Fabrication of ZnO thin films composed of ZnO sub-microrods and improving their electrical properties
Date Issued
2010
Date
2010
Author(s)
Wu, Wen-Hau
Abstract
In our study, we report the fabrication of ZnO thin films composed of ZnO nanorods prepared by the hydrothermal method and the improving of the electrical properties. Firstly, we fabricated the ZnO thin films as seedlayers and ZnO nanorods by using the sol-gel technique and the hydrothermal method separately. Then, we discuss the methods of transferring ZnO nanorods to ZnO thin films composed of nanorods by thermal annealing, argon plasma treatments, excimer laser irradiation, and re-growing. We also study the effects of treatments mentioned above to the ZnO nanorod-composed thin films. Finaly,we fabricated the ZnO/Ag/ZnO multilayer structure with the ZnO layer made by the sol-gel process and the ultra thin silver layer evaporated by a thermal evaporator and introduced the methods to improve the conductivity of this structure.
We investigated the surface morphology of thermally annealed and argon plasma treated ZnO seedlyer and ZnO nanorods by scanning electron microscope (SEM) and atomic force microscope (AFM), measured sheet resistances of the samples by a four point probe technique, revealed the crystalline properties by X-ray diffraction (XRD) and the optical transmittance by a UV-Visible spectrometer. The diameter of grown ZnO nanorods was enlarged by conducting thermal or excimer laser annealing to ZnO seedlayers which affected the conductivity of the ZnO thin films made by those nanorods. Ar plasma treatment and thermal annealing could increase the conductivity of the ZnO thin films by connecting the nanorods and improving the crystalline properties.The ZnO nanorods with thermal annealing at different temperature or excimer laser annealing on the seedlayers are all wurtzite crystal structure and have (002) preferred orientation. The lowest resistivity of 1.17×10-2Ωcm and sheet resistance of 293.3Ω/sq was obtained from the ZnO nanord composed thin films with seedlayer annealed at 900℃ and nanorods treated by thermal annealing and argon plasma treatments.
We also utiltized the excimer laser irradiation to ZnO nanorods and re-growing method to fabricated ZnO nanorods composed thin films at low temperture environment. We adjusted the energy density of excimer laser and irradiation times to connect the nanorods and the sheet resistance was reduced to the value of 1.489×105 Ω/square. The XRD patterns revealed that the highly (002) preferred orientation of excimer laser irradiated ZnO nanorods was preserved. In order to aviod excess excimer laser irradiation destroying the nanorods, we treated the nanorods by argon plasma to connect them first then irradiated by excimer laser. The sheet resitance of samples combined of argon plsama treatment and laser irradiation reduced to 4.48×103Ω/square which is the lowest sheet resistance of ZnO thin films fabricated by low temperature process. On the other hand, we observed that the rod size and desity of re-grown ZnO nanorods could be affected by the rod sizes of base ZnO nanorods and a more large rod size and densely connected morphology of re-grown nanorods grown on the larger base ZnO nanorods would reduce the sheet resistance to the value of 2.271×104 Ω/square. The large sized and compact re-grown nanorods treated by argon plsama showed re-crystallizing at the boundaries of nanorods therefore improved the conductivity and the the sheet resistance reduced to the value of 1.312×104 Ω/square.
In order to supass the conductivity of ZnO nanorod composed thin film with various treatments metioned aboved. We introduced the ZnO/Ag/ZnO multilayer structure with the ZnO layer made by the sol-gel process and the ultra thin silver layer evaporated by a thermal evaporer. The thickness of silver layer on the ZnO/Ag/ZnO structures has a significant impact to light transmission and conductivity. Because of the high resistivity of sol-gel processed ZnO seedlayer and the discontinuity of evaporated silver layer, our ZnO/Ag/ZnO multilayer exhibted high resistivity. But after thermal annealing, the sheet resistance of ZnO/Ag/ZnO multilayer reduced dramaticly to the value of 19.8 Ω/square which was the lowest value through our study and showed good optical transparency over the entire visible lightwavelength range.
Subjects
ZnO
nanorods
hydrothermal method
excimer laser
argon plasma
sheet resistance
Type
thesis
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