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Magnetron sputtered transparent conductive zinc-oxide stabilized amorphous indium oxide thin films on polyethylene terephthalate substrates at ambient temperature
Thin Solid Films 532 (2013) 79–83
Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Magnetron sputtered transparent conductive zinc-oxide stabilized amorphous indium oxide thin films on polyethylene terephthalate substrates at ambient temperature Y. Yan ⁎, X.-F. Zhang, Y.-T. Ding Beijing Institute of Aeronautical Materials (BIAM), P.O. Box 81–83, Beijing 100095, China
a r t i c l e
i n f o
Available online 6 January 2013 Keywords: Transparent conducting oxide Amorphous zinc-stabilized indium oxide Amorphous indium zinc oxide (a-IZO) Room temperature
a b s t r a c t Amorphous transparent conducting zinc-oxide stabilized indium oxide thin films, named amorphous indium zinc oxide (a-IZO), were deposited by direct current magnetron sputtering at ambient temperature on flexible polyethylene terephthalate substrates. It has been demonstrated that the electrical resistivity could attain as low as ~5×10−4 Ω cm, which was noticeably lower than amorphous indium tin oxide films prepared at the same condition, while the visible transmittance exceeded 84% with the refractive index of 1.85–2.00. In our experiments, introduction of oxygen gas appeared to be beneficial to the improvement of the transparency and electrical conductivity. Both free carrier absorption and indirect transition were observed and Burstein–Moss effect proved a-IZO to be a degenerated amorphous semiconductor. However, the linear relation between the optical band gap and the band tail width which usually observed in covalent amorphous semiconductor such as a-Si:H was not conserved. Besides, porosity could greatly determine the resistivity and optical constants for the thickness variation at this deposition condition. Furthermore, a broad photoluminescence peak around 510 nm was identified when more than 1.5 sccm oxygen was introduced. © 2013 Elsevier B.V. All rights reserved.
1. Introduction It is of great interest that, as an excellent host oxide, crystalline and amorphous transparent conducting indium oxide thin films, such as indium tin oxide (ITO), indium molybdenum oxide [1], titanium doped indium oxide [2] and zinc-oxide stabilized indium oxide (namely, indium zinc oxide, IZO) [3], have predominated in both the scientific and technological applications for a long time. Generally, amorphous or nanocrystalline ITO, deposited at ambient temperature from In2O3:SnO2 = 90:10 wt.% as a target, demonstrates several disadvantages compared with crystalline ITO samples prepared under higher temperature, such as poor electrical performance, instability in harsh environments and inferior pattern etch-ability due to a great potential of transformation to the poly-crystalline state. To hurdle these shortcomings, amorphous transparent conducting oxide films, such as a-InGaZnO4 and a-IZO have been developed by Kamiya and Hosono [4], Perkins et al. [3], etc. It was reported that a-IZO films exhibit higher electrical conductivity, together with a very smooth surface, a comparatively low stress level [5] and controllable etching characteristics. Moreover, the a-IZO films show remarkable thermal stability up to 600 °C [3] while keeping the amorphous state. Like yttria-stabilized zirconia, doping with zinc oxide stabilizes the amorphous state of indium oxide thin films within a wide range of temperature and thickness. Warasawa et al. [6] reported that replacement of ZnO:Ga as the window layer by a-IZO in copper indium gallium diselenium solar cells could increase the efficiency of solar ⁎ Corresponding author. Tel.: +86 10 62496499; fax: +86 10 62497654. E-mail address: [email protected] (Y. Yan). 0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2012.12.101
cells. However, even though single composition 10.7 wt.% ZnO in In2O3 is rapidly becoming a de facto standard for the flat panel display industry, it is still necessary to confirm the above good performances because all performances are process-dependent (i.e., target power density, oxygen partial pressure or oxygen ratio, sputtering atmosphere). Furthermore, the supply of target materials is not sufficient compared with conventional ITO target materials. In this work, the influences of oxygen ratios and thicknesses on the structure, morphological, optical and electrical properties of IZO films have been studied. It is found that IZO films deposited on flexible polymeric substrates show good performance which have great potential to be used in a heat-sensitive field. 2. Experimental details IZO thin films were deposited on hard coating side of the flexible 188 μm-thickness polyethylene terephthalate (PET, SH71S type, SKC) and on Si (100) wafer, respectively, using a sintered conducting indium zinc ceramic oxide target (In2O3:ZnO= 88:12 wt.%, Φ70 × 5 mm, purity of 99.99%) by a direct current magnetron sputtering method at ambient temperature. Polished Si wafers were cleaned using ultrasonic cleaning in acetone for 15 min to get rid of organic contaminants, and then rinsed with de-ionized water for 15 min to remove contaminant particles. The distance of the substrate-to-the target was about 80 mm. All samples were fabricated with a power density of 2.325 W/cm 2 and the depostion rate was about 12.5 nm/min. The substrate holder rotated with a velocity of 2 rpm to improve the uniformity. The deposition system was evacuated to less than 3 × 10−3 Pa, using both rotary and
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diffusing vacuum pumping systems. The sputtering was carried out at a pressure of 0.36–0.37 Pa in an Ar and O2 mixture and the Ar flow rate was kept at 30 sccm (standard cubic centimetre/minute). The oxygen was introduced from the chamber wall. The crystallinity of the IZO films was investigated through X-ray diffraction (XRD, D/max 2500, RIGAKU), using a Cu Kα (1.5405 Å) radiation operating at 40 kV and 40 mA, with a grazing incidence angle of 1°. The surface morphology of IZO films was measured by atomic force microscopy (Slover 7, NT-MDT) using the taping mode, and the tip radius is about 15 nm. Sheet resistance of the samples was measured with a four point co-linear probe method and electrical transport properties were evaluated by Hall measurements at room temperature using the van der Pauw geometry. The thickness and optical constants, i.e., refractive index and extinction coefficient were determined from samples on Si substrates by spectroscopic ellipsometry (SE800, SENTECH) based on a step scan analyzer principle using the Drude–Lorentz model in 300–800 nm. The optical transmission and reflection were measured using a UV–vis-NIR double-beam spectrophotometer (Cary 5000, VARIAN) in the wavelength range from 300 to 3000 nm. Reflection measurements were performed with VW configuration using an aluminium mirror at 7° as a reference. Room temperature photoluminescence spectra of the films are recorded using a Raman spectrometer (Spex-1403, SPEX) with excitation wavelength of a He–Cd laser at 325 nm. 3. Results & discussion 3.1. Structural and morphological properties of IZO films The XRD pattern shown in Fig. 1 demonstrates that all the as-deposited IZO films were amorphous with only some characteristic diffraction peaks of the orientated PET substrates and the hard coating on top of them [7,8]. Fig. 2 shows the AFM image of IZO films on PET substrate. A surface with an RMS roughness less than 1.0 nm was obtained, which was beneficial to the organic optoelectronics. 3.2. Optical properties of IZO films The oxygen flow rate has as a profound role in tailoring the physical properties of the IZO films. The carrier generation is greatly affected by oxygen vacancies created. The optical transmission and reflection spectra are shown in Fig. 3 in the wavelength range 300–3000 nm for IZO samples prepared with increasing oxygen flow rate from 0 to 2.5 sccm. It is observed that, in the entire visible and near-infrared (NIR) region, the transmission is increasing gradually with increasing oxygen flow rate. However, with the oxygen flow rate higher than
Fig. 1. XRD profiles of about 230 nm-thickness as-deposited IZO films on PET with oxygen flow rate from 0 to 2.5 sccm.
Fig. 2. AFM image of (the scan area is 2 × 2 μm and the z-scale is 6.0 nm) an IZO film on PET with the oxygen flow rate of 0.5 sccm.
1.0 sccm, no remarkable enhancement in transmission between 500 and 1500 nm could be obtained. As shown in the inset of Fig. 3, a significant red-shift of the absorption edge could be observed. Definitely, the plasma wavelength λp, is defined by the intersecting point of the real and imaginary parts of the complex refractive index or the minimum of the reflection curves [9]. Unfortunately, the interference fringes caused by the μm-scale hard coating on PET made the minimum of the reflection curve difficult to detect. Whereas, the wavelengths at the points of intersection of the transmission and reflection curves, λTR, can be used as an evaluation for the plasma wavelength λp, because it was the watershed between the high reflective and transmissive zones, marked as grey round dots in Fig. 3. It should be noted that the intersecting points for the curves in Fig. 3 with oxygen flow rates of 2.0 and 2.5 sccm are higher than the upper-limit of this spectrometer. Nonetheless, it does not influence the estimation of the red shift of λTR based on the tendency of the transmission and reflection curves in Fig. 3. According to 2
ωp ¼
2
ne e ε0 ε∞ m
ð1Þ
Fig. 3. Transmission and reflection spectra of IZO films on PET with thickness about 230 nm prepared with increasing oxygen flow rate from 0 to 2.5 sccm. The inset shows the magnifier of transmission in the wavelength range of 340–360 nm. Note that the transmissions of IZO films on PET were normalized to the one of bare PET substrate.
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where ne is the carrier concentration in metal-like films, e is electrical charge unit, m⁎ is the effective mass compared with static mass of electron and ε0 and ε∞ are static and high-frequency dielectric constants. Usually, the effective mass is a weak-varied function of the variables [10], then λp ¼
4π2 c2 ε0 ε∞ m ne e2
!1=2 ð2Þ
where c is the velocity of light in vacuum. From Fig. 3, it can be observed that the plasma wavelength increased with increasing oxygen flow rate. According to Eq. (2), it can be concluded that the carrier concentration would decrease gradually when the oxygen flow rate increased which would be confirmed by the results of the Hall measurement indicated in Fig. 7. Based on Eq. (2), the calculated estimated values of the effective masses of conducting electrons were 0.52 m0, 0.57 m0, and 0.60 m0 which were higher than the value reported by Kumar et al. [10]. The absorption coefficient α of IZO films could be calculated using Eq. (3): 1 T ðλÞ 1 1−RðλÞ α ðλÞ ¼ − ln ¼ ln : d 1−RðλÞ d T ðλÞ
ð3Þ
And for amorphous highly degenerated oxide semiconductors based on the assumption of direct allowed transitions to an empty parabolic conduction band, the correlation of the absorption coefficient α and optical band gap Eopt was described by Tauc equation as: 1=2 ðαhυÞ ¼ const: hυ−Eopt
Fig. 5. Tauc plot of as-deposited IZO films on PET with thickness of about 230 nm.
Like classic amorphous semiconductor, such as hydrogenated amorphous silicon (a-Si:H), there exists a weak Urbach absorption region where visible absorption coefficient varies exponentially with the energy of the incident light. As a parameter for characterize the distribution of band tail state, the band tail width, E0, could be determined by E0 ¼
dð lnα Þ −1 : dðℏωÞ
ð5Þ
where d and TIZO = TPET/IZO/TbarePET are the thickness and the normalized transmission of IZO films, respectively. The ultraviolet–visibleNIR absorption spectra of as-deposited a-IZO films at room temperature were shown in Fig. 4 which is very similar with Fig. 1 shown in Ref. [11], as a classic degenerate amorphous oxide semiconductor. The fundamental absorption raised beyond about 3.2 eV and α ∝ λ p (p = 2–3)-type free-carrier absorption is observed below around 1.5 eV. With increasing oxygen flow rate, a tiny red shift in the fundamental edge (from 3.46 eV to 3.42 eV, Tauc plot was shown in Fig. 5), together with a decrease in the free-carrier absorption, suggests a Burstein–Moss effect for which the Fermi level lies in the conduction band. To some extent, the fluctuation of Eopt reflected the uncertainty of amorphous structure. Another absorption edges around 2.0 eV were supposed to the indirect optical band gap. Similar with amorphous ITO films, allowed indirect transition was also observed which indicated the complicated band structure of indium oxide films [12,13].
As shown in Fig. 6 the linear correlation was not conserved between E0 and Eopt which usually appeared in a-Si:H [14]. This phenomenon could be reproduced well and minus Eopt on the right y-axis in Fig. 6 was introduced here only to compare the variation trend of E0 and Eopt when the oxygen flow rate increased. Moreover, the values of E0 of a-IZO films are higher than those of a-Si:H(~0.1 eV). So amorphous IZO films seem to possess several characteristics of both conventional degenerate crystalline ITO and classic amorphous semiconductor(i.e. a-Si:H). The higher value of E0 meant higher density of state at band tail of the amorphous semiconductor, which was detrimental to the electrical properties of a-IZO films especially for the specific resistivity when more than 0.5 sccm oxygen introduced. This could be confirmed by the results via Hall measurements illustrated in Fig. 7. As we know that optical constants can be determined by porosity and intrinsic specific dielectric constant. It has been found that the refractive index changed from 1.85 to 2.00 and the extinction coefficient decreased from 0.15 to 0.01 while the thickness of IZO films increased from about 25 to 230 nm. Comparing this tendency with Fig. 6, it can be concluded that, in this processing conditions, porosity
Fig. 4. Optical absorption spectra of as-deposited a-IZO films with about 230 nm thickness measured at room temperature.
Fig. 6. The band tail width E0 and the minus optical band gap −Eopt of IZO films on PET with the variation of oxygen flow rate.
ð4Þ
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Fig. 8. Hall mobility-carrier density plot of all IZO films on PET compared with the ionized impurity scattering caused only by oxygen vacancy using Brook–Herring–Dingle formula [22].
Fig. 7. Hall effect measurement results of a-IZO films deposited with various oxygen flow rates and different thicknesses at 25, 125 and 230 nm, respectively.
played a prominent role in tuning the electrical and optical properties of a-IZO films. 3.3. Electrical properties of IZO films Generally, targets with different doping concentrations require different amounts of oxygen in the sputtering process to obtain a minimum resistivity in the coating. Therefore, the minimum resistivity in each IZO target is achieved by tuning the oxygen flow rate carefully. Unlike reported that the maximal conductivity was obtained with no oxygen in the sputtering atmosphere [8,15], the appropriate addition of oxygen appears to result in the improvement of both electrical and optical properties of transparent conducting a-IZO films, which is in accordance with results reported by several authors [16–19]. This discrepancy may be mainly caused by different doping concentrations and diverse process configurations. One distinct trait of a-IZO films is its higher Hall mobility arisen from the insensitivity of s states to orientation disorder [4], which would be beneficial for NIR transparency and lower free carrier absorption in the visible region. Hall measurement results are shown in Fig. 7. It is observed that the carrier concentrations decrease and Hall motilities increase when the oxygen flow rate increases from 0 to 2.5 sccm regardless of thickness variation. The opposite trends between the carrier concentrations and Hall motilities would make resistivity reach a minimum at an appropriate oxygen flow rate. In our experiment, the minimal resistivity of about 5 × 10−4 Ω cm was achieved when 1.0 sccm oxygen was introduced. According to defect chemical reaction formula
2−
2O
••
−
↔ O2 ðgÞ þ 2V O þ 4e :
source of carrier generation while the enhancement of transparency and conductivity necessitates small amount of oxygen. However, the origin of conductivity in transparent oxide semiconductors is still controversial. It has been postulated for a long time that transparent conductivity is related to the existence of shallow donor levels near the conduction band, formed by oxygen vacancies. Even in amorphous oxide films, oxygen vacancies are believed to be responsible for controlling the carrier concentration [20]. Nonetheless, the mechanism of carrier generation may not be so simple. Unlike the substitution mechanism in the crystalline ITO, according to the calculated results [21], under the n-type environment, both oxygen vacancy (VO) and indium anti-site on zinc (InZn) have low formation energies and they tend to form a defect complex at a certain condition. In addition, the dominant scattering mechanism was inferred to the ionised impurity scattering based on the plot between the Hall mobility and carrier concentration in Fig. 8 that the mobility increases as the net carrier concentration decreases and the carrier concentration was around 5 × 10 20 cm −3 [22]. Consequently, the dopant with less concentration would be favourable to achieve ever lower electrical resistivity validated by Hara et al. [23] who gained resistivity as low as 2.9 × 10 −4 Ω cm with 130 nm thickness on polycarbonate foils using doping weight ratio of 7.5 wt.% without deliberately heating. As a degenerate metal-like semiconductor, the electron mean free path (eMFP) can be calculated by using 1=3 2 L ¼ 3π ne hμ H =e:
ð7Þ
ð6Þ
It was postulated that the introduction of more oxygen reactive gas annihilates the oxygen vacancies which were presumably the
Fig. 9. Electron mean-free-path variation as a function of thickness (about 25, 125, 230 nm) and oxygen flow rate.
Y. Yan et al. / Thin Solid Films 532 (2013) 79–83
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Unfortunately, the origin of the PL peak is still unknown. It was thought that blue PL can be attributed to oxygen vacancies and indium-oxygen vacancy centres [27]. No such peaks have been discovered in this experiment. Various defects in materials would trigger PL and the results reported in literature are dispersed. Perhaps, combined with other characterizing methods, PL could be a good approach to clarify the defects and carrier generation as well as properties variation in practical applications in the future. 4. Conclusion
Fig. 10. PL spectra of IZO films with the thickness of about 230 nm on PET substrates and PL spectrum of bare PET was also shown here as a reference.
The calculated eMFPs are shown in Fig. 9. The introduction of oxygen gas augmented the eMFP evidently, which corresponds with the Hall mobility values shown in Fig. 6 as eMFP was a measurement of scattering effect for conducting electrons. In many cases, the properties of thin films would vary with increasing thickness. That is because, with the increasing time for a given processing condition, more energies delivered to the growing films, which resulted in the enhancement of crystallinity. Unlike the tendency in ITO [24], IZO films shown amorphous nature with wide range of thickness (these XRD profiles are not shown here). The thickness dependence of electrical resistivity of a-IZO films with different oxygen flow rates was also shown in Fig. 7. The electrical resistivity of semiconductors is determined by carrier concentration and Hall mobility. It can be observed that the electrical resistivity decreased when the thickness increased from about 25 to 230 nm at each rate level of the oxygen flow. The improvement of resistivity can be attributed to the reduction of the density of structural imperfection or porosity. Besides, the enhancement of resistivity was profited from the enhancement of Hall mobility caused by introducing oxygen. Interestingly, Hall motilities were not changed greatly when the thickness increased from 25 to 230 nm. 3.4. Photoluminescence (PL) properties of IZO films PL was associated with defects in material and excitation processes. Consequently, it can be employed to investigate the defect types, states and densities which are crucial for tuning properties of functional thin films. Here it is worth noting that, the analysis should be carried out more carefully because of the mutable nature of these defects, intrinsic or extrinsic. Furthermore, quantitative analysis is very challenging to perform. Normally, PL phenomena were not reported in bulk indium oxide or amorphous indium oxide films. However, UV and/or visible PL were found in crystalline (doped or not) indium oxide films [25] or nanostructures [26]. In our experiments, the emission PL spectra of IZO films deposited on PET substrate with different oxygen flow rates were exhibited in Fig. 10. From the PL curve of bare PET, it could be identified that three distinct PL peaks were located at 368, 387 and 397 nm, which may be caused by PET substrate with hard coating. Meanwhile, band edge emissions of indium oxide also have been reported in this region [27]. So it is ambiguous to extinguish the contributions of these PL peaks. When the oxygen flow rate was less than 1.5 sccm, the PL intensities were quite weak. However, when the oxygen flow rate increased up to 1.5 sccm, an extra broad PL peak appeared at around 510 nm, which was also observed by Hsin et al. in zinc slightly doped indium oxide nanowires [26].
Amorphous IZO thin films were deposited on hard-coating coated PET substrates using a DC magnetron sputtering technique at ambient temperature. The combined transparent and electrical conductive properties were obtained and the optimal resistivity reached 5 × 10−4 Ω cm when the oxygen flow rate was 1 sccm. Moreover, the free carrier absorption, the indirect transition and the Burstein–Moss effect were observed in a-IZO films. However, the linear relation between Eopt and E0 usually found in a-Si:H was not conserved in a-IZO. The interactive relationship between the variation of the band tail width, E0, and the specific resistivity, ρ could be qualitatively established initially. Also, it was indicated that porosity could greatly determine the resistivity and optical constants for the thickness variation at this deposition condition. Besides, a broad PL peak around 510 nm was found when more than 1.5 sccm O2 was introduced during the deposition of IZO films. Acknowledgements This work was supported by the Yicai Youth Fund established by Beijing Institute of Aeronautical Materials, under Grant No. YF53110830. References [1] Y. Yoshida, D.M. Wood, T.A. Gessert, T.J. Couts, Appl. Phys. Lett. 84 (2004) 2097. [2] M.F.A.M. van Hest, M.S. Dabney, J.D. Perkins, D.S. Ginley, M.P. Taylor, Appl. Phys. Lett. 87 (2005) 032111. [3] M.P. Taylor, D.W. Readey, M.F.A.M. van Hest, C.W. Teplin, J.L. Alleman, M.S. Dabney, L.M. Gedvilas, B.M. Keyes, B. To, J.D. Perkins, D.S. Ginley, Adv. Funct. Mater. 18 (2008) 3169. [4] T. Kamiya, H. Hosono, NPG Asia Mater. 2 (2010) 15. [5] T. Sasabayashi, N. Ito, E. Nishimura, M. Kon, P.K. Song, K. Utsumi, A. Kaijo, Y. Shigesato, Thin Solid Films 445 (2003) 219. [6] M. Warasawa, A. Kaijo, M. Sugiyama, Thin Solid Films 520 (2012) 2119. [7] M.M. Nasef, J. Appl. Polym. Sci. 84 (2002) 1949. [8] E.L. Kim, S.K. Jung, S.H. Sohn, D.K. Park, J. Phys. D: Appl. Phys. 40 (2007) 1784 B. [9] M. Fox, Optical Properties of Solids, Oxford Univ. Press, Oxford, 2001, p. 155. [10] B. Kumar, H. Gong, R. Akkipeddi, J. Appl. Phys. 98 (2005) 073703. [11] K. Shimakawa, S. Narushima, H. Hosono, H. Kawazoe, Philos. Mag. Lett. 79 (1999) 755. [12] P. Erhart, A. Klein, R.G. Egdell, K. Albe, Phys. Rev. B 75 (2007) 153205. [13] X. Yin, W. Tang, X. Weng, L. Deng, J. Phys. D: Appl. Phys. 42 (2009) 025104. [14] G.D. Cody, T. Tiedje, B. Abeles, B. Brooks, Y. Goldstein, Phys. Rev. Lett. 47 (1981) 1480. [15] B. Yagliolu, Y.-J. Huang, H.-Y. Yeom, D.C. Paine, Thin Solid Films 496 (2006) 89. [16] Y.S. Rim, S.M. Kim, K.H. Kim, J. Korean Phys. Soc. 54 (2008) 1267. [17] Y.L. Li, D.Y. Lee, S.R. Min, H.N. Cho, J. Kim, C.W. Chung, Jpn. J. Appl. Phys. 47 (2008) 6896. [18] D.Y. Ku, I.H. Kim, I. Lee, K.S. Lee, T.S. Lee, J.-h. Jeong, Thin Solid Films 515 (2006) 1364. [19] Y.B. Xiao, S.M. Kong, E.H. Kim, C.W. Chung, Sol. Energy Mater. Sol. Cells 95 (2011) 264. [20] J.R. Bellingham, W.A. Phillips, C.J. Adkins, J. Phys. Condens. Matter 2 (1990) 6207. [21] H. Peng, J.-H. Song, E.M. Hopper, Q. Zhu, T.O. Mason, A.J. Freeman, Chem. Mater. 24 (2012) 106. [22] N. Ito, Y. Sato, P.K. Song, A. Kaijio, K. Inoue, Y. Shigesato, Thin Solid Films 496 (2006) 99. [23] H. Hara, T. Hanada, T. Shiro, T. Yatabe, J. Vac. Sci. Technol. A 22 (2003) 1726. [24] C.-H. Yang, S.-C. Lee, T.-C. Lin, S.-C. Chen, Thin Solid Films 516 (2008) 1984. [25] M.-S. Lee, W.C. Choi, E.K. Kim, C.K. Kim, S.-K. Kim, Thin Solid Films 279 (1996) 1. [26] C.L. Hsin, J.H. He, L.J. Chen, Appl. Phys. Lett. 88 (2006) 063111. [27] D. Beena, K.J. Lethy, R. Vinodkumar, A.P. Detty, V.P. Mahadevanpillai, V. Ganesan, Optoelectron. Adv. Mater. Rapid Commun. 5 (2011) 1.
Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Magnetron sputtered transparent conductive zinc-oxide stabilized amorphous indium oxide thin films on polyethylene terephthalate substrates at ambient temperature Y. Yan ⁎, X.-F. Zhang, Y.-T. Ding Beijing Institute of Aeronautical Materials (BIAM), P.O. Box 81–83, Beijing 100095, China
a r t i c l e
i n f o
Available online 6 January 2013 Keywords: Transparent conducting oxide Amorphous zinc-stabilized indium oxide Amorphous indium zinc oxide (a-IZO) Room temperature
a b s t r a c t Amorphous transparent conducting zinc-oxide stabilized indium oxide thin films, named amorphous indium zinc oxide (a-IZO), were deposited by direct current magnetron sputtering at ambient temperature on flexible polyethylene terephthalate substrates. It has been demonstrated that the electrical resistivity could attain as low as ~5×10−4 Ω cm, which was noticeably lower than amorphous indium tin oxide films prepared at the same condition, while the visible transmittance exceeded 84% with the refractive index of 1.85–2.00. In our experiments, introduction of oxygen gas appeared to be beneficial to the improvement of the transparency and electrical conductivity. Both free carrier absorption and indirect transition were observed and Burstein–Moss effect proved a-IZO to be a degenerated amorphous semiconductor. However, the linear relation between the optical band gap and the band tail width which usually observed in covalent amorphous semiconductor such as a-Si:H was not conserved. Besides, porosity could greatly determine the resistivity and optical constants for the thickness variation at this deposition condition. Furthermore, a broad photoluminescence peak around 510 nm was identified when more than 1.5 sccm oxygen was introduced. © 2013 Elsevier B.V. All rights reserved.
1. Introduction It is of great interest that, as an excellent host oxide, crystalline and amorphous transparent conducting indium oxide thin films, such as indium tin oxide (ITO), indium molybdenum oxide [1], titanium doped indium oxide [2] and zinc-oxide stabilized indium oxide (namely, indium zinc oxide, IZO) [3], have predominated in both the scientific and technological applications for a long time. Generally, amorphous or nanocrystalline ITO, deposited at ambient temperature from In2O3:SnO2 = 90:10 wt.% as a target, demonstrates several disadvantages compared with crystalline ITO samples prepared under higher temperature, such as poor electrical performance, instability in harsh environments and inferior pattern etch-ability due to a great potential of transformation to the poly-crystalline state. To hurdle these shortcomings, amorphous transparent conducting oxide films, such as a-InGaZnO4 and a-IZO have been developed by Kamiya and Hosono [4], Perkins et al. [3], etc. It was reported that a-IZO films exhibit higher electrical conductivity, together with a very smooth surface, a comparatively low stress level [5] and controllable etching characteristics. Moreover, the a-IZO films show remarkable thermal stability up to 600 °C [3] while keeping the amorphous state. Like yttria-stabilized zirconia, doping with zinc oxide stabilizes the amorphous state of indium oxide thin films within a wide range of temperature and thickness. Warasawa et al. [6] reported that replacement of ZnO:Ga as the window layer by a-IZO in copper indium gallium diselenium solar cells could increase the efficiency of solar ⁎ Corresponding author. Tel.: +86 10 62496499; fax: +86 10 62497654. E-mail address: [email protected] (Y. Yan). 0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2012.12.101
cells. However, even though single composition 10.7 wt.% ZnO in In2O3 is rapidly becoming a de facto standard for the flat panel display industry, it is still necessary to confirm the above good performances because all performances are process-dependent (i.e., target power density, oxygen partial pressure or oxygen ratio, sputtering atmosphere). Furthermore, the supply of target materials is not sufficient compared with conventional ITO target materials. In this work, the influences of oxygen ratios and thicknesses on the structure, morphological, optical and electrical properties of IZO films have been studied. It is found that IZO films deposited on flexible polymeric substrates show good performance which have great potential to be used in a heat-sensitive field. 2. Experimental details IZO thin films were deposited on hard coating side of the flexible 188 μm-thickness polyethylene terephthalate (PET, SH71S type, SKC) and on Si (100) wafer, respectively, using a sintered conducting indium zinc ceramic oxide target (In2O3:ZnO= 88:12 wt.%, Φ70 × 5 mm, purity of 99.99%) by a direct current magnetron sputtering method at ambient temperature. Polished Si wafers were cleaned using ultrasonic cleaning in acetone for 15 min to get rid of organic contaminants, and then rinsed with de-ionized water for 15 min to remove contaminant particles. The distance of the substrate-to-the target was about 80 mm. All samples were fabricated with a power density of 2.325 W/cm 2 and the depostion rate was about 12.5 nm/min. The substrate holder rotated with a velocity of 2 rpm to improve the uniformity. The deposition system was evacuated to less than 3 × 10−3 Pa, using both rotary and
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diffusing vacuum pumping systems. The sputtering was carried out at a pressure of 0.36–0.37 Pa in an Ar and O2 mixture and the Ar flow rate was kept at 30 sccm (standard cubic centimetre/minute). The oxygen was introduced from the chamber wall. The crystallinity of the IZO films was investigated through X-ray diffraction (XRD, D/max 2500, RIGAKU), using a Cu Kα (1.5405 Å) radiation operating at 40 kV and 40 mA, with a grazing incidence angle of 1°. The surface morphology of IZO films was measured by atomic force microscopy (Slover 7, NT-MDT) using the taping mode, and the tip radius is about 15 nm. Sheet resistance of the samples was measured with a four point co-linear probe method and electrical transport properties were evaluated by Hall measurements at room temperature using the van der Pauw geometry. The thickness and optical constants, i.e., refractive index and extinction coefficient were determined from samples on Si substrates by spectroscopic ellipsometry (SE800, SENTECH) based on a step scan analyzer principle using the Drude–Lorentz model in 300–800 nm. The optical transmission and reflection were measured using a UV–vis-NIR double-beam spectrophotometer (Cary 5000, VARIAN) in the wavelength range from 300 to 3000 nm. Reflection measurements were performed with VW configuration using an aluminium mirror at 7° as a reference. Room temperature photoluminescence spectra of the films are recorded using a Raman spectrometer (Spex-1403, SPEX) with excitation wavelength of a He–Cd laser at 325 nm. 3. Results & discussion 3.1. Structural and morphological properties of IZO films The XRD pattern shown in Fig. 1 demonstrates that all the as-deposited IZO films were amorphous with only some characteristic diffraction peaks of the orientated PET substrates and the hard coating on top of them [7,8]. Fig. 2 shows the AFM image of IZO films on PET substrate. A surface with an RMS roughness less than 1.0 nm was obtained, which was beneficial to the organic optoelectronics. 3.2. Optical properties of IZO films The oxygen flow rate has as a profound role in tailoring the physical properties of the IZO films. The carrier generation is greatly affected by oxygen vacancies created. The optical transmission and reflection spectra are shown in Fig. 3 in the wavelength range 300–3000 nm for IZO samples prepared with increasing oxygen flow rate from 0 to 2.5 sccm. It is observed that, in the entire visible and near-infrared (NIR) region, the transmission is increasing gradually with increasing oxygen flow rate. However, with the oxygen flow rate higher than
Fig. 1. XRD profiles of about 230 nm-thickness as-deposited IZO films on PET with oxygen flow rate from 0 to 2.5 sccm.
Fig. 2. AFM image of (the scan area is 2 × 2 μm and the z-scale is 6.0 nm) an IZO film on PET with the oxygen flow rate of 0.5 sccm.
1.0 sccm, no remarkable enhancement in transmission between 500 and 1500 nm could be obtained. As shown in the inset of Fig. 3, a significant red-shift of the absorption edge could be observed. Definitely, the plasma wavelength λp, is defined by the intersecting point of the real and imaginary parts of the complex refractive index or the minimum of the reflection curves [9]. Unfortunately, the interference fringes caused by the μm-scale hard coating on PET made the minimum of the reflection curve difficult to detect. Whereas, the wavelengths at the points of intersection of the transmission and reflection curves, λTR, can be used as an evaluation for the plasma wavelength λp, because it was the watershed between the high reflective and transmissive zones, marked as grey round dots in Fig. 3. It should be noted that the intersecting points for the curves in Fig. 3 with oxygen flow rates of 2.0 and 2.5 sccm are higher than the upper-limit of this spectrometer. Nonetheless, it does not influence the estimation of the red shift of λTR based on the tendency of the transmission and reflection curves in Fig. 3. According to 2
ωp ¼
2
ne e ε0 ε∞ m
ð1Þ
Fig. 3. Transmission and reflection spectra of IZO films on PET with thickness about 230 nm prepared with increasing oxygen flow rate from 0 to 2.5 sccm. The inset shows the magnifier of transmission in the wavelength range of 340–360 nm. Note that the transmissions of IZO films on PET were normalized to the one of bare PET substrate.
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where ne is the carrier concentration in metal-like films, e is electrical charge unit, m⁎ is the effective mass compared with static mass of electron and ε0 and ε∞ are static and high-frequency dielectric constants. Usually, the effective mass is a weak-varied function of the variables [10], then λp ¼
4π2 c2 ε0 ε∞ m ne e2
!1=2 ð2Þ
where c is the velocity of light in vacuum. From Fig. 3, it can be observed that the plasma wavelength increased with increasing oxygen flow rate. According to Eq. (2), it can be concluded that the carrier concentration would decrease gradually when the oxygen flow rate increased which would be confirmed by the results of the Hall measurement indicated in Fig. 7. Based on Eq. (2), the calculated estimated values of the effective masses of conducting electrons were 0.52 m0, 0.57 m0, and 0.60 m0 which were higher than the value reported by Kumar et al. [10]. The absorption coefficient α of IZO films could be calculated using Eq. (3): 1 T ðλÞ 1 1−RðλÞ α ðλÞ ¼ − ln ¼ ln : d 1−RðλÞ d T ðλÞ
ð3Þ
And for amorphous highly degenerated oxide semiconductors based on the assumption of direct allowed transitions to an empty parabolic conduction band, the correlation of the absorption coefficient α and optical band gap Eopt was described by Tauc equation as: 1=2 ðαhυÞ ¼ const: hυ−Eopt
Fig. 5. Tauc plot of as-deposited IZO films on PET with thickness of about 230 nm.
Like classic amorphous semiconductor, such as hydrogenated amorphous silicon (a-Si:H), there exists a weak Urbach absorption region where visible absorption coefficient varies exponentially with the energy of the incident light. As a parameter for characterize the distribution of band tail state, the band tail width, E0, could be determined by E0 ¼
dð lnα Þ −1 : dðℏωÞ
ð5Þ
where d and TIZO = TPET/IZO/TbarePET are the thickness and the normalized transmission of IZO films, respectively. The ultraviolet–visibleNIR absorption spectra of as-deposited a-IZO films at room temperature were shown in Fig. 4 which is very similar with Fig. 1 shown in Ref. [11], as a classic degenerate amorphous oxide semiconductor. The fundamental absorption raised beyond about 3.2 eV and α ∝ λ p (p = 2–3)-type free-carrier absorption is observed below around 1.5 eV. With increasing oxygen flow rate, a tiny red shift in the fundamental edge (from 3.46 eV to 3.42 eV, Tauc plot was shown in Fig. 5), together with a decrease in the free-carrier absorption, suggests a Burstein–Moss effect for which the Fermi level lies in the conduction band. To some extent, the fluctuation of Eopt reflected the uncertainty of amorphous structure. Another absorption edges around 2.0 eV were supposed to the indirect optical band gap. Similar with amorphous ITO films, allowed indirect transition was also observed which indicated the complicated band structure of indium oxide films [12,13].
As shown in Fig. 6 the linear correlation was not conserved between E0 and Eopt which usually appeared in a-Si:H [14]. This phenomenon could be reproduced well and minus Eopt on the right y-axis in Fig. 6 was introduced here only to compare the variation trend of E0 and Eopt when the oxygen flow rate increased. Moreover, the values of E0 of a-IZO films are higher than those of a-Si:H(~0.1 eV). So amorphous IZO films seem to possess several characteristics of both conventional degenerate crystalline ITO and classic amorphous semiconductor(i.e. a-Si:H). The higher value of E0 meant higher density of state at band tail of the amorphous semiconductor, which was detrimental to the electrical properties of a-IZO films especially for the specific resistivity when more than 0.5 sccm oxygen introduced. This could be confirmed by the results via Hall measurements illustrated in Fig. 7. As we know that optical constants can be determined by porosity and intrinsic specific dielectric constant. It has been found that the refractive index changed from 1.85 to 2.00 and the extinction coefficient decreased from 0.15 to 0.01 while the thickness of IZO films increased from about 25 to 230 nm. Comparing this tendency with Fig. 6, it can be concluded that, in this processing conditions, porosity
Fig. 4. Optical absorption spectra of as-deposited a-IZO films with about 230 nm thickness measured at room temperature.
Fig. 6. The band tail width E0 and the minus optical band gap −Eopt of IZO films on PET with the variation of oxygen flow rate.
ð4Þ
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Fig. 8. Hall mobility-carrier density plot of all IZO films on PET compared with the ionized impurity scattering caused only by oxygen vacancy using Brook–Herring–Dingle formula [22].
Fig. 7. Hall effect measurement results of a-IZO films deposited with various oxygen flow rates and different thicknesses at 25, 125 and 230 nm, respectively.
played a prominent role in tuning the electrical and optical properties of a-IZO films. 3.3. Electrical properties of IZO films Generally, targets with different doping concentrations require different amounts of oxygen in the sputtering process to obtain a minimum resistivity in the coating. Therefore, the minimum resistivity in each IZO target is achieved by tuning the oxygen flow rate carefully. Unlike reported that the maximal conductivity was obtained with no oxygen in the sputtering atmosphere [8,15], the appropriate addition of oxygen appears to result in the improvement of both electrical and optical properties of transparent conducting a-IZO films, which is in accordance with results reported by several authors [16–19]. This discrepancy may be mainly caused by different doping concentrations and diverse process configurations. One distinct trait of a-IZO films is its higher Hall mobility arisen from the insensitivity of s states to orientation disorder [4], which would be beneficial for NIR transparency and lower free carrier absorption in the visible region. Hall measurement results are shown in Fig. 7. It is observed that the carrier concentrations decrease and Hall motilities increase when the oxygen flow rate increases from 0 to 2.5 sccm regardless of thickness variation. The opposite trends between the carrier concentrations and Hall motilities would make resistivity reach a minimum at an appropriate oxygen flow rate. In our experiment, the minimal resistivity of about 5 × 10−4 Ω cm was achieved when 1.0 sccm oxygen was introduced. According to defect chemical reaction formula
2−
2O
••
−
↔ O2 ðgÞ þ 2V O þ 4e :
source of carrier generation while the enhancement of transparency and conductivity necessitates small amount of oxygen. However, the origin of conductivity in transparent oxide semiconductors is still controversial. It has been postulated for a long time that transparent conductivity is related to the existence of shallow donor levels near the conduction band, formed by oxygen vacancies. Even in amorphous oxide films, oxygen vacancies are believed to be responsible for controlling the carrier concentration [20]. Nonetheless, the mechanism of carrier generation may not be so simple. Unlike the substitution mechanism in the crystalline ITO, according to the calculated results [21], under the n-type environment, both oxygen vacancy (VO) and indium anti-site on zinc (InZn) have low formation energies and they tend to form a defect complex at a certain condition. In addition, the dominant scattering mechanism was inferred to the ionised impurity scattering based on the plot between the Hall mobility and carrier concentration in Fig. 8 that the mobility increases as the net carrier concentration decreases and the carrier concentration was around 5 × 10 20 cm −3 [22]. Consequently, the dopant with less concentration would be favourable to achieve ever lower electrical resistivity validated by Hara et al. [23] who gained resistivity as low as 2.9 × 10 −4 Ω cm with 130 nm thickness on polycarbonate foils using doping weight ratio of 7.5 wt.% without deliberately heating. As a degenerate metal-like semiconductor, the electron mean free path (eMFP) can be calculated by using 1=3 2 L ¼ 3π ne hμ H =e:
ð7Þ
ð6Þ
It was postulated that the introduction of more oxygen reactive gas annihilates the oxygen vacancies which were presumably the
Fig. 9. Electron mean-free-path variation as a function of thickness (about 25, 125, 230 nm) and oxygen flow rate.
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Unfortunately, the origin of the PL peak is still unknown. It was thought that blue PL can be attributed to oxygen vacancies and indium-oxygen vacancy centres [27]. No such peaks have been discovered in this experiment. Various defects in materials would trigger PL and the results reported in literature are dispersed. Perhaps, combined with other characterizing methods, PL could be a good approach to clarify the defects and carrier generation as well as properties variation in practical applications in the future. 4. Conclusion
Fig. 10. PL spectra of IZO films with the thickness of about 230 nm on PET substrates and PL spectrum of bare PET was also shown here as a reference.
The calculated eMFPs are shown in Fig. 9. The introduction of oxygen gas augmented the eMFP evidently, which corresponds with the Hall mobility values shown in Fig. 6 as eMFP was a measurement of scattering effect for conducting electrons. In many cases, the properties of thin films would vary with increasing thickness. That is because, with the increasing time for a given processing condition, more energies delivered to the growing films, which resulted in the enhancement of crystallinity. Unlike the tendency in ITO [24], IZO films shown amorphous nature with wide range of thickness (these XRD profiles are not shown here). The thickness dependence of electrical resistivity of a-IZO films with different oxygen flow rates was also shown in Fig. 7. The electrical resistivity of semiconductors is determined by carrier concentration and Hall mobility. It can be observed that the electrical resistivity decreased when the thickness increased from about 25 to 230 nm at each rate level of the oxygen flow. The improvement of resistivity can be attributed to the reduction of the density of structural imperfection or porosity. Besides, the enhancement of resistivity was profited from the enhancement of Hall mobility caused by introducing oxygen. Interestingly, Hall motilities were not changed greatly when the thickness increased from 25 to 230 nm. 3.4. Photoluminescence (PL) properties of IZO films PL was associated with defects in material and excitation processes. Consequently, it can be employed to investigate the defect types, states and densities which are crucial for tuning properties of functional thin films. Here it is worth noting that, the analysis should be carried out more carefully because of the mutable nature of these defects, intrinsic or extrinsic. Furthermore, quantitative analysis is very challenging to perform. Normally, PL phenomena were not reported in bulk indium oxide or amorphous indium oxide films. However, UV and/or visible PL were found in crystalline (doped or not) indium oxide films [25] or nanostructures [26]. In our experiments, the emission PL spectra of IZO films deposited on PET substrate with different oxygen flow rates were exhibited in Fig. 10. From the PL curve of bare PET, it could be identified that three distinct PL peaks were located at 368, 387 and 397 nm, which may be caused by PET substrate with hard coating. Meanwhile, band edge emissions of indium oxide also have been reported in this region [27]. So it is ambiguous to extinguish the contributions of these PL peaks. When the oxygen flow rate was less than 1.5 sccm, the PL intensities were quite weak. However, when the oxygen flow rate increased up to 1.5 sccm, an extra broad PL peak appeared at around 510 nm, which was also observed by Hsin et al. in zinc slightly doped indium oxide nanowires [26].
Amorphous IZO thin films were deposited on hard-coating coated PET substrates using a DC magnetron sputtering technique at ambient temperature. The combined transparent and electrical conductive properties were obtained and the optimal resistivity reached 5 × 10−4 Ω cm when the oxygen flow rate was 1 sccm. Moreover, the free carrier absorption, the indirect transition and the Burstein–Moss effect were observed in a-IZO films. However, the linear relation between Eopt and E0 usually found in a-Si:H was not conserved in a-IZO. The interactive relationship between the variation of the band tail width, E0, and the specific resistivity, ρ could be qualitatively established initially. Also, it was indicated that porosity could greatly determine the resistivity and optical constants for the thickness variation at this deposition condition. Besides, a broad PL peak around 510 nm was found when more than 1.5 sccm O2 was introduced during the deposition of IZO films. Acknowledgements This work was supported by the Yicai Youth Fund established by Beijing Institute of Aeronautical Materials, under Grant No. YF53110830. References [1] Y. Yoshida, D.M. Wood, T.A. Gessert, T.J. Couts, Appl. Phys. Lett. 84 (2004) 2097. [2] M.F.A.M. van Hest, M.S. Dabney, J.D. Perkins, D.S. Ginley, M.P. Taylor, Appl. Phys. Lett. 87 (2005) 032111. [3] M.P. Taylor, D.W. Readey, M.F.A.M. van Hest, C.W. Teplin, J.L. Alleman, M.S. Dabney, L.M. Gedvilas, B.M. Keyes, B. To, J.D. Perkins, D.S. Ginley, Adv. Funct. Mater. 18 (2008) 3169. [4] T. Kamiya, H. Hosono, NPG Asia Mater. 2 (2010) 15. [5] T. Sasabayashi, N. Ito, E. Nishimura, M. Kon, P.K. Song, K. Utsumi, A. Kaijo, Y. Shigesato, Thin Solid Films 445 (2003) 219. [6] M. Warasawa, A. Kaijo, M. Sugiyama, Thin Solid Films 520 (2012) 2119. [7] M.M. Nasef, J. Appl. Polym. Sci. 84 (2002) 1949. [8] E.L. Kim, S.K. Jung, S.H. Sohn, D.K. Park, J. Phys. D: Appl. Phys. 40 (2007) 1784 B. [9] M. Fox, Optical Properties of Solids, Oxford Univ. Press, Oxford, 2001, p. 155. [10] B. Kumar, H. Gong, R. Akkipeddi, J. Appl. Phys. 98 (2005) 073703. [11] K. Shimakawa, S. Narushima, H. Hosono, H. Kawazoe, Philos. Mag. Lett. 79 (1999) 755. [12] P. Erhart, A. Klein, R.G. Egdell, K. Albe, Phys. Rev. B 75 (2007) 153205. [13] X. Yin, W. Tang, X. Weng, L. Deng, J. Phys. D: Appl. Phys. 42 (2009) 025104. [14] G.D. Cody, T. Tiedje, B. Abeles, B. Brooks, Y. Goldstein, Phys. Rev. Lett. 47 (1981) 1480. [15] B. Yagliolu, Y.-J. Huang, H.-Y. Yeom, D.C. Paine, Thin Solid Films 496 (2006) 89. [16] Y.S. Rim, S.M. Kim, K.H. Kim, J. Korean Phys. Soc. 54 (2008) 1267. [17] Y.L. Li, D.Y. Lee, S.R. Min, H.N. Cho, J. Kim, C.W. Chung, Jpn. J. Appl. Phys. 47 (2008) 6896. [18] D.Y. Ku, I.H. Kim, I. Lee, K.S. Lee, T.S. Lee, J.-h. Jeong, Thin Solid Films 515 (2006) 1364. [19] Y.B. Xiao, S.M. Kong, E.H. Kim, C.W. Chung, Sol. Energy Mater. Sol. Cells 95 (2011) 264. [20] J.R. Bellingham, W.A. Phillips, C.J. Adkins, J. Phys. Condens. Matter 2 (1990) 6207. [21] H. Peng, J.-H. Song, E.M. Hopper, Q. Zhu, T.O. Mason, A.J. Freeman, Chem. Mater. 24 (2012) 106. [22] N. Ito, Y. Sato, P.K. Song, A. Kaijio, K. Inoue, Y. Shigesato, Thin Solid Films 496 (2006) 99. [23] H. Hara, T. Hanada, T. Shiro, T. Yatabe, J. Vac. Sci. Technol. A 22 (2003) 1726. [24] C.-H. Yang, S.-C. Lee, T.-C. Lin, S.-C. Chen, Thin Solid Films 516 (2008) 1984. [25] M.-S. Lee, W.C. Choi, E.K. Kim, C.K. Kim, S.-K. Kim, Thin Solid Films 279 (1996) 1. [26] C.L. Hsin, J.H. He, L.J. Chen, Appl. Phys. Lett. 88 (2006) 063111. [27] D. Beena, K.J. Lethy, R. Vinodkumar, A.P. Detty, V.P. Mahadevanpillai, V. Ganesan, Optoelectron. Adv. Mater. Rapid Commun. 5 (2011) 1.