and Surface Energy Engineering*

         The theoretical photovoltaic efficiency (PVCE) for GaAs/Si tandem solar cells is 44%, but high T> 400°C during heteroepitaxy and Direct Wafer reduce PVCE to 33%. 

         Nano-Bonding^TM  (NB) via Surface Energy Engineering (SEE) lowers processing T< 220°C. SEE can reverse hydro-affinity, planarizes surfaces at the nano-, micro – and macro-scale and forms far-from-equilibrium 2D Precursor Phases (2D-PP) to cross-bond GaAs to Si via electron exchange. Three Liquid Contact Angle Analysis (3LCAA) measures HA via 3 liquids with different dipole moments, Ion Beam Analysis (IBA) measures oxygen coverage by combining <111> ion channeling with nuclear resonance, and X-Ray Photoelectron Spectroscopy (XPS) characterizes 2D-PPs. Surface Acoustic Wave Microscopy (SAM) and Transmission Electron Microscopy (TEM)  image nano-bonded GaAs/Si interface. 

         3LCAA shows SEE can double the surface energy of GaAs from 30.4 to 60.2 mJ/m² . IBA finds SEE decreases by 50% oxygen coverage on GaAs from 7.2 ± 0.5 to 3.6 ± 0.2 ML. XPS reveals that As₂O₅ is decreased by 13.5%, while As₂O₅ increases by 13.5%. After SEE, SAM shows that 98±1% of GaAs bonds to Si at T≤ 220°C.