Oxygen '96

Early Stages of Oxygen Precipitation in Silicon

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THEORETICAL STUDIES OF VACANCY-OXYGEN AND NITROGEN-OXYGEN DEFECTS IN SILICON

C. P. Ewels (a), R. Jones (a) and S. Öberg (b)

(a) Department of Physics, University of Exeter, Exeter, EX4 4QL, UK
(b) Department of Mathematics, University of Lulea, Lulea, S95187, SWEDEN

Keywords: oxygen, silicon, ab initio theory, vacancy, interstitial, nitrogen, dimer, shallow thermal donor, STD.

An ab initio local density functional cluster method, AIMPRO, is used to examine a variety of different point defect structures in silicon.

Vacancy complexes with multiple oxygen atoms are studied. Substitutional oxygen is found to sit off-site in agreement with experiment, with a single high frequency local vibrational mode (LVM) at 849 cm^-1. VO₂ has D_2d symmetry and a double degenerate high frequency mode at 836 cm^-1. VO₃ has three high frequency modes, whereas V₂O only has one. These results provide support for the assignment of the 889 cm^-1 (300K) local vibrational mode to VO₂. A reaction path is proposed involving binding of single oxygen interstitials to the defect with increasing temperature.

A family of shallow single thermal donors (STDs) has been observed in Cz-silicon that has been either N implanted or grown in a N₂ atmosphere. The presence of oxygen in the defect has been shown and nitrogen inferred indirectly from its presence in the material, however no hyperfine interaction with N is observed. The defects are known to have C_2v symmetry. We examine a variety of different nitrogen-oxygen interstitial complexes, and propose a defect core consisting of a single interstitial nitrogen surrounded by two [011] aligned interstitial oxygen atoms. Such an arrangement serves to minimise the compressive stress along [011] due to oxygen. Now, even though the oxygen bonding is "normal", there is a single shallow donor level which is extremely delocalised. This shallow level originates from a deep level on the Si interstitial but is pushed towards the conduction band by the electrostatic repulsion arising from the polarised oxygen neighbours. The vibrational modes of the defect are reported.

Interstitial oxygen diffuses between bond centred sites at temperatures of around 380 degrees C, with an activation energy of 2.54 eV. However, oxygen loss from solution is observed at temperatures as low at 350 degrees C which suggests a lower activation energy, and rapidly diffusing oxygen dimers may be the cause. We investigate the stability and migration energy of the oxygen interstitial dimer, using a constrained relaxation method. The dimer is more stable than any other configuration, and consists of two neighbouring bond centred interstitials tightly bound to a shared Si atom. The stability is attributed to a chain of [011] aligned polar bonds. We investigate several mechanisms for dimer diffusion including a concerted pair diffusive model as well as a partially dissociated dimer one.

Last modified: Sat Mar 9 14:39:46 GMT 1996 JG