Influence of films thickness and structure on the photo-response of ZnO films

Optics Communications 283 (2010) 1370–1377

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Optics Communications journal homepage: www.elsevier.com/locate/optcom

Influence of films thickness and structure on the photo-response of ZnO films M. Ali Yıldırım a,*, Aytunç Atesß b a b

Department of Science Education, Faculty of Education, Erzincan University, 24030 Erzincan, Turkey Department of Physics, Science Faculty, Atatürk University, Erzurum, Turkey

a r t i c l e

i n f o

Article history: Received 11 November 2009 Received in revised form 3 December 2009 Accepted 4 December 2009

Keywords: ZnO Annealing Film thickness and light effect SILAR

a b s t r a c t ZnO thin films were grown using Successive Ionic Layer Adsorption and Reaction (SILAR) method on glass substrates at room temperature. Annealing temperatures and film thickness effect on the structural, morphological, optical and electrical properties of the films were studied. For this as-deposited films were annealed at 200, 300, 400 and 500 °C for 30 min in oxygen atmosphere. The X-ray diffraction (XRD) and scanning electron microscopy (SEM) studies showed that the films are covered well with glass substrates and have good polycrystalline structure and crystalline levels. The film thickness effect on band gap values was investigated and band gap values were found to be within the range of 3.49–3.19 eV. The annealing temperature and light effect on electrical properties of the films were investigated and it was found that the current increased with increasing light intensity. The resistivity values were found as 105 X-cm for as-deposited films from electrical measurements. The resistivity decreased decuple with annealing temperature and decreased centuple with light emission for annealed films. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction Transparent Conducting Oxides (TCOs), such as zinc oxide, cadmium oxide, indium oxide, tin oxide, etc. have widely been studied for their use in optoelectronic device technology. Due to their optical and electrical properties, TCOs are used for photovoltaic solar cells, phototransistors, liquid crystal display, optical heaters, gas sensors, transparent electrodes and other optoelectronic devices [1]. Of these TCOs, zinc oxide (ZnO) has attracted most attention for various applications such as solar cells, transparent conducting films, chemical sensors, varistors, light-emitting diodes, UV photo detectors, laser diodes [2–5], gas sensors [6,7]. ZnO is n-type, wide direct band gap material that is sensitive to UV region. The large exciton-binding energy of 60 meV and wide band gap energy of 3.37 eV at room temperature make ZnO a promising photonic material for optoelectronic device technology [8]. The resistivity values of ZnO films may be adjusted between 104 and 1012 X-cm by doping and changing the annealing conditions [9–11]. Stoichiometric ZnO films are highly resistive, but these films can be rendered less resistive either by creating oxygen vacancies and Zn interstitials, which act as donors or by doping Al, Ga, In [12,13]. Nowadays almost all the investigations have been done by second harmonic generation (SGH) method without additional alignment of the ZnO nano-structural fragments. The structural parameters and substrates of ZnO films play an important role in the photoinduced optical second harmonic generation [14]. * Corresponding author. Tel.: +90 446 2240089; fax: +90 446 2231901. E-mail address: [email protected] (M. Ali Yıldırım). 0030-4018/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2009.12.009

Different chemical methods such as SILAR [15], Chemical Bath Deposition (CBD) [16], Spray Pyrolysis (SP) [17], Electrodeposition [18], etc. have been used to obtain ZnO films. The SILAR method, introduced by Nicolau [19], is a unique method by which thin films of compound semiconductors can be deposited alternately by means of the dipping substrate into the aqueous solutions of containing ions for each component. The growth of thin films during the SILAR method occurs only heterogeneously on the solid–solution interface due to the intermediate rinsing step between the cation and anion immersions. Therefore, the thickness of the film can easily be controlled by the number of growth cycles used [15,20]. The present paper defines the optimal annealing temperature and film thickness for ZnO thin films have been grown using the SILAR method. For defining the optimal annealing temperature and film thickness, annealing temperature and film thickness effect on the structural, morphological, optical and electrical properties of the films have been studied. Characterization of the films has been done using XRD, SEM, optical absorption measurements and the two-point-probe method.

2. Experimental procedure In this study, ZnO thin films were grown on glass substrates using SILAR method at room temperature and ambient pressure. Aqueous zinc-ammonia complex ions ([Zn(NH3)4]2+) were chosen for the cation precursor, in which using analytical reagents of ZnCl2 and concentrated ammonia (NH3) (25–28%) were used. The concentration values defined for the zinc solution was as 0.1 M and

M. Ali Yıldırım, Aytunç Atesß / Optics Communications 283 (2010) 1370–1377

the molar ratio of Zn:NH3 is 10:1 obtained as a result of several experiments. To deposit ZnO, one SILAR growth cycle involves the four following steps: (1) immersing the substrate in the precursor solution for 15 s to create a thin liquid film containing [Zn(NH3)4]2+ on the substrate; (2) immersing immediately the withdrawn substrates in hot water (90 °C) for 7 s to form a ZnO layer; (3) drying the substrate in the air for 60 s; and (4) rinsing the substrate in a separate beaker for 30 s to remove large and loosely bonded ZnO particles. The film thickness has been defined as 150, 200, 280 and 350 nm by repeating 40, 60, 80 and 100 SILAR growth cycle, respectively. The glass substrates were cleaned ultrasonically for 10 min, first in acetone and then in a 1:1 ethanol: water solution. The substrates were dried and stored in desiccators. For investigation of the annealing effect, the films were annealed at annealing temperature of 200, 300, 400 and 500 °C in an oxygen atmosphere for 30 min. For structural studies, a Rigaku 2200D/Max, X-ray Diffractometer using Cu Ka (k = 1.5405 Angıstrom (Ao)) radiation with 2h of 20°–70° was used. Surface morphology was studied using the Zeiss Supra 50 VP model SEM. In order to study the optical properties of the generated films, the absorption measurements were carried out using a Perkin–Elmer UV/VS Lambda 2S Spectrometer with a wavelength resolution better than ±0.3 nm at room temperature. Electrical characterizations of the films were performed using the two-point-probe method. Indium (In) contacts were used for all electrical measurements. These contacts were formed by evaporation of In dots on the films. For the effect of light on the films, I–V characteristics were measured in dark and under 150 W, 300 W and 500 W light emission. For this measurement, the tungsten halogen lamps with 340– 800 nm wavelengths were used. The halogen lamps were put into a place, which has an about 30 cm height to prevent the heating effect. 3. Result and discussion

½ZnðOHÞ2 ðsÞ ! ZnOðsÞ þ H2 O:

The mechanism of ZnO films formation by the SILAR method can be explained as follows: we have made full use of the thermal decomposition of [Zn(NH3)4]2+ in a neutral aqueous solution, which released ions of Zn2+ into the solution and has been resulted in the formation of ZnO or Zn(OH)2 particles. Eqs. (1)–(4) illustrate the chemical reactions related to the process 

ZnCl2 þ 2NH4 OH $ ZnðOHÞ2 þ 2NHþ4 þ 2Cl :

ð1Þ

The addition of excess ammonia solution reduces Zn2+ ions by producing the complex ions of the type ZnðNH3 Þ2þ n ; n = 4 is the most stable coordination number that avoids precipitation and makes the solution transparent. This process can be explained by the following reaction:

ZnðOHÞ2 þ 4NHþ4 $ ½ZnðNH3 Þ4 2þ þ 2H2 O þ 2Hþ :

ð2Þ

When the glass substrate is immersed in the solution above, these zinc complex ions get adsorbed on the substrate thanks to the attractive forces between ions in the solution and the surface of the substrate. These forces may be Van der Waals, cohesive or chemical attraction [21]. Then, the glass substrate is immersed in hot water, and the [Zn(NH3)4]2+ complex decomposes, forming Zn(OH)2:

½ZnðNH3 Þ4 2þ þ 4H2 O ! ZnðOHÞ2 ðsÞ þ 4NHþ4 þ 2OH :

ð3Þ

As-deposited Zn(OH)2 will transform to ZnO in an aqueous solution at temperatures over 50 °C:

ð4Þ

As a result, ZnO thin film was grown on glass substrate after these processes. 3.2. Structural analysis The structural analysis of ZnO films was carried out by using XRD varying the diffraction angle, 2h from 20° to 70o. The XRD patterns of the as–deposited and annealed films are shown in Fig. 1. The XRD patterns of the films indicate the existence of a ZnO single phase with a hexagonal wurtzite structure. As seen in Fig. 1(a–d), both as-deposited and annealed films have polycrystalline structure with orientation along with different planes. These planes are (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3) and (1 1 2). We observed that all of the films have the same crystal structure but the intensities and full width at half maximum (FWHM) values of these peaks changed with different annealing temperature and film thickness. The diffraction peak intensities increased with an increasing annealing temperature and film thickness. These changes may be attributed to the improvement of crystallinity of the films by annealing temperature and film thickness. It is seen from Fig. 1 that although some peaks disappeared with increasing annealing temperature and film thickness, the (0 0 2) peak is still dominant peak in all films. The best crystallinity is seen in 280 nm film thickness. For all films, it is seen that, the crystallinity begins to break down at 500 °C annealing temperature due to the glass substrate and increasing film thickness especially for 350 nm film thickness. All of these results are in agreement with relevant literature [5,15,16,22]. The structural parameters such as grain size (D), dislocation density (d), FWHM (b) for all films were evaluated by XRD patterns and presented in Table 1. The grain size of the thin films was calculated by XRD patterns using Debye Scherrer’s formula

D¼

3.1. Mechanism of film deposition

1371

0:9k ; b cos h

ð5Þ

where D is the grain size, k is the X-ray wavelength used, b is the angular line width at half-maximum intensity in radians and h is Bragg’s angle. We calculated the grain size and dislocation density of the films using the FWHM of (0 0 2) peak obtained through the Scherrer’s method. Additionally, to have more information on the amount of defects in the films, the dislocation density (d) was evaluated by the formula [23]

d¼

1 D2

:

ð6Þ

Larger D, smaller b and d values indicate better crystallization of the films. As can be seen in Fig. 1 and Table 1, the better crystallization turned out to be 280 nm for film thickness in accordance with to the D, b and d values. 3.3. Surface morphological analysis It is known that the surface properties of the TCO films influence their optical and electrical properties which are important factors in applications to optoelectronic devices. For example, the increase in surface roughness of the films leads to an increase in the propagation loss for surface acoustic wave (SAW) devices and a decrease in the efficiency of photovoltaic solar cells. Therefore, it is very important to investigate the surface morphology of the films. Fig. 2 shows the SEM micrographs of as-deposited ZnO thin films. It is obvious from Fig. 2a that the 150 nm thickness film does not have a smooth and homogeneous surface morphology with holes and cracks. All films are compact, uniform, dense, smooth and well-adhered to the substrates with

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450 375

(a)

o

500 C

500

(b)

o

500 C

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o

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(002)

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Intensity (arb. units)

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(103) (112)

(102)

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o

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450 300 150 750 600

1500 1250 1000 750 500 250 1250 1000 750 500 250 1000 800 600 400 200

o

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750

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o

200 C

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40

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2θ (degrees)

2000 1600 1200 800 400 0 2000 1600 1200 800 400 0 1500 1250 1000 750 500 250 750 600 450 300 150

(c)

o

500 C

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2θ (degrees) 500

(d)

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500 C

400 300 200 (002)

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400 C

(100)

(101) (102)

(110) (103) (112)

o

300 C

o

200 C

As-Grown

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30

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2θ (degrees)

Intensity (arb. units)

Intensity (arb. units)

600 500 400 300 200 100

(110) (103) (112)

1250 1000 750 500 250 900 750 600 450 300 150 900 750 600 450 300 150 700 600 500 400 300 200

(002)

o

400 C

(101) (100) (102)

(110) (103) (112)

o

300 C

o

200 C

As-Grown

20

30

40

50

60

70

2θ (degrees)

Fig. 1. The XRD patterns of as-deposited and annealed at 200, 300, 400 and 500 °C (a)150 nm, (b)200 nm, (c) 280 nm and (d) 350 nm ZnO thin films.

an increasing film thickness and for 280 nm thickness and all of the surface properties are better than the others. For 350 nm thickness (Fig. 2d), the level of surface roughness increases while the film surface deteriorates. Fig. 3 shows the SEM micrographs for annealed films. It can be seen from Fig. 3 that all of the films are homogeneous, dense and smooth without cracks and the grains combine together to form crystalline film by annealing. The surface properties of the films are improved with increasing

annealing temperature. Especially at 300 °C annealing temperature (Fig. 3c2), the film surface is better and more compact, denser and more uniform compared the others. The careful observation shows the presence of well-grown rods and flowerlike structures. Consequently, the surface properties of the ZnO films appear to have changed significantly with annealing temperature and film thickness. These results are in agreement with XRD measurements.

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M. Ali Yıldırım, Aytunç Atesß / Optics Communications 283 (2010) 1370–1377 Table 1 The grain size (D), dislocation density (d) and full width at half maximum (FWHM) values of as-deposited and annealed ZnO thin films. Material

As-deposited

200 °C

300 °C

400 °C

500 °C

ZnO (150 nm)

FWHM D (nm) d  104 (nm2)

0.3588 23.14 18.66

0.2664 31.19 10.27

0.2864 29.01 11.88

0.3018 27.53 13.19

0.3270 25.42 15.47

ZnO (200 nm)

FWHM D (nm) d  104 (nm2)

0.4945 16.80 35.43

0.3576 23.22 18.54

0.2658 31.21 10.26

0.2638 31.49 10.08

0.4064 20.44 23.93

ZnO (280 nm)

FWHM D (nm) d  104 (nm2)

0.3310 25.11 15.86

0.2675 31.06 10.36

0.2522 32.96 9.20

0.2500 33.25 9.04

0.4350 19.10 27.41

ZnO (350 nm)

FWHM D (nm) d  104 (nm2)

0.4606 18.05 30.69

0.2943 28.24 12.53

0.3037 27.36 13.35

0.3269 24.42 16.76

0.3409 24.38 16.82

Fig. 2. SEM images of as-deposited ZnO thin films [150 nm (a), 200 nm (b), 280 nm (c) and 350 nm (d)] at 15 000 magnification.

3.4. Optical characterization Absorption measurements were carried out at room temperature. The energy band gaps of as-deposited and annealed ZnO films were calculated by using the optical absorption spectra. To determine the energy band gap values, we plotted (ahm)2 versus (hm) where a is the absorption coefficient and hm is the photon energy. The theory of interband absorption shows that at the edge of optical absorption the absorption coefficient a varies with the photon energy hm according to Ref. [24]

a¼

A ðhm  Eg Þn ; hm

ð7Þ

where A is a constant, Eg is the optical band gap and n assumes values of 1/2, 2, 3/2 and 3 for allowed direct, allowed indirect, forbid-

den direct and forbidden indirect transitions, respectively. The band gap energies of films were determined by the extrapolation of the linear regions on the energy axis (hm), as shown in Fig. 4. The annealing temperature and film thickness effect on the absorption measurements was investigated. The film quality was broken down at 500 °C annealing temperature due to glass substrate. Thus, optical and electrical properties of the films cannot be studied after 500 °C. The optical absorption spectra of as-deposited and annealed films are shown in Fig. 4. As can be seen from Fig. 4 and Table 2, the band gap energies decreased with increasing annealing temperature. The decrease in band gap with increasing annealing temperature can be attributed to the improvement in the crystals, in morphological changes of the films and in changes of atomic distances and grain size. This result is in agreement with literature, XRD and SEM results [16,20,25–27]. The other reason for

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Fig. 3. SEM images of as-deposited (c) and annealed ZnO thin films [200 °C (c1), 300 °C (c2), 400 °C (c3) and 500 °C (c4)] at 15 000 magnification.

this behaviour, the most annealing treatments cause evaporation of oxygen and the sample become more Zn-rich. Since the O-rich samples usually have lower band gap energy, relatively more Znrich samples after annealing due to the evaporation of excess oxygen from the surface will have higher band gap energies in comparison to the as-deposited samples. On the other hand, we propose that this discrepancy might be resulted from the air annealing conditions where oxygen loss from surface is impossible so that the material may become relatively more O-rich at the annealing temperatures and has lower band gap energy after annealing [28,29]. As can be seen Figs. 4 and 5 and Tables 2, the band gap energies decreased from 3.49 to 3.19 eV with increasing film thickness. There are several reasons for this behaviour, which are given in the literature as impurity levels, and structural changes [16,25,30,31]. The first reason is inoculating the impurity levels

with conductivity band. During the growth process some impurity (oxygen vacancies and/or Zn interstitials, etc.) levels form the edge of conduction band and these levels can inoculate with the conduction band when the thickness increased. There is a possibility of structural defects in the films due to their preparation at room temperature; this could give rise to the allowed states near the conduction band in the forbidden region. In case of thick films, these allowed states may as well merge with the conduction band resulting in the reduction of the band gap. 3.5. Electrical characterization Electrical characterizations of the films were investigated by using the two-point DC probe method. I–V behaviour for as-deposited and annealed films and the effect of light on I–V behaviour of films were investigated. For investigation of the light effect on

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10

4.0x10 ZnO (150nm) As-Deposited o 200 C Annealed o 300 C Annealed o 400 C Annealed

2.7x10

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ZnO (200 nm) As-Deposited o 200 C Annealed o 300 C Annealed o 400 C Annealed

10

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10

-1 2

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10

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(αhν) (eV cm )

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(αhν) (eV cm )

-1 2

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ZnO (350 nm) As-Deposited o 200 C Annealed o 300 C Annealed o 400 C Annealed

2

2

(αhν) (eV cm )

10

-1 2

ZnO (280 nm) As-Deposited o 200 C Annealed o 300 C Annealed o 400 C Annealed

1.1x10

3.2

hν (eV)

9

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hν (eV)

2.8

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3.1

3.2

3.3

3.4

hν (eV)

Fig. 4. Graphs of (ahm)2 versus hm for as-deposited and annealed at 200, 300, 400 °C ZnO films.

Table 2 Annealing temperature and band gap values for as-deposited and annealed ZnO thin films. ZnO (nm)

Eg (eV)

150 200 280 350

As-Deposited

200 °C

300 °C

400 °C

3.49 3.31 3.21 3.19

3.26 3.23 3.11 3.12

3.18 3.17 3.05 3.07

2.99 3.12 2.97 3.05

3.50

ZnO 3.45

Band gap (eV)

3.40 3.35 3.30 3.25 3.20

films, I–V measurements were made in dark and under 150, 300 and 500 W light illumination (see Fig. 6). As can be seen in Fig. 6, irradiation could cause an increase in the current values of both as-deposited and annealed films. The maximum increasing in the current values was observed under illumination at 500 W lights. For both as-deposited and annealed films, the current values increased with increasing annealing temperature. Thus, as-deposited films were found to be more resistant than the annealed films. It can be observed from Fig. 6 and Table 3 that the resistivity values of both as-deposited and annealed films decreased from 105 to 104 X-cm with increasing annealing temperature and reduced to 103 X-cm with light intensity, indicating a semiconductor electrical behaviour. These values are agreement with the literature [10,11,16]. This decrease in resistivity values can be attributed to the improvement of the crystallization and these results agree with the XRD results. Another reason is the defect of oxygen vacancies and/or Zn interstitials. The defect acts as donors in ZnO films. During the annealing process, these impurity (oxygen vacancies and/or Zn interstitials, etc.) levels form the edge of conductivity band. Consequently, these impurity levels are responsible for low resistivity of annealed films. Fig. 7 shows the photo-response characteristics of ZnO film as a function of annealing temperature. The photosensitivity of a semiconductor can be explained as

S¼

3.15 150

200

250

300

350

Film Thickness (nm) Fig. 5. The band gap energy changing with increasing film thickness.

Ilight  Idark ; Idark

ð8Þ

where Ilight and Idark are the currents measured under illumination and in the dark, respectively [32]. As can be seen in Fig. 7, the photosensitivity increased with increasing light intensity. The highest increase was observed under

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2.8x10 -4

ZnO (As-Deposited) Dark 150 Watt Light 300 Watt Light 500 Watt Light

1.8x10

-4

-4

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-4

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ZnO (Annealed at 200°C) Dark 150 Watt Light 300 Watt Light 500 Watt Light

-4

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-5

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100

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-2

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Voltage (V)

Voltage (V)

Fig. 6. I–V behaviour for as-deposited (280 nm) and annealed ZnO thin films at and light effect on I–V behaviour of films.

Table 3 The resistivity values for as-deposited and annealed ZnO thin films.

80

q (X-cm) Dark

As-deposited Annealed at 200 °C Annealed at 300 °C Annealed at 400 °C

150 W Light

300 W Light

500 W Light

70

11.25  105 8.04  105

6.43  105 40.9  104

40.91  104 34.61  104

37.5  104 28.49  104

4

3

3

3

32.14  10

15  104

18.75  10

15  103

7.5  10

6.25  103

3.75  10

3  103

Photosensitivity

ZnO

90

ZnO (V=50 V) 150 Watt Light 300 Watt Light 500 Watt Light

60 50 40 30 20 10

that illuminated 500 W light. Especially at 300 °C annealing temperature, the highest increase in photosensitivity values was observed. As a result, we can say that ZnO film is very sensitive to light [11] and the sensitivity at 300 °C annealing temperature is too high.

0 0

100

200

300

400

o

Annealing Temperature ( C) Fig. 7. The photo-response characteristic of ZnO film as a function of annealing temperature.

4. Conclusions The SILAR method was used to form ZnO thin films on glass substrates at room temperature. For defining the optimal annealing temperature and film thickness, structural, morphological, optical and electrical properties of the films were studied as a function of annealing temperature and film thickness. The XRD and SEM studies showed that as-deposited films have a polycrystalline structure and the crystalline and surface properties of these thin

films are improved with increasing annealing temperature and film thickness. The band gap values decreased with increased annealing temperature and film thickness. The band gap energy values reduced from 3.49 to 3.19 eV with increasing film thickness. It was found that the current values increased with increased annealing temperature and light intensity. As-deposited films were found to be more resistant than the annealed ones. ZnO films are

M. Ali Yıldırım, Aytunç Atesß / Optics Communications 283 (2010) 1370–1377

very sensitive to light. By using all of these results, optimal annealing temperature and film thickness values are defined as 300 °C and 280 nm for this work, respectively. Consequently, we can say that the SILAR method is suitable method for depositing ZnO thin films.

[11] [12] [13] [14]

Acknowledgement

[17]

We would like to acknowledge the financial support given by _ the TUBITAK Foundation, Project No. 107T097. References [1] R.K. Gupta, K. Ghosh, R. Patel, S.R. Mishra, P.K. Kahol, Mater. Lett. 62 (2008) 3373. [2] W.J. Jeong, S.K. Kim, G.C. Park, Thin Solid Films 506 (2006) 180. [3] M. Mikawa, T. Moriga, Y. Sakakibara, Y. Misaki, K. Murai, I. Nakabayashi, K. Tominaga, Mater. Res. Bull. 40 (2005) 1052. [4] S. Lee, E. Shim, H. Kang, S. Pang, J. Kang, Thin Solid Films 473 (2005) 31. [5] F. Yakuphanoglu, Y. Caglar, S. Ilican, M. Caglar, Physica B 394 (2007) 86. [6] P. Mitra, A.P. Chatterjee, H.S. Maiti, Mater. Lett. 35 (1998) 33. [7] B.B. Rao, Mater. Chem. Phys. 64 (2000) 62. [8] S.J. Young, L.W. Ji, S.J. Chang, Y.K. Su, J. Crystal Growth 293 (2006) 43. [9] D. Bao, H. Gu, A. Kuang, Thin Solid Films 312 (1998) 37. [10] B. Ismail, M.A. Abaab, B. Rezig, Thin Solid Films 383 (2001) 92.

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