Christian Cornejo2 1 Michelle Bertram2 1 Timoteo Diaz2 1 Nicole Herbots1 3 Saaketh Narayan4 Jack Day4 Ajit Dhamdhere1 Robert Culbertson3 Rafiqul Islam1 2

2, Arizona State University, Tempe, Arizona, United States
1, Cactus Materials Inc., Tempe, Arizona, United States
3, Arizona State University, Tempe, Arizona, United States
4, BASIS Scottsdale, Scottsdale, Arizona, United States

Bonding heterogeneous semiconductors into monolithic devices increase performance in solar cells, sensors, and opto-electronic devices. Bonding occurs between two surfaces if electrons transfer, forming molecular cross-bonds, called Nano-Bonding™[1]. The possibility of electronic transfer is probed through the surface energy of each material via interactions with electron donors and acceptors. Total surface energy (γT) has been modeled by Van Oss and combines three different interactions: interaction with molecular species or Lifschitz-Van der Waals interactions (γLW), interaction with electron donors (γ+), and with electron acceptors (γ-).

Using Three Liquid Contact Angle Analysis (3LCAA), the three surface energy components are measured with three liquids: 18 MΩ DI Water (polar), Glycerin (polar), and Alpha-Bromonaphthalene (apolar). γT is then computed from γLW, γ+, and γ-, using three liquid contact angles to yield three equations with three unknowns.

On both hydrophobic and hydrophilic surfaces, γT scales linearly with γLW. However, the γT of hydrophobic surfaces is almost entirely due to molecular interactions (γLW), with little contribution from γ+ and γ-.This is consistent for smooth hydrophobic surfaces since few defects and impurities interact with electron donors and acceptors. On the other hand, on very hydrophilic surfaces, the contribution of γ- is very significant and can be as large as γLW, while on hydrophobic samples, γLW is always much larger than γ-.The pairing of a relatively high γ- with a relatively high γ+ is how electron exchange and bonding is enhanced while keeping the surface neutral. Surface modifications that adjust the energy components of each material allow for the design interactions that favor bonding [1].

3LCAA, performed on native oxides of GaAs(100) p-type Te doped, yields consistent surface energies of 37.7 ± 1.7 mJ/m2 corresponding to strongly hydrophobic surfaces. After surface preparation for Nano-Bonding™, GaAs can make strongly hydrophilic reproducible surface energies of 65.5 ± 1.4 mJ/m2. Ion beam analysis (IBA) combines <111> axial channeling with MeV nuclear resonance to show hydrophobic GaAs(100) native oxides have a mixture of about 2/3 Gallium Oxide and 1/3 Arsenic Oxide. After surface preparation, IBA detects the resulting hydrophilic GaAs(100) surface as terminated by an As top layer with a Ga layer underneath.Oxygen on the surface decreases by a factor of two and appears to correspond to an -OH termination. This new surface composition changes GaAs(100) from strongly hydrophobic to strongly hydrophilic. Combined with modification of Si(100) surface energies, such GaAs surfaces can be successfully Nano-Bonding™ to Si(100) in air at temperatures below 200oC [2].

[1] Herbots et al. US Patent 9,018,077 (2015); 9,589,801 (2017).
[2] Herbots et al, US Patent Pending (2017)