  Fermi level pinning at the oxidized (110) surfaces of III-As nanowires (GaAs,
InAs, InGaAs, AlGaAs) is studied. Using scanning gradient Kelvin probe
microscopy, we show that the Fermi level at oxidized cleavage surfaces of
ternary Al$_{x}$Ga$_{1-x}$As (0$\le$x$\le$0.45) and Ga$_{x}$In$_{1-x}$As
(0$\le$x$\le$1) alloys is pinned at the same position of 4.8$\pm$0.1 eV with
regard to the vacuum level. The finding implies a unified mechanism of the
Fermi level pinning for such surfaces. Further investigation, performed by
Raman scattering and photoluminescence spectroscopy, shows that photooxidation
of the Al$_{x}$Ga$_{1-x}$As and Ga$_{x}$In$_{1-x}$As nanowires leads to the
accumulation of an excess arsenic on their crystal surfaces which is
accompanied by a strong decrease of the band-edge photoluminescence intensity.
We conclude that the surface excess arsenic in crystalline or amorphous forms
is responsible for the Fermi level pinning at oxidized (110) surfaces of III-As
nanowires.
