TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Process: The step of forming the molten mixture comprises reducing a silicon halide with a metal aluminum to produce a reduced silicon, heating and melting the reduced silicon, and adding boron to the reduced silicon, where the reduced silicon is solidified under the condition of a temperature gradient provided in one direction in a mold. The reduced silicon is washed with an acid before heating and melting the reduced silicon. The reduced silicon is heated and melted under reduced pressure in the step of heating and melting the reduced silicon; and further heated and melted, and boron is added to it. EXAMPLE - Silicon tetrachloride gas was blown into molten metal aluminum in a heated and molten condition at 1020 degrees C, through a blowing pipe, and reduced. The molten metal aluminum was cooled, and crystallized reduced silicon was cut out. The aluminum content in the reduced silicon was 1100 ppma as an atomic ratio to the silicon element. The reduced silicon was dipped in hydrochloric acid at 80 degrees C for 8 hours. The aluminum, phosphorus and boron content were 10.3 ppma, 0.1 ppma and less than 0.03 ppma, respectively. The acid washed reduced silicon was put into a mold in a furnace and heated and melted at 1510 degrees C. The silicon was kept under reduced pressure of 1 Pa for 12 hours. While the heated and molten condition of the reduced silicon was kept as such, argon gas was introduced into the furnace to create atmospheric pressure, and boron was added to be 0.3 ppma. The reduced silicon was solidified in one direction by moving the mold under the condition of a temperature gradient of 1 degrees C/mm and a solidification speed of 0.4 mm/minute, to give a directional solidified silicon. The part corresponding to the boundary between the liquid phase and solid phase having a solidification fraction of 20%, 50% and 70% was cut out, and the aluminum, phosphorus and boron content in each part was quantitatively determined through inductively coupled plasma mass spectrometry. The obtained directional solidified silicon was cut at the part corresponding to the boundary having a solidification fraction of 70% to remove the crude silicon region, the boron added silicon was obtained as a purified silicon region having aluminum, phosphorus and boron content at 0.14 ppma, 0.06 ppma and 0.25 ppma, respectively.