GaP(0 0 1), (1 1 1)A, and (1 1 1)B surfaces treated by (NH4)(2)S-x solution have been studied by high-resolution X-ray photoelectron spectroscopy (XPS). The surfaces etched by acid solution have small amount of gallium and phosphorus oxides. The oxides grow after the (NH4)(2)S-x treatment. However, the oxide of the (1 1 1)B surface is found to be weak in intensity with and without annealing. Three components of sulfur at 161.2 (I), 162.0 (II), and 163.0 (III) eV in S 2p spectra are found for the surfaces. They can be ascribed to sulfur-bonded to gallium at the surface, sulfur-bonded to Ga below the surface, and that bonded to phosphorus and sulfur, respectively. It is found that the component (II) are dominant for the (1 1 1)A and (1 1 1)B surfaces. The total sulfur component is significantly higher in intensity for the (0 0 1) and (1 1 1)A surfaces than that for the (1 1 1)B. Upon annealing the samples at 400 degreesC, the component (III) for all the samples is tremendously decreased in intensity. The component (II) is still dominant upon annealing the (1 1 1)A surface. The component (I) is increased in intensity for the (1 1 1)B surface, which is caused by conversion from the (II) and/or (III). It is suggested that the chemistry of the sulfurized GaP surfaces is affected by the pre-treatments of the surface, passivation reagents, and surface orientation. The result of the valence band spectra indicates that a flat band of the GaP(0 0 1) surface is obtained by the (NH4)(2)S-x treatment. (C) 2004 Elsevier B.V. All rights reserved.