Defects in silicon

The method of obtaining pure polycrystalline silicon is described, followed by short accounts of how this material is converted into single-crystal form either by the Czochralski (cz) pulling method or the float-zone (FZ) method. It is shown that the silicon contains various impurities including oxygen, carbon, boron and possibly hydrogen. If a silicon nitride crucible is used instead of silica for the growth of cz crystals they contain nitrogen. Crystals also contain structural defects derived from the aggregation of intrinsic point defects as they cool from the melting temperature. The defects and impurities often show a non-homogeneous distribution in the form of helical swirls. Heat treatment of silicon-containing oxygen leads to the clustering of this impurity. At 450 "C there is formation of small complexes that act as shallow donors. Investigations using IR and ESR spectroscopy have so far failed to determine the atomic configuration of the defects. Heating at higher temperatures causes widescale precipitation of oxygen. There are interactions with carbon and there can be formation of silicon carbide precipitates. Contamination from Cu, Au, Fe, etc, can occur during these treatments and methods for gettering these metals are discussed, involving dislocations and silica precipitates. Low-temperature irradiations produce vacancies and self -interstitials which combine with impurities to form complexes on heating from 4 K to 300 K. Certain defects, including vacancies and impurity interstitials, show athermal migration and unexpected electrical properties. Evidence is presented to illustrate the possible charge states of self-interstitials. Damage produced by fast neutrons is discussed next, followed by a brief account of neutron transmutation doping whereby naturally occurring 30Si is converted to 31P by the capture of thermal neutrons. Finally, some aspects of high-temperature diffusion are discussed and attempts are made to correlate the data with that derived from the irradiation studies. It is concluded that self-interstitials are important defects in silicon and oxidation of a surface generates a large flux of non-equilibrium defects which diffuse into the bulk crystal, leading to enhanced impurity diffusion and the growth of structural defects. This review was received in March 1982. 0034-4885/82/101163 +48 $08.00 @ 1982 The Institute of Physics 1163