. Resistivity of the as-grown crystal is up to 4×10¹¹ Ω cm, and IR transmission is up to 45% in the region from 7800 to 450 cm⁻¹. Therefore, the improved growth method is a promising convenient new method for the growth of high quality PbI₂ crystals.     

Crystallization behaviour of an amorphous Ti50Be40Zr10 alloy

The crystallization behaviour of an amorphous Ti₅₀Be₄₀Zr₁₀ alloy during a continuous heating mode from room temperature to 973 K and isothermal annealing at temperatures above the glass transition temperature is examined by differential scanning calorimetry (DSC), small-angle X-ray scattering (SAXS) measurement and large-angle X-ray diffractometry (LAXD). DSC indicated two well-defined exothermic peaks, a slight shoulder at the higher temperature side of the second peak and a small heat evolution at higher temperature. The Kissinger plot for the first and the second peak gives a straight line, from which the apparent activation energy is estimated to be 269 and 413 kJ/mol respectively; the enthalpies for the first and second crystallization process are 1.04 kJ/mol and 4.39 kJ/mol for a heating rate of 20 K/min. The SAXS intensities increase sharply after annealing at about 673 K (corresponding to the first peak in the DSC curves); the scattering is due to the formation of fine-scale crystalline Ti particles by the LAXD. The size of the particles does not change significantly while the number of scattering particles increases, indicating that the reaction is almost nucleation controlled and the growth is very limited. Another crystalline phase would appear in addition to the Ti particles on annealing at temperatures above about 753 K (corresponding to the second peak in the DSC curves), where the SAXS intensities decrease compared with those for only the first-stage of crystallization. The crystalline phase might be a metastable cubic phase with the lattice parameter a₀?0.2994 nm.The sequence in the crystallization of the initial non-crystalline material is amorphous → microcrystalline (MS I) → crystalline (MS II; S III), although the structure of crystalline phase in the final stage (S III) was not identified. It is also likely that cold-rolling does not have a perceptible effect on the crystallization behaviour of the present amorphous alloy.     

Morphology and crystal phase evolution of doctor-blade coated CuInSe2 thin films

CuInSe₂ (CIS) chalcopyrite thin films were prepared using a low-cost, non-vacuum doctor-blade coating and the thermal annealing method. An acetone-based precursor solution containing copper chloride, indium chloride, selenium chloride, and an organic binder was deposited onto a Mo-sputtered soda lime glass substrate using a doctor-blade coating method. After coating, the precursor films were annealed in a quartz tube furnace under low vacuum without the use of a Se atmosphere or reduction conditions. Evolution of the morphology, crystal structure, and thermal decomposition behavior of the films was analyzed by X-ray diffraction, scanning electron microscopy, and thermogravimetric analysis, and the film formation mechanism was suggested. The as-deposited precursor film gradually decomposed with increase in temperature and formed Cu_2−xSe and In₂Se₃ nuclei on the surface of the film. Incorporation of Cu_2−xSe with In₂Se₃ yielded a chalcopyrite CIS phase, which crystallized on annealing above 400 °C. The obtained CIS film showed low-resistive ohmic behavior with a Mo electrode and a high absorption efficiency for visible–infrared (IR) light, making it suitable for use in photovoltaic applications.     

Epitaxial growth of the CoFe2O4 film on SrTiO3 and its characterization

Cobalt ferrite (CoFe₂O₄) thin film is epitaxially grown on (0 0 1) SrTiO₃ (STO) by laser molecular beam epitaxy (LMBE). The growth modes of CoFe₂O₄ (CFO) film are found to be sensitive to laser repetition, the transitions from layer-by-layer mode to Stranski–Krastanov (SK) mode and then to island mode occur at the laser repetition of 3 and 5 Hz at 700 °C, respectively. The X-ray diffraction (XRD) results show that the CFO film on (0 0 1) SrTiO₃ is compressively strained by the underlying substrate and exhibits high crystallinity with a full-width at half-maximum of 0.86°. Microstructural studies indicate that the as-deposited CFO film is c-oriented island structure with rough surface morphology and the magnetic measurements reveal that the compressive strained CoFe₂O₄ film exhibits an enhanced out-of-plane magnetization (190 emu/cm³) with a large coercivity (3.8 kOe).     

Fabrication of nano-crystalline silicon thin film at low temperature by using a neutral beam deposition method

Low temperature (<80 °C) neutral beam deposition (LTNBD) was investigated as a new approach to the fabrication and development of nano-crystalline silicon (nc-Si), which has better properties than that of amorphous silicon (α-Si). The difference between LTNBD and conventional PECVD is that the film formation energy of the nc-Si in LTNBD is supplied by controlled neutral beam energies at a low temperature rather than by heating. Especially, in this study, the characteristics of the nc-Si thin film were investigated by adding 10% of an inert gas such as Ne, Ar or Xe to SiH₄/H₂. Increasing the beam energy resulted in an increase in the deposition rate, but the crystallinity was decreased, due to the increased damage to the substrate. However, the addition of a higher mass inert gas to the gas mixture at a fixed beam energy resulted not only in a higher deposition rate but also in a higher crystallization volume fraction. The high resolution transmission electron microscopy image showed that the grown film is composed of about 10 nm-size grains.     

Bi20TiO32 nanocones prepared from Bi–Ti–O mixture by metalorganic decomposition method

Bi₂₀TiO₃₂ in the form of nanocones are reported for the first time, which have been found during the formation of Bi₂Ti₂O₇ nanocrystals. Bi₂₀TiO₃₂ nanocones were prepared by metalorganic decomposition technique. From X-ray patterns, it was found that Bi₂₀TiO₃₂ is a metastable phase, and can transform gradually into Bi₂Ti₂O₇ phase with the annealing time increasing at a temperature of 550°C. The image of field emission scanning electron microscopy shows that the lengths of the nanocones are up to several micrometers and the diameters of cusps range from 20 to 200 nm. The studies of transmission electron microscopy show that the nanocones are crystalline Bi₂₀TiO₃₂. The growth mechanism of Bi₂₀TiO₃₂ nanocones has been proposed, which is similar to the vapor–liquid–solid growth mechanism.