Oxygen adsorption, dissociation and desorption kinetics at the SnO2 surface is simulated. Both the temperature dependence of equilibrium coverages of various forms of oxygen and their transient behavior in varying temperature are considered. The model is based on our earlier work on rate equation simulations of ionosorbed oxygen, but now refined to include the “bridging” lattice oxygen atoms on the surface. Model for the electrical conductance of porous SnO2 material as a function of temperature and in terms of the effects from surface coverages of different oxygen components is presented. With the present model, we are able to simulate the essential features in the experimental conductance dependence.

Oxide semiconductors form a group of compounds whose specific properties of surfaces and interfaces are used for gas sensing. Our fundamental understanding of the operation principles of these devices is still insufficient. The abundance of phenomena on open oxide–semiconductor surfaces at elevated operation temperatures of the sensors is a central reason for the situation, in addition of the effects originating in the electrode–semiconductor contacts. The exchange of lattice oxygen with the surrounding atmosphere and a possible diffusion of oxygen through oxygen–vacancy donors in n-type oxides, especially at elevated temperatures, have also strong effects on the behaviour of semiconductor gas sensors. Atomistic understanding of surfaces is the basis for the understanding of both the receptor and transducer functions of semiconductor gas sensors. The rutile structure tin dioxide, SnO2, together with its most stable (110) face is the example material here. Especially, we consider the oxygen chemistry at the SnO2 (110) surface together with its connection to dipole layers and band-gap surface states. For example, the role of tin (II) ions at the reduced SnO2 (110) surface is discussed. A “transistor model” is also given to describe the transducing properties of semiconductor gas sensors.

Tin dioxide is a widely used material in gas sensing applications. This is partly due to its stable surface structure and high sensitivity to many gases. The interaction of different gas components with an oxide surface may lead to changes in the lattice oxygen content at the surface in addition to changes in the amount of adsorbed species. The electronic and atomic structures of the surface change with the changes in the lattice oxygen content. This leads to surface relaxation and changes in the surface dipole layer of the ionic surface in addition to changes in the Schottky barrier which is a result of the charge accumulation onto the surface from the bulk of the semiconducting oxide. Changes in both the dipole layer and the Schottky barrier change the work function of the semiconductor and may reflect in its electrical conductivity. Here we have used first-principles calculations based on LDA-SCF to study changes in the electronic and atomic structures of the SnO2(110) surface as a result of oxygen exchange between the lattice and the ambient gas. The transducer function relating the changes at the surface to the changes in the conductivity of a ceramic microstructure is also described by an example.

Surface relaxation of the stoichiometric and reduced SnO2 (110) surfaces is studied with first-principles calculations. Calculations are carried out with two different self-consistent ab initio LDA methods, which lead to similar results. The most prominent feature in the relaxation is that the surface layer oxygens of the reduced surface move outwards about 0.4 Å with respect to the surface tin atoms. The stoichiometric (oxidized) surface is stabilized by the “bridging” oxygen atoms, and therefore, relaxes less. The valence band density-of-states is similar at both surfaces, except that removing bridging oxygens leaves behind electrons that occupy gap states formed at the reduced tin atoms.

A short survey is given of our computational approach for the study of the chemical surface sensitivity of some oxide and sulphide semiconductors. Many oxides and sulphides behave as n-type semiconductors with donors originating from nonstoichiometric defects. Therefore, n-type semiconductors with negligible hole concentrations are considered here. Chemical activity of n-type semiconductor surfaces is based on adsorbate interactions both with conduction-band bulk electrons and with localized electrons trapped at surface states. Some effects may also origin from a possible ionic response of the semiconductor to changing internal electric fields. A dynamical approach with expressions similar to those for the generation-recombination processes (Schockley-Hall-Read theory) is used to describe adsorbate interactions with conduction-band bulk electrons. As practical examples we consider SnO2(110) − 1 × 1 and wurtzite CdS(1010) − 1 × 1 surfaces, both being the most stable faces of their structures, respectively, and nonpolar in the bulk derived geometry. With the SnO2(110) surface we use clusters in calculations of localized electronic surface states originating from different defects, while a slab method is used to calculate the atomic relaxation at the CdS(1010) surface.

A treatment of the surface energy barrier is given for n-type semiconductors in the case of mobile donors. We consider finite grains and solve the Poisson-Boltzmann equation, related to the problem, for one-dimensional (slab shape), two-dimensional (cylindrical-rod shape) and three-dimensional (spherical shape) grain geometries. Analytical solutions are given for the band bending and surface energy barrier in one- and two-dimensional grain geometries in the case of total grain depletion, and a numerical approach was used to calculate the results in spherical grains and also in partially depleted grains. Tin dioxide is used as an example to illustrate grain depletion in ambient oxygen atmosphere in the case of mobile oxygen-vacancy donors.

Atomic geometry and electronic density of states of the wurtzite CdS (10 (1) over bar 0) cleavage surface have been calculated. Calculations were carried out with two different self-consistent ab initio LDA methods leading to similar results. Surface relaxation is found to be strong: cations relax towards bulk and anions outwards from the surface. This is in accordance with experimental observations and other published calculations.

Sintered ceramic semiconductors can be used to monitor the surrounding gas atmosphere. It is based on the conductivity response of the semiconductor to the surface reactions on the grains of porous material. Some nonstoichiometry defect donors, like oxygen vacancies in n-type semiconductors, may be mobile at relatively low temperatures, where semiconductor gas sensors operate. In such case, the donor concentration may considerably decrease in the space charge layer at the semiconductor surface, which may be reflected in the transport properties of electrons through the neck contacts between grains. We model here various neck contacts between spherical grains and compute the electrical potential at the conductive region from the Poisson–Boltzmann equation in cases of mobile single and double donors. The solutions are evaluated numerically with a finite difference method. According to the Schottky-defect model for oxygen vacancies, the surface concentration of donors is kept a fixed parameter in the calculations. The chosen material parameters are those of tin dioxide, which is the key material of semiconductor gas sensors. It is found that mobile donors may strongly modulate the distribution of electrical potential around the neck contacts, and thus, modify the transducing properties of the microstructure of ceramic semiconductor gas sensors.

A cluster approach has been used for the simulation of the adsorption of oxygen and carbon monoxide on SnO2 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 SnO2 (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 SnO2 and CdS clusters, together with levels originating from O2 and CO adsorbates. The shifts of the levels formed by chemisorbed complexes are described in connection with the band scheme of the semiconductor.

Some results are given from a cluster approach for the electronic structure of the SnO2 (110) face together with some oxygen vacancies and 'adsorbates'. Computations are based on ab initio methods, the local-density approximation and atomic orbitals as a basis set. Solutions were calculated self-consistently, but also using a composition of atomic potentials (for some smaller clusters). The atomic-orbital nature (origin) of the cluster levels was traced by projection onto the atomic basis set. The results here refer to a basic cluster [SnO2]13 with 17 surface atoms modelling the SnO2 (110) face and the other 22 atoms in the next five surface layers. The effect of oxygen 'adsorbates' and oxygen vacancies in the few uppermost subsurface layers on the electronic structure was considered. In particular, the focus was on the levels related to oxygen vacancies and originating from Sn 5s orbitals, which are well-known donor levels in the deep bulk, making SnO2 an n-type semiconductor. The results support some other theoretical and experimental predictions that oxygen vacancies behave as neutral defects at or near SnO2 surfaces.

The present study starts with mobile oxygen-vacancy donors in an n-type oxidic semiconductor and considers the height of the energy barrier at the semiconductor surface at different temperatures in oxygen-containing atmosphere. Surface concentrations of the oxygen vacancies are used as input data for the computer simulation and both the diffusion and drift components of the current of these positive donors in the electric field of the space-charge layer are considered. Charge distributions of either single or double donors with the charge distribution of electrons have been the starting point for the solution of Poisson's equation for the electric surface potential. Some rate equations tested in an earlier study are used in the computations to describe the electron transfer between the conduction band of the semiconductor and the oxygen surface species (O2, O2- O-). The concept of mobile oxygen-vacancy donors seems to be a reality, at least in high-temperature applications like TiO2 lambda sensors. An aim of this work was to study the effect of freezing the oxygen-vacancy distribution at a specific temperature on the temperature dependence of the height of the surface energy barrier.

The non-ohmic conductance behaviour of some SnO2-based thick-film sensors has been studied both by a computer simulation and by practical experiments. Current-voltage characteristics have been measured in the voltage range 0–20 V at different temperatures. In the computer simulation, a three-dimensional random barrier network is used to describe polycrystalline SnO2 thick films. The basic unit in the network is a component describing the surface Schottky barrier at intergrain contacts and obeying non-ohmic behaviour. IU characteristics have been calculated at different temperatures for networks with different coordination and with different distributions of barrier heights. Results both of the simulation calculations and of experiments show a strong non-linearity at low temperatures, but this behaviour decrease with increasing temperature. Some comparisons between calculated results and those from analytical expressions are also given in the ohmic range at low voltages.

Rate equation simulatin is used in the present computational approach in order to study the role of different adsorbed oxygen ions (O2− and O−) in controlling the height of the Schottky barrier at the surface of SnO2, a key material in the field of semiconductor gas sensors. Computations are based on the adsorption/desorption model and consider the electron transfer between different oxygen species on the surface and the bulk conduction band. Different values have been tested for both the frequency factors and the activation energies of the rate constants in order to consider the relative population between the O− and O2− ions on the surface at different temperatures, the dependence of the height of the surface Schottky barrier on temperature and oxygen partial pressure, and also the response and recovery times of the barrier heights as a consequence of rapid temperature changes. Comparisons of calculated barrier heights with some empirical values are also given at different temperatures and oxygen partial pressures.

The H2S response of some SnO2-based thick-film gas sensors containing Ag and Al2O3 has been studied in the concentration range 0 to 10 ppm. Several different operational parameters related to the response and recovery times, sensitivity and the interfering effects of NO, CO and H2O have been tested. The response measurements are carried out both in the constant-temperature and temperature-pulsed modes in order to find the advantages of each mode in relation to sensitivity, selectivity and response time. In the case of temperature pulsing, both the response and recovery times are very short compared to those in the constant-temperature mode. The constant-temperature mode, however, has to be used at concentrations below 0.5 ppm, which is about the observation limit in the case of temperature pulsing. The interfering effects of both CO and NO are small in the case of the constant-temperature mode, but very pronounced in the case of temperature pulsing. Some experiments concerning monitoring of H2S as a pollutant in city air have also been conducted with the present sensors.

Conductance responses of some sintered samples of SnO2 prepared in the form of thick films have been studied in the CO concentration range from one ppm upwards in dry synthetic air and also in ambients containing different amounts of humidity. The experiments have been carried out both at equilibrium conditions, by allowing the samples to stabilize at different constant temperatures, and at non-equilibrium conditions, by continuously cycling the sensors between two temperatures. The effects of different electrode materials and of some additives like antimony in SnO2 on the conductance response to CO under equilibrium and non-equilibrium conditions have been studied. In this paper the emphasis is to describe some measurements concerning a peculiar effect of electrode materials (between Au and Pt) and Sb doping on the conductance response to CO in the temperature-pulsed mode of sensor operation. A drastic difference in the non-equilibrium conductance response to CO in ambients containing humidity is found between Sb-doped SnO2 sensors with Au and Pt as electrode materials.