Optical grade Silicon is normally specified with a resistivity of 10 to 40 ohm-cm which is higher than most semiconductor applications. The usual Silicon material, unless otherwise specified is CZ, Czochralski Grade which has a Si-O absorption band at 9μm and so not important if the windows are used in the 3 to 5μm thermal band. Where required FZ, Float Zone material can be supplied which does not have this absorption.
Very high resistivity material can be supplied on special order, particularly for TeraHertz applications.
Silicon is used for windows and lenses in the 2-7µm range, ideal choice for IR LED lenses. The refractive index is near 3.4 throughout the range. Silicon high thermal conductivity and low weight are attractive features in IR or FTIR applications. Silicon is also used as a high reflectivity mirror for lasers.
Due to the strong absorbtion at 9mkm, Silicon is not suitable for use with CO2 lasers as a transmitting optic but is widely used for CO2 mirrors. The density of Silicon is only half that of Germanium and Zinc Selenide.
| Silicon properties | |
| Chemical Formula | Si |
| Molecular Weight | 28.09 |
| Crystal Class | Cubic |
| Lattice Constant, Å | 5.43 |
| Density, g/cm3 at 293 K | 2.329 |
| Dielectric Constant for 9.37 x 109 Hz | 13 |
| Melting Point, K | 1690 |
| Thermal Conductivity, W/(m K) at 125 K at 313 K at 400 K |
598.6 163 105.1 |
| Thermal Expansion, 1/K at 75 K at 293 K at 1400 K |
-0.5 x 10-6 2.6 x 10-6 4.6 x 10-6 |
| Specific Heat, cal/(g K) at 298 K at 1800 K |
0.18 0.253 |
| Debye Temperature, K | 640 |
| Bandgap, eV | 1.1 |
| Solubility in water | None |
| Knoop Hardness, kg/mm2 | 1100 |
| Mohs Hardness | 7 |
| Young’s Modulus, GPa | 130.91 |
| Shear Modulus, GPa | 79.92 |
| Bulk Modulus, GPa | 101.97 |
| Poisson’s Ratio | 0.28 |
Optical grade Silicon – index of refraction
| µm | No | µm | No | µm | No |
| 1.357 | 3.4975 | 1.367 | 3.4962 | 1.395 | 3.4929 |
| 1.5295 | 3.4795 | 1.660 | 3.4696 | 1.709 | 3.4664 |
| 1.813 | 3.4608 | 1.970 | 3.4537 | 2.153 | 3.4476 |
| 2.325 | 3.4430 | 2.714 | 3.4358 | 3.000 | 3.4320 |
| 3.303 | 3.430 | 3.500 | 3.4284 | 4.000 | 3.4257 |
| 4.258 | 3.4245 | 4.500 | 3.4236 | 5.000 | 3.4223 |
| 5.500 | 3.4213 | 6.000 | 3.4202 | 6.500 | 3.4195 |
| 7.000 | 3.4189 | 7.500 | 3.4186 | 8.000 | 3.4184 |
| 8.500 | 3.4182 | 10.00 | 3.4179 | 10.50 | 3.4178 |
| 11.04 | 3.4176 |
