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Effect of Li doping on the structural, optical and electrical properties of spray deposited SnO2 thin films
Thin Solid Films 517 (2009) 6129–6136
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Effect of Li doping on the structural, optical and electrical properties of spray deposited SnO2 thin films D. Paul Joseph, P. Renugambal, M. Saravanan, S. Philip Raja, C. Venkateswaran ⁎ Materials Science Centre, Department of Nuclear Physics, University of Madras, Guindy Campus, Chennai, 600 025, India
a r t i c l e
i n f o
Article history: Received 12 September 2007 Received in revised form 14 April 2009 Accepted 17 April 2009 Available online 3 May 2009 Keywords: Tin oxide Figure of merit Transparent conducting oxides Spray pyrolysis Lithium doping Surface morphology
a b s t r a c t Studies on spray deposited transparent conducting Li doped SnO2 thin films are scarce. Li (0 to 5 wt.%) doped SnO2 thin films spray deposited onto glass substrates at 773 K in air from chloride precursors were studied for their structural, optical and temperature dependent electrical behaviors. X-ray diffraction patterns indicated single phase with polycrystalline nature. Systematic variation in surface morphology on Li doping was examined by scanning electron microscopy and atomic force microscopy. Film thickness, optical band gap (direct and indirect), sheet resistance and figure of merit were computed from spectral transmittance and temperature dependent resistivity data. Lithium doping was found to decrease the value of sheet resistance by an order in magnitude. Activation energy was computed from temperature dependent electrical resistivity data measured in the range 300 to 448 K. The 4 wt.% Li doped SnO2 film was found to have a high value of figure of merit among other films. The results are discussed. © 2009 Elsevier B.V. All rights reserved.
1. Introduction SnO2 is an important material in the family of oxide materials that combine low electrical resistance with high optical transparency in the visible range of the electromagnetic spectrum. This property is essentially used in place where an electrical contact needs to be made without obstructing photons from either entering or escaping the optically active area and in transparent electronics [[1] and references therein]. SnO2 thin films are extensively researched for their use as transparent electrodes in display devices like liquid crystal displays and transparent active layers in SnO2/Si solar cells etc, [2]. Additionally, SnO2 has excellent gas sensing properties, specifically for detecting inflammable and harmful gases like hydrogen (H2), carbon monoxide (CO), and nitrous oxide (NOx) compounds. Nanowires of SnO2 were synthesized and explored for sensing CO [3]. Among the existing transparent conducting oxides (TCO), SnO2 has been well established due to its stability towards atmospheric conditions and low cost. SnO2 is an n-type, wide band-gap semiconductor (Eg = 3.6–3.97 eV) whose electrical properties critically depend upon its oxygen stoichiometry. The conductivity and transparency of SnO2 thin films can be enhanced by way of doping with Sb, In and F. The conductivity of weakly non-stoichiometric [4] SnO2 is supposed to be due to doubly ionized vacancies serving as donors [5]. There are several chemical and physical methods, for the production of SnO2 thin films, like chemical vapor deposition [6], sputtering [7], sol–
⁎ Corresponding author. Tel.: +91 44 22202803. E-mail address: [email protected] (C. Venkateswaran). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.04.047
gel [8], electron beam evaporation [9] and spray pyrolysis [10–12]. Among these, spray pyrolysis is simple, cost effective, reproducible, permits easy doping and mass production. A thorough literature survey indicated very few reports to exist on Li doped SnO2 thin films [13] and powders [14]. The properties of spray deposited SnO2 thin films were found to be dependent on the processing conditions and the nature of precursors used. The precursors play a key role in the formation of film structure, morphology, growth, electrical and optical properties of the deposited material [[15] and references therein]. SnO2 has been doped with a variety of elements for various doping level, by various preparation methods and for different applications [4]. In this paper, we analyze the results of systematic characterization on Li (0 to 5 wt.%) doped SnO2 thin films spray deposited from chloride precursors for their structural, morphological, optical and electrical properties. 2. Experimental details Li (0 to 5 wt.%) doped SnO2 thin films were prepared using a home made spray pyrolysis unit (Fig. 1). Stoichiometric amounts of stannous chloride (SnCl2.2H2O) and LiCl (Li 0 to 5 wt.%) dissolved in 100 ml of de-ionized water along with 3 mL of HCl (to maintain the pH acidic) were used as a precursor solution. The precursor solution was sprayed through a spray gun of inner nozzle diameter, ≈0.3 mm by manually operating a valve onto the ultrasonically cleaned glass substrates. The substrate was maintained at 773 K (±5 K), measured by a K-type thermocouple and controlled electronically. The deposition parameters like the rate of spray solution (1.66 mL/min, with 5 s per spray and with total number of 50 sprays for 4.16 min), nozzle to substrate
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Fig. 3. Crystallite size and film thickness of Li (0 to 5 wt.%) doped SnO2 films as a function of Li ion concentration.
transport data of the films was measured with a four-point probe from 300 K to 448 K.
Fig. 1. Schematic of home made spray pyrolysis unit.
3. Results and discussion distance (35 cm), pressure of compressed and moisture filtered carrier gas (5–6 MPa) were kept constant at the optimized values indicated in the brackets. A rest time of 1 min was provided between each spray. The number of sprays was restricted to 50 since the films turned milky affecting the transparency for 75 sprays. The pyrolytic decomposition of stannous chloride (SnCl2.2H2O) and LiCl forms clusters of Li doped SnO2 and develops into a continuous film (usually Volmer–Weber growth mode) in the presence of water as follows, Δ 2SnO →
2SnCl2 :2H2 O + 2LiCl + 4H2 O
2
+ 2LiO + 6HCl + 3H2 z + 2H2 Oz:
ð1Þ The as-prepared films were highly adherent and transparent. The structure and phase purity were investigated by X-ray diffraction (XRD) with (Xpert high scorer, PANalytical) CuKα radiation with a scan rate of 0.02°/s. The surface morphology of the thin films was studied using a Scanning Electron Microscope (JEOL — JAPAN JSM 840A). The surface analysis was also performed using an Atomic Force microscope (AFM, Model SPI 6800N). The spectral transmittance in the UV–Vis and IR region was recorded with a double beam spectrometer in near normal incident mode (JASCO Corp., V-570, Rev. 1.00). Film thickness and optical band gap were extracted from the spectral transmittance data. The temperature dependent electrical
The XRD patterns of Li (0 to 5 wt.%) doped SnO2 thin films are shown in Fig. 2. The peaks were indexed to the tetragonal SnO2 structure [16] with (110) plane as the 100% peak. The two close lying reflections (112) and (301) were also evident indicating the polycrystalline nature of the films, which may be due to the use of water as a solvent. The calculated values of texture coefficient for (110) and (211) were less than unity confirming the polycrystalline nature of the films. However, the 1 wt.% Li doped SnO2 film presented a possible oriented growth along the (211) plane rather than along the (110) direction. The lattice parameters of undoped SnO2 thin film are, a = 4.738 Å and b = 3.186 Å. Notable change was not observed in lattice parameters with increasing Li ion concentration which is due to the comparable ionic radius of Li+ (0.68 Å) with Sn4+ (0.74 Å). The average crystallite size from the full width at half the maximum of (110) and (211) planes was estimated using Scherrer's relation [17]. The variation of crystallite size with Li concentration is shown in Fig. 3 and Table 1. The 1 wt.% Li doped film has the highest crystallite size of 49 nm, which was found to decrease for further increase in Li concentration. The SEM micrographs of Li (0 to 5 wt.%) doped SnO2 thin films are shown in Fig. 4A to F. The surface of the undoped SnO2 film is continuous with a uniform distribution of faceted bimodal particles.
Table 1 Crystallite size, t, RS, ρ, σ, Ea and Eg (direct and indirect) of Li (0 to 5 wt.%) doped SnO2 thin films. Li %
Crystallite size (nm)
t Rs at (nm) 300 K (Ω/□)
0%
42
336
5.74 × 105 19.29
0.0518
4
1.562
0.6401
1%
49
835
1.87 × 10
2%
45
386
1.88 × 104
0.725
1.377
3%
43
349
2.39 × 104
0.836
1.194
407
2.05 × 10
4
0.834
1.197
3.25 × 10
4
1.815
0.5509
4% Fig. 2. XRD patterns of Li (0 to 5 wt.%) doped SnO2 thin films indicating the polycrystalline nature.
Ea ρ at σ at (eV) 300 K 300 K −1 (Ω cm) (Ω cm)
5%
40 38
557
Ea1 = 0.974 Ea2 = 3.287 Ea1 = 0.419 Ea2 = 1.453 Ea1 = 0.159 Ea2 = 2.067 Ea1 = 0.417 Ea2 = 2.064 Ea1 = 0.689 Ea2 = 2.30 Ea1 = 0.473 Ea2 = 1.759
Egd (eV)
Egind and Ep (eV)
3.906 3.48, 0.12 3.845 3.28, 0.12 3.909 3.50, 0.10 3.906 3.50, 0.10 3.887 3.53, 0.085 3.925 3.52, 0.10
D.P. Joseph et al. / Thin Solid Films 517 (2009) 6129–6136
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Fig. 4. SEM micrographs of Li (0 to 5 wt.%, A to F respectively) doped SnO2 films showing variation in surface morphology.
The particles were found to be less than a micron in size and nearly monodispersed (Fig. 4 A). On doping with 1 wt.% of Li, the SnO2 film was found to have a twisted metal sheet like particle morphology (Fig. 4 B). Similar morphology has been reported by Elangovan et al. [18]. When Li doping increases to 2 and 3 wt.%, the particle morphology becomes spherical (Fig. 4C and D). The 4 wt.% Li doped SnO2 thin film was found to possess monodispersed equiaxed spherical particles with size much less than a micron with nominal agglomeration. The spherical particles grow in size for further increase in Li doping to 5 wt.%. The progressive variation in film morphology may be attributed to the slight variation in deposition conditions and/or due to the difference in precursor concentration resulting in the change in total flux of molecules reaching the substrate and thereby altering the nucleation and growth process [19]. The 2D AFM images of the Li (0 to 5 wt.%) doped SnO2 thin films shown in Fig. 5 (A to F) were measured in dynamic mode over a 1 µm × 1 µm area of the film surface. The 2D images of Li (0 to 5 wt.%) doped SnO2 thin films indicate that the particle size varies with Li concentration. The 1 wt.% Li doped SnO2 film was found to have particles of larger size compared to other films. This also corroborates with the size estimated from XRD results. The 1 wt.% Li doped SnO2 film was found to have the highest crystallite size of 49 nm (Table 1). The particles are not of the same height and are distributed irregularly within the measured region due to the polycrystalline nature of the films. The SnO2 film RMS surface roughness (Fig. 6) is relatively independent of Li doping except for the high value measured for the 1 wt.% Li doped samples.
The spectral transmittance data (Fig. 7) shows that the films were nearly 80% and 70% transparent in the visible region and near IR regions respectively. The observation of interference patterns (Fig. 7) in the transmittance data (Tr) is direct evidence for homogeneous and uniformly thick films [20]. The thickness (t) of the SnO2 films, was calculated using the phase shift (Psh) caused by interference associated with reflections from the front and back surfaces of the film using the relation, Psh = 4πnt = λ
ð2Þ
where, n is the refractive index of the film (n = 1.92 for SnO2), t is the film thickness and λ is the wavelength of two neighboring extrema (a peak and a valley) in the spectra. Since Psh is an integer multiple of π, and neighboring extrema differ by one, two simultaneous equations with only two unknowns were solved to determine film thickness. It is observed that the thickness of the film does not present any trend with Li concentration. The 1 wt.% Li doped SnO2 film was found to have the greatest thickness of 835 nm (Fig. 3) and the largest crystallite size of 49 nm. This may be due to the variation in deposition efficiency, which might have resulted from the diminished mass transport to the substrate and also due to gas convection that pushes the precursor away [21]. The % Tr of all the films for 550 nm measured at 300 K is shown in Fig. 8. The 4 wt.% and 5 wt.% Li doped SnO2 films were found to be more transparent than the undoped SnO2 in that the % Tr at 550 nm for this film was greater than the thinner undoped SnO2 film. This implies that Li doping in SnO2 contributes to the slight
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Fig. 5. 2D AFM images of Li (0 to 5 wt.%, A to F respectively) doped SnO2 films showing 1 wt.% L: doped film to be composed of larger particles among other films.
improvement in transparency, however, the effect of film thickness and the variations in deposition parameters could not be denied. The absorption coefficient has been calculated from the Lambert's formula, α = 1 = t ½lnð1 = Tr Þ
ð3Þ
where, Tr and t are transmittance and the film thickness respectively. The absorption coefficient was found to decrease exponentially with decreasing photon energy, indicating an Urbach characteristic [22]. The band gap of pure SnO2 is widely reported to be of direct transition in nature (3.8–4.3 eV) [4,23]. The direct band gap of Li doped SnO2 films was determined from the (αhν)2 vs hν (Tauc relation) plot by
Fig. 6. Variation of RMS roughness value of Li (0 to 5 wt.%) doped SnO2 films.
extrapolating the linear fit given to the linear region to α = 0 (Fig. 9). The direct band-gap values of Li doped SnO2 are given in Table 1. No remarkable difference or trend was observed in the variation of the value of direct energy gap of Li doped SnO2. Though literature reports indicate the direct band gap of SnO2, there are theoretical predictions for the existence of an indirect band gap for SnO2 with somewhat lower energy [24]. In case of NiO, both direct and indirect band gaps and phonon energies were calculated from the Tauc relation [25]. The (αhν)1/2 vs hν plots obtained for Li (0 to 5 wt.%) doped SnO2 films based on Ref. [25] were found to have a linear region. Two distinct linear fits were made to the linear region for phonon absorption (Egind −Ep) and phonon emission (Egind +Ep) processes. By solving the
Fig. 7. Spectral transmittance plot of Li (0 to 5 wt.%) doped SnO2 thin films.
D.P. Joseph et al. / Thin Solid Films 517 (2009) 6129–6136
Fig. 8. Optical transmittance (at 550 nm) plot of Li (0 to 5 wt.%) doped SnO2 thin films at 300 K.
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Fig. 11. Sheet resistance of SnO2 thin films as a function of Li (0 to 5 wt.%) doping at 300 K. Inset shows the resistivity of the films as a function of temperature.
simultaneous equations formed from the values of the above described fit (Fig. 10), the values of Egind and the phonon energy Ep were obtained and are given in Table 1. All Li doped SnO2 thin films were conducting at 300 K with resistivity values much less than that of undoped SnO2. The four-point probe measurement of sheet resistance (RS) is preferred over the twopoint probe technique since the contact resistance associated with the two-point probe technique is overcome. In the linear four-point probe technique, the current (I) is applied between the outer two leads and the potential difference (V) across the inner two probes is measured. Since negligible contact and spreading resistance are associated with the voltage probes, one can obtain a fairly accurate estimation of RS using the following relation, RS = 4:532ðV = I Þ:
Fig. 9. Band-gap (Egd) estimation of Li (0 to 5 wt.%) doped SnO2 films from Tauc relation.
Fig. 10. Indirect band-gap (Egind) and phonon energy (Ep) estimation of Li (0 to 5 wt.%) doped SnO2 films.
ð4Þ
In the above said configuration, a correction factor of 4.532 was applied for the sample (1 cm × 1 cm) with equally spaced (≈1 mm) probes and the film thickness necessarily being less than the spacing between the probes. The RS of the undoped SnO2 is relatively high
Fig. 12. Variation of resistivity and conductivity at 300 K of Li (0 to 5 wt.%) doped SnO2 films as a function of Li ion concentration.
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Fig. 13. (A to F). Determination of activation energy of Li (0 to 5 wt.%) doped SnO2 films from Arrhenius plot.
(5.74 × 105 Ω/□) compared to that of the Li doped SnO2. Similar sheet resistance values have been reported for spray deposited F doped SnO2 by Acosta et al. [26]. The values of sheet resistance (see Table 1)
of SnO2 films at 300 K as a function of Li ion concentration are shown in Fig. 11. RS was found to decrease by an order in magnitude on doping irrespective of variations in film thickness.
D.P. Joseph et al. / Thin Solid Films 517 (2009) 6129–6136
The results of temperature (T) dependent resistivity (current in plane geometry) from 300 K to 448 K are shown in inset of Fig. 11. The resistivity (ρ) was calculated using the relation, ρ = RS t:
ð5Þ
The resistivity of the Li doped SnO2 films was found to fall with T indicating semiconducting behavior. For increasing T, the resistivity of all the films was found to merge and stay constant at 448 K [Inset of Fig. 11]. The variation in resistivity and conductivity of Li (0 to 5 wt.%) doped SnO2 films at 300 K as a function of Li ion concentration is shown in Fig. 12. The resistivity of pure and stoichiometric SnO2 is very high, approximately 108 Ω cm [13]. The electrical resistivity of intentionally undoped SnO2 grown by many techniques is however significantly less than this value due to non-ideal growth conditions that result in non-stoichiometric, oxygen deficient films [27]. Similar resistivity measurements have also been reported for F:SnO2 in the temperature range 300 to 448 K [28]. SnO2 films deposited by spray pyrolysis technique are more susceptible to oxygen deficiencies [29]. Hence, the use of spray pyrolysis technique leads to the formation of strongly non-stoichiometric SnO2 films resulting in comparatively low resistive films. The conductivity of non-stoichiometric tin oxide films is influenced by the presence of doubly ionized vacancies serving as donors [5]. In TCO thin films, free carriers may interact with different scattering centers such as thermal lattice vibrations, and ionized and neutral impurities. The influence of grain boundaries should also be taken into account [30]. The ionized impurities and grain boundaries are found to be the main scattering mechanisms in TCO thin films [4,31]. The grain boundary is known to possess more defects, impurities, and traps than grain interiors. As the orientation of the grain changes, the density of traps in the direction of current also changes. In interpreting the electrical data of Li doped SnO2, one has to consider the various scattering mechanisms present. Fully polycrystalline Li doped SnO2 is composed of crystallites joined together by grain boundaries, which are the transitional regions between different orientations of neighboring crystallites. Thangaraju had explained transport mechanism in doped SnO2 by two models. First is a charge trapping model [32] assuming that the grain boundaries contain trapping states induced by lattice defects. Second is the dopant segregation model [33] in which grain boundaries are assumed to act as sinks for the preferential segregation of the dopants making the boundary layers inactive. In our case, since all the films are polycrystalline in nature and there is no evidence for segregation of the dopants, the first model is applicable [32]. The activation energies of 0 to 5 wt.% Li doped SnO2 films shown in Fig. 13 A to F were calculated from the slope of the linear fit given to the plot of ln (σ) against 1000 / T. The values of the estimated activation energies are given in Table 1. Two activation energies were estimated for all the films from the conductivity data obtained in the temperature range of 300 to 448 K. The activation energies of the Li doped SnO2 thin films were found to obey the relations, σ = σ 1 expð−ΔEa1 = kB T Þ
6135
regions of two distinct slopes corresponding to low temperature region (I) (up to 373 K) and high temperature region (II) (above 373 K). The existence of two such regions is reported by Viswakarma et al. [36]. The activation energy Ea1 is observed in the temperature range from 300 to 373 K whereas Ea2 is observed above 373 K. Moreover the value of Ea1 and Ea2 of the Li doped SnO2 films was comparably lower than that of the undoped SnO2 film. The values of Ea1 are quite higher than the energy required to excite free carriers from shallow donor levels. Hence the conduction is not due to thermally excited free carriers from the shallow donor levels, instead the high activation energies are indicative of increased scattering due to grain boundaries, defects, impurities etc. in the lower activation energy regime (Region I). The higher activation energy Ea2 above 373 K (Region II) is due to the presence of deep donor levels, which might have resulted from defects and impurities [29]. A plausible explanation for the reduction in activation energy Ea2 of Li doped films compared to undoped films may be the replacement of the Sn atoms at cation sites with relatively lighter Li atoms forming deep donor levels. The figure of merit is an important parameter for evaluating TCO thin films for use in solar cells. Conductivity and transmittance are inversely proportional to each other and should be as high as possible for effective usage. In order to compare the performance of various transparent conductors the most widely used figure of merit as first defined by Haacke [37] is, 10
u = Tr = RS
ð8Þ
where Tr is the transmittance at λ = 550 nm (the wavelength at which solar power conversion is maximized) [37,38] and RS is the sheet resistance. The calculated value of figure of merit (φ) as a function of Li ion concentration is depicted in Fig. 14. The figure of merit for 4 wt.% Li doped SnO2 is found to be maximum. For an ideal TCO, the figure of merit should be as high as possible, but depending on the type of application and requirement, either of the two parameters defining φ has to be compromised. Though maximum transparency and low resistivity is desired for a TCO in general, achievement of both of these properties is difficult. As observed from the electrical transport data by doping Li+ in SnO2, the resistivity is decreased considerably compared to undoped SnO2. The value of φ for Li (4 wt.%) doped SnO2 is very small when compared with the well established indium doped SnO2 TCO. The value of φ of Li (4 wt.%) doped SnO2 film is two orders less than that of fluorine doped tin oxide (FTO) (2.5 × 10− 3 Ω− 1) [31]. However, here the value of φ of
ð6Þ
and σ = σ 2 expð−ΔEa2 = kB T Þ
ð7Þ
where, σ is the d.c. conductivity, σ1 and σ2 are the pre-exponential factors, Ea1 and Ea2 are the activation energies for d.c. conduction, kB is the Boltzmann constant and T is the absolute temperature. The conduction in the range of 300 to 448 K for pure SnO2 was determined to be due to intrinsic semiconducting behavior obeying the Arrhenius law [34,35]. The Arrhenius plot of all the films shows
Fig. 14. Figure of merit of (0 to 5 wt.%) doped SnO2 films as a function of Li ion concentration.
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D.P. Joseph et al. / Thin Solid Films 517 (2009) 6129–6136
Li+ (4 wt.%) doped SnO2 is a better balance between the transmission and sheet resistance resulting in high φ. As far as 4 wt.% Li doped SnO2 is concerned, Li doping into SnO2 prepared under the above mentioned deposition parameters imparts transparency higher than the undoped SnO2 film. Hence, further exploration of Li doped SnO2 could make it a material suitable for versatile applications. 4. Conclusion In summary, Li (0 to 5 wt.%) doped SnO2 thin films were deposited on glass substrates by the spray pyrolysis method. An increase in Li doping was found to change film morphology from a twisted metal, sheet like structure to a spherical one. The resistivity of the Li doped SnO2 films was found to decrease by an order of magnitude with increasing Li concentration. The high transparent conducting oxide figure of merit found for the 4 wt.% Li doped SnO2 film suggests it's exploration for further application. Acknowledgements The authors thank the UGC (X-plan developmental grant sanctioned to the Department of Nuclear Physics) and DST (SR/S5/NM23/2002 dt. 29/10/2002), Govt. of India, for partial financial assistance for the construction of homemade spray pyrolysis unit. We also thank the referees and the editor for improving the clarity of the manuscript through their fruitful comments and suggestions till the final version. One of the authors (DPJ) thanks the CSIR, Govt. of India, for the award of Senior Research Fellowship (2007), Mr. E. Senthil Kumar (IITM) and Mr. A. Sendilkumar (HCU) for their help in various aspects. References [1] M. Batzill, U. Diebold, Prog. Surf. Sci. 79 (2005) 47. [2] S. Colen, Thin Solid Films 77 (1981) 127. [3] M.R. Yang, S.Y. Chu, R.C. Chang, Sens. Actuators, B 122 (2007) 269.
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Effect of Li doping on the structural, optical and electrical properties of spray deposited SnO2 thin films D. Paul Joseph, P. Renugambal, M. Saravanan, S. Philip Raja, C. Venkateswaran ⁎ Materials Science Centre, Department of Nuclear Physics, University of Madras, Guindy Campus, Chennai, 600 025, India
a r t i c l e
i n f o
Article history: Received 12 September 2007 Received in revised form 14 April 2009 Accepted 17 April 2009 Available online 3 May 2009 Keywords: Tin oxide Figure of merit Transparent conducting oxides Spray pyrolysis Lithium doping Surface morphology
a b s t r a c t Studies on spray deposited transparent conducting Li doped SnO2 thin films are scarce. Li (0 to 5 wt.%) doped SnO2 thin films spray deposited onto glass substrates at 773 K in air from chloride precursors were studied for their structural, optical and temperature dependent electrical behaviors. X-ray diffraction patterns indicated single phase with polycrystalline nature. Systematic variation in surface morphology on Li doping was examined by scanning electron microscopy and atomic force microscopy. Film thickness, optical band gap (direct and indirect), sheet resistance and figure of merit were computed from spectral transmittance and temperature dependent resistivity data. Lithium doping was found to decrease the value of sheet resistance by an order in magnitude. Activation energy was computed from temperature dependent electrical resistivity data measured in the range 300 to 448 K. The 4 wt.% Li doped SnO2 film was found to have a high value of figure of merit among other films. The results are discussed. © 2009 Elsevier B.V. All rights reserved.
1. Introduction SnO2 is an important material in the family of oxide materials that combine low electrical resistance with high optical transparency in the visible range of the electromagnetic spectrum. This property is essentially used in place where an electrical contact needs to be made without obstructing photons from either entering or escaping the optically active area and in transparent electronics [[1] and references therein]. SnO2 thin films are extensively researched for their use as transparent electrodes in display devices like liquid crystal displays and transparent active layers in SnO2/Si solar cells etc, [2]. Additionally, SnO2 has excellent gas sensing properties, specifically for detecting inflammable and harmful gases like hydrogen (H2), carbon monoxide (CO), and nitrous oxide (NOx) compounds. Nanowires of SnO2 were synthesized and explored for sensing CO [3]. Among the existing transparent conducting oxides (TCO), SnO2 has been well established due to its stability towards atmospheric conditions and low cost. SnO2 is an n-type, wide band-gap semiconductor (Eg = 3.6–3.97 eV) whose electrical properties critically depend upon its oxygen stoichiometry. The conductivity and transparency of SnO2 thin films can be enhanced by way of doping with Sb, In and F. The conductivity of weakly non-stoichiometric [4] SnO2 is supposed to be due to doubly ionized vacancies serving as donors [5]. There are several chemical and physical methods, for the production of SnO2 thin films, like chemical vapor deposition [6], sputtering [7], sol–
⁎ Corresponding author. Tel.: +91 44 22202803. E-mail address: [email protected] (C. Venkateswaran). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.04.047
gel [8], electron beam evaporation [9] and spray pyrolysis [10–12]. Among these, spray pyrolysis is simple, cost effective, reproducible, permits easy doping and mass production. A thorough literature survey indicated very few reports to exist on Li doped SnO2 thin films [13] and powders [14]. The properties of spray deposited SnO2 thin films were found to be dependent on the processing conditions and the nature of precursors used. The precursors play a key role in the formation of film structure, morphology, growth, electrical and optical properties of the deposited material [[15] and references therein]. SnO2 has been doped with a variety of elements for various doping level, by various preparation methods and for different applications [4]. In this paper, we analyze the results of systematic characterization on Li (0 to 5 wt.%) doped SnO2 thin films spray deposited from chloride precursors for their structural, morphological, optical and electrical properties. 2. Experimental details Li (0 to 5 wt.%) doped SnO2 thin films were prepared using a home made spray pyrolysis unit (Fig. 1). Stoichiometric amounts of stannous chloride (SnCl2.2H2O) and LiCl (Li 0 to 5 wt.%) dissolved in 100 ml of de-ionized water along with 3 mL of HCl (to maintain the pH acidic) were used as a precursor solution. The precursor solution was sprayed through a spray gun of inner nozzle diameter, ≈0.3 mm by manually operating a valve onto the ultrasonically cleaned glass substrates. The substrate was maintained at 773 K (±5 K), measured by a K-type thermocouple and controlled electronically. The deposition parameters like the rate of spray solution (1.66 mL/min, with 5 s per spray and with total number of 50 sprays for 4.16 min), nozzle to substrate
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Fig. 3. Crystallite size and film thickness of Li (0 to 5 wt.%) doped SnO2 films as a function of Li ion concentration.
transport data of the films was measured with a four-point probe from 300 K to 448 K.
Fig. 1. Schematic of home made spray pyrolysis unit.
3. Results and discussion distance (35 cm), pressure of compressed and moisture filtered carrier gas (5–6 MPa) were kept constant at the optimized values indicated in the brackets. A rest time of 1 min was provided between each spray. The number of sprays was restricted to 50 since the films turned milky affecting the transparency for 75 sprays. The pyrolytic decomposition of stannous chloride (SnCl2.2H2O) and LiCl forms clusters of Li doped SnO2 and develops into a continuous film (usually Volmer–Weber growth mode) in the presence of water as follows, Δ 2SnO →
2SnCl2 :2H2 O + 2LiCl + 4H2 O
2
+ 2LiO + 6HCl + 3H2 z + 2H2 Oz:
ð1Þ The as-prepared films were highly adherent and transparent. The structure and phase purity were investigated by X-ray diffraction (XRD) with (Xpert high scorer, PANalytical) CuKα radiation with a scan rate of 0.02°/s. The surface morphology of the thin films was studied using a Scanning Electron Microscope (JEOL — JAPAN JSM 840A). The surface analysis was also performed using an Atomic Force microscope (AFM, Model SPI 6800N). The spectral transmittance in the UV–Vis and IR region was recorded with a double beam spectrometer in near normal incident mode (JASCO Corp., V-570, Rev. 1.00). Film thickness and optical band gap were extracted from the spectral transmittance data. The temperature dependent electrical
The XRD patterns of Li (0 to 5 wt.%) doped SnO2 thin films are shown in Fig. 2. The peaks were indexed to the tetragonal SnO2 structure [16] with (110) plane as the 100% peak. The two close lying reflections (112) and (301) were also evident indicating the polycrystalline nature of the films, which may be due to the use of water as a solvent. The calculated values of texture coefficient for (110) and (211) were less than unity confirming the polycrystalline nature of the films. However, the 1 wt.% Li doped SnO2 film presented a possible oriented growth along the (211) plane rather than along the (110) direction. The lattice parameters of undoped SnO2 thin film are, a = 4.738 Å and b = 3.186 Å. Notable change was not observed in lattice parameters with increasing Li ion concentration which is due to the comparable ionic radius of Li+ (0.68 Å) with Sn4+ (0.74 Å). The average crystallite size from the full width at half the maximum of (110) and (211) planes was estimated using Scherrer's relation [17]. The variation of crystallite size with Li concentration is shown in Fig. 3 and Table 1. The 1 wt.% Li doped film has the highest crystallite size of 49 nm, which was found to decrease for further increase in Li concentration. The SEM micrographs of Li (0 to 5 wt.%) doped SnO2 thin films are shown in Fig. 4A to F. The surface of the undoped SnO2 film is continuous with a uniform distribution of faceted bimodal particles.
Table 1 Crystallite size, t, RS, ρ, σ, Ea and Eg (direct and indirect) of Li (0 to 5 wt.%) doped SnO2 thin films. Li %
Crystallite size (nm)
t Rs at (nm) 300 K (Ω/□)
0%
42
336
5.74 × 105 19.29
0.0518
4
1.562
0.6401
1%
49
835
1.87 × 10
2%
45
386
1.88 × 104
0.725
1.377
3%
43
349
2.39 × 104
0.836
1.194
407
2.05 × 10
4
0.834
1.197
3.25 × 10
4
1.815
0.5509
4% Fig. 2. XRD patterns of Li (0 to 5 wt.%) doped SnO2 thin films indicating the polycrystalline nature.
Ea ρ at σ at (eV) 300 K 300 K −1 (Ω cm) (Ω cm)
5%
40 38
557
Ea1 = 0.974 Ea2 = 3.287 Ea1 = 0.419 Ea2 = 1.453 Ea1 = 0.159 Ea2 = 2.067 Ea1 = 0.417 Ea2 = 2.064 Ea1 = 0.689 Ea2 = 2.30 Ea1 = 0.473 Ea2 = 1.759
Egd (eV)
Egind and Ep (eV)
3.906 3.48, 0.12 3.845 3.28, 0.12 3.909 3.50, 0.10 3.906 3.50, 0.10 3.887 3.53, 0.085 3.925 3.52, 0.10
D.P. Joseph et al. / Thin Solid Films 517 (2009) 6129–6136
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Fig. 4. SEM micrographs of Li (0 to 5 wt.%, A to F respectively) doped SnO2 films showing variation in surface morphology.
The particles were found to be less than a micron in size and nearly monodispersed (Fig. 4 A). On doping with 1 wt.% of Li, the SnO2 film was found to have a twisted metal sheet like particle morphology (Fig. 4 B). Similar morphology has been reported by Elangovan et al. [18]. When Li doping increases to 2 and 3 wt.%, the particle morphology becomes spherical (Fig. 4C and D). The 4 wt.% Li doped SnO2 thin film was found to possess monodispersed equiaxed spherical particles with size much less than a micron with nominal agglomeration. The spherical particles grow in size for further increase in Li doping to 5 wt.%. The progressive variation in film morphology may be attributed to the slight variation in deposition conditions and/or due to the difference in precursor concentration resulting in the change in total flux of molecules reaching the substrate and thereby altering the nucleation and growth process [19]. The 2D AFM images of the Li (0 to 5 wt.%) doped SnO2 thin films shown in Fig. 5 (A to F) were measured in dynamic mode over a 1 µm × 1 µm area of the film surface. The 2D images of Li (0 to 5 wt.%) doped SnO2 thin films indicate that the particle size varies with Li concentration. The 1 wt.% Li doped SnO2 film was found to have particles of larger size compared to other films. This also corroborates with the size estimated from XRD results. The 1 wt.% Li doped SnO2 film was found to have the highest crystallite size of 49 nm (Table 1). The particles are not of the same height and are distributed irregularly within the measured region due to the polycrystalline nature of the films. The SnO2 film RMS surface roughness (Fig. 6) is relatively independent of Li doping except for the high value measured for the 1 wt.% Li doped samples.
The spectral transmittance data (Fig. 7) shows that the films were nearly 80% and 70% transparent in the visible region and near IR regions respectively. The observation of interference patterns (Fig. 7) in the transmittance data (Tr) is direct evidence for homogeneous and uniformly thick films [20]. The thickness (t) of the SnO2 films, was calculated using the phase shift (Psh) caused by interference associated with reflections from the front and back surfaces of the film using the relation, Psh = 4πnt = λ
ð2Þ
where, n is the refractive index of the film (n = 1.92 for SnO2), t is the film thickness and λ is the wavelength of two neighboring extrema (a peak and a valley) in the spectra. Since Psh is an integer multiple of π, and neighboring extrema differ by one, two simultaneous equations with only two unknowns were solved to determine film thickness. It is observed that the thickness of the film does not present any trend with Li concentration. The 1 wt.% Li doped SnO2 film was found to have the greatest thickness of 835 nm (Fig. 3) and the largest crystallite size of 49 nm. This may be due to the variation in deposition efficiency, which might have resulted from the diminished mass transport to the substrate and also due to gas convection that pushes the precursor away [21]. The % Tr of all the films for 550 nm measured at 300 K is shown in Fig. 8. The 4 wt.% and 5 wt.% Li doped SnO2 films were found to be more transparent than the undoped SnO2 in that the % Tr at 550 nm for this film was greater than the thinner undoped SnO2 film. This implies that Li doping in SnO2 contributes to the slight
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Fig. 5. 2D AFM images of Li (0 to 5 wt.%, A to F respectively) doped SnO2 films showing 1 wt.% L: doped film to be composed of larger particles among other films.
improvement in transparency, however, the effect of film thickness and the variations in deposition parameters could not be denied. The absorption coefficient has been calculated from the Lambert's formula, α = 1 = t ½lnð1 = Tr Þ
ð3Þ
where, Tr and t are transmittance and the film thickness respectively. The absorption coefficient was found to decrease exponentially with decreasing photon energy, indicating an Urbach characteristic [22]. The band gap of pure SnO2 is widely reported to be of direct transition in nature (3.8–4.3 eV) [4,23]. The direct band gap of Li doped SnO2 films was determined from the (αhν)2 vs hν (Tauc relation) plot by
Fig. 6. Variation of RMS roughness value of Li (0 to 5 wt.%) doped SnO2 films.
extrapolating the linear fit given to the linear region to α = 0 (Fig. 9). The direct band-gap values of Li doped SnO2 are given in Table 1. No remarkable difference or trend was observed in the variation of the value of direct energy gap of Li doped SnO2. Though literature reports indicate the direct band gap of SnO2, there are theoretical predictions for the existence of an indirect band gap for SnO2 with somewhat lower energy [24]. In case of NiO, both direct and indirect band gaps and phonon energies were calculated from the Tauc relation [25]. The (αhν)1/2 vs hν plots obtained for Li (0 to 5 wt.%) doped SnO2 films based on Ref. [25] were found to have a linear region. Two distinct linear fits were made to the linear region for phonon absorption (Egind −Ep) and phonon emission (Egind +Ep) processes. By solving the
Fig. 7. Spectral transmittance plot of Li (0 to 5 wt.%) doped SnO2 thin films.
D.P. Joseph et al. / Thin Solid Films 517 (2009) 6129–6136
Fig. 8. Optical transmittance (at 550 nm) plot of Li (0 to 5 wt.%) doped SnO2 thin films at 300 K.
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Fig. 11. Sheet resistance of SnO2 thin films as a function of Li (0 to 5 wt.%) doping at 300 K. Inset shows the resistivity of the films as a function of temperature.
simultaneous equations formed from the values of the above described fit (Fig. 10), the values of Egind and the phonon energy Ep were obtained and are given in Table 1. All Li doped SnO2 thin films were conducting at 300 K with resistivity values much less than that of undoped SnO2. The four-point probe measurement of sheet resistance (RS) is preferred over the twopoint probe technique since the contact resistance associated with the two-point probe technique is overcome. In the linear four-point probe technique, the current (I) is applied between the outer two leads and the potential difference (V) across the inner two probes is measured. Since negligible contact and spreading resistance are associated with the voltage probes, one can obtain a fairly accurate estimation of RS using the following relation, RS = 4:532ðV = I Þ:
Fig. 9. Band-gap (Egd) estimation of Li (0 to 5 wt.%) doped SnO2 films from Tauc relation.
Fig. 10. Indirect band-gap (Egind) and phonon energy (Ep) estimation of Li (0 to 5 wt.%) doped SnO2 films.
ð4Þ
In the above said configuration, a correction factor of 4.532 was applied for the sample (1 cm × 1 cm) with equally spaced (≈1 mm) probes and the film thickness necessarily being less than the spacing between the probes. The RS of the undoped SnO2 is relatively high
Fig. 12. Variation of resistivity and conductivity at 300 K of Li (0 to 5 wt.%) doped SnO2 films as a function of Li ion concentration.
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Fig. 13. (A to F). Determination of activation energy of Li (0 to 5 wt.%) doped SnO2 films from Arrhenius plot.
(5.74 × 105 Ω/□) compared to that of the Li doped SnO2. Similar sheet resistance values have been reported for spray deposited F doped SnO2 by Acosta et al. [26]. The values of sheet resistance (see Table 1)
of SnO2 films at 300 K as a function of Li ion concentration are shown in Fig. 11. RS was found to decrease by an order in magnitude on doping irrespective of variations in film thickness.
D.P. Joseph et al. / Thin Solid Films 517 (2009) 6129–6136
The results of temperature (T) dependent resistivity (current in plane geometry) from 300 K to 448 K are shown in inset of Fig. 11. The resistivity (ρ) was calculated using the relation, ρ = RS t:
ð5Þ
The resistivity of the Li doped SnO2 films was found to fall with T indicating semiconducting behavior. For increasing T, the resistivity of all the films was found to merge and stay constant at 448 K [Inset of Fig. 11]. The variation in resistivity and conductivity of Li (0 to 5 wt.%) doped SnO2 films at 300 K as a function of Li ion concentration is shown in Fig. 12. The resistivity of pure and stoichiometric SnO2 is very high, approximately 108 Ω cm [13]. The electrical resistivity of intentionally undoped SnO2 grown by many techniques is however significantly less than this value due to non-ideal growth conditions that result in non-stoichiometric, oxygen deficient films [27]. Similar resistivity measurements have also been reported for F:SnO2 in the temperature range 300 to 448 K [28]. SnO2 films deposited by spray pyrolysis technique are more susceptible to oxygen deficiencies [29]. Hence, the use of spray pyrolysis technique leads to the formation of strongly non-stoichiometric SnO2 films resulting in comparatively low resistive films. The conductivity of non-stoichiometric tin oxide films is influenced by the presence of doubly ionized vacancies serving as donors [5]. In TCO thin films, free carriers may interact with different scattering centers such as thermal lattice vibrations, and ionized and neutral impurities. The influence of grain boundaries should also be taken into account [30]. The ionized impurities and grain boundaries are found to be the main scattering mechanisms in TCO thin films [4,31]. The grain boundary is known to possess more defects, impurities, and traps than grain interiors. As the orientation of the grain changes, the density of traps in the direction of current also changes. In interpreting the electrical data of Li doped SnO2, one has to consider the various scattering mechanisms present. Fully polycrystalline Li doped SnO2 is composed of crystallites joined together by grain boundaries, which are the transitional regions between different orientations of neighboring crystallites. Thangaraju had explained transport mechanism in doped SnO2 by two models. First is a charge trapping model [32] assuming that the grain boundaries contain trapping states induced by lattice defects. Second is the dopant segregation model [33] in which grain boundaries are assumed to act as sinks for the preferential segregation of the dopants making the boundary layers inactive. In our case, since all the films are polycrystalline in nature and there is no evidence for segregation of the dopants, the first model is applicable [32]. The activation energies of 0 to 5 wt.% Li doped SnO2 films shown in Fig. 13 A to F were calculated from the slope of the linear fit given to the plot of ln (σ) against 1000 / T. The values of the estimated activation energies are given in Table 1. Two activation energies were estimated for all the films from the conductivity data obtained in the temperature range of 300 to 448 K. The activation energies of the Li doped SnO2 thin films were found to obey the relations, σ = σ 1 expð−ΔEa1 = kB T Þ
6135
regions of two distinct slopes corresponding to low temperature region (I) (up to 373 K) and high temperature region (II) (above 373 K). The existence of two such regions is reported by Viswakarma et al. [36]. The activation energy Ea1 is observed in the temperature range from 300 to 373 K whereas Ea2 is observed above 373 K. Moreover the value of Ea1 and Ea2 of the Li doped SnO2 films was comparably lower than that of the undoped SnO2 film. The values of Ea1 are quite higher than the energy required to excite free carriers from shallow donor levels. Hence the conduction is not due to thermally excited free carriers from the shallow donor levels, instead the high activation energies are indicative of increased scattering due to grain boundaries, defects, impurities etc. in the lower activation energy regime (Region I). The higher activation energy Ea2 above 373 K (Region II) is due to the presence of deep donor levels, which might have resulted from defects and impurities [29]. A plausible explanation for the reduction in activation energy Ea2 of Li doped films compared to undoped films may be the replacement of the Sn atoms at cation sites with relatively lighter Li atoms forming deep donor levels. The figure of merit is an important parameter for evaluating TCO thin films for use in solar cells. Conductivity and transmittance are inversely proportional to each other and should be as high as possible for effective usage. In order to compare the performance of various transparent conductors the most widely used figure of merit as first defined by Haacke [37] is, 10
u = Tr = RS
ð8Þ
where Tr is the transmittance at λ = 550 nm (the wavelength at which solar power conversion is maximized) [37,38] and RS is the sheet resistance. The calculated value of figure of merit (φ) as a function of Li ion concentration is depicted in Fig. 14. The figure of merit for 4 wt.% Li doped SnO2 is found to be maximum. For an ideal TCO, the figure of merit should be as high as possible, but depending on the type of application and requirement, either of the two parameters defining φ has to be compromised. Though maximum transparency and low resistivity is desired for a TCO in general, achievement of both of these properties is difficult. As observed from the electrical transport data by doping Li+ in SnO2, the resistivity is decreased considerably compared to undoped SnO2. The value of φ for Li (4 wt.%) doped SnO2 is very small when compared with the well established indium doped SnO2 TCO. The value of φ of Li (4 wt.%) doped SnO2 film is two orders less than that of fluorine doped tin oxide (FTO) (2.5 × 10− 3 Ω− 1) [31]. However, here the value of φ of
ð6Þ
and σ = σ 2 expð−ΔEa2 = kB T Þ
ð7Þ
where, σ is the d.c. conductivity, σ1 and σ2 are the pre-exponential factors, Ea1 and Ea2 are the activation energies for d.c. conduction, kB is the Boltzmann constant and T is the absolute temperature. The conduction in the range of 300 to 448 K for pure SnO2 was determined to be due to intrinsic semiconducting behavior obeying the Arrhenius law [34,35]. The Arrhenius plot of all the films shows
Fig. 14. Figure of merit of (0 to 5 wt.%) doped SnO2 films as a function of Li ion concentration.
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Li+ (4 wt.%) doped SnO2 is a better balance between the transmission and sheet resistance resulting in high φ. As far as 4 wt.% Li doped SnO2 is concerned, Li doping into SnO2 prepared under the above mentioned deposition parameters imparts transparency higher than the undoped SnO2 film. Hence, further exploration of Li doped SnO2 could make it a material suitable for versatile applications. 4. Conclusion In summary, Li (0 to 5 wt.%) doped SnO2 thin films were deposited on glass substrates by the spray pyrolysis method. An increase in Li doping was found to change film morphology from a twisted metal, sheet like structure to a spherical one. The resistivity of the Li doped SnO2 films was found to decrease by an order of magnitude with increasing Li concentration. The high transparent conducting oxide figure of merit found for the 4 wt.% Li doped SnO2 film suggests it's exploration for further application. Acknowledgements The authors thank the UGC (X-plan developmental grant sanctioned to the Department of Nuclear Physics) and DST (SR/S5/NM23/2002 dt. 29/10/2002), Govt. of India, for partial financial assistance for the construction of homemade spray pyrolysis unit. We also thank the referees and the editor for improving the clarity of the manuscript through their fruitful comments and suggestions till the final version. One of the authors (DPJ) thanks the CSIR, Govt. of India, for the award of Senior Research Fellowship (2007), Mr. E. Senthil Kumar (IITM) and Mr. A. Sendilkumar (HCU) for their help in various aspects. References [1] M. Batzill, U. Diebold, Prog. Surf. Sci. 79 (2005) 47. [2] S. Colen, Thin Solid Films 77 (1981) 127. [3] M.R. Yang, S.Y. Chu, R.C. Chang, Sens. Actuators, B 122 (2007) 269.
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