Diamond-based Semiconductors Take a Step Foward

Simple doping method for single-crystal diamond semiconductors shows promise

Image: Jung-Hun Seo
Optical image of a diode array on a natural single crystalline diamond plate. (The image looks blurred due to light scattering by the array of small pads on top of the diamond plate.) Inset shows the deposited anode metal on top of a heavy doped silicon nanomembrane that is bonded to natural single crystalline diamond.
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Is the potential of diamond as a semiconductor now being realized? That’s certainly the case if we believe the praise being heaped upon the precious stone by companies such as AKHAN Semiconductor. AKHAN has pronounced that we are now in the “Diamond Age” of semiconductors.

Why? The superior thermal properties of diamonds, compared with those of silicon, are attracting increased attention. Unfortunately, doping diamond-based devices has proven exceptionally difficult, especially when it comes to producing n-type semiconductors. 

Now, in joint research between the University of Wisconsin-Madison and the University of Texas at Arlington, scientists have developed a new method for doping single crystals of diamond; it could help diamond realize its full potential as a semiconductor.

Today’s diamond-doping techniques call for coating the crystal with boron and heating it to 1450 degrees Celsius. The problem with that process is the need to remove the boron coating at the end.

Furthermore, while that method works effectively for diamonds that consist of multiple crystals stuck together, poly-diamond structures have irregularities where the crystal structures meet that make them less attractive as a semiconductor material than groups of single-crystal diamonds. Heretofore, if you wanted to dope single-crystal diamonds, you had to inject boron atoms into the crystals as they were grown. Unfortunately, that approach requires powerful microwaves that degrade the diamond crystal.

But in research described in the Journal of Applied Physics, the joint research team found a way to dope single-crystal diamond with boron but at a comparatively low temperature.

As it turns out, the featured ingredient in their secret sauce was silicon. The researchers found that if they bonded a single-crystal diamond with a piece of silicon that had been doped with boron, then heated it to 800 °C, the boron atoms would migrate from the silicon and attach themselves to the diamond.

This migration occurs because carbon atoms from the diamond shift to fill defects such as atom vacancies in the lattice structure. When they move, they leave vacancies in the diamond lattice structure that the boron atoms ultimately fill.

Perhaps the key feature of this method is that it allows for selective doping that provides a higher level of control when making devices. Creating a diamond with a particular set of properties is achieved by bonding the silicon to a specific spot on the diamond crystal.

While this method addressed p-type doping, which gives the diamonds positive charge carriers, it does not address the n-type doping that remains a more troublesome process to achieve.

If the researchers are going to make devices such as transistors, they will need to overcome that hurdle. But in the meantime, they are focused on developing a simple device using p-type single-crystal diamond semiconductors.

“We feel like we found a very easy, inexpensive, and effective way to do it,” said Zhengqiang (Jack) Ma, one of the authors of the research, in a press release. Ma added that achieving p-type doping is an important step, and might inspire others to find solutions for creating n-type single-crystal diamonds.

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IEEE Spectrum’s nanotechnology blog, featuring news and analysis about the development, applications, and future of science and technology at the nanoscale.