Patterned synthesis and luminescent properties of ZnO nanowire arrays

Abstract

Unintentional contamination arisen from metal catalysts and growing a highly controlled nanostructure ensemble for direct assembly are two challenging issues in bottom-up approach nanotechnology. In this thesis, a series of studies on catalyst-free growth of highly-ordered ZnO NW arrays are described. First, we focus on the growth of ZnO nanowire or nanorod arrays on a conducting aluminum-doped zinc oxide (AZO) buffer layer. Instead of a lattice-mismatched silicon and an insulating aluminum oxide substrate, we choose a conducting buffer layer on silicon substrate strategy for the synthesis. The advantages of using the aforementioned buffer layer are lattice-match, no metal catalyst, and cost-effective. In this approach, the buffer layer assists the nucleation and directs the growth of vertically aligned nanorods. By comparing grown nanorods on different thicknesses of the buffer layer, we have found that the alignment of nanowires is strongly dependent on the quality and crystal orientation of the buffer layer. In addition, the ZnO nanorods grown from our best AZO buffer layer show an improved alignment vertical to the substrate and a narrower diameter distribution. Although the buffer layer facilitates the growth of vertically aligned nanorods, the optical properties of the resulting nanorods have deteriorated. Spatially-resolved cathodoluminescence microscopy measurements show that for nonideal buffer layers the optical quality of nanorods improves along the [0001] direction leading to a lower defect incorporation at the nanorod tip. We have minimized the defect incorporation by proper adjustment of the buffer layer growth. Our results suggest that the quality of the buffer layer and the growth conditions are key parameters to reduce the inhomogeneity in its optical properties. Second, we have developed a rational process for the growth of catalyst-free ZnO NW arrays with a controlled orientation and spatial distribution. By selective patterning of AZO with an insulating mask layer, micro-patterned NW arrays with structures of various patterns such as square, circle, and cross have been acquired by conventional photolithography. Also, the insulating mask provides ease of vertical device fabrication as the current is forbidden to flow through. We have maximized the growth selectivity of ZnO NWs between AZO and the insulating mask by changing the materials and growth conditions. SiO2 deposited by e-beam evaporation shows the best in growth selectivity among other materials including AlN and Al2O3. Our findings show that the types of mask material and growth conditions are critical parameters to acquiring good alignment and growth selectivity for ZnO NW arrays. AFM surface studies of the mask and AZO layers show that surface morphology of these layers influence onedimensional structure nucleation and growth. It is reasonable to expect that a better growth selectivity can be achieved by morphology control. CL measurements on patterned ZnO NW arrays demonstrate that the optical quality of ZnO NWs grown on the AZO layer is better than that on insulating mask which can be attributed to lattice mismatch. Sharp band-edge peak, which dominates the room-temperature PL spectrum of the patterned ZnO NWs, manifests high-quality optical properties. Besides, the I-V characteristics of the ZnO NWs/AZO/p-Si heterostructures exhibit rectifying behavior with a threshold voltage of ~ 3 V and a small reverse leakage current. Patterned and aligned ZnO NW arrays have been demonstrated and they can find applications in optoelectronic devices such as light emitting diodes (LEDs) and photodetectors. Lastly, we have attempted to fabricate arrays of parallel ZnO NWs array devices with the proposed fabrication procedures. In planar device fabrication, preliminary results demonstrate that controlled and aligned NW arrays can be prepared via a simple contact printing method. Further developments may lead to heterogeneous integration of various types of NW, NWs thin film device, and cross-junction fabrication.