Silicon Nitride Crucibles Research Articles

The effect of preliminary heat treatment on the durability of reaction bonded silicon nitride crucibles for solar cells applications

Silicon nitride crucibles have the potential to replace silica crucibles and reduce the cost of silicon crystallization because of their reusability potential. Till date, crucibles’ heat treatment before each use is a prerequisite to achieve non-wetting conditions that is needed to facilitate the ingot release and hence enable reusability. Yet, no studies have examined the heat treatment influence on the crucibles’ durability. The present investigation focuses on the crucibles’ heat treatment and its impact on the crucibles’ lifetime. Repeated heat-treatments of silicon nitride crucibles in the air at above 1100 °C leads to crucible fracture. Therefore, this study identifies the cause and the mechanism of such failures by applying different heat treatment procedures in the air. The mass gain and the oxidation rates of the crucibles at different temperatures are measured via Thermogravimetry (TG) and Differential Thermal Analyzer (DTA). The results show that the porosity and phase distribution along the crucible wall thickness, play a key role in the crucible’s behavior during oxidation. Moreover, excessive internal oxidation in the tested crucibles results in severe thermal stresses which cause cracking during cooling.

Making reusable reaction‐bonded silicon nitride crucibles for silicon casting from kerf‐loss silicon waste

AbstractSilicon kerf loss during wafer slicing and the broken quartz crucibles after silicon casting are two major solid wastes from photovoltaic (PV) industry. Especially, the recycle of kerf‐loss silicon has become an urgent issue because near 100 000 t of solid wastes are generated every year. One of the most meaningful recycle routes of the kerf‐loss silicon is to make silicon nitride crucibles to replace the quartz crucibles. In this study, we demonstrated how this is feasible through acid leaching refining, slip casting, and nitridation. The reaction‐bonded silicon nitride (RBSN) crucibles after oxidation were found pure enough for silicon ingot growth. More importantly, they could be reused after ingot growth. With the present examples, the potential of using the kerf‐loss silicon for fine ceramics is prominent.

Silicon ingot casting using reusable silicon nitride crucibles made from diamond wire sawing kerf-loss silicon

The production of silicon wafers for solar cells generates two major solid wastes. One is the broken quartz crucibles after crystal growth, and the other is the kerf-loss silicon during wafer sawing. In this study, we demonstrated that the reaction bonded silicon nitride crucibles made from the acid leached kerf-loss silicon could be used for silicon casting. The ingots grown from the silicon nitride crucibles had better minority carrier lifetime than that grown from the quartz crucibles; the interstitial oxygen was significantly lower as well. The nitride crucible was reused at least 4 times and remained almost intact.

Crystallization of multicrystalline silicon from reusable silicon nitride crucibles: Material properties and solar cell efficiency

In this paper experimental results on High Performance multicrystalline silicon ingots (HPmc-Si) grown from reusable silicon nitride crucibles are presented. Five ingots are grown from the same crucible and the material is analyzed for carbon and oxygen, and metallic impurities, by means of Fourier transform infrared (FTIR) spectroscopy and Neutron Activation Analysis (NAA), respectively. Microwave Photoconductance Decay (μW-PCD) and Quasi-Steady-State Photoconductance (QSSPC) is applied on samples to reveal the resulting charge carrier lifetime distribution. Full-area aluminum back surface field (BSF) solar cells are processed from 156 mm × 156 mm wafers and the solar cell efficiency is analyzed. Material properties and solar cell efficiencies are comparable to reference material grown from high purity electronic grade (EG) quartz crucibles. Obtained results are discussed with respect to results published recently.

Impurity control in high performance multicrystalline silicon

We report results from a national project about impurities in high performance multicrystalline silicon: Contamination sources, transport routes, interaction with crystal defects and impact on solar cell efficiency parameters. Several ingots were produced in a lab scale furnace. Growth parameters and crucible types were varied, and high purity quartz crucibles were compared to novel silicon nitride crucibles. The material was characterized by a range of methods including FTIR, GDMS, NAA, EBSD, dislocation etching, lifetime analysis. Wafers were processed, and lifetime was measured during the cell processing to analyse the effect of separate processing steps. Models were constructed to simulate the impurity transport during the crystallization. The diffusivity in quartz and silicon nitride crucibles required in the modelling is not generally known, and experiments were performed to evaluate them. Variations in dislocation density have the strongest impact on lifetime, even for this high quality material. The quartz crucible has the highest contamination potential, but the uncertainty in diffusion parameters makes it difficult to conclude if this potential is reached, and for which elements. We therefore make an overview showing which value of the diffusion coefficient shift the contamination dominance between feedstock/crucible/coating. The use of high purity crucibles enables comparable or even higher lifetime material to be made in lab scale compared to industrial settings. Our results indicate that a thin layer of higher purity, lower porosity quartz on the inside of the crucible can be an effective method to transfer this improvement to industrial conditions. The cell processing indicates that the diffusion/gettering sequence profoundly decreases the lifetime by rendering the random high angle grain boundaries electrically active. However, this effect is reversed after firing of the anti‐reflective coating, when the remaining active defects are dislocation clusters and CSL boundaries. Silicon nitride crucibles appear to be a promising low oxygen alternative to traditional quartz crucibles in terms of purity.

Ceramic liner technology for ammonoacidic synthesis

The ammonoacidic crystal growth is a comprehensive method for the synthesis of novel compounds like nitrides or amindes but also for the growth of bulk single crystals like gallium or aluminum nitride for power electronics and photonics. In this report we describe a novel liner technology for growth autoclaves, showing high potential for several research purposes. Thereby the applicability of several ceramic materials as liner materials was investigated for the first time. Moreover, the effectivity of the new apparatus was verified in experimental studies. The described concept based on a silicon nitride crucible is characterized by low costs and diverse possible applications. For example its use is of advantage in fundamental research to explore new nitride materials. Furthermore, it shows high potential for X-ray imaging to investigate basic principles in growth of gallium nitride.

The Interactions of Carbon and Nitrogen in Liquid Silicon

Abstract The interactions between carbon and nitrogen in liquid silicon have been studied experimentally. High purity silicon was melted in silicon nitride crucibles under an Ar atmosphere with a graphite slab inserted in the crucible prior to melting as a carbon source. The system was thus simultaneously equilibrated with Si3N4 and SiC. Samples were extracted in the temperature range 1695–1798 K and analyzed using Leco. It was observed that the simultaneous saturation of nitrogen and carbon caused a significant increase in the solubilities of both elements. The interaction parameters were derived as The solubility of carbon in liquid silicon as a function of temperature and nitrogen content was found to follow: And the solubility of nitrogen in liquid silicon found to follow:

Growth of high-quality multicrystalline Si ingots using noncontact crucible method

Conventional crystal growth methods using silica crucibles cannot control the stress caused by expansion due to the solidification of the Si melt because crucible walls made of silica have insufficient flexibility to reduce the stress. In the case of crystal growth using a silicon nitride crucible, it was reported that an ingot can be more easily released from the crucible. A noncontact crucible method was proposed using conventional silica crucibles that reduces the stress and number of dislocations in Si multicrystalline ingots. We used the present method to grow wafers with only twin boundaries. An ingot with several grains and twin boundaries was realized using a crucible that had not been coated with Si3N4 particles. Several important characteristics were reported such as the presence of a low-temperature region in the Si melt, the possibility of growing large ingots with a diameter close to the crucible diameter, the minority carrier lifetime, the distribution of dislocations, the O concentration and the effect of Si3N4 particles on the crystal structure. Dislocations were almost undetectable in a large area of the cross section when a necking technique was used for the seed growth. The O concentration in ingots grown using crucibles coated with Si3N4 particles was lower than that in an ingot grown using a crucible without a coating of Si3N4 particles. A large ingot with a diameter of 25cm was obtained using a crucible with a diameter of 33cm.

Processing of vacuum heat-treated MgO-densified silicon nitride crucible for molten cast iron handling

Abstract Silicon nitride with 3% MgO powder mix is uniaxially and cold isostatically pressed to form a green Si 3 N 4 crucible. Liquid phase sintering was applied to the green Si 3 N 4 crucible at 1600 °C for 30 min under the nitrogen atmosphere. Intergranular Mg–Si–O–N glass remained between the silicon nitride grains which reacted with the molten metal during melting. This grain boundary glass was removed by vacuum heat treatment at 1575 °C for 5 h. The vacuum heat treated crucible was used to melt cast iron to examine reactions between the molten metal and Si 3 N 4 ceramic crucible. EDX spectra across the Si 3 N 4 –cast iron interface and XRD for silicon nitride sample after cast-iron melting side surface analysis were carried out. Optical microscopy and SEM image analysis were made to examine the interaction between Si 3 N 4 crucible and cast iron melt. Surprisingly, no reaction was observed between the vacuum heat treated crucible and melted cast iron.

Silica versus silicon nitride crucible: Influence of thermophysical properties on the solidification of multi-crystalline silicon by Bridgman technique

In contrast to standard silica crucibles, which can only be used once, crucibles made of silicon nitride have the potential of being used several times for directional solidification of multi-crystalline silicon ingots. In this paper numerical and experimental results on the influence of crucible thermal properties on the solidification of multi-crystalline silicon in a Bridgman furnace are presented. A transient CFD model of the Bridgman furnace has been used for this purpose including all important aspects of heat transfer, e.g. convection, conduction and radiation. Due to the low thermal resistivity of the silicon nitride material more heat is extracted through the crucible bottom, which leads to a prolonged melting time, whereas solidification time is reduced. The prolonged melting time was compensated by replacing the two insulating carbon fibre discs underneath the crucible by alumina fibre material. Finally melting and solidification time is reduced significantly.

Temperature Dependence of the Solubility of Nitrogen in Liquid Silicon Equilibrated with Silicon Nitride

The solubility of nitrogen in liquid silicon equilibrated with silicon nitride and its dependence on the composition of the atmosphere has been studied in the temperature region 1428-1542°C. High purity silicon was melted in silicon nitride crucibles under Ar and N 2 atmospheres. The equilibrium was observed to be established within minutes, after which no evolution of the nitrogen content with time could be observed. The nitrogen transfer between the Si 3 N 4 -crucible and the melt was faster than the transfer via the gas-phase to such an extent that the composition of the atmosphere did not influence the solubility limit. The solubility limit as a function of temperature was found to follow: C N (T, C B ) = 7957.9 * exp(-24376/T) At the melting point of silicon, this equation gives the solubility of nitrogen as 42 ppm mass. From this equation, a thermodynamic derivation led to the following expression for the dissolution energy of nitrogen: ΔG 0 = -1.63 * 10 4 + 26T.

Refinement of .BETA.-Si3N4 Single Crystal Grown from Silicon Melt.

Silicon powder in a reaction-sintered silicon nitride crucible was heated at 1600°C in a nitrogen atomosphere. Clustered β-Si3N4 single crystals were obtained in residual silicon metal after cooling. The silicon residuals were dissolved by chemical purification using a mixture of aqueous HF and HNO3, and subsequently treated and with H2SO4. Scanning electron microscopy observation showed that large β-Si3N4 crystals without Si residuals could be successfully separated by this process.

Nitrogen Solubility in Liquid Silicon

The nitrogen solubility in liquid silicon equilibrated with β-Si3N4 was determined in the temperature range between 1723 and 1873 K. A silicon nitride crucible and an N2–H2 atmosphere (N2:H2=4:1, total pressure=0.1 MPa) were employed for the melting of silicon to secure the Si(l)/Si3N4(s) equilibrium. Nitrogen in silicon was analyzed by the Kjeldahl method. The temperature dependence of nitrogen solubility (CN) in liquid silicon was represented by the following equation:log(CN⁄mass%)=2.410−9759⁄T (±0.24) (T:1723–1873 K).Since oxygen contents in the silicon melt were much smaller than the oxygen solubility, the effect of oxygen in liquid silicon on the nitrogen solubility could be ignored. The nitrogen solubility at the melting point of silicon (1685 K) by extrapolation was around 4 ppm (4×1017 atoms·cm−3), which was one order of magnitude smaller than the literature values reported previously.The standard free energy change for nitrogen dissolution could be represented as:1/2N2=N(mass%, in liquid silicon),ΔG°=−4.45×104+66.4T (J).The correlation between the standard enthalpy change for nitrogen dissolution in liquid silicon and the standard enthalpy of formation of silicon nitride at 298 K per mol of nitrogen agreed with those for other liquid metals. Therefore, the values of nitrogen solubility obtained in the present work were considered to be reasonable.