Multiple Devices Fabricated in a Single Nanowire

Since 1998, researchers in Harvard University 's Lieber Group, within the Dept. of Chemistry and Chemical Biology, have been working to turn fundamental nanotechnology into useful electronic and photonic devices. So far, they have synthesized a variety of new nanocomponents, and shown how those nanocomponents can be used to make several key devices. "We've done a lot of work in terms of synthesis of p- and n-type nanowires, made III-V, II-VI and silicon nanowires, doped them accordingly, and then made LEDs, photodetectors, transistors and logic circuits," explained Mark Gudiksen, a graduate research student in the group. "We've made devices and are thinking about how to make arrays of devices."

In their most recent work, they showed that it's possible to synthesize — in a single nanowire — semiconductor superlattices from group III-V and group IV materials, or n-Si/p-Si and n-InP/p-InP modulation-doped structures. "Previously, we could make heterostructures or p-n junctions by combining several wires, but now, if you can imagine, you can incorporate a higher level of integration by not only enabling these cross-architectures, but then having functionality in individual nanowires as well," Gudiksen said. "It's really a higher level of sophistication in what type of architectures would be possible."

As detailed in the Feb. 7 issue of Nature, the researchers used metal-catalyzed nanowire synthesis to form superlattice structures composed of GaAs and GaP. Here, nanoclusters of gold (Au) are used to control the diameter and, through growth time, the length of the nanowires by way of a vapor-liquid-solid growth process. To create a single junction within the nanowire, the addition of one reactant is stopped during growth, and a second introduced: Repeated switching of the reactants produces nanowire superlattices that can be easily modulation-doped.

In the left hand frame is a false-colored image of a single nanowire, where the colors depict the GaAs (red) and GaP (green) regions. On the right is a three-dimensional image of the photoluminescence from the wire, showing the nano-barcode created by using the alternating GaAs (direct bandgap) and GaP (indirect bandgap) regions. (Source: Harvard University)

GaAs/GaP nanowire superlattices are useful for nano-photonic applications because GaAs is a direct bandgap semiconductor, while GaP has an indirect gap. Photoluminescence imaging of individual nanowires (Figure) shows an emission pattern of three spots separated by dark regions. This pattern is consistent with emission originating from the three GaAs regions, separated by dark GaP regions that act as optical "spacers" — a kind of nano-barcode. The researchers believe that, using materials with a large dielectric contrast, it might be possible to build one-dimensional waveguides with built-in photonic bandgaps (or build cavities for nanowire lasers).

The researchers also fabricated — again in a single wire — silicon p-n junctions, using the Au-nanocluster-catalyzed CVD approach, which could prove useful in highly integrated logic circuits. In a similar manner, they also built p-n junctions in a single InP nanowire, which are essentially nanoscale LEDs.

The advantage of this direct-growth approach to synthesis, compared with the doping of carbon nanotubes (another type of nanotechnology), is that it eliminates the need for lithography, enabling the possibility of bottom-up assembly of complex functional structures. "By the time Moore's Law starts running out," Gudikeson said, "hopefully all the details will be figured out so that these things become practical."

For additional information on emerging technologies, go to www.semiconductor.net/emerging.