A New Low Temperature, Sustainable Process for GaAs on Si Tandem Solar Cells, Solar GaAs, Using Nano-Bonding™ and Surface Energy Engineering*

Solar Cells are a growing source of clean energy. The present work seeks to sustainably increase Photo-Voltaic Efficiency (PVE), by creating Tandem Solar Cells (TSC), via green manufacturing using direct Nano-Bonding™ of GaAs to Si at T < 220°C. 

GaAs-Si TSC absorb across a larger number of wavelengths than Si and GaAs-only SC, with a theoretical maximum PVEθMax of 42.9%, above the PVEθMax for GaAs-only SC (35%) and for Si-only (30.1%) ^[1-5]. Present TSC processes use Hetero-Epitaxy and Direct Wafer Bonding at T > 400°C, leading to defects due to lattice and thermal expansion mismatch, and a realized rPVE of only 30%^[1-4].

To improve rPVE, Nano-Bonding™ uses Surface Energy (γT) Engineering (SEE) to reduce Si and GaAs native oxides, and passivate the resulting surfaces to avoid reoxidation in air. The goal is to create GaAs and Si surfaces ready to bond without using Ultra High Vacuum or T > 220°C and minimize interfacial defects and carrier recombination. SEE modifies γT  to yield passivated planar 2-D  Precursor Phases (2D-PP) which catalyze direct cross -bonding between Si and GaAs when put into ‘nano-contact’ ^[1-5]. SEE achieves surface planarization at the macro-, micro- and nano-scales via wet chemistry to yield 2D-PP. During SEE, hydrophobic GaAs native oxides with γT = 33.4 ± 1 mJ/m2 are etched, then terminated with H +, and rendered hydrophilic with γT = 60 ± 2 mJ/m2.  Hydrophilic Si (100) native oxides are rendered hydrophobic.

Nano-Bonding™ uses Newton rings to monitor nano-contacting during nano-bonding ^[1-3,5].