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Energy and Environment Focus Vol. 4, pp. 12–17, 2015 (www.aspbs.com/efocus)
Effect of Acetic Acid Adding on Structural, Optical and Electrical Properties of Sprayed ZnO Thin Films A. Hadri, C. Nassiri, F. Z. Chafi, M. Loghmarti, and A. Mzerd∗ Physics Department-Faculty of Science-Mohammed V University-Agdal-Rabat, 10080, Morocco
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ABSTRACT Zinc oxide thin solid films were deposited on glass substrates at 350 C, using the chemical spray technique. The effect of acetic acid content in the starting solution, on the structural, morphological optical and electrical properties was studied. X-ray diffraction studies reveal the polycrystalline nature of the films which exhibit the hexagonal wurtzite type with a preferential orientation along [002] direction. Scanning electron microscopy images revealed that the surface morphology of the films changed with various acetic acid adding; many grains of various dimensions were observed depending on the acetic acid adding. The optical transmittance of all the films was found to be higher than 85% in the wavelength region from 400 nm to 800 nm. The sample prepared using spray solution having acetic acid 20 per 200 ml volume exhibits the lowest resistivity of 14 ·cm compared to acetic acid free ZnO of 299 · cm. KEYWORDS: ZnO, Spray Pyrolysis, Thin Films, Acetic Acid, Characterization. large areas.15 Thin film deposition using spray pyrolysis Delivered by Publishing Technology to: unknown involves spraying of the metal salt on to heated substrate. ZnO is a prominent II–VI semi-conductor with aOn: wide IP: 130.88.91.71 Mon, 30 Mar 2015 11:46:43 Droplets sprayed on direct band gap of about 3.3 ev, ahigh excitonAmerican binding Scientific Publishers the substrate surface undergo thermal Copyright: decomposition forming the required compound. Size and energy of 60 mev and higher optical gain (320 cm−1 ) at 1 2 shape of the “grain” depend on the starting solution as well room temperature. It is a native n semi-conductor. Its as on the substrate temperature. Additionally, the depoelectrical conductivity is due to zinc excess at interstitial 3 4 5 sition of ZnO thin films on inexpensive glass substrates position. This can be modified by cationic or anionic could cause an intentional doping effect that occurs during substitution. Besides the high transmittance and outstanddeposition (by the migration of alkaline ions rising from ing electrical properties, ZnO films have the advantages the substrate to the film). Nevertheless, it is still possibleto of cheapness, non-toxicity and environmental friendliness obtain reasonable resistivity values that make chemically over ITO. As a matter of fact, its properties make ZnO an sprayed ZnO thin films suitable for different applications. important material in the manufacture of heat mirrors used Due to the importance of the precursors solution used in in gas stoves, conducting coatings in aircraft glass that the spraying process in determining the properties of the avoid surface icing and as thin film electrodes in amor6 7 thin films, we report in this work a systematic study of the phous silicon based solar cells. Changes in the resistiveffect of acetic acid adding, which can help us further to ity of ZnO thin films upon exposure to gaseous polluants enhance the quality of zinc oxide thin films elaborated by have also lead to the design of very efficient and low cost chemical spray pyrolysis. gas sensors.8 9 Furthermore, the piezoelectricity exhibited in strongly oriented ZnO (002) films is also important in the design and manufacturing of pressure sensors.10 2. EXPERIMENTAL DETAILS ZnO thin films have been prepared using a various depoFour separate solutions of 200 ml each were presition techniques such as, magnetronsputtering,11 chemical pared according to the following procedure. The ZnO vapor deposition,12 pulsed laser deposition13 and chemithin films were deposited from zinc acetate dehydrated cal spray pyrolysis.14 Of all these techniques, the chemical [Zn(CH3 COO)2 –2H2 O] which was previously dissolved in spray pyrolysis has the advantage of low cost and can be 100 ml of dionized water. Subsequently, acetic acid was implemented to produce high quality ZnO thin films on added in different proportions to each solution, specifically 0, 2, 6, 20, and 30 ml. To complete the total volume ∗ Author to whom correspondence should be addressed. dionised water was added. Finally, the solution was stirred Email:
[email protected] for 30 minute until a transparent and homogeneous soluReceived: 17 March 2014 tion was obtained. The final molar concentration of zinc Accepted: 24 June 2014
1. INTRODUCTION
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Energy Environ. Focus 2015, Vol. 4, No. 1
2326-3040/2015/4/012/006
doi:10.1166/eef.2015.1129
Hadri et al.
Effect of Acetic Acid Adding on Structural, Optical and Electrical Properties of Sprayed ZnO Thin Films
3. RESULTS AND DISCUSSION
3.1. Structural Parameters The structural characterizations are very important in explaining optical and electrical properties of thin ZnO films. The X-ray diffraction spectra of samples recorded in 2 angle in the range of 20 –60 are shown in Figure 1. The films exhibited polycrystalline nature in all cases, and fit well to hexagonal type ZnO according to JCPDS data card 01-1136. However, the nature of variation of the peak intensity was observed depending on the caa. In fact, the patterns of the films deposited less than or equal to 2 ml revealed almost all the characteristic peaks that correspond to bulk ZnO, whereas the films deposited at higher caa (e.g., 6, 20 and 30 ml) displayed only one major peak along (002) planes which prevails over all other signals. This behavior could be explained by the variation in the reactant concentration of the solution. As a matter of fact, when the sprayed droplets become closer to the substrate it absorbs heat. As a result, the low boiling solvent evaporate first (water 100 C and acetic acid 118 C). It should also be noted that the melting point of zinc acetate is 237 C. Consequently, the volume of the droplets decreases which lead to an increase of the molar concentration of Zn inside it. This suggests that a poor ZnO formation could be expected in films deposited with solution containing low Energy Environ. Focus, 4, 12–17, 2015
Where I hkl indicates the X-ray diffraction intensity obtained from the film, ‘n’ is the number of diffraction peak considered and I0 hkl is the intensity of the reference diffraction patterns (JCPDS data card 01-1136). It is clear from the definition that the texture coefficient implies the film growth in preferred orientation. Texture coefficients calculated for different orientations namely (100), (002), (101), (102) and (110) are shown in Table I. From this result, we can notice that with acetic acid free sample several TC corresponding to two peaks are higher than 1, which indicate a random orientation of the grains and poor crystallinity of the films. While at higher caa, the corresponding TC values of (002) planes prevails over the Table I. Variation of the texture coefficient with different acetic acid content in the starting solution. Texture coefficient (TC) caa (ml)
(100)
(002)
(101)
(102)
(110)
0 2 6 20 30
0.42 0.70 0.19 0.7 0.09
2.27 2.73 4.19 3.18 2.86
0.87 0.86 0.18 0.28 0.05
1.31 0.42 0.37 0.60 –
0.13 0.29 0.07 0.23 –
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acetate in the starting solution was 0.05 M. The starting solutions is termed as (caa) for denoting the acetic acid content in 200 ml of solution. The substrates were previously cleaned in acetone and methanol, rinced with large amount of distilled water, then dried and finally they were placed over a heated surface. The temperature was measured just above the substrate by a Nikel–Chromel thermocouple. The samples were deposited at a constant substrate temperature of 350 C ± 10 C. Spray rate was held constant at 1.6 ml/min. Compressed air was used as the carrier gas. Air was directly compressed from the atmosphere, using filter to remove water and dust in order to avoid contamination. The nozzle to substrate distance was around 40 cm and the solution and gaz nozzle have a diameter of 0.3 mm. Through this Fig. 1. X-ray diffractograms for ZnO films deposited with differarrangement, it was easy to have stability in the results as ents caa. well as large area deposit. The micro-crystalline structure of deposited ZnO films values of caa. This hypothesis match perfectly with experiwas determined through X-ray diffraction (XRD) on a mental results obtained (caa = 0 ml, caa = 2 ml) and could Siemens D500 Diffractometer. The incident wavelength explain the variation of the thickness seen in Table II. On was 1.54056 Å. A Filmetrics F20 was used to measure the other hand, the thickness has the trend of first increase the thickness of films. The surface morphology was studand then decrease slightly. The thickness of the thin films ied by scanning electron microscope (SEM). The optiwith 20 ml and 30 ml caa are nearly the same and are cal transmittance spectra were obtained using UV-visible higher than the others. It was reported that higher thickspectrophotometer (Perkinelmer Lambda 900), and taking ness could be achieved when the ph is at suitable range into account the glass in the reference beam. Additiondue to its influences on the growth rate.16 Delivered by Publishing Technology to: unknown ally, the electrical resistivity, the carrier concentration and co-efficient TC” was calculated for the difIP: 130.88.91.71 On: Mon, 30The Mar“texture 2015 11:46:43 the mobility were measured, at roomCopyright: temperature, by an Scientific American Publishers ferent planes using the following expression:17 ECOPIA Hall Effect measurement. I hkl/I0 hkl (1) TC = n−1 i=n i=1 I hkl/I0 hkl
Effect of Acetic Acid Adding on Structural, Optical and Electrical Properties of Sprayed ZnO Thin Films
Hadri et al.
Where C11 = 2097 GPa, C12 = 1211 GPa, C13 = 1059 GPa and C33 = 2109 GPa are the elastic stiffness constant of bulk ZnO. The estimated values of stress ( ) in the films are listed in Table II. Strain could be positive (tensile) or negative (compressive) according to Eq. (4). The lattice constant c of ZnO films is larger than the bulk ZnO; all the films exhibit tensile strain. The total stress in 09 the film consists of two components: one is the intrinsic (2) D= cos stress introduced by impurities, defects and lattice distortions in the crystal, and the other is the extrinsic stress Where , and are X-ray wavelength, the Bragg’s introduced by the mismatch between the film and subdiffraction angle and the full with at half maxima (FWHM) strate. The negative values of estimated stress for the films of the peak corresponding to “” value, respectively. The indicate that the lattice constant is elongated as compared average crystallite size is found to increase slightly from to to unstressed powder sample. The negative sign indicates 31.5 to 47.3 nm as the caa increase to 20 ml. The increase that the films are in a state of a compressive stress. From in grain size with acetic acid addition can be understood Table II, it is shown that compressive stress decreases on the basis of elevation of boiling point of the precursor with increase in caa. This suggests that the compressive solution. When acetic acid is used, an increase in resistress is generated during deposition due to the freezing dence time of the drop on the hot substrate enhances the of structural defects at low caa due to the hydroxylation grains growth due to its high boiling point. Further adding of zinc atoms. The increase in caa prevent the precipiof caa decrease slightly the grain size. The same behavior tate (Zn(OH)2) from forming which reduces the structural has been reported by Jiao et al.19 for In doped ZnO (IZO). defects and thus relaxation of the film is observed.While The lattice constants a and c were calculated by using the sample prepared with 30 ml of caa shows an increase 20 the following equation: in the stress. This may be attributed to the slight decrease in the grains size. 4 h2 + hk + k2 l2 1 = + 2 (3) The dislocation density ( ), defined as the length of disPublishing Technology to: unknown d 2 hkl 3 a2Delivered cby IP: 130.88.91.71 On: Mon, location 30 Mar 2015 11:46:43 lines per unit volume of the crystal, was estimated Copyright: Scientific The observed a and c values are shown in Table American II. from thePublishers following relation using the simple approach of In thin films, strains originate mainly to a mismatch Williamson and Smallman:23 between the polycrystalline film and the amorphous sub1 strate and/or differences in coefficient of thermal expan = 2 (6) D sion of the film substrate. The average uniform strain z in the lattice along the c-axis in the randomly oriented The dislocation values are given in Table III. The larger D ZnO films deposited using different caa has been calcuand smaller values indicate better crystallization of the lated with the help of the following expression:21 films. It was observed that the dislocation density of the films are found to decrease with the increase in the caa c − c0 until it reaches its minimum value at 20 ml caa. Hence, z = (4) c0 this sample presents the best crystalline quality.
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rest with a value higher than 1. It is possible to concludean abundance of grains growing inthe preferential (002) planes. This result influences the crystallinity of the prepared samples. Mean crystalline size were calculated for (002) diffraction peak using Debye Scherrer formula:18
Where c0 is the lattice constant for the unstrained ZnO. For hexagonal crystals, the stress ( ) in the plane of the film can be calculated using the biaxial strain model:22
=
2 − C33 C11 + C12 2C12 z C13
(5)
3.2. Morphology Figure 2 shows the surface of ZnO thin films deposited at different caa. All micrographs show a rough texture with randomly disturbed grains, and numerous porous surrounding the grains can be evidenced. From these results,
Table II. Various structural parameters with different acetic acid content in the starting solution, D: Crystallite size, c: Lattice parameter, : strain,
: Stress, : Dislocation density. caa (ml)
D (nm)
a (Å)
c (Å)
(%)
(GPa)
(1014 m−2
Thickness (nm)
Eg (ev)
0 2 6 20 30 JCPDS
31.5 31.6 42.1 47.3 46.9 –
3,192 3,191 3,189 3,183 3,190 –
5.213 5.212 5.208 5.198 5.210 5.176
0.71 0.69 0.62 0.42 0.66 –
−3.24 −3.15 −2.80 −1.93 −2.98 –
101 10 57 45 46 –
690 710 751 899 870
3.25 3.24 3.27 3.29 3.28
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Energy Environ. Focus, 4, 12–17, 2015
Hadri et al.
Effect of Acetic Acid Adding on Structural, Optical and Electrical Properties of Sprayed ZnO Thin Films
Fig. 3. Optical transmittance spectra of the ZnO thin films deposited with different caa.
Energy Environ. Focus, 4, 12–17, 2015
( α h υ )2 (m–2 .ev2 .1010)
3.3. Optical Properties Figure 3 depicts the optical transmission spectra for films grown at different caa, in a wavelength 300–800 nm range. Sharp ultraviolet absorption edges at approximately = 370 nm are observed with a shift to shorter wavelength at high caa. This is mainly attributed to MossBurstein effect.24 As can be seen a region of strong transparency is located between 400 nm and 800 nm. The values of transmission are higher than 85% for all samples. The increase in caa above 2 ml showed interference fringe patterns in their transmittance spectra. This revealed that the films were highly transparent and there was not much scattering or absorption loss at the surface.25 Based on this result, we can conclude that the enhancing
0 ml 2 ml 6 ml 20 ml 30 ml
8000 7000 6000 5000 4000 3000 2000 1000 0 2.5
2.6
2.7
2.8
2.9
3.0
3.1
3.2
3.3
3.4
3.5
Eg (ev) Fig. 4. ( h2 versus h plot of ZnO films with different caa deposited at 350 C.
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in the optical transmission obtained is a result of the film quality, determined by the structural characteristics since it is well known that the scattering induced by nano-sized grains or by the inter-grains regions decrease the film transparency. Therefore concerning both the optical and structural properties of the films, it is apparent that the caa in the starting solution enhance the quality of the films. The extrapolation of the linear portion of the plots of h 2 versus (h ) onto the energy axis gave the band Delivered by Publishing Technology to: unknown gap value (Fig. 4). The obtained values of Eg are varied IP: 130.88.91.71 On: Mon, between 30 Mar 2015 11:46:43 3.24 and 3.28 ev (see Table II). The optical band Copyright: American Scientific Publishers gap shows a variation proportional to the carrier concentration. This behavior suggests that the band gap energy Fig. 2. SEM imagines of (a) 0 ml, (b) 2 ml, (c) 6 ml, (d) 20 ml and (e) is influenced by the carrier concentration, and could be 30 ml acetic acid content on the starting solution. explained based on the Brustein Moss effect.26 As these are degenerate semiconductors, the Fermi level lies within it is quite clear that the grain morphologies are influenced the conduction band where the position depends on the by the variation of the caa. In the case of acid free samdensity of free electrons. Thus, the optical band gap is ple, a poor surface quality is observed. When the caa is given by the energy difference between the Fermi level in less than or equal to 20 ml, it can be seen that, the films the conduction band and valence band. exhibits hexagonal flake with different sizes. But over 20 ml caa, more voids surrounding the grains are clearly visible and reduction in the grain size is observed. Thus 10000 the SEM analysis corroborates the XRD studies. caa 9000
Effect of Acetic Acid Adding on Structural, Optical and Electrical Properties of Sprayed ZnO Thin Films
Table III. caa.
Caa (ml)
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0 2 6 20 30
Electrical parameters of ZnO films prepared with different
Resistivity · cm)
Carrier concentration N (×1016 cm−3 )
Mobility (cm2 · V−1 · S−1 )
299 283 80 14 56
1.8 0.4 2.5 75.3 17.9
1.1 5.2 3 0.5 0.6
Hadri et al.
and the lowest electrical resistivity can be achieved. In our case, this is achieved at 20 ml of acetic acid per 200 ml of the starting solution. In fact, by controlling the amount of acetic acid in the starting solution, it is possible to control the crystal growth along preferential planes. In addition, the optical propertiesare not affected considerably; in contrast to the other physical characteristics. A nearly constant high transmittance (85–90%) is obtained for all prepared films and a slight variation of the band gap is observed. The resistivity drops because of the increase in average grain size. In summary, this study can provide positive guidance for other researchers focus on sprayed ZnO thin films.
3.4. Electrical Properties Hall effects measurements were performed in order to investigate the electrical properties of ZnO thin films. The Acknowledgments: The authors are thankful to Profesmeasured values of resistivity, carrier concentration and sor A. Belayachi, member of Materials and Science Labmobility are shown in Table III. These results demonstrate oratory in Physics Department of Faculty of Sciences of that the resistivity of ZnO thin films strongly depends on Rabat. the caa added to the spray solution. High resistivity of low caa films less than or equal to 2 ml are explained by the randomly oriented grains of ZnO which presents a References and Notes high trap density27 and results in a segregation of Zn at 1. D. C. Look, D. C. Reynolds, J. R. Sizelove, R. L. Jones, C. W. Litton, G. Cantwell, and W. C. Harsch, Solid State Commun. 105, grain boundaries in the form of Zn(OH)2 which is highly 399 (1998). resistive.28 The adding of caa in the starting solution lead 2. Z. K. Tang, G. K. L. Wong, and P. Yu, M. Kawasaki, A. Ohtomo, + to the formation of [ZnCH3 COO ] which can prevent H. Koinuma, and Y. Segawa, Appl. Phys. Lett. 72, 3270 (1998). [Zn2+ ] from forming the zinc hydroxide phases. On the 3. G. Newman, Phys. Status Solidi. B 105, 605 (1981). other hand, Bouderbala et al.29 have reported decrease Technology 4. A. Tiburcio-Silver, J. C. Joubert, and M. Labeau, J. Phy. III France Delivered by aPublishing to: unknown 2, 1287 (1992). of carrier concentration with an increase of the thickness. IP: 130.88.91.71 On: Mon, 30 Mar 2015 11:46:43 5. A. Sanchez-Juarez, A. Tiburcio-Silver, and A. Ortiz, Sol. Energy However, in this work the opposite behavior is seen which Scientific Copyright: American Publishers Mater. Sol. Cells 52, 301 (1998). is due to the acetic acid adding. Hence one could state, that 6. K. L. Chopra, S. Major, and D. K. Pandya, Thin Solid Films 102, 1 the acetic acid adding prevent the formation of hydroxide (1983). 7. M. A. Martínez, J. Herrero, and M. T. Gutierrez, Sol. Energy Mater. zinc oxide which gives a rise to the carrier concentration. Sol. Cells 45, 75 (1997). As a result, the resistivity of the films decreases reaching 8. F.C. Lin, Y. Takao, Y. Shimizu, and M. Egashira, Sens. Actuators, B a minimum value of 14 .cm and the highest carrier con24, 843 (1995). centration of 7.6 1017 cm−3 at 20 ml caa. Further increase 9. A. Dutta, T.K. Chaudhuri, and S. Basu, Mater. Sci. Eng. B 14, 31 in the caa leads to an increase in the resistivity. A pos(1992). 10. S. J Chang, Y. K Su, and Y. P Shei, J. Vac. Sci. Technol. A13, 385 sible explanation for this phenomenon is related to a fact (1995). that excess acetic acid inhibits the electron mobility and 11. S. Youssef, P. Combette, J. Podlecki, R. A. Asmar, and A. Foucaran, 30 low pH decreases the growth rate of the grains. The low Cryst. Growth Des. 9, 1088 (2009). observed mobility values are attributed to the structural 12. G. Y. Zhu, S. L. Gu, S. M. Zhu, S. M. Huang, R. Gu, J. D. Ye, and Y. D. Zheng, J. Cryst. Growth 349, 6 (2012). disorder and grain boundaries, as already suggested by 13. S. K. Lee, and J. Y. Son, Appl. Phys. Lett. 100, 132109 (2012). Thangaraju.31 Considering that even 20 ml of acetic acid 14. A. Elfakir, A. T. Silver, I. Soumahoro, A. Belayachi, A. Douayar, per 200 ml is much more than the volume of few drops and M. Abd-Lefdil, Energy Environ. Focus 2, 277 (2013). 32–36 as it is recommended in some other works. It should 15. P. S. Partil, Mater Chem. Phys. 59, 185 (1999). be noted that the values of electrical resistivity obtained 16. D. Perednis and L. J. Gauckler, J. Electroceram. 14, 103 (2005). 17. A. Douayar, R. Diaz, F. Cherkaoui E. Moursli, G. Schmerber, in the present study are higher than the values reported A. Dinia, and M. Abd-Lefdil, Eur. Phys. J. Appl. Phys. 53, 20501 by PrasadaRao et al. for ZnO layers, deposited at various (2011). 36 substrate temperatures using spray pyrolysis. 18. B. D. Cullity, Elements of X-Ray Diffraction, 2nd edn., Wesley
4. CONCLUSION In this work, we have shown that the acetic acid plays a prominent role on the physical properties of ZnO thin films deposited by the chemical spray technique. It is confirmed that the amount of acetic acid as the stabilizing agent are decisive for ZnO layer properties, and for this series there is an optimum at which the highest optical transparency 16
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