Effect of annealing temperature on the structural and optical properties of Al-doped ZnO films by RF magnetron sputtering Yuebo Wua, b, Bo Huangb, c, Liangtang Zhanga, Suntao Wu*b Department of Mechanical and Electrical Engineering, Xiamen University, Xiamen, Fujian, China 361005；
a
b
Pen-Tung Sah MEMS Research Center, Xiamen University, Xiamen, China 361005； c Department of Physics, Xiamen University, Xiamen, China 361005 ABSTRACT
The Al-doped ZnO (AZO) films were deposited on the glass substrates by RF magnetron sputtering. After the deposition, the films were annealed in N2 at several temperatures from 500℃ to 800℃ for 60 minutes respectively. The crystal structures of the AZO films were characterized and analyzed by X-ray diffraction. The surface morphologies of the films were observed by SEM. The transmission spectra of the films were measured using a spectrophotometer within the range from 200 to 800 nm at room temperature. The results indicate each of the films has a preferential c-axis orientation and the grain size increases with annealing temperature increasing. All the films exhibit a high transmittance in visible region and have sharp ultraviolet absorption characteristics. Keywords：AZO film, Annealing, XRD, SEM, Transmittance
1. INTRODUCTION ZnO film is attracting more and more attention recently, due to its remarkable properties and potential applications in surface wave devices, gas sensor, transparent electrodes for optoelectronic devices, etc. AZO film is a kind of important optoelectronic material. It has many merits such as abundance in natural resource, low cost, non-toxicity, high stability under hydrogen plasma, etc. It has promising applications in many areas such as solar cells, light emitting diodes, photodetectors, etc[1-4]. Many techniques have been used for the fabrication of AZO film, including magnetron sputtering[5,6], metal-organic chemical vapour deposition (MOCVD)[7], pulsed laser deposition (PLD)[8], spray-pyrolysis[9], molecular beam epitaxy (MBE)[10], sol–gel[11] and so on. Among these techniques, magnetron sputtering is convenient and AZO film can be obtained at relatively low substrate temperature and high deposition rate. It has been reported that the effect of annealing on the properties of AZO films by many groups. However, annealing ambient gas species, temperature and time are different for different groups[12-16]. In this work, we investigate the effect of annealing temperature on the structural and optical properties of AZO films deposited by RF magnetron sputtering.
2. EXPERIMENT The AZO films were deposited on the glass substrates by RF magnetron sputtering using a target mixed by ZnO (99.99%) and Al2O3 (3wt%) in a gas mixture of Ar (99.99%) and O2 (99.99%) with Ar/O2 ratio 1:1. During the films deposition, the total pressure was maintained at 1 Pa. The sputtering power was 200W. The substrate temperature was kept at 200℃. The sputtering time was 60 minutes. After the deposition, the films were annealed in N2 at temperatures: 500℃, 600℃, 700℃, 800℃. The annealing time was 60 minutes. These samples were labeled S1, S2, S3, S4 respectively. The crystal structures of the AZO films were analyzed by X-ray diffraction (XRD), using a PANalytical X’pert PRO X-ray diffractometer. The surface morphologies of the films were observed by LEO-1530FE-SEM. The transmission spectra of the films were measured with a Varian Cary-300 spectrophotometer within the range from 200 to 800 nm at room temperature. *E-mail:
[email protected]; Tel: +86-592-2186892; Fax: +86-592-2187196
Sixth International Conference on Thin Film Physics and Applications, edited by Wenzhong Shen, Junhao Chu, Proceedings of SPIE Vol. 6984, 698410, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.792374
Proc. of SPIE Vol. 6984 698410-1 2008 SPIE Digital Library -- Subscriber Archive Copy
3. RESULTS AND DISCUSSION 3.1 Structural property Fig. 1 illustrates the XRD pattern of the sample S2. It appears (002), (103) and (004) peaks. The intensity of (002) peak is much higher than those of (103) and (004) peaks. It indicates that the film is a polycrystallite structure with a preferential c-axis orientation[5]. Fig. 2 is the XRD patterns of the samples S1, S2, S3, S4. It is found that the intensity of (002) peak increases with annealing temperature increasing. The grain size of crystallite is calculated by Scherrer formula[8]:
D=
0.9λ β cos θ
(1)
and the crystallite plane space is calculated by Bragg formula[13]:
λ = 2d sin θ
(2)
Where D is the grain size of crystallite, λ is the X-ray wavelength, β is the full-width at half-maxmium (FWHM) of
a) 0
—
a
'a 0 "I o
o
a)
0
a 0
10
20
30
40
50
60
70
80
20 ( )
0
N)
c) o
0
N)
0
—
0 0)
Intensity/au.
'a 0
N)
(002) peak, θ is the angle of diffraction, d is the crystallite plane space. It can be seen from the calculated results that as annealing temperature increases, the grain size of crystallite augments from 17.7 nm to 34.7 nm, FWHM decreases from 0.47° to 0.24°, the location of (002) peaks is shifted from 34.36° to 34.54°, the crystallite plane space decreases from 0.2608 nm to 0.2595 nm. We think that when annealing temperature increases, some grains merge together to make the grain size of crystallite become bigger and the films become compact.
Fig. 1. XRD pattern of the sample S2
Fig. 2. XRD patterns of the samples
3.2 Surface morphology Fig. 3 shows the surface SEM images of the samples. It is found that annealing temperature has a significant influence on AZO film surface structure. The grain size of surface augments as annealing temperature raises. When annealing temperature rises to 800℃, the grains of surface merge together so that the boundaries of the grains are not obvious. From cross section images we find that all the AZO films grow along the direction perpendicular to the substrate. The results of SEM are in agreements with them of XRD. The thickness of S1, S2, S3, S4 are 1494nm, 1412nm, 1389nm, 1350nm respectively. It is shown that the thickness decreases with annealing temperature increasing.
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(a)
(b)
(c)
(d)
200nm
Mag = 50.00 KX
(==!!!!!
Mag = 50.00 KX
___________
Fig. 3. SEM images of the samples: (a) sample S1, (b) sample S2, (c) sample S3, (d) sample S4
3.3 Optical property Fig. 4 shows the optical transmission spectra of the samples. As shown in Fig. 4, all the AZO films have a high average transmittance of above 80% in visible region and there is a sharp absorption edge in the wavelength range of 350-400 nm. The optical absorption coefficient is given by the following formula[13]:
I = I 0 exp(−α t )
(3)
Where I and I 0 denote the intensities of the transmitted light and the incident light, α is the optical absorption coefficient and t is the film thickness. The optical band gap can be determined by the following formula[14,16]:
α hυ = C (hυ − Eg )1/ 2
(4)
x 1010
100
—9— SI fit I
3.6
—— S2
80
fit 2
—e— S3 fitS .6
S4
60
fit 4
40
20
0.6
0 200
300
400
500
600
700
800
3
3.06
3.1
3.16
3.3
3.26
3.36
3.4
3.46
3.6
hv(e\
A (nm)
Fig. 4. Transmission spectra of the samples
3.2
Fig. 5. Plot of against
(ahυ ) 2 as
a function of photon energy
hυ of the samples
Where C is a constant depending on the electron-hole mobility, hυ is the photon energy and Eg is the optical band gap. Plot of ( ahυ ) as a function of photon energy against hυ of the samples is shown in Fig. 5. The values of the optical 2
band gap are determined by extrapolating the linear portion of the curves to α = 0 . The values of the optical band gap of the samples S1, S2, S3, S4 are 3.29eV, 3.37eV, 3.38eV, 3.44eV respectively. It can be seen that the optical band gap of the samples show a blueshift with annealing temperature increasing. The blueshift is mainly due to Moss-Burstein
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effect[14,15].
4.
CONCLUSION
The AZO films were deposited on the glass substrates by RF magnetron sputtering and annealed in N2 at different temperatures. We discussed the effect of annealing temperature on the structural and optical properties of the films. The results of XRD indicate that all the films have a highly (002) preferential orientation and the grain size of crystallite increases with annealing temperature increasing. The results of SEM exhibit that the grain size augments as annealing temperature raises and the film grows along the direction perpendicular to the substrate. Transmission spectra show that all the films have a high average transmittance in visible region and have sharp ultraviolet absorption characteristics.The optical band gap of the samples show a blueshift with annealing temperature increasing.
ACKNOWLEDGMENTS The work was financially supported by the Ministry of Science and Technology of China (No. 2002CB211807, Science and Technology Project of Fujian province of China (No. 2005H043), International Cooperation Project of the Natural Science Foundation of China (No. 20620130427).
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