Paper EM+SS-TuA7
Controlling GaAs and Si Oxide Surface Energies

Tuesday, October 31, 2017, 4:20 pm, Room 14

Bonding two different semiconductors into a single integrated device can yield economic, medical, and human benefits by increasing performance. Si and GaAs bonding can increase solar cell efficiency and, if the bonding is hermetic, the lifetime of bonded sensors and optoelectronic circuits is extended by reducing percolation. Bonding occurs when the electronic properties of the two surfaces complement each other, to enhance efficient electron transfer.[1] Complementary surfaces can be identified through measurement of their total surface energy, γ^T, since this property can be modeled by Van Oss theory, to consist of three component interaction energies: molecular dipoles (Lifschitz-Van der Waals), γ^LW, electron donors, γ^-, and electron acceptors, γ⁺. Measurements of the total and individual components of the surface energy of Si and GaAs (100) surfaces has been carried out using contact angle measurements of liquid drops with known surface energies, ranging from polar (18 MW water), apolar (α-bromo-naphthalene) to non-polar (glycerin). Accurate reproducible results are obtained using class 100 clean-room environments and analysis of multiple drops of each type of surface energy. This three liquid contact angle analysis (3LCAA) brings a much greater level of sophistication to this well-known and apparently-simple method. When carried out with semiconductor-level control of cleanliness, the contribution of each component to the total surface energy of Si (100) native and non-native oxides has been found to depend linearly on γ^LW. In hydrophobic oxide surfaces, γ^T is due almost entirely to molecular interactions, γ^LW, to within a few % error. Thus, the highly-passivated, thermally-grown SiO₂ surface with few defects or impurities, has a surface energy of 35.7 ± 3 mJ/m² that is entirely explained by γ^LW. However, γ^T can be raised to 57.3 ± 2 mJ/m^2. by generating defects, and unsaturated or dangling bonds that interact with electron acceptors and or donors. This situation applies to heavily-etched, oxide surfaces, or chemically-oxidized surfaces. The contributions from γ^-and γ⁺, raises the total surface energy γ^T up to 40% above that of γ^LW, which is found to remain nearly constant. Similar experiments with GaAs (100) surfaces as a function of surface preparation find that the Si-doped GaAs native oxide to be hydrophobic with a γ^Tof 35 ± 3 mJ/m², with γ^LWcontributing 98 ± 2%, thus close to the entirety of γ^T. This indicates a well-reacted native oxide. [1] Herbots N. et al. US Patent 9,018,077 (2015); 9,589,801 (2017).