|An atomic-force-microscope image shows the surface of a titanium nitride (TiN) film (top). The mean roughness of the film is 0.5 nm. A scanning-electron-microscope image of TiN thin film on sapphire shows a texture indicating multivariant epitaxial (crystalline) growth. (Image: Purdue University)|
West Lafayette, IN--For the first time, a nonmetal -- titanium nitride (TiN) -- has been used as a plasmonic material at visible and near-IR wavelengths, a result of research done at Purdue University.1 A superhard ceramic, TiN can be deposited as a thin film in processes that are standard in the microelectronics industry -- unlike silver and gold plasmonic films, which are more difficult to work with.
Plasmons are electron excitations coupled to light that can direct and manipulate optical signals on the nanoscale, allowing plasmonic circuits a hundredfold smaller than corresponding integrated optical circuits. The result could potentially be optoelectronic devices that operate at unprecedented speed and efficiency.
“We have found that TiN is a promising candidate for an entirely new class of technologies based on plasmonics and metamaterials,” said Alexandra Boltasseva, one of the researchers. “This is particularly compelling because surface plasmons resolve a basic mismatch between wavelength-scale optical devices and the much smaller components of integrated electronic circuits.”
Until now, the best candidates for plasmonic materials were gold and silver. These noble metals, however, are not compatible with standard silicon-manufacturing technologies, limiting their use in commercial products. Silver is the metal with the best optical and surface plasmon properties, but it forms only semicontinuous thin films. Silver also easily degrades in air, which causes a loss of optical signal.
Titanium nitride is already commonly used as a barrier metal in microelectronics and to coat metal surfaces such as medical implants or machine-tooling parts. Its properties allow it to be easily integrated into silicon products.
To test its plasmonic capabilities, the researchers deposited a film of TiN on a sapphire substrate. They were able to confirm that TiN supported the propagation of surface plasmons almost as efficiently as gold. Silver, under perfect conditions, was still more efficient for plasmonic applications, but its acknowledged signal loss limits its practical applications.
To further improve the performance of TiN, the researchers are now looking into molecular-beam epitaxy, which would enable them to grow superlattices. In addition to plasmonics, the researchers also speculate that TiN may have applications in metamaterials.
1. Gururaj V. Naik et al., Optical Materials Express, Vol. 2, Issue 4, p. 478 (2012).