Three-five on silicon

A team of scientists from Philips and the Kavli Institute of Nanoscience Delft has successfully grown III-V semiconductor nanowires on germanium and silicon substrates.

Up till now, III-V semiconductors could not be fabricated on silicon or other group IV materials by the conventional fabrication techniques of thin-film deposition and lithography due to fundamental issues such as lattice and thermal expansion mismatch between the two materials.

The team solved the problem by growing the III-V material in a “bottom up” approach, i.e. instead of growing a layer over the entire substrate and removing the parts that are not needed, III-V materials, in the form of nanowires, are only grown at substrate locations where they are needed.

Because this results in many small individual structures rather than one large connected layer, the mechanical stress with the substrate is relieved and perfect epitaxial growth can be achieved.

The key to achieve this type of growth was the use of the vapour-liquid-solid (VLS) method to grow the semiconducting nanowires. In this method, metal (gold) seeds are deposited (using conventional lithography) at the substrate locations where the nanowires should grow.

Then the semiconductor material is applied to the substrate in vapour form. The vapour dissolves into the metal seeds, and when this mixture becomes oversaturated, growth of the semiconducting material in the form of a nanowire starts.

“Although this process is not new, our team at Philips and the Kavli Institute of Nanoscience was the first to apply it successfully to grow III-V materials on silicon and germanium substrates,” said Dr. Erik Bakkers, senior scientist at Philips Research.

The team showed that perfect epitaxial growth, with atomically smooth interfaces and low contact resistance could be achieved, providing a starting point from which they can explore the use of the materials in devices such as transistors, integrated circuits and light-emitting diodes.

III-V semiconductors are alloys comprising elements from the groups III (e.g. gallium or indium) and V (e.g. arsenic or phosphor) of the periodic table of the elements.

Many of these alloys, for example gallium-arsenide (GaAs) or indium-phosphide (InP) are attractive candidates for high-frequency or optoelectronic applications.

Until now, devices using these materials had to be grown onto substrates of the same class, making them rather exclusive and expensive.

However, the new research shows that InP nanowires can be grown on on silicon or germanium substrates, bringing us closer to the day when III-V materials will be used in the mainstream, silicon-based semiconductor industry.

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