Prijamboedi B. Dielectric Properties of SrTiO3 Thin Film Prepared in a Mixture of 18O2 and 16O2 Gas 2008

7/29/2019 Prijamboedi B. Dielectric Properties of SrTiO3 Thin Film Prepared in a Mixture of 18O2 and 16O2 Gas 2008

http://slidepdf.com/reader/full/prijamboedi-b-dielectric-properties-of-srtio3-thin-film-prepared-in-a-mixture 1/4

Journal of Alloys and Compounds 449 (2008) 48–51

Dielectric properties of SrTiO3 thin film prepared in amixture of 18O2 and 16O2 gas

B. Prijamboedi a,∗,1, H. Takashima a, R. Wang a, A. Shoji a, M. Itoh b

a Nanoelectronics Research Institute of AIST, Umezono 1-1-1, Tsukuba, Ibaraki 305-8568, Japanb Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, Japan

Received 4 November 2005; received in revised form 20 January 2006; accepted 2 February 2006

Available online 16 January 2007

Abstract

We studied the dielectric properties of SrTiO3 (STO) thin film prepared by pulsed laser deposition in a mixture of 18O2 / 16O2 gas during

deposition. We fabricated an STO capacitor from an YBa2Cu3O7 /SrTiO3 /YBa2Cu3O7 multilayer structure by using patterning, ion milling, and

chemical mechanical planarization. The temperature dependence of r of STO thin film prepared in a 70% 18O2 shows Curie–Weiss behavior down

to 2 K without any indication of transition to the ferroelectric state. The frequency dependence of εr at 2.2 K shows that εr is almost constant in the

region between 0.1kHz and 1 MHz. This differs from the behavior of εr of an STO thin film prepared in pure 16O2, which increased as frequency

decreased. The result indicates that only small amount of 18O atoms could be incorporated during deposition, and most of the oxygen atoms were

transferred from the target.

© 2006 Elsevier B.V. All rights reserved.

Keywords: Ferroelectric; Thin films; Laser processing; Dielectric response

1. Introduction

The perovskite-type oxide SrTiO3 (STO) is one of the most

extensively studied materials. This is because STO exhibits

many exotic properties and has many potential applications

in electronic devices such as tunable microwave devices [1],

the gate oxide in CMOS devices [2], and varistors [3]. At

low temperatures, STO exhibits quantum paraelectricity [4].

Ferroelectricity could be induced in STO by Ca substitution

into Sr sites [5], by electric fields [6], and uniaxial stress [7].

Transition to a ferroelectric state has been observed in single

crystals of 18O-isotope-exchanged STO [8]. At low 18O con-

centrations (<26%), a single STO crystal remains in a quantum

paraelectric state but has a higher dielectric constant, εr, than anon-exchanged one [9]. When a dielectric material is used as a

small-scale capacitance temperature sensor [10], a high dielec-

tric constant value with a large variation with temperature is

needed to increase its sensitivity. It is possible that a small-

∗ Corresponding author. Tel.: +62 22 250 2103; fax: +62 22 250 4154.

E-mail address: [email protected] (B. Prijamboedi).1 Present address: Dept. of Chemistry, Bandung Institute of Technology, Ban-

dung, Indonesia.

scale, highly sensitive capacitance temperature sensor could befabricated with 18O-isotope-exchanged STO thin film.

A thin film STO capacitance temperature sensor could be

prepared from the YBa2Cu3O7 /SrTiO3 /YBa2Cu3O7 thin film

multilayer structure using YBa2Cu3O7 (YBCO) as electrodes.

The lattice mismatch between YBCO and STO is small and it

could thus facilitate a good epitaxial growth of film. STO thin

films with single-crystal-like behavior and high dielectric con-

stant have been successfully prepared by PLD and a chemical

mechanical polishing technique [11]. To fabricate 18O-isotope-

exchanged STO thin film, various methods of isotope exchange

could be used. One is post-annealing of the fabricated thin

film. For single crystals, the exchange process was effectively

carried out at a high temperature at around 1050 ◦C [8]. Grow-ing a STO thin film sample on an YBCO electrode layer by

high-temperature annealing is impossible, since a long anneal-

ing process at high temperature could deform the YBCO layer.

Another possibility is to carry out the isotope exchange process

during the deposition process, when the deposited material is

still in the plasma state.

Here, we study the dielectric properties of STO thin film

grown on an YBCO layer prepared by pulsed laser deposition

in a chamber filled with a mixture of the 16O2 and 18O2 gases.

0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.jallcom.2006.02.078

mailto:[email protected]

http://localhost/var/www/apps/conversion/tmp/scratch_3/dx.doi.org/10.1016/j.jallcom.2006.02.078

http://localhost/var/www/apps/conversion/tmp/scratch_3/dx.doi.org/10.1016/j.jallcom.2006.02.078

mailto:[email protected]



B. Prijamboedi et al. / Journal of Alloys and Co mpounds 449 (2008) 48–5 1 49

The dielectric constant data show no transition to the ferroelec-

tric state in samples prepared in 70% 18O2 gas. However, the

dielectric constant does not show a variation with frequency in

the frequency range between 100 Hz and 1 MHz, which indi-

cates that a few 18O atoms could enter the film and affect the

dielectric properties.

2. Experimental

c-axis-oriented YBCO and STO thin films were grown by a pulsed laser

deposition technique. An ArF excimer laser (λ = 193nm) was used as the exci-

tation source and was focused on targets of YBCO ceramic and STO single

crystals. The STO (1 0 0)-oriented substrate was mechanically clamped to a

stainless steel pad and then heated by a halogen lamp. First, the YBCO thin

films 300 nm thick were fabricated on the STO substrate at 745 ◦C in an oxy-

gen pressure of 93Pa. The laser fluence on the target was estimated at 2 J/cm2

and the pulse rate was 2 Hz. The YBCO thin films were then patterned using

standard photolithography and Ar-ion milling to make an YBCO line with a

width of 10m. STO film around 1m thickness was then deposited on the

patterned YBCO layer at the same temperature as that used for the YBCO film

at a laser fluence of 4.5 J/cm2 and pulse rate of 4 Hz. During STO deposition, a

mixture of

18

O2 and

16

O2 gases was flooded into the chamber and the pressureinside the chamber was maintained at 93 Pa. The volume of each gas that leaked

into the chamber was carefully controlled by two mass flow controllers. After

STO deposition, samples were subjectedto a chemical mechanical planarization

process [12] in order to obtain a smooth and flat surface of the STO layer. The

final thickness of the STO layer was around 750–800 nm. Finally, the YBCO

top layer was deposited with the same parameters as the YBCO bottom layer

and then patterned to make six parallel capacitors, each 10 m× 10m. We

prepared STO capacitors in atmospheres containing 0%, 50%, and 70% 18O2

gas.

Dielectric values were measured with an LCR meter (Agilent 4284A) in a

temperature range of 2–300 K in PPMS (Physical Property Measurement Sys-

tem) of Quantum Design. Thetestmeasurementsignal wasset at 5 mVwith zero

DC bias. The frequency range for this measurement was from 100 Hz to 1 MHz.

The dielectric constant can be calculated by using the expression of C = ε0εr A / t ,

where C is the measured capacitance data, εr is the dielectric constant, A is thetotal area of the electrodes, and t is the STO thin film thickness.

3. Results and discussion

The temperature dependences of εr of each STO thin film are

shown in Fig. 1. In STO thin films prepared in 16O2 (sample

1), the dielectric constant follows the Curie–Weiss behavior and

shows saturation below 5 K (low-temperature region). This indi-

cates that the STO thin film has quantum paraelectric behavior.

At 300K, the εr value is about 1700. The dielectric constant at

low temperature measured at 100 Hz is larger than 1.1× 105.

This value is unusually large for an STO thin film. The same

behavior of the dielectric constant properties was also observedin the STO thin film prepared in a mixture of 50% 18O2 and

50% 16O2 (sample 2). In the low-temperature region, the value

of εr measured at low frequency was slightly lower than that of

sample 1, but still high. The dielectric constant values of STO

thin film deposited in atmosphere with 70% 18O2 (sample 3) at

low temperatures were lower than those of samples 1 and 2. In

the low-temperature region, the εr value was around 3× 104.

The dielectric constant varied little against the frequency in this

sample. All of the samples follow the Curie–Weiss behavior and

show a quantum paraelectric state at low temperatures.

The Curie–Weiss behavior of the dielectric data were demon-

strated by plotting 1/ ε0εr (100 kHz) versus temperature (Fig. 2).

Fig. 1. Temperature dependence of dielectric constant of STO thin film mea-

sured ina parallelcapacitor withYBCO/STO/YBCO multilayer structure. Ratios

of 18O2 / 16O2 gas used during deposition: (a) 0%/100%, (b) 50%/50%, (c)

70%/30%.

The temperature dependence of the inverse dielectric data was

linear over a wide temperature range, from 300 to 75 K. The

data then were fitted to the Curie–Weiss formula: εr =C/(T − θ ),

where C is the Curie constant and θ is the Curie temperature.

The Curie temperature for the STO thin film deposited in pure16O2 gas (sample 1) was 26.9 K, and gradually decreased as

the concentration of 16O2 decreased. In samples 2 and 3, the

Curie temperatures were 25.1 and 22.6 K, respectively. The

Curie constant was almost constant in all samples, at around

4.1× 10−6 FK/m.

The dielectric constant and the loss factor or tan δ data at

2.2 K plotted against frequency are shown in Fig. 3. Dielectric

constant dispersion or relaxation is clearly seen in samples 1 and2 between 1 and 10 kHz. The dielectric constant for sample 3 did

not show a large variation in the frequency range from 100 Hz

to 1 MHz. The loss factor for STO thin film deposited in 0% and

50% 18O2 has a peak centered around 5 kHz. The loss factor

for the STO sample deposited in 70% 18O2 tended to increase as

frequencydecreased. Thisindicatesthat the relaxationfrequency

was shifted to thelower frequency region below100 Hz.The loss

factor data also indicate that there is another peak centered above

1 MHz in all samples.

To determine the relaxation mechanism in the STO thin film,

we analyzed the dielectric data using the Cole–Cole formula:

ε

*

= ε∞ + (ε0−

ε∞)/(1−

jωτ )1−␣

, where ε∞ is the permittivity



50 B. Prijamboedi et al. / Journal of Alloys and Compounds 449 (2008) 48–51

Fig. 2. Temperature dependence of 1/ ε0εr data at 100 kHz of STO thin films

deposited in different ratios of 18O2 and 16O2 gas during deposition: (a)

0%/100%, (b) 50%/50%, (c) 70%/30%.

Fig. 3. (a) Frequency dependence of the dielectric constant at 2.2 K of STO thin

film deposited at different 18O2 concentrations. (b) Plot of loss factor or tan δ

vs. frequency at 2.2 K.

Fig. 4. Arrhenius plot of the relaxation time, τ of the STO thin films prepared

in a mixture of 18O2 and 16O2 gas with 18O2 concentrations of 0% and 50%.

at high frequency, ε0 is the static permittivity, ω is the angu-

lar frequency, τ is the relaxation time, and α is the angle of

the semicircular arc. Fitting the results of the data of samples 1

and 2 at high temperature (>75 K) gives the relaxation times atdifferent temperatures shown in Fig. 4. The relaxation time fol-

lows the Arrhenius-type function of τ = τ 0 exp(U / k BT ), where U

is the activation energy. The activation energies obtained from

the data are 11 meV for sample 1 and 3 meV for sample 2.

The τ 0 values are 3.7× 10−6 s for sample 1 and 6.3× 10−6 s

for sample 2. The U and τ 0 values obtained in this study are

comparable with the result from Sr1− x Ca x TiO3 (U = 12 meV,

τ 0 = 3× 10−6 s) [13], which was attributed to the behavior of

the off-center Ca2+ ions on the Sr sites. This indicates that there

are defective states on the Sr sites in the STO thin film. Fur-

thermore, a defect at the Sr site could create an oxygen vacancy

in the surrounding sites and create excess charge. The excessenergy and the physical boundary created by the island growth

of the STO thin film [12] lead to space charge polarization in the

STO film, which gives high dielectric constant values. Thespace

charge polarization gives a large dielectricconstant,much higher

than the contribution from the dipole–dipole polarization [14].

The space charge polarization in this STO film is indicated by

the relaxation time and the large dielectric constant at low tem-

perature exceeding 1× 105, which is far above the maximum

dielectric constant value of an STO single crystal (∼2× 104)

[15].

The dielectric measurement shows that there is no transition

to a ferroelectric state in the STO thin film, as indicated by the

presence of the peak in the dielectric constant data at the transi-tion temperature, T c. Possibly the 18O atoms arenot incorporated

into the deposited STO thin film. However, the deposition in the

mixture of 18O2 and 16O2 gave a dramatic change in the dielec-

tric properties, shifting the dielectric relaxation frequency of

the space charge polarization to a lower value. The STO thin

film deposited in 70% 18O2 shows almost no frequency depen-

dence on the dielectric constant. Another effect observed was

the reduction of the Curie temperature as the 18O2 concentra-

tion was increased in the chamber. Therefore, a small number of 18O atoms could enter the STO crystal structure. Although this

migration did not promote a transition to the ferroelectric state,

it still affects the dielectric properties.



B. Prijamboedi et al. / Journal of Alloys and Co mpounds 449 (2008) 48–5 1 51

4. Conclusions

Parallel capacitors have been fabricated from YBCO/STO/

YBCO thin film multilayer structure by pulsed laser deposition,

patterning, and chemical mechanical planarization technique.

The STO thin film layer was deposited in different composi-

tions of 18O2 and 16O2. Transition to ferroelectric state was not

observed in the STO thin film prepared at a 70% concentra-

tion of 18O2. The dramatic change of the dielectric constant,

which showed a large shift in the relaxation frequency, indicates

that a small amount of 18O atoms could enter the STO crystal

structure.

References

[1] K. Bouzehouane, P. Woodall, B. Marcilhac, A.N. Khodan, D. Crete,

E. Jacquet, J.C. Mage, J.P. Contour, Appl. Phys. Lett. 80 (2002)

109.

[2] K. Eisenbeiser,J.M. Finder,Z. Yu, J. Ramdani,J.A. Curless, J.A.Hallmark,

R. Droopad, W.J. Ooms, L. Salem, S. Bradshaw, C.D. Overgaard, Appl.

Phys. Lett. 76 (2000) 1324.

[3] N. Yamaoka, M. Masuyama, M. Fukui, Am. Ceram. Soc. Bull. 62 (1983)

698.

[4] J.H. Barrett, Phys. Rev. 86 (1952) 118.

[5] J.G. Bednorz, K.A. Muller, Phys. Rev. Lett. 52 (1984) 2289.

[6] P.A. Fleury, J.F. Scott, J.M. Worlock, Phys. Rev. Lett. 21 (1968) 16.

[7] H. Uwe, T. Sakudo, Phys. Rev. B 13 (1976) 271.[8] M. Itoh, R. Wang, Y. Inaguma, T. Yamaguchi, Y.J. Shan, T. Nakamura,

Phys. Rev. Lett. 82 (1999) 3540.

[9] R. Wang, M. Itoh, Phys. Rev. B 64 (2001) 174104.

[10] H. Takashima,R.P.Wang, N. Shirakawa, B. Prijamboedi, A. Shoji, M. Itoh,

Thin Solid Film 486 (2005) 145.

[11] H. Takashima, R. Wang, N. Kasai, A. Shoji, M. Itoh, Appl. Phys. Lett. 83

(2003) 2883.

[12] B. Prijamboedi, H. Takashima, A. Shoji, J. Cryst. Growth 283 (2005) 163.

[13] W. Kleemann, H. Schremmer, Phys. Rev. B 40 (1989) 7428.

[14] A.J. Moulson, J.M. Herbert, Electroceramics, John Wiley & Sons, Chich-

ester, 2003, p. 70.

[15] K.A. Muller, H. Burkard, Phys. Rev. B 19 (1979) 3593.

Documents

Prijamboedi B. Dielectric Properties of SrTiO3 Thin Film Prepared in a Mixture of 18O2 and 16O2 Gas 2008