Optimization of GaAs to Si Cross-Bonding with Minimal Thermal Activation and Compression in Ambient Air through NanoBonding^TM

Recently, GaAs/Si tandem solar cells have proved to be the most efficient in NREL reviews, but their efficiency is lowered by residual oxides and high processing temperatures used (>400°C), which cause defects at the interface. Nano-Bonding (NB) combined Surface Energy Engineering can reduce T≤200°C. SEE optimizes synergistically GaAs and Si by rendering one surface highly hydrophilic, acting as an electron donor, while the second surface is made more hydrophobic. By analyzing 15 GaAs/Si wafer pairs, Si(100) is found to be more easily engineered while GaAs(100) is simply rendered highly reactive and highly hydrophilic via dilute oxide etching. Surface energies measured via Three Liquid Contact Angle Analysis correlate with O coverage on GaAs measured via High Resolution Ion Beam Analysis, and with oxidation states measured via X-Ray Photoelectron Spectroscopy (XPS). SEE reproducibly increases GaAs surface energies by a factor of 2, with a 50% decrease in O coverage from 7 ML to 3.5 ML. XPS shows that O-rich As2O5 decreases while As2O3 increases. Surface Acoustic wave Microscopy show that 98 % of GaAs can nano-bond successfully to Si at low temperatures with light compression, although nano-bonding can occur beyond the area compressed.