Releasing Material for the Growth of Shaped Silicon Crystals

Abstract

A mold coating material for a melt‐shaped crystal growth technique has been developed. After studying the contact angle and reaction between the molten silicon and other various high melting point materials, it was found that for shaped crystal growth with a lower impurity contamination level, a combination coating of Si3N4 and SiO2 is suitable for a mold coating material. A double layer coating was applied to the graphite mold base. The bottom layer in the mold, which had a 100–150 μm thickness, consisted of Si3N4 that had been formed by nitriding a spray‐coated Si powder. The 200 μm thick top layer consisted of a Si3N4‐SiO2 material. It had been produced by oxidizing a sprayed‐on coating consisting of Si3N4 powder mixed in a silanol liquid. Using the mold prepared with these coatings, a melt‐shaped crystal growth technique was carried out by the spinning‐casting process. Polycrystalline silicon sheets, with a 10×10 normalcm rectangular dimension and 0.5 mm thickness the size of a mold cavity, were obtained without adhesion to the mold. The sheet had an oxygen content of 1.7 ppm and a carbon content of 0.33 ppm. The mold was usable repeatedly up to 10 times.

Wetting and adhesion of Si on Si3N4 and BN substrates

“…(2) taking θ C = 49 • , θ ox = 90 • and α ox = 0.1 leads to a value θ Si 3 N 4 = 43 ± 3 • for the intrinsic contact angle of Si on Si 3 N 4 . The equilibrium contact angle obtained in this study is in acceptable agreement with the angles measured by Barsoum and Ownby 7 and by Maeda et al 1 considering the different types of Si 3 N 4 used by the authors (Table 1). In order to decrease P(O 2 ), Li and Hausner 6 performed their experiments in closed crucibles made of Zr or Ta, i.e.…”

“…Nevertheless, BN is of great interest, for instance as crucible material for studying physicochemical and physical properties of molten Si and Si alloys, as it is one of the very few materials (and probably the only one) that are non-wetted by liquid silicon (θ > 90 • ). However, literature data of contact angles indicate a wide range of values, lying between 95 • and 145 • , and contradictory results are obtained for adhesion 1,5,8,9 (Table 2). These literature data justify the work carried out here.…”

“…contact angles θ much higher than 90 • ) which is a favourable condition for obtaining a spontaneous detachment of solidified silicon from the crucible walls under the effect of thermo-mechanical stresses. 1,2 In specific cases, for instance when processing silicon ribbons by solidification on a foreign substrate passing through a liquid bath, good wetting (θ 90 • ) is required for the substrate material. 3 In this case dense materials must be employed in order to avoid infiltration of open porosities by silicon.…”

Wetting Behavior and Reactivity of Molten Silicon with h-BN Substrate at Ultrahigh Temperatures up to 1750 °C

“…It was recognized that the involved mechanism of interaction between molten silicon and ionocovalent oxides (Al 2 O 3 , SiO 2 , MgO, ZrO 2 ) is based on a partial dissolution of a substrate, leading to the oversaturation of the melt with oxygen followed by reprecipitation of SiO 2 product at the interface during cooling (Ref 8). The only exception among all examined Si/ceramic systems is hexagonal boron nitride (h-BN), a ceramic for which non-wetting behavior of Si is observed up to 1500°C (Ref [8][9][10][11][12].…”

Growth velocity and grain size of multicrystalline solar cell silicon

“…Nucleation could initiate from this surface instead of the bottom or the edges of the crucible. Due to the poorer wetting properties between silicon and silica (wetting angle of 951 [28]) compared to silicon and silicon nitride coated silica (wetting angle more likely to be below 901 [3]), there is a higher probability that silicon will nucleate on the silicon nitride coated substrate. The lack of correlation in variation of coating supports this.…”