Structural and optical properties of transparent

Structural and optical properties of transparent polycrystalline ZnO films O. Gencyılmaz, S. Karakaya, F. Atay, O. Ozbas, and I. Akyuz Citation: AIP Conf. Proc. 1476, 216 (2012); doi: 10.1063/1.4751598 View online: http://dx.doi.org/10.1063/1.4751598 View Table of Contents: http://proceedings.aip.org/dbt/dbt.jsp?KEY=APCPCS&Volume=1476&Issue=1 Published by the American Institute of Physics.

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Structural and Optical Properties of Transparent Polycrystalline ZnO Films O. Gencyılmaza,b, S. Karakayaa, F. Ataya, O. Ozbasa and I. Akyuza a

Eskisehir Osmangazi University, Department of Physics, 26480 Eskisehir, Turkey b Cankırı Karatekin University, 18100 Cankırı, Turkey [email protected], [email protected], [email protected]

Zinc oxide is one of the promising transparent conducting oxide materials because of its interesting physical properties. In this work, ZnO thin films have been prepared by ultrasonic spray pyrolysis technique. Structural properties of the films have been characterized by X-ray diffraction. Transmittance spectra have been taken by UV-VIS spectrophotometer. The films have been found to exhibit high transmittance (approximately 85 %). Optical constants of ZnO film such as refractive index (n) and extinction coefficient (k) have been calculated from the transmittance spectra by using envelope method. Also absorption coefficient (α) and thickness of the film have been calculated using this method. Optical method has been used to determine the band gap value of the film. To investigate the surface properties of the film, atomic force microscope images have been taken. Keywords: ZnO, Ultrasonic spray pyrolysis, Envelope method, Optical properties. PACS: 78.20.Ci, 61.05.cp, 68.55.ag, 78.66.Hf, 68.55.jd.

INTRODUCTION ZnO have attracted an enormous interest, due to its high optical transparency in the visible region and low electrical resistivity [1, 2]. Owing to these properties ZnO have been used for several technological applications such as window layer in heterojunction solar cells, light emitting diodes, piezoelectric devices, gas sensors etc. [2-4]. Several techniques have been used to prepare ZnO thin films such as chemical vapor deposition [5], sol gel method [6], thermal evaporation [7], radio frequency sputtering [8] and ultrasonic spray pyrolysis (USP) [9-11]. Among these methods, the USP is one of the most commonly used technique since it is simple, safely and economical method which is convenient for large area coatings. The aim of this work has been to investigate ZnO film’s structural and optical properties. So, the structure of the films was analyzed by using X-ray diffraction patterns. In order to determine the optical constants such as refractive index n and extinction coefficient k and the film thickness, the widely used envelope method has been used.

EXPERIMENTAL DETAILS ZnO thin films were prepared by an USP method. In the USP process, zinc acetate [Zn(CH3COO)2.2H2O] diluted in deionized water (1:1) was selected as the zinc precursor at a concentration of 0.1 M. Also, spray solution were added a few droplets of hydrochloric acid. The solution was sprayed onto a glass substrate held at 573 K, using as carrier gas air at a flow rate of 5 ml /min. Details of the spray pyrolysis technique were published elsewhere [12-13]. The optical properties were obtained using a Perkin Elmer Lambda 2S Shimadzu spectrophotometer in the UV-VIS regions. Swanepoel’s envelope method was employed to evaluate the optical constants such as the absorption coefficient () refractive index (n), extinction coefficient (k) from 2nd International Advances in Applied Physics and Materials Science Congress AIP Conf. Proc. 1476, 216-220 (2012); doi: 10.1063/1.4751598 © 2012 American Institute of Physics 978-0-7354-1085-5/$30.00

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transmittance spectra [14-16]. The thickness of ZnO films was determined from interference fringes of transmission data measured over the visible range. The structural properties were obtained by X-Ray Automatic Diffractometer with CuKα (λ=1.54059 Å radiation). Surface images and roughness values of ZnO films are determined by using atomic force microscopy (AFM) (Park System XE-70 AFM).

RESULTS AND DISCUSSION The structural properties of ZnO films were investigated by XRD diffractometer. Fig. 1 shows measured XRD patterns of ZnO films. XRD measurement exposed that ZnO films obtained were polycrystalline with a hexagonal wurtzite structures and the films grow preferentially along the c-axis orientation. The peak at 2 = 34.72 ° is the most intensive peak and the (002) direction determined for this peak is the preferential orientation of ZnO film. The mean crystallite size of ZnO films was calculated by means of the Scherrer formula [17]; =

0.9

(1)

where D is the grain size of crystallite, is X-ray wavelength, is Bragg diffraction angle, is full width at half maximum of (101) diffraction peak, respectively. The grain size of ZnO films was obtained as 298 nm. The preferential growth orientation was evaluated by using the texture coefficient TC(hkl) expression [18]; (ℎ ) =

1 (ℎ ) / 0(ℎ ) ∑ (ℎ ) / 0(ℎ )

(2)

where I(hkl) is the measured intensity, I0(hkl) is the ASTM standart intensity and N is the reflection number. The spacing between adjacent atomic planes d, full width at half maximum and texture coefficients TC, dislocation intensity, macro stretch are presented in Table 1. TABLE 1. The X-ray diffraction data results of the ZnO films. 2 (degree)

d (Å)

(hkl)

x10-3 (radian)

D(Å)

31.94

2.799

(100)

6.51

224

1.99x10-5

-6.10

150

-5

-5.18

34.64

2.587

(002)

9.63

(line/nm2)

4.45x10

10-3

Fig. 2 shows transmittance spectra of ZnO films. Fig. 2 indicates interference fringes pattern in transmission spectrum. This interference fringes revealed that the ZnO film has smooth surface and there was low scattering and absorption at the film surface. Because of this reason, the film exposed high transparency (90 %) in the visible region.


FIGURE 1. (a) XRD spectra and (b) Transmittance spectra of the ZnO films.

The optical band gap Eg of ZnO films at different preparative parameters was calculated based on the optical spectral absorption using the well-known formula [21];

ℎ − = ℎ

(3)

where A is constant, h is the incident photon energy, and m is a factor whose value depends on the nature of band transition; m=1/2 or m=3/4 for direct allowed and direct forbidden transition, and m=2 or m=3 for indirect allowed and indirect forbidden transition, respectively. The variation of (h )2 versus h for the ZnO films are illustrated in Fig. 2. The information is obtained about surface quality of the films and homogenity by using interfernece fringes of the transmission spectra. If the film’s surface is quality and homogenous, the interference fringes can appear in the transmittance spectra [19, 20]. The envelope method [18] gives a simple solution to calculate the optical parameters of the transparent thin films. Thickness (d), refractive index (n) and extinction coefficient (k) of ZnO film were calculated by the envelope method [16, 18] and results were shown in Fig. 3. The index of refraction n at different wavelengths was calculated using the envelope curve for Tmax (TM) and Tmin (Tm) in the transmission spectra [14]. The refractive index expression is determined from the following equations; 1/2

= + ( 2 − 2 )1/2

=

(2 + 1) ( − ) + 2 2

(4)

where ns is the refractive index of the substrate. TM and Tm are maximum and minimum transmittances at the same wavelength in the fitted envelope curves on the transmittance spectra.


FIGURE 3. Plots of refractive index and extinction coefficient of the ZnO films.

The thickness of the ZnO films is calculated using the following equation [14]: =

1 2 2(1 2 − 2 1 )

(5)

where n1 and n2 are the refractive indices corresponding to wavelengths 1 and 2, respectively [12]. The thickness of the ZnO film was found to be 537 nm. The extinction coefficient k can be calculated from the following formula; 1/2 1 ( − 1)( − ) + 1 = 1/2 ( + 1)( − ) − 1

(6)

where

is the absorption coefficient and d is the film thickness. 1 and 2 are the wavelengths at the two adjacent maxima or minima. Fig 4. displays three-dimensional surface morphology images of ZnO film. The scanning area was 2 m 2 m. The measurements were taken in non-contact mode, 300 kHz frequency and 0.75 Hz scan rate in air at room temperature. Also, root mean square (RMS, Rq) and average (Ra) roughness values were obtained using XEI Version 1.7.1 software. Rq and Ra vaules of ZnO film are obtained as 30 nm and 24 nm, respectively. It is clear that dominant growth mechanism is island like formation for the ZnO film. There are clusters in different height and size through the surfaces of the films. Black regions in these micrographs represent the cracks and voids, where the film continuity suffers.

FIGURE 4. Three-dimensional surface morphology image of the ZnO films.


CONCLUSION Transparent ZnO thin films were successfully prepared by the spray pyrolysis technique on glass substrate. The structural and optical properties of the ZnO films were investigated. The films have the hexagonal wurtzite structure of ZnO and preferred orientation in the (002) plane. The films presented high transparency ( 90 %) in the visible region. Optical constants such as the refractive index n and extinction coefficient k were determined from the transmittance spectra in the UV–VIS regions using the envelope method.

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