Application and development of density functional theory
This thesis concerns developments and applications using the density functional theory (DFT) ab initio electronic structure method. Implementation of a pseudo atomic orbital (PAO) basis set in the linear scaling DFT program CONQUEST is reported and used to test aspects of the linear scaling algorithm. Also a separate study using plane-wave DFT (VASP code) to model the strained growth of Indium Arsenide (InAs) on the (110) surface of Gallium Arsenide (GaAs), in particular the formation of a strain relieving dislocation network, has been performed. Pseudo atomic orbitals are the eigenstates of a pseudo-atom confined to a spherical potential, as used in the SIESTA linear scaling DFT program, and consist of a radial function multiplied by a spherical harmonic. Code to evaluate overlap and kinetic energy matrix elements between PAOs has been written, and tested using Gaussian PAOs, whose overlap integrals can be computed analytically. The PAO code has been integrated into the CON QUEST program and used to perform tests of the linear scaling algorithms on Silicon. Conventional plane wave DFT has been applied to calculate the energetics of a dislocation network in InAs grown on GaAs(110). Both InAs and GaAs have the zinc-blende crystal structure but the lattice constant of InAs is seven percent greater than that of GaAs. Experiments show that during deposition of the InAs by molecular beam epitaxy (MBE) compressive strain leads to formation of a strain relieving dislocation network after a critical amount of InAs coverage. In this thesis DFT is applied to calculate the energetically favoured location for the dislocation core and the resulting structure. In addition the critical InAs coverage necessary for dislocation formation is also calculated and compared to that measured by experiment.