Semiconductor grafting is pivotal for the heterogeneous integration of materials in electronics and optoelectronics, offering isolation of crystal growth from the substrate and enabling combinations of semiconductor heterojunctions with large lattice mismatches. The ultrathin dielectric layer at the heterojunction interface plays a pivotal role in addressing the interface dangling bonds and surface states, essential for effective semiconductor grafting. This study evaluates the effects of ultrathin SiN_x, Al₂O₃, and SiO₂ passivation layers, deposited via atomic layer deposition, on GaN substrates. Photoluminescence (PL) and time-resolved photoluminescence measurements reveal that Al₂O₃ passivation significantly improves carrier lifetimes, reduces surface recombination velocity, and enhances radiative efficiency. The normalized photoluminescence excitation spectra and calculated density of states confirm that Al₂O₃ passivation enhances band edge sharpness, reduces defect-related states, and suppresses undesirable electronic transitions. In addition, the PL peak and full width at half maximum were observed for all passivation materials, demonstrating peak broadening after the deposition. This study highlights the potential of ultrathin surface passivation layers in enhancing GaN-based electronic and optoelectronic device performance, offering insights into the passivation mechanisms and their impact on charge carrier dynamics, which is crucial for developing future high-performance heterojunction devices