Effects of melting parameters and quartz purity on silica glass crucible produced by arc method

Effects of melting parameters and quartz purity on silica glass crucible produced by arc method

We have investigated the effect of hydroxyl (OH) content in fused silica crucible on the scintillation and optical properties of the CsI single crystal, but not limited to, grown by Bridgman technique. For the purpose, 0.1 mol% Tl doped CsI single crystals were grown in crucibles made from fused silica of different grades with OH content varying from 20 ppm to 200 ppm. Silica glass of crucibles was characterized by FTIR and UV–VIS-NIR spectroscopy for the estimation of OH content. Grown crystals were tested for their scintillation performance and a correlation between OH content in silica glass and crystal quality is established. The possibility of ‘OH’ out-diffusion from silica crucible into the melt at higher temperature was further established by temperature dependent study of outgassing from silica crucible by residual gas analyzer (RGA). Further, an optimized process for silica crucible annealing to remove OH (<20 ppm) is proposed to achieve excellent crystal quality of a 5.6% energy resolution at 662 keV without any co-doping in Tl doped CsI.

In photovoltaic industry, silica crucible has an important influence on the quality of single crystal silicon. To obtain a silica glass crucible with large diameter, high uniformity, and low bubble content, two series of crucibles were prepared by the arc melting method, one with various melting parameters (initial power, melting power, and melting time) and crucible sizes, and the other with various high purity quartz crucible. The bubbles inside the crucible wall and pores on the inner surface were all measured using a polarised optical microscope and a portable microscope. The results show that all crucibles have a bubble aggregation area in their inner surface (0–0.4 mm), in which the density and size of bubbles are affected by melting time, melting power, and the distance between the crucible and the graphite electrode. The uniformity of the crucible decreases as the crucible diameter increases (16–28 inches), and the crucible is relatively stable when the initial power is below 400 kW. In final, a silica crucible with large size (diameter of 28 inches) and low bubble content on inner surface (∼50% reduction than that of traditional crucibles) was successfully prepared, which is of great value to the photovoltaic industry.

Currently, the primary materials for fabrication of solar cells are polycrystalline silicon and monocrystalline silicon, with a market share greater than 85% [1]. Solar cells with higher efficiency can be fabricated from monocrystalline silicon, which is usually obtained using the Czochralski (Cz) method [2, 3]. The silica crucibles used in the Cz method are typically made from high-purity amorphous silica. In general, these crucibles consist of two different layers: a transparent layer (Almost bubble free) and a bubble-containing layer (BC layer) [4]. In the outer BC layer, the material contains many bubbles, which decrease transparency. The composition of the gas inside the bubbles remains a matter of debate. It is most likely air, perhaps containing traces of carbon, or water vapour [5]. The inner transparent layer is almost completely transparent, and because this layer is in direct contact with the silicon melt, it is important to keep it free from bubbles throughout the Cz process to ensure that fewer bubbles are released into the melt and subsequently into the silicon ingot.

The silica crucible, which is in direct contact with liquid silicon, has an important impact on the quality of monocrystalline silicon, and silicon wafers with pinholes or dislocations cannot be used for solar-cell fabrication [6]. The industry has therefore devoted extensive efforts to preventing bubbles from entering the melt during the phase of crystal growth [4]. The high-purity quartz sand used to prepare glass crucibles contains various amounts of mineral inclusions (mica, feldspar, etc) and fluid inclusions [7, 8], which can form bubbles at high temperatures. In addition, other gases can affect the quality of monocrystalline silicon. Gas bubbles of SiO and CO can be produced in the melt–crucible interface [9, 10], forming small gas bubbles in the crystal or leading to the generation of dislocations inside the crystal [11]. Argon may also enter the silicon ingot as a protective gas [12]. Reducing the release of bubbles from the transparent layer into the melt during the Cz process can reduce the number of defects in the structure of monocrystalline silicon [13]. In fact, many experiments have been carried out to improve the properties of polycrystalline silicon by improving the purity of fused silica crucibles, but few studies have been reported in the field of single-crystal silicon [14–16]. Therefore, one of the purposes of this experiment is to reduce the bubble content in the transparent layer of crucibles by reducing the impurity element content of quartz.