One step closer to turning diamond into the perfect semiconductor

diamond
Jung-Hun Seo

Original news release was issued by the American Institute of Physics.

It is a relatively well known fact that diamonds could make for pretty great semiconductors. They are thermally conductive, foregoing the need for bulky cooling, and they can handle high voltages and power. Electrical currents also flow through diamonds quickly, meaning that the material would make for energy efficient devices. This is good, since the demand for semiconductors keeps increasing, just like the demand for more efficient electronics that deliver and convert power.

But among the biggest challenges to making diamond-based devices is doping, a process in which other elements are integrated into the semiconductor to change its properties. Because of diamond’s rigid crystalline structure, doping is difficult. Currently, you can dope diamond by coating the crystal with boron and heating it to 1450 degrees Celsius. But it’s difficult to remove the boron coating at the end. This method only works on diamonds consisting of multiple crystals stuck together. Because such polydiamonds have irregularities between the crystals, single-crystals would be superior semiconductors.

You can dope single crystals by injecting boron atoms while growing the crystals artificially. The problem is that the process requires powerful microwaves that can degrade the quality of the crystal.

Now, Zhengqiang ‘Jack’ Ma, an electrical and computer engineering professor at the University of Wisconsin-Madison and his colleagues have found a way to dope single-crystal diamonds with boron at relatively low temperatures and without any degradation. The researchers discovered that if you bond a single-crystal diamond with a piece of silicon doped with boron, and heat it to 800 degrees Celsius, which is low compared to the conventional techniques, the boron atoms will migrate from the silicon to the diamond. It turns out that the boron-doped silicon has defects such as vacancies, where an atom is missing in the lattice structure. Carbon atoms from the diamond will fill those vacancies, leaving empty spots for boron atoms.

This technique also allows for selective doping, which means more control when making devices. You can choose where to dope a single-crystal diamond simply by bonding the silicon to that spot.

The new method only works for P-type doping, where the semiconductor is doped with an element that provides positive charge carriers (in this case, the absence of electrons, called holes).

“We feel like we found a very easy, inexpensive, and effective way to do it,” Ma said. The researchers are already working on a simple device using P-type single-crystal diamond semiconductors.

But to make electronic devices like transistors, you need N-type doping that gives the semiconductor negative charge carriers (electrons). And other barriers remain. Diamond is expensive and single crystals are very small. Still, Ma says, achieving P-type doping is an important step, and might inspire others to find solutions for the remaining challenges. Eventually, he said, single-crystal diamond could be useful everywhere — perfect, for instance, for delivering power through the grid.

Michal Dudic

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