Nanowire Processing

Once nanowires have been grown they must be connected to the outside world to eventually become part of an electronic circuit. However, the high aspect ratio of these objects requires the development of new, nanowire-specific processing techniques. Here, a simplified case of sample processing is described where the growth substrate acts as bottom contact to the nanowires. This section is divided into three parts describing important steps in the formation of source, drain, and gate contacts to the nanowires.
Seed patterning for nanowire growth   In one sense, contact formation starts when the nanowire seeds are defined on the semiconductor surface. To grow semiconductor nanowires at defined locations, the seeds have to be positioned. The most common process for the definition of seeds is electron-beam lithography exposure of a resist layer, metal evaporation, and a lift-off step. Figs. 4 a–c show the cross section of a sample during the seed fabrication process. While the size, periodicity, and thickness of thenanowires do not impose strong demands on the fabrication process,proper surface preparation before seed deposition is crucial for wellcontrolled nanowire growth.
	Figure 4. Seed fabrication process: (a) resist exposure and development; (b) evaporation of Au layer; and (c) lift-off. (d) Scanning electron micrograph of Au seeds on a GaP substrate. The dots have diameters of 30 nm and a height of 15 nm.
If contamination or an oxide layer is present between the seed and the semiconductor surface, the seed may still initiate growth but crystallographic alignment with the substrate may be lost. Development of a standardized process for pregrowth wafer treatment will be an important milestone for reproducible nanowire growth.
Nanowire contacts The most widely used method to define contacts to nanowires is to remove the nanowires from the growth substrate and deposit them on a second substrate. Usually the wires are suspended in a liquid that is then applied to the substrate [16], but ‘dry’ transfer methods also work.  Inspection under an electron microscope then reveals promising candidates for electrical characterization. Contacts are made to individual wires using electron beam lithography, metallization, and lift-off.
	Figure 5. Scanning electron micrographs of GaP nanowires (a) before planarization and (b) after planarization.
However, for up-scaling, an approach that does not rely on a random process step is required. If nanowires are grown epitaxially on the substrate from defined seed locations, the wires can be left in place and contacts can be post-processed to the top of the wires. This requires a planarization step, which can be achieved, for example, by spinning a polymer onto the nanowire sample. Fig. 5 shows (a) as-grown and (b) planarized nanowires. Contacts to the top of the wires can then be formed by metal evaporation.
Continue