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Advances in Control, Chemical Engineering, Civil Engineering and Mechanical Engineering

Effect of deposition temperature on the crystalline structure and surface morphology of ZnO films deposited on p-Si SEVAL AKSOY, YASEMĐN CAGLAR, SALIHA ILICAN, MUJDAT CAGLAR Department of Physics Anadolu University Eskisehir, 26470 TURKEY [email protected] http://www.semiconductorslab.com Abstract: - Zinc oxide (ZnO) films were deposited on p-Si substrates by sol–gel spin coating method. Zinc acetate dihydrate (ZnAc), 2-methoxyethanol and monoethanolamine (MEA) were used as a starting material, solvent and stabilizer, respectively. The films were deposited 500°C, 600°C, 700°C and 800°C for 1 h in air. The effects of deposition temperature on the crystallinity and morphological properties of ZnO films were assessed by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM). The important changes in crystalline structure of the films were observed due to the deposition temperature. The crystallite size, texture coefficient and lattice constant of the films have been calculated. The FESEM measurements showed that the number of particles which has hexagonal structure increased with increasing deposition temperature.

Key-Words: - ZnO, sol–gel, deposition temperature, crystalline structure, FESEM, reflectance certain temperature when the ZnO films are preheated at the same temperature. In this work, ZnO films were prepared by sol–gel spin coating method on (1 0 0) p-type single-crystalline Si substrates. The effect of deposition temperature on crystalline structure and surface morphology of the film was investigated.

1 Introduction Zinc oxide (ZnO) is a wide band gap (3.3 eV) n type semiconductor with a high exciton binding energy (60 meV) at room temperature, which is of interest for a variety of practical applications including transparent conductive coatings, dye-sensitized solar cells, gas sensors, and electro-/photo-luminescent materials. Their absorbing optical, electronic, and mechanical properties are highly creditable for promising nanodevices. For example, epitaxial ZnO films have demonstrated enormous potential for developing blue lasers and light emitting diodes [1–3]. The films have been prepared by various dry processes such as pulsed laser deposition (PLD) [4], metal organic chemical vapor deposition (MOCVD) [5], chemical vapor deposition (CVD) [6], molecular beam epitaxy (MBE) [7], magnetron sputtering [8], and electron beam evaporation [9].Wet processes such as electrochemical deposition [10], spray pyrolysis [11,12], sol–gel [13,14], and hydrothermal method [15– 17] are also valuable for the preparation of the oxide film. There are reports in the literature about the effect of deposition temperature on the structural properties of ZnO films obtained by using sol–gel spin coating method. In the one of them Asghar et al. [18] reported that the effect different annealing temperature on the structural properties of ZnO films which were prepared by evaporation. Wang et al. [19] reported that the the influence preheating and annealing temperatures on the residual stress, grain size and resistivity of the ZnO films. They found that the c-axis orientation is stronger as the annealing temperature increases, and it is weaker after a

ISBN: 978-960-474-251-6

2 Experimental In this study, ZnO films were deposited by a sol–gel process using a spin coating method onto p-Si substrates. Firstly, Si wafer was degreased through RCA cleaning procedure, i.e., a 10 min boiling in NH4OH+H2O2+6 deionized (DI) (18 MΩ DI water), which was followed by a 10 min boiling in HCl+H2O2+6 DI. Before forming an ZnO layer on p-Si substrate, the native oxide on the polish surface of the substrate was removed in HF:H2O (1:10) solution, and finally, the wafer was rinsed in DI water. Zinc acetate dihydrate (ZnAc), 2-methoxyethanol and monoethanolamine (MEA) were used as a starting material, solvent and stabilizer, respectively. The molar ratios of ZnAc to MEA were maintained at 1:1. The sol was stirred at 60oC until a transparent and homogeneous solution is obtained. The sol was dripped on the center of the substrate, which was immediately spun at 4000 rpm for 30 s. After each coating, the coated films were dried at 300°C for 10 min. The coating–drying cycles were repeated ten times. The films were deposited at 500°C, 600°C, 700°C and 800°C for 1 h in air. The thickness of the films were determined with Mettler Toledo MX5 microbalance by using weighing method. X-ray diffraction (XRD) experiments were performed in air

227


where I (h k l) is the measured relative intensity of a plane (h k l). Io (h k l) is the standard intensity of the plane (h k l) taken from the JCPDS data, N is the reflection number and n is the number of diffraction peaks. A sample with randomly oriented crystallite presents TC(h k l) = 1, while the larger this value, the larger abundance of crystallites oriented at the (h k l) direction. It is seen that the highest TC is in (0 0 2) plane for the films.

with a laboratory X-ray powder diffractometer (D8 Advance, BRUKER AXS). The diffractometer reflection of all the films was taken at room temperature. The films were mounted on rotating sample holders (15 rpm). A sealed X-ray tube operated at 40 kV and 40 mA with CuKα radiation was used. Surface morphology was studied using a ZEISS Ultraplus model field emission scanning electron microscope (FESEM). Optical reflectance measurements were recorded with a double beam Shimadzu UV 2450 spectrophotometer with specular reflectance attachment at an incident angle of 5◦.

Table 1. (h k l), 2θ, d, d%, TC values of the ZnO films deposited at different temperatures. Deposition Temperature

3 Result and discussion

500 oC 600 oC 700 oC 800 oC

D=

o

o

700 C o 800 C 30

40

50 2θ (degree)

60

Figure 1. XRD pattern of the ZnO films deposited at different temperatures ( :p-Si substrate).

2θ, d-values and Texture coefficient (TC) values for the films are given in Table 1. The texture coefficient (TC) represents the texture of the particular plane, deviation of which from unity implies the preferred growth. The different texture coefficient TC (h k l) has been calculated from the X-ray data using the well-known formula [20].

I ( hkl ) / I o ( hkl )

TC( hkl ) = N

TC

100 002 101 100 002 101 100 002 101 100 002 101

2.81413 2.60577 2.47700 2.81720 2.60715 2.48100 2.82721 2.61187 2.48558 2.82873 2.60918 2.48259

0.08 2.82 0.10 0.06 2.84 0.09 0.06 2.87 0.07 0.05 2.88 0.07

31.772 34.389 36.237 31.737 34.370 36.176 31.621 34.306 36.107 31.604 34.342 36.152

−1

∑

(2)

The thicknesses of ZnO films deposited at 500°C, 600°C, 700°C and 800°C were found to be 339, 441, 536 and 623 nm, respectively. The thicknesses of the films increased with increasing deposition temperature. As a result, the thickness of the ZnO thin film deposited at high substrate temperature is larger than at low temperature.

(1)

I ( hkl ) / I o ( hkl ) n

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kλ β cos θ

where D is crystallite size, λ is the X-ray wavelength used, β (or FWHM) is the angular line width of half maximum intensity, θ is Bragg’s diffraction angle and k is a constant. The variation of crystallite size with deposition temperature is shown in Fig. 2. It is observed that the crystallite size increases with increasing deposition temperature. This is due to the improvement in the crystallinity of the films.

o

500 C 600 C

d

The crystallite sizes of ZnO films deposited at different temperatures were calculated using Scherrer’s formula [21].

(002)

Intensity (arb. units)

Fig. 1 shows the XRD spectra of the ZnO films deposited at 500°C, 600°C, 700°C and 800°C. The observed indexed peaks in these XRD patterns are fully matched with the corresponding hexagonal wurtzite structure ZnO (zincite, PDF number: 036-1451) Analyses of XRD data reveal peaks corresponding to the (1 0 0), (0 0 2) and (1 0 1) planes of the hexagonal ZnO crystal structure. Intensity of peak for (0 0 2) plane increased with increasing deposition temperature. It is clearly observed that the intensity of ZnO(0 0 2) decreases with increasing deposition temperature.

(hkl) 2θ

228

70

14

60

12

50

10

40

8

30

6

20

4 crystallite size

10

2

Lattice constant values (a and c) are given in Table 2. Lattice parameters were calculated using an analytic method for all the ZnO films [21]. As given in Table 2, c values tend to increased. a/c ratios of the films are 0.62351, 0.62386, 0.62495 and 0.62593 for the films deposited at 500°C, 600°C, 700°C and 800°C, respectively. The Zn–O bond length L is given by

Dislocation density(1/nm2)x10-4

Crystallite size (nm)


2  a2  1   L =  +  − u  c2   3 2   

dislocation density

0

0 500

600

700

800

(3)

Temperature (oC)

where the u parameter is given by (in the wurtzite structure)

Figure 2. Crystallite size and dislocation density values of the ZnO films dependence on deposition temperatures

a2 u = 2 + 0.25 3c

Meanwhile, in the films deposited at high deposition temperature, the number of grains with the caxis orientation is large. So the XRD curve becomes smooth and the intensity of ZnO(0 0 2) peak becomes strong with the increase of deposition temperature. The crystallinity of the specimens becomes well.

and relates to a/c ratio. The Zn–O bond lengths are 1.97825, 1.98006, 1.98600 and 1.98609 for the films deposited at 500°C, 600°C, 700°C and 800°C, respectively. Therefore, the heat treatment has significant effect on Zn–O bond lengths. These results are consistent with the literature [23].The X-ray diffraction studies revealed that the optimized deposition temperature for the ZnO films with preferred (0 0 2) orientation is T = 80oC.

Fig 3. shows the FWHM values of the ZnO films. FWHM values of ZnO film decreased with increasing deposition temperature which indicate that the crystallinity of the films has been improved with increasing deposition temperature. The dislocation density, which represents the amount of defects in the film, is determined from the formula δ=1/D2 [22]. These values are shown in Fig 3. The larger D and smaller FWHM values indicate better crystallization of the film. It is observed that the crystallite size values increase with deposition temperature, which clearly reveals the improved crystallinity. Dislocation densities exhibit a decreasing trend with increasing deposition temperature.

Table 2. Lattice parameters of the ZnO films deposited at different temperatures. Temperature 500 oC 600 oC 700 oC 800 oC

FWHM (deg)

0.4

a (Å)

c (Å)

3.24948 3.25302 3.26458 3.26634

5.21154 5.21430 5.22374 5.21836

FESEM micrographs for the films deposited at 500°C, 600°C, 700°C and 800°C are shown in Fig. 4. As seen in micrograps, the particles size increases with the deposition temperature. The number of particles which has hexagonal structure increased with increasing deposition temperature. As the voids were not observed at 500°C and 600°C. Thus, these films was more homogeneous than others.

0.3

0.2

0.1 400

(4)

500

Figure 3. FWHM temperature.

600 700 800 Temperature (°C)

values

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dependence

900

on deposition

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Figure 5 shows that the cross-sectional FESEM images of the ZnO film deposited at 800°C temperature. This FESEM image was used to estimate the film thickness values. This value is agreed with the obtained from the weighing method. The reflectance spectra of the ZnO films are shown in Fig. 6. The average reflectance values decrease with increasing deposition temperature. 100 o

500 C; Rave 23%

80

o

600 C; Rave 21% o

700 C; Rave 18%

R%

60

o

800 C; Rave 14%

40 20 0 400

500

600

700

800

Wavelength (nm)

Figure 6. Reflectance spectra of the ZnO films deposited at different temperatures.

4 Conclusion ZnO films can be deposited by a sol–gel spin coating method on (1 0 0) p-type silicon substrates. The structural and morphological properties of the ZnO films were influenced by deposition temperature. XRD pattern of ZnO films showed polycrystalline wurtzite with a preferential (0 0 2) orientation. It is observed that the texture coefficient in the ZnO films increases along the (0 0 2) direction with the deposition temperature. We can say from both the structural and morphological results that crystallite size increased with increasing deposition temperature. FESEM images indicated that the film is deposited at 800°C has the highest crystallity size. Reflectance spectra of the ZnO films showed that highest average reflectance value was in 500°C.

Figure 4. FESEM images of the ZnO films deposited at different temperatures.

Acknowledgements This work was supported by Anadolu University Commission of Scientific Research Projects under Grant No. 061039 and 081029.

Figure 5. Cross-sectional FESEM images of the ZnO film deposited at 800°C temperature.

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