PHOTOELECTRON-SPECTROSCOPY OF SILVER DEPOSITION ONTO THE VANDERWAALS FACES OF SNS2-XSEX CRYSTALS

The series of n-type semiconducting single crystals SnS2-xSex (0 ≤ x ≤ 2), which possess a two-dimensional layered structure, have been grown using a chemical vapour transport technique. Photoelectron spectroscopy (XPES, UVPES) has been employed to study the reactivity of the clean SnS2-xSex(001) substrate surfaces (x = 0.0, 0.2, 0.3, 0.5, 1.3, 1.85 and 2.0) towards adsorbed atoms in UHV. Silver adsorption onto the sulphur-rich substrates (x = 0.0, 0.2, 0.3 and 0.5) gave rise to a metallic Ag overlayer contacting the semiconducting SnS2-xSex, substrate. The Schottky barrier formation was accompanied by the formation of some reduced tin in the interfacial region. In contrast to this type of behaviour, the selenium-rich substrates (x = 1.3, 1.85 and 2.0) reacted with the adsorbed Ag atoms, a process akin to an intercalation reaction, to give a new surface phase with Ag atoms situated in the van der Waals gaps of the host SnS2-xSex material. Within this group of "reactive" selenium-rich substrates, a change in the electronic properties of the Ag-reacted phase was observed. The UVPES data suggested that the SnS0.7Se1.3 substrate gave rise to a semiconducting phase after Ag deposition, whereas the SnS0.15Se1.85 and SnSe2 substrates gave rise to a metallic phase. A qualitative impurity band model is presented as a possible interpretation of this change in electronic structure in the Ag-intercalated selenium-rich materials. © 1990.