Physicochemical and Electrochemical Properties of Etched GaP(111)A and GaP(111)B Surfaces
The observable chemical, electrochemical, and morphological features of GaP(111) surfaces after wet chemical etching have been assessed using X-ray photoelectron spectroscopy, atomic force microscopy, and electrochemical impedance spectroscopy. HF(aq), NH4F(aq), H2SO4(aq), HCl(aq), HCl-HNO3(aq), and dilute Br-2 in CH3OH (with and without a follow-up alkaline etch) are all potent etchants for GaP(111) but yield distinctly different GaP(111)A and GaP(111)B surface conditions. Freshly etched GaP(111)B interfaces show higher residual oxide coverages and are generally rougher than similarly treated GaP(111)A surfaces. Differential capacitance measurements of n-GaP(111)A and n-GaP(111)B electrodes show that various etchants result in a wide range of apparent values for the conduction bandedge potentials, spanning approximately 700 mV in acetonitrile. These data show that these etched surfaces are not equivalent and that the observable physicochemical and electrochemical properties of GaP(111) are strongly influenced by the choice of wet chemical etchant. Additionally, X-ray photoelectron spectra show that adsorption of (4-fluorophenyl)methanethiol onto GaP(111) is insensitive to surface orientation or etching method, indicating no dominant covalent interaction between organic thiols and GaP(111) interfaces.