Surface resistance and residual losses of Agdoped YBa2Cu3O7−δ thin films onsapphireR. Pinto, P. R. Apte, M. S. Hegde, and Dhananjay Kumar Citation: Journal of Applied Physics 77, 4116 (1995); doi: 10.1063/1.359496 View online: http://dx.doi.org/10.1063/1.359496 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/77/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Microstructural dependence of penetration depth of Agdoped YBa2Cu3O7−δ thin films probed by atomicforce microscopy Appl. Phys. Lett. 68, 1720 (1996); 10.1063/1.115917 Twodimensional growth model for laserablated Agdoped YBa2Cu3O7−x thin films J. Appl. Phys. 77, 5802 (1995); 10.1063/1.359159 Surface resistance, residual losses, and granularity in Agdoped YBa2Cu3O7−δ thin films J. Appl. Phys. 75, 4258 (1994); 10.1063/1.355965 Large critical currents and improved epitaxy of laser ablated Agdoped YBa2Cu3O7−δ thin films Appl. Phys. Lett. 62, 3522 (1993); 10.1063/1.109014 Improved microwave performance of Agdoped Y1Ba2Cu3O7−δ thin film microstrip resonators J. Appl. Phys. 73, 5105 (1993); 10.1063/1.353783

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

130.113.111.210 On: Tue, 23 Dec 2014 01:39:21

Surface resistance and residual losses of Ag-doped YBa2Cu307-S thin films on sapphire

R. Pinto and P. R. Apte Tata Institute of Fundamental Research, Homi Bhabha Road, Bombay-400 005, India

M. S. Hegde and Dhananjay Kumar Indian Institute of Science, Bangalore- 012, India

(Received 7 July 1994; accepted for publication 6 January 1995)

High-quality Ag-doped YBa&u,O,-, thin films have been grown by laser ablation on R-plane (1102) sapphire without any buffer layer. Thin films have been found to be highly c-axis oriented with T, -90 K, transition width AT< 1 K, and transport .7,= 1.2X IO6 A cme2 at 77 K in self-field conditions. The microwave surface resistance of these films measured on patterned microstrip resonators has been found to be 530 fl at 10 GHz at 77 K which is the lowest reported on unbuffered sapphire. Improved in-plane epitaxy and reduced reaction rate between the substrate and the film caused due to Ag in the film are believed to be responsible for this greatly improved microwave surface resistance. 0 199.5 American Institute of Physics.

Sapphire is the preferred substrate for microwave device applications due to its moderately small dielectric constant (-10 at 10 GHz), low loss tangent (<10e5 at 300 K, <low6 at 77 K), good mechanical strength, and high thermal conductivity. ’ -3 Obviously, therefore, many attempts have been made in the past to grow high-quality YBa,Cu,07-8 (YBCO) thin films on sapphire for microwave passive device applications.‘-lo But the main problem with sapphire is that it reacts with YBCO at temperatures required to realize epi- taxial 1:2:3 phase of YBC0.4*” Furthermore, due to a com- bination of large lattice mismatch between sapphire and YBCO (sapphire has a rhombohedral primitive cell, usually described using a simpler hexagonal unit cell with a,=4759 A and c= 12.991 A, whereas YBCO unit cell has a=382 A, b=3.89 A, and c= 11.68 A)i2 and nonorthogonal axes in the sapphire surface there occur wide dispersions in c axis and in-plane alignments of YBCO film on sapphire. There- fore, YBCO films on unbuffered sapphire reported so far are of very low quality as compared to those on standard sub- strates like (100) SrTiO, or (100) LaAlO,. In fact, the best reported YBCO films on sapphire show an order of magni- tude lower critical current density, J, (at 77 K) and higher microwave surface resistance, R, than those realized in state- of-the-art YBCO films on LaAlO,.

One of the standard solutions to the problem of chemical reactivity of YBCO with sapphire has been through the use of a buffer layer such as Mg0,2 CeOz ,as3 SrTiO, ,6,7 or yttria stabilized zirconia (YSZ).‘*lo Besides inhibiting the reaction at the YBCO-sapphire interface, a buffer layer also provides a gradient for lattice and thermal expansion mismatch. Con- siderable work has been reported on YBCO films on buffer layers on sapphire.“4 However, the best reported YBCO films with Jca 1 X lo6 A cme2 at 77 K with YSZ, MgO, and CeOz bufferst” and R,=850 fl with SrTiO, buffer7 and 530 fl with MgO buffer2 both at 77 K at 10 GHz are not as good as the state-of-the-art YBCO films on LaAlO,.

It was in this context that we thought that doping of YBCO with Ag which has given beneficial results in bulkI and has shown significant improvement in film microstruc-

ture with J, in the range 0.8-1.4X107 A cm-z at 77 K on SrTi03 (Ref. 14) and R,Y as low as 210 ,u.Q at 77 K at 10 GHz on LaAlO, (Refs. 15 and 16) may be the key to realize high quality YBCO films directly on sapphire. Indeed, the results obtained with Ag-doped YBCO films on sapphire have been extremely encouraging with R,=530 pf1 at 77 K at 10 GHz which is the lowest reported on unbuffered sapphire. This implies that microwave circuits with Au on A1203 can be directly replaced with Ag-doped YBCO films on sapphire. This letter describes the first results obtained on the growth and microwave measurements on the Ag-doped YBCO films on sapphire.

Ag-doped YBCO films were in situ grown directly on R-plane (liO2) sapphire using pulsed laser deposition (PLD) technique. 7 wt % Ag-doped YBCO pellets with 15 mm di- ameter and 3 mm thickness and prepared using standard solid state reaction process were used for the growth of films. In situ film growth was carried out using Lambda Physik 301 KrF excimer laser with a repetition rate of 5 Hz in an oxygen pressure of 300 mTorr. The sapphire substrates were of 0.5 mm thickness and were held on the heater using silver paste. The substrate surface temperature was about 700 “C and the film growth rate was -120 &nin at a laser fluence of -3 J cmm2.

Ag-doped YBCO films grown on sapphire were charac- terized by four-probe dc resistance measurements. Transport .I, measurements were carried out on laser patterned micro- bridges of 2000~A-thick films. Shown in Fig. 1 is the dc resistance-temperature plot of Ag-doped YBCO film on sap- phire. The films show a good metallicity with a zero resis- tance transition at 90 K and a transition width -1 K. The films also showed a fairly high transport J,=1.2X106 A cmV2 at 77 K in self-field conditions.

Structural characterization of Ag-doped films on sap- phire was carried out by x-ray diffraction. Shown in Fig. 2 is the x-ray diffraction spectrum of a typical film indicating a high degree of c-axis orientation on R-plane (1102) sapphire.

Microwave measurements were carried out on microstrip resonators fabricated on 10 mmXl0 mmXO.5 mm sapphire

4116 J. Appl. Phys. 77 (8), 15 April 1995 0021-8979/95/77(8)/4116/3/$6.00 Q 1995 American Institute of Physics

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

130.113.111.210 On: Tue, 23 Dec 2014 01:39:21

TEMPERATURE(K)

FIG. I. Resistance vs temperature plot of Ag-doped film on sapphire; Z’,-SO K, AT- 1 K.

substrates using 4000-A-thick Ag-doped YBCO film. The resonators were half-wavelength (h/2) transmission type ob- tained by etching the film into a straight pattern with a line width of 500 p for a 50 fi impedance on sapphire. The resonator length was adjusted to give a resonance at 10 GHz. Both the input/output ports were loosely coupled through capacitive gaps between the resonator and the SMA connec- tor pins such that the measured Q-Q,, the unloaded Q-factor of ‘the resonator. The measurements were carried out using a HP83620A synthesized sweeper and a HP8757C scalar network analyzer with the associated reflectometry setup. Low-temperature measurements were carried out in the range 20-85 K using a closed cycle He cryocooler and a temperature controller. The microwave power levels used for measurements varied from-20 to 10 dBm. Further details of the microstrip resonator measurement set up are described elsewhere.”

As is well known, the Q. of resonators, obtained by calculating the ratio flAf (where f is the resonant frequency and Af is the 3 dB bandwidth of resonant curves), is related to Q, , Qd, and Q, through the expression

5 15 25 35 45 55 65

26' (DEG)

FIG. 2. X-ray diffraction spectrum of 4000-A-thick Ag-doped YBCO film on sapphire.

POWER (dBm1

FIG. 3. Variation of R, of 4000-A-thick Ag-doped YBCO film on sapphire with microwave power measured at 10 GHz at 20 and 77 K.

l/Q,= l/Qo- llQd- l/Q,,

where Q, , Qd, and Q, are Q factors due to conductor, di- electric, and radiation losses, respectively. Since for sapphire tan S (- 1 /QJ< 10m6 at 77 K and since radiation losses have been minimized by providing effective shielding around the device, l/Q,+ 1/Q,-10-5. The values of R, corresponding to various vahtes of Q, were then calculated using the ex- pressions given by Puce1 et al. I7

Shown in Fig. 3 is the variation of R, with microwave power to the resonator at 20 and 77 K. It is evident that although the resonators show a small power dependence at both the temperatures, it is not very significant. The variation of Q. and R, with temperature is shown in Fig. 4 for 0 dBm power. Here, we can consider R, values at two temperatures namely, 77 and 20 K. As seen in the figure, R, at 77 K=530 fl. This is the lowest reported so far on unbuffered sap- phire.

The R, value at 20 K is 330 ,&I, which, although better than 1 mbl at 13 GHz at 4.2 K reported by Char et al. on unbuffered sapphire,4 is higher than 65 ,&I at 10 GHz at 10 K reported by Char et al. on sapphire with SrTiO, buffer.’ The reason for this discrepancy is the presence of Ag at the grain boundaries of Ag-doped YBCO films which introduces the metallic contribution to the residual surface resistance at very low temperatures as shown by us earlier.16

However, the observation of a low R, of 530 ,L& at 10 GHz at 77 K on unbuffered sapphire is important. Obviously, the presence of Ag in films during growth is responsible for the much improved quality of the films on sapphire. We have shown earlier that Ag enhances the oxygen incorporation in the film during growth.t8 This effect, in conjunction with the considerably higher adatom mobility of Ag on the substrate, has been shown to improve the microstructure of Ag-doped YBCO films on LaAJO, by grain enlargement and align- ment. l4 We believe that these two mechanisms are also active on sapphire which effectively reduce the growth temperature by about 50 “C thereby reducing the substrate-film interac- tion. There is also a third mechanism based on the catalytic behavior of Ag which facilitates material transport through a

J. Appl. Phys., Vol. 77, No. 8, 15 April 1995 Pinto et al. 4117 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

130.113.111.210 On: Tue, 23 Dec 2014 01:39:21

800 -

G 3 Q?

400 -

IO GHz OdBm

- 1500

8

- 1000

- 500

ot 1 I I I 1 I 0 20 40 60 77 80 loo0

TEMPERATURE(K)

FIG. 4. Variation of Q, and R, with temperature for 4000-A-thick Agdoped YBCO thin tilm microstrip resonator on unbuffered sapphire obtained at 10 GHz for 0 dBm microwave power.

liquid-phase kind of diffusion and accelerates the formation of the l-2-3 phase. The accelerated formation of the l-2-3 lattice, as observed in bulk Ag-YBCO com- posites, l9 further reduces the substrate-film interaction. Al- though the in-plane alignment in these films is not as good as that realizable in Ag-doped YBCO films on LaAlO, or SrTiO, primarily due to the large lattice mismatch, it is be- lieved that the reduced substrate-film interaction is the ma- jor factor responsible for the much improved quality of these filIIlS.

In conclusion, we have realized high-quality Ag-doped YBCO films on sapphire without buffer layers. The results show an R, value of 530 fl measured at 10 GHz at 77 K which is the lowest reported on unbuffered sapphire and is slightly higher than that of state-of-the-art undoped YBCO films on LaAlO,. With such a low value of R, , Ag-doped YBCO films on sapphire appear to be highly attractive for

superconductive microwave circuits on sapphire which could replace the current Au-on A&O, microwave circuits. Further work is currently being carried out.

‘X. D. Wu, R. E. Muenchausen, N. S. Nogar, A. Pique, R Edwards, B. Wiiens, T. S. Ravi, D. M. Hwang, and C. Y. Chen, Appl. Phys. Lett. 58, 304 (1991).

‘G. C. Liang, R. S. WU.hers, B. F. Cole, S. M. Garrison, M. E. Johansson, W. S. Ruby, and W. G. Lyons, IEEE Trans. Appl. Supercond. 3, 3037 (1993).

“G. C. Liang, R. S. mthers, and B. F. Cole, IEEE Trans. Microwave Theory Tech. MTT-42, 34 (1994).

‘K. Char, D. K. Fork, T. H. Geballe, S. S. Laderman, R. C. Taber, R. D. Jacowitz, F. Bridges, G. A. H. Connell, and J. B. Boyce, Appl. Phys. L&t. 56, 785 (1990).

‘X. D. Wu, k Inam, M. S.,Hegde, B. Wilkens, C. C. Chang, D. M. Hwang, L. Nazar, T. Venkatesan, S. Miura, S. Matsubara, Y. Miyasaka, and N. Shohata, Appl. Phys. L&t. 54, 754 (1989).

bJ. J. Kingston, F. C. Wellstood, P. Lerch, A. H. Miklich, and J. Clarke, Appl. Phys. Lett. 56, 189 (1990).

7K. Char, N. Newman, S. M. Garrison, R. W. Barton, R C. Tabor, S. S. Laderman, and R. D. Jacowitz, AppI. Phys. Lett. 57,409 (1990).

*A. B. Berezin, C. W. Yuan, and A. L. de Lozanne, Appl. Phys. Lett. 57,90 (1990).

‘S. Watanachi, S. Patel, D. T. Shaw, and H. S. Kwok, Appl. Phys. L&t. 55, 295 (1989).

‘OH. Schmidt, K. Hradil, W. Hosler, W. Wersing, G. Gieres, and R. J. See- bock, Appl. Phys. Lett. 59, 222 (1991).

“M Naito R. H. Hammond, B. Oh, M. R. Hahn, J. W. P. Hsu, P. Rosenthal, . , A. F. Marshall, M. R. Beasley, T. H. Geballe, and A. Kapitulnik, I. Mater. Res. 2,713 (1987).

“A. S. Cooper, Acta Cryst. 15, 578 (1962). 13T. H. Tiefel, S. Jin, R. C. Sherwood, M. E. Davis, G. W. Kammlott, P. K.

Gallagher, D. W. Johnson, Jr., R. A. Fastnacht, and W. W. Rhodes, Mater. Lett. 7, 363 (1989).

14D Kumar M. Sharon, R Pinto, P. R. Apte, S. P. Pai, S. C. Purandare, L. C: Gupta,‘and R. viljayaraghavan, Appl. Phys. Lett. 62, 3522 (1993).

“R. Pinto, N. Goyal, S. P. Pai, P. R. Apte, L. C. Gupta, and R. Vijayaragha- van, J. Appl. Phys. 73, 5105 (1993).

16P R Apte, R. Pinto, A. G. Chourey, and S. P. Pai, J. Appl. Phys. 75, 4258 (i99.4).

17R. A. Pucel, D. J. Masse, and C. P. Hartwig, IEEE Trans. Microwave Theory Tech. MTT-16, 342 (1968).

“R Pinto, D. Kumar, S. R Pai, A. G. Chourey, and P. R Apte, Supercond. Sci. Technol. 7, 95 (1994).

“N. L. Wu, H. D. Yu, and S. H. Wei, Appl. Phys. Lett. 64, 2932 (1992).

4118 J. Appl. Phys., Vol. 77, No. 8, 15 April 1995 Pinto ei a/. [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

130.113.111.210 On: Tue, 23 Dec 2014 01:39:21