Studies On The Preparation And Characteristics Of MgB₂ Superconducting Thin Films By E-beam Evaporation And In Situ Annealing

Thin films of MgB₂ show potential for its application in superconducting electronics due to its good electronic properties. A high-quality MgB₂ superconducting thin film is the prerequisite for a superconducting device. The in situ annealing technique of film preparation has advantage in multilayer device. Therefore studies of in situ annealing techniques are important to produce high-quality MgB₂ superconducting thin films for superconducting electronic applications.MgB₂ superconducting thin films on Al₂O₃ and Si substrate were prepared by e-beam evaporation and in situ annealing techniques. The precursor films had a multilayer structure of [B (100A) + Mg (151A)]_N, The repeat number N, depends on the total film thickness required. We chose annealing temperature as: 550℃, 600℃, 625℃, 650℃, 675℃, 700℃, 725℃, 750℃, 800℃, and annealing duration as: 10 minutes, 20 minutes, 30 minutes, 1hour, 2 hours, and 4 hours. Main results are listed as follows.1. Uniform MgB₂ superconducting thin films on Al₂O₃ substrates with an extremely sharp transition width (AT_c) of 0.1K and zero-resistance temperature (T_∞) of 30.3K were prepared by annealing at 630℃for 25 minutes. 0.1K is the smallestâ–³T_c, up to now, in the MgB₂ superconducting films prepared by in situ annealing techniques, indicating a film of high uniformity and purity in its phase.2. MgB₂ superconducting thin films on Si substrates without a buffer layer annealed at 630℃for 25 minutes had a zero-resistance temperature (T_∞) of 32.8K and critical current density (J_c) of 3.3×10⁵A/cm². This transition temperature is the highest one in the MgB₂ superconducting thin films on Si substrates without a buffer layer or seed layer prepared by in situ annealing techniques.3. The procedures of fabricating MgB₂ superconducting thin films deposited by e-beam evaporation were optimized, a) In order to prevent the escape of magnesium in annealing, a boron layer of about 100A was selected as the top layer of the precursor. b) To reduce contamination of the residual oxygen, a magnesium layer of about 100A, functioned as oxygen absorbent, was deposited on the chamber wall before precursor depositing and annealing respectively. Our experiments showed that this could improve effectively transition temperatures of MgB₂ superconducting thin films, c) To block Mg diffusion into Si substrate, the bottom layer of boron (100A) was designed in the precursor and expected to act as a buffer layer of magnesium diffusion into silicon. Although it could not prevent Mg completely diffusion into Si substrate. However, zero-resistance temperature was increased remarkably.4. The effects of different annealing temperatures and annealing durations on superconducting MgB₂ thin films were investigated. It suggests that MgB₂ thin films may produce at an annealing temperature of 550℃to 800℃. The annealing duration depends on the annealing temperature. An optimal duration of 20 minutes to 30 minutes was found for an annealing temperature between 625℃and 800℃. The annealing duration shall be increased when the annealing temperature is below 625℃. For example, the Tco of a MgB₂ superconducting thin film on Si substrate annealed at 550℃was increased by 11.7K when its duration was increased from 30 minutes to 2 hours. But too large annealing duration will result in more of Mg evaporation and diffusion into silicon substrate'then degrade the film quality of MgBe. The high resistivity of MgB₂ superconducting thin films on Si substrates was also discussed. We suggested that the oxygen contamination and the formation of Mg₂Si may be its main cause.