Orientation-dependent electronic properties of n-type GaAs in contact with various redox systems in acetonitrile and methanol
Résumé
The flatband potential of n-GaAdnon-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 (loo), (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 (1 11) 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.