ORIENTATION-DEPENDENT ELECTRONIC-PROPERTIES OF N-TYPE GAAS IN CONTACTWITH 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 elect rolyte (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 observ ed for the [100] and both [111] faces due to the same surface state di stribution for a given solvent. For acetonitrile (methanol), this dist ribution is localized near one-third of the gap (0.1 eV) from the vale nce band edge. In comparison with the [100] face, both [111] faces exh ibit a flatband potential negatively shifted by 0.5-0.7 V. This shift can be correlated with the piezoelectric property of the [111] directi on. 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 r edox species and the GaAs surface are sufficiently weak to hold the id eal character of the [110] surface, i.e. without surface states as for the cleaved surface in UHV conditions.