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Effect of solution molarity on the characteristics of vanadium pentoxide thin film
Applied Surface Science 252 (2006) 8745–8750 www.elsevier.com/locate/apsusc
Effect of solution molarity on the characteristics of vanadium pentoxide thin film Alaa A. Akl * Faculty of Science, Physics Department, El-Minia University, Egypt Received 23 October 2005; received in revised form 7 December 2005; accepted 8 December 2005 Available online 3 February 2006
Abstract Vanadium pentoxide (V2O5) thin films have been prepared by spray pyrolysis technique. The influence of solution molarity on the characteristics of the V2O5 has been investigated. X-ray diffraction analysis (XRD) showed that, the films deposited at 0.1 M were orthorhombic structure with a preferential orientation along h0 0 1i direction. Moreover, the crystallinity was improved by increasing solution molarity. The microstructure parameters have been evaluated by using a single order Voigt profile method. The optical band gaps, determined by using Tauc plot, have been found to be 2.50 0.02 and 2.33 0.02 eV for the direct and indirect allowed transition, respectively. Also the complex optical constants for the wavelength range 300–2500 nm are reported. At room temperature, the dark conductivity as a function of solution molarity showed the range of 5.74 102 0.03 to 3.36 101 0.02 V1 cm1. While at high temperature, the behaviour of electrical conductivity dominated by grain boundaries. The values of activation energy and potential barrier height were 0.156 0.011 and 0.263 0.012 eV, respectively. # 2005 Elsevier B.V. All rights reserved. Keywords: Vanadium pentoxide; Thin films; Spray pyrolysis
1. Introduction Nowadays vanadium pentoxide films are of increasing interest due to their unique features, such as electro-chemical activity, high stability, good specific energy, special layer structure and high capacity as well as excellent thermoelectric property. It can be used as a gas sensor, cathode for solid-state batteries, window for solar cells and as an active material in electrochromic devices [1,2]. There are many numbers of vanadium oxides and suboxides such as VO, V2O3, VO2, V4O9 and V2O5. Most of these oxides are metal–insulator transition, which occurs over a wide range of transition temperature depending on the O/V ratio. Vanadium pentoxide is the most stable compound of the V–O system [3]. Various methods are reported for the preparation of V2O5 thin films such as vacuum evaporation [4,5], RF sputtering [2], chemical vapour deposition [6], electron beam evaporation [7] and pulsed laser deposition (PLD) [8]. However, the deposition of thin film from
* Tel.: +2 86 2348681; fax: +2 86 2363011. E-mail address: [email protected]. 0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.12.076
sprayed solution catch the attention of researchers because of such method has low-cost set-up and has the ability to deposit large area [9]. In this work, the synthesis of V2O5 films by spray pyrolysis technique was investigated at different concentrations of spray solution at constant substrate temperature. The influence of solution molarity on the characteristics (microstructures, optical and electrical properties) of V2O5 thin film is reported. 2. Experimental details At constant temperature, different concentrations of vanadium nitrate [V(NO3)55H2O] namely 0.1, 0.2, 0.3, 0.4 and 0.5 M were sprayed on preheated amorphous glass substrates. The substrates have been chemically and ultrasonically pre-cleaned. In order to get uniform thin films, a suitable condition for the height of the spraying nozzle, the flow rate and the deposition time are chosen and kept unchanged during the deposition process at 27 cm, 5 cm3 min1 and 45 s, respectively. Also, a compressed air of pressure 6 N cm2 has been used as a carrier gas. The substrate temperature was fixed at 350 8C and controlled through a thermocouple.
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Structural characterization of the produced films has been carried out using a JEOL JSDX-60PA diffractometer (Cu Ka ˚ – scanning speed is radiation – Ni-filtered l = 1.54184 A 1 18 min – time constant is 1 s). A range of 2u (from 68 to 728) was scanned, to cover all the possible diffraction patterns of the V2O5 phase. Using the data collected from step-scan mode, the determination of the crystallite/domain size and macrostrain were recorded at step size of 0.028 (2u) and a dwell time 4 s/step. To obtain a profile fitting with good signal, a polycrystalline Si powdered was used for instrumental correction. The optical measurements were studied in the range from 300 to 2500 nm using a UV-310 Pc:UV–VIS-NIR; Shimadzu, double beam spectrophotometer with V–N specular reflection attachment. A laboratory developed computer programs [10] based on solving the exact equations were used to calculate the complex optical constants, absorption coefficient and energy band gap. The electrical conductivity of the films at room temperature was studied using standard two-probe technique. The current–voltage characteristic was measured using Keithly 616 digital electrometer. The film thickness was measured using multiple-beam Fizeau fringes at reflection either with mono and polychromatic light. 3. Results and discussion 3.1. Structural characterization Fig. 1 shows the X-ray diffraction (XRD) patterns for samples prepared at Tsub = 350 8C with different solution molarities, namely 0.1, 0.2, 0.3, 0.4 and 0.5 M. The peaks are indexed in comparing our experimental data with the ASTM cards of X-ray powder. The produced data for all films prepared at different molarities of spray solution showed a good agreement with the data of V2O5 powder file (JCPDS data number 09-0387) which corresponding to the orthorhombic crystalline structure. Indeed, the typical peaks of the polycrystalline phase in V2O5 films appear in XRD spectra. It is observed that the crystallinity of the film increases with increasing the solution molarity. The increase in the intensity of the peaks may be attributed to either grain growth associated with larger thickness or increase in the degree of crystallinity by increasing the solution molarity or both. Moreover, the multi-reflection peaks appears in the 2u range from 12.08 to 34.08 characterized the single phase of V2O5 having the lattice parameters a = 11.470 0.012, ˚ which are very b = 3.568 0.011 and c = 4.373 0.013 A near to the value given in this card (a = 11.510, b = 3.559 and ˚ ). It is also clear that, the films show preferred c = 4.371 A growth along the h0 0 1i direction as the solution molarity increases. The variation of the intensity ratio of the (0 0 2) and (0 0 1) planes as a function of solution molarity are shown in Fig. 2. It is clear that the change in intensity is very pronounced in the molarity range of 0.1–0.5 M. One may conclude that, for any solution molarity, the preferred orientation along both h0 0 1i and h0 0 2i plane is observed.
Fig. 1. XRD patterns of V2O5 thin films sprayed at Tsub = 350 8C with different solution molarities.
3.2. Line broadening analysis Viogt profile analysis was used to calculate the crystallite/ domain size and macrostrain of the prepared V2O5 films [11]. The parameter of interest with the Voigt function is ‘‘the shape parameter’’ which defined as [w = FWHM/integral breadth]. The value of this parameter is used to determine the fractional Lorentzian (Chuchy), bfL and Gaussian, bfG components in the convolute. The Lorentzian and Gaussian components of the integral breadth of pure specimen profile are given by: bfL ¼ bhL bgL and ðbfG Þ2 ¼ ðbhG Þ2 ðbgG Þ2
(1)
where h, f and g are the observed, specimen and instrumental profile function, respectively. The apparent crystallite/domain size e is e ¼ ðbfL Þ1
and
e ¼ ðbfG Þ1=2
(2)
The crystallite size was calculated as an average of the apparent size, which is equivalent to an average volume of the
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Fig. 2. The variation of intensity ratio of the (0 0 2) and (0 0 1) of V2O5 thin films as a function of solution molarities.
Fig. 3. (a and b) The crystallite/domain size and macrostrain of V2O5 thin films as a function of solution molarities.
thickness, measured perpendicular to the reflecting planes. The macrostrain jej is given by,
molecules are ejected with higher kinetic energy and higher momentum leading to make collision with surface, then the molecules recombine with higher mobility energy causing an internal stress, which raises the macrostrain.
jej ¼
Dd 1 ¼ cot u Dð2uÞ do 2
(3)
The macrostrain was calculated as an average of fractional change, Dd/do, in the interplaner spacing, d of the threediffraction planes (2 0 0), (0 0 1) and (0 0 2), where do is that of the standard JCPDS number 09-0387. The calculated crystallite size and macrostrain of the investigated samples are given in Fig. 3(a and b). It is clear that, the crystallite sizes are varied for all samples. The values of crystallite size are near saturated at solution molarity namely 0.2, 0.3 and 0.4 M. But at higher concentration (0.5 M) the crystallite size is sharply decreasing as shown in Fig. 3(a). This means that no variation in crystallite size was observed for films deposited at different solution molarities (0.2, 0.3 and 0.4 M) since this concentration is appropriate for crystal growing on the substrate due to the suitable mobility of ions. But at low concentration (0.1 M) the mobility of ions is insufficient for growing, while at higher concentration (0.5 M) the mobility of ions is higher and lead to crystal agglomerate destruction. Thus, complete grain growth does not occurs, which reduces the formation of crystallite size. The calculated macrostrain for all investigated samples are shown in Fig. 3(b). It is clear that, the macrostrain was the same at solution molarity namely 0.2, 0.3 and 0.4 M. But at concentration of 0.5 M, the macrostrain was increased, since it depends on the elastic constants of the material and nature of internal stress. Moreover, at higher molarity the sprayed
3.3. Optical properties The optical transmission and reflection of the V2O5 prepared films were measured over the wavelength range 300–2500 nm. The spectral variation of transmission and reflection for the V2O5 film deposited onto glass substrate, at Tsub = 350 8C and different solution molarities are shown in Figs. 4 and 5. It was observed that, the surface of the films prepared at solution molarity from 0.2 to 0.5 M appears to be rougher due to the increase of scattered light and decrease of the film transmission and specular reflection. Also, the absorption edge is shifted toward higher energy by decreasing solution molarity from 0.5 to 0.2 M, while at 0.1 M the absorption edge lies in the UV region. From Fig. 4; it is clear that as the solution molarity decreases, the film transmission increases all over the whole spectral region. When concerning the film prepared at solution molarity of 0.1 M higher values of the optical transmission and reflection were observed. From the experimental data, the thickness of films prepared by solution molarity of 0.5 and 0.1 M were 372 2 and 118 2 nm, respectively. This drops of thickness provide the drastic increase of film transmission between them and posses higher absorption coefficient. From the results obtained the film that was prepared at 0.1 M gives the best transmission and
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Fig. 4. Optical transmittance spectra of V2O5 thin films formed at different solution molarities.
reflection spectra, thus this film was chosen to determine the absorption coefficient, a, the optical band gap, Eg and optical constants (n and k). The optical absorption coefficient was evaluated form the relation [12]; aðlÞ ¼
1 f1 RðlÞg2 ln d TðlÞ
(4)
where T and R are the spectral transmittance and reflectance and d is the film thickness. The variation of the absorption coefficient with the incident wavelength at and near absorption edge for V2O5 films is shown in Fig. 6. It is clear that, the absorption coefficient rises abruptly at the edge reaching to the values of approximately 3.5 104 cm1 at l = 650 nm, these values agree with others [4,9]. The extraordinarily high a value (exceeding 104 cm1) reduces material demand and the gas sensing material [13,14].
Fig. 5. Optical reflectance spectra of V2O5 thin films formed at different solution molarities.
Fig. 6. The optical absorption coefficient, a, vs. the wavelength of V2O5 thin film.
The optical band gap was evaluated by using the relation [15,16]: ahn ¼ Aðhn Eg Þm
(5)
where A is a constant, Eg the band gap of the material, hn the incident photon energy and the exponent m determines the type of electronic transitions causing the absorption and take the values (1/2 and 2). The optical band gap calculated by extrapolating the straight line parts of the curves (ahn)1/m = 0, are shown in Fig. 7(a and b). It is found that, the values of optical
Fig. 7. (a and b) Plots of (ahn)2 and (ahn)1/2 vs. photon energy for V2O5 thin film.
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band gap for direct and indirect allowed transitions are 2.50 0.02 and 2.33 0.02 eV, respectively. The obtained values of direct and indirect transition agree with the values reported by several authors [8,9,17,18] for crystalline V2O5 thin film. The complex optical constants (refractive index, n, and extinction coefficient, k) of the prepared film have been calculated from the corrected T and R using a developed computation programs [10]. The programs involve bivariant search technique based on minimizing jDRj2 and jDTj2, where: jDRj2 ¼ jRexp Rcalc j2 jDTj2 ¼ jTexp Tcalc j2
(6)
The subscripts exp and calc are the experimental and calculated results, respectively. Both the transmittance Tcalc and reflectance Rcalc are given by Murmann’s exact equations [19]. Fig. 8(a) shows the spectral variation of the refractive index, n against wavelength, l. The n values drops from 2.64 0.03 to 1.82 0.02 when the wavelength changed from 400 to 2500 nm, respectively. Thus the maximum value of refractive index is observed at short wavelengths (l = 400 nm), which is good agreement with the data obtained by others [20,21] for like phase. Fig. 8(b) illustrates the corresponding data for the extinction coefficient, k versus wavelength, l, according to Fresnel equation [22]: T l k ¼ ln (7) 1 R 4pd The k value markedly decreases as wavelength increases in the range from 300 to 600 nm and has a very large magnitude about 0.87 0.01 at 300 nm. This value of k is attributed to the fundamental band gap and the very low transmittance at short wavelength. The information, which used in the calculations is mainly obtained from reflectance so that surface effects tend to dominate [21].
Fig. 8. (a and b) Variation of refractive index, n, and extinction coefficient, k, vs. wavelength, l, for V2O5 thin film.
Fig. 9. Variation of dc conductivity with different solution molarity for V2O5 thin films.
3.4. Electrical properties For the electrical conductivity, two ways of investigation were done, one of them at room temperature and the other at different temperatures. The room temperature electrical conductivity of V2O5 films was studied by two-probe technique. Fig. 9 represents the variation of electrical conductivity against film molarity. It is observed that, all asdeposited films have low conductivity. The hot-point probe method showed that all the investigated V2O5 films were n-type semiconductors. Also at room temperature, the experimental data showed that, the electrical conductivity tend to decrease with increasing film thickness and reached to constant value at 3.36 101 V1 cm1 for the film thickness 372 2 nm [23]. At different temperatures, the variation of conductivity against reciprocal absolute temperature presented at the temperature range 300–650 K was investigated. Fig. 10(a) shows the plot of ln s versus T1. It is observed that, the electrical conductivity increases monotonically with increasing temperature. This behaviour of the electrical conductivity can be explained by the effect of charge carrier trapping in grain boundaries, which create a depopulated area of these charge carriers due to their capture at traps present at the grain boundaries [24]. When the trapped charge carriers (N) increased and reached to the critical acceptors or donors concentration (N*), the grains are completely depopulated. In this case the electrical conductivity, s, at the grain boundaries can be estimated from the formula [24]: Eg =2 þ eT s ¼ AT exp (8) KT
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orientation along h0 0 1i direction. The values of crystallite size increased with increasing solution molarity from 0.1 to 0.4 M. Furthermore, increasing of molarity showed a significant decrease of crystallite size. Also, the macrostrain are affected by solution molarity but it was showed the same values for 0.2, 0.3 and 0.4 M. The optical band gap determined by using Tauc plot, has been d in ¼ 2:50 0:02 eV and Eopt ¼ 2:33 0:02 eV found to be Eopt for the direct and indirect allowed transition, respectively. The refractive index, n values drops from 2.64 0.03 to 1.82 0.02 when the wavelength changed from 400 to 2500 nm, respectively. The extinction coefficient, k markedly decreases as wavelength increases in the range from 300 to 600 nm and has a very large magnitude about 0.87 0.01 at 300 nm. The room temperature electrical conductivity showed that, all at deposited films have low conductivity. But at different temperatures the electrical conductivity increases monotonically with increasing temperature. In addition, the activation energy and the potential barrier height were found to be 0.156 0.011 and 0.263 0.012 eV, respectively. References
Fig. 10. (a and b) Plots of ln s vs. 1000/T and ln sT1/2 vs. 1000/T for V2O5 thin film.
where A is the constant, Eg the band gap, eT the energy of trap states in the grain boundary, K the Boltzman constant and T is the absolute temperature. Moreover, the activation energy was deduced from the slope of the curve and gives the value of 0.156 0.011 eV. When the grains are partially depopulated (N > N*), the electrical conductivity at the grain boundaries follows the formula [25]: s ¼ BT
1=2
qFB exp KT
(9)
where B is the constant and qFB is the potential barrier. Fig. 10(b) shows the electrical conductivity based on the model of partially depopulated grains which gives a good correlation with the experimental results obtained in the high temperature range. The potential barrier height FB in the grain boundary, which calculated from the slope gives the value 0.263 0.012 eV. 4. Conclusions In the present investigation vanadium pentoxide films deposited by spray pyrolysis at different solution molarities exhibit some interesting microstructural, optical and electrical properties. The X-ray diffraction (XRD) revealed that, the prepared films at solution molarity 0.1 M and Tsub = 350 8C was polycrystalline orthorhombic structure with a preferential
[1] M. Benmoussa, A. Outzourhit, A. Bennouna, E.L. Ameziane, Thin Solid Films 405 (2002) 11. [2] L. Ottaviano, A. Pennisi, F. Simone, A.M. Salvi, Opt. Mater. 27 (2004) 307. [3] P. Chatterjee, S.P. Sen Gupta, S. Suchitra, Bull. Mater. Sci. 24 (2001) 173. [4] G. Wu, K. Du, C. Xia, X. Kun, J. Shen, B. Zhou, J. Wang, Thin Solid Films 485 (2005) 284. [5] R.T. Rajendra Kumar, B. Karunagaran, V. Senthil Kumar, Y.L. Jeyachandran, D. Mangalaraj, Sa.K. Narayandass, Mater. Sci. Semi. Proc. 6 (2003) 543. [6] B.N.B. Thi, T.P. Minh, B. Simona, J. Appl. Phys. 80 (1996) 7041. [7] K.V. Madhuri, K. Srinivasa Rao, S. Uthanna, B.S. Naidu, O.M. Hussan, J. Indian Inst. Sci 81 (2001) 653. [8] C.V. Ramana, B.S. Naidu, O.M. Hussain, R. Pinto, J. Phys. D: Appl. Phys. 34 (2001) L35. [9] A. Bouzidi, N. Benramdane, A. Nakrela, C. Mathieu, B. Khelifa, R. Desfeux, A. Da Costa, Mater. Sci. Eng. B95 (2002) 141. [10] S.A. Mahmoud, Egypt J. Sol. 19 (1996) 221. [11] H.P. Klug, L.E. Alexander, X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials, second ed., Wiley, New York, 1974. [12] G. Eranna, B.C. Joshi, D.P. Ranthala, R.P. Gupta, Crit. Rev. Sol. St. Mater. Sci. 29 (2004) 111. [13] S. Oliver, K. Colbow, Sens. Actuators B: Chem. 21 (1994) 151. [14] K.L. Chopra, Thin Film Phenomena, McGraw-Hill, New York, 1969. [15] N. Kenny, C.R. Kannewurf, D.H. Whitmore, J. Phys. Chem. Solids 27 (1966) 1237. [16] G.A. Khan, C.A. Hogarth, J. Mater. Sci. 26 (1991) 412. [17] T. Maruyama, T. Kawaguchi, Thin Solid Films 188 (1990) 323. [18] B. Yagoubi, C.A. Hogarth, J. Mater. Sci. 26 (1991) 579. [19] O.S. Heavens, Optical Properties of Thin Films, Dover, New York, 1965; O.S. Heavens, Thin Film Physics, Methuen, London, 1970. [20] E. Masetti, F. Varsano, F. Decker, A. Krasilnikova, Electrochim. Acta 46 (2001) 2085. [21] T. Rechardson, K. von Rottkay, J. Slack, F. Michalak, M. Rubin, Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, CA94720, USA. [22] A.L. Pergament, E.L. Kazakova, G.B. Stefanovich, J. Phys. D: Appl. Phys. 35 (2002) 2187. [23] K.V. Madhuri, K. Srinivasa Rao, S. Uthanna, B.S. Naidu, O.M. Hussain, J. Indian Inst. Sci. 81 (2001) 653. [24] J.W. Seto, J. Appl. Phys. 46 (1975) 5247. [25] G. Baccarini, B.et. Ricco, G. Spadini, J. Appl. Phys. 49 (1978) 5565.
Effect of solution molarity on the characteristics of vanadium pentoxide thin film Alaa A. Akl * Faculty of Science, Physics Department, El-Minia University, Egypt Received 23 October 2005; received in revised form 7 December 2005; accepted 8 December 2005 Available online 3 February 2006
Abstract Vanadium pentoxide (V2O5) thin films have been prepared by spray pyrolysis technique. The influence of solution molarity on the characteristics of the V2O5 has been investigated. X-ray diffraction analysis (XRD) showed that, the films deposited at 0.1 M were orthorhombic structure with a preferential orientation along h0 0 1i direction. Moreover, the crystallinity was improved by increasing solution molarity. The microstructure parameters have been evaluated by using a single order Voigt profile method. The optical band gaps, determined by using Tauc plot, have been found to be 2.50 0.02 and 2.33 0.02 eV for the direct and indirect allowed transition, respectively. Also the complex optical constants for the wavelength range 300–2500 nm are reported. At room temperature, the dark conductivity as a function of solution molarity showed the range of 5.74 102 0.03 to 3.36 101 0.02 V1 cm1. While at high temperature, the behaviour of electrical conductivity dominated by grain boundaries. The values of activation energy and potential barrier height were 0.156 0.011 and 0.263 0.012 eV, respectively. # 2005 Elsevier B.V. All rights reserved. Keywords: Vanadium pentoxide; Thin films; Spray pyrolysis
1. Introduction Nowadays vanadium pentoxide films are of increasing interest due to their unique features, such as electro-chemical activity, high stability, good specific energy, special layer structure and high capacity as well as excellent thermoelectric property. It can be used as a gas sensor, cathode for solid-state batteries, window for solar cells and as an active material in electrochromic devices [1,2]. There are many numbers of vanadium oxides and suboxides such as VO, V2O3, VO2, V4O9 and V2O5. Most of these oxides are metal–insulator transition, which occurs over a wide range of transition temperature depending on the O/V ratio. Vanadium pentoxide is the most stable compound of the V–O system [3]. Various methods are reported for the preparation of V2O5 thin films such as vacuum evaporation [4,5], RF sputtering [2], chemical vapour deposition [6], electron beam evaporation [7] and pulsed laser deposition (PLD) [8]. However, the deposition of thin film from
* Tel.: +2 86 2348681; fax: +2 86 2363011. E-mail address: [email protected]. 0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.12.076
sprayed solution catch the attention of researchers because of such method has low-cost set-up and has the ability to deposit large area [9]. In this work, the synthesis of V2O5 films by spray pyrolysis technique was investigated at different concentrations of spray solution at constant substrate temperature. The influence of solution molarity on the characteristics (microstructures, optical and electrical properties) of V2O5 thin film is reported. 2. Experimental details At constant temperature, different concentrations of vanadium nitrate [V(NO3)55H2O] namely 0.1, 0.2, 0.3, 0.4 and 0.5 M were sprayed on preheated amorphous glass substrates. The substrates have been chemically and ultrasonically pre-cleaned. In order to get uniform thin films, a suitable condition for the height of the spraying nozzle, the flow rate and the deposition time are chosen and kept unchanged during the deposition process at 27 cm, 5 cm3 min1 and 45 s, respectively. Also, a compressed air of pressure 6 N cm2 has been used as a carrier gas. The substrate temperature was fixed at 350 8C and controlled through a thermocouple.
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Structural characterization of the produced films has been carried out using a JEOL JSDX-60PA diffractometer (Cu Ka ˚ – scanning speed is radiation – Ni-filtered l = 1.54184 A 1 18 min – time constant is 1 s). A range of 2u (from 68 to 728) was scanned, to cover all the possible diffraction patterns of the V2O5 phase. Using the data collected from step-scan mode, the determination of the crystallite/domain size and macrostrain were recorded at step size of 0.028 (2u) and a dwell time 4 s/step. To obtain a profile fitting with good signal, a polycrystalline Si powdered was used for instrumental correction. The optical measurements were studied in the range from 300 to 2500 nm using a UV-310 Pc:UV–VIS-NIR; Shimadzu, double beam spectrophotometer with V–N specular reflection attachment. A laboratory developed computer programs [10] based on solving the exact equations were used to calculate the complex optical constants, absorption coefficient and energy band gap. The electrical conductivity of the films at room temperature was studied using standard two-probe technique. The current–voltage characteristic was measured using Keithly 616 digital electrometer. The film thickness was measured using multiple-beam Fizeau fringes at reflection either with mono and polychromatic light. 3. Results and discussion 3.1. Structural characterization Fig. 1 shows the X-ray diffraction (XRD) patterns for samples prepared at Tsub = 350 8C with different solution molarities, namely 0.1, 0.2, 0.3, 0.4 and 0.5 M. The peaks are indexed in comparing our experimental data with the ASTM cards of X-ray powder. The produced data for all films prepared at different molarities of spray solution showed a good agreement with the data of V2O5 powder file (JCPDS data number 09-0387) which corresponding to the orthorhombic crystalline structure. Indeed, the typical peaks of the polycrystalline phase in V2O5 films appear in XRD spectra. It is observed that the crystallinity of the film increases with increasing the solution molarity. The increase in the intensity of the peaks may be attributed to either grain growth associated with larger thickness or increase in the degree of crystallinity by increasing the solution molarity or both. Moreover, the multi-reflection peaks appears in the 2u range from 12.08 to 34.08 characterized the single phase of V2O5 having the lattice parameters a = 11.470 0.012, ˚ which are very b = 3.568 0.011 and c = 4.373 0.013 A near to the value given in this card (a = 11.510, b = 3.559 and ˚ ). It is also clear that, the films show preferred c = 4.371 A growth along the h0 0 1i direction as the solution molarity increases. The variation of the intensity ratio of the (0 0 2) and (0 0 1) planes as a function of solution molarity are shown in Fig. 2. It is clear that the change in intensity is very pronounced in the molarity range of 0.1–0.5 M. One may conclude that, for any solution molarity, the preferred orientation along both h0 0 1i and h0 0 2i plane is observed.
Fig. 1. XRD patterns of V2O5 thin films sprayed at Tsub = 350 8C with different solution molarities.
3.2. Line broadening analysis Viogt profile analysis was used to calculate the crystallite/ domain size and macrostrain of the prepared V2O5 films [11]. The parameter of interest with the Voigt function is ‘‘the shape parameter’’ which defined as [w = FWHM/integral breadth]. The value of this parameter is used to determine the fractional Lorentzian (Chuchy), bfL and Gaussian, bfG components in the convolute. The Lorentzian and Gaussian components of the integral breadth of pure specimen profile are given by: bfL ¼ bhL bgL and ðbfG Þ2 ¼ ðbhG Þ2 ðbgG Þ2
(1)
where h, f and g are the observed, specimen and instrumental profile function, respectively. The apparent crystallite/domain size e is e ¼ ðbfL Þ1
and
e ¼ ðbfG Þ1=2
(2)
The crystallite size was calculated as an average of the apparent size, which is equivalent to an average volume of the
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Fig. 2. The variation of intensity ratio of the (0 0 2) and (0 0 1) of V2O5 thin films as a function of solution molarities.
Fig. 3. (a and b) The crystallite/domain size and macrostrain of V2O5 thin films as a function of solution molarities.
thickness, measured perpendicular to the reflecting planes. The macrostrain jej is given by,
molecules are ejected with higher kinetic energy and higher momentum leading to make collision with surface, then the molecules recombine with higher mobility energy causing an internal stress, which raises the macrostrain.
jej ¼
Dd 1 ¼ cot u Dð2uÞ do 2
(3)
The macrostrain was calculated as an average of fractional change, Dd/do, in the interplaner spacing, d of the threediffraction planes (2 0 0), (0 0 1) and (0 0 2), where do is that of the standard JCPDS number 09-0387. The calculated crystallite size and macrostrain of the investigated samples are given in Fig. 3(a and b). It is clear that, the crystallite sizes are varied for all samples. The values of crystallite size are near saturated at solution molarity namely 0.2, 0.3 and 0.4 M. But at higher concentration (0.5 M) the crystallite size is sharply decreasing as shown in Fig. 3(a). This means that no variation in crystallite size was observed for films deposited at different solution molarities (0.2, 0.3 and 0.4 M) since this concentration is appropriate for crystal growing on the substrate due to the suitable mobility of ions. But at low concentration (0.1 M) the mobility of ions is insufficient for growing, while at higher concentration (0.5 M) the mobility of ions is higher and lead to crystal agglomerate destruction. Thus, complete grain growth does not occurs, which reduces the formation of crystallite size. The calculated macrostrain for all investigated samples are shown in Fig. 3(b). It is clear that, the macrostrain was the same at solution molarity namely 0.2, 0.3 and 0.4 M. But at concentration of 0.5 M, the macrostrain was increased, since it depends on the elastic constants of the material and nature of internal stress. Moreover, at higher molarity the sprayed
3.3. Optical properties The optical transmission and reflection of the V2O5 prepared films were measured over the wavelength range 300–2500 nm. The spectral variation of transmission and reflection for the V2O5 film deposited onto glass substrate, at Tsub = 350 8C and different solution molarities are shown in Figs. 4 and 5. It was observed that, the surface of the films prepared at solution molarity from 0.2 to 0.5 M appears to be rougher due to the increase of scattered light and decrease of the film transmission and specular reflection. Also, the absorption edge is shifted toward higher energy by decreasing solution molarity from 0.5 to 0.2 M, while at 0.1 M the absorption edge lies in the UV region. From Fig. 4; it is clear that as the solution molarity decreases, the film transmission increases all over the whole spectral region. When concerning the film prepared at solution molarity of 0.1 M higher values of the optical transmission and reflection were observed. From the experimental data, the thickness of films prepared by solution molarity of 0.5 and 0.1 M were 372 2 and 118 2 nm, respectively. This drops of thickness provide the drastic increase of film transmission between them and posses higher absorption coefficient. From the results obtained the film that was prepared at 0.1 M gives the best transmission and
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Fig. 4. Optical transmittance spectra of V2O5 thin films formed at different solution molarities.
reflection spectra, thus this film was chosen to determine the absorption coefficient, a, the optical band gap, Eg and optical constants (n and k). The optical absorption coefficient was evaluated form the relation [12]; aðlÞ ¼
1 f1 RðlÞg2 ln d TðlÞ
(4)
where T and R are the spectral transmittance and reflectance and d is the film thickness. The variation of the absorption coefficient with the incident wavelength at and near absorption edge for V2O5 films is shown in Fig. 6. It is clear that, the absorption coefficient rises abruptly at the edge reaching to the values of approximately 3.5 104 cm1 at l = 650 nm, these values agree with others [4,9]. The extraordinarily high a value (exceeding 104 cm1) reduces material demand and the gas sensing material [13,14].
Fig. 5. Optical reflectance spectra of V2O5 thin films formed at different solution molarities.
Fig. 6. The optical absorption coefficient, a, vs. the wavelength of V2O5 thin film.
The optical band gap was evaluated by using the relation [15,16]: ahn ¼ Aðhn Eg Þm
(5)
where A is a constant, Eg the band gap of the material, hn the incident photon energy and the exponent m determines the type of electronic transitions causing the absorption and take the values (1/2 and 2). The optical band gap calculated by extrapolating the straight line parts of the curves (ahn)1/m = 0, are shown in Fig. 7(a and b). It is found that, the values of optical
Fig. 7. (a and b) Plots of (ahn)2 and (ahn)1/2 vs. photon energy for V2O5 thin film.
A.A. Akl / Applied Surface Science 252 (2006) 8745–8750
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band gap for direct and indirect allowed transitions are 2.50 0.02 and 2.33 0.02 eV, respectively. The obtained values of direct and indirect transition agree with the values reported by several authors [8,9,17,18] for crystalline V2O5 thin film. The complex optical constants (refractive index, n, and extinction coefficient, k) of the prepared film have been calculated from the corrected T and R using a developed computation programs [10]. The programs involve bivariant search technique based on minimizing jDRj2 and jDTj2, where: jDRj2 ¼ jRexp Rcalc j2 jDTj2 ¼ jTexp Tcalc j2
(6)
The subscripts exp and calc are the experimental and calculated results, respectively. Both the transmittance Tcalc and reflectance Rcalc are given by Murmann’s exact equations [19]. Fig. 8(a) shows the spectral variation of the refractive index, n against wavelength, l. The n values drops from 2.64 0.03 to 1.82 0.02 when the wavelength changed from 400 to 2500 nm, respectively. Thus the maximum value of refractive index is observed at short wavelengths (l = 400 nm), which is good agreement with the data obtained by others [20,21] for like phase. Fig. 8(b) illustrates the corresponding data for the extinction coefficient, k versus wavelength, l, according to Fresnel equation [22]: T l k ¼ ln (7) 1 R 4pd The k value markedly decreases as wavelength increases in the range from 300 to 600 nm and has a very large magnitude about 0.87 0.01 at 300 nm. This value of k is attributed to the fundamental band gap and the very low transmittance at short wavelength. The information, which used in the calculations is mainly obtained from reflectance so that surface effects tend to dominate [21].
Fig. 8. (a and b) Variation of refractive index, n, and extinction coefficient, k, vs. wavelength, l, for V2O5 thin film.
Fig. 9. Variation of dc conductivity with different solution molarity for V2O5 thin films.
3.4. Electrical properties For the electrical conductivity, two ways of investigation were done, one of them at room temperature and the other at different temperatures. The room temperature electrical conductivity of V2O5 films was studied by two-probe technique. Fig. 9 represents the variation of electrical conductivity against film molarity. It is observed that, all asdeposited films have low conductivity. The hot-point probe method showed that all the investigated V2O5 films were n-type semiconductors. Also at room temperature, the experimental data showed that, the electrical conductivity tend to decrease with increasing film thickness and reached to constant value at 3.36 101 V1 cm1 for the film thickness 372 2 nm [23]. At different temperatures, the variation of conductivity against reciprocal absolute temperature presented at the temperature range 300–650 K was investigated. Fig. 10(a) shows the plot of ln s versus T1. It is observed that, the electrical conductivity increases monotonically with increasing temperature. This behaviour of the electrical conductivity can be explained by the effect of charge carrier trapping in grain boundaries, which create a depopulated area of these charge carriers due to their capture at traps present at the grain boundaries [24]. When the trapped charge carriers (N) increased and reached to the critical acceptors or donors concentration (N*), the grains are completely depopulated. In this case the electrical conductivity, s, at the grain boundaries can be estimated from the formula [24]: Eg =2 þ eT s ¼ AT exp (8) KT
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orientation along h0 0 1i direction. The values of crystallite size increased with increasing solution molarity from 0.1 to 0.4 M. Furthermore, increasing of molarity showed a significant decrease of crystallite size. Also, the macrostrain are affected by solution molarity but it was showed the same values for 0.2, 0.3 and 0.4 M. The optical band gap determined by using Tauc plot, has been d in ¼ 2:50 0:02 eV and Eopt ¼ 2:33 0:02 eV found to be Eopt for the direct and indirect allowed transition, respectively. The refractive index, n values drops from 2.64 0.03 to 1.82 0.02 when the wavelength changed from 400 to 2500 nm, respectively. The extinction coefficient, k markedly decreases as wavelength increases in the range from 300 to 600 nm and has a very large magnitude about 0.87 0.01 at 300 nm. The room temperature electrical conductivity showed that, all at deposited films have low conductivity. But at different temperatures the electrical conductivity increases monotonically with increasing temperature. In addition, the activation energy and the potential barrier height were found to be 0.156 0.011 and 0.263 0.012 eV, respectively. References
Fig. 10. (a and b) Plots of ln s vs. 1000/T and ln sT1/2 vs. 1000/T for V2O5 thin film.
where A is the constant, Eg the band gap, eT the energy of trap states in the grain boundary, K the Boltzman constant and T is the absolute temperature. Moreover, the activation energy was deduced from the slope of the curve and gives the value of 0.156 0.011 eV. When the grains are partially depopulated (N > N*), the electrical conductivity at the grain boundaries follows the formula [25]: s ¼ BT
1=2
qFB exp KT
(9)
where B is the constant and qFB is the potential barrier. Fig. 10(b) shows the electrical conductivity based on the model of partially depopulated grains which gives a good correlation with the experimental results obtained in the high temperature range. The potential barrier height FB in the grain boundary, which calculated from the slope gives the value 0.263 0.012 eV. 4. Conclusions In the present investigation vanadium pentoxide films deposited by spray pyrolysis at different solution molarities exhibit some interesting microstructural, optical and electrical properties. The X-ray diffraction (XRD) revealed that, the prepared films at solution molarity 0.1 M and Tsub = 350 8C was polycrystalline orthorhombic structure with a preferential
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