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.