<title>Optimization of thin-film gas sensors for environmental monitoring through theoretical modeling</title>

Abstract

ABSTRACT The task of optimization of gas sensor characteristics such as absolute value and temperature range of sensitivity, timeresponse and selectivity is one of the most important problems of gas sensor design. Unfortunately, at present time thedecision of this problem has empirical character, which is not effective, because of multi-factor task. We propose anotherapproach to GS optimization, which is based on the theoretical modeling of gas sensing characteristics in the framework ofcheimsortiona1 views. Such approach is more effective as it permits both to understand and to predict the influence ofsurface and bulk xrameters of metal oxide films on sensing characteristics.The conclusions about optimum rameters combination of SnO thin films were made. Many of these conclusions haveexperimental confirmation.Keywords: SnO; thin film; gas sensors; simulation; chemisorption; optimization. 1. INTRODUCTION Problem of optimization is a key factor in design and fabrication of any electronic sensor. In the case of gas sensors (GS) ithas some specific moments and peculiarities due to absence of strict quantitative theory, which would describe GSoperation. Some of these semi-quantitative approaches one can find elsewhere14. Since the number of physical andchemical xirameters, which characterize the sensor properties are large, and some of them are controlled with difficulty, theproblem of optimization today has empirical character and remains a kind of an art. Therefore, present situation in the fieldof gas sensorics5 is characterized by searching of adequate theoretical models, which can promote GS optimization. Herethe term optimization means the achievement of necessary or the most available values of sensitivity, selectivity andtransient times of GS at given conditions. In our previous works we developed thin film GS quantitative model, based onchemisorptional theory of Volkenstein9. This model allowed both to understand the main moments of GS behavior and togive some recommendations to optimization of GS technology.2. TREATMENT OF GS PARAMETERS AND CHARACTERISTICS2.1. Integral parameters determining gas sensitivity characteristicsOne can select the following main physical xwameters, which influence on GS characteristics, and that are neededto control both during manufacturing process and after various treatments. They are:1) geometric and structural parameters of the film:-length, width, thickness and porosity of the film;-effective size of grains or crystallites, area of inter-grain contacts;-crystallographic structure and orientation of grains, crystallites (energetic parameters of adsorpUod AID energies; positions and distributions oflocal electronic levels in band gap of semiconductor due to chemisorption and so on)3) composition or bulk and surface stoichiometry of the film (these parameters determine the electron concentration,initial surface potential, own surface charge through the number of oxygen vacancies)4) parameters of the impurities and additives: type of impurity; effective size and surface density of clusters; bulk andsurface concentration; electrical activity, etc. Metal catalysts particles are the main controlled additives. Fromuncontrolled impurities it is required to isolate carbon and its compounds, various forms of water and interferingchemisorbed species between water and carbon-contained species.