Fabrication And Properties Of Epitaxial Stannate Thin-films And Heterojunctions With The Perovskite Structure

Transparent and conductive oxide (TCOs) thin films are widely used in solar cell, flat-panel displays and light-emitting diodes, etc., due to their unique properties of optical transparency in the visible range and controllable electrical conductivity from almost insulating to degenerate semiconducting behaviors. The common TCOs films, such as ITO, doped SnO2, and ZnO, are usually used in polycrystalline or amorphous form, and all these applications use transparent conductive oxides as simple passive electrical or optical coatings. However, with the continual effort to search for new TCOs materials and especially the discovery of p-type TCOs, a new application field of"transparent electronics"was created. On the other hand, with the rapid development of full-oxide electronic devices, the research and application of all-perovskite heterojunction appear very important, which will make strict demands on lattice parameter, interface and compatibility of different TCOs materials. It is therefore becoming more important and interesting to search for new TCOs films with perovskite structures.In this dissertation, we have grown epitaxially new doped-SrSnO3 and BaSnO3 TCOs films with perovskite structures by laser ablation, and the structural, electrical, and optical properties of films as functions of doped contents and deposition conditions were studied in detail. Stannate- manganate heterojunctions exhibiting very good rectifying property were also fabricated, and rectifying properties and magnetoresistance effects were analyzed and discussed.This dissertation consists of five chapters. The arrangement of the chapters and the main contents in each chapter are presented as follows:In chapter 1, we make a brief introduction to the basic properties, research history and applications in all fields of the transparent and conductive oxide thin films, and the research progress of full transparent devices and manganite-based heterojunctions were also reviewed briefly.In chapter 2, the samples preparation method, samples characterizations and samples measurement are introduced, including the principle and method of the pulsed laser deposition (PLD), the operational principle and methods of characterization of x-ray diffraction, scanning electron microscope, atomic force microscope and x-ray photoelectron spectroscopy, and test principles and methods of Magnetic Property Measurement System (MPMS) and Physical Property Measurement System (PPMS).In chapter 3, the effects of doping of Sb on Sn site and Nd on Sr site on the structural, electrical and optical properties of SrSnO3 films have been extensively studied. It was found that these films exhibited high optical transmittance above 90% in the wavelength region of 300-2500nm, and behaved as an n-type semiconductor with good conductivity. The Sb-doped SrSnO3 films showed a cubic perovskite structure with a lattice constant of about 4.04 ? and a direct allowed band gap of 4.53eV. The minimal resistivity of 21m?cm was observed at room temperature in the 5% Nd-doped SrSnO3 films and affected by the growth temperature. All results show that new TCOs films with perovskite structures were obtained by doping of SrSnO3.In chapter 4, we report the structural, electrical, and optical properties of epitaxial Ba(SbxSn1-x)O3 (x=0-0.30) (BSSO) films grown on SrTiO3(001) substrates by pulsed laser deposition method. Investigation reveals that the transport and optical characteristics of BSSO films depend very sensitively on Sb-doped contents. Temperature-dependent resistivity measurements show that at low Sb contents (x=0.03, 0.07) the metal-semiconductor transition occurs at 150K and 80K, respectively, and the semiconductor behavior appears in high doped (x=0.15, 0.30) films. The transmittance decreases significantly from about 80% to near zero in the visible region and the optical band gap shifts from 3.48eV to 4.0eV with increasing Sb contents in films. The room-temperature lowest resistivity of 2.43m?cm with carrier density and mobility of 1.65×1021cm-3 and 1.75cm2/V s was obtained in films with doping at x =0.07, All results show that low Sb-doped BSSO films with perovskite structures are new type TCOs films exhibiting good conductivity and high optical transmittance.In chapter 5, two heterojunctions with different structures were fabricated: one p-n junction has a n-LSSO/p-LCMO/NdGaO3(001) structure, and another structure of p-i-n junction is p-LCMO/i-SrSnO3/n-LSSO/SrTiO3. Both of them show very good rectifying behavior over a temperature range of 10-310K. The charge ordering behavior of La0.67Ca0.33MnO3 was induced by the in-plane angle mismatch between LCMO and NdGaO3. Charge ordering phase at the interface of LCMO/SrSnO3 was melted when the magnetic field and electric field is large enough, producing very large negative magnetoresistance (up to -93% under a field of 3T). In another p-i-n junction there was no charge ordering phase. A crossover of MR from negative to positive is observed with increasing bias voltage, which is ascribed to the dominant spin character changing from the majority spin carriers to the minority spin carriers at the LCMO/SSO interface.