Nanowires grow junctions ready for transistors

Nanowires grow junctions ready for transistors
US researchers have developed a synthetic method for growing semiconductor nanowires made from alternating blocks of silicon and silicon germanium on a single wire, forming the basic junctions that will be needed to make transistors.

Dr Peidong Yang, co-founder of Nanosys, a US company focused on the development of nanotechnology-enabled systems, and professor of chemistry at the University of California in Berkeley, led the research team.

"By growing these so-called superlattice nanowires, one puts junctions and interfaces between two different semiconductors," said Dr Yang. "These semiconductor interfaces are critical for many of the devices we use today, including LEDs, transistors and lasers.

"Now we can think about making devices and systems on single nanowires, which would have a much smaller footprint than the conventional two-dimensional thin film devices."

A hybrid pulsed laser ablation/chemical vapour deposition method is used to create various heterostructures on individual nano-wires in a 'custom-made' fashion.

By keeping the nanocrystal-line growth constant and changing the chemical composition as a function of the growth time, the team can control the crystal growth on a nanometre scale just by programming a computer.

This can be seen in a scanning transmission electron microscope image of two nanowires in a bright-field mode. The dark stripes along the wires originate from the periodic deposition of the SiGe and Si segments, with the SiGe sections appearing darker than pure Si.

The team believes the technology will enable the creation of various devices such as p-n junctions, coupled quantum dot structures and heterostructured bipolar transistors on a single nanowire, leading to the construction of nano-scale electronic circuits and light- emitting devices.

At the moment, the abruptness of the Si/SiGe interfaces in the nanowires is not ideal. The team believes that incorporating more precise and faster vapour dosing/switching schemes such as beam processes could im-prove them.