Grain Boundary Conduction Control in Polycrystalline Semiconductors

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In this research project, we have investigated the electrical conduction in polycrystalline semiconductors, with emphasis on the electrical conduction of polycrystalline tin oxide (SnO_2) thin films deposited on glass substrate by the chemical vopr deposition method. For the purpose we have constructed an automatic measurement system of Hall effect in a wide temperature range by using a computer. In this system we were able to measure Hall effect of a series of SnO_2 polycrystalline thin films in the temperature range from 15 K to 300 K, and also analyze automatically data of carrier concentration and mobility of SnO_2 films. As a result, we have obtained the following results: i) F impurities in SnO_2 films deposit at grain boundaries and reduce carrier scattering, resulting in the reduction of resistivity of SnO_2 films, ii) the mobility of electrons in SnO_2 films is determined by the scattering due to neutral and ionized impurities of F in SnO_2 which are locarized at the region of grain boundaries in this polycrybtalline semicondcutors, iii) we have also tried to dope other impurities such as 3d trandition metals or rare earth metals to find a new impurity in SnO_2 films with low resistivity, but at present could not find an effective impurity for the purpose.