Asymmetric Activation-Driven Quasi-Direct Bonding of InP/LiNbO₃ Heterointerfaces for Scalable Quantum Photonics

Lithium niobate (LiNbO₃) has emerged as a powerful phase-stable and efficient platform for precise light manipulation but lacks deterministic single-photon sources. Hybrid integration with InP via plasma-activated bonding offers a viable solution to achieve high-precision aligned (sub-50 nm) on-chip quantum emission. Although plasma enhances surface metastability and lowers the covalent bond formation barrier, its limited subsurface modulation (several to tens of nanometers) restricts interfacial polymerization, hindering evanescent coupling heterointerface formation. An asymmetric-activation driven quasi-direct bonding strategy to construct InP/LiNbO₃ heterointerfaces by the targeted introduction of an ultrathin (10 nm) Al₂O₃ layer on the InP surface is proposed. Combined with the asymmetric plasma activation, the flawless interface with a bonding strength of 8.3 MPa is achieved at 80 °C, demonstrating the effectiveness of Al₂O₃ to balance the InP surface metastability and subsurface relaxation. Moreover, the void-free interface further demonstrates the feasibility of Al₂O₃ as the outgassing micro-channel to promote the diffusion of gas by-products. In addition, a hybrid quantum photonic device is designed to simulate the influence of Al₂O₃ on the evanescent coupling efficiency. This work emphasizes the critical role of Al₂O₃ in synergistic control of surface metastability and subsurface modulation, advancing scalable heterogeneous integration for quantum photonics.