Evolution of Au nanoparticles in c-plane GaN under the heavy ion implantation and their optical properties

Abstract

The modification of GaN with Au nanoparticles is a promising way for manipulating optical properties in optoelectronics and enhancing the sensitivity of GaN-based substrates used in Surface Enhanced Raman Spectroscopy. Ion implantation is an attractive method for the preparation of high-purity metal nanoparticles on the surface or within the bulk of solids, although this process in crystals is not yet fully understood. Here we study the specific stages of Au nanoparticle formation in c-plane GaN crystals implanted with 1.85 MeV Au ions to the fluence range of 1.5×1016 - 7×1016 cm−2. The implanted samples were annealed at 800 °C in an ammonia atmosphere for 20 minutes to support the nucleation of Au nanoparticles and prevent surface decomposition. The structural and optical properties of the samples were investigated by a combination of Rutherford backscattering spectroscopy in channeling mode (RBS-C), Transmission Electron Microscopy (TEM), Raman spectroscopy, photoluminescence (PL) and diffuse-reflectance spectroscopy (DRS). The two damaged regions, namely at the depth and at the surface, were identified. The depth region exhibits saturation of damage, unlike the surface region reaching an amorphous state for the highest implantation fluence. RBS-C revealed multimodal Au depth profiles for higher ion fluences, which were subsequently redistributed after thermal annealing due to the formation of Au nanoparticles, as confirmed by TEM. Au nanoparticles with fcc structure have been successfully synthesized with the Au-ion implantation fluence of 5×1016 cm−2 with the sizes mostly of 4 – 20 nm resulting in increased optical scattering and absorption within the spectral range of 500 – 700 nm associated with surface plasmon resonance (SPR) and the emergence of blue (2.8 eV) and red (2 eV) photoluminescence bands linked to the combined effect of SPR and implantation-induced defects.