ABSTRACT
The unique physical, mechanical, chemical, optical, and electronic properties of hexagonal boron nitride (hBN) make it a promising 2D material for electronic, optoelectronic, nanophotonic, and quantum devices. Here, the changes in hBN's properties induced by isotopic purification in both boron and nitrogen are reported. Previous studies on isotopically pure hBN have focused on purifying the boron isotope concentration in hBN from its natural concentration (≈20 at% 10 B, 80 at% 11 B) while using naturally abundant nitrogen (99.6 at% 14 N, 0.4 at% 15 N), that is, almost pure 14 N. In this study, the class of isotopically purified hBN crystals to 15 N is extended. Crystals in the four configurations, namely h10 B14 N, h11 B14 N, h10 B15 N, and h11 B15 N, are grown by the metal flux method using boron and nitrogen single isotope (> 99%) enriched sources, with nickel plus chromium as the solvent. In-depth Raman and photoluminescence spectroscopies demonstrate the high quality of the monoisotopic hBN crystals with vibrational and optical properties of the 15 N-purified crystals at the state-of-the-art of currently available 14 N-purified hBN. The growth of high-quality h10 B14 N, h11 B14 N, h10 B15 N, and h11 B15 N opens exciting perspectives for thermal conductivity control in heat management, as well as for advanced functionalities in quantum technologies.
ABSTRACT
The presence of metastable Bernal stacking boron nitride is verified by combining second harmonic generation (SHG) and photoluminescence (PL) spectroscopy. The scanning confocal cryomicroscope, operating in the deep-ultraviolet range, shows a one-to-one correlation between inversion symmetry breaking probed by SHG and the detection of an intense PL line at â¼6.035 eV, the specific signature of the noncentrosymmetric Bernal stacking. The coherent character of the Bernal phase in boron nitride crystals is demonstrated by two-photon excitation spectroscopy. Direct and indirect excitons are simultaneously detected in the emission spectrum; they are quasi-degenerate, in agreement with theoretical predictions for Bernal boron nitride. The transition from AA' to AB stacking is characterized by an intense emission from stacking faults at the grain boundaries of hexagonal and Bernal boron nitride crystals.
ABSTRACT
Silicon carbide (SiC) sublimation is the most promising option to achieve transfer-free graphene at the wafer-scale. We investigated the initial growth stages from the buffer layer to monolayer graphene on SiC(0001) as a function of annealing temperature at low argon pressure (10 mbar). A buffer layer, fully covering the SiC substrate, forms when the substrate is annealed at 1600 °C. Graphene formation starts from the step edges of the SiC substrate at higher temperature (1700 °C). The spatial homogeneity of the monolayer graphene was observed at 1750 °C, as characterized by Raman spectroscopy and magneto-transport. Raman spectroscopy mapping indicated an AG-graphene/AG-HOPG ratio of around 3.3%, which is very close to the experimental value reported for a graphene monolayer. Transport measurements from room temperature down to 1.7 K indicated slightly p-doped samples (p ≃ 1010 cm-2) and confirmed both continuity and thickness of the monolayer graphene film. Successive growth processes have confirmed the reproducibility and homogeneity of these monolayer films.
