Dislocations in silicon

Abstract: The topic of this thesis is theoretical studies of the electrical and structural properties in the elemental semiconductor silicon. Because of the numerous and powerful applications of semiconductors, e.g., rectifiers, transistors, solar cells and lasers, they have been the focus of huge amounts of research. Electronic devices necessitate use of almost pure semiconductor materials, normally in the form of a single crystal. In this crystal an accurately measured amount, usually extremely small, of a foreign dopant has been included to control its electrical properties. The properties of a device may be distributed by unwanted defects. Dislocations are examples of defects that degrade the performance of an electronic device, and they may even cause breakdown. This is due to the influence of dislocations on density, mobility and, most importantly, lifetime of the electrical carriers in the material. In newer semiconductor materials, such as III-V compounds, Si-Ge alloys, heterostructures and strained layers, dislocations cause problems. Dislocations in silicon are amenable more than most other solids to many different characterisation techniques and theoretical approaches. Therefore, a thorough understanding of dislocations in silicon is valuable, and an important milestone in the longer journey towards a comprehensive description of dislocations in all semiconductors. This thesis presents theoretical calculations focused on dislocations and some dislocation-related defects in silicon. Firstly, the electronic states of intrinsic stacking faults bounded by dislocations are investigated. Such stacking faults are one constituent part of commonly occurring extended dislocations in silicon. Secondly, the electronic and atomic structure of vacancies interacting with dislocations is investigated. This is important since vacancies are gettered by dislocations. Finally dislocation- dislocation interaction and its effects on the core structure of the so- called 90' partial dislocation are investigated. In order to study dislocation-dislocation interactions, a new and self-constitent method to apply periodic boundary conditions is introduced.