Browsing by Author "Havu, Ville, D.Sc., Aalto University, Department of Applied Physics, Finland"

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Computational study of isovalent group-V impurity doping and native point defects of III-V compound semiconductors
(2014) Virkkala, Ville
School of Science | Doctoral dissertation (article-based)
In the 1990s it was observed that the optical properties of III-V compound semiconductors can be significantly modified by substituting the V component by just a small fraction of nitrogen causing the narrowing of the band gap of the material. This observation led to a huge interest towards these alloys. The interest stems from the fact that these materials can be used in high-efficiency solar cell applications by tuning the band gap of the alloy to harvest photons with certain energy as well as a base material for long-wavelenght lasers. Currently the most widely accepted theories explain the band gap narrowing with increasing nitrogen concentration by the interaction between the host material states and the nitrogen-induced states, which causes either the host material conduction band edge or nitrogen-induced states close to the bottom of the conduction band to shift into lower energies. However, it is also often suggested that the nitrogen-nitrogen interactions in these alloys lead to the broadening of the nitrogen-induced states which would then cause the band gap narrowing, even though the actual mechanism for the nitrogen-nitrogen interactions is unknown. Alongside with the band gap engineering of III-V-N compound semiconductors, defects play a crucial role in the electrical properties in these materials. GaSb is a promising material for optoelectronic applications and undoped GaSb is known to be of p-type irrespective of growth conditions. The p-type conductivity is often connected to native point defects, but the origin of p-type conductivity is still largely debated. The main result of this thesis is the development and implementation of a quantum mechanical model to describe the nitrogen-nitrogen interactions and the resulting broadening of the nitro-gen-induced states in III-V compound semiconductors. The model is further extended to GaAs1-xBix alloys. Our model reproduces qualitatively and quantitatively the experimentally observed band gap behavior as a function of nitrogen or bismuth concentration. In this thesis also the native point defect energetics in GaSb is studied using an advanced hybrid functional of the density functional theory. Special focus is given to an accurate treatment of charged defects by implementing a recently published correction scheme for electrostatic energy in periodic systems.