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Speeding Up Computing By Introducing Defects In Solids

Researchers probe the effects of defects in solids to improve the speed of electronic devices

Many of the solid materials used in electronic devices (such as semiconductor devices usually made out of silicon) develop defects in their crystal structure when the crystals are grown. Scientists have for long used these defects to their advantage, and have even wilfully introduced defects to achieve interesting material properties. For instance, electrical properties of semiconductors are improved by adding a few atoms of a different element to it.

 In a recent study, a team of researchers led by Dr Gopal Dixit at the Indian Institute of Technology Bombay (IIT Bombay) and Dr Angel Rubio at Max Planck Institute for the Structure and Dynamics of Matter, Hamburg, Germany, have analysed how defects in hexagonal boron nitride can help in boosting the performance of electronic devices. The study was published in the journal npj Computational Materials.

Defects in solids arise when the regular arrangement of atoms in the crystal structure are hampered in different ways. For example, removing an atom from the structure or replacing it with an atom of another element can be considered a crystal defect. Physicists study the effect of light on crystal defects by simulating them on computers, but till now, such studies have had some limitations.

Previous studies have looked at defects in only one dimension, along a chain of atoms, but realistic materials cannot be described in a single dimension. On the other hand, using the numerical methods that can tackle complicated defects present in actual materials require so much computing power that it becomes infeasible. “But we were able to strike a practical and realistic balance in our simulations,” says Dr Dixit.

The researchers studied the numerical models of boron nitride having a two-dimensional, hexagonal crystal structure. They focussed on a fundamental property of matter, called spin, which comes in two variants — “up” and “down”. Pristine hexagonal boron nitride has zero net spin since its boron and nitrogen atoms have opposing spins of the same quantity that balance each other. But, when they created two kinds of defects in the hexagonal boron nitride model, either by removing the boron atom or another by removing the nitrogen atom, the material had a net up or down spin.

The researchers then carried out computer simulations to study the effect of shining intense short pulses of laser light on the defected materials. In solids, electrons occupy various energy levels, collectively called energy bands. They found that the electrons of the defected solids oscillate at many times the frequency of the laser light. The output ‘spectrum’, or the distribution of the intensity of light emitted by the solid plotted against its frequency, was different for the pristine solid and its defected versions. Moreover, the output spectrum also varied for the two types of defects since they carried opposite kinds of net spin.

Probing further, the team found that mutual interactions between the electrons also plays a role in how the defected solids behave under the influence of laser. The combined effect resulted in strongly enhanced light output at frequencies very different from the frequency of the incident laser. At frequencies thrice that of the laser frequency, the output intensity is similar for the two defects. However, when the output light frequency is more than triple, the output intensities vary for the two distinct defects. This, the researchers say, is also expected because the energy bands of the electrons are different for the two types of defects.

Instead of removing the boron or the nitrogen atom, the researchers also studied the effect of substituting them with a carbon atom, just like in a process called doping, very commonly used for most semiconductor devices. While replacing the boron atom with a carbon atom was similar to the effect of removing a nitrogen atom completely, they found that replacing the nitrogen atom with a carbon atom had an effect similar to taking out a boron atom completely.

The speed of devices such as microprocessor chips is naturally limited by the speed of transfer of electronic signals, which happens via electrons. “For the first time, we have shown that the spin of the electrons can be used to control their oscillations at frequencies that are much higher than the current frequencies. This can help in improving the clock speed of processors by at least a thousand times, by shining laser pulses on carefully defected materials,” says Dr Dixit.

Given how quickly technology is adapting to fundamental science, the researchers feel that their idea can be used to produce new and faster processors in the next decade. Their research opens up new avenues for probing the effect of more complex defects and more realistic solids.

Article written by

দেবদত্ত পাল। Debdutta Paul

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