Laser Deposited YBa2Cu3O7-x Thin Films and Heterostructures: Growth and Characterization

Doktorsavhandling, 1993

YBa2Cu3O7-x (YBCO) superconducting thin films and heterostructures were made by laser deposition and electrical and microstructural properties were studied. Electric field effect devices were manufactured and investigated. YBCO films were deposited on several different substrates, such as MgO, yttriastabilized ZrO2 (YSZ), SrTiO3, LaAlO3, and NdGaO3, and were optimized with respect to critical temperature (Tc), critical current (jc), and microwave surface resistance (RS). Tc:s in excess of 90 K were measured for films on all substrates, and a jc of 5x106 A/cm2 at 77 K and an RS of 85 µ.OMEGA. at 4.2 K and 10 GHz were obtained on YSZ. A detailed study was performed on films on YSZ substrates. It was found that the crystallographic orientation relations between the film and the substrate depended strongly on the deposition temperature and the substrate morphology. It was concluded that steps in the substrate surface induced a graphoepitaxial alignment of the film. The steps also eliminated disorder in the films. The integration of YBCO in different heterostructures is essential for many applications. Heterostructures in the form of YBCO/insulator/YBCO trilayers were deposited and characterized. Single films of NdGaO3 and PrGaO3, and multilayers of SrTiO3 and PrGaO3, were used as insulators. The trilayers with single-film insulators occasionally showed high insulation, ~109 .OMEGA.cm at 300 K for NdGaO3, but often suffered from pinholes causing short circuits. An interaction layer was also observed between YBCO and PrGaO3. These problems were overcome by introducing thin and equally spaced SrTiO3 films in the PrGaO3 and at the YBCO/PrGaO3 interfaces. It was found that voids developed in the PrGaO3 above a critical thickness of ~40 nm. The voids were eliminated by limiting the PrGaO3 thickness, by means of the intermediate SrTiO3 films. These SrTiO3/PrGaO3-insulator trilayers showed an insulation resistivity of 2x109 .OMEGA:cm at 77 K for interface areas up to 1500 µm2. Outgrowths commonly observed on the YBCO bottom layers did not affect the insulation, but were continuously covered by the SrTiO3/PrGaO3. A Tc of 88 K and a jc of ~106 A/cm2 at 77 K were measured for the top YBCO layer. The electric field effect was investigated in polycrystalline YBCO films (~200 nm) deposited on amorphous buffer layers, and in ultrathin (~4.5 nm) YBCO films deposited on Pr1Ba2Cu3O7-x templates. Pb(Zr,Ti)O3 (500 nm) was used as gate insulator on the polycrystalline films and a 7% increase of the normal resistance close to Tc (86.5 K) was observed at a gate voltage of + 1.8 V. The ultrathin films had a Tc of 50 K and a jc of 106 A/cm2 at 4.2 K. The gate insulator consisted of crystalline SrTiO3 (80 nm) and amorphous SrTiO3 (300 nm), and the gate area was ~800 µm2. A maximum jc modulation of 2.4% was observed when gate voltages of ±80 V were applied. jc was modulated at temperatures between 13.5-40 K, and was enhanced and depressed by negative and positive gate voltages respectively.