Kompleksowe badania centrów rekombinacji promienistej w monokryształach heksagonalnego azotku boru z zastosowaniem spektroskopii wysokociśnieniowej i czasowo rozdzielonej wspartej analizą teoretyczną

The project “Comprehensive study of radiative recombination centers in hexagonal boron nitride single crystals using high-pressure and time-resolved spectroscopy supported by theoretical analysis” is devoted to an innovative and unique approach to resolving the mystery of radiative recombination centers whose emission is observed in photoluminescence (PL) of hexagonal boron nitride (h-BN) crystals. H-BN is one of the key two-dimensional (2D) materials, which are intensively studied both due to the abundance of novel physical phenomena and from the point of view of numerous advanced applications. It is composed of boron and nitrogen ions forming flat sp2 covalent bonds with a honeycomb structure inside the layers and weak van der Waals bonds between the layers, creating a three-dimensional crystal with a wide indirect bandgap of ~6 eV, unlike semi-metallic graphite or graphene with a similar structure. This large bandgap accommodates numerous optically active electronic states of structural defects and impurities that are abundant in currently grown crystals or epitaxial layers. Many of them act as bright single photon sources with various photon energies from 5.5 eV down to ~1.5 eV or as efficient ultraviolet emitters with extreme thermal stability. However, to fully control and develop on-demand unique applications of h-BN, the nature of radiative recombination centers should be identified. Despite intensive experimental and theoretical research, this is still one of the unresolved issues and none of the currently proposed defects assignments are conclusive. The main goal of the project is to perform a detailed study of optically active defects in h-BN. To achieve this goal, it is proposed to use a new comprehensive approach including: 1. High Nitrogen Pressure Solution growth of h-BN single crystals, first nominally pure, then with a controlled content of selected impurities (C, Mg, Ge, O), with different growth parameters (pressure, temperature, composition of the growth solution) for the needs of specific research experiments. 2. Microstructural-defect characterization of the obtained crystals using X-ray diffraction (XRD) for detailed analysis of lattice constants, thicknesses and composition of the layers, and strain distribution in the crystals, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) for detailed analysis of layer thicknesses and homogeneity, energy dispersive X-ray spectrometry (EDX) and secondary ion mass spectrometry (SIMS) for detailed defect characterization of the grown crystals. 3. Optical properties characterization that will include measurements of pressure and temperature dependences of PL, PL decay kinetics, PLE spectra, cathodoluminescence, and Raman spectra. In particular, we propose using the diamond/sapphire anvil cell high-pressure spectroscopy to get a deeper insight into the luminescent properties of point defects in h-BN in the whole visible-deep UV spectral range The high-pressure spectroscopy is a very efficient experimental tool for the identification of defect types in materials which allows obtaining new and valuable data that are difficult or not possible to get by other methods. By changing the pressure, we can change in a controlled and smooth way both the energetic structure of the band states of the host crystal and the energy levels of defects. Therefore this technique is very useful to distinguish band-to-band radiative transitions or transitions involving shallow-level defects from those, in which the deep defect states participate. Additional information on the nature of observed emission will be extracted from temperature-dependent PL or time-resolved PL measurements. 4. Theoretical analysis: ab initio simulations of h-BN crystals with different defect structures, based on the density functional method; calculations will be carried out using several models with varying detail levels to receive a precise description and complete understanding of the observed effects. The final result of the project will be the new knowledge on growing high-quality h-BN crystals, the energy structure of selected impurities in the h-BN host, and their impact on optical properties of the investigated material leading to a better understanding and improved control of occurring emission properties, which cannot be sufficiently controlled at present. These results will allow the design of h-BN crystals with given emission properties.