Tuomo S. Rantala

Surface relaxation of the (110) face of rutile SnO2

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 A 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.

Electronic structure of SnO2 (110) surface

Abstract The electronic structure of the stoichiometric and reduced SnO2 (110) surfaces is studied with first-principles calculations. Calculations are carried out with two complementary self-consistent ab initio–DFT–GGA methods. Surface relaxation is considered, where the most prominent feature turns out to be the surface layer in-plane oxygen displacement of the reduced surface outwards, about 0.4 A with respect to the surface layer tin atoms. The electronic structure of the relaxed surfaces is considered in terms of atomic orbitals and rehybridization, and the surface band structure. The bands are flat at the stoichiometric surface, but strong dispersion occurs at the reduced surface. The dispersion results in electronic levels into the band gap, which have also been experimentally observed.

Kinetic Monte Carlo simulation of oxygen exchange of SnO2 surface

Abstract 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.

Atomistic understanding of semiconductor gas sensors

Abstract 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, SnO 2 , together with its most stable (110) face is the example material here. Especially, we consider the oxygen chemistry at the SnO 2 (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 SnO 2 (110) surface is discussed. A “transistor model” is also given to describe the transducing properties of semiconductor gas sensors.

Computational approaches to the chemical sensitivity of semiconducting tin dioxide

Abstract Some computational approaches to the chemical sensitivity of semiconducting tin dioxide are presented. Chemical sensitivity is often observed using conductance measurement. Therefore, the potential energy barriers in grain contacts between adjacent grains of a polycrystalline semiconductor are the key parameters for transducing the chemical surface sensitivity into the conductance response. The rate equation model describes the electronic exchange between the adsorbed oxygen species and the bulk conduction band of a semiconductor. It predicts the type of the major negative oxygen ion (O2− or O−) at the surface as a function of temperature in agreement with experimental findings. The grain geometry has only a small effect on the potential energy barrier at the surface of finite grains. Even a small neck contact between grains, in the case of mobile donors, decreases strongly the potential energy barrier between grains compared to that in the case of an open grain contact. Results from Monte Carlo simulations with random barrier networks reveal that the current–voltage characteristic of a polycrystalline semiconductor is non-linear at higher voltages and the non-linearity of the network increases with increasing width of the barrier distributions. Electronic-structure calculations with clusters give qualitative information on the role of oxygen vacancies in different atomic planes in SnO2 and its unrelaxed and unreconstructed (110) surface.

Rate equation simulation of the height of Schottky barriers at the surface of oxidic semiconductors

Abstract Rate equation simulatin is used in the present computational approach in order to study the role of different adsorbed oxygen ions (O 2 − and O − ) in controlling the height of the Schottky barrier at the surface of SnO 2 , 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 O 2 − 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 influence of relative humidity on the response of tin oxide gas sensors to carbon monoxide

Abstract It is well known that the selectivity of tin oxide gas sensors is not very good. In particular, if the sensors are submitted to real life conditions, their response becomes ambiguous because of ambient water vapour. There are several techniques to cope with this problem. A first method to decrease the influence of the relative humidity is to incorporate additives into the tin oxide. However, with this method it is impossible to eliminate the influence of water to an acceptable level. Another method is to use an array of sensors and to convert the response of this array by means of mathematical algorithms, in order to obtain a response that is insensitive to water vapour. The method presented in this paper consists of switching a single tin oxide gas sensor between two well-defined temperatures. With this method, it is possible to decrease the sensitivity to water vapour to an acceptable level, while the high sensitivity to CO is unaffected.

Computational studies for the interpretation of gas response of SnO2(110) surface

Abstract 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 SnO 2 (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.

Sensitivity and selectivity of doped SnO2 thick-film sensors to H2S in the constant- and pulsed-temperature modes

Abstract 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.

Some effects of mobile donors on electron trapping at semiconductor surfaces

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.