Zinc Oxide Bulk and Nanorods
Abstract: Zinc Oxide (ZnO) has many promising properties for optoelectronics, sensor applications, transparent electronics etc. To mention a few, ZnO has a large exciton binding energy (60 meV at room temperature) and a direct wide bandgap energy of 3.37 eV. In addition, ZnO is piezoelectric and shows more resistance to radiation damage than Si and GaN. The green luminescence band, or deep band emission (DBE), in ZnO has been studied for decades, but no clear consensus has been made. The first part of this thesis has been addressed to the issue of the origin of the DBE. Both an ion implantation and a systematic annealing study was performed. In the ion implantation study both Zn and O ions were implanted in different concentrations in bulk single crystal ZnO wafers. The photoluminescence (PL) intensity of the DBE decreased rapidly with increased concentration of implanted Zn ions, whereas the DBE intensity only slightly decreased with increased concentration of the implanted O ions. The comparison of DBE intensities together with PL measurements of the as-grown samples led to the conclusion that zinc vacancies (VZn) are involved in the DBE. In the annealing study single crystalline bulk ZnO wafers were annealed in different atmospheres, especially O- and Zn- rich atmospheres, in a systematic way. All samples were characterized with PL measurement performed from 27 K to room temperature. A striking correlation between the position of the DBE and the annealing condition was observed. In particular, for samples annealed in Zn-rich atmospheres the DBE peak position was located at 2.53 eV at room temperature, whereas annealing in O-rich conditions resulted in a DBE peak position located at 2.35 eV. The former peak position was attributed to VO and the later to VZn. Furthermore, both the VZn- and the VO-related PL band exhibited characteristic features when the measurement temperature was decreased. The peak position of the VZn-related band increased with decreased measurement temperature, while that of VO decreased. Secondly, phonon replicas were clearly observed in the DBE spectra in the sample containing VZn. Finally, for the VZn-enriched samples the decay curves showed strong wavelength dependence and generally slower decay components as compared to the sample enriched with VO. The results showed that the DBE is composed of at least two components, VZn and VO. Both of them have their own characteristic features promoting defect identification. In the second part of the thesis growth and properties of ZnO nanorods are discussed. Both the vapor-liquid-solid (VLS) and the aqueous chemical growth method were used. Laser action was observed at room temperature from vertically aligned nanorods under optical pumping. Secondly, Schottky contacts were demonstrated on ZnO nanorods grown on glass substrates. The best Schottky contact was the as deposited Pd/ZnO contact with an ideality factor of 1.74 pm 0.43 and a barrier height of 0.67 pm 0.09 eV. The presence of surface states due to the high evaporation pressure is probably the reason for the high ideality factor. Finally buckling of ZnO nanorods under uniaxial compression was studied. The critical load was found to be 477 ?N. The Young’s modulus together with the critical buckling stress and strain for single nanorods was calculated using the Euler (for long nanorods) and Johnson (for intermediate nanorods) buckling models.
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