Reduced microwave losses of YBa2Cu3O7−δ thin films on electro-optic LiNbO3 crystals

We report on the growth of epitaxial YBa2Cu3O7 thin films on X-cut LiNbO3 single crystals. The use of double CeO2/YSZ buffer layers allows a single in-plane orientation of YBa2Cu3O7, and results in superior superconducting properties. In particular, surface resistance Rs values of 1.4 mΩ have been measured at 8 GHz and 65 K. The attainment of such low values of Rs constitutes a key step toward the incorporation of high Tc materials as electrodes in photonic and acoustic devices.We report on the growth of epitaxial YBa2Cu3O7 thin films on X-cut LiNbO3 single crystals. The use of double CeO2/YSZ buffer layers allows a single in-plane orientation of YBa2Cu3O7, and results in superior superconducting properties. In particular, surface resistance Rs values of 1.4 mΩ have been measured at 8 GHz and 65 K. The attainment of such low values of Rs constitutes a key step toward the incorporation of high Tc materials as electrodes in photonic and acoustic devices.

We report on the growth of epitaxial YBa 2 Cu 3 O 7 thin films on X-cut LiNbO 3 single crystals.The use of double CeO 2 /YSZ buffer layers allows a single in-plane orientation of YBa 2 Cu 3 O 7 , and results in superior superconducting properties.In particular, surface resistance R s values of 1.4 m⍀ have been measured at 8 GHz and 65 K.The attainment of such low values of R s constitutes a key step toward the incorporation of high T c materials as electrodes in photonic and acoustic devices.© 2002 American Institute of Physics.͓DOI: 10.1063/1.1515372͔LiNbO 3 ͑LNO͒ is an essential element of many electronic components, such as electro-optical modulators and surface acoustic wave devices, 1,2 due to its excellent ferroelectric and electro-optic properties.In most of these devices, a radiofrequency ͑rf͒ field is applied by using metal electrodes, in order to suitably modify the mechanical, optical, or electrical properties of LNO.If the metal electrodes are substituted by superconductors, the performances of the devices would be significantly improved.It has been predicted 3 that an enhancement of the bandwidth, operating speed, and electrode efficiency are to be expected, due to the much lower surface resistance of superconducting materials.For such a purpose, the growth of epitaxial thin films of high temperature superconductors ͑HTSs͒ on LNO crystals, with high critical current and low rf losses, is necessary.
The growth of high quality epitaxial YBa 2 Cu 3 O 7Ϫ␦ ͑YBCO͒ films on LNO crystals is a difficult problem, due to the poor structural matching of both materials, and to the unstability of the LNO surface at the high temperature and low pressure required for the epitaxial growth of HTS materials.There have been several reports of YBCO films grown on LNO, most of them using buffer layers. 4 -9Usually, the microstructure and superconducting properties of these films are rather poor, due to the mentioned growth problems.Their rf properties have been scarcely studied, in spite of being a crucial parameter to evaluate the potential applications of these films; there is only one report of surface resistance measurements: high values of 450 m⍀ at 18 GHz and 70 K were measured in Ref. 8 for YBCO films grown on LNO with several buffer layers.Therefore, in spite of the promising potentiality [1][2][3] for improving performances of electrooptic devices, several problems related to materials properties have hampered the implementation of HTS in such devices.
In a previous work, 7 we reported the growth and properties of epitaxial YBCO films grown on X-cut LNO crystals by rf magnetron sputtering, using yttria-stabilized zirconia ͑YSZ͒ buffer layers.Below the superconducting transition, resonators made with these films displayed quality factors 2.8 times larger than those of equivalent Au resonators at 7 GHz and 55 K. 3 However, ͑001͒ YBCO thin films grown on YSZ usually display two in-plane orientations, forming 45°g rain boundaries which set a limit to the superconducting performances of the films.
In this communication we report on the superconducting and rf properties of improved YBCO films, grown on CeO 2 /YSZ buffer layers.The additional CeO 2 layer has been used to overcome the above mentioned difficulty; it allows a single in-plane orientation of YBCO films, and consequently better superconducting properties are obtained.Their high critical current and low surface resistance values make them very promising candidates to substitute Au electrodes in several electro-optic and electroacoustic devices operating at liquid nitrogen temperatures.
Epitaxial YBCO/CeO 2 /YSZ heterostructures were grown on X-cut LNO crystals by pulsed laser deposition using a KrF excimer laser.The total thickness of the double buffer was about 110 nm; YBCO films with thickness t between 95 and 380 nm were studied.Details on the growth conditions and structural characterization may be found elsewhere. 10X-ray diffraction patterns reveal the high quality of the buffer and superconducting layers.Specially, the scans ͑Fig.1͒ indicate that YBCO grows epitaxially, with a single in-plane orientation.
YBCO films display good transport properties, with a resistance ratio R(300 K)/R(100 K) of 2.8, as it may be seen in the inset on Fig. 2. The typical transition width is below 1.5 K, and zero resistance is attained usually above 85 K.
Hysteresis cycles (5 KрTр77 K) were recorded with a superconducting quantum interference device magnetometer, and the critical current J c was calculated using the critical state model.J c is found to decay exponentially with magnetic field, as commonly reported in YBCO films. 11Figure 2 shows the field dependence of J c measured at 77 K, for three YBCO thicknesses.As expected, J c of the thinner film (t ϭ95 nm) displays higher values, likely due to changes in the microstructure of the films; Atomic force microscopy analyses reveal indeed a surface degradation as t increases.Thicker films display lower J c values, weakly dependent on thickness; rf measurements described below were performed on the thicker films, since tϾ is required.
At 5 K and zero field, J c of the 95 nm film reaches 6 ϫ10 6 A/cm 2 .This value is nearly 1 order of magnitude better than those reported for YBCO films grown directly on LNO, 6 and only a factor of 5 lower than those of optimal thin films grown on SrTiO 3 single crystals. 11Low field J c values of 3ϫ10 5 A/cm 2 are obtained at 77 K; these are nearly 1 order of magnitude lower than those of films on SrTiO 3 .Typical values of 2ϫ10 5 A/cm 2 have been reported at 77 K, also from inductive measurements, for YBCO films grown on Y-cut LNO both by pulsed laser deposition and sputtering. 4,6Only Tiwari et al. reported a higher value, J c Ͼ10 6 A/cm 2 , from transport measurements. 5he rf losses of the films were measured with a rutileloaded cavity, following the procedure described in Ref. 12, with two heterostructures grown on 10ϫ10 mm 2 LNO crystals, with YBCO thickness of 230 nm.This resonator was chosen-instead of the coplanar one used in Ref. 3-to avoid the contribution of the dielectric losses of LNO and the buffer layers.The inset in Fig. 3 displays the unloaded quality factor Q as a function of temperature.At 65 K, the resonant frequency is 8 GHz and Qϭ3.7ϫ10 4 ; this is 4.7 times better than that of an identical Cu resonator at the same temperature.Additionally, measurements have been performed on two 700 nm thick YBCO films grown on MgO by a commercial supplier to assure that, when measuring our samples, Q is dominated by the surface resistance of the YBCO and not by the dielectric loss of the rutile.Taking into account the rutile losses, 13 the measured R s at 65 K becomes 2.3 m⍀.After correction for the finite thickness of the film, using the procedure described in Ref. 14 and assuming a temperature-dependent penetration depth with L (0) ϭ150 nm, 15 the value of R s is reduced to 1.4 m⍀ ͑see Fig. 3͒.The actual R s might be even lower, since the penetration depth can be expected to be longer, and thus the finite thickness correction would be more important.On the other hand, to rule out a significant nonlinear behavior of the rf loss in our films, we have measured the quality factor of the resonator in a 30 dB range of the source power ͑from Ϫ16 to 14 dB m͒ at 60 and 70 K and found no significant changes. 16e emphasize that the R s value of our films is nearly 2 orders of magnitude lower than the previously reported one for YBCO films grown on LNO, 8 and only a factor of 4 -10 higher than the R s values of the best YBCO films grown on LaAlO 3 .
In conclusion, we have demonstrated the epitaxial growth of high quality epitaxial YBCO thin films on X-cut LNO substrates using a double CeO 2 /YSZ buffer layer.The good properties of the films, with critical currents above 3 ϫ10 5 A/cm 2 at 77 K and surface resistance of 1.4 m⍀ at 65 K and 8 GHz, are extremely encouraging in order to improve the performances of LNO-based devices by using them as substitutes of metallic electrodes, operating at cryogenic temperatures.More precisely, the X cut of LNO crystals, used in this work, is suitable for some photonic components like electro-optical modulators, and thus the here reported results constitute a key step toward the integration of HTS in these devices.This work has been financed by the Spanish MCyT ͑MAT99-0984 Project͒.L.F. wishes to also thank the MCyT for her contract.

FIG. 1 .
FIG.1.scan of the ͑103͒ peak of YBCO corresponding to a YBCO/CeO 2 /YSZ heterostructure grown on X-cut LNO; the scan of the ͑012͒ peak of LNO is also shown.