The photoelectrochemical properties of n-GaAs are studied in acidified
chloride electrolytes of different compositions, with the objective o
f preventing its photocorrosion. After irradiation of a n-GaAs electro
de, under fixed anodic potential, current-voltage curves (J-V) experie
nce two modifications: a negative shift in the onset potential of the
photocurrent, Vp, and a dark cathodic peak at similar to-0.38 V (satur
ated calomel electrode, SCE). During irradiation at +0.4 V (SCE) in an
acidified chloride solution, the illumination current increased with
time rather nonlinearly. In an electrolyte composed of 2.5 M H2SO4 and
6.0 M LiCl at +0.4 V (SCE) the illumination current reached a saturat
ion value of 18 mA cm(-2) after similar to 40 min of irradiation. The
relationship between the illumination current density and the square r
oot of the irradiation time is linear; however, it has a changing slop
e at certain time. Therefore, according to the predictions of the diff
usivity equation solution, the electrode reactions are limited by diff
usion-controlled processes. The origin of these processes is due to th
e preferential dissolution of surface gallium atoms and the formation
of gallium vacancies. As a result of this dissolution, the surface of
GaAs is enriched with arsenic. The negative shift in V-p is attributed
to the junction between the arsenic-rich layer and the GaAs surface.
Furthermore, the cathodic peak at similar to-0.98 V (SCE) is assigned
to either the adsorption of chloride ions; on this arsenic layer or th
e reduction of arsenic chloride on the surface. Regarding the stabilit
y of the photogenerated elemental arsenic in contact with the electrol
ytes, the values of the standard redox potentials as well as the shape
of the illumination current-time curves (J-t) suggest that arsenic is
oxidized to AS(2)O(3) in electrolytes of relatively low acid concentr
ation. On the other hand, in electrolytes having H2SO4 concentrations
as high as 2.5 M in the presence of high chloride concentrations, arse
nic chloride, arsenic oxychloride, as well as elemental arsenic are ex
pected to exist on GaAs surface. The diffusion coefficient of gallium
ions or vacancies, as calculated from the value of the slope of the cu
rves of illumination current versus the square root of irradiation tim
e in a 2.5 M H2SO4 + 6.0 M LiCl mixture, is zero after 40 min of irrad
iation. Therefore it is expected that the photodissolution of gallium
ions has completely stopped.