Far-infrared studies of shallow thermal donors and dilute impurities in high-purity silicon

Silicon is a candidate material for the test masses of future gravitational-wave detectors; it also has applications in high-resolution infrared spectroscopy in astronomy. We have measured the temperature-dependent infrared transmission of high-purity silicon samples having impurity concentrations of ~10^15 /cm^3. Measurements were made in a frequency range from 10–2000 cm^-1 at temperatures from 10–300K. At 10 K, silicon is transparent in the far infrared (10–600 cm^-1 ) apart from narrow absorption lines caused by residual impurities. In our samples, these absorption lines are mostly due to hydrogen-like transitions in interstitial oxygen occurring in the form of thermal double donors. The oxygen is introduced during the Czochralski growth method and occurs as an uneven distribution of oxygen throughout the boule as determined by how and where the Si melt makes contact with the fused-silica crucible. At higher temperatures, the electrons are ionized by the thermal energy in the crystal and become free electrons, causing a Drude-like response in the far infrared. Using the results of our transmission spectra, we can determine the type of impurities in the sample and their concentrations.