ORIENTATION-DEPENDENT ELECTRONIC-PROPERTIES OF N-TYPE GAAS IN CONTACT WITH VARIOUS REDOX SYSTEMS IN ACETONITRILE AND METHANOL

The flatband potential of n-GaAs/non-aqueous electrolyte contacts has been measured as a function of (i) the redox energy level of the electrolyte (over 2 V in acetonitrile and over 1.3 V in methanol) and (ii) the crystallographic orientation, considering the [100], [111]As, [111]Ga and [110] faces. The experimental conditions have been selected to prevent oxide formation. A strong Fermi level pinning (FLP) is observed for the [100] and both [111] faces due to the same surface state distribution for a given solvent. For acetonitrile (methanol), this distribution is localized near one-third of the gap (0.1 eV) from the valence band edge. In comparison with the [100] face, both [111] faces exhibit a flatband potential negatively shifted by 0.5-0.7 V. This shift can be correlated with the piezoelectric property of the [111] direction. On the contrary, for the [110] face there is no FLP. The flatband potential is independent of the redox level when this latter is varied over all the GaAs gap. This means that the interactions between the redox species and the GaAs surface are sufficiently weak to hold the ideal character of the [110] surface, i.e. without surface states as for the cleaved surface in UHV conditions.