Thermodynamics of boron in a silicon melt
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I.
INTRODUCTION
S O L A R energy will shortly be in great demand, since it is inexhaustible and cleaner than any conventional energy resources. Silicon is the most widely used material to convert solar energy into electricity. At present, expensive off-grade silicon for semiconductor (SEG-Si) is used for a solar cell, since silicon for solar cell cannot be produced from inexpensive metallurgical grade silicon (MG-Si). Many studies on refining MG-Si for producing inexpensive solar cells have been conducted. However, they have not been successful in effectively removing boron, which is similar to silicon in physical property and decreases the photovolataic efficiency of a solar cell, although various trials have been reported.t~-51 The authors ~61have reported the removal of boron using basic fluxes such as CaO-CaFz-SiOz, CaO-MgO-SiO2, and CaO-BaO-SiOz systems. In the present study, the activity coefficient of boron in silicon melt was determined by using a chemical equilibration technique, in order to thermodynamically estimate the removal of boron in silicon by using basic fluxes.
partial pressure of nitrogen according to Eq. [3]. Since a small amount of B and N in Si affects the activity coefficient of B in Si, the interaction parameters between B and N in Si have been determined by changing partial pressure of nitrogen from 1.01 • 104 to 1.01 X 105 Pa at 1723 K. Furthermore, the temperature dependence of the activity coefficient of B in Si is also measured from 1723 to 1923 K at PN2 = 1.01 • 105 Pa. Twelve grams of Si in a BN crucible (25-ram OD, 20-mm ID, 50-mm length, purity of 99.5 pct) was held in a graphite holder and equilibrated at temperatures ranging from 1723 to 1923 K controlled within -+0.5 K in a deoxidized Ar and N2 mixture. A chunk of highpurity polycrystalline silicon was used as a specimen in the experiments. The equilibration time was predetermined to be 48 hours. After equilibration, the graphite holder was quickly withdrawn from the furnace and the sample was cooled in argon. The B content of Si was analyzed by radiofrequency inductively coupled plasma (ICP) emission spectrometry and the N content by a LECO* analyzer. *LECO is a trademark of Leco Corporation, St. Joseph, MI.
II.
EXPERIMENTAL
IlL
A molten Si-B alloy was equilibrated in a BN crucible with nitrogen having a constant partial pressure. The reaction of formation of BN and its equilibrium constant are expressed by Eqs. [1] and [3], respectively.
1 B (1 in Si) + ~ N2 (g) = BN (s) AG~ = -299,000 + 108T ann KI -
. ul/2
aB --N2
J/mol t71
[1] [2]
1 ol/~
(Yn'XB)'--N~
[3]
where aaN is the activity of BN, which is unity in the present study, aa and YB are the activity and activity coefficient of B in Si, respectively, relative to pure liquid boron, and PN2 is the partial pressure of nitrogen. The activity coefficient of B in Si can be obtained by measuring B content in Si at constant temperature and RYOUJI NOGUCHI, formerly Graduate Student, Department of Metallurgy, The University of Tokyo, is with SONY, Tokyo, Japan. KICHIY
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