The Electrochemical Hydroxylation of H-Terminated <i>n</i> -Si as Revealed by a Generalized Phase Analysis of EIS: Measuring Band Shift and Band Bending

The generalized phase element (gpe) method is a differential impedance analysis, built upon the concept of a frequency dependent phase element αg, which can be used for instantaneous in situ measurements of reactive electrochemical interfaces. During the impedance evolution of a n-Si/sulfate electrolyte under a dark positive scan of the potential E, the αg(E) curve revealed a two-step hydroxylation reaction occurring at negative potentials. This study has shown that, both the αg(E) curve and the dispersion-free capacitance Cg(E) curve generated by the gpe analysis allows for an independent and precise determination of the Helmholtz capacitance and the flat band potential, and consequent decoupling of the band bending from the flat band shift caused by the hydroxylation reaction. The interfacial energetics indicate that the electrochemical hydroxylation of the H-terminated Si involves an electron from the conduction band, a dangling- bond state, and a valence-band hole─even though the surface states appear energetically closer to the conduction band. The innovative gpe analysis shows that the capacitance of these surface states and their frequency response is consistent with a three-level distribution, with energy positions that vary based on occupancy.