Effect of annealing on the structural, optical and ... - Springer Link

Indian J. Phys., Vol. 85, No. 9, pp. 1381-1391, September, 2011

Effect of annealing on the structural, optical and electrical properties of ZnO thin films by spray pyrolysis 1

T Prasada Rao1, M C Santhosh Kumar1* and V Ganesan2

Advanced Materials Laboratory, Department of Physics, National Institute of Technology, Tiruchirappalli-620 015, Tamil Nadu, India 2 UGC-DAE Consortium for Scientific Research, Indore-452 001, Madhya Pradesh, India E-mail : [email protected] Received 15 June 2010, accepted 8 October 2010

Abstract : Zinc oxide thin films have been deposited on glass substrates at a substrate temperature of 673 K by spray pyrolysis. The samples are annealed in ambient atmosphere at various temperatures. The effect of annealing on structural, electrical, and optical properties of ZnO films has been investigated. X-ray diffraction patterns show that crystallinity of the ZnO films has been improved after annealing. The morphology of ZnO thin films is studied by atomic force microscopy. The tensile strain (compressive stress) is found to decrease with increase in annealing temperature which indicates the relaxation of tensile strain in ZnO thin films. A decrease in energy band gap is observed with increase of annealing temperature. The mechanism of blue-green luminescence of ZnO thin film has been analyzed. The resistivity is found to decrease with annealing temperature. Keywords : ZnO, strain, electrical properties, optical properties, photoluminescence. PACS Nos. : 68.37.Ps, 68.55.Ag, 73.61.Ga, 78.55.Et, 78.40.Fy

1. Introduction ZnO is amongst the most widely studied of all metal oxide systems and has recently become a very popular material due to its great potential for optoelectronics applications. The wide direct band gap of 3.3 eV and large exciton binding energy ~60 meV at room temperature [1] is especially attractive for optoelectronic, nonlinear optics and electro-optics applications [2]. Moreover, the binding energy of the exciton of ZnO (60 meV) is larger than its competitor GaN (25 meV) at room temperature making it attractive for exciton-related device applications [3], dye-sensitized oxide semiconductor solar cells [4] and lasing devices operating at high temperatures and in harsh environments [5]. Various deposition techniques have been widely used to produce *Corresponding Author

© 2011 IACS

!&

T Prasada Rao, M C Santhosh Kumar and V Ganesan

ZnO thin films. However, seeking the most reliable and economic deposition technique is the main goal. The most intensively studied techniques include RF magnetron sputtering [6], chemical vapor deposition (CVD) [7], sol-gel method [8], thermal evaporation [9] and spray pyrolysis [10]. Among these, spray pyrolysis is a widely used technique. Spray pyrolysis has been developed as a powerful tool to prepare various kinds of thin films such as metal oxides, superconducting materials, and nanophase materials. In comparison with other chemical deposition techniques, spray pyrolysis has several advantages such as high purity, excellent control of chemical uniformity, and stoichiometry in multi-component system. Other advantage of the spray pyrolysis method is that it can be adapted easily for production of large-area films. In this study, ZnO thin films were deposited by spray pyrolysis on glass substrates. The influence of annealing on the structural, optical and electrical properties of ZnO thin films was investigated. 2. Experiments Spray pyrolysis is an effective method for the deposition of thin films of metallic oxides, as is the case of the ZnO. In this deposition technique, a starting solution, containing Zn precursor, was sprayed by means of a nozzle, assisted by a carrier gas over a hot substrate. When the fine droplets arrive at the substrate, the solid compounds react to form a new chemical compound. ZnO thin films were deposited onto ultrasonically cleaned glass substrates using the spray pyrolysis method at a substrate temperature of 673 K. A solution of 0.1 M Zn(CH3CO2)2 was used as a precursor, prepared by dissolving in a mixture of deionized water and ethanol. In this mixture, ethanol concentration was 10 ml in 100 ml solution. A small amount of acetic acid was added to aqueous solutions to prevent the formation of hydroxides. Ethanol is used as one of the oxidizing agents because of their volatility and thus facilitating quick transformation of the precursor mist into vapor form, which is an important criterion for obtaining good quality films. The nozzle was at a distance of 20 cm from the substrate during deposition. The solution flow rate was held constant at 3 ml/min. Air was used as the carrier gas, at the pressure of 2 bar. When aerosol droplets come close to the substrates, a pyrolytic process occurs and highly adherent ZnO films were produced. The ZnO thin films with thickness of 325 nm were deposited at a substrate temperature of 673 K. As-prepared samples were annealed at 573 K and 723 K in ambient atmosphere for one hour. The film thickness was measured using Stylus profile meter. The structural properties were studied by X-ray diffraction measurements (XRD) using Rigaku D/Max ULTIMA III diffractometer with CuKa radiation (l = 1.5406 Å). The average dimensions of crystallites were determined by the Scherrer method from the broadening of the diffraction peaks. The surface morphology profiles of the samples were recorded using Nanoscope-E instrument in contact mode with a Si3N4 cantilever of atomic microscope

Effect of annealing on the structural, optical and electrical properties of ZnO thin films by spray pyrolysis

!&!

(AFM). The optical measurements of the ZnO thin films were carried out at room temperature using Shimadzu UV-1700 spectrophotometer in the wavelength range 300 to 1100 nm. PL measurements were performed using the 325 nm line from a Xenon pulse lamp as the excitation source and a UV-visible photomultiplier tube as detector. Hall measurements were carried out using ECOPIA HMS-3000 Hall measurement system. 3. Results and discussion 3.1. Structural characteristics : Figure 1 shows the X-ray diffractograms of ZnO films prepared at 673 K on glass substrates along with films annealed at different temperatures. The presence of (1 0 0), (0 0 2), (1 0 1), and (1 1 0) peaks indicate a random orientation of the hexagonal crystallites with polycrystalline nature. Ohyama et al [11] reported c-axis oriented solgel ZnO film on glass substrate. According to them, the orientation of ZnO films on glass substrates is strongly influenced by the precursor chemistry and heat-treatment procedures. Mechanism of the c-axis orientation of ZnO on amorphous substrate has been reported by Fujihara et al [12]. In the present work, the experimental conditions appear not to be suitable for the occurrence of the preferential orientation of the ZnO thin film on glass. In the randomly oriented films, the nucleation and crystal growth 1000

600 400 200

(100)

annealed at 723 K

(002) (101)

800

(110)

0 Intensity (a.u.)

300 annealed at 573 K 200 100 300 as prepared at 673 K 200

100

0 30

45

60

75

2q (degree)

Figure 1. XRD patterns of ZnO thin films.

!&"


may have occurred throughout the films without being initiated exclusively on the substrate surface. The lattice constants ‘a’ and ‘c’ were calculated using the following equation :

1

=

d2hkl

4 éê h 2 + hk + k 2 ùú l 2 . + ú c2 a2 3 êë û

(1)

The observed ‘a’ and ‘c’ values (Table 1) are in good agreement with the standard values taken from the Joint Committee of Powder Diffraction Standards (JCPDS) card 75-0576. The increase in annealing temperature is likely to drive the modification of Table 1. Variation of lattice parameters, roughness, particle size and stress of ZnO thin films. Lattice parameters

Grain size (nm)

Roughness

Stress

(nm)

(GPa)

a (Å)

c (Å)

c/a

XRD

AFM

As-prepared

3.248

5.209

1.603

15.0

169.8

18.3

–1.36

Annealed at 573 K

3.249

5.204

1.601

19.7

171.6

20.8

–0.86

Annealed at 723 K

3. 248

5.198

1.600

27.2

192.4

29.1

–0.31

3.242

5.194

1.602

–

–

–

–

JCPDS

the grain boundary configuration. It is well-known that the shape and size of inorganic functional materials have an important role on their electrical and optical properties [13]. The crystallite size (D) of the samples was estimated using the Scherrer formula [14] :

D=

0.9l , b cos q

(2)

where l, q and b are X-ray wavelength, the Bragg’s diffraction angle and the full width at half maximum (FWHM) of the peak corresponding to the ‘‘q’’ value respectively. The values of b and q of the XRD peaks are estimated by Gaussian fitting. The average uniform strain ezz in the lattice along the c-axis in the ZnO films annealed at different temperatures have been estimated from the lattice parameters using the following expression [15] : ezz =

(c - c 0 ) ´ 100% c0

(3)

where c is the lattice parameter of the strained ZnO films calculated from X-ray diffraction data and c0 is the unstrained lattice parameter of ZnO. For hexagonal crystals, the stress (s) in the plane of the film can be calculated using the biaxial strain model [16] : s film =

2 - C33 (C11 + C12 ) 2C13 × ezz C13

(4)


!&#

where C11 = 209.7 GPa, C12 = 121.1 GPa, C13 =105.1 GPa, and C33 = 210.9 Gpa are the elastic stiffness constants of bulk ZnO. The estimated values of stress ‘‘s’’ in the films, grown at different annealing temperatures, are listed in Table 1. The value of s is negative, demonstrating that the stress in the deposited films is compressive in the direction of the c-axis. It is well known that the biaxial stress in thin film originates from intrinsic and extrinsic stresses. The intrinsic stress is associated with defects and impurities. The extrinsic stress is mainly related to lattice mismatch, and thermal expansion coeifficent mismatch [17,18]. It is difficult to calculate the stress introduced by lattice mismatch between ZnO film and glass substrate because glass is amorphous material. The thermal expansion coefficients can be considered as a constant because substrate temperature maintained at a constant value of 673 K. Intrinsic stress has its origin in the imperfection of the crystallites during growth. Several growth parameters, such as deposition temperature, deposition pressure and gas mixture would contribute to the intrinsic stress. These imperfections cause lattice disorder, which generates the intrinsic stress in the films. As can be seen, in general, the values of the strain and the stress decrease with increasing annealing temperature. This is due to the fact that the atoms get higher energy to adjust their position in the lattice during annealing. Thus the stress tends to relax in the films after annealing at higher temperatures. The correlation between the average grain size and the stress (Table 1) demonstrates that the value of the stress decreases with the increase of grain size. It is observed that there is an increment in the grain size with annealing temperature. The grain boundary scattering is one of the primary factors influencing the electrical properties of thin films. In this regard, changing grain sizes imply that grain size effect would also contribute significantly influence electrical properties of these ZnO films. In order to explore the dependence of structural properties on the annealing temperature, we have investigated the changes in structural characteristics such as the surface morphology by means of AFM. The images are obtained in contacting mode taken over a scale of 2 mm × 2 mm. Figure 2 shows AFM micrographs of ZnO films. Though the films are all visually similar and smooth, the nature of their morphology is quite different. The mean grain size and root-mean-square roughness obtained from AFM image are shown in Table 1. The mean grain size and roughness increasing with annealing temperature. The mean crystallite size obtained using Scherrer’s formula are smaller than the dimension of grains observed in AFM images, indicating these grains are probably an aggregate of many crystallites. 3.2. Optical properties : The transmittance and reflectance spectra of as-grown films in the wavelength range of 300–1100 nm are shown in Fig. 3. The spectra clearly exhibit a shift in band edge due to the variation of annealing temperature, with a transparency of about 80% in the

!&$


(a)

(b)

(c)

Figure 2. AFM micrographs (a) as-prepared, (b) annealed at 573 K and (c) annealed at 723 K. 100

100 as prepared anealed at 723 K anealed at 573 K

80

60

T (%)

60 as prepared

R (%)

80

40

40 anealed at 573 K anealed at 723 K

20

20

0

0 300

400

500

600 700 800 Wavelength (nm)

900

1000

1100

Figure 3. Transmission and reflection spectra of the ZnO thin films.

visible range above 400 nm. Sharp ultraviolet absorption edges at approximately l = 380 nm are observed with the absorption edge being shifted to longer wavelength at higher annealing temperature. The absorption coefficient a is calculated from the relation [19] : T = (1 - R ) exp(-at )

(5)


!&%

where T is the transmittance, R is the reflectance and t is the film thickness. The optical band gap of the films is determined by applying the Tauc model in the high absorption region [20] : ahu = A hu - E g

n

(6)

where hn is the photon energy, Eg the optical band gap and A is a constant. For n = ½ the transition data provide the best linear curve in the band edge region, implying the transition is direct in nature. The band gaps of the films have been calculated using Tauc’s plot of (ahn)2 vs. hn (shown in Fig. 4) and by extrapolating the linear portion of the absorption edge to find the intercept with energy axis. Band gap of asprepared sample is 3.30 eV, which decreases to 3.27 eV for the sample annealed at 723 K. Dependence of optical band gap Eg on annealing temperature is shown in Fig. 5. The band gap of ZnO films found to decrease with increase in annealing temperature. Another way of understanding this decrease in band gap is based on the compressive stress relaxation occurred in ZnO films. Since compressive stress relaxation occurred in the lattice is expected to provide a narrow band gap because of the decrease in repulsion between the oxygen 2p and the zinc 4s bands [15,21]. Therefore, it can be concluded that the reduction in band gap is due to stress relaxation in the films. 4 × 1011

(ahu)2 (m2eV2)

3 × 1011

as prepered annealed at 573 K annealed at 723 K

2 × 1011

1 × 1011

0 3.0

3.1

3.2

3.3

3.4

3.5

hn (eV)

Figure 4. Variation of (ahn)2 vs. hn of the ZnO thin films.

3.3. Photoluminescence : Generally, the luminescence property of the films has a close relationship with the film crystallinity. The PL emission spectra of as-deposited ZnO as well as annealed samples are shown in Fig. 6. In all the samples four emission bands, 419 nm (2.96 eV), 444 nm (2.79 eV), 491 nm (2.52 eV) and 530 nm (2.34 eV) are present. The violet luminescence (2.96 eV) is probably due to radiative defects related to the interface traps existing at the grain boundaries and emitted from the radiative transition

!&&


3.300

0.30

3.295

0.20 3.285 0.15

2.280 2.275

Strain (%)

Eg (eV)

0.25 3.290

0.10

2.270 200

300

400 500 600 Annealing Temperature (K)

700

0.05 800

Figure 5. Variation of band gap (Eg) and strain as a function of annealing temperature. 350

419 nm as prepered annealed at 573 K annealed at 723 K

300

Intensity (cps)

250 200

444 nm

150 491 nm

100

530 nm

50 0 300

350

400

450 500 550 Wavelength (nm)

600

650

Figure 6. Photoluminescence spectra of ZnO thin films.

between this level and the valence band [22]. The films may have more grain boundary defects emitting the violet luminescence of higher intensity because they have smaller grains and larger grain boundary area. However, if the grains are not preferentially oriented, the emitted light may not be effectively detected, or the grain boundary may produce different kinds of defects such as non-radiative defects [23]. The sample annealed at 723 K shows much higher intensities of luminescence than the asdeposited sample as well as annealed sample at 573 K. This can be understood from the improvement in the crystalline quality of three sets of samples, evident from the in the XRD results. It is thus expected that the sample annealed at 723 K will probably have more improved stoichiometry with less oxygen vacancies. The greenpeak near 2.52 eV is a widely observed defect related emission in ZnO [24]. It is well understood that PL spectra depend on the stoichiometry and the microstructure of the films. ZnO is non-stoichiometric oxide containing oxygen vacancy (VO) and reduced


!&'

interstitial zinc species. These defects formed when the zinc acetate was transformed into ZnO in the spray pyrolysis process. Zinc acetate is a reductant whose decomposition to most stable compounds needs oxygen. ZnO is Zn-rich due to the rapid evaporation of water which is the source of oxygen in the growth process and the low concentration of oxygen in the air near the substrate according to the ideal gas law [25]. In our work, the rapid evaporation-oxidation process, VO should be generated because of partially incomplete oxidation and crystallization. This means that ZnO, which formed by the pyrolysis of zinc acetate, has high density of VO [25]. Further, the high density of VO can cause lattice distortion and there by responsible for the higher compressive (1.36 GPa) intrinsic stress in films. As a result, PL intensity is low for as deposited films. 3.4. Electrical properties : The Hall-effect measurements were performed in order to investigate the electrical properties of the ZnO thin films. The room temperature values of resistivity and mobility are given in Table 2. As-prepared and annealed ZnO films are n-type in Table 2. Resistivity, carrier concentration and mobility of ZnO thin films. Sample As-prepared Annealed at 573 K Annealed at 723 K

Resistivity (r) (W.cm)

Carrier concentration (n) (cm–3)

Mobility (cm 2/Vs)

Carrier type

3.20 × 103 2.90 × 103 1.55 × 103

7.27 × 1015 7.56 × 1015 1.83 × 1016

0.26 0.28 0.22

n-type n-type n-type

nature. In general as-deposited and annealed samples showed high resistivity (103 W.cm). This high resistivity of the samples is attributed to chemisorptions of oxygen at grain boundaries [26]. This may be possible, since air was used as carrier gas during deposition. To interpret the conductivity behavior of ZnO thin films, we can assume that the resistivity of these films comes from the sum of resistivities of the grain boundary, impurity and strain. The grain boundary barrier depends on the microvoids, inter-grain distance, grain size, impurity concentration, crystallinity, and non coordinated atoms at the grain boundaries [27]. The contribution of grain boundary region to the electronic properties increases as the grains become smaller; there are large number of grains and hence large area of the grain surface, which resulted in higher resistivity of the thin films. Lattice contribution is expected through the intrinsic stress. The compressed lattice is expected to provide a wider band gap because of the increased repulsion between the oxygen 2p and the zinc 4s bands [21]. The results also show that the poorer conducting films have higher stress, while the samples with better conductivity have lower stress. Therefore, the decrease in the room-temperature resistivity with increase of annealing temperature is attributed to the increase of grain size as well as the decrease of the stress. This can be understood from grain size variation observed from the AFM images. As the grain size increases

!'


with the annealing temperature; the grain boundary the scattering is diminished by the reduction of the number of grain boundaries [28]. Additionally, from the XRD spectra it is evident that the films annealed at higher temerature films have a better crystallinity. As a result of the enhancement of both morphological and structural characteristics of ZnO films, the electrical properties are also improved. The resistivity reduction with increase of annealing temperature is mainly attributed to reduction in grain boundary scattering. This leads to an increase in carrier concentration. 4. Conclusion In this study, the influence of the annealing temperature on the structural, optical, and electrical properties of ZnO thin films grown by the spray pyrolysis method on glass substrates were investigated. With the increase of annealing temperature, it was observed that there is a corresponding increase in grain growth and crystallinity of ZnO thin films deposited on glass substrates. The XRD spectra indicate that the films are polycrystalline in nature. The XRD studies show that the films deposited at substrate temperature of 673 K have large stress, which relaxes as the temperature of the annealing is increased. All the films show transmittance about 80% in the visible region. The optical band gap energy of ZnO thin film after the annealing process was found to decrease. The resistivity was found to decrease with annealing temperature. Acknowledgments Authors are thankful to the Director, National Institute of Technology, Tiruchirappalli for providing financial support for this work through the research seed money. References [1]

W Y Liang and A D Yoffe Phys. Rev. Lett. 20 59 (1968)

[2]

I V Kityk, J Ebothe, A Elchichou, M Addou, A Bougrine and B Sahraoui Phys. Status Solidi B234 553 (2002)

[3]

F K Shan, B C Shin, S C Kim and Y S Yu J. Eur. Ceram. Soc. 24 1861 (2004)

[4]

K Hara, T Horiguchi, T Kinoshita, K Sayama, H Sugihara and H Arakawa Sol. Energy Mater. Sol. Cells 64 115 (2000)

[5]

T Makino, C H Chia, T T Nguen, Y Segawa, M Kawasaki, A Ohtomo, K Tamura and H Koinuma Appl. Phys. Lett. 77 1405 (2000)

[6]

Z Xingwen, L Yongqiang, L Ye, L Yingwei and X Yiben Vacuum 81 502 (2006); B Saha, R Thapa, S N Das and K K Chattopadhyay Indian J. Phys. 84 681 (2010); B Saha, R Thapa, S Jana and K K Chattopadhyay Indian J. Phys. 84 1341 (2010)

[7]

S Fay, U Kroll, C Bucher, E Vallat-Sauvain and A Shah Sol. Energy. Mat. Sol. Cells. 86 385 (2005)

[8]

R Ghosh, G K. Paul and D Basak Mater. Res. Bull. 40 1905 (2005)

[9]

Mihaela, G G Rusu, Sylvie Dabos-Seignon and Mihaela Rusu Appl. Surf. Sci. 25 4179 (2008)

[10]

D F Paraguay, L W Estrada, D R Acosta, M E Andrade and MiKi Yoshida Thin Solid Films. 350 192 (1999)

[11]

M Ohyama, H Kozuka and T Yoko J. Am. Ceram. Soc. 81 1622 (1998)

[12]

S Fujihara, C Sasaki and T Kimura Appl. Surf. Sci. 180 341 (2001)


!'

[13]

Y Zhang and Y D Li J. Phys. Chem. B108 17805 (2004)

[14]

B D Cullity and S R Stock Elements of X-Ray Diffraction, (New Jersey : Prentice Hall) p 619 (2001)

[15]

T Prasada Rao, M C Santhosh Kumar, S Anbumozhi Angayarkanni and M Ashok J. Alloys and Compounds. 485 413 (2009)

[16]

M K Puchet, P Y Timbrell and R N Lamb J. Vac. Sci. Technol. A14 2220 (1996)

[17]

W Water and S Y Chu Mater. Lett. 55 67 (2002)

[18]

Y F Li, B Yao, Y M Lu, C X Cong, Z Z Zhang, Y Q Gai, C J Zheng, B H Li, Z P Wei, D Z Shen, X W Fan, L Xiao, S C Xu and Y Liu Appl. Phys. Lett. 91 021915 (2007)

[19]

K L Chopra Thin Film Phenomena (New York : McGraw-Hill) p 734 (1969)

[20]

D Jiles Introduction to the Electronic Properties of Materials (New York : Chapman and Hall) p180 (1994)

[21]

R Ghosh, D Basak and S Fujihara J. Appl. Phys. 96 2689 (2004)

[22]

B J Jin, S Im and SY Lee Thin Solid Films 366 107 (2000)

[23]

K Prabakar, Choongmo Kim and Chongmu Lee Cryst. Res. Technol. 40 1150 (2005)

[24]

D C Look and B Claflin Phys. Status Solidi B241 624 (2004)

[25]

Khedidja Bouzid, Abdelkader Djelloul, Noureddine Bouzid and Jamal Bougdira Phys. Status Solidi A206 106 (2009)

[26]

Shih-Chia Chang J. Vac. Sci. Technol. 17 366 (1980)

[27]

Xiaotao Hao, Jin Ma, Deheng Zhang, Tianlin Yang, Honglei Ma, Yingge Yang, Chuanfu Cheng and Jie Huang Appl. Surf. Sci. 183 137 (2001)

[28]

M de la L Olvera, H Go Mez and A Maldonado Solar Energy Materials and Solar Cells 91 1449 (2007)