Fabrication of magnesium diboride (MgB2) thin films at STI
Since the discovery of superconductivity in MgB2, there has been marked progress in the ability to grow thin films of this material by the techniques of pulsed laser deposition, molecular beam epitaxy, and hybrid physical chemical vapor deposition. However, the efficacy of most of these methods is deleteriously affected by the particular challenges of MgB2 film growth, meaning they must employ sub-optimal growth temperatures or undesirable ex situ or in situ annealing procedures. Numerous difficulties thus remain regarding the routine deposition of completely in situ, smooth, robust, high-quality films which are useful for applications. In addition, the deposition method should also be compatible with multilayer deposition, double-sided deposition, and the growth of films on large-area substrates of technological interest.
STI has developed a patented growth technique for MgB2 thin films which offers several advantages over existing methods and is the first deposition method to enable the production of high-quality MgB2 films for real-world applications.
STI films possess among the highest TC values and lowest resistivity values reported. The TC values of our MgB2 films are 38 to >39 K, on par with the bulk value. The transition to the superconducting state is sharp, with widths ΔTC of ~0.2 K. These observations indicate that the films are clean. Films grown by our technique also have outstanding RF properties. The surface resistance (Rs ) values of our films are among the lowest reported in the literature.
Our growth method enables fabrication of high-quality films on a number of different substrates, including single-crystal r-plane sapphire, c-plane sapphire, m-plane sapphire, 4H-SiC, MgO, LaAlO3 , NdGaO3 , LaGaO3, LSAT, SrTiO3 , YSZ, Si, and GaN. We have also grown high-quality films on titanium, niobium, and flexible metal and dielectric substrates. The technique enables deposition of high-quality films on any substrate for which there is not a chemical reaction with Mg or B. We have developed MgB2 thin film technologies for flexible flat ribbon cables, superconducting electronics, and superconducting cavities. At present we can deposit MgB2 wafers up to 4" in diameter. These are the largest-area MgB2 films available; moreover our apparatus is scalable to much larger sizes.