The millimeter wave surface resistance measurements of Y-Ba-Cu-O films epitaxially grown on non-buffered sapphire substrates

ICEC 14 Proceedings

THE MILLIMetER WAVE SURFACE RESIST~LNCE MEASUREMENTS OF Y-Ba-Cu-O FILMS EPITAXII~LLY GROWN ON NON-BUFFERED SAPPHIRE SUBSTRATES Oleg Pustylnik+ Viatcheslav Vratskikh*,# Sergey Vasiljev# Igor Voinovsky+ Oleg Khimenko+

Aleksandr

Dymnikov+

+ Kiev University, Vladimirskaya 61, Kiev 252017, Ukraine * Institute for Thermophysics, Kutateladze I, Novosibirsk 630090, Russia # Radian Enterprise, Lavrentjev 6, Novosibirsk 630090, Russia

The improved value of surface resistance of epitaxial Y-Ba-Cu-O films directly grown on r-plane sapphire have been achieved. The surface resistance of the films was 1.6 mOhm at 35GHz,6.1 mOhm at 65 GHz and 31 ~ h m at 135 GHz,that is essentially lower than surface resistance of copper films.The microstrip resonator fabricated with film of Rs= i0 mOhm at 35 GHz is described.The prospects of further improvement of microwave characteristics of epitaxial Y-Ba-Cu-O films on sapphire substrates is discussed.

INT~DUC~ION Epitaxial Y-Ba-Cu-O (123) films of high quality have been grown on substrates such as SrTi05, MgO, YSZ and LaAlO~by various deposition techniques. It is known, however,that each of these substrates represent some kind of compromise [I]. From practical point of view, the substrate like sapphire is the most desirable material for 123 films because of the low microwave loss, high mechanical strength, high thermal conductivity, high Debye temperature, good surface chemistry and low cost. The lattice mismatch and reaction of Ba with sapphire are the main obstacles to epi- taxial growth of 123 films on sapphire. Epitaxial Y-Ba-Cu-O films on sapphire with good critical and transport properties were demonstrated by Char et al [l],but the weak epitaxy and high surface resistance of the films have led to conclusion that it is necessary to use a buffer layer between the substrate and 123 film to improve the epitaxial match and to reduce chemical reactions between substrate and film [2]. Here we report the substantial improvement of microwave properties of 123 films epi- taxially grown on non-buffered sapphire by laser ablation technique. Film preparation ~ r deposition system will be described in details elsewhere. Briefly, 123 targets were ablated by many-mode Nd:YAG laser. The pulse energy was 300 m J , the pulse duration 20 ns, the average beam energy density 4 J/cm=.Thedistance of the target and the substrate was 6 cm and the screen was placed between target and substrate in order to protect the film surface from droplets. At a pulse repetition rate of I0 Hz the average growth rate was 0~3 nm/s .The oxygen pressure was 0,;6 Torr and the substrate temperature, measured with accuracy 5 C~ was 755-770°C. The relatively high oxygen pressure is necessary to produce large clusters and to reduce in such way the chemical activity of Ba on the substrate surface. The elevated substrate temperature is required for surface mobility of large clusters. The films quality was sensitive both to substrate temperature and oxygen pressure. Substrates Among 1500 experiments required for optimization of film growth conditions only 300 were carried out with the "fresh" substrates. In other cases 123 films were removed by etching and used again so that any substrate was utilized two or three times. In research aimed at optimization of growth conditions, the assessment of the films quality was carried out by dc transport measurements. No difference in critical properties (Tc, ATc, Jc and R3OO/RIO0) for Ist, 2nd and 3rd deposition run with the

620

Cryogenics 1992 Vol 32 ICEC Supplement

ICEC 14

Proceedings

same substrate was found. In opposite, our recent x-ray measurements reveal the strong dependence of volume percent of grains misaligned in a-b plane on substrate surface history and surface preparation procedure. The summarized volume percent of grains misaligned at various angles was varied from 0,5 to 5 % for different samples. For most samples, the distribution of misaligned at 45 grains was not uniform and varied from 0,1% to 1,5%. These observations indicates that the state of substrate surface in most cases was not perfect. Film characterization The x-ray diffraction pattern of our films contains only (00~) sharp peaks showing, that the film is highly oriented with the c-axis perpendicular to the substrate. The FWHM of the [005] rocking curve was in the range of 1.3-2.5= . Some general parameters of samples, used for microwave measurements and device fabrication are represented in Table I. Table 1

General parameters of samples.

Sample

Tc (R=O), K

n T, K

A

89

0.5

B

90

0.5

88


ref. [I]

* ** +

Jc, A/cm

R3OO/RIO0

6 * > I0 at 77K 6* > I0 at 77K 6** 5.4.10 at 4.2K

Thickness, nm

Rs,mOhm at 13 GHz

Rs,mO~m at 35 GHz

+

3

170

3

250

3

200

0.22 at 55K + 1.31 at 55K

1.6 at 55K i0 at 55K

1.00 at 4.2K

7.6 at 4.2K

do - transport measurements, magnetization measurements, calculated with quadratic frequency dependence.

Microwave measurements The measurements of the temperature dependence of microwave surface resistance were carried out at 34 GHz, 65 GHz and 134 GHz using a method of a cylindrical copper host cavity. The cavities were excited in the TEOII mode. In detail this method was reported in work [3]. Moreover, the temperature dependence of Rs was calculated from the measurements of the unloaded quality factor Qo of the cavity. Ring resonator was designed at 32.7 GHz and excited in TMII mode with an impedance of 50 Ohm. The ring diameter was chosen for the fifth harmonic. The structure of the resonator is plotted on Figure 2. Temperature variation between I0 and 150 K was provided by warming the cavity in vacuum in helium cryogenic system. The temperature was measured with a semiconductor resistor with an absolute accuracy of ± 0.I K. Figure 1 shows Rs vs T for our best 170 nm film (sample A in Table 1 ) measured at 34, 65 and 134 GHz.For comparison at the same figure by solid line is plotted the temperature dependence of the surface resistance of the copper cavity walls. The low residual resistance for all three frequency bands, taking into account that the film thickness was lower than penetration depth of magnetic field, shows that the film is of high quality. The frequency dependence of the surface resistance determined from our data for T= 70 K and T= 20 K indicates a good agreement with quadratic dependence. The calculated value of Rs at 13 GHz using quadratic frequency dependence of Rs is represented in Table 1 <0.22 mOhm at 13 GHz and 55 K). Device .fabrication The 123 film was patterned with photolithographic technique and etched with dilute phosphoric acid. The film on sapphire was coated with 0.7 micron @H383 positive photoresist. The photoresist was then baked in 90 ~ oven for 15 min and afterwards was

Cryogenics 1992 Vol 32 ICEC Supplement

621

ICEC 14 Proceedings

exposed 42 s using a I:i contact printer using UF lamp. Then I:i diluted developer was used to develop the photoresist for about i0 s and the substrate was rinsed thoroughly with deionized water. A second bake in a < 1 0 0 ~ over for 30 min stabilized the resist for optimized linewidth control during the 123 wet etch in a diluted solution phosphoric acid. After this etch, the photoresist was stripped in acetone. Figure 2 shows the temperature dependence of the surface resistance of the film (sample B in Table I) used for microstrip resonator fabrication, that was measured by resonator host cavity method before the lithography and 'the temperature dependence of microstrip resonator Q-factor of the patterned film. CONCLUSION We consider the obtained results as the base for further systematic experiments. Taking into account the nonuniformity of our samples, the poor quality of substrate preparation on present state, the spatial nonuniformity of laser beam, the fact that measured films were not thick enough, the absence of grains misoriented in a-b plane in some parts of the best films, we can conclude that further improvement of microwave properties of 123 films epit~ially grown on non-buffered sapphire is still possible. This work is partly supported by High Educational Ministry of Ukraine, from 21.03.91

Contract N.93

REFERENCES Char,K. et al, Properties of Phys.Lett.(1990) 56 595-598

epitaxial Y-Ba-Cu-O thin films on sapphire Appl.

Rowell,J.H., Research advances and needs related to HTSC electronics Supercond. Sci.Technol.(1991) 4 51-58 Kessler,J.R.and Gering,J.M., Use of a Fabry-Perot resonator for the measurement of the surface resistance of high Tc supercondcctors at mm wave frequencies Int.J.IR and MM waves(1990) ii 151-164

622

Cryogenics 1992 Vol 32 ICEC Supplement

ICEC 14 Proceedings

10

4

t Rs,mOhrn 10

'~

~0

~

,~.

....................................

"~-z " ' - - ' I F

....

(;u.

34

GHz

}

t0

--"--T---T-'--I~'T--T--T--I'--T'--I"- "1 -'l.'---T---r"' I

•0

Figure 1

6 0

T - - [ ' - T - - T - 1--"~---T" q

T, !K

I t 0

The temperature dependence of the surface resistence of the sample A ( ~ - 3 4 GHz, o - 65 GHz, ~- 1.34 GHz)

. oo i{ Oo

.1_ L ._.L_L...L._J..,.[

. I.,.L

3 . . L.,L__.I_.A..__.L . L _ L . L

,l...J__L,J_.~_|__.k

Rs,

f .

,'161

'J-=)l

] 0

4

q ' 10 --1 -i

:'

mOhm , f~ ~.. ~:.

L'>

r

"~

-i

"]

2 0 0 ,-]

:i

10

0

-I

-1

I:,

,->

I

J] I

p

~

I

10

4

1

0 _~ --r-r--r--r-m-v

'lO Figure 2

4

r"x-r-rr--;mT--r-r 60 T,K

I - r ; "r ; t"10

ti

The temperature dependence of the surface resistence and Q-factor of the sample B. ( ~ -surface resistence of at 3 4 G H z of the unpatterned film, = - the unloaded q-factor of the mierostrip r ~ resonator at 32.7 GHz )

Cryogenics 1992 Vol 32 ICEC Supplement

623