Abstract
Progressive improvement in the TCO characteristics of ITO (In2O3:Sn) thin films grown at substrate temperature (TS ∼ 232 °C) near the melting point of Sn has been pursued via optimization of gas pressure in the RF magnetron sputtering system. The optical transmission characteristics virtually remain consistently impressive on varying the gas pressure; however, crystallinity improves with significant dominance along 〈222〉 crystallographic orientation at an optimum gas pressure of 10 mTorr. Simultaneously, enhanced doping of the In2O3 network by Sn4+ active dopants along with optimum reducing condition attained at 10 mTorr pressure in the magnetron plasma provide maxima of both concentration and mobility of the charge carriers in the ITO films prepared at each substrate temperature in the range 200–350 °C and lead to a very high TCO figure of merit characteristics. At lower gas pressure below 10 mTorr, although dopant incorporation increases, formation of enhanced number of electrically inactive dopants (Sn2+) (reduced Sn4+/Sn2+ ratio) deteriorates the electrical characteristics of the ITO films. On lowering of gas pressure beyond the optimum reducing condition in the plasma, changes in the nature of decay of the concentration and mobility of the charge carriers depend sensitively on the substrate temperature. The carrier concentration reduces rapidly at higher TS while carrier mobility diminishes significantly at lower TS. Accordingly, dopant concentration and the oxidation state of the In2O3:Sn matrix together control the carrier characteristics in a complex manner which is again sensitively influenced by the substrate temperature and the working gas pressure of the magnetron plasma. Significantly superior ohmic contact of the optimum ITO film with c-Si wafer evolves due to narrowing of the junction barrier height (φB) arising by virtue of increased carrier concentration in ITO, thanks to effective doping by Sn4+. Improved c-Si/ITO junction characteristics admit supplementary application feasibility of ITO films in devices fabrication, particularly as transparent electrodes in c-Si p-n junction solar cells.
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