Growth of cubic InN films with high phase purity by pulsed laser deposition

Cubic InN films have been grown on MgO (1 0 0) substrates with cubic GaN buffer layers by pulsed laser deposition (PLD). It has been found that cubic InN (1 0 0) films grow on the GaN (1 0 0)/MgO (1 0 0) structure with an in-plane epitaxial relationship of [0 0 1]_InN∥[0 0 1]_GaN∥[0 0 1]MgO. The phase purity of a cubic InN film grown at 440 °C was as high as 99% that can probably be attributed to the enhanced surface migration of film precursors in case of PLD. These results indicate that PLD is a suitable technique for the growth of high-quality cubic InN films, and will makes it possible to fabricate optical and electron devices based on cubic InN films.     

An easy sol–gel route for deposition of oriented Ln2Ti2O7 (Ln=La,Nd) films on SrTiO3 substrates

The effect of the orientation of SrTiO₃ (STO) substrates, (1 0 0) or (1 1 0), on the growth of La₂Ti₂O₇ (LTO) and Nd₂Ti₂O₇ (NTO) thin films was investigated. The films were deposited via a sol–gel process coupled to the spin-coating technique. Depending on the substrate orientation, a similar effect was observed on the structural properties for both the LTO and NTO thin films. For (1 1 0)-oriented STO substrate, the films are preferentially (0 0 1) oriented while for (1 0 0)-oriented STO substrates, the orientation is mainly (0 1 2). Those matching appear to be in good agreement with the compatibility between the film/substrate crystal lattices, in relation with the existence of perovskite slabs in Ln₂Ti₂O₇ (Ln=rare earth) compounds. In these latter, a slight disorientation of the (0 1 2) planes with respect to the surface of the substrate was measured. This tilting is even more marked in the case of NTO. The surface morphology was studied by atomic force microscopy.     

Growth and process modeling studies of nickel-catalyzed metalorganic chemical vapor deposition of GaN nanowires

A combination of experimental and computational fluid dynamics-based reactor modeling studies were utilized to study the effects of process conditions on GaN nanowire growth by metalorganic chemical vapor deposition (MOCVD) in an isothermal tube reactor. The GaN nanowires were synthesized on (0 0 0 1) sapphire substrates using nickel thin films as a catalyst. GaN nanowire growth was observed over a furnace temperature range of 800–900 °C at V/III ratios ranging from 33 to 67 and was found to be strongly dependent on the position of the substrate relative to the group III inlet tube. The modeling studies revealed that nanowire growth consistently occurred in a region in the reactor where the GaN thin-film deposition rate was reduced and the gas phase consisted primarily of intermediate species produced by the reaction and decomposition of trimethylgallium–ammonia adduct compounds. The GaN nanowires exhibited a predominant [1 1 2¯ 0] growth direction. Photoluminescence measurements revealed an increase in the GaN near-band edge emission intensity and a reduction in the deep-level yellow luminescence with increasing growth temperature and V/III ratio.     

Influence of growth temperature on the selective area MOVPE of InAs nanowires on GaAs (1 1 1) B using N2 carrier gas

The influence of temperature on selective area (SA) InAs nanowire growth was investigated for metal-organic vapor phase epitaxy (MOVPE) using N₂ as the carrier gas and (1 1 1) B GaAs substrates. In contrast to the growth temperature range – below 600 °C – reported for hydrogen ambient, the optimal growth temperature between 650 and 700 °C was 100 K higher than the optimal ones for H₂ carrier gas. At these temperatures, nanowires with aspect ratios of about 80 and a symmetric hexagonal shape were obtained. The results found are attributed to the physical and chemical properties of the carrier gas.     

Low-temperature/high-temperature AlN superlattice buffer layers for high-quality AlxGa1−xN on Si (1 1 1)

We present MOVPE-grown, high-quality Al_xGa_1−x N layers with Al content up to x=0.65 on Si (1 1 1) substrates. Crack-free layers with smooth surface and low defect density are obtained with optimized AlN-based seeding and buffer layers. High-temperature AlN seeding layers and (low temperature (LT)/high temperature (HT)) AlN-based superlattices (SLs) as buffer layers are efficient in reducing the dislocation density and in-plane residual strain. The crystalline quality of Al_xGa_1−xN was characterized by high-resolution X-ray diffraction (XRD). With optimized AlN-based seeding and SL buffer layers, best ω-FWHMs of the (0 0 0 2) reflection of 540 and 1400 arcsec for the (1 0 1¯ 0) reflection were achieved for a ∼1-μm-thick Al_0.1Ga_0.9N layer and 1010 and 1560 arcsec for the (0 0 0 2) and (1 0 1¯ 0) reflection of a ∼500-nm-thick Al_0.65Ga_0.35N layer. AFM and FE-SEM measurements were used to study the surface morphology and TEM cross-section measurements to determine the dislocation behaviour. With a high crystalline quality and good optical properties, Al_xGa_1−x N layers can be applied to grow electronic and optoelectronic device structures on silicon substrates in further investigations.     

Sol–gel preparation and photoluminescence properties of Ce-activated Y3Al5O12 nano-sized powders

Trivalent cerium-doped yttrium aluminum garnet (YAG:Ce³⁺) nanoparticles were successfully prepared by a facile sol–gel technique. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy were used to characterize the as-prepared phosphors. Well-crystallized fine nanoparticles were obtained at 1000 °C. Single-crystal nanoparticles with irregular shapes were obtained, with crystallite sizes ranging between 20 and 60 nm. PL intensity of the particles increased monotonically with decreasing Ce doping concentration and showed the maximum value at 0.1 at%.     

The character of FeMn-1# powder catalyst and its influence on the synthesis of diamond

In this paper industrial diamond crystal was synthesized using FeMn-1# powder catalyst in China-type cubic high-pressure apparatus at 5.7 GPa and 1400–1600 °C. The growth feature of diamond in the graphite–FeMn-1# system was researched. Optical microscope observation showed that all the diamond crystals were light yellow octahedral with grain size of 0.3–0.5 mm. There are also plenty of bubbles in the crystals. By SEM, we can see that the surface of diamond is smooth and the crystal is intact. Mössbauer spectrum was used to detect the impurity in the diamonds.     

Synthesis and characterization of ilmenite NiTiO3 and CoTiO3 prepared by a modified Pechini method

Powders of ilmenite structure NiTiO₃ and CoTiO₃ were prepared by a simple method based on the modified Pechini process. The raw compounds and citric acid (CA) were mixed in ethanol (EA) with the molar ratio Ni(Co)/Ti/CA/EA = 1/1/1/7.5. The DTA curve shows exothermic peaks only around 300-350 °C and 600 °C, which correspond to the decomposition of the organic compound and direct crystallization of the ilmenite phase. X-ray diffraction patterns indicated that the ilmenite phase was successfully synthesized as the Ni(Co)-Ti precursor calcined above 600-900 °C for 3 h, and the activation energies of NiTiO₃ and CoTiO₃ were calculated to be about 8.84 and 13.23 kJ/mol. TEM bright field images showed that the grain sizes of powders of NiTiO₃ and CoTiO₃ at 600-900 °C were estimated to be about 10-250 and 20-200 nm depending on the nature of the aggregate. The samples of NiTiO₃ calcined at 600-800 °C have the larger specific surface area of 31.51, 18.78, and 6.01 m²/g, respectively. The UV-Vis diffuse reflectance spectra show the optical band gaps of NiTiO₃ and CoTiO₃ as 3.02 and 2.43 eV.     

Growth and characterization of epitaxial Fe0.8Ga0.2/0.69PMN-0.31PT heterostructures

Fe_0.8Ga_0.2 films were deposited on bulk single-crystal (0 0 1) 0.69PMN-0.31PT substrates by DC magnetron sputtering to make magnetoelectric bilayer composites. Films deposited at temperatures below 600 °C were X-ray amorphous. Films deposited at temperatures of 600 °C and higher exhibited a single-crystal (0 0 1) disordered BCC structure. The crystalline FeGa films demonstrate a 45° twisted cube-on-cube epitaxial relationship with the PMN–PT substrates. Heterostructures with an X-ray amorphous FeGa film exhibited zero magnetoelectric response. Heterostructures with a 990 nm epitaxial FeGa film exhibited a large inverse magnetoelectric voltage coefficient of 13.4 (G cm)/V.     

Formation of twinned lamellas with the application of static magnetic fields during semi-continuous casting of Al–0.24 wt%Fe alloy

Al–0.24 wt%Fe was semi-continuously cast with the application of static magnetic field of 0.1 and 0.2 T. The magnetic field transfers the microstructure from the columnar grains growing in the 〈1 0 0〉 direction into twinned lamellar crystals growing in the 〈1 1 0〉 direction. The lamellas possess the morphology and the crystallographic growth direction of the “feathery crystals”. However, the “feathers” (secondary arms) that branch off the twinning plane only start to form at 0.2 T. Under the two strengths of the magnetic field applied, no side arms in the twinning planes were observed. The increase of the lamellar spacing with the magnetic field is due to the initiation of the secondary arms.     

Growth kinetics of SiGe/Si superlattices on bulk and silicon-on-insulator substrates for multi-channel devices

We have studied in reduced pressure chemical vapor deposition the growth kinetics of Si and Si_0.8Ge_0.2 on bulk Si(0 0 1) and on silicon-on-insulator (145 nm buried oxide/20 nm Si over-layer) substrates. For this, we have grown at 650 °C, 20 Torr 19 periods (Si_0.8Ge_0.2 19 nm/Si 32 nm) superlattices on both types of substrates that we have studied in secondary ion mass spectrometry, X-ray diffraction and cross-sectional transmission electron microscopy. The Si and SiGe growth rates together with the Ge content are steady on bulk Si(0 0 1), with mean values around 9.5 nm min⁻¹ and 20.2%, respectively. In contrast, growth rates decrease from ∼9.5 nm min⁻¹ down to values around 7.0 nm min⁻¹ (SiGe) and 6.3 nm min⁻¹ (Si), when the deposited thickness on SOI increases from 0 up to slightly more than 100 nm. They then go back up to values around 8.8–9.0 nm min⁻¹ as the thickness increases from 100 up to 400 nm. They then slowly decrease to values around 8.4–8.6 nm min⁻¹ as the thickness increases from 400 up to 800 nm. The Ge concentration follows on SOI exactly the opposite trend: an increase from 19.9% (0 nm) up to 20.6% (∼100 nm) followed by a decrease to values around 20.1% (400 nm) then a slow re-increase up to 20.4% (800 nm). These fluctuations are most likely due to the following SOI surface temperature variations: from 650 °C down to 638 °C (100 nm), back up to 648 °C (400 nm) followed by a slow decrease to 646 °C (800 nm). These data curves will be most useful to grow on conventional SOI substrates large number of periods, regular Si/Si_0.8Ge_0.2 superlattices that will serve as the core of multi-channel or three-dimensional nano-wires field effect transistors.     

Polymorphism and morphology of calcium carbonate precipitated in mixed solvents of ethylene glycol and water

The influence of (mono) ethylene glycol (MEG) on polymorphism and the resulting morphology of calcium carbonate have been studied for activity-based supersaturation ratios in the range of 3–10 and temperatures from 25–80 °C in mixed solvents of ethylene glycol and water at ratios of 0–90 wt%. The presence of a co-solvent in the solution affects the supersaturation, because of changes in the activity coefficients and the solubility of the salt, a fact that is usually not accounted for in similar studies in the literature. In the present study, the effect of the solvent was isolated from the accompanying change in the supersaturation. MEG was found to affect both the polymorphic abundance in the precipitates, the morphology of the particles and the transformation rates. High concentrations of MEG favoured the precipitation of vaterite and higher temperatures promoted the formation of aragonite. The particle size was reduced in experiments with high MEG concentrations at supersaturations ratios comparable to water solutions, illustrating that the nucleation rate is affected by the co-solvent. The morphology of the calcium carbonate particles was changed at various conditions of MEG concentrations and temperatures from cubes of calcite to spherical, flower-like and dumbbell particles of vaterite and aragonite needles. MEG prolongs the transformation time of metastable polymorphs and the effect was shown to be caused by the solvent itself, probably as a result of kinetic stabilization by delaying the growth rate of the more stable polymorphs.     

Effects of initial stages on the crystal quality of nonpolar a-plane AlN on r-plane sapphire by low-pressure HVPE

A 4–6 μm thick a-plane (1 1 2¯ 0) AlN was grown on r-plane sapphire substrate by low-pressure hydride vapor phase epitaxy (LP-HVPE), using a direct growth without any nitridation and buffer layer, a single-step nitridation growth, a two-step nitridation growth and a two-step buffer growth method. For the two-step buffer growth procedure, smoother surface is observed with the lower full widths at half maximum (FWHM) of X-ray rocking curves (XRC) compared with the other two kinds of nitridation procedures. A smaller FWHM of in-plane XRC peak anisotropy features are reversed, which is consistent with the smaller in-plane stress anisotropic distribution in a-plane AlN, when the two-step nitridation or buffer growth method is used. In four kinds of initial growth procedures, the two-step buffer method is the suitable method for the growth of a-plane AlN by HVPE with the high crystal quality and more isotropic distribution.     

Epitaxial NiO (1 0 0) and NiO (1 1 1) films grown by atomic layer deposition

Epitaxial NiO (1 1 1) and NiO (1 0 0) films have been grown by atomic layer deposition on both MgO (1 0 0) and α-Al₂O₃ (0 0 l) substrates at temperatures as low as 200 °C by using bis(2,2,6,6-tetramethyl-3,5-heptanedionato)Ni(II) and water as precursors. The films grown on the MgO (1 0 0) substrate show the expected cube on cube growth while the NiO (1 1 1) films grow with a twin rotated 180° on the α-Al₂O₃ (0 0 l) substrate surface. The films had columnar microstructures on both substrate types. The single grains were running throughout the whole film thickness and were significantly smaller in the direction parallel to the surface. Thin NiO (1 1 1) films can be grown with high crystal quality with a FWHM of 0.02–0.05° in the rocking curve measurements.     

Metal organic vapour phase epitaxy of MgO films grown on c-plane Sapphire

Metal organic vapour phase epitaxy (MOVPE) has been used to epitaxially grow MgO films on c-plane sapphire substrates. Bismethylcyclopentadienyl magnesium (MCP₂Mg) and nitrous oxide (N₂O) were used as the magnesium and the oxygen source, respectively, with nitrogen (N₂) as the carrier gas. The dependence of the growth rate on the partial pressure of magnesium and on the growth temperature was investigated. The growth rate increases with the magnesium partial pressure. The morphological and structural properties of MgO films were investigated using atomic force microscopy and X-ray diffraction. The structural properties are strongly dependent on the growth temperature in the range 400–800 °C. (1 1 1)-oriented MgO layers are observed at growth temperatures above 600 °C whereas no diffraction peak is found at lower growth temperatures. The atomic force microscopy (AFM) images reveal a smooth surface morphology.     

Sol-gel synthesis of porous and dense silica microspheres

An acidic silica sol (35 ± 2 wt% equivalent SiO₂) having a gelling time of 9-10 min has been used as an aqueous phase for obtaining a w/o emulsion in CCl₄ as oil phase in presence of a surfactant, Tween 80. The silica sol was allowed to form gel at room temperature via polycondensation among the -Si-OH groups forming the porous silica gel microspheres. The surface area of the microspheres heated at 500°, 700° and 900 °C was found to be 227 m²/g, 167 m²/g and 81 m²/g indicating the gradual densification. The decreased surface area and unchanged -Si-O-Si- asymmetric stretching vibration at 1084 cm⁻¹ up to 700 °C probably indicate the formation of extensive cross-linked gel structure in the microsphere. The appearance of the -Si-O-Si- asymmetric stretching vibration at 1104 cm⁻¹ and the absence of porosity while heating at 1000 °C indicate the formation of dense silica glass microspheres.     

Growth of dl-malic acid-doped ammonium dihydrogen phosphate crystal and its characterization

Single crystals of dl-malic acid-doped ammonium dihydrogen phosphate have been grown using slow evaporation method and also by Sankaranarayanan–Ramasamy (SR) method with the vision to improve the properties of the ADP crystals. The characterization of grown crystals was made by single-crystal X-ray diffraction, powder X-ray diffraction, UV–vis. spectroscopy, Fourier transform infrared spectroscopy (FTIR), differential thermal analysis (DTA), Vicker's microhardness, dielectric measurements, high-resolution X-ray diffraction (HRXRD) and second-harmonic studies. Structural difference between pure and doped crystal has been studied by XRD analysis. Functional groups were identified by FTIR spectroscopy. The grown crystals were found to be transparent in the entire visible region. Decomposition temperatures of the grown crystals were measured by DTA. Vicker's hardness study carried out on (0 0 1) face at room temperature shows increased hardness of the doped crystals and SR-method-grown crystals. Dielectric measurements reveal that SR-method-grown DLM-doped ADP crystals have low dielectric loss. Crystalline perfection of the grown crystals is analyzed using HRXRD. Preliminary measurements indicate that the second harmonic generation efficiency of the doped crystals at a fundamental wavelength of 1064 nm is roughly 1.5 times greater than that of pure ADP.     

Thermodynamics of GaAs nanowire MBE growth with gold droplets

The thermodynamics of growth conditions of GaAs nanowires using gold droplets is analyzed. Equilibrium conditions for steady-state growth using experimental molecular beam epitaxy (MBE) impinging molecular flows, as previously published, are calculated in the range 793–893 K. These show that: (i) the tie line for Ga liquidus composition in equilibrium with GaAs(s) is in the 0.4–0.6 mole fraction range, close to the GaAu–GaAs pseudo-binary section, (ii) the As content of the droplet is in the 0.2–0.4×10⁻³ mole fraction range and (iii) the growth rate is mainly governed by the contact angle that determines the droplet section. Different cooling conditions are analyzed using the Scheil–Guliver assumptions to compare final phases after solidification, as analyzed by X-ray diffraction (XRD), with our calculations. The agreement is very good and this feature demonstrates that quasi-equilibrium conditions prevail in the growth process of nanowires.     

Growth of cadmium mercury thiocyanate single crystals using acetone–water mixed solvent and their characterization studies

Cadmium mercury tetrathiocyanate single crystals were grown from acetone–water (4:1) mixed solvent by slow evaporation solution technique. The structure of the grown crystal was confirmed by single-crystal X-ray diffraction analysis. The crystalline perfection of cadmium mercury thiocyanate (CMTC) was ascertained by high-resolution X-ray diffraction (HRXRD) analysis. The presence of functional groups and the coordination of thiocyanate ion in the CMTC compound were confirmed by laser Raman spectroscopy. The optical transparency of CMTC single crystal was studied by UV–vis spectroscopy. Single-shot surface laser damage threshold of CMTC estimated using an Nd-YAG laser of wavelength 1064 nm was found to be 4.59 GW/cm² and second harmonic generation (SHG) output power was estimated through the Kurtz powder method. Dielectric constant and dielectric loss for various frequencies and temperatures were performed on CMTC single crystal.     

Structure of Ni/SiO2 films prepared by sol-gel dip coating

The sol-gel dip coating technique has been used to deposit composite oxide films (NiO)_x(SiO₂)_1−x with x = 0.1 on silicon wafers. Single and multilayer coatings allowed a variation of the film thickness from 70 to 400 nm. Film morphology, atomic structure and atomic composition have been investigated by transmission electron microscopy (TEM) and Rutherford backscattering spectrometry (RBS). The local environment of the Ni atoms was characterized by extended X-ray absorption fine structure (EXAFS). The samples were studied in the as-prepared state and after annealing in H₂ at 600 °C for 1 h. The structural and chemical state evolution of clusters present inside the silica matrix is discussed in terms of out-of-equilibrium reaction processes specific to low-dimensional objects and superficial effects.