Mechanism of GaAs surface sulfidation
The mechanism of GaAs sulfidation under illumination and potentiodynamic polarization was investigated in acidified thiourea (TU) electrolytes. Sulfidation generated smooth surfaces, as revealed by scanning electron microscopy and atomic force microscopy images; but analysis by inductively coupled plasma - mass spectroscopy (ICP-MS) of spent electrolytes showed that this was in part due to GaAs dissolution. The initial step in sulfidation occurred through formation of elemental arsenic which then reacted with TU and forms As(III) sulfide, which was subsequently oxidized into As(V) sulfide and finally to arsenic sulfate. X- Ray photoelectron spectroscopy (XPS) demonstrated the initial formation of elemental As (XPS peak at 42 eV of As - As bond). XPS also showed three S 2p doublets at 162.4 eV which were assigned to (As(III) - S), 164.4 eV for (As(V) - S) and 169.1 eV for (S - O). The intensity of the XPS peak due to As(V) - S (S 2p at 164.4 eV) increased with prolonged exposure to sulfidation by the electrolyte, indicating the oxidation of As(III) into As(V) sulfide. Furthermore, the intensity of the S - O bond (S 2p at 169.1 eV) decreased with time, presumably due to arsenic sulfate dissolution. Both XPS and ICP-MS studies revealed that arsenic species were preferentially segregated on the surface but Ga ions diffused towards the electrode bulk.