PhD Thesis


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.


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.
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