Effect of Electroless Copper on the Growth of ZnO Nanowires
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ZnO nanowires along with ZnO thin films were obtained on copper-metallized silicon substrates using an radio frequency-reactive sputter-deposition technique. Residual tensile stresses were found in both the copper layer and the ZnO layer. The ZnO nanowires were observed exclusively at the grain boundaries of the ZnO thin films. The average diameter of ZnO nanowires varies only slightly with the ZnO deposition time, while the average length increases linearly with the ZnO deposition time. Based on the observations a growth model involving stress-assisted diffusion of copper and reaction-controlled catalytic growth of ZnO nanowires is suggested.
I. INTRODUCTION
II. EXPERIMENTAL
ZnO nanostructures with excellent optoelectronic properties are useful for light-emitting diode,1,2 semiconductor, fluorescence, and optical probe.3 Among the nanostructures, ZnO nanowires and nanorods are usually synthesized using vapor-phase oxidation of metallic Zn powders,4,5 vapor phase transport process via catalyzed epitaxial crystal growth,6,7 chemical reactions in confine spaces provided by alumina templates with nanochannels,8 and aqueous growth method.9 Irrespective of the forms and growth techniques, successful growth of ZnO nanowires and nanorods typically requires the use of a single-crystal substrate, such as sapphire or diamond. In previous papers, we have reported the growth of ZnO nanowires on a non-single-crystal substrate using a conventional reactive sputter deposition technique.10,11 It was found that ZnO nanowires can be grown on copper metallized silicon wafers. However, the copper has to be prepared by electroless plating. The use of other techniques such as sputter deposition for the preparation of copper metallization failed to produce ZnO nanowires. Although the presence of a copper layer was found to be decisive in obtaining the ZnO nanowires, correlation of the processing parameters and the growth of ZnO nanowires is required for further understanding of the growth of ZnO nanowires. As a result, in this study, we have further explored the role of electroless copper by investigating the residual stresses in both the copper and the ZnO thin films, and the preferred growth sites of ZnO nanowires. A formation mechanism based on the consideration of the residual stress and copper as the catalyst is discussed
As-received p-type, (100) silicon wafers were used as virgin substrates. The wafers were first deposited with a thin layer of Ti using a direct current (dc) sputterdeposition technique. The Ti layer is used as a diffusion barrier. The Ti-deposited silicon wafers were electrolessplated with copper. The detailed plating process and the electroless bath composition can be found elsewhere.12 The electroless copper plating was performed in a batch manner. In a batch, four identical Ti-deposited Si coupons were immersed in the bath at the same time. The coupons were subsequently removed from the bath one by one after 1, 2, 3, and 4 min of plating time. Substrates thus obtained were then subjected to ZnO deposition in
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