Application of scanning tunnelling microscopy to semiconductor/electrolyte interfaces

Abstract

To study the application of in situ electrochemical scanning tunnelling microscopy (STM) measurements at semiconductor electrode surfaces, the tip current has been measured as a function of the semiconductor potential. It was demonstrated that the surface treatment affects very strongly the potential window for STM measurements on n-GaAs. At an ‘etched’ n-GaAs electrode, the tip current was maintained at a preset value when the potential of n-GaAs was more negative than –250mV and more positive than +700 mV. STM images of n-GaAs in the two potential regions are presented to confirm that imaging is really possible in these regions. While no surface change was observed in the negative-potential region, the surface morphology changed with time in the positive-potential region. If the surface was treated with RuCl₃ solution, the preset tip current flowed when the potential was more negative than –100 mV and more positive than +100 mV. On the other hand, if the surface was treated with (NH₄)₂S–S solution, the preset tip current flowed only when the potential was more negative than –1000 mV, which is very close to the flat-band potential. These results are explained using an energy diagram and the importance of the surface-state density for STM characterisation is stressed. The ‘ruthenium treatment’ and the ‘sulphur treatment’ are considered to introduce and remove, respectively, the surface states. Results for CdS are also presented.