Native and hydrogen-plasma induced shallow traps in hydrothermally grown ZnO crystals have been investigated by charge-based deep level transient spectroscopy (Q-DLTS), photoluminescence and cathodoluminescence microanalysis. The as-grown ZnO exhibit
s a trap state at 23 meV, while H-doped ZnO produced by plasma doping shows two levels at 22 meV and 11 meV below the conduction band. As-grown ZnO displays the expected thermal decay of bound excitons with increasing temperature from 7 K, while we observed an anomalous behaviour of the excitonic emission in H-doped ZnO, in which its intensity increases with increasing temperature in the range 140-300 K. Based on a multitude of optical results, a qualitative model is developed which explains the Y line structural defects, which act as an electron trap with an activation energy of 11 meV, being responsible for the anomalous temperature-dependent cathodoluminescence of H-doped ZnO.
New insights into controlling nanowire merging phenomena are demonstrated in growth of thin ZnO nanowires using monodispersed Au colloidal nanoparticles as catalyst. Both nanowire diameter and density were found to be strongly dependent on the densit
y of Au nanoparticles. Structural analysis and spectral cathodoluminescence imaging of the c-plane nanowire cross-sections reveal that thin isolated nanowires growing from the Au nanoparticles begin to merge and coalesce with neighbouring nanowires to form larger nanowires when their separation reaches a threshold distance. Green luminescence, which is originated from the remnants of constituent nanowires before merging, is detected at the core of fused nanowires. The distribution of nanowire diameters and green emission were found to be strongly dependent on the density of the Au nanoparticles. The merging phenomenon is attributed to electrostatic interactions between nanowire c-facets during growth and well-described by a cantilever bending model.
X-ray absorption near-edge spectroscopy (XANES), photoluminescence, cathodoluminescence and Raman spectroscopy have been used to investigate the chemical states of nitrogen dopants in ZnO nanowires. It is found that nitrogen exists in multiple states
: NO, NZn and loosely bound N2 molecule. The work establishes a direct link between a donor-acceptor pair (DAP) emission at 3.232 eV and the concentration of loosely bound N2. These results confirm that N2 at Zn site is a potential candidate for producing a shallow acceptor state in N-doped ZnO as theoretically predicted by Lambrecht and Boonchun [Phys. Rev. B 87, 195207 (2013)]. Additionally, shallow acceptor states arising from NO complexes have been ruled out in this study.
