We provide the highest quality grade silicon for the semiconductor industry. Below is a recent quote from a PHD candidate out of a Texas University. You can purchase such wafers at our online store.
"Could you please advise if the above type of silicon wafers will work for IR spectra under transmission mode too?"
Wafers in Question:
100mm Undoped (100) 500um DSP FZ 10,000-20,000 ohm-cm
100mm Intrinsic (100) 500um DSP FZ >20,000 ohm-cm
"Semiconductor grade Silicon has nil absorption in IR range 1.0 to 6.0µm. In the 6.0 to 25µm range there are several absorption bands. Furthermore Silicon has a rather large refractive index in this range so that in air reflections are significant (so that even with nil absorption the transmission is still only about 52%).
You need wafers polished on both sides (P/P), The smallest size that they normally come in is 2" diameter. Normally they are 0.3mm thick but we do have some that are 2mm thick and thicker.
Wafer resistivity >10 Ohmcm (whatever the dopant) is enough for Absorption to be nil in the 1.0 to 6.0µm range. However, the wafers marked FZ have nil Oxygen content, in contrast to wafers that are not so marked, which contain ~20ppma of dissolved Oxygen. That makes a difference in the vicinity of 7µm where there is a noticeable Oxygen Absorption band.
Crystallographic wafer orientation,  or  has no effect on IR optical properties of Silicon."