26 may 2015 - LMGP Seminar : Joseph Kioseoglou - Nanostructures, Surfaces, Interfaces and Defects at the atomic scale

Joseph Kioseoglou - Assist. Prof.
Physics Department, Aristotle University of Thessaloniki - Greece

Abstract

Intensive research efforts in the field of III-nitride semiconductor optoelectronic devices are concentrated in the extension of their operational range from the deep ultraviolet to the infrared wavelengths. III-nitride core/shell nanowires (NWs) grown along the polar direction comprise defect free non-polar interfaces, even for greatly lattice mismatched materials, offering significant improvements in the light-extraction efficiency compared with their planar counterparts. In this work GaN/AlN and InN/GaN core/shell NWs are investigated with Molecular Dynamics and DFT codes. Structural models of several thousands of atoms with multiple shell-to-NW rations are examined. The variations of the a lattice constants along [10-10] though the middle and through the edge and [11-20] are calculated and the results show an variation which is dependent on the direction. The bandgaps increase exponentially with the shell-to-NW ratio and reach a maximum value for approximately 0.5.

The influence of strain on the energetics and the electronic properties of NWs consisted of a GaN base part followed by a superlattice part of successive InxGa1-xN nanodisks (NDs) (x ranging from 3% to 25%) separated by GaN spacers is investigated. InxGa1-xN/GaN supercells were modelled and simulations were implemented with the LAMMPS code for Molecular Dynamics, using a bond-order interatomic potential under the III-species environment approach, and with DFT calculations.

It has been found by both simulation approaches that among three possible types of strain (biaxial, hydrostatic and uniaxial), the biaxially strained NW superlattice is the one with the lowest excess energy for all indium concentrations. However the energy difference between biaxially and hydrostatically strained states for In concentration below 10% in the NDs is small. It is deduced that up to ~10% of In, the hydrostatic strain state is competitive with the biaxial and above this value the preferable strain model is the biaxial one. Hence, hydrostatic and biaxial strain components should be both considered in the embedded NDs and they are of different physical origin. The biaxial strain originates from growth on lattice mismatched layers, while the hydrostatic strain component originates from the lateral surfaces.