Electrochemical impedance spectroscopy of carboxylic-acid terminal alkanethiol self assembled monolayers on GaAs substrates

Adsorbate-induced charge depolarization can influence the organization of self assembled monolayers (SAMs) on semiconductor surfaces, especially as a function of the SAM functional group, SAM length and substrate dopant level and type. Based on systematic differences in the frequency response of the electrochemical impedance and phase data for carboxylic acid (COOH) terminal alkanethiol monolayers of varying alkane chain length assembled on GaAs substrates of different dopant level and type, we assessed the relative monolayer quality through fits to an appropriate equivalent circuit analog to compare the proportion of defects and SAM-induced semiconductor depolarization. At the open circuit potential in the NaCl–phosphate buffer, while SAMs on p+ GaAs were of marginally better quality than those on p GaAs, SAMs on n+ GaAs exhibited a far superior quality than those formed on n GaAs. COOH-terminal SAMs of longer chain lengths formed higher quality monolayers at all the substrate doping levels. COOH-terminal SAM modified n and n+ GaAs surfaces were passivated and stable over a wider voltage range than SAM modified p and p+ GaAs surfaces, from cyclic voltammetry. The poorer quality of COOH-terminal SAMs formed on GaAs substrates at the lower doping levels is attributed to the disorder as a result of the enhanced degree of charge depolarization at these surfaces, as substantiated by systematic variations in the space charge capacitance upon SAM modification that suggest a negative surface dipole.