Boron and Phosphorus Determination in Low Resistivity Solar‐Grade Silicon

Abstract

A technique is developed to analyze boron and phosphorus concentrations in silicon by determining net carrier concentrations from Hall‐effect measurements. The goal of the technique is to measure these impurities in purified metallurgical silicon (PMS) which is to be ultimately used for fabricating solar cells. Net carrier concentrations are measured on two wafers cut from a second‐generation Czochralski ingot pulled from PMS. Boron and phosphorus concentrations in the PMS and the ingots are calculated from the net carrier concentrations by applying the segregation equations of the impurities. The accuracy of the boron measurements between 1 and 30 ppma is equivalent to that obtained on similar samples analyzed by multiple‐pass float zoning. Phosphorus levels can be measured down to about 20% of that of the boron concentrations. Excellent precision of the Hall‐effect technique is demonstrated. Sensitivity calculations show that net carrier concentration is the most critical parameter affecting the accuracy of the method. Overall, the technique is considered to be sufficient for making quality‐control decisions with respect to boron and phosphorus levels in PMS and ingots grown from it.

Compositional Analysis of Silicon for Solar Cells

“…Hall effect and float zone.--There are only two methods that appear to be reliable enough to provide accurate, precise analyses for boron and phosphorus at concentrations covering the range from sub-ppma to about 100 ppma. These are the float-zone, resistivity method that is traditionally used for evaluating semiconductor-grade polycrystalline silicon, and the Hall-effect technique (5).…”

Microstructure of metallurgical grade silicon and its acid leaching behaviour by alloying with calcium

“…Amongst other detected phases were Al-Fe-Ti-Si, Al-Ca-Fe-Si, Al-Ca-Cu-Fe-Si, Ca-Cu-Si, Fe-Si-Ti, Ca-Fe-Si; some of them are most likely non-equilibrium phases (Meteleva-Fischer et al, 2012a). Leaching using water only (Hunt et al, 1976a(Hunt et al, , 1976b would not be efficient and much more time consuming, taking into account the low solubility in water of the abovementioned impurity phases. Thus, acid leaching is addressed to impurities precipitated in inclusions, and for MG-Si with iron as a major impurity hydrofluoric acid solution is more effective.…”

“…Quantitative results, selection of leaching agents and optimum conditions of the leaching process, obtained by various authors, differed remarkably from each other. Numerous acids (nitric (Santos et al, 1990), sulphuric (Voos, 1961), hydrochloric (Santos et al, 1990;Zhang et al, 2009) and hydrofluoric (Voos, 1961;Santos et al, 1990;Ma et al, 2009)), their mixtures (Dietl, 1983;Norman et al, 1985), and even water (Santos et al, 1990;Hunt et al, 1976aHunt et al, , 1976bChu and Chu, 1983) were found in different studies as efficient leaching agents. Double-step leaching of MG-Si at elevated temperature using hydrochloric acid (16%, 5 h, 80uC) and a relatively coarse particle fraction (116 mm), followed by hydrofluoric acid (2?5%, 2 h, 80uC), removed y85% of the impurities and resulted in 99?9% pure upgraded metallurgical grade silicon (UMG-Si) (Santos et al, 1990).…”

“…Shimpo et al (2004) reduced the phosphorus content in silicon to 16?7 ppmw by alloying with calcium followed by acid leaching. Particle size was found to be important and grinding to below the grain size was preferred (Santos et al, 1990;Zhang et al, 2009;Ma et al, 2009;Dietl, 1983;Hunt et al, 1976aHunt et al, , 1976bChu and Chu, 1983). At the same time large 3-12 cm pieces of MG-Si with calcium as a dominant impurity were purified up to 99?95% by a combination of two leaching steps: treatment with a water solution of iron (III) chloride or iron (III) chloride and hydrochloric acid, followed by 5% hydrofluoric acid or mixed hydrofluoric/ nitric acid solution (Ceccaroli and Friestad, 2005).…”

Determination of boron in silicon without use of additional complexing agents

“…The remarkable importance of boron as an analyte in various matrices is reected in the history of analytical chemistry by a vast spectrum of detection methods focusing on this element. Detection methods include the Hall effect, 3 spectrophotometry with various complexing agents [4][5][6][7][8] providing competitive detection limits down to the ng mL À1 range in liquids, infrared spectrometry 9 and many more. 10 Today, mainly mass spectrometric methods, sometimes in combination with isotope dilution 11 are used, since these techniques offer very high precision, low limits of determination and additional isotopic information.…”