Diamond is nature’s hardest material. But on many expectations, it also has great potential as excellent electronic equipment. A joint research team led by Hong Kong City University (CityU) demonstrated for the first time the large and uniform elastic tensile stress of microfabricated diamond arrays using a nanomechanical approach. Their discoveries showed the potential of strained diamonds as prime candidates for advanced functional devices in microelectronics, photonics, and quantum information technology.
The research was co-led by Dr Lu Yang, associate professor in the Department of Mechanical Engineering (MNE) at CityU and researchers from the Massachusetts Institute of Technology (MIT) and Harbin Institute of Technology (HIT). Their results were recently published in the scientific journal Science, titled “Achieving High Uniform Tensile Elasticity in Microfabricated Diamond.”
“This is the first time that the extremely high and uniform elasticity of diamond has been shown by tensile experiments. Our results demonstrate the possibility of developing electronic devices through “deep elastic deformation engineering” of microfabricated diamond structures, ”said Dr. Lu.
Diamond: “Mount Everest” of electronic materials
Well known for its hardness, industrial applications of diamonds are typically 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 electric charge carriers, high resistance to breakdown and ultra-wide bandgap. Band gap is a key property in semiconductors, and wide band gap allows the operation of high power or high frequency devices. “This is why diamond can be considered as the ‘Mount Everest’ of electronic materials, possessing all of these excellent properties,” said Dr Lu.
However, the wide band gap and tight crystal structure of diamond makes it difficult to “dop”, a common way to modulate the electronic properties of semiconductors during production, thus hampering industrial application of diamond in electronic and optoelectronic devices. . A potential alternative is through “strain engineering”, ie applying a very large lattice strain, to change the electronic band structure and associated functional properties. But it was considered “impossible” for diamond due to its extremely high hardness.
Then, in 2018, Dr Lu and colleagues discovered that, surprisingly, nano diamond can be elastically bent with unexpectedly strong local stress. This finding suggests that changing the physical properties of diamond by engineering elastic deformations may be possible. On this basis, the latest study showed how this phenomenon can be used to develop functional diamond devices.
Uniform tensile stress across the sample
The team first microfabricated single crystal diamond samples from a solid single crystal diamond. The samples were in the shape of a bridge – about a micrometer long and 300 nanometers wide, with the two wider ends for gripping (see image: tensile tension of the diamond bridges). The diamond bridges were then uniaxially stretched in a well-controlled manner in an electron microscope. Under continuous and controllable load-unload cycles of quantitative tensile testing, the diamond bridges demonstrated a large, very uniform elastic strain of approximately 7.5% strain across the gauge section of the specimen, rather than to deform to a localized zone in bending. And they returned to their original shape after unloading.
By further optimizing the sample geometry using the American Society for Testing and Materials (ASTM) standard, they achieved a maximum uniform tensile strain of up to 9.7%, which even exceeded the maximum local value of the 2018 study, and was close to the theoretical value. elastic limit of diamond. More importantly, to demonstrate the concept of a strained diamond device, the team also performed elastic deformation of microfabricated diamond arrays.
Adjustment of the band gap by elastic strains
The team then performed density functional theory (DFT) calculations to estimate the impact of 0 to 12% elastic strain on the electronic properties of diamond. Simulation results indicated that the diamond bandgap generally decreased as tensile stress increased, with the highest bandgap reduction rate falling from about 5 eV to 3 eV at a strain of about 9. % along a specific crystal orientation. The team performed an electron energy loss spectroscopy analysis on a pre-stressed diamond sample and verified this downward trend in the bandgap.
The results of their calculations also showed that, interestingly, the bandgap could go from indirect to direct with tensile strains greater than 9% along another crystal orientation. The direct bandgap in the semiconductor means that an electron can directly emit a photon, allowing many optoelectronic applications with higher efficiency.
These discoveries are a first step in achieving deep elastic deformation engineering of microfabricated diamonds. Through a nanomechanical approach, the team demonstrated that the structure of the diamond band can be altered and, more importantly, these changes can be continuous and reversible, allowing different applications, of micro / nanoelectromechanical systems (MEMS / NEMS), to distortion transistors, to the novel. optoelectronic and quantum technologies. “I believe that a new era for diamonds is upon us,” said Dr Lu.
Research at CityU was funded by the Hong Kong Research Grants Council and the National Natural Science Foundation of China.