High-resolution XPS analysis of GaP(0 0 1), (1 1 1)A, and (1 1 1)B surfaces passivated by (NH4)2Sx solution

GaP(0 0 1), (1 1 1)A, and (1 1 1)B surfaces treated by (NH4)2Sx 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)2Sx 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 8C, 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)2Sx treatment. # 2004 Elsevier B.V. All rights reserved.