Defects in zinc oxide nanowires : a study of the charge trapping dynamics and mechanisms
Date of Issue2013
School of Physical and Mathematical Sciences
Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University
The key to the design and optimization of novel nanoscale semiconductor devices for optoelectronic applications is through a deep understanding of how to control or tailor the charge carrier dynamics. Such control could be achieved by means of hybrid nanostructures fabrication of fabrication, doping and even through defects engineering. Hence, investigations into the role of defects on the charge carrier dynamics (i.e., relaxation, trapping, recombination and separation) occurring in the femtoseconds to picoseconds timescale in semiconductors has attracted considerable attention in the recent years. In this thesis, ultrafast spectroscopy techniques, such as time resolved photoluminescence (TRPL), transient absorption spectroscopy (TAS), z-scan technique were performed on unintentionally-doped (un-doped) as-grown and post-fabrication thermally annealed ZnO nanowires (NWs) as well as the intentionally Cu-doped ZnO These NW samples were synthesized by the vapor transport method. The origin of the green emission (GE), recombination mechanism and the charge-trapping dynamics to the GE-centers, Cu ions, as well as various chemisorbed species were investigated. Our results revealed that the GE in un-doped ZnO NWs involves the transitions of the electrons in the conduction band and/or shallow (delocalized) donor levels with holes trapped at the GE-centers (i.e. VZnO di-vacancies) that are located at 0.88 eV above the valence band maximum. Importantly, an ultrafast excitonic Auger-type hole-trapping process to the GE-centers occurring in a sub-ps timescale was also uncovered by TAS – shedding new light on the mechanism behind the fast and efficient charge trapping of photoexcited carriers. In addition, it was found that mechanisms of charge trapping and the PL transients from the GE band in the un-doped and Cu-doped ZnO NWs are significantly different even though the spectral shape and positions are nearly identical. In the former, ultrafast hole trapping in the sub-ps timescale occurs in the un-doped ZnO while the charge trapping time from the ZnO host to the Cu dopant in the latter occurs within 38 ps. Lastly, it was uncovered that the surface hole or electron traps have strong influence on the charge carrier dynamics in ZnO NWs. It was found that photoexcited holes were captured by surface chemisorbed within a few hundred ps in the air-annealed samples; and photoexcited holes are trapped by occurring within tens of ps in the H2-annealed ZnO NWs. These fast carrier trapping processes introduced by annealing effectively compete with the ZnO band-edge emission (BE) dynamics and provide efficient photo-excited charge separations in ZnO NWs. These findings provide valuable information on the dynamic interplay between various trapping centers in ZnO NWs and suggest a strategy for tailoring the ultrafast carrier dynamics of ZnO NW-based devices.
DRNTU::Science::Physics::Optics and light