Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 120 (2014) 297–303
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Spectroscopic study and optical and electrical properties of Ti-doped ZnO thin films by spray pyrolysis R. Sridhar a, C. Manoharan a,⇑, S. Ramalingam b, S. Dhanapandian a, M. Bououdina c,d a
Department of Physics, Annamalai University, Annamalai Nagar 608 002, India Department of Physics, A.V.C. College, Mayiladuthurai, Tamilnadu, India c Nanotechnology Centre, University of Bahrain, PO Box 32038, Bahrain d Department of Physics, College of Science, University of Bahrain, PO Box 32038, Bahrain b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The titanium-doped zinc oxide
Zinc oxide films were doped with different concentrations of Ti on glass substrates at 400 °C by spray pyrolysis technique. The films exhibited single phase ZnO for low concentrations of Ti. Wurtzite ZnO peaks were observed at higher doping concentration with decreased crystallinity. Crystallite size, strain and dislocation density were evaluated from the X-ray diffraction data. Surface morphology of the films indicated that a remarkable decrease in grain size with increasing of Ti concentration. The band gap of the films was found to be increased from 3.20 eV to 3.32 eV as the concentration of Ti doping increases. The resistivity of the films decreased from 9 105 X cm to 9 104 X cm with the increase of Ti doping concentration.
(Ti-doped ZnO) thin films were deposited onto glass substrate by spray pyrolysis technique. The physical properties of these films were characterized by XRD, AFM and PL measurements. The crystallographic parameters; microstrain, dislocation density, and lattice constant were estimated. The band gap of the films was found to be increased from 3.20 eV to 3.32 eV as the concentration of Ti doping increased.
a r t i c l e
i n f o
Article history: Received 23 August 2013 Received in revised form 16 September 2013 Accepted 29 September 2013 Available online 11 October 2013 Keywords: ZnO thin films Ti doping Raman Photoluminescence Resistivity
a b s t r a c t Zinc oxide films were doped with different concentrations of Ti on glass substrates at 400 °C by spray pyrolysis technique. The films exhibited single phase ZnO for low concentrations of Ti. Wurtzite ZnO peaks were observed at higher doping concentration with decreased crystallinity. Crystallite size, strain and dislocation density were evaluated from the X-ray diffraction data. Surface morphology of the films indicated that a remarkable decrease in grain size with increasing of Ti concentration. The band gap of the films was found to be increased from 3.20 eV to 3.32 eV as the concentration of Ti doping increases. The resistivity of the films decreased from 9 105 X cm to 9 104 X cm with the increase of Ti doping concentration. Both Raman spectroscopy and room temperature photoluminescence exhibited characteristic peaks that confirmed the formation of ZnO phase. Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved.
⇑ Corresponding author. Tel.: +91 9443787811; fax: +91 04144 226181. E-mail address: [email protected] (C. Manoharan). 1386-1425/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.09.149
298
R. Sridhar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 120 (2014) 297–303
Introduction As a semiconductor, ZnO has a band gap similar to that of TiO2. ZnO has aroused much attention because of its high photocatalytic activity in some organic photodegradation reactions [1–3]. Ding et al. [4] and Jiang et al. [5] have reported that composite ZnO– TiO2 shows better activity than TiO2 in dye photodegradation. On the contrary, Marci and co-workers [6,7] have prepared polycrystalline ZnO/TiO2 solids using home prepared TiO2 (anatase) or TiO2 (rutile) as support. The authors found that the coupling of ZnO and TiO2 semiconducting powders are not so beneficial to enhance the photo reactivity for the studied reaction. Additionally, extensive studies have been devoted to the effects of doping ZnO films with various elements such as Si [8], Ga [9], Al [10], Cu [11], Li [12] and Mg [13]. Pure ZnO has excellent conductivity as it contains high concentration of native defect (oxygen vacancy or zinc interstitials). At high temperature, pure ZnO thin films are not stable chemically and electrically. The performance of ZnO thin films can be greatly modified and improved by appropriate impurity doping. Modification of ZnO with transient metals (Al, In, Ga, Ti) provides a successful and cost effective alternative. In order to create free electrons and enhance n-type conductivity, two criteria should be satisfied: (i) the doping ion should be smaller than, or equal to, the diameter of the host ion; (ii) the ion of the dopant should have higher valency than that of the host atom [14]. The ionic radius of Ti4+ (0.068 nm) should be smaller than that of Zn2+ (0.074 nm), and hence Ti4+ ions can replace Zn2+ ions at substitutional sites [15]. In the present study, ZnO films doped with small Ti concentration were deposited on glass substrate. Structural, microstructural, optical and electrical properties were investigated. V–I characteristics were also measured.
Experimental methods Both un-doped and Ti-doped ZnO films were deposited on microscopic glass substrates using spray pyrolysis technique systematically by controlling the deposition parameters. For deposition, 0.1 M of zinc acetylacetonate Zn(AcAc) was dissolved in ethanol and sprayed onto microscopic glass substrates with dimensions of 75 25 mm2 at fixed substrate temperature; Ts = 400 °C. In order to dope ZnO with Ti, titanyl acetylacetonate Ti(AcAc) solution was added to the starting solution. The solution was sprayed using different doping concentrations ranging from 0.1% to 0.9%. Prior to deposition, the substrates were first cleaned with water bath, followed by dipping in con.HCl, acetone and ethanol successively. Finally, the substrates were rinsed in deionised water and allowed to dry in a hot air oven. In spray unit, the substrate temperature was maintained by using a heater, and it was controlled by a feedback circuit. During the spray, the substrate temperature was kept constant (400 °C) with an accuracy of ±5 °C. Spray head and substrate heater were kept inside a chamber, provided with an exhaust fan, for removing gaseous by-products and vapors from the solvent. The spray head was allowed to move in the X–Y plane using the microcontroller stepper motor, in order to achieve a uniform coating on the substrate. The spray head could scan an area of 200 200 mm2 with X-movement at a speed of 20 mm/s and Y-movement in steps of 5 mm/s simultaneously. The spray unit had the provision for controlling the spray rate of the solution as well as the pressure of the carrier gas. The microcontroller device was linked with PC through the serial port in which the data of each spray could be stored. The values of optimized parameters are given below: Substrate temperature: 400 °C. Spray time: 15 min.
Solution flow rate: 3 ml/min. Air flow rate: 1 kg/cm2. Spray nozzle to substrate distance: 20 cm. After deposition, the films were allowed to cool slowly to room temperature and washed with deionized water and then dried. The structural characterization of the deposited films was carried out by X-ray diffraction technique on JEOL JDX-803 a diffrac0 tometer (monochromatic Cu Ka radiation, k = 1.5406 Å A). The XRD patterns were recorded in 2h interval from 10° to 80° in steps of 0.05° at room temperature. The surface morphology was studied by Scanning Electron Microscopy (SEM) using JEOL-JES-1600 with a magnification of 10 K. Morphological and topography observations were carried out using Atomic Force microscope (AFM) Nano Surf Easy Scan 2. Optical absorption spectra were recorded in the range 350–900 nm using Varian-Cary 500 scan double-beam spectrophotometer. Raman spectra were recorded at room temperature using Renishaw laser Raman spectrometer and 18 mW He–Ne laser. Photoluminescence spectrum (PL) was studied at room temperature with a wavelength of 375 nm as the excitation source using Plorolog3-HORIBA JOBIN YVON. Results and discussion X-ray diffraction (XRD) analysis Fig. 1 shows the evolution of XRD patterns of ZnO:Ti thin films deposited on glass substrate. It is noticed that the strongest lines in the entire pattern are the principle lines (1 0 0), (0 0 2) and (1 0 1) of the hexagonal wurzite ZnO, in agreement with JCPDS standard card No. 75-0576. On doping with Ti up to 0.7%, no additional peaks are observed. This is attributed to the fact that the concentration of the dopant Ti is low. When the doping concentration is further increased above 0.7%, it is observed that the crystallinity becomes lower. The crystallite size is evaluated from peak profile using full width at half maximum (FWHM) of the (1 0 1) plane through Scherer’s formula:
D¼
Kk b cos h
where K = 0.94 is the shape factor, k is the X-ray wavelength of Cu Ka radiation, h is the Bragg angle and b is FWHM of the corresponding peak. The lattice strain (e) is calculated using the relation:
e¼
b cos h 4
Fig. 1. XRD patterns of deposited ZnO films with different Ti concentration.
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The value of dislocation density (d) is calculated using the following equation:
d¼
1 D2
Parameters like grain size (crystallite size), strain and dislocation density are estimated and are shown in Table 1. It is noticed that the grain size decreases linearly with Ti concentration (Fig. 2a), while both lattice strain and dislocation density increase. Generally, the continuous grain size reduction is occurred from 39 nm for un-doped ZnO to 12 nm after 0.9% Ti doping. The lattice strain value increases smoothly from 9 for pure ZnO to about 13104 for 0.3% Ti doping then drastically increases around 26 104 for 0.5% to remain almost constant for both 0.7% and 0.9%, see Fig. 2b. Similarly, dislocations density increases smoothly from about 7 1014 lines/m2 for un-doped ZnO to about 13 1014 lines/m2 for 0.3%, to drastically become 3 times higher about 39 1014 lines/m2, then increases monotically for higher concentrations, see Fig. 2c.
Fig. 2. EDS spectrum of ZnO film doped with 0.9% Ti.
Value of band gap of pure ZnO film is 2.20 eV which is small when compared to that of bulk value; 3.37 eV [17]. This can be attributed to structural and micro structural effects. The refractive index is calculated at different wavelengths using the following relation:
Microstructure and chemical analysis Composition of the films was studied by EDAX. Peaks corresponding to Ti, Zn and O elements were presented along with Si peaks coming from the glass substrate, see Fig. 3a. Table 2 shows the atomic percentage of the elements in the films. Fig. 3b shows SEM images of Ti-doped ZnO films. It observed that the surface morphology was gradually changed with increase of Ti doping concentration; the grains were homogeneously distributed over the entire surface for pure ZnO, which deteriorates afterwards. This result agrees well with XRD analysis, both crystallinity and the average grain size decreased by increasing the concentration of the dopant. Spherical-type grains are observed for all compositions and the average grain size decreases. The effect of 0.5% of Ti doping was analysed by AFM (Fig. 4a) and compared with pure ZnO. The image was observed with RMS surface roughness in a scanning size of 10 10 lm2. From images, it could be noticed that the roughness value decreases from 3.1 for un-doped ZnO to 2.6 nm after doping with 0.5% of Ti. From the micrographs, the observed high roughness value for pure ZnO may be due to the higher crystallinity of the film. The above results are in good agreement with the higher crystallinity analysed through XRD patterns. Optical properties Absorption spectra of Ti-doped ZnO films reported in Fig. 5 indicate that the value of the average transmittance varies in the range 80–95% in the visible region then decreases suddenly in the UV and near visible regions, to settle in the region 70–85%. The value of the band gap of the films is obtained from Tauc’s plot, see Fig. 6. The estimated values reported in Table 3 indicate that the band gap gradually increases with increasing Ti concentration. Similar result is reported in the literature [16].
n¼
1 þ R1=2 1 R1=2
The extinction coefficient (k) can be obtained from the expression:
k¼
ak 4p
The optical properties of a sprayed film depend on its refractive index (n) and extinction coefficient (k) values, see Fig. 7a and b. Figure 7a shows the variation of refractive index with different wavelengths of the ZnO films doped with different concentrations of Ti. From the result, it is clear that, the refractive index increases in the UV region and decreased gradually from 2.7 to 1.8 and then slightly increases in the higher wavelength region (high transmission range). Low refractive index occurs due to successive internal reflections or to the trapped photon energy within the grain boundary. It is also attributed to the variety of different impurities and defects that may form within the film. With the increase of Ti doping concentration in ZnO films, structural defects level is very high and this implies that there is a strong optical scattering in the films. The variation of extinction coefficient with wavelength is shown in Fig. 7b. It is observed that the extinction coefficient decreases with the increase of Ti content in ZnO films. The fall in the extinction coefficient may be due to the absorption of light at the grain boundaries. From the figure, it is clear that the value of k decreases rapidly with increasing wavelength from 350 to 400 nm and after that it slightly increases. The dielectric constant (e) of the films is calculated using the relationship: 1=2
e ¼ e1 þ ie2 ¼ ðe21 þ e22 Þ
where e1 and e2 are the real and imaginary parts of the dielectric constant. The values of e1 and e2 for different incident photon energies can be obtained from the values of ‘n’ and ‘k’ using the relations
Table 1 Microstructural parameters of ZnO films doped with different Ti concentration. Doping concentration (%)
Crystallite size (D) (nm)
Lattice strain (e) (104)
Dislocation density (d) (1014 lines/m2)
0.0 0.1 0.3 0.5 0.7 0.9
39 36 28 16 14 12
9.02 9.30 12.89 25.65 26.97 28.65
6.57 7.71 12.75 39.06 51.02 69.44
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Fig. 3. SEM micrographs of ZnO films (a) un-doped and doped with Ti (b) 0.1% (c) 0.5% Ti and (d) 0.9% Ti.
Table 2 Composition of ZnO films doped with different Ti concentration. Doping concentration (%)
O–K
Zn–L
Ti–K
0.1 0.5 0.9
76.65 77.42 86.83
23.29 22.44 12.94
0.06 0.15 0.23
e1 ¼ n2 k2 and e2 ¼ 2nk Fig. 8 shows the variation of the real (e1) and imaginary (e2) parts of the dielectric constant for Ti doped ZnO films. From the graphs, it is revealed that the values of the real part are higher than that of the imaginary part. From the optical data, it is noticed that the refractive index (n), the extinction coefficient (k), the real (e1) and imaginary (e2) parts of the dielectric constant follow the same behavior. Fig. 5. Transmission spectra of films doped with different Ti concentration.
Electrical properties Electrical properties are studied at room temperature, by evaporating gold ohmic contact on the edges of the top surface
of Ti-doped ZnO films. Linear characteristics are observed for all compositions, see Fig. 9. Resistance is computed from the slope
Fig. 4. AFM images of ZnO films (a) undoped and doped with 0.5% Ti.
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Raman spectroscopy
Fig. 6. Tauc’s plot of ZnO films with different Ti concentration.
Table 3 Band gap of ZnO films doped with different Ti concentration. Dopant concentration (%)
Band gap (eV)
0.0 0.1 0.3 0.5 0.7 0.9
3.20 3.22 3.26 3.27 3.28 3.32
of the graphs. It can be noticed that resistivity of the films decreases with the increase of Ti concentration. The calculated values of the resistivity of the films are 9 105, 7 105, 4 105, 2 105 and 9 104 X cm for the pure ZnO and doped with 0%, 1%, 3%, 5% and 9%, respectively. Ti4+ ions substituted Zn2+ ions within the crystal lattice induce positive TiZn charges in the material. In order to maintain electrical neutrality, two negative electrons are induced to compensate the excess positive charges. Hence, the resistivity slightly decreases due to the increase of the free electrons in the film. However, when more Ti is incorporated, the resistivity suddenly increases. On the other hand, the residual stress arises simultaneously as well.
Fig. 10 shows room temperature Raman spectra of ZnO films doped with different I concentrations. A1 and E1 modes can be split into transverse (TO) and longitudinal optical (LO) phonons. Fundamental optical modes of ZnO wurtzite structure such as A1(TO), High E1(LO), (EHigh ELow are observed using Raman scattering 2 ) and E2 2 at 383, 576, 325 and 435 cm1, respectively [18–20]. The EHigh 2 mode of non-polar optical phonons is the characteristic peak of the hexagonal wurtzite phase. E1(LO) mode is related to the structural defects (VO, Zinc interstitials, free carriers, etc.) within ZnO lattice. Most of these multiple phonon modes were strongly related to local vibrational modes associated with point defects and their corresponding optical processes are correlated [21]. The presence of an intense EHigh mode in the Raman spectrum 2 indicates that the synthesized ZnO nanostructures are highly crystalline with a hexagonal wurtzite phase. However, the presence of the intense multiple phonon modes and a weak EHigh mode in the 2 Raman spectrum will indicate that the synthesized Ti-doped ZnO nanostructures are poorly crystalline, which is in agreement with XRD analysis and SEM as well as AFM observations. Ti-doped ZnO nanoparticles show a suppressed E1(LO) mode related to the VO-related defects.
Photoluminescence (PL) analysis Fig. 11 shows room temperature PL spectra of Ti-doped ZnO system. Peaks are observed at 424, 450, 525 and 675 nm. It can be noticed that the PL intensity decreases with the increase of Ti concentration. The PL peak for un-doped ZnO occurred at 420 nm and is red shifted with Ti doping. The blue band at 450 nm, which is related with interstitial Zn defects emission, has been described in detail elsewhere [22,23]. Generally, the stoichiometry of undoped ZnO is deviated from the unity (1) and usually contains more Zn interstitials and oxygen vacancies, and hence strong blue emission peak is observed. As for Ti-doped ZnO, various Ti impurities will affect the concentration of the interstitial Zn and O vacancies. The valency of Ti can be +2 and +4 in Ti-doped zinc oxide films. The radius of Ti2+, Ti4+ and Zn2+ ions are 0.094, 0.068 and 0.074 nm, respectively. Thus Ti impurities are mainly included as Ti2+ substitute, Ti4+
Fig. 7. Variations of (a) refractive index and (b) extinction coefficient as a function of wavelength for ZnO films doped with different Ti concentration.
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Fig. 8. Variation of (a) real and (b) imaginary part of dielectric constants as a function of wavelength for ZnO films doped with different Ti concentration.
Fig. 11. Room temperature photoluminescence spectra of ZnO films doped with different Ti concentration. Fig. 9. V–I characteristics of ZnO films doped with different Ti concentration.
affect the concentration of the interstitial Zn and O vacancies; both may increase with increasing Ti4+ substitute because ZnO is a selfassembled oxide compound. The intensity of the emission peaks decrease by increasing concentration of dopant. This may be occurring due to the probability that excess of substitutional Ti 2+ ions is present more than the interstitial Ti4+ ions which coexist within ZnO films. Conclusion
Fig. 10. Room temperature Raman spectra of ZnO films doped with different Ti concentration.
substitute and Ti4+ interstitial. As Ti concentration is increased, Ti2+ substitute and Ti4+ interstitial coexisted within ZnO films, which will
The results of this work clearly indicate that nanocrystalline Tidoped ZnO films can be deposited by spray pyrolysis technique. XRD, SEM, AFM, Raman and PL measurements confirm the formation of ZnO phase over the entire Ti concentration (0.1–0.9%) and that crystallinity is greatly affected by Ti doping content. It is evident that increasing Ti doping leads to a decrease of crystallinity. Additionally, it is found that with the increase of Ti doping concentration, the value of band gap increases slightly from 3.20 eV to 3.32 eV whereas the resistivity decreases from 9 105 to 9 104 X cm.
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Acknowledgement The authors thank the University Grants commission, New Delhi, for the financial support through research Grant No. 42-860/ 2013. References [1] M.R. Hoffmann, S.T. Martin, W. Choi, W. Bahnemann, Chem. Rev. 95 (1995) 69– 96. [2] S. Sakthivel, B. Neppolian, M.V. Shankar, B. Arabindoo, M. Palanichamy, V. Murugesan, Sol. Energy Mater. Sol. Cells 77 (2003) 65–82. [3] Y.J. Jang, C. Simer, T. Ohm, Mater. Res. Bull. 41 (2006) 67–77. [4] S.W. Ding, L.Y. Wang, S.Y. Zhang, S.J. Liu, Y. Ding, Y.C. Liu, et al., Chin. J. Inorg. Chem. 19 (2003) 631. [5] H.B. Jiang, L. Gao, Q.H. Zhang, J. Inorg. Mater. 18 (2003) 695–701. [6] G. Marci, V. Augugliaro, M.J. Lopez-Munoz, C. Martin, L. Palmisano, V. Rives, et al., J. Phys. Chem. B 105 (2001) 1026. [7] G. Marci, V. Augugliaro, M.J. Lopez-Munoz, C. Martin, L. Palmisano, V. Rives, J. Phys. Chem. B 105 (2001) 1033. [8] T. Minami, H. Sato, H. Nanto, S. Takada, Jpn. J. Appl. Phys. 125 (1986) 1776. [9] J.K. Kim, S.J. Yun, J.M. Lee, J.W. Lim, Curr. Appl. Phys. 10 (2010) 451.
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Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Spectroscopic study and optical and electrical properties of Ti-doped ZnO thin films by spray pyrolysis R. Sridhar a, C. Manoharan a,⇑, S. Ramalingam b, S. Dhanapandian a, M. Bououdina c,d a
Department of Physics, Annamalai University, Annamalai Nagar 608 002, India Department of Physics, A.V.C. College, Mayiladuthurai, Tamilnadu, India c Nanotechnology Centre, University of Bahrain, PO Box 32038, Bahrain d Department of Physics, College of Science, University of Bahrain, PO Box 32038, Bahrain b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The titanium-doped zinc oxide
Zinc oxide films were doped with different concentrations of Ti on glass substrates at 400 °C by spray pyrolysis technique. The films exhibited single phase ZnO for low concentrations of Ti. Wurtzite ZnO peaks were observed at higher doping concentration with decreased crystallinity. Crystallite size, strain and dislocation density were evaluated from the X-ray diffraction data. Surface morphology of the films indicated that a remarkable decrease in grain size with increasing of Ti concentration. The band gap of the films was found to be increased from 3.20 eV to 3.32 eV as the concentration of Ti doping increases. The resistivity of the films decreased from 9 105 X cm to 9 104 X cm with the increase of Ti doping concentration.
(Ti-doped ZnO) thin films were deposited onto glass substrate by spray pyrolysis technique. The physical properties of these films were characterized by XRD, AFM and PL measurements. The crystallographic parameters; microstrain, dislocation density, and lattice constant were estimated. The band gap of the films was found to be increased from 3.20 eV to 3.32 eV as the concentration of Ti doping increased.
a r t i c l e
i n f o
Article history: Received 23 August 2013 Received in revised form 16 September 2013 Accepted 29 September 2013 Available online 11 October 2013 Keywords: ZnO thin films Ti doping Raman Photoluminescence Resistivity
a b s t r a c t Zinc oxide films were doped with different concentrations of Ti on glass substrates at 400 °C by spray pyrolysis technique. The films exhibited single phase ZnO for low concentrations of Ti. Wurtzite ZnO peaks were observed at higher doping concentration with decreased crystallinity. Crystallite size, strain and dislocation density were evaluated from the X-ray diffraction data. Surface morphology of the films indicated that a remarkable decrease in grain size with increasing of Ti concentration. The band gap of the films was found to be increased from 3.20 eV to 3.32 eV as the concentration of Ti doping increases. The resistivity of the films decreased from 9 105 X cm to 9 104 X cm with the increase of Ti doping concentration. Both Raman spectroscopy and room temperature photoluminescence exhibited characteristic peaks that confirmed the formation of ZnO phase. Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved.
⇑ Corresponding author. Tel.: +91 9443787811; fax: +91 04144 226181. E-mail address: [email protected] (C. Manoharan). 1386-1425/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.09.149
298
R. Sridhar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 120 (2014) 297–303
Introduction As a semiconductor, ZnO has a band gap similar to that of TiO2. ZnO has aroused much attention because of its high photocatalytic activity in some organic photodegradation reactions [1–3]. Ding et al. [4] and Jiang et al. [5] have reported that composite ZnO– TiO2 shows better activity than TiO2 in dye photodegradation. On the contrary, Marci and co-workers [6,7] have prepared polycrystalline ZnO/TiO2 solids using home prepared TiO2 (anatase) or TiO2 (rutile) as support. The authors found that the coupling of ZnO and TiO2 semiconducting powders are not so beneficial to enhance the photo reactivity for the studied reaction. Additionally, extensive studies have been devoted to the effects of doping ZnO films with various elements such as Si [8], Ga [9], Al [10], Cu [11], Li [12] and Mg [13]. Pure ZnO has excellent conductivity as it contains high concentration of native defect (oxygen vacancy or zinc interstitials). At high temperature, pure ZnO thin films are not stable chemically and electrically. The performance of ZnO thin films can be greatly modified and improved by appropriate impurity doping. Modification of ZnO with transient metals (Al, In, Ga, Ti) provides a successful and cost effective alternative. In order to create free electrons and enhance n-type conductivity, two criteria should be satisfied: (i) the doping ion should be smaller than, or equal to, the diameter of the host ion; (ii) the ion of the dopant should have higher valency than that of the host atom [14]. The ionic radius of Ti4+ (0.068 nm) should be smaller than that of Zn2+ (0.074 nm), and hence Ti4+ ions can replace Zn2+ ions at substitutional sites [15]. In the present study, ZnO films doped with small Ti concentration were deposited on glass substrate. Structural, microstructural, optical and electrical properties were investigated. V–I characteristics were also measured.
Experimental methods Both un-doped and Ti-doped ZnO films were deposited on microscopic glass substrates using spray pyrolysis technique systematically by controlling the deposition parameters. For deposition, 0.1 M of zinc acetylacetonate Zn(AcAc) was dissolved in ethanol and sprayed onto microscopic glass substrates with dimensions of 75 25 mm2 at fixed substrate temperature; Ts = 400 °C. In order to dope ZnO with Ti, titanyl acetylacetonate Ti(AcAc) solution was added to the starting solution. The solution was sprayed using different doping concentrations ranging from 0.1% to 0.9%. Prior to deposition, the substrates were first cleaned with water bath, followed by dipping in con.HCl, acetone and ethanol successively. Finally, the substrates were rinsed in deionised water and allowed to dry in a hot air oven. In spray unit, the substrate temperature was maintained by using a heater, and it was controlled by a feedback circuit. During the spray, the substrate temperature was kept constant (400 °C) with an accuracy of ±5 °C. Spray head and substrate heater were kept inside a chamber, provided with an exhaust fan, for removing gaseous by-products and vapors from the solvent. The spray head was allowed to move in the X–Y plane using the microcontroller stepper motor, in order to achieve a uniform coating on the substrate. The spray head could scan an area of 200 200 mm2 with X-movement at a speed of 20 mm/s and Y-movement in steps of 5 mm/s simultaneously. The spray unit had the provision for controlling the spray rate of the solution as well as the pressure of the carrier gas. The microcontroller device was linked with PC through the serial port in which the data of each spray could be stored. The values of optimized parameters are given below: Substrate temperature: 400 °C. Spray time: 15 min.
Solution flow rate: 3 ml/min. Air flow rate: 1 kg/cm2. Spray nozzle to substrate distance: 20 cm. After deposition, the films were allowed to cool slowly to room temperature and washed with deionized water and then dried. The structural characterization of the deposited films was carried out by X-ray diffraction technique on JEOL JDX-803 a diffrac0 tometer (monochromatic Cu Ka radiation, k = 1.5406 Å A). The XRD patterns were recorded in 2h interval from 10° to 80° in steps of 0.05° at room temperature. The surface morphology was studied by Scanning Electron Microscopy (SEM) using JEOL-JES-1600 with a magnification of 10 K. Morphological and topography observations were carried out using Atomic Force microscope (AFM) Nano Surf Easy Scan 2. Optical absorption spectra were recorded in the range 350–900 nm using Varian-Cary 500 scan double-beam spectrophotometer. Raman spectra were recorded at room temperature using Renishaw laser Raman spectrometer and 18 mW He–Ne laser. Photoluminescence spectrum (PL) was studied at room temperature with a wavelength of 375 nm as the excitation source using Plorolog3-HORIBA JOBIN YVON. Results and discussion X-ray diffraction (XRD) analysis Fig. 1 shows the evolution of XRD patterns of ZnO:Ti thin films deposited on glass substrate. It is noticed that the strongest lines in the entire pattern are the principle lines (1 0 0), (0 0 2) and (1 0 1) of the hexagonal wurzite ZnO, in agreement with JCPDS standard card No. 75-0576. On doping with Ti up to 0.7%, no additional peaks are observed. This is attributed to the fact that the concentration of the dopant Ti is low. When the doping concentration is further increased above 0.7%, it is observed that the crystallinity becomes lower. The crystallite size is evaluated from peak profile using full width at half maximum (FWHM) of the (1 0 1) plane through Scherer’s formula:
D¼
Kk b cos h
where K = 0.94 is the shape factor, k is the X-ray wavelength of Cu Ka radiation, h is the Bragg angle and b is FWHM of the corresponding peak. The lattice strain (e) is calculated using the relation:
e¼
b cos h 4
Fig. 1. XRD patterns of deposited ZnO films with different Ti concentration.
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The value of dislocation density (d) is calculated using the following equation:
d¼
1 D2
Parameters like grain size (crystallite size), strain and dislocation density are estimated and are shown in Table 1. It is noticed that the grain size decreases linearly with Ti concentration (Fig. 2a), while both lattice strain and dislocation density increase. Generally, the continuous grain size reduction is occurred from 39 nm for un-doped ZnO to 12 nm after 0.9% Ti doping. The lattice strain value increases smoothly from 9 for pure ZnO to about 13104 for 0.3% Ti doping then drastically increases around 26 104 for 0.5% to remain almost constant for both 0.7% and 0.9%, see Fig. 2b. Similarly, dislocations density increases smoothly from about 7 1014 lines/m2 for un-doped ZnO to about 13 1014 lines/m2 for 0.3%, to drastically become 3 times higher about 39 1014 lines/m2, then increases monotically for higher concentrations, see Fig. 2c.
Fig. 2. EDS spectrum of ZnO film doped with 0.9% Ti.
Value of band gap of pure ZnO film is 2.20 eV which is small when compared to that of bulk value; 3.37 eV [17]. This can be attributed to structural and micro structural effects. The refractive index is calculated at different wavelengths using the following relation:
Microstructure and chemical analysis Composition of the films was studied by EDAX. Peaks corresponding to Ti, Zn and O elements were presented along with Si peaks coming from the glass substrate, see Fig. 3a. Table 2 shows the atomic percentage of the elements in the films. Fig. 3b shows SEM images of Ti-doped ZnO films. It observed that the surface morphology was gradually changed with increase of Ti doping concentration; the grains were homogeneously distributed over the entire surface for pure ZnO, which deteriorates afterwards. This result agrees well with XRD analysis, both crystallinity and the average grain size decreased by increasing the concentration of the dopant. Spherical-type grains are observed for all compositions and the average grain size decreases. The effect of 0.5% of Ti doping was analysed by AFM (Fig. 4a) and compared with pure ZnO. The image was observed with RMS surface roughness in a scanning size of 10 10 lm2. From images, it could be noticed that the roughness value decreases from 3.1 for un-doped ZnO to 2.6 nm after doping with 0.5% of Ti. From the micrographs, the observed high roughness value for pure ZnO may be due to the higher crystallinity of the film. The above results are in good agreement with the higher crystallinity analysed through XRD patterns. Optical properties Absorption spectra of Ti-doped ZnO films reported in Fig. 5 indicate that the value of the average transmittance varies in the range 80–95% in the visible region then decreases suddenly in the UV and near visible regions, to settle in the region 70–85%. The value of the band gap of the films is obtained from Tauc’s plot, see Fig. 6. The estimated values reported in Table 3 indicate that the band gap gradually increases with increasing Ti concentration. Similar result is reported in the literature [16].
n¼
1 þ R1=2 1 R1=2
The extinction coefficient (k) can be obtained from the expression:
k¼
ak 4p
The optical properties of a sprayed film depend on its refractive index (n) and extinction coefficient (k) values, see Fig. 7a and b. Figure 7a shows the variation of refractive index with different wavelengths of the ZnO films doped with different concentrations of Ti. From the result, it is clear that, the refractive index increases in the UV region and decreased gradually from 2.7 to 1.8 and then slightly increases in the higher wavelength region (high transmission range). Low refractive index occurs due to successive internal reflections or to the trapped photon energy within the grain boundary. It is also attributed to the variety of different impurities and defects that may form within the film. With the increase of Ti doping concentration in ZnO films, structural defects level is very high and this implies that there is a strong optical scattering in the films. The variation of extinction coefficient with wavelength is shown in Fig. 7b. It is observed that the extinction coefficient decreases with the increase of Ti content in ZnO films. The fall in the extinction coefficient may be due to the absorption of light at the grain boundaries. From the figure, it is clear that the value of k decreases rapidly with increasing wavelength from 350 to 400 nm and after that it slightly increases. The dielectric constant (e) of the films is calculated using the relationship: 1=2
e ¼ e1 þ ie2 ¼ ðe21 þ e22 Þ
where e1 and e2 are the real and imaginary parts of the dielectric constant. The values of e1 and e2 for different incident photon energies can be obtained from the values of ‘n’ and ‘k’ using the relations
Table 1 Microstructural parameters of ZnO films doped with different Ti concentration. Doping concentration (%)
Crystallite size (D) (nm)
Lattice strain (e) (104)
Dislocation density (d) (1014 lines/m2)
0.0 0.1 0.3 0.5 0.7 0.9
39 36 28 16 14 12
9.02 9.30 12.89 25.65 26.97 28.65
6.57 7.71 12.75 39.06 51.02 69.44
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Fig. 3. SEM micrographs of ZnO films (a) un-doped and doped with Ti (b) 0.1% (c) 0.5% Ti and (d) 0.9% Ti.
Table 2 Composition of ZnO films doped with different Ti concentration. Doping concentration (%)
O–K
Zn–L
Ti–K
0.1 0.5 0.9
76.65 77.42 86.83
23.29 22.44 12.94
0.06 0.15 0.23
e1 ¼ n2 k2 and e2 ¼ 2nk Fig. 8 shows the variation of the real (e1) and imaginary (e2) parts of the dielectric constant for Ti doped ZnO films. From the graphs, it is revealed that the values of the real part are higher than that of the imaginary part. From the optical data, it is noticed that the refractive index (n), the extinction coefficient (k), the real (e1) and imaginary (e2) parts of the dielectric constant follow the same behavior. Fig. 5. Transmission spectra of films doped with different Ti concentration.
Electrical properties Electrical properties are studied at room temperature, by evaporating gold ohmic contact on the edges of the top surface
of Ti-doped ZnO films. Linear characteristics are observed for all compositions, see Fig. 9. Resistance is computed from the slope
Fig. 4. AFM images of ZnO films (a) undoped and doped with 0.5% Ti.
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Raman spectroscopy
Fig. 6. Tauc’s plot of ZnO films with different Ti concentration.
Table 3 Band gap of ZnO films doped with different Ti concentration. Dopant concentration (%)
Band gap (eV)
0.0 0.1 0.3 0.5 0.7 0.9
3.20 3.22 3.26 3.27 3.28 3.32
of the graphs. It can be noticed that resistivity of the films decreases with the increase of Ti concentration. The calculated values of the resistivity of the films are 9 105, 7 105, 4 105, 2 105 and 9 104 X cm for the pure ZnO and doped with 0%, 1%, 3%, 5% and 9%, respectively. Ti4+ ions substituted Zn2+ ions within the crystal lattice induce positive TiZn charges in the material. In order to maintain electrical neutrality, two negative electrons are induced to compensate the excess positive charges. Hence, the resistivity slightly decreases due to the increase of the free electrons in the film. However, when more Ti is incorporated, the resistivity suddenly increases. On the other hand, the residual stress arises simultaneously as well.
Fig. 10 shows room temperature Raman spectra of ZnO films doped with different I concentrations. A1 and E1 modes can be split into transverse (TO) and longitudinal optical (LO) phonons. Fundamental optical modes of ZnO wurtzite structure such as A1(TO), High E1(LO), (EHigh ELow are observed using Raman scattering 2 ) and E2 2 at 383, 576, 325 and 435 cm1, respectively [18–20]. The EHigh 2 mode of non-polar optical phonons is the characteristic peak of the hexagonal wurtzite phase. E1(LO) mode is related to the structural defects (VO, Zinc interstitials, free carriers, etc.) within ZnO lattice. Most of these multiple phonon modes were strongly related to local vibrational modes associated with point defects and their corresponding optical processes are correlated [21]. The presence of an intense EHigh mode in the Raman spectrum 2 indicates that the synthesized ZnO nanostructures are highly crystalline with a hexagonal wurtzite phase. However, the presence of the intense multiple phonon modes and a weak EHigh mode in the 2 Raman spectrum will indicate that the synthesized Ti-doped ZnO nanostructures are poorly crystalline, which is in agreement with XRD analysis and SEM as well as AFM observations. Ti-doped ZnO nanoparticles show a suppressed E1(LO) mode related to the VO-related defects.
Photoluminescence (PL) analysis Fig. 11 shows room temperature PL spectra of Ti-doped ZnO system. Peaks are observed at 424, 450, 525 and 675 nm. It can be noticed that the PL intensity decreases with the increase of Ti concentration. The PL peak for un-doped ZnO occurred at 420 nm and is red shifted with Ti doping. The blue band at 450 nm, which is related with interstitial Zn defects emission, has been described in detail elsewhere [22,23]. Generally, the stoichiometry of undoped ZnO is deviated from the unity (1) and usually contains more Zn interstitials and oxygen vacancies, and hence strong blue emission peak is observed. As for Ti-doped ZnO, various Ti impurities will affect the concentration of the interstitial Zn and O vacancies. The valency of Ti can be +2 and +4 in Ti-doped zinc oxide films. The radius of Ti2+, Ti4+ and Zn2+ ions are 0.094, 0.068 and 0.074 nm, respectively. Thus Ti impurities are mainly included as Ti2+ substitute, Ti4+
Fig. 7. Variations of (a) refractive index and (b) extinction coefficient as a function of wavelength for ZnO films doped with different Ti concentration.
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Fig. 8. Variation of (a) real and (b) imaginary part of dielectric constants as a function of wavelength for ZnO films doped with different Ti concentration.
Fig. 11. Room temperature photoluminescence spectra of ZnO films doped with different Ti concentration. Fig. 9. V–I characteristics of ZnO films doped with different Ti concentration.
affect the concentration of the interstitial Zn and O vacancies; both may increase with increasing Ti4+ substitute because ZnO is a selfassembled oxide compound. The intensity of the emission peaks decrease by increasing concentration of dopant. This may be occurring due to the probability that excess of substitutional Ti 2+ ions is present more than the interstitial Ti4+ ions which coexist within ZnO films. Conclusion
Fig. 10. Room temperature Raman spectra of ZnO films doped with different Ti concentration.
substitute and Ti4+ interstitial. As Ti concentration is increased, Ti2+ substitute and Ti4+ interstitial coexisted within ZnO films, which will
The results of this work clearly indicate that nanocrystalline Tidoped ZnO films can be deposited by spray pyrolysis technique. XRD, SEM, AFM, Raman and PL measurements confirm the formation of ZnO phase over the entire Ti concentration (0.1–0.9%) and that crystallinity is greatly affected by Ti doping content. It is evident that increasing Ti doping leads to a decrease of crystallinity. Additionally, it is found that with the increase of Ti doping concentration, the value of band gap increases slightly from 3.20 eV to 3.32 eV whereas the resistivity decreases from 9 105 to 9 104 X cm.
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