Calculations of defect related properties in semiconductors

Sammanfattning: Many of the most important and useful properties of crystalline materials, such as mechanical strength and electrical resistance, are determined by the presence of lattice defects and impurities. Well known examples are dislocations which control plastic flow and make metals ductile, and dopant atoms which control the extrinsic conduction in semiconductors and insulators. In fact the electronic device industry is based on controlled introduction of specific impurities in semiconductors and insulators and methods to avoid or eliminate destructive defects . Thus improvements of existing solid state devices and the invention of new ones are the driving forces for the large amount of experimental and theoretical investigations performed on defects and solids today. In this thesis dislocations and impurities in the most important semiconductors are analysed with mathematical methods, ranging from a method based on isotropic elasticity theory, interatomic potentials and a semi-empirical quantum-mechanical prescription, here applied on dislocations, to an ab initio quantum mechanical method by which properties of defects in crystalline solids can be obtained from first principles, that is, can be derived from the knowledge of only the atomic numbers and masses of constituent atoms, here applied on impurities. The ab initio method is based on Local Density Functional Theory, which provides a many-electron description. The semi-empirical method has been applied on dislocations in Gallium-Arsenide and Cadmium-Telluride. The ab initio method has been applied on various defects in Silicon and Gallium-Arsenide. For Silicon we have investigated interstitial Oxygen complexes, substitutional Carbon and Boron, and intersitial Carbon-Oxygen complexes. For Gallium- Arsenide de the investigation includes the Carbon impurity and the Carbon-Hydrogen complex, the Boron double acceptors, and the so called DX-centre in GaAs:Si. The interaction of hydrogen with impurities in semiconductors has also been investigated. We have also calculated the diffusion barrier of Oxygen in Silicon with excellent results. Our work shows clearly that Local Density Functional theory can provide useful information about defect structures, dynamical properties and diffusion processes in solids.