Submitted by António Luís Santos Ferreira-Resende to the University of Exeter
as a thesis for the degree of Doctor of Philosophy in Physics, November 1999.
ABSTRACT
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Theoretical investigations into deep-level defects in crystalline
silicon are presented in this thesis. The calculations are carried
out using the AIMPRO code, an ab initio pseudopotential local
spin density method applied to large hydrogen terminated clusters
containing up to 346 atoms.
By definition, deep-level defects are those with localised states,
i.e., states with decaying wavefunctions in real space. As a
result, these defects can, and often do, give rise to a number of
levels lying within the silicon bandgap. Due to the fact that the
presence of these levels within the Si forbidden band can dramatically
change the optical and electrical properties of Si integrated devices,
the understanding of their microscopic properties is of paramount
importance.
Two distinct types of deep-level defects are investigated:
radiation-induced defects and transition-metal (TM) related
defects. Most of these defects are unstable against Jahn-Teller
distortions. Since their electrical properties are highly sensitive to
their atomic arrangement, it is therefore necessary to correctly
describe their ground state configurations.
The back-bone of this thesis is a novel theoretical approach to the
calculation of the electrical level of deep-level defects.
This method has allowed the successful characterisation a number of
common defects in radiation damaged Si material, like VO
(A-centre), VOH, CiP, CiOi, CiCs-H
(T-centre), etc.
The method is then applied to the study of the structural and
electrical properties of the lattice di-vacancy. As a result, we
confirm the predictions of Watkins and Corbett for the structural
properties of paramagnetic V2 defects. Accordingly, these
defects undergo a strong Jahn-Teller distortion by pairing of
four of the six atoms constituting the defect.
This is followed by a study on the structural, vibrational and
electrical properties of vacancy-hydrogen-related defects
produced by low-temperature proton implantation.
Finally, the method is applied to the analysis of the structural and
electrical properties of substitutional transition-metal centres-gold,
silver, platinum and palladium. The main goal of these calculations is
the study of the influence on the electrical properties of these
defects of bringing atomic hydrogen close to the defect's
core. Concerning the structure of the TM-H defects, we find no
evidence for a direct interaction between the hydrogen impurity and
the TM ion. In accordance with the vacancy model of Watkins,
the TM ions do seem to interact weakly with the surroundings, in an
attempt to reproduce their environment as isolated species. Despite
an enlargement of the vacancy cage, the hydrogen atoms sit at
`anti-bonding' lattice positions, being back-bonded to the Si vacancy
atoms. No electrically inactive TM-Hn, with
n = 1...4, were found and an alternative model for the
neutralisation of the electrical properties of these centres is
proposed.
The entire thesis is available in the following formats:
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