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Removal of Boron from Silicon by Solvent

Refining Using Ferrosilicon Alloys

Leili Tafaghodi Khajavi, Kazuki Morita, Takeshi Yoshikawa and Mansoor Barati

Version Post-print/Accepted Manuscript

Citation

(published version) Khajavi, L.T., Morita, K., Yoshikawa, T. et al. Metall and Materi Trans B (2015) 46: 615. https://doi.org/10.1007/s11663-014-0236-3

Publisher’s statement This is a post-peer-review, pre-copyedit version of an article published in Metallurgical and Materials Transactions B. The final authenticated version is available online at: http://dx.doi.org/10.1007/s11663-014-0236-3

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Removal of boron from silicon by solvent refining using

ferrosilicon alloys

L Tafaghodi Khajavi1, K Morita2, T Yoshikawa2, M Barati1

1 Department of Materials Science and Engineering, University of Toronto, 184 College Street,

Suite 140; Toronto, Ontario M5S 3E4, Canada 2Institute of Industrial Science, The University of Tokyo, 4-6-1, Komaba, Meguro-ku, Tokyo

153-8505, Japan

E-mail:leili.tafaghodikhajavi@utoronto.ca

Abstract

The distribution of boron between purified solid silicon and iron-silicon melt was evaluated to investigate

the possibility of boron removal from silicon by solvent refining with iron-silicon alloys. The distribution

coefficient, defined as the ratio of the mole fraction of boron in solid to that of liquid, was found to be

strongly dependent on boron concentration. Solvent refining at lower temperatures resulted in smaller

distribution coefficient values. The boron removal percentages for the lowest boron concentration

examined in this study were, 70% (1583 K), 65% (1533 K), and 65% (1483 K). The values obtained for

interaction parameter of boron on iron in solid silicon are as following: -813± 53 (1583 K), -830 ± 92

(1533 K), -863 ± 91 (1483 K). Lower temperature resulted in smaller distribution coefficient and higher

silicon yield.

Keywords: silicon, thermodynamics, solvent refining, impurity, boron

1. Introduction

Silicon is the most common base material for semiconductor devices. It accounts for over 90 percent of

today's PV materials [1-3]. Ultra pure silicon (9N) for solid state electronic devices is industrially

prepared by the distillation and subsequent thermal decomposition of volatile silicon compounds [4-6].

Considering the cost and energy required for the above process and the difference in silicon specification

requirements for application in microelectronics and in PV industry (6-7N), an interest has grown to

develop technologies for mass production of low cost solar grade silicon. Metallurgical techniques have

been particularly focused on because of their ability to deliver large production rates at low cost [3, 4, 7-

12]. Solvent refining is one of the techniques that have been considered as a cost efficient, less energy

intensive purification step for producing solar grade silicon. Basically the process employs an alloying

element as a solvent for crude Si, from which pure Si crystals are later precipitated upon controlled

cooling and solidification. The purification that takes place during crystal growth is mainly due to

impurity rejection by the solidification front. Previously, Al [7, 8, 13-17], Cu [18-20], Ni [21], Sb [22]

and Sn [23] have been used as the alloying elements, or impurity getter, in solvent refining of silicon.

In a new approach to metallurgical refining of silicon by a combination of solvent refining and heavy

media separation, iron has been successfully utilized as the getter [10, 11, 24]. Unlike most metallic

impurities, phosphorus and boron have relatively large segregation coefficients [4] that render them

unresponsive to solidification refining processes. Consequently it is crucial to study the feasibility and

efficiency of phosphorus and boron removal through solvent refining with iron. In a recent study [25],

thermodynamics of phosphorus distribution between purified silicon crystals and the iron-silicon melt has

been investigated. Aiming to establish the thermodynamic fundamentals of the process, the present study

was performed to investigate the distribution coefficient of boron between silicon and the alloy melt.

Furthermore, the interaction parameter between iron and boron was estimated to determine the affinity of

iron for boron.

2. Materials and Methods

Sample preparation involves melting silicon, iron and boron together to form an alloy with the desired

composition. According to iron-silicon phase diagram (Figure 1), the binary alloy should contain more

than 58.2 wt% silicon in order to allow the formation of primary silicon crystals during cooling to the

eutectic temperature. Considering this minimum required silicon content, the solvent refining process was

performed on silicon-iron-boron alloys with ~80 wt% silicon and ~20 wt% iron.

Figure 1. Iron-silicon phase diagram [26]

The presence of impurity elements in the prepared alloy can influence the thermodynamic properties of

boron; consequently it is critical to avoid the introduction of other impurities from the possible sources.

Therefore, electrolytic iron and high purity silicon with +99.9985% metal were used as the starting

materials. Quartz crucibles were considered suitable to avoid contamination from the crucible. An inert

atmosphere was maintained inside the furnace by passing high purity argon (< 2 ppm oxygen) to avoid

sample oxidation. Argon gas passed through Drierite drying column and heated titanium sponge to

remove any trace of humidity and oxygen before entering the furnace. A schematic representation of the

experimental setup is depicted in Figure 2.

Figure 2. Schematic representation of the experimental apparatus.

Each sample was heated up to 1873K followed by a dwell for 3 hours and then slowly cooled down to

1583K at the rate of 0.5K/min. The crucible was kept at 1583K for 1 hour and finally quenched in water.

Different quenching temperatures (1533 and 1483K) were also examined to study the effect of

temperature on boron distribution between Si crystals and Fe-Si alloy melt.

The solidified alloy was crushed and ground to -37 μm (Mesh No. 400). The next stage involved

separation of Si and alloy particles by leaching in a HF solution. The leaching solution consisted of 10

vol% HF, 20 vol% acetic acid and 70 vol% deionized water. Si-Fe alloy dissolves in HF while silicon

particles remain intact. After filtration, the Si product was digested for ICP analysis. The digestion took

place in a Teflon beaker and began with the addition of nitric and sulfuric acid to the sample followed by

gradual addition of hydrofluoric acid. HF was added drop by drop while the beaker was kept in a water

bath cooled with ice packs. Water was added to the solution after the completion of sample dissolution.

Many researchers have mentioned low recovery of boron for environmental, agricultural, and geological

samples with silicate matrix [27-29]. Gradual addition of HF, cooling down the solution and adding water

after the completion of the digestion are practices that prevent boron loss during digestion. For each

experimental condition, one sample including both phases was also digested using the same procedure

before performing ICP-AES analysis.

The presence of even a very small amount of the alloy phase which was not completely washed away by

acid will significantly affect the Fe concentration in Si. Consequently the concentration of Fe in Si was

measured using EPMA. The value was obtained by averaging the concentration from several points in the

silicon phase. The measurements were done with the accelerating voltage of 20 kV, sample current of 280

nA and counting time of 90 minutes. To ensure accuracy, extreme conditions including large beam

current (280 nA), and very long counting time (1800sec) were used for the calibration process.

3. Result and Discussion

A typical SEM micrograph of a sample quenched at 1483 K is presented in Figure 3. The darker phase is

the silicon crystals, while the brittle phase with lighter color is the iron-silicon alloy. Size distribution of

silicon crystals in silicon-iron system and its effect on silicon purification has been studied elsewhere

[10]. As mentioned earlier the samples in the current study were quenched above eutectic temperature

while the quenching temperature in the previous study was below eutectic. Furthermore the samples in

this study contain 80wt.% silicon while samples with 72 wt.% silicon were used by Esfahani and Barati

[10]. It should be noted that different chemical composition and temperature profile of the samples in this

study can result in different size distribution of the silicon crystals compared to aforementioned study.

The concentration of boron in the whole sample and in solid silicon phase was measured using ICP-AES.

Using the two values, the boron content of iron-silicon melt was calculated based on mass balance. The

mass fractions of the alloy melt that was calculated based on the Fe-Si phase diagram are as following:

0.49 (1483K), 0.57 (1533K ) and 0.64 (1583K).

For each sample, the average of 3 replicates was used for reporting. Generally the relative standard

deviation for the boron concentration of each sample was below 5%. In the case of EPMA measurements

the relative standard deviation for iron concentration in most of the samples was below 12%. The

chemical analysis of the samples is presented in Table 1.

Figure 3. Backscattered SEM image of a sample quenched at 1483 K (1210 °C).

3.1. Distribution Coefficient of Boron

The distribution coefficient of boron between solid silicon and iron-silicon alloy melt is defined as:

B in solid Si

BB in Fe Si melt

XK

X

(1)

where K is the distribution coefficient and X is the mole fraction.

In order to examine whether the equilibrium condition is reached after the samples experienced the

proposed heating/cooling profile, different dwell times (0.5, 1, and 2 h) were applied for several samples

experiencing similar maximum temperature and cooling rates. As the diffusion coefficient increases with

temperature, the quenching temperature was set to 1483 K to ensure that the samples quenched from

higher temperatures have reached equilibrium state as well. The only difference between the samples was

the duration of the final dwell, which was set to 0.5, 1 and 2 hours. The initial boron content of these

samples was 520- 620ppm. Figure 4 shows the distribution coefficient of boron calculated for these

samples, indicating that equilibrium has been established within one hour.

Figure 4. Distribution coefficient of boron at different dwell times at 1483 K (1210 °C).

Table 1. Chemical analysis of the samples based on ICP-AES and EPMA measurements.

Sample Temperature (K) CB in Si (ppmw) CFe in Si(ppmw) CB in Alloy(ppmw) CB initial(ppmw)

1 1483 362 2.0 956 653

2 1483 529 3.1 1030 776

3 1483 721 5.4 1330 1022

4 1483 1770 20.0 2330 2048

5 1533 297 1.4 581 459

6 1533 510 3.5 935 752

7 1533 534 7.2 1070 838

8 1533 1090 8.6 1510 1329

9 1533 1820 26.1 2410 2156

10 1583 235 1.8 424 356

11 1583 553 4.6 898 774

12 1583 645 6.3 1000 872

13 1583 1150 7.3 1520 1385

14 1583 1970 61.6 2250 2148

Figure 5 shows the distribution coefficient of boron at different temperatures and boron concentrations.

According to the data obtained, the distribution coefficient of boron is not only a function of temperature

but also a function of boron concentration. It means that the liquidus and/or solidus surfaces of Si-Fe-B

phase diagram that determine the distribution coefficient of boron are curved in the range of our

experiments. It is clear that the distribution coefficient increases with increasing boron content.

Furthermore it can be seen that the distribution coefficient increases with temperature.

Figure 5. Distribution coefficient of boron between solid silicon and iron-silicon melt.

It should be noted that for concentrations approximately below 2000 ppm that is tested in the current

study, the distribution coefficient is smaller than 0.8 which is boron’s distribution coefficient between

solid and liquid silicon [30].

3.2. Boron Removal and Silicon Yield

According to the results obtained for the distribution coefficient, B removal is favoured at lower

temperatures and concentrations. At the same time, lower temperatures lead to higher silicon yield for a

given alloy mass. The extent of boron removal from silicon at each temperature is calculated based on

mass balance, considering the initial and final weight of silicon. The theoretical yield of silicon at each

temperature is calculated from Fe-Si phase diagram using lever rule. The removal percentage of B is

calculated using the following equation:

𝐵𝑜𝑟𝑜𝑛 𝑟𝑒𝑚𝑜𝑣𝑎𝑙 (%) = 100(1 −𝑋𝑆𝑖(𝑝𝑢𝑟𝑖𝑓𝑖𝑒𝑑) × 𝑋𝐵 𝑖𝑛 𝑠𝑜𝑙𝑖𝑑 𝑆𝑖

𝑋𝑆𝑖 (𝑖𝑛𝑖𝑡𝑖𝑎𝑙) × 𝑋𝐵(𝑖𝑛𝑖𝑡𝑖𝑎𝑙)) (2)

The variations of theoretical silicon yield and boron removal percentage with temperature are presented in

Figure 6. Although the distribution coefficient of boron increases with temperature, the boron removal

percentage showed a small rise with temperature. This trend was observed due to the decrease in silicon

yield at higher temperatures. In other words, despite the constant initial Si content, the fraction of purified

Si varies with temperature.

Figure 6. Dependence of boron removal and silicon yield on temperature.

3.3. Comparison of Effective Boron Removal between the Proposed Solvent Refining method

and Directional Solidification

In order to compare boron removal in the current solvent refining method with that of directional

solidification it should be noted that a significant amount of the impurity getter (iron) is added in the case

of solvent refining and subsequently the yield of silicon is different from that of directional solidification.

Therefore a legitimate comparison would require that the yield of silicon in the directional solidification

be equal to that in the solvent refining method.

The concentration profile of boron in an ingot subjected to directional solidification process can be

calculated based on Scheil’s equation [31].

( 1)0 (1 ) k

sC k C f (3)

where k is the distribution coefficient, C0 is the initial concentration of the impurity, and f is the solidified

fraction of silicon. By integrating Equation 3 from 0 to x, the average concentration of the impurity for a

certain fraction of the ingot (x) can be determined using Equation 4.

0

x

S

ave

C df

Cx

(4)

In the current solvent refining process at 1483 K where the smallest distribution coefficient was

obtained, the theoretical yield of silicon is 0.51. Since the initial concentration of Si in the alloy

is 80wt%, the actual yield is = 0.51/0.8= 0.64. Considering the same yield for directional

solidification and kB = 0.8, the average boron content of the refined ingot can be calculated as

0.87C0. This is 2-3 times larger than the B content of silicon produced in the proposed solvent

refining, with 65-70% removal of B. Figure 7 shows the concentration of B vs. the solidified

fraction (Equation 3) considering equal yield for directional solidification (0.64) and kB = 0.8.

The average B concentration calculated using Equation 4 is also depicted in Figure 7.

Figure 7. Boron concentration profile for a sample with 64 wt pct silicon yield based on Scheil equation.

3.4. Interaction Parameter of Boron on Iron

From fundamental point of view, it is important to determine the affinity between boron and iron, as the

solubility of boron in silicon is influenced by the presence of iron. Thus, the interaction between these

elements in solid silicon was studied by evaluating the interaction parameter of boron on iron.

Due to the very small variation in the boron content of the iron-silicon melt, the activity of iron in the

alloy melt is considered to be constant at each temperature. Subsequently the activity of iron in solid

silicon is also constant. The equilibrium between Si and Fe-Si melt dictates that:

(5)

The activity coefficient of iron in solid silicon can be presented as Equation 6.

ln ln Fe BFeinsolid Si Feinsolid Si Feinsolid Si Feinsolid Si Feinsolid Si Binsolid SiX X (6)

ln ln

ln ln

( .)

fusFe

Fe in Fe Si melt Fein solid Si

Fein solid Si Fein solid Si

Ga a

RT

X

C const

where Feinsolid Si is the activity coefficient of iron at its infinite dilution relative to pure solid iron,

FeFeinsolid Si and

BFeinsolid Si are the self interaction parameter of iron and the interaction parameter of

boron on iron respectively. Considering the small content of iron in solid silicon, self interaction

parameter of iron can be neglected and Equation 7 can be obtained by substituting ln Feinsolid Si from

Equation 6 into Equation 5.

ln lnBFeinsolid Si Feinsolid Si Binsolid Si Feinsolid SiX X C (7)

BFe insolid Si can be determined from the slope of the linear regression of ln Feinsolid SiX vs.

Binsolid SiX , as depicted in Figure 8. The intercepts (shown with open marks) of the lines plotted in

Figure 8 are equal to the solid solubility of iron in silicon which is equal to 3×10−7 (1483 K), 4×10−7

(1533K), and 5×10−7 (1583KC) [32]. Similar approach has been employed to determine the interaction

parameter of phosphorus on aluminum while in silicon [14].

Figure 8. Evaluation of interaction parameter of boron on iron in solid silicon.

The values obtained for interaction parameter of boron on iron are as follow: -813 ± 53 (1583 K), -830 ±

92 (1533 K), -863 ± 91 (1483 K). The negative values of interaction parameter indicate the strong

attractive interaction between iron and boron. The absolute value of the interaction parameter slightly

decrease with temperature, indicating a weaker affinity between the two (while in Si) at higher

temperatures. It should be noted that the dependence of the interaction parameter on temperature is not

significant.

It is worthwhile noting that pairing between donors and acceptors in silicon is a well known phenomenon

and a substantial body of literature is available on iron and boron pairing in silicon [33-41] which shows

the affinity between the two.

It is often more convenient to express the concentrations of the solutes in terms of weight percentages,

accordingly the values obtained for the interaction parameter, BFe insolid Si , can be converted to

interaction coefficient, BFe insolid Sie , using Equation 8 [41].

230*B B Si BBFe insolid Si Fe insolid Si

Si Si

MW MWMWe

MW MW

(8)

where MW is the molecular weight of the element.

The values obtained for interaction coefficient of boron on iron in solid silicon are as following: -9.19 ±

0.60 (1583 K), -9.38 ± 1.04 (1533 K), -9.76 ± 1.03 (1483 K).

4. Conclusions

The distribution coefficient of boron between solid silicon and the iron-silicon alloy melt increases with

boron content and temperature. Considering equal yield of silicon, the average boron content in the Si

from solvent refining method is 2-3 times smaller than that of the conventional directional solidification

technique.

Large negative values were obtained for interaction parameter of boron on iron, indicating strong affinity

between the two elements while in silicon. With increasing temperature, a slight decrease in the affinity

between iron and boron in silicon was observed.

Acknowledgement

This research is partly supported by Natural Sciences and Engineering Research Council of Canada

(NSERC).

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