A INTRODUCTION

Oxygen is present in all silicon crystals, either pulled from a melt contained in a SiO ₂ crucible (Czochralski or CZ silicon) or grown by the float-zone (FZ) method. With the latter method, the O concentration is typically 10 ¹⁵ cm ^-3 or 20 ppba, compared to about 10 ¹⁸ cm ^-3 in most CZ crystals. At room temperature, as-grown CZ crystals are supersaturated with isolated oxygen (O _i) which is the dominant form, but due to the reactivity of oxygen, complexes of O with some foreign atoms are also found. Irradiation of silicon with energetic particles or photons also produces complexes between oxygen and lattice defects and foreign atoms. O-related electrically active thermal donors (TDs) are produced by annealing CZ silicon from approximately 400 to 550 C. Various forms of O precipitates are formed by annealing, the morphology and composition depending mainly on the annealing temperature. Above 1100 C, the precipitates begin to dissolve when the solubility of oxygen becomes comparable with the O _i concentration in the material.

Vibrational absorption due to Oi, O-related complexes and defects and silica precipitates are observed in the mid IR. Low-temperature electronic absorption due to TDs can also be observed in the same spectral region and at lower frequencies, depending on the location of the Fermi level. The creation of localised excitons bound to O-related complexes can also be produced by IR absorption in the...