Synthesis And Properties Of In-doped SnO₂ Thin Film

Tin oxide (SnO2) thin film with a wide direct bandgap (Eg= 3.6eV) is a typical n-type metal-oxide semiconductor with stable physical and chemical properties such as its excellent electrical conductivity, superior optical property, good thermal and anti-corrosion properties. So it is widely used for many purposes such gas-sensing materials, solar cells, chemical electrodes, transparent conductive film, etc. Since the carrier concentration in the SnO2 thin films is decided by oxygen vacancies, the relatively high resistivity is resulted and difficult to be controlled. So SnO2 films doped with metals, such as indium, aluminum, etc. had been used to significantly improve the electrical resistivity and maintain a good optical transmittance.These transparent thin films have not only good optical and electrical properties, but also can be used to as the sensing materials to monitor some harmful gases for its excellent performance.Indium-doped tin dioxide, known as ITO, is a useful semiconductor oxide in the electronic applications. ITO is also an increasingly important material for the gas sensors application. Lots of techniques had been employed for ITO thin films preparation including:sputtering, evaporation and so on. This paper use sol-gel technique to prepare ITO thin films for gas sensor application. Poly (ethylene glycol) had been used as an additive to improve the quality of ITO thin films. This method is simple, low cost. The main content of this thesis is as below:(1) Indium-doped tin dioxide (ITO) thin films with various concentration of In have been successfully prepared on silicon wafer by sol-gel spin-coating technique, using C2H5OH, anhydrous SnCl4 and InCl3 as raw materials. In addition, polyethylene glycol (PEG) with different concentrations and molecular weight has been introduced. The influence of the processing paramerters such as the annealing temperature, PEG concentrations, etc on the ITO had been studied. The ITO films were characterized by X-ray (XRD), scanning electron microscopy (SEM), UV-vis spectrometer, etc.The study indicated that the benefits of introduction of PEG include:to form the porous thin films, to improve the surface flatness and to lowering the sheet resistance of the ITO thin films.(2) Argon plasma calcination had been used instead of the conventional thermal annealing to form the ITO thin films at room temperature. The influence of the plasma parameters such as the plasma power, processing time on the ITO thin film surface properties and nanostructures had been studied and characterized by scanning electron microscopy (SEM), infrared analysis (FTIR), field emission gun scanning electron microscope (FEGSEM). It is demonstrated that with argon plama calcination, ITO thin films with good performance can be fabricated at room temperature.(3) The influence of the annealing atmosphere on the ITO films was investigated. the structure of ITO films on the different annealing atmosphere were characterized by scanning electron microscopy (SEM) and the films'resistivity were studied by SZT-2 four-probe meter. The results showed that the higher oxygen content in the annealing environment, the worse the film surface structure, the greater the resistivity.The PEG-doped ITO thin films have the porous structure, which can be used as the gas-sensing material.