Lopez Gejo, Francisco; (2003) Quantum mechanical models of point defects in gate dielectrics. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This Thesis studies the limits of applicability of state of the art electronic structure calculations, for predictive modelling and simulation of properties of dielectric materials. These materials play a fundamental role in microelectronics technology, as gate insulators in MOS devices. Current devices are largely based on the properties of the Si/SiO2 system. Large attention has been paid to the change in the energy barrier experienced by carriers at the junction, as the oxide layer becomes thinner. We have calculated the valence band offset at the Si/SiO2 interface, directly from first-principles calculations of different models of Si/SiO2 junction. We studied the dependence of the results on the choice of Hamiltonian and basis set, and found that the best results are obtained when using the B3LYP scheme and basis sets containing polarisation functions. We have shown that the interface states are confined in a region of the oxide whose dimensions do not depend on the thickness of the dielectric layer. Hydrogen is believed to play a fundamental role in the processes that lead to the breakdown of the dielectric. We have calculated the stable sites for atomic hydrogen inside silica using a Density Functional Theory scheme. We found two shallow but stable minima in the same channel of the quartz structure, indicating that some type of interaction exists between the impurity and the host. We found that Hydrogen actually becomes polarised. The results, however, are critically affected by two of the approximations of the method: the approximative treatment of the exchange-correlation interaction, and the classic treatment of hydrogen nuclei. We have also calculated the isotropic hyperfine interaction parameters for H inside quartz, that can be compared directly with data from EPR experiments. The results, however, appeared to be very sensitive to the quality of the basis set. Materials with higher dielectric constant than that of SiO2 are being considered as alternative gate dielectrics. Their reliability of the material, however, depends critically on how easily defects can be generated, and on the ability of these defects to trap charge carriers that can then tunnel through the material. We have studied the energies of formation and the ionization potential and electron affinities of cation and anion vacancies, as well as substitutional Zr inside HfO2. All these properties are defined as ground-state properties, and corrected for the underestimation of the band gap, typical of the Density Functional Theory scheme employed. Energy of formation of O vacancies is much lower than that of cation vacancies. The O vacancies are shown to be able to trap electrons when positively charged, and holes when neutral. First-principle techniques are very computationally demanding methods. Alternatively, semi-empirical methods can be employed to explore the properties of systems whose sizes are out of the range of the ab initio schemes. We have updated an implementation of the INDO method in order to allow the study of systems containing several hundreds of atoms in single processor machines. A set of parameters for studying compounds containing Si, O, N and H using this technique has also been developed and tested. It allows the description of bulk structures of Si/SiO2, Si3N4 and SiON with differences of 0.1 A and 5° with respect to experimental bond-lengths and angles, respectively.

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