Infrared Spectroscopy of 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 effectiveness of these devices relies heavily on the transparency of Si in the infrared region. The temperature-dependent infrared transmission has been measured for high-purity silicon samples with a range of impurity concentrations, ranging from below 10¹² to 10¹⁵ per cc. Measurements were made in a frequency range from 10–10,000 cm^-1 and temperatures from 10–300K. The mid infrared region (600–2000 cm^-1) is dominated by multiphonon absorption, while the far infrared (10–600 cm^-1) and the near infrared (2000–9000 cm^-1) are transparent with the exception of absorption lines in the far infrared caused by residual impurities. These absorption lines are mostly due to hydrogen-like transitions in phosphorus and boron and will have a significant effect on the optimal operation of the Si device. With increasing temperature, we are able to observe thermal population of excited states of the impurity atom and eventually of the continuum. Using the results of our transmission spectra we are able to determine the concentration of the impurities in the sample.