Thermal transport and structural improvements due to annealing of wafer bonded β-Ga₂O₃|4H-SiC

The impact of postbond annealing on the structural and thermal characteristics of 130 nm thick exfoliated (201) β-Ga₂O₃ (via H⁺ ion implantation) wafer bonded to (0001) 4H-SiC was studied. Thirty nanometer amorphous-Al₂O₃ was grown on the β-Ga₂O₃ substrates prior to bonding as an interlayer between β-Ga₂O₃ and 4H-SiC. The surface activated bonding technique was utilized for bonding, which induces a thin nanometer amorphous interfacial region at the bonded interface (Al₂O₃|4H-SiC). We demonstrate annealing the bonded structure at 800 °C up to 1 h is beneficial: (1) the removal of residual strain in the exfoliated β-Ga₂O₃ layer that was due to the exfoliation implant, (2) reduction of lattice mosaicity in the β-Ga₂O₃ layer, (3) nearly complete recrystallization of the amorphous bonded interfacial region, and (4) partial recrystallization of the initially amorphous-Al₂O₃ interlayer. The thermal characteristics correspondingly improve with the improvement in structural characteristics. The thermal conductivity of the as-bonded β-Ga₂O₃ layer was 2.9 W/m K, and the thermal boundary conductance of the bonded interface was 66 MW/m² K. After annealing at 800 °C for 1 h, triple-axis x-ray diffraction ω:2θ measurements showed a reduction in strain for the β-Ga₂O₃ layer and the symmetric (201) rocking curve widths. We simultaneously observe a doubling of the β-Ga₂O₃ thermal conductivity to 6.0 W/m K and a 20% increase in the thermal boundary conductance. However, upon further annealing up to 10 h and fully recrystallizing both the Al₂O₃ interlayer and bonded interface, the thermal boundary conductance dropped by ∼30%. This preliminary result suggests that crystalline heterointerfaces may not necessarily be the most optimal interfacial structure for thermal transport.