SURFACE ARSENIC ENRICHMENT OF N-GAAS PHOTOANODES IN CONCENTRATED ACIDIC CHLORIDE SOLUTIONS

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.