A Mott-Schottky Analysis of Mesoporous Silicon in Aqueous Electrolyte by Electrochemical Impedance Spectroscopy

Nanoporosity in silicon leads to completely new functionalities of this mainstream semiconductor. In recent years, it has been shown that filling the pores with aqueous electrolytes in addition opens a particularly wide field for modifying and achieving active control of these functionalities, e.g., for electrochemo-mechanical actuation and tunable photonics, or for the design of on-chip supercapacitors. However, a mechanistic understanding of these new features has been hampered by the lack of a detailed characterization of the electrochemical behavior of mesoporous silicon in aqueous electrolytes. Here, the capacitive, potential-controlled charging of the electrical double layer in a mesoporous silicon electrode (pore diameter $7\,\mathrm{nm}$) imbibed with perchloric acid solution is studied by electrochemical impedance spectroscopy. Thorough measurements with detailed explanations of the observed phenomena lead to a comprehensive understanding of the capacitive properties of porous silicon. An analysis based on the Mott-Schottky equation allows general conclusions to be drawn about the state of the band structure within the pore walls. Essential parameters such as the flat band potential, the doping density and the width of the space charge region can be determined. A comparison with bulk silicon shows that the flat band potential in particular is significantly altered by the introduction of nanopores, as it shifts from $1.4\pm0.1\,\mathrm{V}$ to $1.9\pm0.2\,\mathrm{V}$. Overall, this study provides a unique insight into the electrochemical processes, especially the electrical double layer charging, of nanoporous semiconductor electrodes.