Defect Complexes in Silicon: Electronic Structures and Positron Annihilation

In silicon processing technology one of the most important current objectives is to achieve a controlled impurity doping in the crystal. Point defects and defect complexes present in the crystal influence in an important way the electrical activity and the diffusion properties of the dopants. In this thesis, defect complexes in silicon are studied by using quantum-mechanical electronic-structure calculations and by modeling positron annihilation experiments. The electronic-structure calculations are based on the density-functional theory and its state-of-the-art implementations, such as a plane-wave pseudopotential computer code. For the calculation of the momentum density of annihilating electron-positron pairs a new method is presented and tested. It is based on a two-particle description of the correlated pair so that the contact density depends explicitly on the whole spatial distribution of the electron state in question. The new method is found to be superior to the state-independent methods for the momentum density and provides a basis for identifying defect complexes with different chemical surroundings from their momentum distribution fingerprint. In this work, the computational methods are used to study the positron annihilation characteristics at small vacancy clusters in silicon and the properties of typical dopant atoms, which include arsenic and boron. In highly arsenic-doped silicon an electrically inactive defect complex consisting of a vacancy decorated by three arsenic atoms is identified. In boron-doped silicon the defect structures containing one boron atom are analyzed and an estimate is given for the activation energy of boron diffusion.