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Ordered High-Density Si [100] Nanowire Arrays Epitaxially Grown by Bottom Imprint Method By Zhang Zhang,* Tomohiro Shimizu, Stephan Senz,* and Ulrich Go¨sele Among all 1D nanomaterials, silicon nanowires (SiNWs) in particular attracted attention because of their potential usage in nanoelectronic, optoelectronic, and thermolelectronic devices.[1–3] Therefore, considerable effort has been devoted to developing the controlled synthesis of SiNWs by various techniques.[4] Vapor–liquid–solid (VLS) is one widely used method that can grow epitaxial nanowires on single-crystal substrates with the help of metal catalysts.[5,6] However, using a VLS chemical vapor deposition (CVD) process, mainly three epitaxial growth directions on bare silicon substrates were obtained, which do not include the h100i growth direction.[6] Considering conventional Si micro/nanoelectronics based on Si (100) wafers, it is meaningful to realize vertically grown high-density epitaxial Si [100] nanowire arrays on Si (100) wafers. Especially for devices using vertical-nanowire arrays, ordering and size distribution are also important parameters. Anodic aluminum oxide (AAO) membranes are widely used as templates to prepare various nanostructures.[7,8] Much effort has been devoted to improving the ordering of thin AAO templates with their adjustable pore size and thickness.[9] The method discovered by Masuda et al. relies on self-ordering of pores at the bottom of porous alumina channels after a long first anodization step, which can give rise to a self-assembled honeycomb array of uniformly sized parallel channels.[10] An ordered AAO template can be obtained in a second anodization after selective etching off the first anodized one.[11] Lithographic methods allow the fabrication of nanowire arrays with controlled size and welldefined growth positions through patterned templates. Longrange ordered AAO templates have already been achieved by focused ion beam (FIB) patterning of Al films deposited on silicon substrates,[12] although this process is limited by high costs and the long time required for sequential writing. In order to use the well-defined AAO templates after two-step anodization to direct the epitaxial growth of Si nanowire arrays, the most important issue is a good interface between the grown nanowires and the substrate. Without an interface to the substrate, SiNWs have already been synthesized inside the channels of AAO templates.[13,14] Au catalyst seeds were inserted into the middle of pores of AAO before the CVD process, which formed SiNWs inside the pores, with both ends capped by Au.[14] This method allows poor control over the crystallographic growth direction without epitaxial growth, and the SiNWs released from the template have been difficult to integrate into vertical-array
¨sele [*] Z. Zhang, Dr. T. Shimizu, Dr. S. Senz, Prof. U. Go Max Planck Institute of Microstructure Physics Weinberg 2, 06120 Halle (Germany) E-mail:
[email protected]
DOI: 10.1002/adma.200802156
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devices. Lombardi et al. transferred an AAO membrane onto a Si (111) substrate as mask of Au catalyst seeds. Subsequently, they successfully prepared vertically grown epitaxial Si [111] nanowire arrays with high density and well-defined size distribution.[15] However, this approach is not possible for Si [100] nanowires. To realize the [100] growth direction, our previous work successfully utilized directly anodized AAO templates to grow epitaxial Si [100] nanowires vertically on Si (100) substrates. We successfully resolved the task of direct contacting between Au and silicon at the bottom of the pores with the help of HF-assisted electroless deposition.[16] However, for the direct deposition of an Al layer on a silicon wafer, a high-enough thickness for the long-time first anodization is not easy to achieve, and is important for the ordering. The following anodization plus chemical etching result in unwanted alteration of the Si-surface morphology.[17] Therefore, up to now, epitaxial growth of vertical high-density Si [100] nanowire arrays in AAO templates with well-ordered pores and a narrow size distribution has not been achieved. In this paper, we report a novel approach, which we call bottom imprint (BI) method, for growing high-density epitaxial Si [100] nanowire arrays on Si (100) substrates with well-defined ordering as well as a narrow size-distribution. In comparison with directly anodized AAO templates on silicon substrates, thin AAO membranes using long-time first anodization are more convenient for our purpose. Highly ordered AAO membranes can be produced by the well-developed two-step anodization of highpurity Al foils.[10,11] Furthermore, with the pore-opened thin film structure transferred onto the prepared silicon substrate, we can avoid any chemical contamination and surface-morphology modification. The central idea of our approach is to bond thin pore-through AAO membranes directly onto Si (100) substrates covered by pre-deposited Au layers, forming conformal contact. Au is a soft metal compared with alumina, both Young’s modulus and hardness decrease as the temperature and indentation load increase.[18] Under proper temperature and load, it is possible to imprint the harder bottom structure of the AAO template into the Au layer to form homogeneously separated nanoparticle arrays. Finally, in a UHV-CVD process, SiNWs catalyzed by the VLS mechanism grow epitaxially inside the pores with the same ordering and size as the AAO template. The thin (
