New Approach Makes Diamonds Viable for Advanced Microelectronics

Among all materials, diamond is said to be the hardest one in nature. However, out of several expectations, diamond also has excellent potential as a superior electronic material.

For the first time, a collaborative research group has used a nanomechanical approach to reveal the large and even tensile elastic straining of microfabricated diamond arrays. The team was headed by the City University of Hong Kong (CityU).

The researchers’ findings have demonstrated that strained diamonds could serve as major candidates for next-generation functional devices used in photonics, microelectronics, and quantum information technologies.

The study was jointly headed by Dr Lu Yang, an Associate Professor in the Department of Mechanical Engineering (MNE) at CityU, and also by scientists from the Massachusetts Institute of Technology (MIT) and Harbin Institute of Technology (HIT).

The team’s findings were recently reported in the leading scientific journal Science, in a paper titled “Achieving large uniform tensile elasticity in microfabricated diamond.”

This is the first time showing the extremely large, uniform elasticity of diamond by tensile experiments. Our findings demonstrate the possibility of developing electronic devices through ‘deep elastic strain engineering’ of microfabricated diamond structures.

Lu Yang, Associate Professor, Department of Mechanical Engineering, City University of Hong Kong

Diamond: “Mount Everest” of Electronic Materials

Diamonds are renowned for their excellent hardness. Industrial applications of these materials often involve drilling, cutting, or grinding. Nonetheless, diamond is also regarded as a high-performance photonic and electronic material because of its excellent electric charge carrier mobility, ultra-high thermal conductivity, ultra-wide bandgap, and high breakdown strength.

In semiconductors, the bandgap is known to be a crucial property, and a broad bandgap facilitates the operation of high-frequency or high-power devices.

That’s why diamond can be considered as ‘Mount Everest’ of electronic materials, possessing all these excellent properties.

Lu Yang, Associate Professor, Department of Mechanical Engineering, City University of Hong Kong

But diamonds have a tight crystal structure and a large bandgap that make it hard to “dope”—a standard method used to modulate the electronic properties of semiconductors at the time of production”. This limits the industrial applications of diamonds in optoelectronic and electronic devices.

“Strain engineering” offers a viable option in which an extremely large lattice strain is applied to modify the electronic band structure and the related functional characteristics. But due to its ultra-high hardness, this approach was deemed to be “impossible” for diamonds.

Then, back in 2018, Dr Lu and his colleagues unexpectedly found that nanoscale diamonds can be elastically twisted using an unexpected large local strain. This finding indicates that elastic strain engineering could be used to change the physical characteristics of diamonds. Based on this finding, the new study demonstrated how this phenomenon can be used for designing functional diamond devices.

Uniform Tensile Straining Across the Sample

Initially, the team microfabricated samples of a single-crystalline diamond from a solid diamond single crystals. These samples had a bridge-like shape—measuring around 1 µm in length and 300 nm in width, with two wider ends meant for gripping.

Then, under an electron microscope, the diamond bridges were uniaxially expanded in a well-controlled way. Under controllable and continuous loading-unloading cycles of quantitative tensile tests, the diamond bridges exhibited a large and highly uniform elastic deformation of around 7.5% strain across the entire gauge section of the sample, instead of deforming at a localized region in bending. After unloading, the diamond bridges regained their original shape.

The team also employed the American Society for Testing and Materials (ASTM) standard to further improve the sample geometry and achieved the highest uniform tensile strain of around 9.7%.

This value even exceeded the highest local value in the study performed in 2018 and was almost the same as the hypothetical elastic limit of diamonds. More significantly, to show the concept of strained diamond devices, the researchers also achieved elastic straining of microfabricated diamond arrays.

Tuning the Bandgap by Elastic Strains

The researchers subsequently carried out density functional theory (DFT) calculations to determine the effect of elastic straining between 0% and 12% on the electronic properties of the diamond.

The simulation outcomes indicated that the diamond bandgap usually decreased with the increase in tensile strain, while the largest bandgap reduction rate decreased from around 5 eV to 3 eV at about 9% strain along a certain crystalline orientation. The researchers also used a pre-strained diamond sample on which they carried out an electron energy-loss spectroscopy analysis and then validated the decreasing trend of this bandgap.

The team’s calculation outcomes also demonstrated that, fascinatingly, the bandgap can potentially convert from indirect to direct with the tensile strains being greater than 9% along an additional crystalline orientation.

In the case of semiconductors, a direct bandgap implies that a single electron can instantly produce a photon, enabling several optoelectronic applications with greater efficiency.

Such findings represent an early step in realizing deep elastic strain engineering of microfabricated diamonds. Through this nanomechanical method, the researchers effectively showed that it is possible to change the band structure of a diamond, and more significantly, such changes can be reversible and continuous, enabling a range of applications, such as strain-engineered transistors, micro/nanoelectromechanical systems (MEMS/NEMS), and innovative quantum and optoelectronic technologies.

I believe a new era for diamond is ahead of us.

Lu Yang, Associate Professor, Department of Mechanical Engineering, City University of Hong Kong

Dr Lu, Dr Alice Hu, who is also from the Department of Mechanical Engineering at CityU, Professor Li Ju from MIT, and Professor Zhu Jiaqi from HIT are the corresponding authors of the study.

The co-first authors of the study are Dang Chaoqun, Ph.D. graduate, and Dr Chou Jyh-Pin, a former postdoctoral fellow from the Department of Mechanical Engineering at CityU, Dr Dai Bing from HIT, and Chou Chang-Ti from National Chiao Tung University.

The research team also included Dr Fan Rong and Lin Weitong from CityU. Other collaborating scientists are from the Lawrence Berkeley National Laboratory, University of California, Berkeley, and Southern University of Science and Technology.

The Hong Kong Research Grants Council and the National Natural Science Foundation of China have funded the study performed at CityU.

Journal Reference:

Dang, C., et al. (2021) Achieving large uniform tensile elasticity in microfabricated diamond. Science. doi.org/10.1126/science.abc4174.