GaAs to Si Direct Wafer Bonding at T ≤ 220 °C in Ambient Air Via Nano-Bonding™ and Surface Energy Engineering (SEE)

When different semiconductors are integrated into hetero-junctions, native oxides generate interfacial defects and cause electronic recombination. Two state-of-the-art integration methods, hetero-epitaxy and Direct Wafer Bonding (DWB), require temperatures, T > 400 °C to reduce native oxides. However, T > 400 °C leads to defects due to lattice and thermal expansion mismatches. In this work, DWB temperatures are lowered via Nano-Bonding™ (NB) at T ≤ 220 °C and P ≤ 60 kPa (9 psi). NB uses Surface Energy Engineering (SEE) at 300 K to modify surface energies (γ^T) to far-from-equilibrium states, so cross-bonding occurs with little thermal activation and compression. SEE modifies γ^T and hydro-affinity (HA) via chemical etching, planarization, and termination that are optimized to yield 2-D Precursor Phases (2D-PP) metastable in ambient air and highly planar at the nano- and micro- scales. Complementary 2D-PPs nano-contact via carrier exchange from donor 2D-PP surfaces to acceptor ones. Here, NB models and SEE are applied to the DWB of GaAs to Si for photovoltaics. SEE modifies (1) the initial γ^T0 and HA⁰ measured via Three Liquid Contact Angle Analysis, (2) the oxygen coverage measured via High Resolution Ion Beam Analysis, and (3) the oxidation states measured via X-Ray Photoelectron Spectroscopy. SEE etches hydrophobic GaAs oxides with γ^T = 33.4 ± 1 mJ/m², and terminates GaAs (100) with H⁺, rendering GaAs hydrophilic with γ^T = 60 ± 2 mJ/m². Similarly, hydrophilic Si native oxides are etched into hydrophobic SiO₄H₂. H⁺- GaAs nano-bonds reproducibly to Si, as measured via Surface Acoustic Wave Microscopy, validating the NB model and SEE design.