A team of scientists from Philips and the Kavli Institute of Nanoscience in Delft, the Netherlands, has successfully grown III-V semiconductor nanowires on germanium and silicon substrates. III-V semiconductors cannot be fabricated on silicon or other group IV materials by conventional thin-film deposition and lithographic structuring.
Fundamental issues such as lattice and thermal expansion mismatch prevent growth. Expansion mismatch prevents a growth mode in which the crystallographic structure of the substrate is copied in the layer grown on top ("epitaxial growth"). This mode is essential to produce the right material properties, like a low interface resistance.
The team eliminated this problem by growing the III-V material using a "bottom up" approach. Instead of growing a layer over the entire substrate and removing the parts not required, III-V materials in the form of nanowires were grown only at substrate locations where they were needed. This produces many small individual structures rather than one large connected layer, relieving the mechanical stress within the substrate and enabling perfect epitaxial growth.
The key to achieving this type of growth was the use of the vapor-liquid-solid (VLS) method to grow the semiconducting nanowires. This technique uses conventional lithography to deposit gold seeds at the substrate locations where the nanowires need to grow. Then, the semiconductor material is applied to the substrate in vapor form. The vapor dissolves into the metal seeds. When this mixture becomes oversaturated, the semiconducting material starts to grow as nanowires.
"Although this process is not new, the team at Philips and the Kavli Institute was the first to apply it successfully to grow III-V materials on silicon and germanium substrates," said Erik Bakkers, senior scientist with Philips Research.
The team showed that perfect epitaxial growth, with atomically smooth interfaces and low contact resistance, is possible. This provides a strong base from which to explore the application of these materials in transistors, ICs, and LEDs.
III-V semiconductors are alloys comprising elements from groups III and V of the periodic table. Many of these alloys, like gallium arsenide or indium phosphide, are attractive candidates for high-frequency or optoelectronic applications. Until now, devices using these materials were grown onto substrates of the same class, making them expensive.