New method raises heat on crystal defects

A team of researchers from Osaka University in Japan, the Institute for High Pressure Physics and the Institute for Nuclear Research of the Russian Academy of Sciences in Russia and TU Dresden in Germany have discovered an effective method for removing lattice defects from crystals. They report this method in a paper in the Journal of Physics: Materials.

Boron is a semiconductor with a variety of crystal structures, but all of them have large amounts of lattice defects that spoil the state of crystalline order. In this study, the team were able to create an ordered phase of boron by first adding hydrogen (hydrogenation) at high temperatures and then conducting dehydrogenation by low-temperature annealing. This new method comes out of theoretical work by the research groups in Japan and Germany of a phenomenon that the Russian groups discovered in experiments.

Lattice defects are present in all crystalline materials and influence many of their electrical properties. The proper use of lattice defects is useful for the electronic applications of semiconductors, because their electrical conductivity can be enhanced by doping to produce n (negative)-type or p (positive)-type semiconductors.

This control of lattice defects is called ‘valence electron control’ and is achieved by placing dopants (impurities) into the atom sites. However, impurity atoms that occupy the spaces between the atom sites, known as interstitial sites, are not useful for controlling valence electrons.

In a boron crystal, not only are many atoms randomly arranged in the interstitial sites, but its crystal structure is too hard for the interstitial atoms to reach more desirable sites. To render boron crystals as excellent semiconductor materials, it is necessary to rearrange the randomly distributed boron atoms into an ordered structure.

To do this, the researchers created α-tetragonal (α-T) boron crystal at a high temperature and pressure, and added a large amount of hydrogen as dopant. The obtained samples had a lot of defects, with the boron atoms and hydrogen atoms in the interstitial sites randomly arranged. But the researchers then found that by recovering the samples to ambient conditions and annealing them at moderate temperatures, they could simultaneously remove the hydrogen atoms and order the interstitial boron atoms. This indicates that the random arrangement of interstitial atoms becomes an ordered structure, and represents the first time that an ordered boron crystal with a large unit cell (containing more than 50 boron atoms) was obtained.

Generally, a crystal adopts a more ordered structure at low temperatures. But crystallization usually occurs at high temperatures, which induce many defects, and these defects are then solidified at low temperatures. In this case, by incorporating volatile hydrogen atoms into the boron crystal and then releasing them by annealing, the researchers were able to induce the boron atoms to migrate and form an ordered arrangement. This transition is a kind of ‘cooperative phenomenon’ between two different changes: diffusion of hydrogen and the ordering of the host atoms.

"In addition to boron, our method of removing defects can also be applied to carbon-based materials, such as fullerene, which are very hard and have a high melting point," says Koun Shirai, an associate professor at Osaka University. "Because removing defects from these hard materials is difficult, our recipe will be an efficient method of removing defects for other semiconductor materials as well."

This story is adapted from material from Osaka University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.