This study compares the surface energies of and effect of doping on native oxides of Si(100), Si(111) and GaAs(100), and effects from doping, via Three Liquid Contact Angle Analysis (3LCAA) and the Van Oss Theory.
In 3LCAA, contact angles are measured from three liquids (18 MΩ deionized water, glycerine, and alpha-bromonaphthalene) in a class 100 hood. A new automated Drop and Reflection Operative Program (DROP) uses polynomial curve fitting to determine the contact angle of both drops and its reflection. DROP removes subjectivity in image analysis, yielding more reproducible contact angles. When compared with the Sessile Drop method, DROP yields 10-50% lower standard deviations for sets of 20-30 drops.
Native oxide/Si(100) wafers with a low bulk p+-doping of 5 x 1013 B/cm3 (7-13 Ωcm) is hydrophilic and yields a total surface energy density γT of 53 ± 2 mJ/m2. This is 9% larger than the γT for native SiO2/Si(111) n-doped, which yields 48 ± 2 mJ/m2 across four Si(111) wafers. Ion Beam Analysis (IBA) using O2 nuclear resonance combined with MeV ion channeling detects that the native oxides on Si(111) contain 1.4 times more oxygen than Si(100). A decrease in the amount of native SiO2 on Si(100) would increase its surface energy. Measurements of the Lifshitz-Van der Waals surface energy component γLW for native SiO2/Si(100) yield 36 ± 0.4 mJ/m2 equal to γLW for native SiO2/Si(111), 36 ± 0.6 mJ/m2. Measurements of the electron acceptor surface energy component γ+ for native SiO2/Si(100) yield 34 ± 2 mJ/m2 which is greater than γ+ for native SiO2/Si(111), 28 ± 3 mJ/m2, by 21%.
Native oxides on Si(100) with a high bulk p+-doping level of 5 x 1019 B/cm3 (.01-.02 Ohm cm) yield γT of 39 ± 1 mJ/m2. Consistent with a lower γT, it is more hydrophobic. Higher doping leads to a 20% larger γT, with a relative error less than 3%. IBA using O2 nuclear resonance combined with MeV ion channeling is used again to establish that it is due to a difference in native oxide thickness, in this case caused by doping rather than crystal structure .
In comparison, the surface energy density of native oxides of GaAs(100) is found to always fall around 38 ± 2 mJ/m2 as measured on four GaAs wafers with different dopants and doping levels. The consistently hydrophobic native oxides found on various GaAs surfaces correlates with consistent native oxide composition and thickness.
3LCAA and IBA can detect, with an accuracy of a few percent, changes in the surface energy of native oxides due to surface composition variations arising from doping or crystal structure.