NOVEL APPLICATION OF SCANNING TUNNELING MICROSCOPY - TIP CURRENT VOLTAMMETRY OF N-GAAS AND P-GAP IN ELECTROLYTE SOLUTION

Scanning tunneling microscopy (STM) has been modified for use on semiconductors in an electrolyte solution providing a new technique, tip current voltammetry (TCV), for characterization of semiconductor/electrolyte interfaces. It provides information about chemical reactions on the surface, the interface energetics, and the current transport mechanisms. The TCV technique has been applied to the study of n-GaAs (with two different doping levels) and p-GaP in 2 mM NaOH. The tip current is measured, while scanning the potential on the semiconductor electrode versus a reference electrode, keeping the tip at constant potential versus the reference electrode. With the semiconductor potential in the accumulation region, tunneling is obtained in the normal way. When changing the potential into the depletion region, the distance between tip and electrode surface goes to zero, the instrument goes into "contact mode". In the depletion region the p-GaP and n-GaAs samples behaved differently. At the p-GaP sample the tip current drastically dropped at the flat-band potential, while at the n-GaAs sample the "tunneling mode" is maintained even at depletion, + 700 mV from the flat-band potential. At the very highly doped n-GaAs sample, a new tunneling mode is obtained again when the semiconductor potential reaches the inversion region. The tip current voltammograms are affected by surface reactions and by illumination. The transition potential between tunneling and contact modes is shifted negatively at an oxidized surface and returns to its original value if the electrode is held at negative potentials. © 1990.