IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 11, NO. 1, MARCH 2001 3419

Microwave and Structural Properties of Y B ~ , C U , ~ , - ~ Films on R-cut Sapphire Buffered

with Post-annealed CeO, Layer

W. I. Yang, J. H. Lee, J. M. Jang, J. S. Ryu, Jung Hur and Sang Young Lee

Abstract-Effects of the post-annealing temperature of CeOz buffer layer on the microwave and structural properties of YBCO films on Ce0,-buffered sapphire (CbS) were investigated using YBCO films grown on CeOz buffer layer post-annealed at temperatures of 950 - 1100 "C. YBCO films on post-annealed CbS appeared to have better properties than those on as-grown CbS with regard to the morphological, structural and microwave properties when the YBCO films were prepared on CeOz buffer layer post-annealed at temperatures of 1000 - 1050 "C. The TE,,, mode Q of a rutile-loaded cavity resonator with YBCO films on CbS post-annealed at 1000 - 1050 "C showed the unloaded Q as high as 1.8 x 105 at 77 K at 8.6 GHz, a value more than 2 times the corresponding value with the YBCO films replaced by those on as-grown CbS. Also, the decrease in the unloaded Q of a microstrip resonator made of the YBCO film on CbS post-annealed at 1000 "C appeared less than 3 O/O at 60 K for input power increase of 10 dBm, while the corresponding value was about 24 O/O for a microstrip resonator prepared from the YBCO film on as-grown CbS.

Zn&x Terms-YBa&u,O,, film, microwave properties, CeO, buffer layer, structures.

I. INTRODUCTION

YB~,CU,O~.~ (YBCO) films on Ce0,-buffered r-cut sapphire have been studied for their applicability for high power microwave devices. Advantages in using sapphire include the high thermal conductivity of sapphire, its extremely low loss tangent [2] and the availability of large sapphire substrates with diameter of more than 4 inches. It is possible to grow YBCO film epitaxially on Ce0,-buffered sapphire. The surface resistance (Rs,yBco) of YBCO films on CbS have been reported low enough to meet the requirements for microwave devices in transmitter modules [ 11. Recently, we reported that both Rs,yBco, the surface morphology and crystal structures of YBCO films on CbS could be greatly

Manuscript received September 18, 2000. This work was supported in part by Korean Ministry of Science and Technology and Ministry of Information and Communication under the university research program.

Sang Young Lee, W. I. Yang, J. H. Lee, J. M. Jang, and J. S. Ryu are with the Department of Physics and Center for Advanced Materials and Devices, Konkuk University, Seoul 143-701, Korea (Telephone : +82-2- 450-3 166, e-mail : sylee@kkucc.konkuk.ac.kr).

Jung Hur is with the Department of Electronic Engineering, Konkuk University, Seoul 143-701, Korea.

improved by using CbS post-annealed at 1000 "C, with the typical peak-to-valley surface roughness of about 3 nm and R,,yBco of about - 300 @ at 77 K and 10 GHz for 300 nm- thick YBCO films [3]. However, the relations between the post-annealing temperature of the CeO, layer and the properties of YBCO films deposited on the CeO, layer have not been studied yet.

This paper describes microwave and structural properties of YBCO films grown on post-annealed CbS at various temperatures. Microwave properties of YBCO films on post- annealed CbS were compared with each other using TEoiI mode rutile-loaded resonators and microstrip ring resonators. Enhanced properties of YBCO films on post-annealed CbS are described.

11. EXPERIMENTAL

45 nm-thick CeO, buffer layers were grown in-situ on 12 x 12 x 0.5 mm3 r-cut sapphire using an on-axis RF magnetron sputtering method. The post-annealed CeO, buffer layer was prepared by heat treatment of the as-grown CeO, buffer layers at 950 - 1100 "C for an hour inside a furnace. Later 400 nm-thick YBCO films were deposited on CbS by an off- axis dc magnetron sputtering method at the rate of 1.1 nm/sec. Detailed descriptions for the deposition conditions for YBCO and CeO, are described elsewhere. X-ray diffraction (XRD), atomic force microscope (AFM), and Scanning Electron Microscope were used for investigating the structures of the CeO, buffer layers and the YBCO films. To study the microwave properties of the YBCO films, a TFi,,, mode rutile-loaded cavity resonator with YBCO endplates was prepared using a rutile rod sandwiched between two YBCO endplates of a cavity made of oxygen-free high purity copper (OFHC). The temperature dependence of the unloaded Q (Qo) of the mol, mode resonator was measured in the experiments. The rutile rod, supplied by Crystec, has the dimensions of 3.88 mm and 2.73 mm for the diameter and height, respectively, with the corresponding values of 9 mm and 2.73 mm for the dimensions of the cylindrical cavity. 320 nm- thick YBCO films were also prepared on as-grown CbS, post-annealed CbS at 1000 "C and LaAlO,, respectively, which were later patterned into the form of a microstrip ring resonator with silver ground for studying the microwave power dependence of Q,. The inner and outer diameters of the microstrip ring resonator are 1.70 mm and 1.79 mm,

1051-8223/01$10.00 0 2001 IEEE

3420

respectively. The surface resistance of the YBCO films could be calculated from the measured Qo. A HP8510C network analyzer was used in measuring the microwave properties. Details for measurement procedures have been reported elsewhere [4 - 61.

111. RESULTS AND DISCUSSION

A. Surface Morphology and Structures of CeO, layers and YBCO Plms

AFM data for the 45 nm-thick CeO, layers revealed that the peak-to-valley distance in the surface profile (henceforth called 'the R-factor') became smaller when the as-grown CeO, layers were post-annealed at temperatures higher than 950 "C. For instance, the R-factor of the as-grown sample Ce- A was 3.8 nm, which became as small as 0.92 nm (Ce-Pl), 1.5 nm (Ce-P2), 0.69 nm (Ce-P3) and 1.4 nm (Ce-P4) after post-annealing of sample Ce-A at the temperature of 950 "C, 1000 "C, 1050 "C and 1100 "C, respectively. XRD data of both as-grown and post-annealed CeO, buffer

layers showed only (100) peaks, indicating epitaxial growth of CeO, layers on r-cut sapphire. Also, the rocking curve data for the (400) peak appeared smaller than that of as-grown CeO,, revealing improved crystalline structures of CeO, after the post-annealing. YBCO films with 400 nm thickness were prepared on as-grown CbS (sample YBCO-A) and on post-annealed CbS (samples YBCO-P1 - YBCO-P4). Ce-A and Ce-P1 - Ce-P4 were used for this purpose, respectively. XRD data revealed that all the YBCO films were epitaxially grown along the caxis regardless of the post-annealing treatment for the CeO, buffer layers. Figure 1 shows the XRD data for a YBCO film on a post-annealed CbS at 1000 "C (sample YBCO-P2), where only (001) peaks are seen with the FWHM of (8 - 28) reflex of (005) peak being 0.19". Am for the (005) peak of the YBCO film appeared to be 0.48" for sample YBCO-P2 as seen in the inset of Fig. 1, which is much smaller than Am = 0.65" for the YBCO film on as- grown CeO, (sample YBCO-A). Am s for the (005) peak of YBCO films on post-annealed CbS at 1050 "C and 1100 "C

"10 20 30 40 50 60 70 80 90 100 28 (degree)

Fig. 1 . X-ray diffraction data of a 400 nm-thick YBCO film (sample YBCO-P2) on r-cut sapphire buffered with a 45 nm-thick CeOz layer post- annealed at 1000 'C. Inset: w-scan of the (005) peak of sample YBCO-P2. The FWHM value is 0.48O.

(samples YBCO-P3 and YBCO-P4) also appeared much smaller than that of YBCO-A with the respective value of 0.46 " and 0.47 ", revealing significant improvements in the crystalline quality of YBCO films on post-annealed CbS. Also, the resistance data of YBCO films on post-annealed CbS appeared better than those of YBCO on as-grown CbS. For instance, the transition width ( A n and R(300K)/R(IOOK) ratio are 1 K and 3, respectively, for YBCO-P2 - YBCO-P4, while the corresponding values are 2 K and 2.8, respectively, for YBCO-A. Also, the surface smoothness of YBCOs on post-annealed CbS appeared significantly improved compared to that of YBCOs on as-grown CbS. AFM pictures revealed that the R-factor was 96 nm for sample YBCO-A, while the values became significantly smaller with the magnitudes of 9.1 nm, 4.5 nm, 7.9 nm and 8.6 nm for samples YBCO-P1 - YBCO-P4, respectively. Figures 2(a) - (d) show AFM images of samples YBCO-A, YBCO-PI, YBCO-P2 and YBCO-P4. Detailed descriptions for the YBCO films are listed in Table 1. As seen in Fig. 2(a), the observed high R-factors for YBCO on as-grown CbS are mainly attributed to large outgrowths of YBCO. Meanwhile, outgrowths were rarely observed for YBCO films on post- annealed CbSs at 1000 - 1050 "C as seen in Fig. 2(c) for sample YBCO-P2, allowing very small R-factors. It is noted that no microcracks were observed in the SEM images of all the YBCO samples.

Our results are consistent with our previous reports on 140 nm-thick and 300 nm-thick YBCO films on post-annealed CbSs, where the R-factor appeared to be as small as 3.2 nm for the 300 nm-thick YBCO film on post-annealed CbS at 1000 "C [3]. We note here that the R-factor of the 400 nm- thick YBCO film on CbS became the smallest when the CbS was post-annealed at 1000 "C, with those of the YBCO films on CbSs post-annealed at higher temperatures of 1050 "C and 1100 "C appearing a little higher. Our observation is also similar to what has previously been reported for YBCO films

Table 1 . Data for 300 nm-thick YBCO films on r-cut sapphire substrates buffered with as-grown and post-annealed 45 nm-thick CeO, layers. The post-annealing temperature for the CeO, buffer layer is from 950 OC to 1100 a c .

Sample Smoothness 20-FWHM Rocking curve No. (4 (degree) FWHM(degree)

Many hillocks (200) 0.25 (200) 0.5 YBCO-A 963 (400) 0.5 (400) 0.57

(005) 0.65 (005) 0.2 rms : 284 (200) 0.25 (200) 0.52 (400) 0.43 (400) 0.5 (0051 0.19 (0051 0.6

27 rms : 91

Y BCO-P 1

" C (200) 0.25 (200) 0.5 (400) 0.42 (400) 0.5 (005) 0.19 (005) 0.48

' t J

YBCO-PZ rms : 1 1

(200) 0.52 (200) 0.23 (400) 0.42 (400) 0.49 (005) 0.19 (005) 0.46

79 rms : 31

YBCO-P3

(200) 0.4 (200) 0.24 (400) 0.38 (400) 0.42 (005) 0.16 (005) 0.47

86 rms: 19

YBCO-P4

342 1

I

Fig. 2. AFM pictures of 400 nm-thick YBCO films on r-cut sapphire buffered with (a) an as-grown 45 nm-thick CeOz layer (YBCO-A) , (b) a post-annealed 45 nm-thick CeOz layer at 950 "C (YBCO-Pl), (c) a post- annealed 45 nm-thick CeOz layer at 1000 "C (YBCO-E), (d) a post- annealed 45 nm-thick CeO, laver at 1100 "C fYBCO-P4).

on post annealed MgO [7], However, the mest reasons for the improved surface smoothness of YBCO on post-annealed CbS have not been addressed yet.

B. Microwave Properties of YBCO Jilms on Post-annealed CbS

We also measured the "Eoll mode Qo of the rutilsloaded cavity resonator with the endplates replaced by samples YBCO-PI - P4. Figure 3 shows Qo vs temperature data for the rutile-loaded resonator with YBCO films used as the endplates. The resonant &equency is about 8.5 - 8.7 GHz. In the figure, Qo appears as high as 470000,21oooO, and 72000 at 40 K, 60 K and 77 K, with sample YBCO-A used as the endplates. When sample YBCO-A vim replaced by another sample YBCO-P3. Qo became as high as 620000, 280000, and 180000 with the values significantly larger than the corresponding Qo for sample YBCO-A. Also, Qo appeared as high as 690000,230OO0, and 110000 at 30 K, 60 Kand 77 K, with sample YBCO-P2 used as the endplates. In Fig. 3, we see that the microwave properties of YBCO on CbS could be improved when the CbS was post-annealed at temperatures of

10'

0

io4

0 asgrown 0 Q5O0C : A looooc :

TemPe-e 6) I . I . I . I . , . , . , . , .

20 30 40 50 60 70 80 90 100 Temperature (K)

Fig. 3. The temperature dependence of the T b l l mode Qo of the rutile- loaded cavity resonator with YBCO endplates. Each YBCO sample can be identified by the preparing conditions of the CeOz buffer layer.

1000 - 1050 "C. Meanwhile the rutile-loaded cavity resonator with YBCO-P1 and YBCO-P4 appeared to have lower Q, than that with YBCO-A, even though their crystal structures appeared better than those of YBCO on as-grown CbS.

Based on our observations described above, we also compared the microwave power dependence of YBCO films on CbS post-annealed at 1000 "C with those of YBCO films on as-grown CbS and LAO. For this purpose, three microstrip ring resonators were prepared using 320 nm-thick YBCO films grown on as-grown CbS (YBCO-AR), CbS post- annealed at 1000 "C (YBCO-PR) and LAO (YBCO-LR), respectively, for which the power dependence of the resonator Q were measured and compared with each other. Figure 4(a) shows the Qo vs temperature data for samples YBCO-AR, YBCO-PR and YBCO-LR for input powers of 0 dBm and 10 a m . In the figure, we see that Qo is the highest for YBCO-PR with the value of 7900,6600 and 5 100 at 30 K, 60 K and 77 K, respectively, at the resonant &quency of 11.2 GI& which are significantly highex than the corresponding values for YBCO-AR and YBCO-LR Moreover, while the Qovalues of YBCO-AR and YBCO-LR decreased as much as 24 % and 12 % for input power increase of 10 a m , respectively, the decrease in the corresponding value appeared less than 4 % for YBCO-PR At 60 K, the decrease in Qo of YBCO-PR appeared even smaller, showing a decrease of 2.6 %. This observation is very interesting, considering that YBCO on CbS can be used for fabricating microwave transmitter modules dealing with high power and that commercialized microwave devices made of high-T, superconductor films are currently designed to operate at 60 K. From the measured QOt we also calculated the effective microwave surfirce resistance (&@j for YBCO- PR, Yl3CO-AR and YBCO-LR with the loss tangent of sapphire and LAO assumed to be 1 x lo4 and 1 x lo5, respectively. From the calculation, Rs@turned out to be 520 pi2 and 1150 pi2 at 77 K at 11.2 GHz for YBCO-PR and

‘a’ 8000 -

6000 - CY a

0 0

3 0 4000:

3 2000- . 0

n A

As-grown(l0 dBm) Annealing ( 0 dBm) Annealing (IO dBm) LAO( 0 dBm)

” r A LAO(l0 dBm) I 10 20 30 40 50 60 70 80 90 100

Temperature (K)

l o - tb ’ i o ’ i o ’ i o ’ i o ’ 60 ’ i o ‘ S O ’ i o , * i o Temperature (K)

Fig. 4. (a) The temperature dependence of microstrip resonator Qo of samples YBCO-PR, YBCO-AR and YBCO-LR. (b) Rsd vs T data for samples YBCO-PR, YBCO-AR and YBCO-LR at IO GHz. Inset: Rs vs P data for samples YBCO-PR, YBCO-AR and YBCO-LR at IO GHz. Note that the meaning of each symbol is the same as in Fig. 4(a).

YBCO-AR, respectively, and 540 pC2 at 77 K at 7.9 GHz for YBCO-LR. We could also calculate R:8at 10 GHz using the relation of R, cc f ’, which are 410 p&2 and 920 pL2 for YBCO-PR and YJ3CO-AR, respectively, and 850 pL2 for YESCO-LR. The inset of Fig. 4(b) shows the calculated RSu vs T data for the YBCO films used for the microstrip resonator. Since the film thickness of 320 nm is comparable to the penetration depth (A) of YBCO at 77 K, we used the so called ‘phenomenological equivalence method’ to take the radiation loss into account in calculating intrinsic surface resistance (R,) from the measured Rf [5, 6, 81. From the calculation, R, at 10 GHz appeared to be 240 p2 and 510 at 77 K for YBCO-PR and YBCO-AR, respectively, and 500 p2 at 77 K for YBCO-LR. The relation AV) = A,.[l - (T/TJ2J’ was used in calculating Rs with Tc = 89 K and the assumed value A,= 200 nm. The calculated R, vs T data are displayed in Fig. 4(b).

SUMMARY

Summarizing, the surface smoothness of 400 nm-thick YBCO films on r-cut sapphire buffered with post-annealed

CeO, (CbS) at 950 - 1100 “C appeared significantly improved compared to those on as-grown CbS.

Microwave properties of YBCO films on post-annealed CbS appeared superior to those of YBCO films on as-grown CbS when the post-annealing temperature for the CeO, layer is between 1000 “C and 1100 “C. The “E$,, mode Q of a rutile-loaded cavity resonator with YBCO films on CbS post- annealed at 1000 - 1050 “C showed the unloaded Q more than 2 times of the corresponding value with the YBCO films replaced by those on as-grown CbS. Also, the microwave properties of a microstrip resonator made of 320 nm-thick YBCO film on post-annealed Ce0,-buffered sapphire at 1000 “C appeared to have superior power dependence to those on YBCO film on as-grown CbS, with the decrease in the unloaded Q of only 2.6 % at 60 K for input power increase of 10 dBm for the former, and about 24 YO for the latter. The observed intrinsic surface resistance of the 320 nm-thick YBCO film appeared to be as low as 240 pC2 at 10 GHz and 77 K.

ACKNOWLEDGMENT

The authors express sincere thanks to Y. B. KO and Y. S. Chung for their helps in preparing the samples and Jung Hun Lee and J. Lim for their helps in preparing this manuscript.

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