Surface Arsenic Enrichment of n-GaAs Photoanodes in Concentrated Acidic Chloride Solutions

Abstract

The photoelectrochemical properties of n-GaAs are studied in acidified chloride electrolytes of different compositions, with the objective of preventing its photocorrosion. After irradiation of a n-GaAs electrode, under fixed anodic potential, current−voltage curves (J−V) experience two modifications: a negative shift in the onset potential of the photocurrent, Vp, and a dark cathodic peak at ∼−0.38 V (saturated 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 saturation value of 18 mA cm-2 after ∼40 min of irradiation. The relationship between the illumination current density and the square root of the irradiation time is linear; however, it has a changing slope at certain time. Therefore, according to the predictions of the diffusivity equation solution, the electrode reactions are limited by diffusion-controlled processes. The origin of these processes is due to the 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 ∼−0.38 V (SCE) is assigned to either the adsorption of chloride ions on this arsenic layer or the reduction of arsenic chloride on the surface. Regarding the stability of the photogenerated elemental arsenic in contact with the electrolytes, 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 As2O3 in electrolytes of relatively low acid concentration. On the other hand, in electrolytes having H2SO4 concentrations as high as 2.5 M in the presence of high chloride concentrations, arsenic chloride, arsenic oxychloride, as well as elemental arsenic are expected to exist on GaAs surface. The diffusion coefficient of gallium ions or vacancies, as calculated from the value of the slope of the curves of illumination current versus the square root of irradiation time in a 2.5 M H2SO4 + 6.0 M LiCl mixture, is zero after 40 min of irradiation. Therefore it is expected that the photodissolution of gallium ions has completely stopped.

Prevention of gallium arsenide photocorrosion by an epoxy adhesion layer

“…Etching in H 2 SO 4 /H 2 O 2 also leaves the surface of GaAs rich in arsenic that is also oxidized. The surface of GaAs is thus composed of an oxide layer not necessarily in the same stoichiometric ratio of Ga 2 O 3 and As 2 O 3 [34,35]. Although, GaAs itself is a semiconductor but its oxides are insulators.…”