Properties Of ZnO Thin Films And Heterostructures With Perovskite Oxides

Wurtize ZnO is a direct wide-band gap semiconductor. It is attractive because of itspotential applications in many fields including blue-ultraviolet light emitting diodes,optoelectronic devices, diluted magnetic semiconductors, gas sensors, transparentconducting oxides as well as transparent thin film transistors. Due to its lack ofinversion symmetry, wurtzite ZnO is a polar oxide with a polar direction along the caxis. The spontaneous polarization along the c-axis will affect the structural, optical,electrical, and chemical properties of ZnO thin films and nano-structures. In this thesis,we succeeded in fabricating the epitaxial ZnO thin films and heterojunctions withperovskite oxides. Our focus is mainly on the influence of polarity on the properties ofthe ZnO thin film and heterostructures. Our investigations are of importance fordeveloping new functional ZnO related devices.With the method of pulsed laser deposition, nonpolar (110)(a plane) and polar(001)(c-plane) oriented ZnO epitaxial thin films can be grown simultaneously on the(001) and (110) oriented SrTiO3substrates, respectively. Although all the growthparameters are the same, the resistivity of the a-plane ZnO film is much higher than thatof the c-plane film. We attribute the larger resistivity of a-plane ZnO thin films to thebarrier height at the grain boundaries induced by the in-plane spontaneous polarization,which enhances the scattering of electrons passing through the grain boundaries. Ourexperiment data can be well fitted by the theoretical model, yielding a barrier height ofabout0.22eV at room temperature.Nb-doped SrTiO3(NSTO) is conductive. Both nonpolar ZnO(110)/NSTO(001) andpolar ZnO(001)/NSTO(110) epitaxial heterostructures with good rectifyingcharacteristics are fabricated. We found that at the same voltage bias, the current ofnonpolar ZnO(110)/NSTO(001) heterostructure is larger than that of the polarZnO(001)/NSTO(110) heterostructure. We proposed that during the growth of the polarZnO thin films on the NSTO substrates, the inevitable temperature gradient across theNSTO substrate generates a layer of excess negative charges on the substrate surfacedue to the significant pyroelectric effect of NSTO. Consequently, the positively chargedZn ions should be deposited first on the NSTO substrate. The as-prepared ZnO film is terminated by O layer on the free surface with the spontaneous polarization pointing tothe surface. The negative bound polarization charges at the interface would bend theenergy band upward, and increase the barrier height of the heterostructure. So thec-ZnO/NSTO(110) heterostructure showed larger resistance than the other. Theheterostructures of ZnO with p-type perovskite oxides, LSMO and LCMO, also showgood rectifying property. The current of ZnO/LCMO/NSTO(001) heterostructure islarger than that of the ZnO/LCMO/NSTO(110) heterostructure, which can be explainedby the influence of polarity of ZnO as well.Giant negative magnetoresistance was observed in the heterostructure of ZnO(110)/LCMO/NSTO(001). In order to investigate the mechanism of magetoresistance,we also made LCMO/NSTO heterostructures with different orientations. Both LCMO/NSTO(001) and LCMO/NSTO(110) heterostructure showed good rectifying propertiesand significant magnetoresistance. We also find that the magnetoresisatnce of theformer heterostructure is much larger than that of the latter. The underlying mechanismneeds further investigations.