The influence of oxygen vacancy behaviors during a cooling process in semiconductor gas sensors is discussed by the numerical analysis method based on the gradient-distributed oxygen vacancy model. A diffusion equation is established to describe the behaviors of oxygen vacancies, which follows the effects of diffusion and exclusion in the cooling process. Numerical analysis is introduced to find the accurate solutions of the diffusion equation. The solutions illustrate the oxygen vacancy distribution profiles, which are dependent on the cooling rate as well as the temperature interval of the cooling process. The gas-sensing characteristics of reduced resistance and response are calculated. Both of them, together with oxygen vacancy distribution, show the grain size effects and the re-annealing effect. It is found that the properties of gas sensors can be controlled or adjusted by the designed cooling process. The proposed model provides a possibility for sensor characteristics simulations, which may be beneficial for the design of gas sensors. A quantitative interpretation on the gas-sensing mechanism of semiconductors has been contributed.
The first principle calculation based on density function theory was used to investigate the crystallite size effect of ZnO semiconductor, the size of which was controlled from 0.325 to 1.625 nm. The formation energy of oxygen vacancy increased from 3.855 to 6.256 eV, showing incremental difficulties in the formation of defects. The densities of states of ZnO super cells with complete and defective ( 110) surface were calculated, concluding positive dependences of carrier mobility and conductivity on crystallite size. The oxygen species of O − and O 2 − were considered as adsorbates on ZnO super cells with defective (110) surface. In case of crystallite size below 1.3 nm, the adsorption energy of O − increased with crystallite size, while the one of O 2 − kept constant, inferring that the oxygen adsorption was dominated by O − . A competitive adsorption between both species was found when crystallite size was over 1.3 nm. The adsorption of O − was inhibited by the incremental adsorption energy of O 2 − , which was the dominated species on ZnO crystallite surface.
Background: Oxygen behaviors play essential roles on the receptor function in the gas-sensing mechanism of SnO2 semiconductors, the size effect of which is a fundamental phenomenon for the development of gas sensors. Objective: This article discusses the size effect on the oxygen behaviors in the gas-sensitive SnO2 semiconductor. Methods: : The first principle calculation was used to investigate size effect on formation of oxygen vacancies and adsorption of oxygen species in the SnO2 semiconductor. The electrical characteristics of conductivity, band gap and electron transfer in SnO2 crystallites were analyzed by density of states and the Mulliken population. Results: The defect of surface bridge oxygen has the lowest formation energy and it is most likely to form in the SnO2 semiconductor. The adsorption energies for O- and O2- are from 1.717 to 3.791 eV and 2.371 to 4.683 eV, respectively. The Mulliken population distribution illustrates that O 2p orbit captures the electrons from the orbits of Sn 5s and 5p as well as O 2s Conclusion: The formation energies of oxygen defects in complete and defective SnO2 super cells are of positive correlation with crystallite size. The carrier concentration and conductivity are improved by the incremental crystallite size. The adsorption energies of O- and O2- species on defective SnO2 super cells increase with crystallite size. With the assistance of connecting Sn atoms, the adsorbates of O- and O2- are able to capture electrons from the inner region of crystallites, resulting in an expansion of depletion layer
The oxygen vacancies (VO) play an essential role in the gas-sensing mechanism of semiconductor devices. A diffusion equation is established to describe the VO behaviors during a cooling process based on the model of gradient-distributed oxygen vacancies. Numerical solutions of the diffusion equation are found to illustrate the VO distribution in grains. The gradient of VO distribution is of negative dependence on the cooling rate, which also influences the average VO density in the depletion layer. The migration of oxygen vacancies in cooling process could be interrupted by quenching and it is restarted by the re-annealing process. The VO distributing process is illustrated by three stages from initial uniform distribution to final gradient profile via a transient stage. The influence of VO distribution on gas-sensing characteristics of semiconductor grains is discussed. Potential opportunities are found to control the gas sensor characteristics by a designed annealing process.
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