Structural transformations on the AsxS100-x nanolayers: X-ray photoelectron and electron microscopy investigations


Background. The research of chalcogenide glassy (ChG) materials formed a general understanding of electronic phenomena in disordered structures. The numerous investigations of their fundamental physical and chemical properties have been already studied. Unique structural, electronic and optical properties determined their various applications. Photosensitivity is the main feature of chalcogenide glasses for fabrication phase-change memory, tailoring direct waveguides and grating patterning.
Materials and methods. Amorphous AsxS100-x (x = 40, 45, 50) were prepared by thermal vacuum evaporation from appropriate bulk glass powders onto silicon wafers. The surface chemical composition and structure of material were investigated by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) on as-deposited ChG thin film samples and samples being illuminated by near bandgap laser (λ = 532 nm, photon energy ~ 2.4 eV, p = 25 mW), under ambient conditions.
Results. All spectra of S 2p electron core level were fitted by two components: S-As2 and S-SAs s.u. The As 3d core level spectra of As40S60 and As50S50 nanolayers contains As-S3, As-S2As and As-SAs2 s.u. As45S55 structure is characterized by the presence of the fourth, As-As3 component apart from previous mentioned three. Near-bandgap laser light illumination of AsxS100-x (x = 40, 45, 50) samples causes decreasing of the contribution of components with the homopolar S-S bonds in all samples, decreasing of components with homopolar As-As bonds in the structure of As40S60 and As50S50 nanolayers and increasing of concentration of components with As-As bonds in As45S55 films.
Conclusion. The local and molecular structure of the surface layers of AsxS100-x thin films (x = 40, 45, 50) and their structural transformations induced by coherent laser irradiation with energy close to band gap energy, with using XPS and SEM were investigated. An essential difference between the surface stoichiometry and the composition of the appropriate glasses has been found. Established that this behavior caused by peculiarities of the molecular component of the gas phase during deposition. This indicates that the type of molecules in a pair plays a decisive role in forming composition of the film. Laser irradiation reduces the concentration of homopolar S-S bonds in the structure of all AsxS100-x nanolayers (x = 40, 45, 50). However a decreasing of the As-As homopolar bond concentration induced by laser light was observed only on samples of As40S60 and As50S50 films. During illumination significant contribution of As-S2As and As-SAs2 s.u. and the appiarence of As-enriched As-As3 s.u. was seen in the case of As45S55 thin film sample. SEM photos of the surface of the films confirm the ordering of the structure of the films during processing

Keywords: X-ray photoelectron spectroscopy, amorphous film, laser irradiation, scanning electron microscopy, structural units