Passivation of AlGaAs(100) surfaces with ammonium sulfide solutions

X-ray photoelectron spectroscopy is used to study interaction of the native-oxide-covered Al$_{0.3}$Ga$_{0.7}$As(100) surfaces with different ammonium sulfide solutions. The most effective removal of the native oxide layer and chemical passivation is achieved after treatment with diluted aqueous ammonium sulfide solution with volume concentration of about 4% prepared from the so-called aged ammonium sulfide. The treated surface contains tiny amount of elemental arsenic as well as residual gallium and aluminum oxides and is covered with the passivating layer consisting of arsenic sulfides. After treatment with the solution of the same concentration prepared from the freshly-opened ammonium sulfide the sulfur atoms are hardly adsorbed at the surface and arsenic sulfides are not formed. In addition, it is shown that after interaction of the concentrated ($\sim$44 vol.%) aged ammonium sulfide and 4% solution of aged ammonium sulfide in 2-propyl alcohol, the surface alloy stoichiometry is disturbed due to removal of gallium atoms from the surface and the surface becomes covered with a relatively thick layer of aluminum oxide.