Electrophysical properties of sintered polycrystalline semiconductors

As of now such semiconductor oxides as ZnO, Sn02 and Ti02 are most widely used as operational sensor elements. This is initially explained by the vast amount of experimental data gathered for above compounds and on the other hand by the importance of their being used as catalysts in various reactions. Finally, this can be explained by the fact that they are most suitable from the stand-point of requirements 

Zinc oxide is a thoroughly studied typical semiconductor of n-type with the width of forbidden band of 3.2 eV, dielectric constant being 10. Centers responsible for the dope electric conductivity in ZnO are provided by interstitial Zn atoms as well as by oxygen vacancies whose total concentration vary within limits 10 - 10 cm. Electron mobility in monocrystals of ZnO at ambient temperature amounts to 200 cm -s . The depth of donor levels corresponding to interstitial Zn and oxygen vacancies under the bottom of conductivity band is several hundredth of electron volt [18]. 

Semiconductor films of ZnO used as operational elements are obtained by oxidation at - 500 - 600°C in the jet of purified oxygen of zinc film deposited at vacuum iP 10 Torr) on substrates made of fused quartz with subsequent sintering at - 350°C at high vacuum conditions [34]. As it was concluded in paper [17] the sintered polycrystalline sample obtained in such a manner should not be considered as a set of various separate crystallites touching each other but rather as a monolithic pattern in which microcrystals with diameter of 1-10 pm are linked with each other by bridges with length and thickness of the order of 0,1 pm (see Fig. 2.4). 

In order to interpret correctly the results of electrophysical measurements conducted on vacuum - sintered ZnO semiconductor films one should answer the question concerning the origin of contacts between specific crystallites controlling the electric conductivity of the material. This was accomplished in paper [37] using the method of prerelaxation VAC to run a comparative analysis of mechanisms of charge transfer in thin sintered (in vacuum) films and pressed polycrystalline ZnO samples. 

The field dependence of electric conductivity of a thin sintered film (/) and a pressed ZnO sample (2) [37] 

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