pubs.acs.org/crystal Published on Web 07/12/2010 r 2010 American Chemical Society DOI: 10.1021/cg901506v 2010, Vol. 10 3297–3301 Large-Area Oblique-Aligned ZnO Nanowires through a Continuously Bent Columnar Buffer: Growth, Microstructure, and Antireflection Jun-Han Huang, † Cheng-Ying Chen, ‡ Yi-Feng Lai, † Yu-I Shih, † Yuh-Chieh Lin, † Jr-Hau He, ‡ and Chuan-Pu Liu* ,† † Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan, Republic of China, and ‡ Institute of Photonics and Optoelectronics, and Department of Electrical Engineering, National Taiwan University, Taipei 10617, Taiwan, Republic of China Received December 8, 2009; Revised Manuscript Received June 19, 2010 ABSTRACT: We demonstrate a novel defect-induced bending mechanism for a modified oblique-angle deposition (OAD) system, where different defect density was introduced to accommodate the mass difference between the shadowed and exposed surfaces, leading to continuous structural bending. Oblique angle sputtering and hydrothermal processes were employed for growth of inclined ZnO nanowire arrays on ZnO bent columns. Transmission electron microscopy images reveal that a dislocation network was introduced to accommodate the mass difference in bent columns, and the bending angle could be controlled by growth temperature. Nanowires were then grown along the tangent lines of the bent column tips. The bent column curvature and limited space determine the nanowire growth direction. The reflectance measurements demonstrate that the oblique-aligned ZnO nanowire arrays are an excellent candidate for antireflection coatings, showing the significant suppression of reflectance of 87.5% and 90.0% for polished Si under TE and TM polarization, respectively. The interference oscillations of reflectance show the optical anisotropy of oblique-aligned ZnO nanowire arrays, which is dependent on the angle range of nanowire direction. First discovered in 1959, 1,2 oblique-angle deposition (OAD) has since developed well and become a technique to grow anisotropic nanostructure. Moreover, in the recent two decades, more hierarchical nanostructures into three-dimensions have been developed based on the OAD-related technology, such as helical, 3-5 direction and symmetry control, 6,7 and branched structures. 8-10 The control over the direction of the growth front of nanostructures provides an additional degree of freedom to design for complex nanostructures, thus enabling applica- tions in new fields, such as hydrogen or energy storage, 11,12 mechanical or biological sensors, 13,14 mechanical components, 15 field emissions, 16 photonic crystals, 17,18 enhanced birefringence, 19 enhanced light extraction of light emitting diodes, 20 and anti- reflection for solar cells. 21,22 However, the OAD technology usually grows poor crystallinity of the nanostructures and allows a limited range of the nanowire growth direction, which represent the major weaknesses of the technology for some applications, especially in optoelectronic devices. The OAD deposited nanostructures usually grew toward the incident source direction via a shadowing effect and limited adatom diffusion. 23,24 Therefore, most OAD experiments were requested to operate at low energetic growth conditions, so that adatoms can merely migrate a short distance by shadowing. That is the reason why the nanostructures were aligned only with the incident source direction and usually grew in poor crystal quality. The oblique angle of the grown nanostructures relative to the substrate surface normal is far smaller than the incident flux angle, following the common predictions from the experimental tangent rule 25 or the theoretical Tait’s rule. 26 So far, only a few reports have obtained oblique structures with good crystallinity. 27,28 To achieve oblique nanowire arrays aligned in an even larger range of angles with single crystallinity is apparently very challenging, though the breakthrough can lead to expanding the potential applications into optoelectronic de- vices. On the other hand, emerging nanofabrication technology has enabled materials to be engineered to meet desired anti- reflection (AR) characteristics, 29 however through complicated procedures. Very recently, ZnO nanostructures were shown to bring exciting possibilities for next-generation AR coatings (ARCs) to suppress the Fresnel reflection effectively. 30 In this report, we demonstrate an interesting approach by a combined method of modified OAD and hydrothermal growth to grow oblique ZnO nanowire arrays at any angle with single crystal- linity, whose growth mechanism differs from the typical OAD. Furthermore, reflectance measurements have been examined to demonstrate the oblique-aligned ZnO nanowire arrays as an excellent ARC for photovoltaic application. The correlation between the oblique angle of nanowires and reflectance has been discussed. This growth mechanism is universal and can be applied to other materials for more applications, in principle. An oblique-angle sputter system followed by hydrothermal growth was used to grow a direction-controlled ZnO nanowire array on Si(100). A pure ZnO target (99.99%) was used as the sputtering source. At first, a thin ZnO buffer with c-axis vertically aligned was deposited at a 30° oblique angle with 1 rpm substrate rotation in argon at 410 °C. Subsequently, a layer with bent columns was designed on the buffer in a reduced atmosphere with 20% hydrogen/argon mixture gas; with the oblique angle, R, set at 30° relative to the surface normal, no substrate rotation was applied during this step. Three growth temperatures were attempted, which were 460 °C, 320 °C, and 265 °C. For the final hydrothermal process, the samples were grown in a solution of 0.005 M zinc acetate dehydrate (Zn(CH 3 COO) 2 3 2H 2 O) and 0.005 M hexamethylenetetramine (HMT C 6 H 12 N 4 ) in the ratio of 1:1, heated at 81 °C for 2 h. TEM samples were prepared with a SMI 3050 focused ion beam (FIB). JEOL-7000 field-emission scanning electron microscopy (SEM) was operated at 10 keV to image nanostructure morphology. JEOL-2100F field-emission transmission electron microscopy (TEM) at 200 keV and X-ray diffraction (XRD) spectroscopy were employed to characterize the microstructure. Optical reflectance measurements were per- formed at the angle of incidence (AOI) of 8° for TE and TM polarization in the wavelength ranges of 200 to 850 nm with a standard UV-vis-NIR spectrophotometer (JASCO V-670). The reflection of a collimated incident light beam was measured by collecting the specularly reflected cone of light within an acceptance angle of 8°. *Corresponding author. E-mail:cpliu@mail.ncku.edu.tw. Telephone: 886-6-2757575-62943. Fax: 886-6-2346290.