GaAs-Si Tandem Solar Cells Formed At T\textless 500K by Nano-Bonding via Surface Energy Engineering

The theoretical Photo-Voltaic Efficiency (PVE) of GaAs/Si tandem solar cells (SC) is 44{\%}, but practically is only 33{\%} because current technology uses hetero-epitaxy or Direct Wafer Bonding at T \textgreater 700 K. Hence Nano-Bonding (NB) at T \textless 500K is used here instead to planarize GaAs and Si at the nano-, micro- and macro-scale via far-from-equilibrium surface phases via Surface Energy Engineering (SEE), then `nano-contacts' GaAs to Si via light mechanical compression 0- 70 kPa at T \textless 500 K to nano-bond. SEE reverses initial hydro-affinity (HA) of GaAs from hydrophobic to hydrophilic, and vice-versa for Si. HA and Surface Energy SE are measured by Three Liquid Contact Angle Analysis. High Resolution Ion Beam Analysis (HR-IBA to yield absolute Oxygen coverage. X-Ray Photoelectron Spectroscopy yields surface stoichiometry. The SE of GaAs increases via SEE from 30.4\textpm 1 mJ/m$^{2\, }$to 60\textpm 2 mJ/m$^{2}$, while IBA detects a 50{\%} decrease in Oxygen coverage from 7 ML to 3.5 ML. XPS shows that SEE decreases O-rich As$_{2}$O$_{5}$ on GaAs(100) decreases by at least 13.5{\%}, while As$_{2}$O$_{3}$ increases by at least 13.5{\%}. Next, nano-bonded GaAs/Si interfaces are then imaged via Surface Acoustic wave Microscopy (SAM) and Transmission Electron Microscopy. SAM shows than 98\textpm 1{\%} of GaAs nano- bonds successfully to Si at T \textless 500K