Electronic materials “Mount Everest”: Stretching Diamond for next-generation microelectronics

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The diamond is the hardest substance in existence.

Expected by many, it also has great potential as an excellent electronic content.

The massive, uniform tensile elastic strain of microfabricated diamond arrays using the nanomechanical approach has been demonstrated for the first time by a collaborative research team led by the City University of Hong Kong (CityU).

Their findings have highlighted the potential of stretched diamonds in microelectronics, photonics and quantum information technology as prime candidates for advanced functional devices.

Dr. Lu Yang, Associate Professor in the Department of Mechanical Engineering (MNE) at CityU and scientists from the Massachusetts Institute of Technology (MIT) and the Harbin Institute of Technology, led the research (HIT).

Their findings were recently published in “Achieving large uniform tensile elasticity in microfabricated diamond.” in the prestigious journal Science.

“This is the first time we have demonstrated the extremely large uniform elasticity of diamond through tensile testing. Our results show the possibility of developing electronic devices through ‘deep elastic strain engineering’ of microfabricated diamond structures,” Dr. Lu said.

“Mount Everest”Mount Everest
Diamonds, known for their hardness, are most widely used for cutting, drilling or grinding in the industry.

However, due to its ultra-high thermal conductivity, exceptional electrical carrier stability, high dielectric strength and ultra-wide band gap, diamond is also considered a high-performance material for electronics and photonics.

In semiconductors, bandgap is a key property and a large bandgap allows high-power or high-frequency devices to work. “Therefore, diamond can be considered the ‘Mount Everest’ of electronic materials because it has all these excellent properties,” said Dr. Lu.

The wide band gap and narrow crystal structure of diamonds, however, make it difficult to “dope,” a common method of modulating semiconductor electronic properties during manufacturing, which hinders the industrial application of diamonds in electronic and optoelectronic devices.

“strain engineering,” which is the application of very large lattice strains to change the structure of the electronic band and associated functional properties, is a potential alternative.

But because of its extremely high hardness, that was considered “impossible” for diamonds.

Then, in 2018, Dr. Lu and his collaborators discovered that with unexpectedly large local pressure, nanoscale diamonds can shockingly be elastically bent.

This discovery indicates that it could be possible to alter the physical properties of diamonds by elastic strain engineering.

Building on this, the latest research has shown how practical diamond devices can be produced using this phenomenon.

Uniform tensile strain around the sample

Microfabricated single-crystal diamond samples from a solid single crystal diamond were first developed by the team.

The samples had a bridge-like design, about one micrometer long and 300 nanometers wide, with a broader grip on both ends (see Fig. 2).

Then the diamond bridges under an electron microscope were uniaxially extended in a controlled manner. The diamond bridges exhibited a very uniform, broad elastic deformation of about 7.5 percent strain over the whole measured cross-section of the sample under continuous and controllable loading and unloading cycles in the quantitative tensile test, instead of deforming at a localized region during bending.

And after unloading, they recovered to their original form.

They obtained a maximum uniform tensile strain of up to 9.7 percent, which surpassed even the maximum localized value in the 2018 study and was similar to the theoretical elastic limit of diamonds, by further refining the sample geometry using the American Society for Testing and Materials (ASTM) norm. More significantly, to demonstrate the idea of the stretched diamond device, the team also realized the elastic strain of microfabricated diamond arrays.

Tuning by elastic stretching of the band gap.
To estimate the effects of elastic strain from 0 to 12 percent on the diamond’s electronic properties, the team then performed density functional theory (DFT) calculations.

Simulation results showed that with increasing strain, the diamond band gap typically decreases, with the largest band gap decreasing from about 5 eV to 3 eV occurring in a specific crystalline orientation at approximately 9 percent strain.

An electron energy loss was carried out by the team

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