Journal of the Korean Physical Society, Vol. 67, No. 3, August 2015, pp. 563∼567
Structural and Electronic Properties of Indium-tin-oxide Sol-gel Films for Various Post-annealing Treatment Il Wan Seo, Miru Noh, Y. S. Lee,∗ J-H. Park and J-S. Chung Department of Physics, Soongsil University, Seoul 156-743, Korea
J. H. Park IT Convergence and Components and Material Research Laboratory, Electronics and Telecommunications Research Institute, Daejon 305-700, Korea
Hyuk Jin Kim Department of Energy & Environmental System Engineering, University of Seoul, Seoul 130-743, Korea
Young Jun Chang Department of Physics, University of Seoul, Seoul 130-743, Korea and Department of Energy & Environmental System Engineering, University of Seoul, Seoul 130-743, Korea (Received 26 May 2015, in final form 22 June 2015) We investigated the change in the electronic properties of indium-tin-oxide sol-gel thin films for various annealing temperatures (Tanneal ) up to 800 ◦ C. The X-ray diffraction (XRD) measurement showed that the crystallinity was enhanced continuously with increasing Tanneal . From the electrodynamic analysis performed by using spectroscopic ellipsometry, in accord with the XRD result, the charge carrier density of the films was found to increase, and resultantly the DC conductivity was found to increase with thermal annealing at a higher Tanneal . The optical absorption in the visible region was rather sizable in relation to the quite broad feature of the interband transition near 4 eV. This implies that high-temperature thermal annealing may induce some defects inside the film. Our findings for the thermally-annealed sol-gel films are compared with the optical properties of the photo-annealed sol-gel film and the commercially-used film fabricated by using a sputtering technique. PACS numbers: 78.20.-e, 78.20.Cj, 78.20.Bh, 78.40.-q Keywords: Indium tin oxide, Thermal annealing, Spectroscopic ellipsometry, Sol-gel DOI: 10.3938/jkps.67.563
oxygen vacancies [6] as well as the substitution of Sn4+ for In3+ . The best Sn doping concentration is by own to be about 10%. The fabrication of ITO films by using the sol-gel wetprocessing method has attracted much attention for its advantages of simple manufactures, nano-scale thickness controllability, low cost, and the large-area film fabrication [7,8]. One of main procedures in the sol-gel method is the annealing step at a relatively high temperature for crystallization and densification of the sol-gel ITO films. Unfortunately, this thermal annealing (TA) process appears to be incompatible for expanding the scope of its application printable electrode of low-cost on some heat-sensitive plastic substrates. As an alternative to thermal annealing, the photo-assisted annealing method is a possibility [9,10]. Very recently, we used an excimerlaser annealing (ELA) treatment for densification and crystallization of sol-gel ITO films [11]. While we found
I. INTRODUCTION A transparent conductive oxide (TCO) film is an industrially important material due to its useful application as a transparent electrode to optical electronic devices, e.g., flat panel displays and solar cell [1–3]. The greatest feature of a TCO is its combination of high transparency in the visible region and low electric resistance. Indium tin oxide (ITO) is one of the most utilized TCO materials [4]. The resistivity (ρDC ) of ITO is low as 10−3 cm or less, and the extinction coefficient (k) in the visible range is as small as or less than 0.001. It has a wide optical band gap with a band gap energy (Eg ) of 3.8 eV [5]. Its fairly low resistivity results from the high carrier concentration (n ∼ 1021 /cm3 ) generated by ∗ E-mail:
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Journal of the Korean Physical Society, Vol. 67, No. 3, August 2015
significant enhancements of the optical and the electric properties of the sol-gel ITO films via the ELA process, the detailed mechanism of ELA is not yet understood. If the physical quality of the ELA-treated film is to be superior to that of the TA-treated film, a comparative study of the changes in the physical properties of the ITO sol-gel film for the TA and the ELA processes must be performed. In this research, we investigated the change in the electronic properties of ITO sol-gel films with variating annealing temperature (Tanneal ) up to 800 ◦ C. While the optical and the electrical properties of the films are improved with higher-Tanneal TA treatment, the moderate absorption in the visible remains even for the highestTanneal film, which is very easily distinguished from the strong suppression of visible absorption in the ELAtreated film.
II. EXPERIMENT The ITO films were fabricated by using the sol-gel method. The metal precursors of ITO, In(NO3 )3 · xH2 O and SnCl2 , were dissolved in 2-methoxyethanol (2-ME) at 0.3 M. The mixing ratio of the In and the Sn solutions was 9:1, which is known to be the optimal ratio for a transparent conducting behavior. With the SiO2 (300 nm)-covered Si substrates, we performed spin-coating at a speed of 3000 rpm for 30 seconds. After the spincoating, the films were pre-annealed at 150 ◦ C for 30 min on a hotplate (as-spun film). Then, we annealed the as-spun films at annealing temperatures 400, 600, and 800 ◦ C for 2 hours (TA-ITO film). X-ray diffraction (XRD) patterns were measured with a Bruker-AXS Discover D8 system with a Cu target Xray tube. The X-ray beam was focused to a parallel beam by using a Gobel mirror to enhance the intensity for the thin-film measurement. We performed the 2θ scans at a 3◦ grazing incidence. The scan speed was 0.1 step/sec with increments of 0.05◦ . For the spectroscopic study on the ITO films, we performed spectroscopic ellipsometry (SE) with various incident light angles (60◦ , 65◦ , and 70◦ ) in the range of 0.74 eV to 5.5 eV at room temperature. The measured ellipsometry angular spectra Ψ(ω) and Δ(ω) were fitted with the three-layer model (ITO film + SiO2 (300 nm) + Si single crystal) to obtain the complex optical constants, e.g., the complex refractive index, n (ω) = n + ik, and the complex conductivity spectra, σ (ω) [≡ σ1 (ω) + iσ2 (ω)], of the ITO films. The fitting spectra were composed of Drude and Lorentz oscillator modes. We also measured the normal-incidence reflectivity spectra R(ω) for the spectral range 0.1 − 6 eV and found that the measured R(ω) agreed with the corresponding reflectivity spectra calculated from the ellipsometry data. With a combination of the SE and the reflectivity measurements, we determined the optical constants of our ITO films in the photon energy region
Fig. 1. (Color online) XRD 2θ scan data for the as-spun and the TA-treated ITO films annealed at Tanneal = 400 ◦ C, 600 ◦ C, and 800 ◦ C. Inset: the integrated intensity of the (222) peak as a function of Tanneal .
from 0.1 eV to 6 eV.
III. RESULTS AND DISCUSSION Figure 1 shows XRD 2θ scan results at 3◦ grazing incidence for the TA-ITO films annealed at Tanneal = 400 ◦ C, 600 ◦ C, and 800 ◦ C. For comparison, the data of the as-spun film are included. In contrast to the case of the as-spun film, the XRD peaks were clearly observed in the TA-ITO films, indicative of the crystallization of the ITO compound by the thermal annealing. The dominant peaks near 2θ = 30◦ and 35◦ were assigned as the ITO (222) and ITO (400) peaks, respectively [12]. As the Tanneal was increased, the XRD peaks developed. For quantitative analysis, we estimated the integrated intensities (Iint ) of the XRD peaks, and we display them in the inset of Fig. 1. Increase Iint implies that the crystallinity of the film is enhanced with increasing Tanneal . Concurrently, the full widths at half maxima (FWHMs) of the peaks became narrower, which is indicative of large-sized grains. According to the Scherrer formula [13], we estimated the grain sizes to be 9.1 ˚ A for Tanneal = 400 ◦ C ◦ ˚ and 12.2 A for Tanneal = 800 C. The lattice constant of the films with cubic structures was estimated to be 10.13 ˚ A without any sizable change in Tanneal. This value is quite comparable to the fully-relaxed bulk value [14]. Figure 2 shows the real parts of optical conductivity spectra σ1 (ω) of the TA-ITO films annealed at different Tanneal . In contrast to the case of the as-spun film where one cannot see any meaningful electronic response in the measured spectral range (thick dashed line in Fig. 2(a)), the spectra of the TA-ITO films commonly
Structural and Electronic Properties of Indium-tin-oxide Sol-gel Films · · · – Il Wan Seo et al.
Fig. 2. Real part of the optical conductivity spectra of the TA-ITO films for varying Tanneal . In (a), the dotted curve represents the data for the as-spun film. Arrows mark the optical bandgap energies. Inset: optical bandgap of the TAITO films as a function of Tanneal . For the as-spun film, the pre-annealing temperature was considered as Tanneal .
show three features: a coherent mode in the low energy region, transparency in the visible region, and interband transitions above 4 eV. As Tanneal increases, the coherent mode develops continously. The result of a detailed analysis on the coherent mode will be discussed later. The optical spectra at energies above 4 eV show two interband transitions, which correspond to the transitions of the acceptor level and conduction band from the O 2p band [15]. With increasing Tanneal , the lower energy of the interband near 4.5 eV becomes narrower but its peak position remains unchanged. As a result, the optical bandgap, which was estimated from the crossing point of abscissa with a linear extrapolation of σ1 (ω), increased (the inset of Fig. 1(c)). Also, the weak absorption in the visible, originating from the lower energy tail of the interband transition, was suppressed gradually. On the other hand, the higher energy of the interband transition shifts to lower energies. While the detailed mechanism is yet to be understood, some theoretical studies have reported that the conduction bands shift down with the compound structure turning from an amorphous to a crystalline phase [16]. To obtain some insight into the electrodynamics in the ITO films, we derived the electrodynamic parameters of the coherent mode by using the conventional Drude model. According to the Drude model, σ1 (ω) is
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Fig. 3. Electrodynamic parameters as functions of Tanneal : (a) charge carrier density (n), (b) scattering rate (ΓD ), and (c) DC conductivity (σDC ). (d) The film’s thicknesses (d) estimated from the spectroscopic ellipsometry analysis. For the as-spun film, the pre-annealing temperature was considered as Tanneal .
expressed as follows: σ1 (ω) =
ωp2 ΓD . 4π Γ2D + ω 2
(1)
Here, ωp is the plasma frequency of the charge carrier and ΓD is the scattering rate of the charge carrier. The square of the plasma frequency ωp2 corresponds to n/m∗ (n: carrier density, m∗ : effective mass), so it may be converted to n, by assuming that m∗ to be unity. The fitting result is shown in Fig. 3. With increasing Tanneal , the carrier density (n) is increased whereas the scattering rate is not changed significantly. Thus, the gradual enhancement of the dc conductivity is attributed mainly to the increase in the carrier density in our films. The film’s thickness is seen to be decreased by the thermal annealing process. The film’s thickness was estimated from the spectroscopic ellipsometry analysis. The thickness of the ITO film is reduced from 53 nm for the as-spun film to ∼20 nm for the TA-ITO films, as shown in Fig. 3(c). This behavior is associated with the densification of the ITO film due to the decomposition of the complex organics in the precursor. The film thickness is noted not to depend strongly on the Tanneal . This implies that Tanneal = 400 ◦ C appears to be high enough for the densification of the ITO film. Our results show that both the structural and the electrical properties of our ITO films are enhanced continuously as Tanneal is increased up to 800 ◦ C. Some previous studies reported that the resistance minimum occurred near Tanneal = 500 − 600 ◦ C because the tin oxide and the indium oxides might be reduced and/or an additional scattering channel might be open at higher Tanneal [17, 18]. In a practical fabrication procedure, TA at Tanneal
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Journal of the Korean Physical Society, Vol. 67, No. 3, August 2015
Table 1. Summary of the electrodynamic parameters and the optical features of commercially-available ITO film (GEOMATEC, Japan), TA-ITO film (Tanneal = 400 ◦ C and 800 ◦ C), and ELA-ITO film (JL = 240 mJ/cm2 ).
Sample Commercial ITO TA-ITO (400 ◦ C) TA-ITO (800 ◦ C) ELA-ITO (JL = 240 mJ/cm2 )
Carrier density (n) (×1020 cm−3 )
Scattering rate (ΓDC ) (cm−1 )
Extinction coefficient (k) at 2.5eV
Band gap (Eg ) (cm−2 )
DC conductivity (σDC ) (Ω−1 cm−1 )
20.7
1030
0.0013
3.89
3150
2.0
675
0.054
3.74
450
6.8
580
0.021
3.82
1750
4.8
680
0.0012
3.67
710
Fig. 5. (Color online) Real parts of the optical conductivity spectra of commercially-available ITO film (GEOMATEC, Japan), TA-ITO film (Tanneal = 400 ◦ C and 800 ◦ C), and ELA-ITO film (JL = 240 mJ/cm2 ). The spectra in the xaxis range below 1 eV are plotted logarithmically.
Fig. 4. (Color online) Complex optical constants, (a) refractive index (n(ω)) and (b) extinction coefficient (k(ω)), with varying Tanneal .
= 400 ◦ C has often been employed [10]. These variations indicate that the optimal temperature for thermal annealing appears to depend on the details of the precursor and the annealing procedure. Figure 4 shows the complex optical constants, refractive index (n(ω)) and extinction coefficients (k(ω)) of the ITO films. In Fig. 4(a), the low-energy suppression in n(ω) indicates the existence of charge carriers. The
weak peak structure near 4 eV indicates the interband transition. As Tanneal is increased, the n value in the visible region decreases slightly. Figure 4(b) exhibits the k(ω) of the ITO films. The k(ω) spectra clearly follow the Drude mode at energies below 1 eV and the interband transition features at energies above 4 eV. As Tanneal is increased, the k value in the visible region decreases gradually down to less than 0.02, leading to better transparency. Finally, we compare the optical properties of the TAITO film with those of the ELA-ITO films and the commercially-supplied ITO films fabricated by using the sputtering technique. Figure 5 shows the σ1 (ω) of these three films. The commercially-available ITO film shows a typical TCO behavior: a well-developed coherent mode
Structural and Electronic Properties of Indium-tin-oxide Sol-gel Films · · · – Il Wan Seo et al.
and very small k value in the visible region. Compared with the case of the commercially-available ITO film, our sol-gel films show lower conductivity by a factor of 2 − 3 even though the scattering rate is much smaller. The suppression of charge carrier scattering may originate from a reduction in the electron-electron scattering at a relatively lower charge carrier density. The electrical property of the ELA-ITO film is noted to be comparable to that of the TA-ITO film. This implies that the ELA process may be an alternative to the high temperature TA treatment. On the other hand, the interband transition feature in the ELA-ITO film is much weaker than those of the other films. The development of the electronic structure appears not yet to be completed. Another notable point is that the lower energy tail of the interband transition is sizable in the TA-ITO film. Band-edge broadening has been observed when defects exist inside the samples. In this sense, thermal annealing may generate more defects, e.g., oxygen vacancies, in the film, compared to other films. The optical parameters of the ITO films examined in the current study are summarized in Table 1.
IV. SUMMARY We investigated the change in the electronic properties of ITO sol-gel thin films with varying annealing temperature (Tanneal ). The X-ray diffraction (XRD) measurement showed that the crystallinity was enhanced with increasing Tanneal . From the spectroscopic ellipsometry, in accord with the XRD result, the electric transport was found to be improved continuously with increasing Tanneal , comparable to the case of an excimer-laserannealed film. The optical absorption in the visible region was weak, but still detectable, which is in stark contrast to negligible absorption in the excimer-laserannealed ITO sol-gel films. While our sol-gel films have some room for improvements in their transport and optical properties compared with commercially-used ITO film, excimer-laser-annealed film films could be rivals to the thermally-annealed ones.
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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Korea government (MOE) (NRF-2013R1A1A2012281, NRF2013R1A1A2007239, and NRF-2014R1A1A1002868).
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