Kinetics of crystal growth of mirabilite in aqueous supersaturated solutions

The crystallization of sodium sulfate decahydrate (Na₂SO₄·10H₂O, mirabilite) from supersaturated solutions was investigated using stable supersaturated solutions seeded with mirabilite seed crystals. The experiments were done in batch, stirred reactors in which the supersaturated solutions were prepared either by dissolution of sodium sulfate anhydrous at 32 °C followed by cooling to 18 or 20 °C or by mixing equal volumes of equimolar ammonium sulfate and sodium hydroxide solutions at 20 °C. Inoculation of the solutions supersaturated only with respect to mirabilite with seed crystals was accompanied with temperature increase of the thermostated solution. Despite the fact that crystal growth was initiated with seed crystals, the process started past the lapse of induction times inversely proportional to the solution supersaturation. The rates of crystal growth were measured both from the temperature rise and from the concentration–time profiles, which were linearly correlated. The measured crystal growth rates showed a parabolic dependence on supersaturation at low supersaturations. For higher values this dependence changed to linear, a behavior consistent with the BCF spiral crystal growth model. The morphology of the crystals growing at 20 °C showed typical prismatic habit, while at 18 °C when crystallized from cooled sodium sulfate solutions changes in the crystal habit to a leaf like morphology were observed.     

Growth of thick AlN epilayers with droplet-free and atomically smooth surface by plasma-assisted molecular beam epitaxy using laser reflectometry monitoring

Low-temperature (<750 °C) growth of thick AlN epilayers on c-sapphire by plasma-assisted molecular-beam epitaxy under the Al-rich conditions (F_Al/F_N?<1.4) is reported here. Short periodic Al-flux interruptions controlled precisely by laser reflectometry ensure continuous growth of droplet-free and atomically smooth AlN films (rms<2 ML over 4 μm²) with a growth rate governed by the activated nitrogen flux. Lateral spreading of small accumulated Al clusters with their subsequent incorporation into the AlN layer during the Al-flux interruptions is supposed to be facilitated by activated nitrogen radicals. Strong influence of the remaining Al droplets on the subsequent growth of AlGaN/AlN superlattices is also demonstrated.     

Analysis of SiC crystal sublimation growth by fully coupled compressible multi-phase flow simulation

A fully coupled compressible multi-phase flow solver was developed to effectively design a large furnace for producing large-size SiC crystals. Compressible effect, convection and buoyancy effects, flow coupling between argon gas and species, and the Stefan effect are included. A small and experimental furnace is used to validate the solver. First, the essentiality of 2D flow calculation and the significance of incorporating buoyancy effect and gas convection, the Stefan effect, and flow interaction between argon gas and species were investigated by numerical results. Then the effects of argon gas on deposition rate, growth rate, graphitization on the powder source, and supersaturation and stoichiometry on the seed were analyzed. Finally, the advantages of an extra chamber design were explained, and improvement of growth rate was validated by the present solver.     

Seeded growth of AlN on N- and Al-polar 〈0001〉 AlN seeds by physical vapor transport

We demonstrated seeded growth of AlN on large-area Al- and N-polar <0 0 0 1>-oriented AlN seeds using the physical vapor transport method (PVT). In both cases, crystals having a diameter of 15 mm were obtained from 5 mm seeds. Based on growth step and terrace width analyses, it was found that the N-polar face was suitable for growth within a large window of growth parameters while the Al-polar seeds yielded high-quality crystals only at low supersaturation.     

Transmission electron microscopy investigation of AlN growth on Si(111)

AlN layers with a thickness of 250 nm were grown by plasma-assisted gas source molecular-beam epitaxy on Si(111) at substrate temperatures between 600 °C and 900 °C. The surface morphology and microstructure of the AlN layers were analyzed by scanning and transmission electron microscopy. Different defect types are observed in the AlN layers and at the AlN/Si(111) interfaces as a function of the temperature: inclusions of pure Al in the Si-substrate, crystallites of the cubic AlN phase, dislocations, stacking faults and inversion domain boundaries. The formation and concentration of the defects depends strongly on the substrate temperature during the growth. X-ray diffraction rocking curves for the (0002) reflection yield minimum full width at half maximum values for the sample grown at the 900 °C under Al-rich conditions indicating optimum structural quality. However, the discussion of the entity of defects will show that a more differentiated view is required to assess the overall quality of the AlN layers.     

Crystal growth of ZrW2O8 and its optical and mechanical characterization

ZrW₂O₈ is known for its isotropic negative thermal expansion over a wide of range of temperature from ?272 to 777 °C. However, ZrW₂O₈ melts incongruently at 1257 °C and is stable only over a short temperature interval between 1105 and 1257 °C. This makes the growth of single crystals a formidable challenge. In order to study the intrinsic properties of this compound, a repeatable, viable single crystal growth strategy is required. Here we report a simple, self-seeding, self-fluxing single crystal growth process which resulted in single crystals of ZrW₂O₈ up to about 4 mm in size. Grown crystals were characterized by X-ray diffraction (XRD), Raman spectroscopy and Fourier transform infrared (FTIR) spectroscopy. Mechanical properties of the crystals were studied using nanoindentation.     

In situ crystallization of lithium disilicate glass: Effect of pressure on crystal growth rate

Crystallization of a Li₂O · 2SiO₂ (LS₂) glass subjected to a uniform hydrostatic pressure of 4.5 and 6 GPa was investigated up to a temperature of 750 °C. The density of the compressed glass is ∼2% greater at 4.5 GPa than 1 atm and, depending on the processing temperature, up to 10% greater at 6 GPa. Crystal growth rates investigated as a function of temperature and pressure show that lithium disilicate crystal growth is an order of magnitude slower at 4.5 GPa than 1 atm resulting in a shift of +45 °C (±10 °C) in the growth rate curve at high pressure compared to 1 atm conditions. At 6 GPa lithium disilicate crystallization is suppressed entirely, while a new high pressure lithium metasilicate crystallizes at temperatures 95 °C (±10 °C) higher than those reported for lithium disilicate crystallization at 1 atm. The observed decrease in crystal growth rate with increasing pressure for the lithium disilicate glass up to 750 °C is attributed to an increase in viscosity with pressure associated with fundamental changes in glass structure accommodating densification.     

Vacancy behavior in Czochralski silicon growth

The vacancy defect behavior in silicon crystal growth has been investigated by kinetic Monte Carlo and continuum simulations. The vacancy concentration distributions in silicon crystal were obtained from continuum model simulations corresponding to experimental conditions and then the vacancy clusters distribution obtained from kinetic lattice Monte Carlo simulations. At the temperature above 1470 K, the diffusivity was almost constant because the clusters were not formed. In the clustering temperature region (1370–1270 K), the larger clusters were generated at the higher temperature, the smaller clusters were generated at the lower temperature. While the vacancy concentration led to increase in the number of clusters, the mean size of clusters was irrespective of the vacancy concentrations. Clustering phenomena were very susceptible to temperature and vacancy concentrations. The total number of vacancy clusters was linearly proportion to the crystal pull rate. The radial distribution of clusters obtained from multi-scale simulations was in good agreement with the distribution of voids in the experimental data.     

Vertical Bridgman growth and characterization of large ZnGeP2 single crystals

The growth of ZnGeP₂ crystals by seeded Vertical Bridgman method was studied. High-quality near-stoichiometric ZnGeP₂ single crystals obtained were of 20–30 mm in diameter and 90–120 mm in length. By selection of the seed crystallographic orientation the single crystal ingots without cracks and twins were grown, as shown by X-ray diffraction. The infrared transmission property of the ZnGeP₂ crystals was studied by the calculated optical absorption coefficient spectra. The results showed that after thermal annealing of the crystals the optical absorption coefficient was ～0.10 cm^?1 at 2.05 μm, and ～0.01 cm^?1 at 3–8 μm. The rocking curves patterns of the (4 0 0) reflection demonstrated that the as-grown single crystals possessed a good structural quality. The composition of the crystals was close to the ideal stoichiometry ratio of 1:1:2. The low-loss typical ZnGeP₂ samples of 6 mm×6 mm×15 mm in sizes were cut from the annealed ingots for optical parametric oscillation experiments. The output power of 3.2 W was obtained at 3–5 μm when the incident pumping power of 2.05 μm laser was 9.4 W, and the corresponding slope efficiency and the conversion efficiency were 44% and 34%, respectively.     

Preparation and characterization of CdGeAs2 crystal by modified vertical Bridgman method

A large, crack-free CdGeAs₂ single crystal measuring 15 mm in diameter and 45 mm in length was grown in a vertical three-zone tubular furnace by a modified vertical Bridgman method, i.e. quasi-seed technique with small temperature gradient and descending quartz ampoule. High-purity, single phase CdGeAs₂ polycrystallite for crystal growth was synthesized using a rocking furnace with temperature oscillation techniques. Various measuring means, including X-ray diffractometer(XRD), Fourier transform infrared spectroscopy(FTIR), and Field emission scanning electron microscope(FE-SEM) were adopted to characterize the as-grown crystal. It is found that the cleavage plane of the as-grown crystal is {1 0 1} face; the crystal is integrated in structure and crystallized well; etch pits in the shape of pentagon on (1 1 2) face have been observed for the first time using the new preferential etchant we prepared. All these results encouragingly indicate that the modified vertical Bridgman method is a convenient and effective way for high quality CdGeAs₂ crystal growth.     

CFD modelling of single crystal growth of potassium dihydrogen phosphate (KDP) from binary water solution at 30 °C

Crystal growth rate depends on both diffusion and surface reaction. In industrial crystallizers, there exist conditions for diffusion-controlled growth and surface reaction-controlled growth. Using mathematical modelling and experimental information obtained from growth studies of single crystals, it is possible to separate these phenomena and study how they are affected by concentration, slip velocities of particles, temperature and finally estimate the parameters for crystal growth models.In this study, a power-law growth model using activity-based driving force is created. Computational fluid dynamics (CFD) was used to evaluate the thickness of a diffusion layer around the crystal. Parameters of the crystal growth model were estimated using a non-linear optimization package KINFIT. Experimental data on growth rate of the (1 0 1) face of a potassium dihydrogen phosphate (KDP) single crystal and simulated data on the thickness of a diffusion layer at the same crystal face were used in parameter estimation. The new surface reaction model was implemented into the CFD code. The model was used to study the effect of flow direction on growth rate of the whole crystal with various slip velocities and solute concentrations.The developed method itself is valid in general but the parameters of crystal growth model are dependent on the system. In this study, the model parameters were estimated and verified for KDP crystal growth from binary water solution.     

Polarities of AlN films and underlying 3C-SiC intermediate layers grown on (1 1 1) Si substrates

The hydride vapor phase epitaxy (HVPE) of {0 0 0 1} AlN films on {1 1 1} Si substrates covered with epitaxial {1 1 1} cubic SiC (3C-SiC intermediate layers) was carried out. 3C-SiC intermediate layers are essential to obtain high-quality AlN films on Si substrates, because specular AlN films are obtained with 3C-SiC intermediate layers, whereas rough AlN films are obtained without 3C-SiC intermediate layers. We determined the polarities of AlN films and the underlying 3C-SiC intermediate layers by convergent beam electron diffraction (CBED) using transmission electron microscopy. For the first time, the polarities of the AlN films and the 3C-SiC intermediate layers were determined as Al and Si polarities, respectively. The AlN films were hardly etched by aqueous KOH solution, thereby indicating Al polarity. This supports the results obtained by CBED. The result is also consistent with electrostatic arguments. An interfacial structure was proposed. The 3C-SiC intermediate layers are promising for the HVPE of AlN films on Si substrates.     

Bulk GaN single crystals: growth conditions by flux method

Hexagonal GaN platelet crystals with a size of 1–4 mm have been grown by a Li-based flux method. The influence of growth conditions such as the molar ratio of starting materials, temperature, pressure, the position of Li₃N in the crucible on the growth of GaN single crystals was studied. The quality of GaN single crystal was checked by optical microscope and X-ray rocking curve.     

Rapid Z-plate seed regeneration of large size KDP crystal from solution

Regeneration process of a 330×330×20 mm³ Z-plate seed is carried out in a 1.5 metric tonnage volume crystallizer that placed in a water bath of temperature fluctuation less than ±0.02 °C within 10 days. The surface of the whole crystal was restored by the formation of a box-like structure filled with growth solution, and then the transparent layer of perfect tetragonal KDP crystal without inclusions, crack and milky regions just like those produced by traditional slow cooling technique can be grown from solution. After the regeneration, the height of KDP crystal is merely 0.5 times the side of plate seed. We found it that the optical transmission and laser damage threshold of the KDP crystals we grown are not significantly different from those of KDP crystals grown by traditional method.     

Single crystal growth and physical properties of superconducting ferro-pnictides Ba(Fe,Co)2As2 grown using self-flux and Bridgman techniques

We have employed the high temperature solutions growth technique and the Bridgman technique to grow cm-size high-quality single crystals of pristine BaFe₂As₂ (Ba122) and its Co-doped superconducting variants. In the first approach, self-flux (Fe, Co)As was used to achieve a homogeneous melt of composition Ba(Fe, Co)_3.1As_3.1 at T=1463 K. The melt was then cooled slowly under a temperature gradient in a double-wall crucible assembly to obtain large and flux-free single crystals of Ba(Fe, Co)₂As₂. In the second approach, single crystals were grown directly from a stoichiometric Ba(Fe, Co)₂As₂ melt at T=1723 K employing the Bridgman technique associated with a vertical tube furnace. Using both techniques single crystals with lateral dimensions up to 25×10 mm² and thickness up to 1 mm were obtained. Details of the two methods are given and a comparative study of the magnetic and transport properties of the single crystals obtained using the two methods is presented.     

Investigation of directional solidified Al–Ti alloy

Al–1 wt% Ti alloy was directionally solidified upwards under argon atmosphere under the two conditions; with different temperature gradients (G = 2.20–5.82 K/mm) at a constant growth rate (V = 8.30 μm/s) and with different growth rates (V = 8.30–498.60 μm/s) at a constant temperature gradient (G = 5.82 K/mm) in a Bridgman furnace. The dependence of characteristic microstructure parameters such as primary dendrite arm spacing (λ₁), secondary dendrite arm spacing (λ₂), dendrite tip radius (R) and mushy zone depth (d) on the velocity of crystal growth and the temperature gradient were determined by using a linear regression analysis. A detailed analysis of microstructure development with models of dendritic solidification and with previous similar experimental works on dendritic growth for binary alloys were also made.     

Molecular beam epitaxy as a growth technique for achieving free-standing zinc-blende GaN and wurtzite AlxGa1-xN

Currently there is a high level of interest in the development of ultraviolet (UV) light sources for solid-state lighting, optical sensors, surface decontamination and water purification. III-V semiconductor UV LEDs are now successfully manufactured using the AlGaN material system; however, their efficiency is still low. The majority of UV LEDs require Al_xGa_1-xN layers with compositions in the mid-range between AlN and GaN. Because there is a significant difference in the lattice parameters of GaN and AlN, Al_xGa_1-xN substrates would be preferable to those of either GaN or AlN for many ultraviolet device applications. However, the growth of Al_xGa_1-xN bulk crystals by any standard bulk growth techniques has not been developed so far.There are very strong electric polarization fields inside the wurtzite (hexagonal) group III-nitride structures. The charge separation within quantum wells leads to a significant reduction in the efficiency of optoelectronic device structures. Therefore, the growth of non-polar and semi-polar group III-nitride structures has been the subject of considerable interest recently. A direct way to eliminate polarization effects is to use non-polar (001) zinc-blende (cubic) III-nitride layers. However, attempts to grow zinc-blende GaN bulk crystals by any standard bulk growth techniques were not successful.Molecular beam epitaxy (MBE) is normally regarded as an epitaxial technique for the growth of very thin layers with monolayer control of their thickness. In this study we have used plasma-assisted molecular beam epitaxy (PA-MBE) and have produced for the first time free-standing layers of zinc-blende GaN up to 100 μm in thickness and up to 3-inch in diameter. We have shown that our newly developed PA-MBE process for the growth of zinc-blende GaN layers can also be used to achieve free-standing wurtzite Al_xGa_1-xN wafers. Zinc-blende and wurtzite Al_xGa_1-xN polytypes can be grown on different orientations of GaAs substrates - (001) and (111)B respectively. We have subsequently removed the GaAs using a chemical etch in order to produce free-standing GaN and Al_xGa_1-xN wafers. At a thickness of ～30 µm, free-standing GaN and Al_xGa_1-xN wafers can easily be handled without cracking. Therefore, free-standing GaN and Al_xGa_1-xN wafers with thicknesses in the 30–100 μm range may be used as substrates for further growth of GaN and Al_xGa_1-xN-based structures and devices.We have compared different RF nitrogen plasma sources for the growth of thick nitride Al_xGa_1-xN films including a standard HD25 source from Oxford Applied Research and a novel high efficiency source from Riber. We have investigated a wide range of the growth rates from 0.2 to 3 µm/h. The use of highly efficient nitrogen RF plasma sources makes PA-MBE a potentially viable commercial process, since free-standing films can be achieved in a single day.Our results have demonstrated that MBE may be competitive with the other group III-nitrides bulk growth techniques in several important areas including production of free-standing zinc-blende (cubic) (Al)GaN and of free-standing wurtzite (hexagonal) AlGaN.     

Effect of deposition pressure on microstructure and properties of hydrogenated carbon nitride films prepared by DC-RF-PECVD

Hydrogenated carbon nitride (a-CN:H films) were deposited on n-type (1 0 0) silicon substrates making use of dual direct current radio frequency plasma enhanced chemical vapor deposition (DC-RF-PECVD), at working pressure of 2–20 Pa, using a mixed gas of CH₄ and N₂ as the source gas. The growth rate, composition, bonding structure of the deposited films were characterized by means of XPS and FTIR, and the mechanical properties of the deposited films were investigated by nano-indentation test. It was found that the parameters for the DC-RF-PECVD process had significant effects on the growth rate, structure and properties of the deposited films. The growth rate of the deposited films increased at first with increasing deposition pressure, then saturated with further increase of the deposition pressure. The N/C ratio inside the deposited films increased with increasing working pressure except that it was as much as 0.50 at a working pressure of 5.0 Pa. The nano-hardness of the films decreased with increasing deposition pressure. CN radicals were remarkably formed in the deposited films at higher pressures, and their contents are related to the nitrogen concentrations in the deposited films.     

PVT法氮化铝晶体铝面、氮面生长对比分析

在氮气环境下用PVT方法生长氮化铝过程中,氮面和铝面由于表面化学性质不同,生长的主要化学反应速度存在差异。原子在生长表面的迁移能力不同造成单晶表面生长方式差异较大。在基本相同条件下(生长温度、生长温差、生长气压、类似的籽晶、同一台生长设备)进行了铝、氮面氮化铝单晶晶体生长。为了更明显地表现铝氮面的差异,将同一片籽晶分为两半,翻转其中一半让铝氮面同时生长。铝面生长较好的区域形成了明显的晶畴,而氮面生长时生长较好的部分出现了明显的生长台阶,并出现了晶畴边界被生长台阶湮灭的生长现象,进一步通过AFM观测到铝面生长台阶平整但被缺陷所阻隔,晶畴发育明显为各晶畴独立生长。氮面生长台阶没有铝面规则但连续性较强,在原来晶畴边界位置也出现了连续的生长台阶(或台阶簇)。所以籽晶质量不高时氮面生长更容易提高晶体质量,后续的XRD测量结果也证明了氮面生长后的晶体质量明显高于铝面生长的晶体质量。     

Growth of corundum single crystals by the top-seeded solution growth technique (TSSG)

Single crystals of corundum were grown by the top-seeded solution growth technique from a cryolite, Na₃AlF₆, solvent. The relationship between the growth rate (mg/h) of a crystal and the temperature difference (= supersaturation) or the rotation rate of a seed crystal was investigated, and optimum growth conditions for obtaining single crystals with good quality are discussed.