The heterogeneous integration of gallium nitride (GaN) with silicon (Si) substrates presents a transformative pathway for advancing next-generation high-power, high-frequency, and optoelectronic devices, leveraging the superior electronic properties of GaN and the mature, cost-effective manufacturing platform of Si. However, this integration is fundamentally challenged by a significant lattice mismatch and a substantial difference in coefficients of thermal expansion between the two materials. These inherent material incompatibilities induce severe interfacial thermo-mechanical stresses during post-bonding thermal processes, which can lead to wafer bowing, interface delamination, crack propagation, and ultimately, device failure. The work aims to systematically investigate the atomic-scale bonding mechanisms and interfacial microstructure evolution in GaN/Si heterostructures fabricated via SAB technology, thus elucidating how the SAB process mediates the formidable material disparities, correlating the nanoscale interfacial features with the macroscopic bonding quality and mechanical strength, and ultimately establishing a process-structure-property relationship that guides the fabrication of reliable heterogeneously integrated devices.
To achieve these objectives, 4-inch GaN and Si wafers were employed as the starting materials. The key innovation in sample preparation was the surface activation pretreatment. This involved exposing the mirror-polished GaN and Si surfaces to a beam of Ar ions in a vacuum environment. The wafers were then brought into intimate contact for pre-bonding at room temperature. A critical annealing step was performed in a controlled atmosphere furnace, with a particular focus on identifying the optimal temperature, which was determined to be 500 ℃ based on a series of parametric studies. The interfacial integrity and bonding rate were initially evaluated non-destructively through scanning acoustic microscopy (SAM). The core microstructural and chemical analysis was conducted via advanced transmission electron microscopy (TEM). Cross-sectional TEM lamellae were prepared via focused ion beam (FIB) milling. High-resolution TEM (HRTEM) imaging was used to resolve the atomic structure at the bonded interface. Complementary scanning TEM (STEM) mode coupled with energy-dispersive X-ray spectroscopy (EDS) was employed to perform precise line scans and elemental mapping across the interface, providing quantitative data on compositional gradients and interdiffusion profiles.
The SAM analysis showed that non-activated bonding samples exhibited poor bonding quality with large, contiguous un-bonded areas, while SAB-treated samples showed excellent bonding uniformity with only sporadic, point-like defects. Mechanical tests confirmed that the maximum bonding strength for samples annealed at 500 ℃ was 10.5 MPa. The TEM analysis revealed the nanoscale origin of this superior performance. At the heart of the GaN/Si interface, a distinct, continuous, and remarkably uniform amorphous interlayer with an average thickness of approximately 5 nm was consistently observed. This interlayer displayed no long-range crystallographic order, separating the single-crystal lattices of GaN and Si. The STEM-EDS line scans provided compelling evidence of active atomic intermixing. The intensity profiles for Ga and Si did not exhibit an abrupt step-function change at the nominal interface. Instead, they demonstrated a gradual transition over a width of 8 nm, indicating mutual interdiffusion. Furthermore, signals for oxygen (O) and nitrogen (N) were also present and intermingled within this interfacial zone. The oxygen signal peaked within the amorphous interlayer, confirming its role as a key constituent. This combination of an amorphous microstructure and elemental interdiffusion constituted the defining characteristic of the SAB-formed interface.
In conclusion, this work conclusively demonstrates that the formation of a nanoscale amorphous interlayer coupled with interfacial element interdiffusion is the fundamental mechanism responsible for successful, high-strength GaN/Si integration via surface-activated bonding. This work provides a comprehensive and coherent atomic-to-macroscopic understanding of SAB technology for GaN/Si integration. It is highlighted that the deliberate creation of a disordered, graded interfacial nanocomposite is the key to overcoming the challenges of heterogeneous integration, offering vital insights for the design and fabrication of robust wide-bandgap semiconductor devices on silicon platforms.
Key words
GaN/Si heterojunction interface /
interfacial stress /
interfacial amorphous layer /
interdiffusion of elements /
wafer bonding /
surface activation /
hydroxylation
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Funding
National Natural Science Foundation of China (62204050); Natural Science Foundation of the Jiangsu Higher Education Institutions of China (23KJB430036); Wuxi University Research Start-up Fund for Introduced Talents (2026r016)
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