Abstract
In this thesis, I have studied the optical properties of hexagonal boron nitride (hBN) monolayers and polytypes using hyperspectral microscopy in the deep-ultraviolet (deep-UV). In the last decade, hBN has emerged as a pivotal material for efficient deep-UV optoelectronics exhibiting a strong emission at a typical wavelength of 210nm. Interestingly, hBN is a graphite-like layered material that consists of weakly stacked honeycomb atomic layers of tightly bound boron and nitrogen atoms, the so-called hBN monolayers. As many other 2D materials, the layered structure of hBN offers great flexibility in the atomic configuration allowing the layers separation down to a single hBN monolayer or the existence of various stackings leading to the formation of hBN polytypes. The degrees of freedom permitted by the layered structure of hBN, namely the reduction of dimensionality and the layers stacking, may influence the hBN properties through the alteration of the interlayer coupling, which disappears for a single hBN monolayer. This results in a diversity of optoelectronic properties that could be tailored by controlling the thickness and stacking arrangements of hBN layers for specific applications. However, hBN monolayers and polytypes are typically found with lateral sizes of 10-100µm in hBN flakes. Thus, studying their optical properties requires spatially-resolved luminescence microscopy in the deep UV which represents a real technical challenge. Consequently, the optical properties of monolayer hBN are still poorly known while not yet been reported for hBN polytypes.During my thesis, I have developed an original hyperspectral photoluminescence (PL) microscope operating in deep-UV at cryogenic temperatures (4K) to study the optical properties of hBN monolayers and polytypes. The microscope reaches a spatial resolution of 250nm in the deep-UV allowing me to study sub-micrometric systems as hBN monolayers or polytypes. On the one hand, this microscope has allowed me to study the optical properties of exfoliated and epitaxial hBN monolayers at the microscopic scale in the deep-UV. My results unraveled the coexistence of PL and second-order Raman signals at 6.1eV induced by the quasi-resonant excitation in monolayer hBN. This leads me to a complete and universal description of the emission spectrum at 6.1eV, providing better insight into the long-debated nature of the optical bandgap of monolayer hBN. On the other hand, I used the microscope to isolate for the first time a hBN polytype corresponding to the so-called bernal boron nitride (AB-hBN) which allowed me to perform an in-depth study of its optical properties. I thus unraveled the impact of polytypes on the optical properties of hBN, leading to the emergence of exotic properties in AB-hBN such as the simultaneous direct and indirect bandgap emission. Notably, this allowed me to demonstrate experimentally for the first time the impact of the light-matter coupling on the phonon-dephasing at the origin of the thermal broadening. Furthermore, by studying the impact of stacking on the emission spectrum corresponding to the so-called "4.1eV defect" in hBN, I could conclude on the potential microscopic nature of this defect. My results provide a better understanding about the effect of structural modifications of the layered structure on the optical properties of hBN.