2962 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 15, NO. 2, JUNE 2005 Control of Porosity and Composition in Large-Area YBCO Films to Achieve Micrometer Thickness and High on Sapphire Substrates Katherine D. Bagarinao, Hirofumi Yamasaki, Jiacai C. Nie, Mariappan Murugesan, Haruhiko Obara, and Yoshihiko Nakagawa Abstract—Relatively thick YBCO thin films (thickness ) ideal for fault current limiter as well as microwave applications have been successfully prepared by large-area pulsed laser deposition (PLD) on -buffered sapphire substrates. The attainment of an unusually high film thickness (up to 2.0 ) without microcracking is attributed in part to the presence of pores correlated with yttrium-rich composition in the films. The effect of using YBCO targets with varied Y:Ba:Cu ratios on film porosity and surface morphology was investigated in detail. Using the optimum target composition, uniform critical current densities ranging from at to at have been achieved. Characterization of a film with showed low mi- crowave surface resistance values [ and at 22 GHz] comparable to the best YBCO films reported by other studies. Index Terms— , large-area, sapphire, YBCO. I. INTRODUCTION A MONG the available substrates for YBCO films, sap- phire is the most ideal candidate for applications in high power/high current and microwave applications, primarily due to its availability in large areas (up to 8 in diameter), good mechanical strength, and superior properties such as very high thermal conductivity and very low microwave loss tangent ( at 10 GHz). YBCO films with high current-carrying ca- pacity, as denoted by high critical current per unit width ( , where , ), are necessary to increase the nominal power for practical utilization in resistive fault current limiter applications [1], [2]. Moreover, thicknesses in the range of are desired to improve performance as microwave components [3]. However, due to the differences in the thermal expansion coefficients between YBCO and sapphire, microcracking occurs in the film beyond a critical thickness of [4]. Microcracking results to a drastic decrease in the value of and limits the Manuscript received October 5, 2004. This work has been carried out as a part of the Super-ACE project (R&D of fundamental technologies for supercon- ducting AC power equipment) of the Ministry of Economy, Trade, and Industry (METI). The authors are with the National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8568, Japan (e-mail: kathy@ni.aist.go.jp). Digital Object Identifier 10.1109/TASC.2005.848682 Fig. 1. SEM images of YBCO films deposited using various target compositions. (a) Target 1, ; (b) Target 2, ; (c) Target 3, ; (d) Target 1, ; (e) Target 2, ; (f) Target 3, . thickness of YBCO that can be grown on sapphire. This im- poses a severe limitation on the value of the critical current that can be achieved in YBCO films. In a recent study, we have shown that microcracking can be prevented in YBCO films grown under conditions unique to a large-area PLD system [5]. There are three unique properties of the films that enabled the attainment of a film thickness greater than 1 , namely: 1) yttrium-rich composition, 2) high porosity, and 3) high defect density. Whereas porosity was considered beneficial to increase film thickness without microcracking, we have observed that porous films also possess relatively low values of . Furthermore, porous films also tend to degrade faster due to exposure to air and humidity. In this study, we seek to elucidate the relationship among target composition, porosity, surface morphology, and in particular, . We will demonstrate that an effective control of porosity and composition in YBCO films can enable the attainment of relatively high even for thick films. II. EXPERIMENTAL PROCEDURE Both buffer layers and YBCO films were deposited on -cut sapphire substrates using a large-area PLD system utilizing a KrF excimer laser source (248 nm, Lambda Physik LPX 305i). buffer layers were deposited at a rate of 1–2 nm/min, with thicknesses ranging from 30–50 nm. These were further subjected to a high-temperature annealing 1051-8223/$20.00 © 2005 IEEE