AC conductivity studies on electron irradiated ZnO: Li thin films

AC conductivity studies on electron irradiated ZnO: Li thin films

Balaji Biradar1, V M Jali1, a , Murali B2 and S B Krupanidhi2

1Department of Physics, Gulbarga University, Gulbarga 585 106, India

2Materials Research Centre, Indian Institute of Science, Bangalore 560 012, India

Abstract – In this article, we report the AC conductivity studies on electron irradiated Li doped Zinc Oxide thin films. The thin films in the geometry Al/LZO/Pt-Si were irradiated with 8 MeV electrons at different fluence viz. 3×1012, 3×1013 and 3×1014 electrons/cm2. The conductivity of pristine and irradiated thin films was measured in the temperature range 300350K. Conductivity increased with fluence. The order parameter s was found to decrease with fluence. The activation energy value calculated from Arrhenius plot increased with fluence. It increased from 0.15 to 0.55 eV for the highest fluence.

Keywords:

Li doped ZnO thin films, electron irradiation and ac conductivity

1. INTRODUCTION

   Zinc oxide is a wide band gap oxide semiconductor with hexagonal crystal structure (P63mc). It finds application in solar cells, gas sensors, LCDs, heat mirror and SAW devices because of its versatile properties [1]. The charge transport and dielectric relaxation phenomena are the vital characteristics for the thin film devices, in lieu of both practical and scientific reasons. It is known that material properties are essentially controlled by the inherent defects or charge carriers, which are created during the synthesis itself. Therefore, it is significant to investigate the charge carrier transport mechanism in these materials. The defects may also come from the electron irradiation when such devices are exposed to such radiations. The radiation induced charges will affect the charge distribution and thereby alter the local fields near the defects, the interface and the localized charges, which will be echoed as changes in device properties. It is of vital importance to understand the precise influence of irradiation induced charge carriers on the properties of the materials in the presence of the inherent charge carriers [2]. ZnO is considered to be the radiation hard material; there are several reports on irradiation induced changes in the properties of ZnO [3-5]. Znv, Zni, OV and Oi types of defects may be formed. To the best of our knowledge, there are no reports on the AC conductivity studies on electron irradiated Lithium doped ZnO thin films. Hence, an attempt was made to study the conducting properties of LZO thin films.

2. SYNTHESIS AND e-IRRADIATION

   The synthesis of the thin films was reported elsewhere [6]. The thin films were in M-I-M configuration (Al/Li: ZnO/Pt-Si). The deposited films were characterized for AC conductivity measurements using a KEITHLEY 3330 LCZ Meter. The well characterized films were exposed to 8 MeV electron irradiation in air at room temperature with fluence of 3×1012, 3×1013 and 3×1014 electrons/cm2 by using Microtron Accelerator. Post irradiation characterizations of the LZO thin films were carried out.

3. RESULTS AND DISCUSSION

   1. AC Conductivity

      Variation of AC conductivity with frequency of LZO thin films before and after irradiation is presented in fig.1. Before e-irradiation (Fig.1a), the AC conductivity had very weak frequency dependence at measured temperatures. However, it increased with temperature. After e-irradiation, it increased with an increase in fluence as shown in fig.2-I, at any given temperature. Material loses its dielectric/insulating behavior exhibiting enhanced conducting nature. The frequency dependence of AC conductivity is given by the power law,

      ac n

      A (T )[sin(n 1) / 2]s

      where An is a constant. The frequency dependent s is a characteristic parameter that represents the many body interactions of the electrons, charges and impurities. It depends on the material temperature (T) and varies from 0 to 1. For ideal Debye type samples, it is equal to 1. fig.2- II shows the variation of s parameter for both the unirradiated and irradiated LZO thin films, which was obtained from the linear regions of the corresponding plots in fig.1. Before irradiation, the exponent s decreased with increase in temperature. We anticipate that the temperature increase could lead to the increase in the contribution from deep traps and when the temperature is high enough for the band-to-band transition to govern the conduction process, s approaches zero, as electronic conduction is frequency independent. This may be attributed to the greater interaction between the dipoles participating in the polarization process. After irradiation, the s parameter decreased with an increase in the delivered fluence; it could be due to the enhancement in the contribution from many

      body interactions, because of the creation of large number

      -6

      1kHz 10kHz

      -6

      (a)

      of radiation induced defects.

      -9

      -12

      100kHz

      -9

      -12

      -4 (a)

      (b)

      -4

      -15

      ln

      ac

      -18

      -21

      -15

      ln

      ac

      -18

      -21 1kHz

      10kHz

      -6 -6

      -24

      -24

      (b)

      100kHz

      log( )

      ac

      log( )

      ac

      2.7 2.8 2.9 3.0 3.1 3.2 3.3 2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.3

      -8 -8

      -6 1000/T

      -6 1000/T

      (d)

      -10

      -12

      22

      33 44

      logf

      310K 320K

      330K

      340 K

      350 K

      55

      -10

      -12

      310 K 320 K

      330 K

      340 K

      350 K

      2 3 4 5

      logf

      -9

      -12

      ln

      ac

      -15

      -18

      -9

      ln

      ac

      -12

      -15

      -18

      (c)

      -4

      (d)

      -4

      -21 1kHz

      10kHz

      -21

      1kHz

      10kHz

      ac

      ac

      -6 -6

      -24 (c)

      100kHz

      -24

      100kHz

      log( )

      log( )

      -8 -8

      2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.3

      1000/T

      2.7 2.8 2.9 3.0 3.1 3.2 3.3

      1000/T

      -10

      310 K

      -10

      Fig. 3. Arrhenius plot, before and after electron irradiation at 10 kHz (a)

      320 K

      330 K 310 K

      Unirradiated (b) 3 X 1012

      (c) 3 X 1013

      (d) 3 X 1014

      electron/cm2 fluence.

      -12

      340 K

      2 3 4 5

      logf

      -12

      320 K

      2 3 4 5

      logf

4. CONCLUSIONS

Fig. 1. Variation of AC conductivity with frequency at room temperature of Zn0.93Li0.07O thin film before and after electron irradiation at 10 kHz (a)Unirradiated (b) 3 X 1012 (c) 3 X 1013 (d) 3 X 1014 electron/cm2 fluence.

(a)

(b)

The AC conductivity of the thin films increased with an increase in the fluence and it was ascribed to the presence of radiation induced trapped charges in the LZO thin film. The order parameter s decreased after irradiation. The

-4 (c)

(d)

log( )

ac

-5

-6

-7

1.5

S

1.0

0.5

0.0

(a)

(b)

(c)

(d)

activation energy values indicated the creation of shallow type defect levels after e-irradiation.

ACKNOWLEDGMENT:

Two of the authors (BB, VMJ) are thankful to Dr. Ganesh Sanjeev, Microtron Centre, Mangalore University,

2 3 4 5

logf

300 320 340 360

Temparature (K)

Mangalore (India) for extending the irradiation facilities.

Fig. 2. I) Variation of AC conductivity with electron fluence. II) Variation of s parameter with temperature for Zn0.93Li0.07O thin film before and after electron rradiation at 10 kHz (a) Unirradiated (b) 3 X 1012 (c) 3 X 1013 (d)

3 X 1014 electron/cm2 fluence.

1. Arrhenius plot

In order to understand the obtained results in terms of the activation energies, the Arrhenius plot (lnac vs. 1000/T) at three selected frequencies of 1, 10 and 100 kHz are shown in fig. 3 for both unirradiated and irradiated LZO thin films. The activation energy for the LZO thin films increased after irradiation as follows: for unirradiated film it was 0.15eV, after irradiation with the fluence 3 x 1012 and 3 x 1013electrons/cm2 it got increased to 0.50 eV and for the fluence 3 x 1014 electrons/cm2 it increased to 0.55eV, corresponding to the shallow traps. The increase in the activation energy after irradiation was due to the trapping of charge carriers by the radiation induced defects/shallow traps.

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3. O. Fukuoka, N Matsunami, M Tazawa, T. Shimura, M. Sataka, H Sugai, and S Okayasu,

   Irradiation effects with 100 MeV Xe ions on optical properties of Al-doped ZnO films Nucl. Instr. and Meth. Phys. Res. B, Vol. 250, pp. 295299, June 2006.

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   The effect of high-energy electron irradiation on ZnO-based ohmic and Schottky contactsSemicond. Sci. Technol., Vol. 21, pp. 16561660,August 2006.

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