METHOD FOR IMPROVING THE SURFACE SMOOTHNESS, THE CRYSTAL STRUCTURE AND THE MICROWAVE SURFACE RESISTANCE OF YBa2CU307-8 HIGH-TEMPERATURE SUPERCONDUCTOR FILMS GROWN ON CeO2-BUFFERED r-CUT SAPPHIRE SUBSTRATES

BACKGROUND OF THE INVENTION Field of the Invention This invention generally relates to a method for improving the properties of Yl3a2Cu307 8 high temperature superconductor films on CeO2-buffered r-cut sapphire substrates. In particular, the present invention is directed to a method for improving the surface smoothness, the crystal structure and the microwave surface resistance of a Y-Ba, Cu, O,-, film by post-annealing a CeO,-buffered r-cut sapphire substrate before growing YBa2Cu3O7-# high temperature superconductor film on the substrate, and for obtaining atomically smooth as-grown CeO2-buffer layer on r-cut sapphire.

Description of the Prior Art To date, YBa2Cu3O7-# (hereinafter, referred to as"YBCO") superconductor films grown on (100) LaAlO3 (hereinafter"LAO") substrate have been widely used for manufacturing microwave elements for receivers. The reasons are that the YBCO film grown on a LAO substrate has a low surface resistance of about 200 HQ at 10 GHz and 77 K, the loss tangent of the LAO substrate has a relatively small value, and the permittivity of the LAO substrate is greater than 25. Thus, the integration of a circuit through the use of YBCO can be obtained.

However, in cases where the magnitude of a signal processed by microwave elements is large such as in a transmitter module, problems of uneven heat distribution may be occur when the heat conductivity is low as in a LAO. When a YBCO film is grown onto a sapphire substrate, the heat conductivity of the sapphire substrate is higher by

twenty times than that of the LAO substrate, and the loss tangent of the sapphire is very low in about 10-5 and 10-7 at 300 K and 77 K. In addition, for a sapphire substrate, it is possible that a diameter of more than 4 inches can be provided whereas in case of a LAO substrate, a diameter of only about 2 inches can be achieved. Further, there is additional advantage as a YBCO film can be epitaxially grown on an r-cut sapphire substrate with use of an epitaxially grown CeO, film as the buffer layer. The CeO2 buffer layer has the lattice constant corresponding to that of the YBCO and also prevents the reaction between the YBCO and the sapphire. The arrangement of lattices on r-plane [or (1102) plane] is nearly rectangular. The spaces between the lattices in the directions [1011] and [1210] are 0.512 nm and 0.4759 nm, respectively. On this point, the differences in lattice constant of sapphire and lattice constant ou Ce- in both directions are 5.7% and 13.7%, respectively, which shows the large differences between them. Nevertheless, since the difference in the lattice constant of CeO, and lattice constant of the YBCO is very small (less than 1%), it is possible to grow YBCO films on CeO2 epitaxially. In addition, the fact that the coefficients of thermal expansion of the YBCO film (ayaco) and sapphire (aA, 2O3) are ayBCO= 13 x 10-6 K-l and aA, 2O3 = 6 x 10-6 K-'with the coefficient of thermal expansion of CeO2 (αCeO2) of αCeO2 = 11.6 x 10-6 K-', the value between ayaco and OCCeO2l also shows effectiveness of using the CeO2 film as a buffer layer.

However, problems exist in the prior art for growth of a high temperature superconductor YBCO film on a CeO2-buffered r-cut sapphire substrate (hereinafter "CbS"). Problems include that the surface roughness and the crystal structure of the YBCO film grown on CbS is generally much worse than those of the YBCO film grown on single crytal substrates such as LAO and SrTiO3 (STO) substrates due to differences in the surface properties between the single crystal substrates and the CeO, buffered- sapphire, and that the microwave surface resistance of YBCO film on CbS is generally higher than that of YBCO film on single crystal substrate such as LAO.

SUMMARY OF THE INVENTION In order to solve the above-noted problems in the art, it is an object of the

present invention to provide a method for improving the surface property, the crystal structure and the microwave surface resistance of thin film when high temperature superconductor YBCO film is grown on CbS, which is useful for preparing high-quality YBCO films for electronic applications.

The object is achieved by a method that improves the surface property and the crystal structure of high temperature superconductor YBCO thin film grown on CbS by preparing atomically smooth as-grown CbS, post-annealing the CbS at 960°C ~ 1050°C, preferably at 980 C w 1020 C and then growing the YBCO thin film. The atomically smooth as-grown CbS is prepared by growing the CbS at the deposition rate of 0.2-1.2 nm/sec, preferably at nm/sec.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1A shows an AFM picture of a 30 nm-thick CeO2 film grown on r-cut sapphire substrate before post-annealing of the same.

FIG. 1B shows an AFM picture of a 30 nm-thick CeO2 film grown on r-cut sapphire substrate after post-annealing of the same.

FIG. 2A shows an AFM picture of an 80 nm-thick CeO, film grown on r-cut sapphire substrate before post-annealing of the same.

FIG. 2B shows an AFM picture of an 80 nm-thick CeO2 film grown on r-cut sapphire substrate after post-annealing of the same.

FIG. 3A shows an AFM picture of a 45 nm-thick CeO, film grown on r-cut sapphire substrate before post-annealing of the same.

FIG. 3B shows an AFM picture of a 45 nm-thick CeO2 film grown on r-cut sapphire substrate after post-annealing of the same.

FIG. 4A shows an experimental result from X-ray diffraction analysis of a 45 nm- thick CeO2 film grown on r-cut sapphire substrate before post-annealing surface of the same.

FIG. 4B shows an experimental result from X-ray diffraction analysis of a 45 nm- thick Ce- film grown on r-cut sapphire substrate after post-annealing of the same.

FIG. 5 shows co-scan measurement data after growing a 45 nm-thick CeO2 film

on a r-cut sapphire substrate at 780 C and then post-annealing the film.

FIG. 6 shows X-ray diffraction analysis data of a 300-nm thick YBCO film grown on a post-annealed 45 nm-thick CeO2 buffered r-cut sapphire substrate.

FIG. 7isco-scan measurement data of (005) peak of a 300 nm-thick YBCO film grown on a post-annealed CeO, buffered r-cut sapphire substrate.

FIG. 8A presents dc resistance measurement data for YBCO films grown on an as-grown 45 nm-thick CeO2 buffered r-cut sapphire substrate.

FIG. 8B presents dc resistance measurement data for YBCO films grown on a post-annealed CeO2 buffered r-cut sapphire.

FIG. 9A is an AFM picture of the surface of a 140 nm-thick YBCO film grown on an as-grown 45 nm-thick CeO, buffered r-cut sapphire substrate.

FIG. 9B is an AFM picture of the surface of a 140 nm-thick YBCO film grown on a post-annealed CeO2 buffered r-cut sapphire.

FIG. louais an AFM picture of the surface of a 300 nm-thick YBCO film grown on an as-grown 45 nm-thick CeO2 buffered r-cut sapphire substrate.

FIG. lobais an AFM picture of the surface of a 300 nm-thick YBCO film grown on a post-annealed CeO, buffered r-cut sapphire.

FIG. 11 presents data for the temperature dependence of the microwave surface resistance (Rs) of a YBCO film on an as-grown CbS and that on an post-annealed CbS at the frequency of 8.6-8.7 GHz, respectively. Rs was calculated from the measured Qo in the inset. The inset shows TEo,, mode unloaded Q (Qo) of a rutile-loaded cavity resonator with two 300 nm-thick YBCO films prepared on the same substrate used as the endplates at the top and bottom of the cavity.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The invention comprise a method for growing YBCO thin film after post- annealing the CbS at 960C 1050 C in order to improve the surface property, the crystal structure and the microwave surface resistance of high temperature superconductor YBCO film on CbS.

The CeO2 buffer layer was grown on r-cut sapphire substrate by on-axis rf-

sputtering method using a target having the same composition as the grown thin film.

The diameter and thickness of the target are 50 mm and 4 mm, respectively. A ratio of partial pressures of argon gas and oxygen gas is used in a range from 3: 1 to 10: 1. The total gas pressure used in thin-film growth is 40 to 100 mtorr. During the film growth, the temperature of the substrate is set at 780 C and silver paste is used in order to improve the thermal contact between the substrate and heater.

A YBCO thin film was grown by the dc-magnetron sputtering method using a single target having the same composition as the YBCO film. A CbS substrate was used for the growth of the YBCO thin film. The temperature of the substrate was 730 C, entire pressure of gas was 100 mTorr, ratio of partial pressure of argon and oxygen was 4: 1 and thickness of as-grown YBCO thin film was about 100 to 300 nm.

The structures of the CeO2 buffer layers and YBCO films were analyzed by methods of X-ray diffraction analysis, atomic force microscope (AFM) and scanning electron microscope (SEM). In order to measure electrical characteristics for the YBCO thin film, direct-current resistances and microwave surface resistances were measured.

Particular structures and functions of the invention will now be described in preferable embodiments in conjunction with the accompanying drawings. The embodiments of the invention are described for purposes of the illustration and not by way of limitation.

Example 1 Surface properties according to post-annealin of CbS In the present embodiment, a CeO2 buffer layer was grown on a r-cut sapphire substrate using an on-axis rf-sputtering method and a target having the same composition as the grown thin film was used. After the CeO2 thin film was grown under a partial pressure ratio of argon to oxygen at 3: 1 and deposition rate of thin film at 1.5-4 nm/min, the change in the surface condition of CbS thin films is shown according to whether the post-annealing of CbS was performed at 1000 C. The R- factor in Table 1 was obtained from measured results of the surface morphology using AFM, which means a difference in the height between the highest point and the lowest

point in the measured surface region. As the thickness of the thin film increased from 30 nm to 80 nm, the magnitude of the R-factor increased from 17 nm to 44 nm (see Figs.

2A).1Aand Table 1 Changes in the roughness of CbS thin films when post-annealing of CbS was performed at 1000 C. Sample No. TotalPost-peak-to-peakDeposition (Å) rate(Å/min) roughnessAnnealing (Å)(mTorr)* Ce-1 800 40 40 X 437 Ce-2 600 30 70X381 05. 7 Ce-3 400 20 40 X 305 Ce-4 300 15 40 173 73 * :"O"denotes post-annealed film, and"X"denotes in-situ as-grown film.

However, it will be appreciated that the R-factor of CeO2 thin film having thickness of 30nm undergoes improvement from 17nm to 7nm and R-factor of CeO2 thin film having thickness of 80nm undergoes improvement from 44nm to 1.3nm when it is passed through post-annealing process under the oxygen environment of 1000 C with respect to as-grown CeO2 thin film (referring to Figs. 1B and 2B). Also, it is confirmed that in case of CeO2 thin film having thickness of 60nm grown at a deposition rate of 3 nm/min at a total pressure of 70mTorr, the thin film have a very small R-factor of 0.57 nm after post-annealing process at 1000 C although the R-factor of the as- grown CeO2 film have the value of 38 nm.

Example 2 Surface properties of CbS depending on post-annealing process of CbS with CeO2 prepared at a low deposition rate In this embodiment, the R-factor of CeO2 thin film, prepared under the same

patial gas pressure ratio as in Example 1 with the total gas pressure of 70 to 100 mTorr, and grown at a deposition rate of 0.75 nm/min, have a very small value of about 0.67 nm as shown in Table 2. This is improved to 0.28 nm after post-annealing process at 1000 C similar to Example 1 (see Figs. 3A and 3B).

Table 2 Changes in the roughness of CbS grown with CeO2 deposition rate of 0.75 nm/min depending on post-annealing process at 1000 C. Sample No. TotalPost-peak-to-peakDeposition (Å) annealingroughnessPressure (mtorr)- (A) Ce-5 450 7.5 70 6 7 Ce-6 450 7.5 100 X 3.3 ..

* :"O"denotes post-annealed film, and"X"denotes as-grown film.

Referring to Figs. 4A and 4B that show the experimental results of X-ray diffraction analysis for the CeO2 film samples in Table 2, only (100) peaks were observed from the thin films regardless of the post-annealing treatment. This shows that CeO2 thin films of the cubic structure were epitaxially grown well along the c-axis. Similar results were revealed for as-grown CeO2 thin films of 30-100 nm thickness. In Fig. 4, the full width half maximum (FWHM) of A (8-2A) reflex for the (200) peak appeared to have the small value of 0.2-0.26°, and FWHM for the (400) peak appeared to have the value of about 0.4°. Also, in Fig. 5, Ac3 from the rocking curve data for the (400) peak [the same value with (004) peak in cubic structure] of CeO2 thin films (Ce-5 sample) after the post-annealing process appeared to have the small value between 0.4- 0.5°. Thus, the values of Ac3 and A (6-26) appeared to be not much different regardless of the changes in the deposition rate and total gas pressure, with the post-annealing process for CeO, thin film at high temperature not affecting the CeO, thin films structure significantly.

Thus, it was confirmed that the surface smoothness of as-grown CeO2 thin film (less than 40 nm) was effectively improved by reducing the deposition rate of CeO2.

Example 3 Properties of YBCO film according to whether post-annealing of CbS was performed or not In this embodiment, CeO2 film was used as a buffer layer to grow YBCO film on the r-cut sapphire substrate. The effects of the post-annealing of the CeO2 buffer layer at the oxygen atmosphere to the structural and electrical characteristics of YBCO film grown on the CeO, buffer layer were tested. The measured results for the surface morphology of the YBCO films are shown in Table 3. CbS as in the condition grown in the film growth chamber (hereinafter"as-grown CbS") and CbS which is the as-grown CbS post-annealed at the oxygen atmosphere (hereinafter"post-annealed CbS") were used. The R-factors of each buffer layer were 0.67 nm and 0.28 nm (Ce-5 Sample of Example 2). The thickness of YBCO films grown on each CbS were 140 nm and 300 nm, respectively.

Table 3 Changes in the surface roughness of YBCO film grown on the as-grown CbS and the post-annealed CbS with a 45 nm-thick CeO2 buffer layer ThicknessTotalPost-Roughnesspeak-to-peakSampleNo. Annealing(RMS)*2roughness(Å)pressure (Å)(Å)(mTorr)*1 YBCO-1a 1400 100 X 207 480 YBCO-1b 1400 100 O 6.5 23 YBCO-2a 3000 100 X 305 1125 YBCO-2b 3000 100 O 11 32 *1 :"0"denotes a case where post-annealed CbS was used, and"X"denotes use of as- grown CbS.

*2: Value of Root Mean Square.

X-ray diffraction analysis shows that c-axis growth of YBCO thin films are successfully effected whether or not the used CbS boards are post-annealed. Fig. 6 shows that only (00 peaks are observed and that the YBCO thin films are epitaxially

grown as a result of x-ray diffraction analysis of 300 nm-thick YBCO thin film (sample YBCO-2b in Table 3) grown on a post-annealed CbS with a 45 nm-thick CeO, buffer layer.

As shown in Fig. 7, D and the FWHM of D (0-2 0) reflex for the (005) peak of YBCO thin film (YBCO-2b) grown on the post-annealed CbS are 0.47° and 0.16°, respectively. Meanwhile, the values for D the FWHM of D (9-2 9) reflex for the (005) peak are 0.69 and 0. 19, respectively, for YBCO film (YBCO-2a) grown on as-grown CbS. Thus, it shows that the crystal structure of YBCO thin film grown on the post-annealed CbS is much better than that of YBCO film on as-grown CbS.

The differences between the transition width (dut) and R (300K)/R (100K) obtained from the dc resistance data of YBCO grown on as-grown CbS and those of YBCO grown on the post-annealed CbS were very little, as shown in Figs. 8A and 8B.

Here DT denotes the difference between the onset temperature at which they begin to be superconductive and the zero-resistance temperature at which zero resistance is encountered, with R (300K)/R (100K), the ratio of the resistance at 300K to the value at 100K. In the figure, DT of the YBCO film grown on the post-annealed CbS was slightly less than that of YBCO film grown on as-grown CbS. In addition, R (300K)/R (100K) the YBCO film grown on the post-annealed CbS was slightly larger than that of YBCO film grown on as-grown CbS.

Above all, the R-factor of YBCO film on CbS was dramatically improved when the CbS was post-annealed at high temperatures before deposition of YBCO. As shown in Figs. 9A and 10A, the R-factors of the YBCO films having the thickness of 140 nm and 300 nm, deposited on as-grown CbS, were as large as 48 nm and 112 nm, respectively. However, the corresponding R-factors of the YBCO films on post- annealed CbS appeared to be significantly improved, with the respective value of 2.3 nm and 3.2 nm for the 140 nm-thick and the 300 nm-thick YBCO films, as shown in Figs. 9B and lOB.

The surface resistance of the YBCO films on post-annealed CbS also showed that the YBCO films would be very useful for fabrication of microwave elements for transceiver modules. In this example, the surface resistance was measured by using a

rutile-loaded cavity resonator with two same YBCO thin films placed as the endplates of the cavity. The inset of Fig. 11 shows the dependence of the TE,,,,-mode unloaded Q of the rutile-loaded resonator with two YBCO films (YBCO-2a) on as-grown CbS and two other ones (YBCO-2b) on post-annealed CbS used as the two endplates, respectively. Here, the values of the measured resonance frequencies are 8.6-8.7 GHz at 40-77 K. The microwave surface resisntace (Rs) could be calculated from the measured TEj-mode unloaded Q. The values were 54,110 and 230 jj. Q at 40,60 and 77 K, respectively, for Rs of the YBCO film (YBCO-2b) deposited on post-annealed CbS, which are significantly smaller than the corresponding values of 83,130,720 pQ for the YBCO film (YBCO-2a) grown on as-grown CbS. For reference, the values of 1.25 x 10-6,3.17 x 10-6, and 6.5 x 10-6 were used for the loss tangent of the rutile at 40, 60, and 77 K, respectively, with 5.22 x 10'S/m used for the conductivity of the copper.

As described above, the present invention enables to reduce the surface roughness of YBCO high temperature superconductor films grown on CbS to 1/6 of or less than the value that can be obtained using the conventional technologies, by changing the surface structure and the smoothness of CbS. Furthermore, the present invention enables to improve the crystal structure and the microwave surface resistance of YBCO films on CbS significantly. These are the three essential features of the present invention. In this regard, the present invention is applicable for fabrication of various microwave elements of high performance for transceiver modules, and highly applicable to the high temperature superconductor industry.