Well-ordered ZnO nanowire arrays on GaN substrate fabricated via nanosphere lithography
Introduction
Synthesis of ZnO nanowires with precise control of their alignment, distribution and aspect ratio is highly desirable for their potential applications in sensor arrays, high-efficiency photonic devices, near-UV lasers, and for assembling complex three-dimensional nanoscale systems ([1], and refs therein). A straightforward approach for this purpose is to create metal nanoparticles, which are used as catalyst templates for the subsequent guided vapor–liquid–solid (VLS) growth of nanowires [2]. In the past few years, a number of approaches have been used to obtain nanoscale-patterned metal catalysts for the fabrication of ZnO nanowires arrays. They include electron beam lithography (EBL) [3], soft-photolithography [4], and mask lithography by porous alumina [5], self-assembled micro- or nanospheres [6], [7], [8]. EBL is known as a relatively complicated and costly method, thus unsuitable for large-scale fabrication. In contrast, imprint and nanosphere lithography (NSL) tend to be more promising as they are less costly techniques with a much higher throughput. Recently, several groups [6], [7], [8] reported the large-scale fabrication of ZnO nanowires templated by NSL. However, the ZnO nanowires in these reports are either not patterned in nanoscale because of the interconnection of the printed Au [6], or not truly vertically aligned [7], [8] due to unoptimized growth conditions and/or imperfect lattice matching between the sapphire substrates and ZnO nanowires. These drawbacks might hinder the consideration of such nanowire arrays from device applications.
GaN and ZnO have a similar fundamental bandgap energy (∼3.4 eV), the same wurtzite crystal symmetry, and a low misfit of the lattice constant (1.9%). This, as well as the availability of high-quality p-doped GaN films, makes GaN a good candidate for both epitaxial growth [9], [10] and device applications of ZnO nanowires. Recently, we reported a new template method for fabrication of perfect arranged arrays of ZnO nanowires or pillars on GaN epilayers, in which Au nanotube membranes were used as a lithography mask to produce ordered arrays of Au nanodots [10]. In this contribution, we apply the same growth method using a modified NSL technique. The obtained ZnO nanowires have a uniform vertical alignment to the substrate, and nanoscale pattern (i.e., nearly a single wire at each catalytic Au dot site) as defined by the mask.
Section snippets
Experiments
The whole fabrication process is schematically illustrated in Fig. 1. First, aqueous solution of 488 nm polystyrene (PS) nanospheres (Nanoparticles GmBH, Germany) was diluted in methanol and spin-coated onto a glass substrate. The glass substrates were cleaned in NH4OH/H2O2/H2O (1:1:5) at 80 °C for 30 min and stored in water until usage. The PS spheres self-assemble into a monolayer (or bilayer depending on dilution degree and spin speed) structure [7], [8], [11] (Fig. 1(a)). Afterwards, a ∼50 nm
Results and discussion
NSL has been a widely used technique for structuring substrates with a hexagonal array of various nanomaterials, including ferroelectric [13], magnetic [14], [15], and metal dots for growth of carbon nanotubes [16], [17] and semiconductor nanowires [6], [7], [8], etc. However, a direct deposition of PS nanospheres on GaN was found to be difficult due to the hydrophobicity of GaN. To address this problem, we applied the known “mask transfer” [12] technique as illustrated in Fig. 1, i.e., the
Conclusion
By using nanosphere lithography and mask transfer technique, as well as optimized vapor-phase growth conditions, we fabricated well-ordered and hexagonal-patterned ZnO nanowire arrays in a large scale on epitaxial GaN substrates. Single nanowires at individual catalytic sites are achieved. When the nanospheres are controlled to be mono- or bilayers, the resulting ZnO nanowires have different diameter, geometry and interwire distance. Cross-sectional TEM analysis confirms the vapor–liquid–solid
Acknowledgements
We thank M. Hopfe for TEM sample preparation and F. Heyroth for help with SEM.
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