Optical spectroscopy of impurity atoms in semiconducting Nanowires

Sammanfattning: In a small and confined one dimensional geometry of nanowire, even few defects can influence the performance of the devices. Main objective of this thesis is to conduct photoluminescence study of such defect states in GaAs nanowires, which we will first introduce by diffusing copper impurity atoms, in very low concentrations. In low concentrations, ideally single atom in a wire, the photoluminescence outcomes is expected to be influenced by the position of copper impurities in the wire. Therefore, these defect states can act as local probes in the material and provide useful information about the recombination kinetics. Single defects can be used to understand the effect of dielectric screening on the ionization energies of the impurities, within a nanowire. As a part of this thesis, PL studies of high quality GaAs nanowires, which demonstrated benchmark solar cell performance, were conducted. Experiments were conducted to identify the conditions for diffusing copper in GaAs at low temperatures. As a part of this thesis, a dedicated PL setup, optimized for defect studies was assembled and a positioning scheme was developed to perform PL measurements on same nanowires before and after the diffusion. Band gap luminescence peak for as grown GaAs nanowires was found ~10meV red shifted from theoretical values, and an attempt has been made to provide reason out the same. By comparing luminescence peak, before and after diffusion, associated with copper acceptor states in GaAs, it was confirmed that copper diffusion was successfully achieved at lower temperatures of 450oC. But, the possibility of creating single defect states was limited by background contamination of copper in our samples.