V. Golovanov

A comparison of conductance behaviour between SnO2 and CdS gas-sensitive films

Abstract The conductance behaviours of SnO 2 and CdS semiconducting films as a function of temperature and after rapid changes in temperature and in oxygen partial pressure are compared. Experimental results are given at different partial pressure of oxygen in dry mixtures with nitrogen in the temperature ranges 350–850 K for SnO 2 and 300–550 K for CdS. In the case of conductance response to rapid changes in oxygen partial pressure, results are given for both an increase and a decrease of the partial pressure. Some results are also given for SnO 2 films doped with aluminium and antimony. SnO 2 samples are in the form of thick films, which together with gold electrodes are screen printed on an alumina substrate. Spray pyrolysis is used for the deposition of CdS thin films from CdCl 2 and (NH 2 ) 2 CS solutions on glass substrates. A similar sigmoidal variation of conductance during heating is found for both SnO 2 and CdS films with a shift of 200 K and more in the temperature scale for this variation between the two films. The results are discussed in the light of conduction models where the surface barriers at intergranular contacts dominate the film resistance.

Different thick-film methods in printing of one-electrode semiconductor gas sensors

Abstract Different thick-film printing techniques have been used for the fabrication of one-electrode semiconductor gas sensors in the form of thick films on insulating alumina substrate. In a typical one-electrode sensor construction, a thin platinum wire (diameter 20 μm) spiral is embedded inside a sintered oxide semiconductor button. The platinum wire spiral is replaced by a platinum thick-film resistor in our prototype sensor, and the oxide semiconductor is screen printed over the platinum resistor. Both screen printing and gravure off-set printing (pad printing) were used for the printing of platinum thick-film resistors. Tin dioxide, an n-type semiconductor, was used as the sensing (shunting)_thick-film layer over the platinum resistor, and diferent amounts of either silver or antimony were used as additives in SnO2. H2S and CO at different concentrations in synthetic air were used to test the response properties of two different sensor types with, respectively, screen-printed and pad-printed platinum thick-film resistors.

Influence of chemical composition on sensitivity and signal reproducibility of CdS sensors of oxygen

Abstract It is well known that due to the high sensitivity of CdS to oxygen chemisorption, thin films of this semiconductor can be used as oxygen sensors. In this work CdS sensors are produced by an electrohydrodynamic spray of liquid onto glass substrates heated up to the equilibrium temperature of the deposition process. Three types of CdS film with different ratios of initial solvent components have been investigated: Cd:S = (a) 10, (b) 1.0 and (c) 0.1. It is established from XPS quantification that the Cd:S ratio is increased in the film produced from an initial solution with higher Cd:S ratio. Simultaneously the total amount of oxygen on the sensor surface is increased. The obtained experimental data demonstrate that an elevation of oxygen concentration is caused by chemisorption, i.e., the chemisorption sensitivity is increased in the case of higher Cd:S ratios.

Dependence of sensitivity and reproducibility of CdS oxygen sensors

Abstract The electrical conductance of the CdS-based sensors increased or decreased when exposed to CO or SO 2 gases, depending on the operation temperature and stoichiometric composition of the film. A correlation between the IR spectrum and inversion of the chemisorbed complex charge state was observed. This is considered to be a favourable basis for the detection of atmospheric pollutants in ambient air, by combinaton of two sensor systems.

A cluster approach for the adsorption of oxygen and carbon monoxide on SnO2 and CdS surfaces

Abstract A cluster approach has been used for the simulation of the adsorption of oxygen and carbon monoxide on SnO 2 and CdS surfaces. Computations are based on an ab initio method, the local-density approximation and atomic orbitals as a basis set. Solutions are calculated self consistently. The atomic orbital nature (origin) of the cluster levels is traced by the projection onto the atomic basis set. The results refer to clusters modelling SnO 2 (110) and CdS (101−0) faces without any surface reconstructions. Energy levels related to metallic surface species with different coordination are described for the basic SnO 2 and CdS clusters, together with levels originating from O 2 and CO adsorbates. The shifts of the levels formed by chemisorbed complexes are described in connection with the band scheme of the semiconductor.

Surface spectroscopy study of CdSe and CdS thin-film oxygen sensors

Two prmmpal forms of oxygen chemrsorptlon on the surface of CdSe and CdS gas sensors have been revealed by X-ray photoelectron spectroscopy and thermal desorptlon mass spectrometry expenments For the chemlsorbed Oz-, the characteristic bmdmg energy of the 0 Is electrons IS found to be 5318 f. 0 2 eV and for the chemlsorbed O-, 531 1 &O 2 eV The concentration of the chemlsorbed oxygen Increases when the surface of the mvestlgated sensor IS enriched or doped wth metal atoms Kevwordr Cadmmm selemde, Cadnuum sulphlde, Oxygen sensors, Surface spectroscopy, Thm films

Interaction between collective and local subsystems in semiconductor surface-active structures

Abstract Two different mechanisms have been suggested to describe the interaction between the chemisorbed molecule and semiconductor surface during equilibrium disturbance. The correlation between the sorption subsystem, characterized by local quantum-mechanical parameters of a chemisorbed complex, and the electronic subsystem, described by collective characteristics of the crystal, has been analysed using the example of oxygen interaction with CdS and SnO 2 polycrystalline films.

Comparative study of differently coordinated surface sites: CdxS- and SnxWOy-based gas sensors

Abstract The electronic structure of chemisorption complexes on the surface of oxidic α-Sn x WO y - and Cd x S-based gas sensors with different elemental composition were comparatively studied by XPS, CEMS and IR-spectroscopy combined with conductivity measurements. The surface sites of Cd and Sn with different coordination were characterized by chemisorption of CO. It was found, differing from the case in a well-oxidized SnO 2 , that electrical conductance of the Sn x WO y and Cd x S films appeared to either increase or decrease when exposed to CO depended on concentration of the surface Sn and Cd ions, the partial pressure of the analyzed gas and the operating temperature. It is argued that the surface metallic species on the investigated films are responsible for oxidizing and reducing types of chemisorption reactions. The relative location of energy levels, formed in the band gap by external orbitals of the chemisorbed complex to the Fermi energy was considered as the determining factor for the observed different chemisorption mechanisms.