Electrical Properties of Ultra Thin Tin Oxide Films
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at the same time, film thickness of TCO is
Generally resistivity increases drastically with decreasing the thickness. We call films in the thickness range of less than 20nm as "Ultra thin films" to distinguish them from "bulk films" whose thickness is more than 100nm. We studied electrical properties of "Ultra thin Tin Oxide films" in relation to fluorine doping. In this paper, we describe the resistivity control and improvement in thermal stability of Ultra thin Tin Oxide films. EXPERIMENT Deposition systems APCVD apparatus have been developed to deposit Fluorine doped Tin Oxide (Sn02:F) films[5]. An in-line type horizontal furnace was utilized for deposition. Heating up, film deposition and cooling down were proceeded successively for mobile substrates. Film deposition was performed by the gas injectors. Source gases impinge on substrates passing the injectors by a metal conveyer belt. Each Injector has 5 gas nozzles which supply uniform gas flow for full width of substrates. Since Injector nozzles and substrates were fully apart by 15mm, source gases mix together on the substrate surface and react.
Substrates Common Soda-Lime glass of 1.lmm thickness was used as a substrate. It is generally 393 Mat. Res. Soc. Symp. Proc. Vol. 558 0 2000 Materials Research Society
known that electrical properties of TCO films are degraded by migration of alkali metal ions from the glass. To impede the migration a silica layer of 50nm was deposited onto the glass surface preceding to TCO deposition. Starting materials and Deposition conditions Films were deposited through the hydrolysis reaction from stannic chloride and water. Hydrofluoric acid and methanol was used to dope fluorine into SnO2 films. Input reactants, SnCI4, H20 and CH3OH were stored in temperature controlled tank. Gas transportation was performed by bubbling method utilizing Nitrogen as carrier gas. Molar flux of each reactant can be calculated by specifying temperature and N2 flow rate. Typical deposition conditions are summarized in Table I Measurement and Characterization Although film thickness is generally measured using stylus instruments, error in measurements increases for thickness less than 20nm because of instrumental limitation.. We examined a relationship between optical transmission and film thickness measured using a stylus instrument (Dektak3O3O) to improve reproducibility of measurement. As shown in Fig.1, optical transmission increases with decreasing film thickness. Based on the relationship, we evaluated film thickness by transmission at 550 rim, The electrical properties, the resistivity, Hall mobility and carrier concentration, were measured by Van der Pauw method at room temperature. The observation of surface morphology of TCO was carried out by using scanning electron microscopy (SEM). SIMS measurement was carried out to evaluate fluorine concentration in the films. RESULTS SEM observation Fig.2 shows a SEM image of film surface of the TCO film which is 20nm in thickness. The SEM image revealed the film consists of micro grains of less
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