Mrs Boston 2002

Alternate Current Conduction and Impedance Spectroscopy of Laser Ablated BaZro3 Thin Films V. Rajasekarakumar, P. Victor, R. Ranjith, S. Saha1, S. B. Krupanidhi, S. Rajagopalan2 and A. K. Tyagi2. Materials Research Center, Indian Institute of Science, Bangalore- 560 012, India. 1 Materials Science Division, Argonne Laboratory, Argonne, IL, USA. 2

Materials Science Division, IGCAR, Kalpakkam.

MRS fall meeting, Boston, USA, dec 2, 2002.

INTRODUCTION:
Presently there had been demand for monolithic microwave integrated circuit technologies (MMIC) for its essential use in the microwave communication applications, such as mobile phones, satellite communications and global positioning systems.
Paraelectric materials used as resonators and filters in microwave circuits require high dielectric constants, low dielectric losses (tanδ), and low temperature coefficient of resonance frequency (τf).
Since 1970’s, in bulk several materials including BaZrO3 have been developed for the microwave applications. BaZrO3 also finds its potential use in the protonic conductors and recently there had been extensive research performed on this material.
Polycrystalline Barium zirconate (BZ) thin films were grown using a KrF excimer laser ablation technique. A dense target with single phase of BZ ceramic was used as a target. The films were deposited on platinum coated silicon substrates.
Barium Zirconate thin films were deposited at an operating pressure between 20 to 100 mTorr in the ultra high pure oxygen gas ambience.
The structural and stoichiometry was confirmed by x-ray diffraction technique (XRD) and energy-dispersive x- ray analysis (EDAX) respectively.
The interface has been analysed by Secondary ion mass spectrometry (SIMS).
The Metal – Insulator – Metal (MIM) capacitor has been formed to perform to electrical characterization on the BZ thin films.

Structure and Stoichiometry Effect of substrate temperature in the crystallinity

*

(220)

(211)

(200) PtKα

Intensity (arb. unit)

* PtK β

(111)

*

(110)


BZ Films were grown at different substrate

0

650 C

*

0

600 C 0

550 C

*

pow 20

30

40

2θ

50

60

temperature at a constant partial pressure of 50 mTorr.
The perovskite phase evolves at 5500C and at 6000C the enhancement of (110), (111) and (211) planes are observed.
All the single phase BZ thin films exhibited polycrystalline nature for the thin films deposited upto 6000C.
Further increment of substrate temperature to 6500C, there is a tendency for the orientation along (110) direction with the disappearance of other prominent perovskite peaks.
This is accordance with the Thorton’s model which suggests that at higher substrate temperatures the thin films tend to exhibit columnar structure and higher orientation along a favourable plane.

0

800 C

0

775 C

0

750 C 20

30

40

50

2θ

60

(220)

(211)

90 min_ 20 mTorr PtKβ

(200)

PtKα

PtKβ

Intensity(arb.unit)

(110)

Effect of ex-situ annealing temperature in crystallinity
The BZ thin films deposited at the substrate temperature of 4000C in the pressure of 20 mTorr and subsequently annealed at different temperatures for 90 minutes.
The perovskite peak enhanced with increasing of temperatures upto 7750C.

Depth profile of in-situ annealed BZ thin film using SIMS. 5

10

1 -> 138Ba 2 -> 16O 3 -> 90Zr 4 -> 195Pt

2

*

1 4

10

* *

2 3

10

[C/s]

3 4

2

10

4

3

1

10

1

Tp 0

10

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Thickness (µm)

Type of deposition : In-situ Oxygen partial pressure: 50 mTorr Annealing temperature : 650 0 C


The interface is sharp and almost no diffusion of Ba and Zr in to platinum layer when compared to the thickness of the platinum thickness.
The peaks (*) observed in all elements (Ba, Zr, O) reveals that there is a fixed proportion of the constituent species of the expected phase. So the film growth at the interface is good. Moreover, the stoichiometry and sputtering rate are different at the interface which has lead to humps in the region.
Oxygen concentration increases after platinum layer because we have an inter-face of (Pt/ TiO2/ SiO2/ Si) in our substrate.
Tp shows the thickness of the platinum layer (0.18 µm).

Depth profile of Ex-situ annealed BZ thin film using SIMS. 5

10

1 -> 138Ba 2 -> 16O 3 -> 90Zr 4 -> 195Pt

#

1

2

*

4

10

4 2 3

10

*

[C/s]

3 2

4

10

1 1

10

0

10

0.0

0.2

0.4

0.6

0.8

1.0

Thickness (µm)

Deposition pressure : 50 mTorr Substrate temperature : 400 0 C Annealing time : 90 min. Annealing temperature: 600 0 C


The interface is not sharp and all the elements are interdiffused.
In the (*) region both Zr and Oxygen have a uniform rise, whereas Ba falls down suddenly ( Zirconium oxide have formed or segregated).
Hump in the * region due to interface effects.
Flat nature was observed in the region (//). It has given the conclusion of compound formation (ZrO2) at the interface, a flat nature reveals that we have constituents species of definite proportion.
In # area there is a hump in oxygen, that due to TiO2 interface.
The diffusion length is around 0.16 µm.

Dielectric Properties Dielectric constant and loss factor dispersion With frequency at room temperature 100

-1

10

tanδ

At 100 kHz frequency in room Temperature tanδ

ε'

75

⇓

50

Dielectric constant ⇒ 24.4 Loss factor ⇒0.03 25 -2

10

2

10

3

10

4

10

frequency (Hz)

5

10

AC electrical properties Ac conductivity plot as a function of frequency for different sample temperatures
The plot at low temperature

T= 100 T=150 T= 175 T= 200 T= 225 T= 250 T= 275 T= 300

-7

10

-8

σ ac (Ω.cm)

-1

10

-9

10

-10

10

2

10

3

10

Frequency(Hz)

4

10

5

10

respond to the power law.
The power law dependency corresponds to the short range hopping of charge carriers through trap sites separated by energy barriers of varied heights.
The plot for the different temperature at high frequency region converges with one another, since the electronic conduction which is dominant in that region is independent of frequency.

Arrhenius plot of ac conductivity Vs (1000/T) in BZ thin film.
The plot contain two different regions corresponds

-7

10

100kHz

10kHz

LnσT(ω) (S.cm-1)

-8

10

1kHz

-9

10

0.1kHz -10

10

1.5

2.0

2.5

3.0 -1

1000/T (K )

3.5

to two different activation energies.
At lower temperature the obtained σac is independent of temperature upto 1270C. It’s probably due to the saturation in the number of liberated electrons from the donor states.
Beyond 1270C, the shape of the curve becomes more steeper and linear with the calculated activation energy of 1.5 eV. This might be attributed either to the oxygen vacancy motion or due to the deep trap space charge conduction mechanism.
The activation energy was considerably lower than the band gap energy, which implies that the conduction in this range of temperatures were dominated by the charge carriers other than electrons, possibly by ionic charge carriers such as oxygen vacancies.
The non linear shape of the Arrhenius plot indicated that, at different temperatures different mechanisms involved in the ac conduction process.

Impedance Analysis Imaginary part of the impedance Vs frequency 2.0x10

Imaginary part of the modulus Vs frequency

7

-5

1.5x10

0

1.5x10

7

7

125 150 175 200 225 250 275 300

''

5.0x10

T= 125 C 0 T= 150 C 0 T= 175 C 0 T= 200 C 0 T= 225 C 0 T= 250 C 0 T= 275 C 0 T= 300 C

-5

1.0x10

M''

Z (Ω)

1.0x10

T= T= T= T= T= T= T= T=

-6

5.0x10

6

0.0

0.0

10

2

10

3

10

4

10

5

Frequency (Hz)


No coincidence of peak frequency of Z″ and M″ ⇒ Non Debye type relaxation phenomenon.

2

10

10

3

4

10

5

10

Frequency (Hz)


Peak found to shift with increase of temperature towards high frequency ⇒ Single RC combination

This type of asymmetric behavior in real materials largely determined by the Johnscher’s power law

Conclusions:
Single phase polycrystalline Barium Zirconate thin films were deposited on the Pt coated Si substrates by pulsed excimer laser ablation technique.
The SIMS analysis exhibit a sharp transition existing at the substrate – film interface indicating that here was no interdiffusion in the case of in-situ annealed thin films.
In the case of ex-situ films the interdiffusion and segregation of ZrO2 at the interface was observed.
The calculated activation energy from the ac analysis was 1.5 eV and the activation energy is attributed to the oxygen vacancy migration.
The impedance spectroscopy reveals that the relaxation behavior was more like Johnscher’s model.