43995-AC10
Boride Thin Films and Heterostructures Using Hybrid Physical-Chemical Vapor Deposition
Magnesium diboride, MgB2, is an exciting superconductor. It is a conventional BCS superconductor, in which the Cooper pairs are formed through electron-phonon coupling, with a high transition temperature Tc of 39 K. It has multiple bands with weak interband impurity scattering: the 2-dimensional σ bands and the 3-dimensional π bands. They couple to the B-B stretch modes of E2g symmetry with different strengths, resulting in different superconducting energy gaps. For electronic applications, the high Tc of MgB2 allows operation of MgB2 devices and circuits above 20 K, substantially reducing the cryogenic requirements compared to the Nb-based superconducting electronics, which have to operate at 4.2 K. For applications in high magnetic field, carbon-alloyed MgB2 films have shown higher upper critical field Hc2 values than those of the Nb-based superconductors at all temperatures. MgB2 is of particular interest for magnets in cryogen-free magnetic resonance imaging (MRI) systems. It is recognized that MgB2 is an attractive material for RF cavity applications. The high Tc and low resistivity allows for lower surface resistance than Nb, the currently used superconductor for RF cavities. MgB2 has much higher critical field Hc than Nb, which can lead to higher breakdown field in RF cavities.
For MgB2 devices and circuits, we have made in the past planar MgB2-TiB2-MgB2 SNS Josephson junctions that show excellent properties above 30 K, and trilayer MgB2-barrier-Pb SIS Josephson junctions that show both gaps and excellent MgB2-barrier interface properties. The next step is to fabricate trilayer MgB2-barrier-MgB2 junctions. In this PRF project, we have collaborated closely with Prof. Buhrman's group at Cornell on X-Ray Photoelectron Spectroscopy (XPS) studies on the natural MgO barrier layer on MgB2 films grown by hybrid physical-chemical vapor deposition (HPCVD) used in our MgB2 Josephson junctions. Although this natural MgO barrier produces excellent MgB2-barrier-Pb Josephson junctions, it can not survive the deposition of the MgB2 electrode. Recently, we have used rf-sputtered MgO layer as the barrier to produce MgB2-barrier-MgB2 junctions. In small size junctions (4 μm x 4 μm to 70 μm x 70 μm), we have successfully produced Josephson tunnel junctions showing supercurrent Jc as high as 2 kA/cm2, which remains finite up to near 40 K. The 3 nm MgO barrier survived the deposition of the top MgB2 electrode. Excellent Shapiro steps and Fraunhofer patterns were observed, indicating high quality Josephson tunneling. This is an important breakthrough that laid the foundation for MgB2 superconducting digital circuits that work above 20 K.
In the past, we have produced carbon alloyed MgB2 films using bis(methylcyclopentadienyl)magnesium, ((MeCp)2Mg), a metalorganic magnesium precursor, during the HPCVD deposition. Transmission electron microscope (TEM) results show that the carbon-alloyed MgB2 films contain grain boundaries between the carbon doped MgB2 grains. These films showed extraordinarily high Hc2 over 60 T, much higher than any MgB2 samples reported by other groups. However, the grain boundaries in such films reduce the critical current density. In this PRF project, we have studied to replace (MeCp)2Mg with Trimethylboron, (TMB, B(CH3)3), as the carbon source. TMB has much simpler structure than (MeCp)2Mg and can break up more completely. The results indicate that using TMB to dope the MgB2 films with carbon is a better approach for getting more uniform films. While Tc is suppressed by carbon doping, the resistivity increases with carbon content in a much slower pace than in carbon-alloyed MgB2 films using (MeCp)2Mg. After optimization of the growth conditions, we have achieved the best Hc2 result ever reported: a high dHc2/dT slope at Tc as high as 8T/K. A film with 36 K Tc has a dHc2/dT of 6T/K. Using this to extrapolate to zero temperature, Hc2(0) would be over 200T. This result is extraordinary if it can be confirmed by high magnetic field measurement. The difference of the new samples from previous samples is that the growth temperature and the growth rate are both lower, and the new measurements were done in patterned samples. The new results open up a new direction for the study of the mechanism of high Hc2 in carbon doped/alloyed films.
