Surface Structure and Morphology of CdS Thin Films Deposited by Spray Pyrolysis
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V GOLOVANOV*, V. LANTIrO, S.LEPPAVUORI, A. UUSIMAKI, J. REMES and J. FRANTTI Microelectronics Laboratory, University of Oulu, Linnanmaa, 90570 Oulu, Finland *Permanent address: Faculty of Physics, Odessa State University, Pastera 42, 270100 Odessa, Ukraine
ABSTRACT CdS thin films with different compositions were deposited by the electrospray pyrolysis of CdCI2 and (NH 2 )2CS solutions mixed in different proportions. The diverse morphology of the films was analyzed using AFM/SEM observations. The relative atomic concentrations of Cd and S species in the bulk, and at the surface of the films were estimated using EDS and XPS quantification routines, respectively. It was found that the surfaces of all types of investigated films were enriched by metallic species and the atomic surface ratio of Cd/S was between 1.6 and 3.2. Chemisorption of oxygen and carbon monoxide on the film surfaces at low temperatures between 290 K and 500 K has been studied in order to get information about Cd surface sites on the chemisorption. Metallic surface species are considered as basic centres for the chemisorption of CO and 02 gases. Three different oxygen species with different binding energies for O1 s electrons were found on the film surfaces. The electronic structure of the surface complex CO, together with its influence on the charge transfer reactions during excitation are discussed in terms of a model based on the correlation between collective electronic characteristics of the crystal and local quantum-mechanical parameters of the chemisorbed complex.
INTRODUCTION It is well known that many II-VI compound semiconductors can be used for gas sensing applications. The basic characteristics of such gas sensors are described by their collective parameters (conductivity, surface potential, etc.), which change during chemisorption processes, and can be interpreted within the band scheme of the semiconductor [1]. However, an atomic-scale understanding of the sensing mechanism and, particularly, the role of different surface structures for adsorption-desorption processes is not clear at the moment. For instance, two different models are commonly considered to explain the role of metallic surface additives during chemisorption and catalysis in connection with electronic charge-transfer reactions. Metallic surface defects may act as specific sites for chemisorption and/or dissociation of molecules to be detected ("geometric factor") or they may pin the Fermi energy (Fermi energy control or "electronic factor"). Therefore, chemisorption of oxygen and carbon monoxide on a "prototype material" such as CdS in the form of thin films was studied in a low-temperature operation interval (290-500 K). Indeed, CdS films provide a suitable model system because 1) they show high photo and chemi-sensitivity and 2) it is possible to vary widely the element composition at the surface and the Fermi-level position in the band gap. XPS (X-Ray Photoelectron Spectroscopy) has proved to be unrivalled in giving direct information on the composition and structur
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