Oxygen '96
Early Stages of Oxygen Precipitation in Silicon
THEORETICAL STUDIES OF VACANCY-OXYGEN AND NITROGEN-OXYGEN DEFECTS IN
SILICON
(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
D2d 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 C2v 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.
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Last modified: Sat Mar 9 14:39:46 GMT 1996
JG