Eindhoven, the Netherlands - A team of scientists from Philips and the Kavli Institute of Nanoscience Delft has announced to be the world's first to successfully demonstrate the growth of III-V semiconductor nanowires on germanium and silicon substrates. In their article published in today's issue of Nature Materials, they present a detailed study of this important step towards the integration of the superior high frequency and optoelectronic properties of III-V semiconductors with the huge silicon technology base of the semiconductor industry.
Up till now, III-V semiconductors could not be fabricated on silicon or other group IV materials by the conventional fabrication route of thin-film deposition and lithographic structuring. This is caused by fundamental issues such as lattice and thermal expansion mismatch, which prevent a growth mode in which the crystallographic structure of the substrate is copied in the layer grown on top ("epitaxial growth"). This growth mode is essential to produce the required material properties such as a low interface resistance, which is important for future electronic devices such as transistors and light-emitting diodes.
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, is only grown at substrate locations where it is needed. Because this results in many small individual structures rather than one large connected layer, the mechanical stress with the substrate is relieved easier and perfect epitaxial growth can indeed be achieved.
The key to achieve this type of growth was the use of the vapor-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 vapor form. The vapor 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 of 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 and first author of the article in Nature Materials.
The team showed that perfect epitaxial growth, with atomically smooth interfaces and low contact resistance could be reached, providing an excellent base to explore these 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 (e.g. high-bandwidth connectivity) or optoelectronic (e.g. integrated optics or LEDs) applications. Until now, devices using these materials are grown onto substrates of the same class, making them rather exclusive and expensive. In the article in Nature Materials, the scientists describe the growth of InP nanowires on germanium substrates. In the meantime, the team has also succeeded in growing InP on silicon substrates, setting an important step towards the application of III-V materials in the mainstream, silicon-based semiconductor industry.