APPLICATION OF SCANNING-TUNNELING-MICROSCOPY TO SEMICONDUCTOR ELECTROLYTE INTERFACES

To study the application of in situ electrochemical scanning tunnellin g microscopy (STM) measurements at semiconductor electrode surfaces, t he tip current has been measured as a function of the semiconductor po tential. It was demonstrated that the surface treatment affects very s trongly the potential window for STM measurements on n-GaAs. At an 'et ched' n-GaAs electrode, the tip current was maintained at a preset val ue when the potential of n-GaAs was more negative than -250 mV and mor e positive than +700 mV. STM images of n-GaAs in the two potential reg ions are presented to confirm that imaging is really possible in these regions. While no surface change was observed in the negative-potenti al region, the surface morphology changed with time in the positive-po tential region. If the surface was treated with RuCl3 Solution, the pr eset 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 onl y when the potential was more negative than -1000 mV, which is very cl ose to the flat-band potential. these results are explained using an e nergy diagram and the importance of the surface-state density for STM characterisation is stressed. The 'ruthenium treatment' and the 'sulph ur treatment' are considered to introduce and remove, respectively, th e surface states. Results for CdS are also presented.