Effects of interfacial energetics on the effective surface recombination velocity of Si/liquid contacts

Photoconductivity decay data have been obtained for NH4F(aq)-etched Si(111) and for air-oxidized Si(111) surfaces in contact with solutions of methanol, tetrahydrofuran (THF), or acetonitrile containing either ferrocene(+/0) (Fc(+/0)), [bis(pentamethylcyclopentadienyl)iron](+/0) (Me(10)Fc(+/0)), iodine (I-2), or cobaltocene(+/0) (CoCp2+/0). Carrier decay measurements were made under both low-level and high-level injection conditions using a contactless rf photoconductivity decay apparatus. When in contact with electrolyte solutions having either very positive (Fc(+/0), I-2/I-) or relatively negative (CoCp2+/0) Nernstian redox potentials with respect to the conduction-band edge of Si, Si surfaces exhibited low effective surface recombination velocities. In contrast, surfaces that were exposed only to N-2(g) ambients or to electrolyte solutions that contained a mild oxidant (such as Me(10)Fc(+/0)) showed differing rf photoconductivity decay behavior depending on their different surface chemistry. Specifically, surfaces that possessed Si-OCH3 bonds, produced by reaction of H-terminated Si with CH3OH-Fc(+/0), showed lower surface recombination velocities in contact with N-2(g) or in contact with CH3OH-Me(10)Fc(+/0) solutions than did NH4F(aq)-etched, air-exposed H-terminated Si(111) surfaces in contact with the same ambients. Furthermore, the CH3OH-Fc(+/0)-treated surfaces showed lower surface recombination velocities than surfaces containing Si-I bonds, which were formed by the reaction of H-terminated Si surfaces with CH3OH-I-2 or THF-I-2 solutions. These results can all be consistently explained through reference to the electrochemistry of Si/liquid contacts. In conjunction with prior measurements of the near-surface channel conductance for p(+)-n-p(+) Si structures in contact with CH3OH-Fc(+/0) solutions, the data reveal that formation of an inversion layer (i.e., an accumulation of holes at the surface) on n-type Si, and not a reduced density of surface electrical trap sites, is primarily responsible for the long charge carrier lifetimes observed for Si surfaces in contact with CH3OH or THF electrolytes containing I-2 or Fc(+/0). Similarly, formation of an accumulation layer (i.e., an accumulation of electrons at the surface) consistently explains the low effective surface recombination velocity observed for the Si/CH3OH-CoCp2 and Si/CH3CN-CoCp2 contacts. Detailed digital simulations of the photoconductivity decay dynamics for semiconductors that are in conditions of inversion or depletion while in contact with redox-active electrolytes support these conclusions.