Physical and chemical processes in ISFETs with chalcogenide membranes

Abstract Chemical and physical properties of thin chalcogenide films (As 2 Se 3 :Cu) in ISFETs and EIS sysems have been investigated. Chemical sensitivity, selectivity and response time of a pCu-ISFET with chalcogenide membrane are similar to those of a conventional pCu-selective chalcogenide glass electrode (Yu. G. Vlasov and E.A. Bychkov, Ion-selective Electrode Rev., 9 (1987) 5–93). Investigation of the dependences of physical properties of chalcogenide films on the copper content of the films and on the pCu of electrolyte solutions revealed: (i) the spatial distribution of electrical charges in chalcogenide films and their optical constants (refractive index and absorption coefficient) depend strongly on the copper concentration in the film; (ii) the electrical and optical properties of chalcogenide films change drastically during the first exposure to an electrolyte solution containing copper ions; (iii) these changes can be unambiguously described as the formation of the two-layer structure of the chalcogenide film, electrical and optical properties of the outer layer (approximately 100 nm thick) corresponding to the properties of the films with lower copper concentration as compared to the inner part of the film. It is concluded that the formation of a modified surface layer characterised by high mobilities of species in it allows the ion-exchange mechanism of chalcogenide membrane/electrolyte solution interface potential generation to be realised.