Effects of helium ion irradiation and thermal annealing on the optical and structural properties of hexagonal boron nitride

Hexagonal boron nitride (h-BN) has emerged as a promising two-dimensional material for quantum and optoelectronic applications, with its unique ability to host engineered defects enabling single-photon emission and spin manipulation. This study investigates defect formation in h-BN using focused helium ion beam (He⁺ FIB) irradiation and post-annealing treatments. We demonstrate that helium ion irradiation at doses up to 2 × 10⁹ ions/μm² does not induce phase transitions or amorphization. Spectroscopic analyses, including differential reflectance spectroscopy (DRS), photoluminescence (PL), and Raman spectroscopy, reveal substantial defect formation and structural modifications. Notably, the irradiation induces a softening of in-plane and interlayer phonon modes, characterized by frequency redshifts of 10.5 cm⁻¹ and 3.2 cm⁻¹, respectively. While high-temperature thermal annealing mitigates lattice defects and facilitates single-photon emission, the E_2g peak width remains 38% broader and the shear mode peak width is 60% broader compared to pre-annealing conditions in the Raman spectra, indicating residual structural degradation. These findings provide insights into defect engineering mechanisms in h-BN, offering guidance for optimizing processing conditions and advancing quantum and optoelectronic device technologies.