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

Silicon has uses as test-mass material for future gravitational-wave detectors and in high-resolution infrared spectroscopy in astronomy. The transparency of Si in the infrared region is critical to the effectiveness of these new devices. The temperature-dependent infrared transmission has been measured for high-purity silicon samples with impurity concentrations of ~10¹⁵ per cc. Measurements were made in a frequency range from 10–2000 cm^-1 and 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. These absorption lines are mostly due to hydrogen-like transitions in donor and acceptor atoms as well as interstitial oxygen and will have a significant effect on the optimal operation of the Si device. At higher temperatures the electrons are ionized by the thermal energy in the crystal and become free elections, 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 concentration.