Seminarium Fizyki Ciała Stałego

The discoveries of graphene, topological insulators and other novel materials have opened a whole new area, namely two-dimensional electronics. This new frontier has spurred the rediscovery of boron nitride (BN), a member of the family of III-N compounds that has much promise for modern electronics. In this talk the hexagonal BN (hBN) will be introduced. In particular the use of high pressure for its synthesis and crystal growth as well for identification of selected defect centers will be highlighted.Like carbon, BN forms crystalline structures with different atomic configurations at different pressure-temperature conditions. At relatively low pressures, BN is thermodynamically stable in its hexagonal graphite-like structure, hBN. This form features monoatomic layers strongly bound in a honeycomb pattern, like graphene, and inter-bonded with weak van der Waals forces.
The hBN, is well established, having been widely deployed for many years. It has found use as a crucible material, in the form of sintered ceramics that benefit from its high thermal and chemical stability, and as a solid lubricant, thanks to its layered structure. Now new opportunities beckon, with hBN under investigation as a potential platform for 2D electronics and quantum technology.
While graphite and hBN have similarities in their crystalline form, they differ in electrical conductivity, with hBN behaving as a strongly insulating crystal. This strongly insulating characteristic is to be expected, given that hBN has a wide bandgap of 6 eV. In monolayer hBN this gap is direct, but it shifts to indirect as the number of layers increases. Surprisingly, regardless of whether this material has a direct or indirect band gap, the UV optical emission produced by hBN is extremely efficient.
Employed in its two-dimensional form as very thin structures, hBN can be deployed as an excellent insulator or tunnelling dielectric barrier in devices based on graphene and other 2D heterostructures.Important opportunities arise in hBN due to defects within the material. These imperfections enable very interesting physical systems that provide single-photon emitters, or centres hosting quantum spin states with a long coherence time. The results of using high pressure for identification of important ultraviolet color centers in hBN will be presented.
For both fundamental studies and the pursuit of new applications, it is critical to produce high-quality crystalline hBN, as this holds the key to uncovering its fascinating properties, as well as evaluating the predictability of theoretical models. Unfortunately, it is far from easy to grow crystalline hBN. Due to a melting temperature that exceeds 3000 °C, hBN, unlike silicon and GaAs, cannot be grown from its stoichiometric melt. Due to this limitation, two leading methods have emerged for the crystallization of hBN. One of them, involves very high pressures of 5 GPa or more, and metallic solutions, containing the likes of Ba, Mg and Ni. In contrast to GaN and InN, hBN does not require a high nitrogen pressure to suppress its decomposition at high temperatures. So the alternative –is based on growth on the surfaces of molten transition metal alloys containing boron, and involves a flow of nitrogen gas at atmospheric pressure. For both approaches, crystals are limited to the order of 1 mm in size. Another concern is the very small thickness of the hBN crystals grown on metal surfaces under atmospheric N2 pressure. To avoid this, we have focused recently, on the high N2 pressure approach to the crystal growth of hBN. Our expectation is that through the controlled increase of nitrogen solubility in molten metals, we will enhance the growth of hBN in the c-direction in the case of surface crystallization, and enable the production of thicker crystals. Our ultimate goal is to establish a new approach, where in contrast to crystallization of hBN on metal surfaces, crystals will be grown in the solution volume. That’s mirroring the technique that we honed for the production of high-quality single crystals of GaN with dimensions of more than 1cm. New emerging high pressure approaches: zone melting and ammonothermal will be also introduced.