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Effect of substrate temperature on optical, structural and electrical properties of FeSe thin films deposited by spray pyrolysis technique
Journal of Physics and Chemistry of Solids ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Effect of substrate temperature on optical, structural and electrical properties of FeSe thin films deposited by spray pyrolysis technique A.U. Ubale n, Y.S. Sakhare Nanostructured Thin Film Materials Laboratory, Department of Physics, Government Vidarbha Institute of Science and Humanities, VMV Road, Amravati 444604, Maharashtra, India
art ic l e i nf o
a b s t r a c t
Article history: Received 15 January 2013 Accepted 5 May 2013
The versatile spray pyrolysis technique was employed to prepare thin films of iron selenide on glass substrates at different substrate temperatures. The deposition temperature was varied between 473 and 673 K. The asdeposited films were characterized by X-ray diffraction (XRD), SEM, optical and electrical characterization techniques. The X-ray studies reveal that the films are nanocrystalline with tetragonal structure and exhibit (101) preferred orientation. The SEM and AFM studies indicate that the film surface is homogenous with no cracks or pinholes and well covers the glass substrate. The film thickness was found to vary from 110 to 230 nm with substrate temperature. The optical band gap was found to decrease from 2.92 to 2.68 eV depending on deposition temperature. The resistivity of p-type FeSe film is of the order of 8 104 Ω cm and it decreases to 1.5 104 Ω cm as substrate temperature is increased from 473 K to 673 K. & 2013 Elsevier Ltd. All rights reserved.
Keywords: A. Thin Films D. Electrical properties
1. Introduction The global warming, rising oil prices and a steadily rising demand for electricity in developing countries clearly shows prime a need of development and utilization of clean and renewable energy sources. The development of economic solar photovoltaic devices is the main motivation for the development of nanostructured thin film. During the last few years, the synthesis of nano-structured iron selenide materials has attracted considerable attention due to their novel physical and chemical properties and potential applications in constructing nanoscale electronic and optoelectronic devices. The nanocrystalline thin films attracted significant attention of the scientists not only due to the growing technological applications, but also because of the interest in quantum confinement effects that arises in it. They exhibit physical and chemical properties that are neither characteristics of the atoms nor bulk counterparts. Various sophisticated chemical and physical methods are employed to grow nanocrystalline thin films but out of them, spray pyrolysis is a simple, convenient and economic to grow nanocrystalline FeSe thin films [1]. While development of such chemical method requires optimization of various preparative parameters like, substrate temperature, spray rate, concentration of the solution etc as they affect various physical and chemical properties of the deposited material. Chen et al. [2] have deposited nanostructured FeSe thin films by electrochemical deposition method on indium doped tin oxide coated conducting glass substrates. The growth of epitaxial tetragonal iron selenide thin films n
Corresponding author. Tel.: +91 721 2531706; fax: +91 721 2531705. E-mail address: [email protected] (A.U. Ubale).
on SrTiO3 and MgO substrates was reported by Chan et al. [3] by pulsed laser deposition technique. The deposition temperature and annealing process are found to be critical for achieving tetragonal phase of FeSe. Lin et al. [4] have reported superconducting FeSex thin film by selenization technique. The influence of selenization temperature on film components was studied. Thanikaikarasan et al. [5] have electrodeposited FeSe thin films on ITO substrates at various bath temperatures from 30 to 90 1C in an aqueous electrolytic bath containing FeSO4 and SeO2. Xu et al. [6] have reported (001) orientated FeSe thin film by metal organic vapour deposition method on Si substrate. The structural analysis confirm single tetragonal phase of FeSe. Hamdadou et al. [7] have grown FeSe thin films by selenization of iron thin films. The films grown by this method shows tetragonal FeSe with some FeSe2 crystallites. Liu et al. [8] have reported FeSe thin film on GaAs (001) substrate by low pressure metal organic chemical vapor deposition. The X-ray diffraction measurement indicated that the sample was preferentially oriented with tetragonal structure. In the present work nanocrystallines FeSe thin films were prepared by spray pyrolysis method at different substrate temperatures to investigate its effect on physical properties.
2. Experimental details The aqueous solutions of Fe (NO3)3 9H2O, SeO2 (Prepared by dissolving Se metal powder in HNO3) and acetic acid were prepared by dissolving appropriate amount of these salts (AR grade) in double distilled water. The spray solution was prepared by mixing 20 ml 0.12 M Fe(NO3)3 9H2O, 10 ml of 0.12 M CH3-COOH and 20 ml of
0022-3697/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jpcs.2013.05.006
i Please cite this article as: A.U. Ubale, Y.S. Sakhare, J. Phys. Chem. Solids, Effect of substrate temperature on optical, structural and electrical properties of FeSe thin films deposited by spray pyrolysis technique (2013), http://dx.doi.org/10.1016/j.jpcs.2013.05.006
3. Result and discussion 3.1. Film formation mechanism The spray pyrolysis technique is basically a chemical deposition technique in which fine droplets of sprayed solution undergo pyrolytic decomposition to form film at hot substrate. Each droplet containing solvent and solute passes through different temperature zones formed between hot substrate and nozzle. The process of thermal decomposition depends upon the substrate temperature. It was observed that the films deposited at lower substrate temperature (o473 K) are non uniform and powdery. As a result at lower temperature the sprayed droplets forms less adhesive powdery films giving lower thickness. However, at higher temperature (4573 K), the solvent vaporizes well before reaching the hot substrate causing lower thickness (Fig. 1). Also, at higher temperature there is possibility that only bigger droplets reaches to the hot substrate giving porous morphology. In the present case it was found that the films deposited at 573 K temperature are uniform and well adherent to the glass substrate. 3.2. Structural studies The structural analysis of FeSe thin films was carried out using CuKα radiation of wavelength 0.154 nm. The XRD patterns were 240
Thickness (nm)
220 200 180 160 140 120 100 190
240
290
340
390
Substrates temperature (K) Fig. 1. Variation of FeSe films thickness (nm) with substrate temperatures.
473 K FeSe: JCPDS card 85-0735 002
523 K
Intensity (Arbitrary unit)
573 K 623 K 200
673 K 212
0.13 M freshly prepared SeO2 solution in a 100 ml beaker. In the present work, the substrate temperature was varied between 473 and 673 K to investigate its effect on structural, optical and electrical properties of FeSe. The 30 ml of solution was sprayed at the rate 5 ml/ min onto hot substrates by using compressed air as carrier gas. The distance between nozzle and substrate was kept 25 cm. The thickness of the films prepared at various substrate temperatures was measured by the gravimetric method, by considering the bulk density of iron selenide. The structural characterization of FeSe was carried out by analyzing XRD patterns obtained by using an X-ray diffractometer PANalytical X'Pert PRO MRD with CuKα radiation of wavelength 0.154 nm. The surface morphology and energy dispersive X-ray analysis of the spray deposited FeSe thin films was carried out by using a JOEL'S JSM-7600F scanning electron microscope attached with EDX setup. The AFM measurements were performed with diInnova from Veeco Instruments. The electrical resistivity of the film was measured as a function of temperature in the range 303–573 K using dc two point probe method. The optical absorption spectra of the films were measured by JASCO V-530 UV/visible spectrophotometer, in the 300–900 nm wavelength range, with glass substrate as a reference. The thermoelectric power (TEP) of FeSe thin films prepared by varying substrate temperature between 303 and 573 K was measured in dark. Silver contacts were made for the resistivity and TEP measurements.
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2 (degree) Fig. 2. X-ray diffraction pattern of FeSe thin films deposited at various substrate temperatures.
recorded by varying diffraction angle between 201 and 801. Fig. 2 shows the typical X-ray diffraction patterns of FeSe thin films deposited at various substrate temperatures. The films are nanocrystalline in nature with tetragonal structure [2,9]. The observed and standard 2θ and d value are in good agreement which confirms that the deposited films are tetragonal FeSe. The (101) and (200) peaks are observed for all the films. The (002) and (212) peaks are observed at higher deposition temperature. The peak intensity and number of peaks increases with substrate temperature and is attributed to the improved crystallinity of the film. The average grain size of FeSe was determined by using Scherer's formula d¼
0:9λ β cos θ
ð1Þ
where d is the grain size, λ is the wavelength of X-ray used, β the angular line width at half-maximum intensity and θ is Bragg's angle. The crystallite size of FeSe change from 21 to 53 nm as substrate temperature is increased from 473 to 573 K. For further rize in temperature from 523 to 573 K the crystallite size decreases from 53 to 48 nm which may be due to re-evaporation of film [10]. 3.3. Surface morphology Fig. 3 shows the SEM micrographs of FeSe thin films on glass substrates deposited at various substrate temperatures. The deposited films has homogenous surface without creaks or holes. At lower substrate temperature film surface is more homogenous and the circular grain growth with agglomeration is improved with rise in substrate temperature. Above 573 K substrate temperature the film surface shows porous nature. The compositional analysis of the FeSe thin film was carried out by EDAX technique. Fig. 4 shows a typical EDAX pattern of FeSe thin film deposited at 573 K substrate temperature. The elemental analysis was carried out only for Fe and Se element. The average atomic percentage of Fe:Se was 52:48 showing enrichment of Fe in FeSe film. The threedimensional surface morphology of the spray deposited FeSe thin films was investigated by taking atomic force micrographs. Fig. 5 (F) and (H) shows the 3D and 2D AFM images of FeSe thin films deposited at 473 and 573 K substrate temperature. The AFM micrographs show total coverage of the substrate with uniform spherical grains of FeSe. The spherical grains with relatively smooth surface are observed for films deposited at lower temperature. However, the surface morphology of FeSe film deposited at higher temperature is improved which may be due to complete thermal decomposition of spared droplets. The morphology of film
i Please cite this article as: A.U. Ubale, Y.S. Sakhare, J. Phys. Chem. Solids, Effect of substrate temperature on optical, structural and electrical properties of FeSe thin films deposited by spray pyrolysis technique (2013), http://dx.doi.org/10.1016/j.jpcs.2013.05.006
A.U. Ubale, Y.S. Sakhare / Journal of Physics and Chemistry of Solids ∎ (∎∎∎∎) ∎∎∎–∎∎∎
3
100 nm
100 nm
100 nm
100 nm
100 nm
Fig. 3. SEM images of FeSe thin films deposited at various substrate temperatures.
deposited at higher temperature is more compact with densely packed spherical grains. 3.4. Optical absorption studies Study of material by means of optical absorption gives information about band gap energy and band structure. In the present investigation, optical absorption in FeSe thin films deposited at different substrate temperatures was studied in the wavelength range 300–900 nm. Fig. 6 shows the optical absorption curves as a function of wavelength for the FeSe films deposited at different
temperatures. The lower absorption observed in the films is attributed to the less scattering effect structural homogeneity and better crystallites. The absorption edge is shifted towards shorter wavelength side as the deposition temperature is increased. The plots of (αhν)2 versus hν for FeSe thin films are shown in Fig. 7. The linear nature of the plots suggests that the mode of optical transition in these films is of direct type. It is observed that the band gap of FeSe is increased from 2.68 to 2.92 eV (Fig.8) as the deposition temperature was decreased from 623 to 473 (Table 1). This may be due to improvement in crystalline nature of film with deposition temperature.
i Please cite this article as: A.U. Ubale, Y.S. Sakhare, J. Phys. Chem. Solids, Effect of substrate temperature on optical, structural and electrical properties of FeSe thin films deposited by spray pyrolysis technique (2013), http://dx.doi.org/10.1016/j.jpcs.2013.05.006
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is increased from 473 to 623 K (Fig. 10). This observation is attributed to the size dependent effects observed in semiconductor thin films [11,12]. The thermal activation energy is calculated using the relation.
3.5. Electrical characterizations The variation of electrical resistivity of the spray deposited FeSe thin films was measured in the temperature range of 303–503 K. Fig. 9 shows the plot of log ρ versus reciprocal of temperature (1/T). The resistivity of FeSe thin film decreases with increase in temperature indicating its semiconducting nature. The high value of resistivity at low deposition temperature is attributed to the fine grains of FeSe. The decrease in resistivity at higher deposition temperature is due to the increased grain size and reduced grain boundary scattering of FeSe. The room temperature electrical resistivity of the FeSe thin films is of the order 104 Ω cm and it decreases from 8 104 to 1.5 104 Ω cm as deposition temperature
Ea ρ ¼ ρo exp KT
ð2Þ
3 473 K 523 K 573 K
2
Fe Se αt
623 K 673 K
1
Fe
Fe
Se
0 340
440
540
640
740
840
Wavelength λ (nm) Fig. 6. Plots of optical absorption (αt) versus wavelength (nm) for FeSe thin films deposited at various substrate temperatures.
Fig. 4. Typical EDAX spectra of FeSe thin film deposited at 573 K temperature.
20 μm
0.447 μm 20 μm
10 μm
20 μm 10 μm -0.324 μm
10 μm
00 μm 00 μm
10 μm
20 μm
10 μm
20 μm
00 μm
20 μm 0.418 μm
20 μm
20 μm
-0.311 μm 10 μm
10 μm
10 μm
00 μm
00 μm 00 μm
Fig. 5. The 2D and 3D-AFM image of FeSe thin films deposited at various substrate temperature. (A) K and (B) K.
i Please cite this article as: A.U. Ubale, Y.S. Sakhare, J. Phys. Chem. Solids, Effect of substrate temperature on optical, structural and electrical properties of FeSe thin films deposited by spray pyrolysis technique (2013), http://dx.doi.org/10.1016/j.jpcs.2013.05.006
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30
Log (ρ)
(αhν)2 × 10-10 (eV-cm)2
5
523 K 573 K
20
673 K 1 0 1.9
10
1.8
2.3
2.8
2.7
2.9
3.1
3.3
-1
9
3.3
Resistivity (ρ)
7.5
3
Energy Band gap (eV)
2.5
Fig. 9. Variation of resistivity (Ω-cm) of FeSe at 373 K temperature with deposition temperature.
Fig. 7. Plots of (αhυ)2 versus hυ for FeSe thin films deposited at various substrate temperatures.
6 4.5 3 1.5
2.9 0 470
520
570
620
670
Substrate temperatures (K)
2.8
Fig. 10. Variation of log (ρ) versus 1/T 103 (K−1) for different thickness of FeSe films deposited at various substrate temperatures.
2.7
2.6 470
520
570
620
670
Substrate temperatures (K) Fig. 8. Variation Optical band gap energy (eV) of FeSe film with deposition temperature.
Film thickness (nm)
Standard
estimated and are presented in Table 2. In the low temperature regime, activation energy changes from 0.17 to 0.10 eV and in the high temperature regime from 1.12 to 0.80 eV as the deposition temperature was increased from 473 to 623 K (Fig. 11). 3.6. Thermo-emf measurement
Table 1 Comparison of standard and observed JCPDS data for FeSe film.
215
2.3
(1/T) × 10 (K )
hν (eV)
230
2.1
3
1.3
185
3 2
15
0
152
4
623 K
5
110
473 K 523 K 573 K 623 K 673 K
6
473 K 25
5
Observed
hkl
2θ (deg)
d (Å)
2θ (deg)
d (Å)
28.681 48.308 28.681 48.308 28.681 48.308 28.681 32.424 48.308 64.817 28.681 32.424 48.308 64.817
3.11 1.882 3.11 1.882 3.11 1.882 3.11 2.759 1.882 1.437 3.11 2.759 1.882 1.437
28.567 48.304 28.66 48.3 28.42 48.226 28.44 32.434 48.304 64.888 28.622 32.435 48.32 64.812
3.113 1.878 3.109 1.887 3.112 1.881 3.114 2.752 1.887 1.436 3.109 2.767 1.882 1.439
101 200 101 200 101 200 101 002 200 212 101 002 200 212
where ρo is the constant, K is Boltzmann constant; T is the absolute temperature and Ea is the activation energy. The activation energies in the corresponding low and high temperature regimes were
The dependence of thermo-emf on temperature gradient applied across the film is depicted in Fig. 12. The thermo-emf was measured as a function of temperature difference in the range 25–225 K. The polarity of the thermo-emf generated was positive at the cold end with respect to the hot end which confirmed that iron selenide thin films are of p-type.
4. Conclusions The structural, optical and electrical properties of the spray deposited FeSe thin films have been found to be influenced by the deposition temperature. The deposited films are nanocrystalline with tetragonal structure and highly textured. The preferred orientation is along (101) direction and becomes prominent with substrate temperature. The SEM and AFM study shows that the film morphology is influenced by the substrate temperature. The optical band gap of FeSe was decreased from 2.92 to 2.68 eV as the deposition temperature was increased from 473 to 623 K. The variation of electrical properties with temperature confirms semiconducting nature of FeSe. The thermo emf developed between
i Please cite this article as: A.U. Ubale, Y.S. Sakhare, J. Phys. Chem. Solids, Effect of substrate temperature on optical, structural and electrical properties of FeSe thin films deposited by spray pyrolysis technique (2013), http://dx.doi.org/10.1016/j.jpcs.2013.05.006
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Table 2 Grain size, electrical resistivity at 373 K, band gap energy and activation energy values of FeSe thin film of different thickness. Film
Substrate temperature (K)
F G H I J
473 523 573 623 673
Thickness (nm)
110 152 185 230 215
Band gap energy (eV)
Activation energy (eV)
2.92 2.8 2.75 2.68 2.7
Low temperature region
High temperature region
0.17 0.15 0.14 0.1 0.1
1.12 1 0.83 0.8 0.82
1.2
Activation Energy (eV)
Low temperature
ρ 104 (Ω-cm)
21 29 40 53 48
8 5.8 4 1.5 2.2
Acknowledgment
High temperature
1
The authors are thankful to UGC, WRO, Pune (India), for financial support.
0.8 0.6 0.4
References
0.2 0 470
520
570
620
670
Substrate temperatures (K) Fig. 11. Variation of activation energy (eV) of FeSe with substrate temperature.
6 473 K 5
Thermo-emf (mV)
Grain size (nm)
4 3
523 K 573 K 623 K 673 K
2 1 0 25
75
125
175
225
Temperature difference (K) Fig. 12. Variation of thermoemf (mV) versus temperature differences across the FeSe thin films deposited at various substrate temperatures.
the two ends of the film demonstrates that the FeSe thin films deposited at various substrate temperatures are of p-type.
[1] A.U. Ubale, Y.S. Sakhare, M.R. Belkhedkar, Synthesis and characterization of spray deposited nanostructured FeSe thin films, Mater. Lett. 92 (2013) 111. [2] P.Y. Chen, S.F. Hu, R.S. Liu, C.Y Huang, Electrodeposition of nano-dimensioned FeSe, Thin solid Films 519 (2011) 8397. [3] Li Chen, F.T. Chen, Y. Zhu, Z. Bi, H. Wang, Enhanced superconducting properties in epitaxial FeSe thin films with self-assembled Fe3O4 nanoparticles, Physica C 471 (2011) 515. [4] C.Q. Lin, T.H. Ling, H.Z. Qiang, L. Wang, Q. Fei, A novel selenization technique for fabrication of superconducting FeSex thin film, Physica C 471 (2011) 428. [5] S. Thanikaikarasan, T. Mahalingam, K. Sundaram, A. Kathalingam, Yong Deak Kim, Taekyu Kim, Growth and characterization of electrosynthesized iron selenide thin films, Vacuum 83 (2009) 1066. [6] X.J Wu, Z.Z. Zhang, J.Y. Zhang, Z.G. Jua, D.Z. Shen, B.H. Lia, C.X. Shan, Y.M. Lu, Structural and electrical characterizations of single tetragonal FeSe on Si substrate, J. Cryst. Growth 300 (2007) 483. [7] N. Hamdadou, J.C. Bernede, A. Khelil, Preparation of iron selenide films by selenization technique, J. Cryst. Growth 241 (2002) 313. [8] K.W. Liu, J.Y. Zhang, D.Z. Shen, C.X. Shan, B.H. Li, Y.M. Lu, X.W. Fan, Electronic and magnetic properties of FeSe thin film prepared on GaAs (001) substrate by metal-organic chemical vapor deposition, Appl. Phys. Lett. 90 (2007) 262503. [9] Q.J. Feng, D.Z. Shen, J.Y. Zhang, C.X. Shan, Y.M. Lu, Y.C. Lin, X.W. Fan, H.W. Liang, Ferromagnetic FeSe: Structural, electrical, and magnetic properties, Appl. Phys. Lett. 88 (2006) 012505. [10] A.V Mohdkar, S.M. Pawar, K.V. Rajpure, P.S. Patil, C.H. Bhosale, J.H. Kim, Temperature dependent properties of spray deposited ITO thin films, J. Therm. Spray Technol. 19 (3) (2010) 531. [11] A.U. Ubale, V.P. Deshpande, Thickness dependent structural, electrical and optical properties of nanostructred zinc oxide thin films deposited by chemical spray technique, Optoelect. Adv. Mater. Rap. Commun. 5 (2) (2011) 119. [12] A.U. Ubale, Effect of complexing agent on growth process and properties of nanostructure Bi2S3 thin films deposited by chemical bath deposition method, Mater. Chem. Phys. 121 (2010) 555.
i Please cite this article as: A.U. Ubale, Y.S. Sakhare, J. Phys. Chem. Solids, Effect of substrate temperature on optical, structural and electrical properties of FeSe thin films deposited by spray pyrolysis technique (2013), http://dx.doi.org/10.1016/j.jpcs.2013.05.006
Contents lists available at SciVerse ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Effect of substrate temperature on optical, structural and electrical properties of FeSe thin films deposited by spray pyrolysis technique A.U. Ubale n, Y.S. Sakhare Nanostructured Thin Film Materials Laboratory, Department of Physics, Government Vidarbha Institute of Science and Humanities, VMV Road, Amravati 444604, Maharashtra, India
art ic l e i nf o
a b s t r a c t
Article history: Received 15 January 2013 Accepted 5 May 2013
The versatile spray pyrolysis technique was employed to prepare thin films of iron selenide on glass substrates at different substrate temperatures. The deposition temperature was varied between 473 and 673 K. The asdeposited films were characterized by X-ray diffraction (XRD), SEM, optical and electrical characterization techniques. The X-ray studies reveal that the films are nanocrystalline with tetragonal structure and exhibit (101) preferred orientation. The SEM and AFM studies indicate that the film surface is homogenous with no cracks or pinholes and well covers the glass substrate. The film thickness was found to vary from 110 to 230 nm with substrate temperature. The optical band gap was found to decrease from 2.92 to 2.68 eV depending on deposition temperature. The resistivity of p-type FeSe film is of the order of 8 104 Ω cm and it decreases to 1.5 104 Ω cm as substrate temperature is increased from 473 K to 673 K. & 2013 Elsevier Ltd. All rights reserved.
Keywords: A. Thin Films D. Electrical properties
1. Introduction The global warming, rising oil prices and a steadily rising demand for electricity in developing countries clearly shows prime a need of development and utilization of clean and renewable energy sources. The development of economic solar photovoltaic devices is the main motivation for the development of nanostructured thin film. During the last few years, the synthesis of nano-structured iron selenide materials has attracted considerable attention due to their novel physical and chemical properties and potential applications in constructing nanoscale electronic and optoelectronic devices. The nanocrystalline thin films attracted significant attention of the scientists not only due to the growing technological applications, but also because of the interest in quantum confinement effects that arises in it. They exhibit physical and chemical properties that are neither characteristics of the atoms nor bulk counterparts. Various sophisticated chemical and physical methods are employed to grow nanocrystalline thin films but out of them, spray pyrolysis is a simple, convenient and economic to grow nanocrystalline FeSe thin films [1]. While development of such chemical method requires optimization of various preparative parameters like, substrate temperature, spray rate, concentration of the solution etc as they affect various physical and chemical properties of the deposited material. Chen et al. [2] have deposited nanostructured FeSe thin films by electrochemical deposition method on indium doped tin oxide coated conducting glass substrates. The growth of epitaxial tetragonal iron selenide thin films n
Corresponding author. Tel.: +91 721 2531706; fax: +91 721 2531705. E-mail address: [email protected] (A.U. Ubale).
on SrTiO3 and MgO substrates was reported by Chan et al. [3] by pulsed laser deposition technique. The deposition temperature and annealing process are found to be critical for achieving tetragonal phase of FeSe. Lin et al. [4] have reported superconducting FeSex thin film by selenization technique. The influence of selenization temperature on film components was studied. Thanikaikarasan et al. [5] have electrodeposited FeSe thin films on ITO substrates at various bath temperatures from 30 to 90 1C in an aqueous electrolytic bath containing FeSO4 and SeO2. Xu et al. [6] have reported (001) orientated FeSe thin film by metal organic vapour deposition method on Si substrate. The structural analysis confirm single tetragonal phase of FeSe. Hamdadou et al. [7] have grown FeSe thin films by selenization of iron thin films. The films grown by this method shows tetragonal FeSe with some FeSe2 crystallites. Liu et al. [8] have reported FeSe thin film on GaAs (001) substrate by low pressure metal organic chemical vapor deposition. The X-ray diffraction measurement indicated that the sample was preferentially oriented with tetragonal structure. In the present work nanocrystallines FeSe thin films were prepared by spray pyrolysis method at different substrate temperatures to investigate its effect on physical properties.
2. Experimental details The aqueous solutions of Fe (NO3)3 9H2O, SeO2 (Prepared by dissolving Se metal powder in HNO3) and acetic acid were prepared by dissolving appropriate amount of these salts (AR grade) in double distilled water. The spray solution was prepared by mixing 20 ml 0.12 M Fe(NO3)3 9H2O, 10 ml of 0.12 M CH3-COOH and 20 ml of
0022-3697/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jpcs.2013.05.006
i Please cite this article as: A.U. Ubale, Y.S. Sakhare, J. Phys. Chem. Solids, Effect of substrate temperature on optical, structural and electrical properties of FeSe thin films deposited by spray pyrolysis technique (2013), http://dx.doi.org/10.1016/j.jpcs.2013.05.006
3. Result and discussion 3.1. Film formation mechanism The spray pyrolysis technique is basically a chemical deposition technique in which fine droplets of sprayed solution undergo pyrolytic decomposition to form film at hot substrate. Each droplet containing solvent and solute passes through different temperature zones formed between hot substrate and nozzle. The process of thermal decomposition depends upon the substrate temperature. It was observed that the films deposited at lower substrate temperature (o473 K) are non uniform and powdery. As a result at lower temperature the sprayed droplets forms less adhesive powdery films giving lower thickness. However, at higher temperature (4573 K), the solvent vaporizes well before reaching the hot substrate causing lower thickness (Fig. 1). Also, at higher temperature there is possibility that only bigger droplets reaches to the hot substrate giving porous morphology. In the present case it was found that the films deposited at 573 K temperature are uniform and well adherent to the glass substrate. 3.2. Structural studies The structural analysis of FeSe thin films was carried out using CuKα radiation of wavelength 0.154 nm. The XRD patterns were 240
Thickness (nm)
220 200 180 160 140 120 100 190
240
290
340
390
Substrates temperature (K) Fig. 1. Variation of FeSe films thickness (nm) with substrate temperatures.
473 K FeSe: JCPDS card 85-0735 002
523 K
Intensity (Arbitrary unit)
573 K 623 K 200
673 K 212
0.13 M freshly prepared SeO2 solution in a 100 ml beaker. In the present work, the substrate temperature was varied between 473 and 673 K to investigate its effect on structural, optical and electrical properties of FeSe. The 30 ml of solution was sprayed at the rate 5 ml/ min onto hot substrates by using compressed air as carrier gas. The distance between nozzle and substrate was kept 25 cm. The thickness of the films prepared at various substrate temperatures was measured by the gravimetric method, by considering the bulk density of iron selenide. The structural characterization of FeSe was carried out by analyzing XRD patterns obtained by using an X-ray diffractometer PANalytical X'Pert PRO MRD with CuKα radiation of wavelength 0.154 nm. The surface morphology and energy dispersive X-ray analysis of the spray deposited FeSe thin films was carried out by using a JOEL'S JSM-7600F scanning electron microscope attached with EDX setup. The AFM measurements were performed with diInnova from Veeco Instruments. The electrical resistivity of the film was measured as a function of temperature in the range 303–573 K using dc two point probe method. The optical absorption spectra of the films were measured by JASCO V-530 UV/visible spectrophotometer, in the 300–900 nm wavelength range, with glass substrate as a reference. The thermoelectric power (TEP) of FeSe thin films prepared by varying substrate temperature between 303 and 573 K was measured in dark. Silver contacts were made for the resistivity and TEP measurements.
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80
2 (degree) Fig. 2. X-ray diffraction pattern of FeSe thin films deposited at various substrate temperatures.
recorded by varying diffraction angle between 201 and 801. Fig. 2 shows the typical X-ray diffraction patterns of FeSe thin films deposited at various substrate temperatures. The films are nanocrystalline in nature with tetragonal structure [2,9]. The observed and standard 2θ and d value are in good agreement which confirms that the deposited films are tetragonal FeSe. The (101) and (200) peaks are observed for all the films. The (002) and (212) peaks are observed at higher deposition temperature. The peak intensity and number of peaks increases with substrate temperature and is attributed to the improved crystallinity of the film. The average grain size of FeSe was determined by using Scherer's formula d¼
0:9λ β cos θ
ð1Þ
where d is the grain size, λ is the wavelength of X-ray used, β the angular line width at half-maximum intensity and θ is Bragg's angle. The crystallite size of FeSe change from 21 to 53 nm as substrate temperature is increased from 473 to 573 K. For further rize in temperature from 523 to 573 K the crystallite size decreases from 53 to 48 nm which may be due to re-evaporation of film [10]. 3.3. Surface morphology Fig. 3 shows the SEM micrographs of FeSe thin films on glass substrates deposited at various substrate temperatures. The deposited films has homogenous surface without creaks or holes. At lower substrate temperature film surface is more homogenous and the circular grain growth with agglomeration is improved with rise in substrate temperature. Above 573 K substrate temperature the film surface shows porous nature. The compositional analysis of the FeSe thin film was carried out by EDAX technique. Fig. 4 shows a typical EDAX pattern of FeSe thin film deposited at 573 K substrate temperature. The elemental analysis was carried out only for Fe and Se element. The average atomic percentage of Fe:Se was 52:48 showing enrichment of Fe in FeSe film. The threedimensional surface morphology of the spray deposited FeSe thin films was investigated by taking atomic force micrographs. Fig. 5 (F) and (H) shows the 3D and 2D AFM images of FeSe thin films deposited at 473 and 573 K substrate temperature. The AFM micrographs show total coverage of the substrate with uniform spherical grains of FeSe. The spherical grains with relatively smooth surface are observed for films deposited at lower temperature. However, the surface morphology of FeSe film deposited at higher temperature is improved which may be due to complete thermal decomposition of spared droplets. The morphology of film
i Please cite this article as: A.U. Ubale, Y.S. Sakhare, J. Phys. Chem. Solids, Effect of substrate temperature on optical, structural and electrical properties of FeSe thin films deposited by spray pyrolysis technique (2013), http://dx.doi.org/10.1016/j.jpcs.2013.05.006
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100 nm
100 nm
100 nm
100 nm
100 nm
Fig. 3. SEM images of FeSe thin films deposited at various substrate temperatures.
deposited at higher temperature is more compact with densely packed spherical grains. 3.4. Optical absorption studies Study of material by means of optical absorption gives information about band gap energy and band structure. In the present investigation, optical absorption in FeSe thin films deposited at different substrate temperatures was studied in the wavelength range 300–900 nm. Fig. 6 shows the optical absorption curves as a function of wavelength for the FeSe films deposited at different
temperatures. The lower absorption observed in the films is attributed to the less scattering effect structural homogeneity and better crystallites. The absorption edge is shifted towards shorter wavelength side as the deposition temperature is increased. The plots of (αhν)2 versus hν for FeSe thin films are shown in Fig. 7. The linear nature of the plots suggests that the mode of optical transition in these films is of direct type. It is observed that the band gap of FeSe is increased from 2.68 to 2.92 eV (Fig.8) as the deposition temperature was decreased from 623 to 473 (Table 1). This may be due to improvement in crystalline nature of film with deposition temperature.
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is increased from 473 to 623 K (Fig. 10). This observation is attributed to the size dependent effects observed in semiconductor thin films [11,12]. The thermal activation energy is calculated using the relation.
3.5. Electrical characterizations The variation of electrical resistivity of the spray deposited FeSe thin films was measured in the temperature range of 303–503 K. Fig. 9 shows the plot of log ρ versus reciprocal of temperature (1/T). The resistivity of FeSe thin film decreases with increase in temperature indicating its semiconducting nature. The high value of resistivity at low deposition temperature is attributed to the fine grains of FeSe. The decrease in resistivity at higher deposition temperature is due to the increased grain size and reduced grain boundary scattering of FeSe. The room temperature electrical resistivity of the FeSe thin films is of the order 104 Ω cm and it decreases from 8 104 to 1.5 104 Ω cm as deposition temperature
Ea ρ ¼ ρo exp KT
ð2Þ
3 473 K 523 K 573 K
2
Fe Se αt
623 K 673 K
1
Fe
Fe
Se
0 340
440
540
640
740
840
Wavelength λ (nm) Fig. 6. Plots of optical absorption (αt) versus wavelength (nm) for FeSe thin films deposited at various substrate temperatures.
Fig. 4. Typical EDAX spectra of FeSe thin film deposited at 573 K temperature.
20 μm
0.447 μm 20 μm
10 μm
20 μm 10 μm -0.324 μm
10 μm
00 μm 00 μm
10 μm
20 μm
10 μm
20 μm
00 μm
20 μm 0.418 μm
20 μm
20 μm
-0.311 μm 10 μm
10 μm
10 μm
00 μm
00 μm 00 μm
Fig. 5. The 2D and 3D-AFM image of FeSe thin films deposited at various substrate temperature. (A) K and (B) K.
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30
Log (ρ)
(αhν)2 × 10-10 (eV-cm)2
5
523 K 573 K
20
673 K 1 0 1.9
10
1.8
2.3
2.8
2.7
2.9
3.1
3.3
-1
9
3.3
Resistivity (ρ)
7.5
3
Energy Band gap (eV)
2.5
Fig. 9. Variation of resistivity (Ω-cm) of FeSe at 373 K temperature with deposition temperature.
Fig. 7. Plots of (αhυ)2 versus hυ for FeSe thin films deposited at various substrate temperatures.
6 4.5 3 1.5
2.9 0 470
520
570
620
670
Substrate temperatures (K)
2.8
Fig. 10. Variation of log (ρ) versus 1/T 103 (K−1) for different thickness of FeSe films deposited at various substrate temperatures.
2.7
2.6 470
520
570
620
670
Substrate temperatures (K) Fig. 8. Variation Optical band gap energy (eV) of FeSe film with deposition temperature.
Film thickness (nm)
Standard
estimated and are presented in Table 2. In the low temperature regime, activation energy changes from 0.17 to 0.10 eV and in the high temperature regime from 1.12 to 0.80 eV as the deposition temperature was increased from 473 to 623 K (Fig. 11). 3.6. Thermo-emf measurement
Table 1 Comparison of standard and observed JCPDS data for FeSe film.
215
2.3
(1/T) × 10 (K )
hν (eV)
230
2.1
3
1.3
185
3 2
15
0
152
4
623 K
5
110
473 K 523 K 573 K 623 K 673 K
6
473 K 25
5
Observed
hkl
2θ (deg)
d (Å)
2θ (deg)
d (Å)
28.681 48.308 28.681 48.308 28.681 48.308 28.681 32.424 48.308 64.817 28.681 32.424 48.308 64.817
3.11 1.882 3.11 1.882 3.11 1.882 3.11 2.759 1.882 1.437 3.11 2.759 1.882 1.437
28.567 48.304 28.66 48.3 28.42 48.226 28.44 32.434 48.304 64.888 28.622 32.435 48.32 64.812
3.113 1.878 3.109 1.887 3.112 1.881 3.114 2.752 1.887 1.436 3.109 2.767 1.882 1.439
101 200 101 200 101 200 101 002 200 212 101 002 200 212
where ρo is the constant, K is Boltzmann constant; T is the absolute temperature and Ea is the activation energy. The activation energies in the corresponding low and high temperature regimes were
The dependence of thermo-emf on temperature gradient applied across the film is depicted in Fig. 12. The thermo-emf was measured as a function of temperature difference in the range 25–225 K. The polarity of the thermo-emf generated was positive at the cold end with respect to the hot end which confirmed that iron selenide thin films are of p-type.
4. Conclusions The structural, optical and electrical properties of the spray deposited FeSe thin films have been found to be influenced by the deposition temperature. The deposited films are nanocrystalline with tetragonal structure and highly textured. The preferred orientation is along (101) direction and becomes prominent with substrate temperature. The SEM and AFM study shows that the film morphology is influenced by the substrate temperature. The optical band gap of FeSe was decreased from 2.92 to 2.68 eV as the deposition temperature was increased from 473 to 623 K. The variation of electrical properties with temperature confirms semiconducting nature of FeSe. The thermo emf developed between
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Table 2 Grain size, electrical resistivity at 373 K, band gap energy and activation energy values of FeSe thin film of different thickness. Film
Substrate temperature (K)
F G H I J
473 523 573 623 673
Thickness (nm)
110 152 185 230 215
Band gap energy (eV)
Activation energy (eV)
2.92 2.8 2.75 2.68 2.7
Low temperature region
High temperature region
0.17 0.15 0.14 0.1 0.1
1.12 1 0.83 0.8 0.82
1.2
Activation Energy (eV)
Low temperature
ρ 104 (Ω-cm)
21 29 40 53 48
8 5.8 4 1.5 2.2
Acknowledgment
High temperature
1
The authors are thankful to UGC, WRO, Pune (India), for financial support.
0.8 0.6 0.4
References
0.2 0 470
520
570
620
670
Substrate temperatures (K) Fig. 11. Variation of activation energy (eV) of FeSe with substrate temperature.
6 473 K 5
Thermo-emf (mV)
Grain size (nm)
4 3
523 K 573 K 623 K 673 K
2 1 0 25
75
125
175
225
Temperature difference (K) Fig. 12. Variation of thermoemf (mV) versus temperature differences across the FeSe thin films deposited at various substrate temperatures.
the two ends of the film demonstrates that the FeSe thin films deposited at various substrate temperatures are of p-type.
[1] A.U. Ubale, Y.S. Sakhare, M.R. Belkhedkar, Synthesis and characterization of spray deposited nanostructured FeSe thin films, Mater. Lett. 92 (2013) 111. [2] P.Y. Chen, S.F. Hu, R.S. Liu, C.Y Huang, Electrodeposition of nano-dimensioned FeSe, Thin solid Films 519 (2011) 8397. [3] Li Chen, F.T. Chen, Y. Zhu, Z. Bi, H. Wang, Enhanced superconducting properties in epitaxial FeSe thin films with self-assembled Fe3O4 nanoparticles, Physica C 471 (2011) 515. [4] C.Q. Lin, T.H. Ling, H.Z. Qiang, L. Wang, Q. Fei, A novel selenization technique for fabrication of superconducting FeSex thin film, Physica C 471 (2011) 428. [5] S. Thanikaikarasan, T. Mahalingam, K. Sundaram, A. Kathalingam, Yong Deak Kim, Taekyu Kim, Growth and characterization of electrosynthesized iron selenide thin films, Vacuum 83 (2009) 1066. [6] X.J Wu, Z.Z. Zhang, J.Y. Zhang, Z.G. Jua, D.Z. Shen, B.H. Lia, C.X. Shan, Y.M. Lu, Structural and electrical characterizations of single tetragonal FeSe on Si substrate, J. Cryst. Growth 300 (2007) 483. [7] N. Hamdadou, J.C. Bernede, A. Khelil, Preparation of iron selenide films by selenization technique, J. Cryst. Growth 241 (2002) 313. [8] K.W. Liu, J.Y. Zhang, D.Z. Shen, C.X. Shan, B.H. Li, Y.M. Lu, X.W. Fan, Electronic and magnetic properties of FeSe thin film prepared on GaAs (001) substrate by metal-organic chemical vapor deposition, Appl. Phys. Lett. 90 (2007) 262503. [9] Q.J. Feng, D.Z. Shen, J.Y. Zhang, C.X. Shan, Y.M. Lu, Y.C. Lin, X.W. Fan, H.W. Liang, Ferromagnetic FeSe: Structural, electrical, and magnetic properties, Appl. Phys. Lett. 88 (2006) 012505. [10] A.V Mohdkar, S.M. Pawar, K.V. Rajpure, P.S. Patil, C.H. Bhosale, J.H. Kim, Temperature dependent properties of spray deposited ITO thin films, J. Therm. Spray Technol. 19 (3) (2010) 531. [11] A.U. Ubale, V.P. Deshpande, Thickness dependent structural, electrical and optical properties of nanostructred zinc oxide thin films deposited by chemical spray technique, Optoelect. Adv. Mater. Rap. Commun. 5 (2) (2011) 119. [12] A.U. Ubale, Effect of complexing agent on growth process and properties of nanostructure Bi2S3 thin films deposited by chemical bath deposition method, Mater. Chem. Phys. 121 (2010) 555.
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