Defects And Optical Properties Of ZnO Micro Column Arrays

Luminescence of short wavelength semiconductor plays a significant role in next generation of semiconductor lighting, UV communication and optical storage. Zinc Oxide is a direct band gap semiconductor with the band gap of 3.37 eV. Besides, due to the large exciton binding energy of 60 meV, which is far more than the room temperature, ZnO is an excellent material for the preparation of short wavelength devices. With the development of ZnO-based functional application, ZnO requires more specific control on its micro nano structure and performance, urging us to carry out systematically study on its controllable preparation and luminescence properties.However, the intrinsic defects and the luminescence mechanism of ZnO are still under debate, besides, the modulation effect brought by depositon of metal on ZnO are still not clear by far.On account of these issues, we use hydrothermal growth method to study the intrinsic defects, luminescence mechanisms and enhancement methods of ZnO, the main contents of this paper are listed below:1. We investigated the annealing condition of different temperatures and atmospheres on photoluminescence properties of ZnO micro column arrays,and we adjust the excitation wavelength to study the recombination path of photogenerated carriers. We found that the yellow-orange emission originates from radiative recombination of electrons from conduction band to interstitial oxygen and the green emission results from recombination of conduction band electrons with oxygen vacancy. Annealing in reduced atmosphere of hydrogen results in decreased interstitial oxygen, bule shift and enhanced near band edge emission; while annealing in oxided atmosphere will introduce oxygen to diffuse in and form interstitial oxygen. High annealing temperature of 900? leads to the formation of oxygen vacancy despite of the atmosphere.Therefore, by choosing the annealing atmosphere and temperature we can manipulate the type of defects in obtained ZnO.2. We revealed the influence of two most commonly used precursors in hydrothermal growth of ZnO on its morphology,photoluminescence properties and intrinsic defects. Due to the adsorption of acetate ions on Zn2+polar surface, the deposition of zinc ion groups on c axis was hindered, thus inhibiting the longitudinal growth of A-ZnO, results in smaller length to diameter ratios and more zinc vacancies compared with N-ZnO. The blue-green band of 470 - 500 nm originating from zinc vacancy in PL spectra,the signal of g = 1.96 in electron paramagnetic resonance spectra together with the obtained Zn/O lattice atom ratio from X-ray photoelectron spectroscopy, prove that A-ZnO possesses zinc vacancies, besides, the presence of yellow-orange emission despite of the annealing condition in PL spectra of N-ZnO indicates that zinc nitrate precursor introduces intrinsic interstitial oxygen while zinc acetate precursor brings in intrinsic zinc vacancy.3. We deposite different metals on as-grown and annealed ZnO to modify the optical properties. Different mechanisms of the change in PL spectra were given. Easily formed ohmic contaction between Ti and ZnO with and without annealing facilitate the electron transfer from Ti to conduction band of ZnO,enhancing both near band edge and defect emissions. As for Au-coated ZnO,the Schottky barrier height of Au/ZnO interface impeded the transfer of electrons and facilitated the Au surface plasmon resonance enhanced green band emission; while high temperature annealing process induced defects can lower the barrier height, benifiting the electron transfer from SPR level of Au to conduction band of ZnO, leading to enhanced near band edge emission of ZnO. The deposition of Al can bule-shift and significantly enhance the near band edge emission of annealed instead of as-grown ZnO, and the defect-related emission quenched completely.4. The effect of Al capping on the enhancement of the near band edge emission of ZnO was investigated by low-temperature and temperature-dependent photoluminescence spectra. The surface Plasmons of Al have stronger coupling effect on less localized excitons in ZnO. After the deposition of Al,the intensity of enhanced near band edge emission of ZnO increased with increasing temperature, presenting a negative temperature quenching behavior.By fitting the temperature dependence of the intergrated near band edge emission of ZnO with and without Al capping, we calculated the activation energy of non-radiative recombination and a new pathway of radiative recombination brought by the coupling effect of surface Plasmon resonance of Al nanoparticles with excitions in ZnO. Based on the the calculation results,we explained the resonance mechanism of Al-capped ZnO.