Silicon

Forming ultrapure silicon

• Silica sand mixed with carbon and heated in electric arc furnace (>2000C) to give ~99% pure Si (Metallurgical grade Si (MGS)) $\text{ce}SiO2+2C\Rightarrow Si+2CO(g)$
• Grind, add HCl heat to 300C. Main impurities (Fe, P, B) form e.g $\text{ce}FeCl3$ which are removed by distillation. $\text{ce}Si+3HCl\Rightarrow SiHCl3+H2(g)$
• Trichlorosilane is decomposed on hot Si rods ($1000C$) $\text{ce}2SiHCl3+2H2(g)\Rightarrow 2Si(s)+6HCl(g)$
• This produces polycrystaline electronic grade Si (EGS) - 99.999999% pure!
• These are some of the purest materials on the planet. To make electronic devices we then deliberately add impurities – known as doping (more on that later)

Czochralski (CZ) crystal growth allows an ingot or boule

The boule is ground to identify crystallographic orientation and doping – large wafers have grooves which identify orientation only