Density Functional Modelling of Point Defects in Semiconductors
Chris Ewels, PhD Thesis, July 1997
Models are proposed for three different types of shallow thermal donor in silicon, NiO2i, (CH)iO4i and AlsO4i. This includes a new mechanism for converting otherwise deep level defects into shallow donor level centres, through electrostatic compression via neighbouring oxygen atoms. A reconstruction mechanism is also found whereby (CH)iO4i can transform into a deep state defect.
Several thermal donor models are examined, and a centre consisting of a `di-y-lid', O4i is shown to account for most of the observed experimental properties. Higher order thermal donor formation is discussed, as well as the role of hydrogen and silicon self-interstitials in thermal donor behaviour.
Rapid oxygen diffusion is examined in the context of the oxygen dimer, O2i. A puckered dimer structure is shown to be stable with 16O modes in good agreement with experiment. A low energy migration path for dimer diffusion is determined. The role of the dimer in creating other oxygen-based point defects in silicon is discussed.
Oxygen complexes with nitrogen are also modelled, and the most common N/O defect is shown to be N2iOi, consisting of bond centred oxygen neighbouring an interstitial nitrogen square. A NiOi complex is also identified and correlated with experimental data. Various vacancy--oxygen complexes are studied and their anomalous formation discussed. The structure VO2 is unambiguously assigned to experimental infra-red absorption at 889 cm-1.
Finally the interaction between hydrogen, Group-II elements and vacancies in III-V materials is examined, particularly InP. VH4 in InP is shown to be a single shallow donor, responsible for Fe charge compensation observed in InP:Fe-H. Trends in structure with varying Group-II element and III-V material are examined.
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