3.1. Global distributions of impurities in DS furnaces at 50% crystalline fraction
The O and C impurities in the DS furnace mainly include SiO and CO impurities in argon and O and C impurities in silicon melt and silicon crystal. The distributions of impurities in the furnaces at 50% crystalline fraction are shown in Fig. 2, in which Fig. 2(a) shows the distributions of SiO impurity in the argon and O impurity in the silicon, and Fig. 2(b) shows the distributions of CO impurity in the argon and C impurity in the silicon. From Fig. 2(a), it can be seen that the maximum value of SiO concentration appears at the corner of the free surface and the side wall of the crucible, where the concentration of O impurity is the highest and the flow velocity of argon gas is the lowest. Compared with G6, the maximum value of the SiO concentration in the argon gas in the G7 furnace increases, and the O impurity distribution in the silicon is less uniform. It can be seen from Fig. 2(b) that the C impurity has an evident segregation phenomenon at the crystal-melt interface. This is because the segregation coefficient of C in silicon is low (0.07), and most of the C atoms are discharged into the silicon melt during the solidification process. Compared with before the upgrade, there is a smaller concentration gradient of C impurity in the furnace after the upgrade. Generally speaking, the concentration of SiO and CO in argon ranges from 10− 9 mol·cm− 3 to 10− 8 mol·cm− 3, and the concentration of O and C in silicon ranges from 1016 atom·cm− 3 to 1018 atom·cm− 3.
3.2. Distributions of O and C in silicon at 100% crystalline fraction
Figure 3 shows the comparison of concentration distributions of O and C impurities in silicon ingot before and after size upgrade. As shown in Fig. 3(a), the distributions of O and C impurities in silicon before the size upgrade are relatively uniform, and the O and C impurities in the horizontal direction are basically at the same concentration level. However, as shown in Fig. 3(b), the O impurity concentration in the horizontal direction is less uniform after the size upgrade, especially at the middle horizontal position of the silicon ingot, it spans several orders of magnitude. This is because after the upgrade, the temperature difference in the silicon area only slightly increases, the radial temperature difference increases by about 1 ~ 2 K, and the flow intensity in the center of silicon melt does not significantly increase. Moreover, with the increase of crucible siz, the diffusion of impurities in the center of melt is relatively weakened. It is more difficult for impurities to reach the central area from the wall and the impurities distributions are more uneven in the horizontal direction. The numerical results show that the average concentrations of O and C impurities in the silicon crystal in the G7 furnace are reduced compared with the G6 furnace, in which the average O concentration decreases by 6.7% and the average C concentration decreases by 7.3%.
3.3. Argon flow structure above the free surface of the silicon melt
As a protective gas in the crystalline silicon ingot furnace, Argon is not only used for cooling but also has the function of removing impurities. Therefore, the argon flow tends to affect the distributions of SiO and CO impurities. Figure 4 shows the flow structure of argon near the upper side of the free surface in G6 (left) and G7 (right) furnaces. As seen from the figure, argon gas first enters the furnace vertically from the argon tube, scours the center of the free surface, then turns and flows along the free surface to the side wall of the crucible, and flows upward along the side wall of the crucible. Finally, a part of argon flows out from the gap between the graphite cover and the quartz crucible, and the other part of argon flows back to the free surface along the surface of the cover, where a vortex is formed. Compared with the G6 furnace, the distance between the graphite cover and the free surface of melt in the G7 furnace is closer. The argon gas flows out more easily from the side outlet and takes away a lot of the SiO gas. At the same time, the smaller space in the vertical direction makes the argon gas better maintain the vertical flow, which significantly reduces the accumulation of SiO in the corners and the CO flowing back from the outlet. The above is exactly the expected result of the furnace upgrade, which takes away more SiO and reduces the dissolution of CO.
3.4. Evaporation rate of SiO and dissolution rate of CO at different crystalline fractions
Figure 5 shows the concentration of SiO and CO at the free surface at different crystalline fractions. As shown in Fig. 5(a), the concentration of SiO in the radial direction of the free surface appears to be low at the center and high at the edges. At different crystallization fractions, the SiO concentration at the free surface of the G7 furnace is slightly higher than that of the G6 furnace, and the CO concentration is lower than that of the G6 furnace. This is because the distance between the G7 furnace and graphite cover plate on the free surface of silicon melt is smaller, and the flow structure of argon has changed, which may have an impact on the argon flow rate and SiO evaporation rate at the free surface, and make more SiO evaporate from the free surface. In Fig. 5(b), contrary to the law of SiO concentration, the CO concentration at the free surface appears to be high at the center and low at both ends. This is because CO first reaches the center with the backflow of argon, where the concentration of CO is the highest. The CO is produced by the chemical reaction on the surfaces of high-temperature graphite such as heaters and cover, and flows over the surface of the silicon melt with the backflow of argon. The CO concentration at the free surface of the G7 furnace is lower than that of the G6 furnace under different solidification fractions. The lower CO concentration indicates a narrowing of the space above the free surface, making it more difficult to store the CO brought along with the argon reflux here, and indirectly reducing the dissolution of C atoms into the silicon melt.
3.5. Impurities distributions of O and C in silicon at different crystalline fractions
The distributions of O and C in the silicon region at different growth stages in the G7 furnace after size upgrade were investigated. Figure 6 shows the distributions of O and C impurities in silicon at 20% and 80% crystalline fractions. It can be seen from Fig. 6(a) that the highest concentration of C impurity occurs in the center of the silicon melt at 20% crystalline fractions, which is due to the effect of the argon vortex above the free surface of the melt, as we described earlier, the maximum CO dissolution rate is located at the center of the free surface, which means that more CO dissolves in the center. However, at the crystalline fraction is 80% in Fig. 6(b), the content of C impurity at the crystal-melt interface is the highest, and the position of the highest concentration of C impurity is transferred. This is because, with the growth of the crystal, the contact area between silicon melt and crucible decreases, and the surface area of the O source decreases, so that the number of O atoms entering the silicon melt also decreases, resulting in the decrease of SiO evaporation and CO dissolution on the free surface, the high concentration region of C at the free surface disappears. In addition, the C impurity has an obvious delamination phenomenon near the crystal-melt interface, which is due to the low segregation coefficient of C, so that most of the C atoms are discharged into the silicon melt, and only a small amount of C atoms enter the silicon crystal. With the progress of crystal growth, the C atoms near the crystal-melt interface continue to accumulate, and the position of the highest concentration of C impurity shifts from the center of the free surface to the vicinity of the crystal-melt interface from the initial stage. At different crystalline fractions, the distribution law of O has no apparent change.
3.6. Distributions of O and C on the central axis of silicon at different crystalline fractions
Figure 7 shows the concentrations distributions of O and C impurities on the central axis of silicon in the G7 furnace at different crystalline fractions. It can be seen from Fig. 7(a) that the distribution law of O impurity along the central axis of the silicon region is as follows: the concentration of O impurity at the bottom is higher, mainly concentrated at the bottom of the silicon ingot. As the height on the axis increases, the O impurity concentration gradually decreases and stabilizes at about 2.0×1017 atom·cm− 3. It can be seen from Fig. 7(b) that the C impurity concentration increases gradually from the bottom to the top on the central axis of the silicon region. In addition, with the increase of crystalline fraction, the concentration of C impurity at the same position also increased significantly.
3.7. Average distributions of O and C impurities in silicon at different crystalline fractions
Figure 8 shows the changes in the average concentrations of O and C impurities in silicon melt and silicon crystal at different crystalline fractions. As shown in Fig. 8(a), with the increase of crystalline fraction, the average concentration of C impurity in the silicon melt increases continuously, while the concentration of O impurity in the silicon melt does not change significantly. The increase of C impurity concentration is caused by the dissolution of CO at the free surface of the melt and the diffusion of C impurities into the silicon melt at the crystal-melt interface. Figure 8(b) shows the change in the average concentrations of O and C impurities in the silicon crystal. With the increase of crystalline fraction, the average concentration of C impurity in silicon melt increases, while the average concentration of O impurity decreases gradually. The reason is that with the increase of crystalline fraction, the contact area between high-temperature silicon melt and crucible decreases, and the number of O atoms entering the silicon melt decreases. The increasing average concentration of C in the silicon crystal is because most of the C atoms are concentrated in the silicon melt. With the increase of the crystalline fraction, more and more C atoms from the outside dissolve into the silicon region. Therefore, the average concentration of C impurity in the silicon crystal also increases gradually. Compared with the G6 furnace, the average concentrations of O and C in the silicon crystal of the G7 furnace are reduced after the crystal is fully grown, among which the average O concentration is reduced by 6.7%, and the average C concentration is reduced by 7.3%.