Crucible Wall Research Articles

Harmonicity and anharmonicity of phonon and surface phonon-polariton in high symmetry directions in wurtzite AlN

We report on the potential of self-nucleated AlN single crystals as tunable near-field infrared sources. A self-nucleated AlN crystal was grown with appropriate care to ensure minimal contact with crucible walls or other crystals. The grown crystal exhibits natural AlN growth characteristics with several well-developed facets of different orientations. The characteristics of surface-phonon-polariton (SPhP) modes on the developed crystal facets have been investigated. Reflectivity spectra were recorded from five facets of different orientations. The measured spectra were analyzed by a model taking into account the dependence of harmonicity and anharmonicity of the excited zone center optical phonons on the surface orientation. Consequently, the dielectric properties that determine the condition of existence, dispersion relations, and lifetimes of the SPhP modes were accurately retrieved. The dielectric functions were determined as a function of the angle of incidence and used to compute the characteristics of the SPhP modes on each of the measured facets. We found that facets of different orientations exhibit SPhP modes of different frequencies and lifetimes, which makes the investigated self-nucleated crystal potential candidates for tunable near-field infrared sources.

Grain boundary engineering of high performance multicrystalline silicon: Control of iron contamination at the ingot edge

The high performance multicrystalline (HPMC) silicon material with the feature of small and uniform grains has already been widely adopted in photovoltaic industry nowadays. However, the HPMC silicon ingots still suffer a comparatively lower minority carrier lifetime at the ingot edges induced by Fe in-diffusion. Here, we have engineered the grain boundaries (GBs) to control the low carrier lifetime zone at the HPMC silicon ingot edges, based on the grain nucleation enhanced by silicon powder coating at the crucible walls. The resultant GBs with high density paralleling to the crucible walls can getter Fe impurity, and meanwhile become the barriers for Fe diffusion. Therefore, the detrimental effect of interstitial Fe impurity on the carrier lifetime of edge wafers is sufficiently reduced and the performance of corresponding solar cells is improved. The solar cells have a narrower distribution in the performance, which is beneficial for the stability and durability of solar cells and modules. This growth concept using GBs to control the behaviors of Fe diffused from the crucible walls is interesting for photovoltaic application.

Numerical and experimental investigation of heat transfer process in electromagnetically driven flow within a vacuum induction furnace

The paper presents an advanced numerical study of the operation of the vacuum induction furnace. The two-way coupling of electromagnetic and fluid dynamics fields was developed to accurately predict the temperature distribution within the crucible melt and walls that were simplified to two-dimensional axisymmetric domain. To define the heat transfer inside the charge, both radiative and convective heat fluxes were taken into consideration. To solve electromagnetic problem, a set of differential Maxwell equations with appropriate boundary conditions were specified. Numerical simulations were performed for several cases to examine the influence of inductor position and power on the coupled processes inside the crucible. The proposed mathematical model was validated against experimental data obtained in the real vacuum induction furnace. The measurements were performed using non-intrusive contactless methods with infrared and high-speed cameras. Validation of free surface showed clearly that numerical results were within the standard deviation for two variants of coil input power. The obtained results of heat transfer within the crucible indicated importance of the radiative heat transfer, especially that of the charge. Moreover, the proposed model showed a good agreement in terms of charge temperature and temperature profile on the crucible wall with a relative error lower than 5%.

Solidification of Undercooled Melts of Al-Based Alloys on Earth and in Space

Containerless processing of droplets and drops by atomization and electromagnetic levitation are applied to undercool metallic melts and alloys prior to solidification. Heterogeneous nucleation on crucible walls is completely avoided giving access to large undercoolings. Experiments are performed both under terrestrial (1 g) conditions and in reduced gravity (µg) as well. Microgravity conditions are realized by the free fall of small droplets during atomization of a spray of droplets, individual drops in a drop tube and by electromagnetic levitation of drops during parabolic flights, sounding rocket missions, and using the electro-magnetic levitator multi-user facility on board the International Space Station. The comparison of both sets of experiments in 1 g and µg leads to an estimation of the influence of forced convection on dendrite growth kinetics and microstructure evolution.

Defect related radiative recombination in mono-like crystalline silicon wafers

We report on studies of sub-bandgap defect related photoluminescence (DRL) signals originating from radiative recombination through traps in the bandgap of cooled mono-like silicon wafers. Spectrally resolved photoluminescence (SPL) and multivariate curve resolution (MCR) have been used in combination, to study the behaviour of sub-bandgap photoluminescence (PL) emissions in wafers cut from different heights in a pilot-scale mono-like silicon ingot. No DRL signals were found in the main mono-like body. Strong defect related sub-bandgap emissions correlating with heavily dislocated areas, are found directly above some of the seed junctions. The DRL signal exhibits a correlation with the number of axis with small angle misalignment in the junctions of the seeds. The signal conventionally labelled D1 (0.80 eV) decreases with ingot height. A mechanism relating this signal to oxygen is proposed. The signals D3 (0.94 eV) and D4 (1.00 eV) are found to co-occur, supporting previous studies, and similarly to the D2 (0.87 eV) signal, their strength is found to increase with ingot height. As the content of the transition metal impurities in the ingot is supposed to increase with height, this supports a reported link between the D3 and D4 signals with Fe, as well as a link between D2 and other impurities. An emission previously found in multicrystalline material and labelled D07 (0.70 eV), is found to solely exist as the only DRL signal recorded by us in parasitic crystals, growing into the main mono-like ingot from the crucible walls. This contradicts the common notion that the D1–D4 signals are strongly related to, and always follow dislocations. Total photoluminescence spectrum (right) and distribution (left) of the PL signal with centre energy 0.70 eV emanating from the parasitic crystals growing into the bulk mono-like Si crystal from the crucible walls.

Electronic Quality Improvement of Highly Defective Quasi‐Mono Silicon Material by Phosphorus Diffusion Gettering

Quasi‐mono silicon (QM‐Si) attracts interest as a substrate material for silicon device processing with the promise to yield single‐crystalline silicon quality with multicrystalline silicon cost. A significant barrier to widespread implementation of QM‐Si is ingot edge‐contamination caused by the seed material and crucible walls during crystal growth. This work aims to recover the scrap material in QM‐Si manufacturing with a process easily adaptable to semiconductor device manufacturing. A phosphorus diffusion process at 870 °C for 60 min significantly improves the electronic quality of a QM‐Si wafer cut from a contaminated edge brick. The harmonic minority carrier recombination lifetime of the wafer, a key predictor of ultimate device performance, experiences a tenfold increase from 17 to 178 µs, which makes the scrap QM‐Si material usable for device fabrication. Local areas with suboptimal (<50 µs) lifetimes remaining can be further improved by a high temperature anneal before the phosphorus diffusion process.

Synthesis and crystal structure of a new aluminum-silicon-nitride phosphor containing boron, Ba5B2Al4Si32N52:Eu

Single crystals of Ba5B2Al4Si32N52:Eu were grown on the wall of a boron nitride crucible by heating a starting mixture of binary nitrides at 2050°C and a N2 pressure of 0.85MPa. The fundamental reflections of X-ray diffraction (XRD) for the crystals were indexed with triclinic cell parameters, a=9.7879(11) Å, b=9.7920(11) Å, c=12.7226(15) Å, α=96.074(4)°, β=112.330(3)°, and γ=94.080(4)°. Streak lines were observed between the fundamental reflections in the direction of the c* axis in the oscillation XRD images and selected area electron diffraction (SAED) patterns, indicating stacking faults in the structure. The atomic images of stacking faults with a slip system of (0 0 1)[−1 1 0]/3, and displacement of a Ba atom layer with (0 0 1)[−1 −1 0]/6 were observed with a scanning transmission electron microscope (STEM). The models of the basic (normal-stacking) structure with space group P1 and local structures of the stacking faults are herein presented. The single crystals emitted blue light with a peak wavelength of 472nm and a full width at half maximum of 78nm under 365nm excitation.

Facile synthesis of silicon nitride nanowires with flexible mechanical properties and with diameters controlled by flow rate

Ultralong Si3N4 nanowires (NWs) were successfully synthesized with size controlled in N2 gas by using an efficient method. The diameters of the Si3N4 NWs increased when the flow rate of N2 gas increased, with average diameters of 290 nm from flow rates of 100 ml/min, 343 nm from flow rates of 200 ml/min and 425 nm from flow rates of 400 ml/min. Young’s modulus was found to rely strongly on the diameters of the Si3N4 NWs, decreasing from approximately 526.0 GPa to 321.9 GPa; as the diameters increased from 360 nm to 960 nm. These findings provide a promising method for tailoring these mechanical properties of the NWs in a controlled manner over a wide range of Young’s modulus values. Vapour-liquid-solid (VLS) mechanisms were used to model the growth of Si3N4 NWs on the inner wall of an alumina crucible and on the surface of the powder mixture. Alumina may be an effective mediator of NW growth that plays an important role in controlling the concentrations of Si-containing reactants to support the growth of NWs on the inner wall of the alumina crucible. This approach offers a valuable means for preparing ultralong Si3N4 NWs doped with Al with unique properties.

Numerical Study of the Thermal and Flow Fields during the Growth Process of 800 kg and 1600 kg Silicon Feedstock

Two-dimensional (2D) transient numerical simulations are performed to investigate the evolution of the thermal and flow fields during the growth of multi-crystalline silicon ingots with two different silicon feedstock capacities, 800 kg and 1600 kg. The simulation results show that there are differences in the structure of the melt flow. In the 1600 kg case, there is a reduction in the concavity of the crystal-melt interface near the crucible wall and an increase in the convexity of the interface at higher solidification fractions. Moreover, the Voronkov ratios, which are indicative of the formation of defects, become lower during the solidification process.

Global simulation of an RF Czochralski furnace during different stages of germanium single crystal growth, part II: to investigate the effect of the crucible's relative position against the RF coil on the isotherms, flow fields and thermo-elastic stresses

By changing the vertical relative position of the crucible against the induction coil, the flow and thermal fields in the melt are seriously affected. The distribution of heat generation along the crucible vertical inner wall is assumed in three cases. A downward shift of the crucible leads to an increase in the heat generation rate near the crucible rim and its reduction near the bottom, while the temperature distribution along the crucible bottom wall varies monotonically with the transmission of the crucible. With downward movement of the crucible and relocation of the hottest region from bottom to top, a reduction in the maximum temperature of 17 degrees can be seen and the freezing amount of liquid on the floor rapidly increased. In contrast, if the crucible is pulled upward, the crucible vertical wall's temperature decreases from bottom to top, due to heat generation by the induced electromagnetic field, and some melt was frozen at the melt free surface. Also, there were two eddy currents in opposite directions within the melt which improve the heat transfer to the crystal–melt interface and the convexity of the germanium crystal decreased. Changes in the manner of thermo-elastic stress distribution against the crucible's relative movement have also been reported.

Effect of Different Cooling Rates Condition on Thermal Profile and Microstructure of Aluminium 6061

Thermal analysis and microstructure characterization provide information regarding material thermal profiles and microstructure formation. Wrought aluminium alloys offer significant advantages in terms of higher ultimate tensile strength (UTS) and yield strength but relatively poor fluidity properties. The objective of this experiment presented in this paper was to understand the relationship between solidification rate, metallurgical behaviour, and fraction phase growth of wrought aluminium 6061. This information was crucial and important to the foundry industry to understand the material behaviour that will help to cast wrought aluminium 6061. Thermal analysis and microstructure of wrought aluminium 6061 on different cooling conditions are present in this paper. In this work, Aluminium 6061 heated and melted in a graphite crucible at a temperature of 800°C. Two thermocouples located at the centre and 20 mm from the graphite crucible wall. Slow cooling rate condition experiment rig was developed by placing graphite crucible into a chamber with kaowool insulation. Normal cooling rate condition was developed by allowing the molten solidify at room temperature. Fast cooling rate condition was prepared by applying a forced airflow over the graphite crucible. The slow, normal, and high cooling rates were calculated at 0.03°C/s, 0.2°C/s and 0.3°C/s respectively. Cooling curve analysis was performed to predict various areas of solidification phase and fraction solid. In Addition, the microstructure formation was observed, recorded, and compared between different cooling conditions. The results show slow cooling rate condition formation of eutectic and solidus temperatures occurred far from liquidus temperature. The eutectic and solidus temperature was increased with the increment of the cooling rate. Furthermore, the DCP temperature of slow cooling rate condition at 638.3°C was the lowest while gives wider temperature range corresponding to the fraction solid percentage increment. Meanwhile, an increase in cooling rate refined the microstructure, improved the grain circularity and at the same time reduced the aspect ratio.

Control of melt-crystal interface shape during sapphire crystal growth by heat exchanger method

We numerically investigate the melt-crystal interface shape during the early stage of the solidification process when the crystal diameter increases. The contact angle between the melt-crystal interface and the crucible bottom wall is found obtuse during this stage, which is unfavorable for the crystal quality. We found that the obtuse contact angle is caused by the thermal resistance difference between the sapphire crystal and melt as well as the insufficient cooling effect of the crucible bottom. Two approaches are proposed to suppress the obtuse contact angle. The first approach is to increase the emissivity of the outer surface of crucible bottom. The second approach is to install a heat shield near the crucible bottom. The reduction of the emissivity of the heat shield is also favorable for the suppression of the obtuse contact angle. Compared with the increase of the emissivity of the crucible bottom, the installation of a heat shield is a more effective approach to prevent the appearance of an obtuse contact angle for the sake of reliability since a molybdenum heat shield can be reused and will not induce other impurities.

Defect characterization of β-Ga2O3 single crystals grown by vertical Bridgman method

The characteristics of structural defects observed on (100) wafers in β-Ga2O3 single crystals grown by directional solidification in a vertical Bridgman furnace were studied in terms of crystal growth conditions. No high-dislocation-density regions near the wafer periphery were observed owing to the lack of adhesion between the as-grown crystal ingot surface and the crucible inner wall, and directional solidification growth in a crucible with a very low temperature gradient resulted in β-Ga2O3 single crystals with a low mean dislocation density of 2.3 × 103 cm−2. Line-shaped defects up to 150 µm long in the [010] direction were detected at a mean density of 0.5 × 102 cm−2, which decreased with decreasing growth rate. The line-shaped defect structure and formation mechanism were discussed.

Applications of novel effects derived from Si ingot growth inside Si melt without contact with crucible wall using noncontact crucible method to high-efficiency solar cells

The noncontact crucible (NOC) method was proposed for obtaining Si single bulk crystals with a large diameter and volume using a cast furnace and solar cells with high conversion efficiency and yield. This method has several novel characteristics that originate from its key feature that ingots can be grown inside a Si melt without contact with a crucible wall. Si ingots for solar cells were grown by utilizing the merits resulting from these characteristics. Single ingots with high quality were grown by the NOC method after furnace cleaning, and the minority carrier lifetime was measured to investigate reduction of the number of impurities. A p-type ingot with a convex growth interface in the growth direction was also grown after furnace cleaning. For p-type solar cells prepared using wafers cut from the ingot, the highest and average conversion efficiencies were 19.14% and 19.0%, respectively, which were obtained using the same solar cell structure and process as those employed to obtain a conversion efficiency of 19.1% for a p-type Czochralski (CZ) wafer. Using the cast furnace, solar cells with a conversion efficiency and yield as high as those of CZ solar cells were obtained by the NOC method.

Formation mechanism and properties of twinned structures in (111) seeded directionally solidified solar grade silicon

The growth structure of photovoltaic multicrystalline silicon formed by directional solidification presents a high fraction of Σ3 and higher order twins. Previous studies proposed that these complex structures are formed by a succession of 2D nucleation events of Σ3 twins on {111} growth facets at the triple line formed by their intersection with the crucible wall, another crystal, or the surface. In this work, we report the reproducible formation of multiple twinned domains inside solar grade Si single crystals grown by directional solidification above a (111) seed. These domains start on a Σ3 twin nucleated inside the crystal bulk, and systematically develop into similar twinned structures characterized by a ternary arrangement of grains in Σ3, Σ9, and Σ27 relationship. The mechanism of formation of the initial twin nucleus is discussed, and a scenario is proposed for the processes of subsequent multiple twinning. The growth competition between twin grains is shown to promote the appearance of incoherent twin boundaries, and dislocations near grain boundaries and in the twin grains themselves. The electrical activity of Σ-boundaries is measured, and the correlation between the structure of the defects and the resulting detrimental electrical activity is then discussed.

Modeling of poly-crystalline silicon ingot crystallization during casting and theoretical suggestion for ingot quality improvement

Finite element (FE) and cellular automaton (CA) models are used in order to predict the microstructure of Poly-Crystalline Silicon (PC-Si) ingots during casting in a directional solidification furnace. During solidification, the 3D-FE model is used to simulate thermal field inside the furnace, while the nucleation and growth of PC-Si are simulated by the modified version of CA model. In this model both nucleation at silicon-crucible interface and nucleation of equiaxed grains on impurities are modeled. The origin of impurity is considered SiC particles when carbon segregates during the solidification and precipitates as SiC particles if carbon solubility limit is reached. The model is evaluated with experimental data for different parameters, such as: the temperature profile at the top and bottom of the PC-Si, grain size, and the height of solidified silicon during solidification. A one-dimensional form of grain growth is observed at the crucible-silicon interface, while a more two-dimensional form of growth is predicted at higher silicon height. The microstructure of PC-Si shows that the grain size increases as a function of PC-Si ingot height. The effects of the cooling rate and competitive growth on the final microstructure of PC-Si are also investigated. The results show that less nucleation sites are formed at the bottom of the crucible with a slower cooling rate, which leads to a larger grain size. Furthermore, the grains at the crucible side walls tend grow inward at a lower cooling rate, which can significantly change the microstructure of PC-Si. This study shows that the competitive growth phenomena plays an important role in the final microstructure of silicon ingots. A simulated model is proposed which shows that by changing the density and location of nucleation sites it is possible to achieve more effective control on final PC-Si micro structure. This simulation helps to explain the crystal behavior observed in ingots cast under different conditions.

Growth of β-Ga2O3 single crystals using vertical Bridgman method in ambient air

A new approach to β-Ga2O3 single crystal growth was studied, using the vertical Bridgman (VB) method in ambient air, while measuring the β-Ga2O3 melting temperature and investigating the effects of crucible composition and shape. β-Ga2O3 single crystals 25mm in diameter were grown in platinum–rhodium alloy crucibles in ambient air, with no adhesion of the crystals to the crucible wall. Single crystal growth without a crystal seed was realized by (100) faceted growth with a growth direction perpendicular to the (100) faceted plane.

Nonstoichiometry and luminescent properties of ZnSe crystals grown from the melt at high pressures

50mm diameter ZnSe crystals have been grown from the melt by a vertical Bridgman technique at 100atm argon pressure in a graphite crucible. 3D impurities concentration and nonstoichiometry mappings of the grown crystals have been defined by ICP-MS and a direct physic-chemical method, correspondingly. Photoluminescence mapping of the analyzed crystal has been done. It was found out that along the crystal height the nonstoichiometry changed from Se excess over stoichiometrical composition in the cone (bottom) part to Zn excess in the tail (upper) part passing through the stoichiometrical composition in the cylindrical part of the crystal. Metal impurities concentrated in the upper part of the crystal. The gas-forming impurities (H, C, O, N, F) had stochastic distribution but Cl impurity concentrated in the crystal peripheral part (near the crucible walls). It was found out that the as-grown crystal had a single wide PL peal with maximum of 583nm. A proposal about complex structure luminescent center based on Cl dopant an overstoichiometric Se has been made.

Measuring thermal conductivity of powders with differential scanning calorimetry

This paper simplifies a recently proposed method for measuring the thermal conductivity of powders using differential scanning calorimetry (DSC) (Sanchez-Rodriguez et al. in J Therm Anal Calorim 121:469–473, 2015). With this method, a crucible is filled with powder and a spherical metal reference is partially sunk into it. The thermal resistance between the metal and the crucible wall at the metal melting point is obtained from the DSC melting peak slope. In the simplified method outlined in this paper, a cylindrical pan is substituted for the original hemispherical crucible. The equivalence of both methods is demonstrated with alumina powder and commercial cylindrical crucibles of several sizes and aspect ratios.

Torsional vibration measurement of the viscosity of a metallic melt

The liquid Co91B9 alloy is used as an example to study the influence of boundary conditions at the upper melt boundary on the results of viscosity measurements using torsional vibrations. Specific features related to film effects and wetting phenomena are shown to appear in the temperature dependence of logarithmic decrement. To exclude the influence of these effects and phenomena, viscosity measurements should be performed under the experimental conditions where the melt to be studied is in closed volume and the internal crucible walls are fully wetted. The temperature dependence of the kinematic viscosity of the Co91B9 melt that is obtained under such experimental conditions has a monotonic character.