Far-infrared and annealing studies of thermal donors in high-purity silicon

We have measured the temperature-dependent infrared transmission of high-purity silicon samples having impurity concentrations of ~10¹⁵ /cm³. Measurements were made in a frequency range from 10–10,000 cm^-1 at temperatures from 10–300 K. At 10 K, silicon is transparent in the far infrared (10–600 cm^-1) apart from narrow absorption lines caused by residual oxygen impurities known as thermal double donors (TDD). 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. There is also absorption caused by several vibrational modes of Si₂O in the far and mid infrared regions. The oxygen is introduced by the amorphous silica crucible during the Czochralski growth process and occurs as an uneven distribution of oxygen throughout the boule. Samples were annealed between 450-700 C in vacuum and a significant effect on the TDD oxygen but not on the vibrational oxygen concentration was observed. This leads us to believe that the thermal history of the boule plays an important role in determining the structure of the oxygen impurity.