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Pulsed laser deposition growth of bismuth oxide superconductors

Sajjadi, Amir; (1995) Pulsed laser deposition growth of bismuth oxide superconductors. Doctoral thesis (Ph.D.), University College London (United Kingdom). Green open access

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Abstract

The discovery of high temperature superconductivity in cuprate systems such as YBaCuO, Bi-Sr-Ca-Cu-0, and T1-Ba-Ca-Cu-0 has led to an unprecedented explosion in research in the field because of their potential for technological application. The prospect of integrating these materials in electronic devices depends on the ability to grow good quality thin films. One of the methods of film fabrication increasingly in use is pulsed laser deposition (PLD) technique. This thesis reports on the growth and properties of the Bi2Sr2Can-1.CunO4+2n+[delta] system (n=l, 2) thin films by PLD. In order to deposit high temperature superconducting films uniformly over a large area a new deposition geometry is suggested. The geometry involves tilting the target by an angle [theta] away from its normal horizontal position. A detailed modelling with supporting experimental results of the new geometry is presented. The method is shown to produce homogenous films over an area of several square centimetre. It is also shown that the film uniformity and thickness is dependent on the angle of tilt, [theta], and the substrate-target distance, a. Ex-situ and in-situ thin film growth of the 2212 phase of the Bi-Sr-Ca-Cu-O system by PLD on MgO substrates is demonstrated. For ex-situ process, the growth parameters such as annealing temperature and duration are optimised. It is shown that the annealing temperature is crucial in obtaining a highly c-axis 2212 film. Also, the superconducting properties of the films are shown to be determined by the annealing period. With in-situ processing it is shown that phase control of BiPbSrCaCuO films is possible by varying the substrate temperature and oxygen pressure. Superconducting thin films of the Pb-doped Bi-Sr-Ca-Cu-O system (2223 phase) on MgO substrates by PLD are demonstrated. In the initial programme of thin film growth of the 2223 phase, BiPbSrCaCuO films were deposited from a single (BiPb)SrCaCuO target. It is shown, however, that this approach is not effective in obtaining a high proportion of the 2223 phase ([greater than] 50[percent]). This is found to be due to the evaporation of Pb at an early annealing stages from the film. To overcome this problem an approach of depositing multilayers of PbO between several layers of BiPbSrCaCuO is utilised. Using the multilayered (BiPb)SrCaCuO/PbO technique Pb could in sufficient quantity be doped in the BiSrCaCuO films to facilitate the growth of the 2223 phase. It is demonstrated that with the multilayered method, superconducting Bi2Sr2Ca2Cu3O10+[delta] films containing more than 85[percent] 2223 phase could be grown by annealing at 854[degrees]C for 15 hours in air. The effect of target composition on the thin film growth of the 2223 phase is also investigated. It is shown that an excess Ca and Cu over the stoichiometric composition is effective in promoting the formation of the 2223 phase significantly, however, excess Cu is found to also favour the formation of the (Sr14-x Cax)Cu24O41-y phase. In this study it is also demonstrated that the growth of the 2223 phase depends strongly on the annealing temperature and duration and that the temperature range in which the 2223 phase is produced, in the multilayered BiPbSrCaCuO/PbO films, preferentially is exceedingly narrow. The optimum annealing temperature and duration for the formation of the 2223 phase with the multilayered technique is found to be 854[degrees]C ([plus-minus]1[degrees]C) for 15 hours in air. The effect of Ar:O2 annealing of the multilayered films on the formation of the 2223 phase is also investigated. It is shown that Ar:O2 annealing lowers the melting point of the BiPbSrCaCuO films considerably. The critical current density of the 2223 BiPbSrCaCuO films is found to be dependent on the proportion of the (Sr,Ca)14Cu24O41 phase in the films. It is shown that the presence of the (Sr,Ca)14Cu24O41 phase lowers the critical current value of the film considerably. Critical current density values as high as 3.7x103 and 1.0x104 A/cm2, in zero field at 70K were obtained for the 2223 BiPbSrCaCuO films. The successful patterning of 2212 and 2223 films by wet etching process for device application is demonstrated. The design and development of a superconducting microstrip filter for space applications using the 2223 films is attempted. Although the device does not fully show the filter characteristic, the measured response is very similar in shape to the simulated response, though no sharp band-pass response is measured.

Type: Thesis (Doctoral)
Qualification: Ph.D.
Title: Pulsed laser deposition growth of bismuth oxide superconductors
Open access status: An open access version is available from UCL Discovery
Language: English
Additional information: Thesis digitised by ProQuest.
Keywords: (UMI)AAI10017246; Applied sciences
URI: https://discovery.ucl.ac.uk/id/eprint/10099796
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