Extensible diamond for new generation microelectronics

Diamond is the heaviest material in nature. But, from many expectations, it also has great potential as an excellent electronic material. A joint research team led by the City University of Hong Kong (CityU) demonstrated for the first time the uniform tensile elastic traction of microfabricated diamond matrices through the nanomechanical approach. Their discoveries have shown the potential of taut diamonds as major candidates for advanced functional devices in microelectronics, photonics and quantum information technologies.

The research was led by Dr. Lu Yang, an associate professor in the Department of Mechanical Engineering (MNE) at CityU and researchers at the Massachusetts Institute of Technology (MIT) and Harbin Institute of Technology (HIT). Their findings were recently published in the prestigious scientific journal Science, entitled ‘Obtaining high uniform tensile elasticity in microfabricated diamond’.

“This is the first time that shows the extremely high and uniform elasticity of diamond through tensile experiments. Our findings demonstrate the possibility of developing electronic devices by” deep elastic deformation engineering “of microfabricated diamond structures,” said Dr. Lu.

Diamond: “Mount Everest” of electronic materials

Known for its hardness, industrial applications of diamonds are usually cutting, drilling or grinding. But diamond is also considered a high performance electronic and photonic material due to its ultra-high thermal conductivity, exceptional mobility of the electric charge carrier, high damage resistance and ultra-wide bandwidth. Bandgap is a key property in semiconductor, and wide bandgap allows the operation of high power or high frequency devices. “Therefore, diamond can be considered the ‘Mount Everest’ of electronic materials, which possesses all these excellent properties,” said Dr. Lu.

However, the large bandwidth and tight crystal structure of the diamond make it difficult to “drug”, a common way of modulating the electronic properties of semiconductors during production, thus making it difficult for the industrial application of diamond in electronic and optoelectronic devices. A potential alternative is through “deformation engineering”, ie the application of a very large network strain, to change the structure of the electronic band and the associated functional properties. But it was considered “impossible” for diamond due to its extremely high hardness.

Then, in 2018, Dr. Lu and his collaborators discovered that, surprisingly, the nano-scale diamond can be flexibly bent with an unexpectedly large local strain. This finding suggests that changing the physical properties of diamond may be possible through elastic engineering. Based on this, the latest study showed how this phenomenon can be used for the development of functional diamond devices.

Uniform tensile tension on the sample

The team first sampled microcrystalline monocrystalline diamond from a single solid diamond crystal. The samples were bridge-shaped – about a micrometer long and 300 nanometers wide, with both ends wider for grip (See image: Traction strain of diamond bridges). The diamond bridges were then stretched uniaxially in a well-controlled manner under an electron microscope. In the continuous and controllable loading-unloading cycles of the quantitative tensile tests, the diamond bridges showed a very uniform, large elastic deformation, with a deformation of about 7.5% over the entire section of the specimen gauge, rather than the deformation. an area located in the bend. And they regained their original shape after downloading.

Further optimizing the sample geometry using the American Society for Testing and Materials (ASTM) standard, they achieved a maximum uniform tensile stress of up to 9.7%, which even exceeded the maximum local value in the 2018 study and was close to theory. the elastic limit of the diamond. More importantly, to demonstrate the concept of tensioned diamond device, the team also performed elastic tensioning of microfabricated diamond matrices.

Bandgap adjustment by elastic straps

The team then performed functional density theory (DFT) calculations to estimate the impact of elastic deformation from 0 to 12% on the electronic properties of the diamond. The simulation results indicated that the band gap of the diamond generally decreased as the tensile stress increased, with the highest rate of bandwidth reduction down from about 5 eV to 3 eV at about 9% strain. along a specific crystalline orientation. The team performed an electron loss spectroscopy analysis on a pre-tensioned diamond sample and checked this bandgap decrease trend.

Their calculation results also showed that, interestingly, the bandwidth can change from indirect to direct with traction strains greater than 9% along another crystalline orientation. The bandwidth directly in the semiconductor means that an electron can emit a photon directly, allowing many optoelectronic applications with greater efficiency.

These findings are an early step in the engineering of deep elastic deformation of microfabricated diamonds. Through the nanomechanical approach, the team demonstrated that the structure of the diamond band can be changed and, more importantly, these changes can be continuous and reversible, allowing different applications, from micro / nanoelectromechanical systems (MEMS / NEMS), voltage engineering transistors, to new optoelectronic and quantum technologies. “I think a new era of diamond is ahead of us,” said Dr. Lu.

Provided by City University of Hong Kong