Superconducting MgB 2 thin films were prepared on Al 2 O 3 (0001) and MgO(100) substrates. Boron thin films were deposited by the electron-beam evaporation followed by post-annealing process with magnesium. Proper post annealing conditions were investigated to grow good superconducting MgB 2 thin films. The X-ray diffraction patterns showed randomly orientated growth of MgB 2 phase in our thin films. The surface morphology was examined by scanning electron microscope (SEM) and atomic force microscope (AFM).Critical current density (J c ) measured by transport method was about 10 7 A/cm 2 at 15 K, and superconducting transition temperature (T c ) was ~ 39 K in the MgB 2 thin films on Al 2 O 3 .PACS numbers: 74.25. Fy, 74.60.Jg, 74.70.Ad, 74.76.Db a) Corresponding author, e-mail: smoon@LG-Elite.com 2 The recent discovery of the superconductivity above 39 K in magnesium boride (MgB 2 ) material attracts many researchers in scientific as well as technical reasons. 1 This material seems to have conventional BCS type superconductivity, and it has the simple structure. 2 The possibility of making good superconducting MgB 2 wires with low-cost was reported by several groups, because the weaklink problem between grains does not seem to be the case in this material. [3][4][5] The possibility of the electronic device application is also wide open, since it becomes possible to operate devices made of MgB 2 thin films with a low-cost refrigerator because of the higher T c ~ 39 K than other conventional superconductors. In addition, it may be easy to make very reliable electronic devices and Josephson junctions with this material, because of its simpler crystal structure and longer coherence length compared with the oxide superconductors. To make electronic devices, MgB 2 thin films with good superconducting properties are essential. Several groups have reported MgB 2 thin films made by pulsed laser deposition (PLD) method or e-beam evaporation followed by post annealing. [6][7][8][9][10][11] In this paper, we report our results on the growth of MgB 2 thin films by the electron-beam evaporation method followed by post-annealing process. The evaporation method has an advantage to the PLD method to make large area thin films. We investigated optimum growth conditions to make good superconducting MgB 2 thin films on Al 2 O 3 (0001) and MgO(100) substrates. Some superconducting transport properties (T c and J c ) and surface morphology of the MgB 2 thin films were also investigated.To make MgB 2 thin films, we have started with the boron thin films deposited on the substrates, similar to the MgB 2 wire formation by Canfield et al. 4 The boron thin film was deposited by the electron beam evaporation from boron source in crucible. We have used two different substrate temperatures for the boron deposition, room temperature and 750 °C. The background pressure of the deposition chamber was below 1 x 10 -6 Torr. Typically 250 ~ 300 nm thick boron films were made with the deposition rate of ~ 2 Å /sec. The boron films as deposited...
Since the discovery of superconductivity in MgB 2 1 considerable progress has been made in determining the physical properties of the material, which are promising for bulk conductors 2-5 . Tunneling studies [6][7][8][9] show that the material is reasonably isotropic and has a well-developed s-wave energy gap (∆), implying that electronic devices based on MgB 2 could operate close to 30K. Although a number of groups have reported the formation of thin films by post-reaction of precursors 10-14 , heterostructure growth is likely to require considerable technological development, making single-layer device structures of most immediate interest. MgB 2 is unlike the cuprate superconductors in that grain boundaries do not form good Josephson junctions, and although a SQUID based on MgB 2 nanobridges has been fabricated 15 , the nanobridges themselves do not show junction-like properties. Here we report the successful creation of planar MgB 2 junctions by localised ion damage in thin films. The critical current (I C ) of these devices is strongly modulated by applied microwave radiation and magnetic field. The product of the critical current and normal state resistance (I C R N ) is remarkably high, implying a potential for very high frequency applications.Our film deposition technique has been described elsewhere. 12 Briefly, B films were deposited at room temperature on to (0001) sapphire substrates by electron beam evaporation and then ex-situ annealed in a Mg vapour at 850°C for 30 minutes to produce MgB 2 films with a final thickness of 100nm and a critical temperature (T C ) of 36K. We deposited a 20nm Au film onto the films by dc magnetron sputtering in order to protect the MgB 2 from contact with water during subsequent processing. Tracks and contact pads were patterned in the bilayer film using standard photolithography and broad beam Ar-ion milling. The T C of the patterned tracks was approximately 35K. In order to fabricate the junctions, the film was then transferred to a focused ion beam system (FIB) (Philips-FEI Inc. FIB 200). Chips were wirebonded to enable the resistance of the tracks to be monitored during the FIB milling process. 16 The barrier was defined by writing 50nm wide cuts across the width of tracks using a 4pA 30kV Ga ion beam. The depth of the cut was calibrated by comparing the cut time to that required to completely sever a track; since the Au mills more than order of magnitude faster than the MgB 2 it could be ignored in calibrating the cut depth. Compared to other materials used to fabricate SNS junctions using this technique, 17 MgB 2 offers the advantage of a relatively low milling rate and hence excellent depth control due to the low atomic mass of its constituents and high melting point.The devices were measured in dip probes placed in 4 He storage dewars. The probes are equipped with coils to provide a magnetic field perpendicular to the substrate plane, and with a microwave antenna. The inset to Fig. 1 shows the resistance against temperature for two tracks on the same substrate,...
Ion-induced secondary electron emission from the MgO protecting layer of an AC plasma display panel (AC-PDP) and the successive back-scattering of secondary electrons toward the cathode due to high gas pressure have been studied theoretically. A correlation between the intrinsic secondary electron emission coefficient (γ i ) and the MgO surface properties such as band gap energy and defect states in the band gap was found. The behavior of the effective secondary electron emission coefficient (γ eff ) of various ion species as a function of the reduced electric field (E/P) has also been investigated. These relationships can be applied to AC-PDP manufacturing in order to enhance the luminous efficiency and to reduce the breakdown voltage of AC-PDPs.
The Raman spectrum of the superconductor MgB 2 has been measured as a function of the T c of the film. A striking correlation is observed between the T c onset and the frequency of the E 2g mode. Analysis of the data with the McMillan formula provides clear experimental evidence for the collapse of the electron-phonon coupling at the temperature predicted for the convergence of two superconducting gaps into one observable gap. This gives indirect evidence of the convergence of the two gaps and direct evidence of a transition to an Since the discovery of superconductivity in MgB 2 ͑Ref. 1͒, a large volume of work has been directed towards developing and improving the quality of MgB 2 thin films. 2 The temperature of the onset of superconductivity (T c ) in these films has varied considerably, although there is no consensus on the mechanism behind the T c suppression. Nonetheless, theoretical models suggest that a two-band superconductivity model is needed to explain the high T c in MgB 2 . 3-6 As interband scattering increases perhaps due to impurities in the boron layer, the two gaps in MgB 2 are predicted to converge into one gap with ⌬ ϭ⌬ ϭ4.1 meV; 6 this transition is predicted to occur as the T c is suppressed below 25 K. 6 Measurements of the phonon energies in YBa 2 Cu 3 O 7Ϫ␦ helped the understanding of the relation between the crystal structure and the T c in this material. 7 Therefore, similar measurements made on MgB 2 films with different T c 's may help to elucidate the mechanisms behind the suppression of the critical temperatures in MgB 2 , while casting light on the nature of the two-band superconductivity.Numerical calculations indicate that in MgB 2 there are two phonon modes involving vibrations along the c axis (A 2u and B 1g ) and two doubly degenerate modes with vibrations only in the ab plane (E 1u and E 2g ); 8 for the P 6 mmm space group to which MgB 2 belongs, only the E 2g mode is Raman active. 9 Predictions for the frequency of the E 2g mode vary between 515 cm Ϫ1 ͑64 meV͒ ͑Ref. 3͒ and 665 cm Ϫ1 ͑82 meV͒, 10 while experimental studies have shown a broad Raman mode between 600 cm Ϫ1 and 630 cm Ϫ1 . 9,11-18 The consensus is that this mode is the E 2g mode, however, there remains controversy about this assignment, 9,14 with Kunc et al. 9 concluding that this mode was due to a ''contaminant phase at the sample surface.'' In this communication, Raman spectroscopy has been used to characterize the quality of MgB 2 films both in terms of impurity phases and disorder. The effects of a suppressed T c on the phonon mode at ϳ600 cm Ϫ1 were studied. Both magnesium stoichiometry and oxygen contamination may play a role in determining the T c . 19,20 It has been shown previously that as-grown films of MgB 2 can have significant magnesium concentration gradients through the thickness of the film. 21This effect is equally likely where a precursor boron film has been annealed in magnesium vapor. Therefore, measurement of the superconducting properties of a post annealed boron film through the thickness...
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