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Effects of Interfacial Energetics on the Effective Surface Recombination Velocity of Si/Liquid Contacts

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journal contribution
posted on 22.02.2002, 00:00 by Florian Gstrein, David J. Michalak, William J. Royea, Nathan S. Lewis
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 (Me10Fc+/0), iodine (I2), 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, I2/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 N2(g) ambients or to electrolyte solutions that contained a mild oxidant (such as Me10Fc+/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 N2(g) or in contact with CH3OH−Me10Fc+/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−I2 or THF−I2 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 I2 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.