Theory of isolated dopants in GaAs (110) surfaces

In this thesis we perform a range of highly accurate density functional theory (DFT) calculations for a GaAs (110) slab containing almost all of the near-surface single atomic dopants from groups III, IV and V of the periodic table. We look in detail at the relaxed geometry and local density of states of the doped surface, and using the theory of Tersoff and Hamann we generate STM images of the different dopant systems. Where possible we compare to experimental results obtaining excellent qualitative and quantitative agreement, with bond lengths and shifts in STM contrast agreeing to within 0.03 Å and 0.09 Å respectively. We are able to show very clear trends in both the relaxed positions and STM image contrasts for the range of dopants. These trends are determined by the covalent radius of the dopants. Dopants with larger radii relax out of the surface and ones with smaller radii relax into the surface, and these relaxations cause the different contrasts in the STM images. These trends fit very well with existing results for nitrogen and silicon doped systems, and also allow us to fill in the gaps for those systems that have not been as thoroughly investigated. Our analysis applies equally across the three groups of dopants from the periodic table covering isovalent, donor and acceptor cases. By developing a geometrical model based on the covalent radii of the dopants and host atoms, we show how the covalent radius determines the geometry of the surface, which in turn determines the contrast seen in the STM images. Using this model we are able to explain and predict the relaxation and STM images for all the dopants in this work to a high degree of accuracy without relying on DFT simulations.