Linear-scaling density functional theory simulations of polar semiconductor nanorods

Abstract

Binary polar semiconductors in the wurtzite structure can be grown in the form of nanorods aligned along the wurtzite [0001] direction. In such structures, very large dipole moments have been observed experimentally. We have studied the distribution of charge in GaAs nanorods, so as to elucidate the origin of the dipole moments. To make contact with the realistic experimental regime, we need to model systems of several thousand atoms, necessitating the use of a Linear-Scaling formulation of DFT. We use the ONETEP code, which combines the benefits of O(N) computational effort and plane-wave accuracy. We show that both the direction and magnitude of the dipole moment of a nanorod, and its electric field, depend sensitively on how its surfaces are terminated and do not depend strongly on the spontaneous polarization of the underlying lattice. Furthermore, we observe that the Fermi level for an isolated nanorod always coincides with a significant density of electronic surface states at the polar surfaces, which are either mid-gap states or band-edge states. These states pin the Fermi level, and therefore fix the potential difference across the rod. We provide evidence that this effect can have a determining influence on the polarity of nanorods, and has consequences for the way a rod responds to changes in its surface chemistry.