Propriedades de transporte elétrico em baixas temperaturas em heteroestruturas Zn1-xCdxO/CdO crescidas pela técnica Spray Pirólise

Abstract

In this thesis we study how the electrical transport properties in Zn1-xCdxO/CdO, heterostructures grown by spray pyrolysis, modified by doping and growing on substrates of glass and silicon. In the glass substrate, the x-doping values in the heterostructures were 0.50; 0.60; 0.75 and 0.95 and in silicon 0.60 and 0.95. CdO and Zn0,40Cd0,60O films were also grown on glass and silicon to analyze the contribution of these layers to heterostructures. From the diffractograms obtained from all the samples, it was verified that all are polycrystalline with phase-centered cubic structure (CFC) and that in the heterostructures grown on glass, the crystallite size is larger with the increase of doping. From the scanning electron microscopy (SEM) images of the surface of the samples, it was found that those grown on a glass substrate are less rough and those grown on silicon have domes on the surface. From the measurements of Hall effects it was observed that the samples are n-type independent of the substrate, having a high concentration of carriers being higher in the samples grown in glass, while the mobility was higher in the samples grown in silicon varying in up to four orders of magnitude. guarantee compared to those grown in glass. The high mobility in the samples grown on silicon was attributed to the domes. Electrical characterization and magnetotransport measurements were performed at temperatures ranging from 1.9 to 300 K and magnetic fields up to 9 T. All samples grown on glass showed metal-insulator transition (TMI) with different transition temperatures. The TMI observed in samples grown on glass is due to the degree of disorder being known as an Anderson-type transition. The heterostructure grown on silicon Zn0.40Cd0.60O/CdO also presented the TMI, but of the Mott type, which was verified by measuring resistance as a function of temperature (RT) applying a magnetic field. The Zn0.05Cd0.95O/CdO heterostructure and the CdO film grown on silicon showed insulating behavior in the RT curves throughout the analyzed temperature range. In the magnetoresistance (MR) curves, all samples grown on glass and the CdO film grown on silicon showed negative magnetoresistance (MRN) due to the weak localization effect. The MR curves of these samples were fitted using the Kawabata 3D model and from the fit it was obtained the phase coherence length and that the mechanism of electrical transport at low temperatures is the electron-electron interaction. As for the MR curves of the heterostructure Zn0.40Cd0.60O/CdO grown on silicon, it can be seen that the MR curves are totally influenced by the substrate since this sample exhibited positive magnetoresistance (MRP) while in the glass it was MRN. Although this sample exhibited high mobility, no wobbling pattern in the MR curves was detected due to high roughness. The Zn0.40Cd0.60O film measurements showed that the MRP exhibited by the Zn0.40Cd0.60O/CdO heterostructure grown on silicon comes from the upper layer. From all the results obtained, it was possible to verify that the heterostructures presented better electrical properties than the CdO films and that the heterostructure with silicon substrate is much more sensitive to the application of magnetic field than the one grown on glass.