With the demand for ever smaller and more powerful laptops and smartphones, there is a continuing need to develop new electronic materials. Diamond offers an exciting prospect in the search for new, hardier materials that can be employed in extreme and novel environments.
A team of researchers in Moscow have taken a significant step toward diamond-based electronics—which are useful for high-temperature, high-power microwave electronics—with work detailed in a recent issue of Crystal Growth & Design (doi:10.1021/acs.cgd.5b01520). Using previous advances in growing high-quality diamond on metal as a starting point, Stanislav Evlashin of the Russian Academy of Sciences (RAS) and Lomonosov Moscow State University and colleagues sought to further improve the processing and properties of metal films on diamond. They focused their study on the heteroepitaxy of nickel-based (Ni-Cu, Ni-Cu-Cr, and Ni-W) alloys on the surface of diamond.
Speaking about the inception of the work, Evlashin says, “The main idea to ‘flip’ the task of the growth of a diamond on metals to the growth of heteroepitaxial layers of metals on a diamond belongs to Nikolay Suetin [of Lomonosov Moscow State University].” This “flip” allowed the use of magnetron sputtering, a commercially available and economical technique. But growing ultrathin high-quality solid metal films on diamond presented other challenges.
Due to its chemical stability, metal adhesion to diamond tends to be poor. One solution is to form a metal carbide, although these often have undesirable properties. Iridium adheres to diamond without forming a carbide, but Evlashin and his team found that their iridium films were of poor quality—strain caused by the large mismatch between the film and substrate lattices ruined the film. Roman Khmelnitsky (RAS) suggested nickel-based alloys, which have a lattice parameter that is nearly identical to that of diamond, thereby solving the mismatch problem. They hoped that heteroepitaxy would provide sufficient adhesion. These alloys also should not form carbides at the relatively low growth temperatures. With a few carefully chosen Ni-Cu alloys, Evlashin and his colleagues grew and characterized several films.
Scanning electron micrographs of heteroepitaxial films on diamond. Credit: Crystal Growth & Design.
“The big surprise was that heteroepitaxy had been achieved at a temperature of 290°C, and the parameters of the crystal lattice of [the diamond substrate] and the films differed by less than 0.5%,” Evlashin says. They had achieved high-quality, ultrathin heteroepitaxial films at a much lower temperature than expected. Furthermore, the combination of magnetron sputtering and low deposition temperature reduced fabrication costs dramatically, which is encouraging for future work. And the 0.5% mismatch is especially good—“It’s a record,” Evlashin says.
The vitality of diamond electronics depends on the ability to create durable and effective metal contacts on diamond. But this work also has applications in radiation sensing, and may have impact as far reaching as nanophotonics and quantum computing. Evlashin and colleagues have opened the door to a new avenue of research and discovery in electronic materials.
