Nanowires by Step Decoration
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i(100) (2 X 1) surface, or by adsorption of alkali and alkaline earth atoms. Wires perpendicular to a surface have been obt a i n e d by electroplating metals into Channels across a thin film (e.g., etched nuclear tracks in a polymer 4 and anodized pores in alumina ). Whiskers can be obtained by seeding them at the surface.6 Here, we focus on concepts that utilize a stepped surface as a template (Figure l 7 ' 8 ). Adsorbed atoms or molecules have a t e n d e n c y to stick to the step edges, where they find extra bonding partners for lateral bonds. Ideally, one hopes to achieve a step-flow g r o w t h mode, 9 where stripes grow along the step edges in a row-by-row fashion (Figu r e l a ) . Such a m e t h o d is attractive because it allows for two controllable process parameters: (1) the coverage determines the stripe width, as in many of the other self-assembly methods; and (2) the miscut angle of the Substrate determines the spacing between the stripes. Simple step-flow growth can be extended into a more "universal" method 8 that emulates Silicon microlithography on the nanoscale (Figure lb). Initially, an inert mask is produced by step-flow growth and acts as the equivalent of the photo-
Step-Flow Growth
resist. Selective deposition (or etching) in the areas not protected by the mask produces features with controllable vertical aspect ratios. Silicon-based techniques are able to take advantage of a vast array of well-tested processing methods. In the following sections, we will explore how such an idealized view holds up at real surfaces. We will encounter a wide variety of g r o w t h modes when s t u d y i n g the s e l f - a s s e m b l y of o n e dimensional objects in a two-dimensional world. Not only wires, but also linear ar rays of dots can be fabricated with dimensions in the single-digit-nanometer regime and areal densities of a few teradots per Square inch. Silicon Substrates will be considered in detail. In all of these studies, new chemical-imaging methods are used in combination with scanning tunneling microscopy (STM) for observing the redistribution of adsorbate and Substrate atoms on the nanometer scale.
Growth Modes at Stepped Surfaces The idea of obtaining nanowires by step-flow growth was tested early on for the GaAs/GaAlAs System. 9 Step-flow growth proceeds rather well for these materials because they are structurally and chemically matched. This method can be extended to more heterogeneous adsorbate/substrate combinations, as d e m o n s t r a t e d for c o m b i n a t i o n s of metals.7,810"13 It requires a Substrate with high surface energy (W, Mo) and an ad sorbate with low surface energy (Cu) that does not alloy with the Substrate. Fig ure 2 shows 3-nm-wide Cu wires on a stepped Mo(110) surface as an example. Step-flow growth is not the only Op tion. A surprising variety of additional g r o w t h modes can be identified and classified, as shown in Figure 3. All of the familiär equilibrium growth modes
[b] "Universal" Growth Method Selective deposition
• Coverage -> Stripe width
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