MICROELECTRONIC devices of the future may be grown from the ground up like trees rather than carved into silicon. That’s the possibility raised by the growth of forests of semiconducting “nanotrees”. They have properties that promise to overcome one of the most fundamental problems facing the microelectronics industry – maintaining the quality of signals within devices as they get smaller.
The smallest features that are routinely carved in silicon today are 130 nanometres across. But when electrons pass through these features in microelectronic devices, they tend to be scattered by imperfections in the structure of the material. In large devices the effect of this scattering is negligible but it becomes increasingly important as devices approach the nanoscale because the scattering causes signals to interfere. “You can’t keep the electrons where they need to be,” says Knut Deppert of Lund University in Sweden.
One way round this is to build nanoscale semiconductors and circuits from the ground up by depositing materials such as gallium phosphide on a substrate, molecule-by-molecule. Nanowires have already been built in this way. They behave as ideal one-dimensional crystals which do not have the imperfections of etched circuits and so allow electrons to travel “ballistically” – in straight lines without scattering.
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Now, Lars Samuelson, also of Lund University, and Deppert and colleagues, have built a more complex structure – a nanoscale tree in which the trunk and each branch have the same properties as a single nanowire.
First, the researchers deposit an array of gold nanoparticles, each about 50 nanometres in diameter, on a substrate of gallium phosphide (GaP). The substrate is placed in a chamber filled with trimethylgallium and phosphine gases and heated to 500 °C. The gold catalyses the formation of GaP molecules, which are attracted to the substrate and so migrate to the bottom of the gold particle. Layer upon layer of these GaP molecules gradually build up beneath the gold particle to form a 1-micrometre-tall trunk with the blob of gold perched on top.
To grow branches on these trees, the team sprayed the trunks with an aerosol of even smaller charged gold particles, each about 30 nanometres in diameter (Nature Materials, DOI: 10.1038/nmat1133). The researchers are able to control the spacing and number of particles that stick to each trunk using electric fields to guide the charged particles into place. They cannot, however, control exactly where the particles land. “It’s like in nature, where you don’t exactly know where you’ll get the next branch on a tree,” says Deppert.
The trunks are then placed back in the original chamber and the growing process repeated to create branches of gallium phosphide (see Graphic). By using different combinations of gases in the chamber, the team can also grow branches of indium phosphide on trunks of gallium phosphide, or even have segments of the branches made of gallium arsenide phosphide, another semiconductor. This is significant because by controlling where the different types of semiconductors form, the researchers say it should be possible to make the branches behave as diodes or transistors.
Peidong Yang, an expert on nanowires at the University of California, Berkeley, is impressed. “This is a very nice piece of work with implications for making solar cells of high-energy conversion efficiency.”
Samuelson’s team has already begun work on turning a forest of these nanoscale trees into solar cells. The idea is to grow “leaves” at the end of branches using a photovoltaic material that creates a current when bombarded by photons. This approach would dramatically increase the effective surface area of the cell making it far more efficient than traditional photovoltaic materials. However, the team is keeping tight-lipped about the details while it patents the technology.
