Silicon (Si) is grown by Czochralski pulling techniques (CZ) and contains some oxygen that causes an absorption band at 9 microns. To avoid this, material can be prepared by a Float-Zone (FZ) process. Optical silicon is generally lightly doped (5 to 40 ohm cm) for best transmission above 10 microns, and doping is usually boron (p-type) and phosphorus (n-type). After doping silicon has a further pass band: 30 to 100 microns which is effective only in very high resistivity uncompensated material.

CZ Silicon is commonly used as substrate material for infrared reflectors and windows in the 1.5~8 micron region. The strong absorption band at 9 microns makes it unsuitable for CO₂ laser transmission applications, but it is frequently used for laser mirrors because of its high thermal conductivity and low density. Application as window, lens in the 1.5~8 μm region; Mirror for CO₂ laser and spectrometer applications.

Si Mirror grade
Any dopant, any conductivity, any orientation, not warranted with respect to transmission, suitable for mirror substrates.

Si Optical grade
Czohralski (CZ), P type doped with Boron, <111> or <100>, Resistivity 5 - 40 ohmcm
Floatzone (FZ), N type doped with Phosphorus, <111>, Resistivity > 50, preferably > 500 ohmcm, the absorption at 9 microns is absent.