Epitaxy of Hg-HTS Superconducting Films and Fabrication of Hg-HTS Microwave Devices
The discovery of the first high temperature superconuctor (HTS) by Bednoz and Miller in 1986, which won them the Nobel prize, triggered a worldwide effort in HTS research because of the tremendous potential of these materials in applications. Among many other HTSs, Hg-based HTS (HgBa2Can-1CunO2n+2, n=1,2,3,�), or Hg-HTSs, have the highest superconducting transition temperature Tc of 135 K. The highly volatile nature of the Hg-based compounds, however, makes epitaxy of Hg-HTS films the toughest challenge so far in the HTS material research. We have developed several new processes including an alkaline doping assisted process to promote liquid phase formation which accelerates the formation of Hg-HTS phase, and a fast temperature ramping process to bring the processing temperature directly to the window so as to minimize the formation of the impurity phases. Using these new processes, we have demonstrated high-quality Hg-1223 films with Tc>130K and followed with many interesting studies on these films.
Despite the many exciting results on Hg-HTS films that her group achieved early on, two problems hindered further progress: poor run-to-run reproducibility and severe film/substrate reaction. These generic problems associated with epitaxy of volatile compounds in conventional material processing prompted us to invent a non-conventional cation exchange process. This process employs a precursor matrix of similar crystalline structure and chemical composition to the desired material, but without the volatile cations such as Hg. By providing perturbations to the guest cations on the sites near the final sites of the volatile cations, the guest cations can be replaced with volatile cations without collapsing the crystal structure, like an �atomic surgery� over an existing crystal lattice. The microscopic mechanism of the cation exchange has been a focus of our group in recent years and question we would like to answer is how it occurs at microscopic scale, what are the relevant processing parameters, and can it be applied to design a new material using exiting ones.
The increased research interest in the microwave applications of high temperature superconductors (HTS) was brought about by the perceived potential on the marketability of superconducting electronics, especially in the wireless communications industry. Excellent surface morphology and high reproducibility of the HgBa2CaCu2O6+d (Hg-1212) thin films made from cation exchange process has motivated us to develop microwave bandpass filters, which can be operated at above 77 K and therefore are much more cost effective. High quality Hg-1212 three-pole filters have been successfully fabricated. The filters exhibited lower insertion loss even at higher operating temperature, compared with bandpass filters made of YBCO and copper with the same mask. The power handling capability of the filter was characterized by monitoring the third-order intermodulation signals. The power scaling of IM3 products in the Hg-1212 filter with regard to input power was around 2.8:1, well consistent with theoretical predicted scaling: 3:1. The IP3 of the Hg-1212 filter was 51 dBm at 90K, which was comparable to that of YBCO at 77K. This improved performance was attributed mainly to its higher Tc, which makes Hg-1212 a promising alternative material for passive microwave devices.