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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 14 — Jul. 15, 2013
  • pp: 16670–16682
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Broadband terahertz conductivity and optical transmission of indium-tin-oxide (ITO) nanomaterials

Chan-Shan Yang, Chan-Ming Chang, Po-Han Chen, Peichen Yu, and Ci-Ling Pan  »View Author Affiliations


Optics Express, Vol. 21, Issue 14, pp. 16670-16682 (2013)
http://dx.doi.org/10.1364/OE.21.016670


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Abstract

Indium-tin-oxide (ITO) nanorods (NRs) and nanowhiskers (NWhs) were fabricated by an electron-beam glancing-angle deposition (GLAD) system. These nanomaterials are of interests as transparent conducting electrodes in various devices. Two terahertz (THz) time-domain spectrometers (TDS) with combined spectral coverage from 0.15 to 9.00 THz were used. These allow accurate determination of the optical and electrical properties of such ITO nanomaterials in the frequency range from 0.20 to 4.00 THz. Together with Fourier transform infrared spectroscopic (FTIR) measurements, we found that the THz and far-infrared transmittance of these nanomaterials can be as high as 70% up to 15 THz, as opposed to about 9% for sputtered ITO thin films. The complex conductivities of ITO NRs, NWhs as well films are well fitted by the Drude-Smith model. Taking into account that the volume filling factors of both type of nanomaterials are nearly same, mobilities, and DC conductivities of ITO NWhs are higher than those of NRs due to less severe carrier localization effects in the former. On the other hand, mobilities of sputtered ITO thin films are poorer than ITO nanomaterials because of larger concentration of dopant ions in films, which causes stronger carrier scattering. We note further that consideration of the extreme values of Re{σ} and Im{σ} as well the inflection points, which are functions of the carrier scattering time (τ) and the expectation value of cosine of the scattering angle (γ), provide additional criteria for accessing the accuracy of the extraction of electrical parameters of non-Drude-like materials using THz-TDS. Our studies so far indicate ITO NWhs with heights of ~1000 nm show outstanding transmittance and good electrical characteristics for applications such as transparent conducting electrodes of THz Devices.

© 2013 OSA

1. Introduction

Indium-tin-oxide (ITO), one kind of heavily-doped transparent conductive oxides (TCOs), has been widely employed as transparent conducting electrode and direct-Ohmic contact layers in optoelectronic devices, due to its high transmittance and low resistivity in the visible [1

1. J. W. Shim, H. Cheun, J. Meyer, C. Fuentes-Hernandez, A. Dindar, Y. H. Zhou, D. K. Hwang, A. Kahn, and B. Kippelen, “Polyvinylpyrrolidone-modified indium tin oxide as an electron-collecting electrode for inverted polymer solar cells,” Appl. Phys. Lett. 101(7), 073303 (2012). [CrossRef]

5

5. C. K. Choi, K. D. Kihm, and A. E. English, “Optoelectric biosensor using indium-tin-oxide electrodes,” Opt. Lett. 32(11), 1405–1407 (2007). [CrossRef] [PubMed]

]. More recently, ITO nanomaterials, e.g. nanocolumn, nanorods (NRs), nanowires, nanodots, and nanowhiskers (NWhs), are reported to have the omnidirectional, broadband anti-reflective (AR) characteristics, and superhydrophilicity for improving the function of solar cells [6

6. P. Yu, C.-H. Chang, C.-H. Chiu, C.-S. Yang, J.-C. Yu, H.-C. Kuo, S.-H. Hsu, and Y.-C. Chang, “Efficiency enhancement of GaAs photovoltaics employing antireflective indium tin oxide nanocolumns,” Adv. Mater. 21(16), 1618–1621 (2009). [CrossRef]

12

12. C.-H. Chang, P. Yu, M.-H. Hsu, P.-C. Tseng, W.-L. Chang, W.-C. Sun, W.-C. Hsu, S.-H. Hsu, and Y.-C. Chang, “Combined micro- and nano-scale surface textures for enhanced near-infrared light harvesting in silicon photovoltaics,” Nanotechnology 22(9), 095201 (2011). [CrossRef] [PubMed]

], light emitting diodes (LEDs) [13

13. C. H. Chiu, P. Yu, C. H. Chang, C. S. Yang, M. H. Hsu, H. C. Kuo, and M. A. Tsai, “Oblique electron-beam evaporation of distinctive indium-tin-oxide nanorods for enhanced light extraction from InGaN/GaN light emitting diodes,” Opt. Express 17(23), 21250–21256 (2009). [CrossRef] [PubMed]

, 14

14. T. H. Seo, K. J. Lee, A. H. Park, C.-H. Hong, E.-K. Suh, S. J. Chae, Y. H. Lee, T. V. Cuong, V. H. Pham, J. S. Chung, E. J. Kim, and S.-R. Jeon, “Enhanced light output power of near UV light emitting diodes with graphene / indium tin oxide nanodot nodes for transparent and current spreading electrode,” Opt. Express 19(23), 23111–23117 (2011). [CrossRef] [PubMed]

], organic LEDs [15

15. Y. Y. Kee, S. S. Tan, T. K. Yong, C. H. Nee, S. S. Yap, T. Y. Tou, G. Sáfrán, Z. E. Horváth, J. P. Moscatello, and Y. K. Yap, “Low-temperature synthesis of indium tin oxide nanowires as the transparent electrodes for organic light emitting devices,” Nanotechnology 23(2), 025706 (2012). [CrossRef] [PubMed]

], and displays [16

16. S. H. Lee and N. Y. Ha, “Nanostructured indium-tin-oxide films fabricated by all-solution processing for functional transparent electrodes,” Opt. Express 19(22), 21803–21808 (2011). [CrossRef] [PubMed]

]. Further, the ITO NWhs were found to exhibit superb terahertz (THz) transparency, mobility, and comparable conductivities to sputtered thin films [17

17. C.-S. Yang, M.-H. Lin, C.-H. Chang, P. Yu, J.-M. Shieh, C.-H. Shen, O. Wada, and C.-L. Pan, “Non-Drude behavior in indium-tin-oxide nanowhiskers and thin films by transmission and reflection THz time-domain spectroscopy,” IEEE J. Quantum Electron. , accepted (2013).

]. Previously, ITO thin films were employed in optoelectronic devices for manipulation of the THz radiation [18

18. T. Bauer, J. S. Kolb, T. Löffler, E. Mohler, U. C. Pernisz, and H. G. Roskos, “Indium-tin-oxide-coated glass as dichroic mirror for far-infrared electromagnetic radiation,” J. Appl. Phys. 92(4), 2210–2212 (2002). [CrossRef]

21

21. D. G. Cooke and P. U. Jepsen, “Optical modulation of terahertz pulses in a parallel plate waveguide,” Opt. Express 16(19), 15123–15129 (2008). [CrossRef] [PubMed]

]. Unfortunately, because of the issue of high plasma frequencies (several hundred THz), the sputtered ITO thin film exhibits high reflectance in the infrared and THz region [17

17. C.-S. Yang, M.-H. Lin, C.-H. Chang, P. Yu, J.-M. Shieh, C.-H. Shen, O. Wada, and C.-L. Pan, “Non-Drude behavior in indium-tin-oxide nanowhiskers and thin films by transmission and reflection THz time-domain spectroscopy,” IEEE J. Quantum Electron. , accepted (2013).

, 22

22. C. H. Chang, P. Yu, and C. S. Yang, “Broadband and omnidirectional antireflection from conductive indium-tin-oxide nanocolumns prepared by glancing-angle deposition with nitrogen,” Appl. Phys. Lett. 94(5), 051114 (2009). [CrossRef]

]. Therefore, ITO nanomaterials mentioned previously are interesting alternatives as transparent electrodes for THz devices. Several studies have reported the (visible) optical [6

6. P. Yu, C.-H. Chang, C.-H. Chiu, C.-S. Yang, J.-C. Yu, H.-C. Kuo, S.-H. Hsu, and Y.-C. Chang, “Efficiency enhancement of GaAs photovoltaics employing antireflective indium tin oxide nanocolumns,” Adv. Mater. 21(16), 1618–1621 (2009). [CrossRef]

, 9

9. C.-H. Chang, M.-H. Hsu, P.-C. Tseng, P. Yu, W.-L. Chang, W.-C. Sun, and W.-C. Hsu, “Enhanced angular characteristics of indium tin oxide nanowhisker-coated silicon solar cells,” Opt. Express 19(S3Suppl 3), A219–A224 (2011). [CrossRef] [PubMed]

, 12

12. C.-H. Chang, P. Yu, M.-H. Hsu, P.-C. Tseng, W.-L. Chang, W.-C. Sun, W.-C. Hsu, S.-H. Hsu, and Y.-C. Chang, “Combined micro- and nano-scale surface textures for enhanced near-infrared light harvesting in silicon photovoltaics,” Nanotechnology 22(9), 095201 (2011). [CrossRef] [PubMed]

, 13

13. C. H. Chiu, P. Yu, C. H. Chang, C. S. Yang, M. H. Hsu, H. C. Kuo, and M. A. Tsai, “Oblique electron-beam evaporation of distinctive indium-tin-oxide nanorods for enhanced light extraction from InGaN/GaN light emitting diodes,” Opt. Express 17(23), 21250–21256 (2009). [CrossRef] [PubMed]

, 23

23. I. Hamberg, A. Hjortsberg, and C. G. Granqvist, “High quality transparent heat reflectors of reactively evaporated indium tin oxide,” Appl. Phys. Lett. 40(5), 362–364 (1982). [CrossRef]

], and electrical properties of ITO nanomaterials [24

24. Q. Wan, Z. T. Song, S. L. Feng, and T. H. Wang, “Single-crystalline tin-doped indium oxide whiskers: synthesis and characterization,” Appl. Phys. Lett. 85(20), 4759–4761 (2004). [CrossRef]

26

26. J. Gao, R. Chen, D. H. Li, L. Jiang, J. C. Ye, X. C. Ma, X. D. Chen, Q. H. Xiong, H. D. Sun, and T. Wu, “UV light emitting transparent conducting tin-doped indium oxide (ITO) nanowires,” Nanotechnology 22(19), 195706 (2011). [CrossRef] [PubMed]

]. However, the conventional methods of characterizing electrical properties of materials can damage the morphology of nanomaterials because of the need of electrical contacts, or just provide information on the mobility and resistivity of an individual nanostructure [24

24. Q. Wan, Z. T. Song, S. L. Feng, and T. H. Wang, “Single-crystalline tin-doped indium oxide whiskers: synthesis and characterization,” Appl. Phys. Lett. 85(20), 4759–4761 (2004). [CrossRef]

26

26. J. Gao, R. Chen, D. H. Li, L. Jiang, J. C. Ye, X. C. Ma, X. D. Chen, Q. H. Xiong, H. D. Sun, and T. Wu, “UV light emitting transparent conducting tin-doped indium oxide (ITO) nanowires,” Nanotechnology 22(19), 195706 (2011). [CrossRef] [PubMed]

].

Recently, broadband THz emission systems based on a gaseous plasma with incommensurate dual-color (ω-2ω) excitation has been demonstrated [35

35. I.-C. Ho, X. Guo, and X.-C. Zhang, “Design and performance of reflective terahertz air-biased-coherent-detection for time-domain spectroscopy,” Opt. Express 18(3), 2872–2883 (2010). [CrossRef] [PubMed]

37

37. N. Vieweg, B. M. Fischer, M. Reuter, P. Kula, R. Dabrowski, M. A. Celik, G. Frenking, M. Koch, and P. U. Jepsen, “Ultrabroadband terahertz spectroscopy of a liquid crystal,” Opt. Express 20(27), 28249–28256 (2012). [CrossRef] [PubMed]

]. Such systems have known been increasingly used in the THz-TDS system to study materials, e.g., nonlinear crystals [35

35. I.-C. Ho, X. Guo, and X.-C. Zhang, “Design and performance of reflective terahertz air-biased-coherent-detection for time-domain spectroscopy,” Opt. Express 18(3), 2872–2883 (2010). [CrossRef] [PubMed]

], explosives [36

36. B. Clough, J. Liu, and X.-C. Zhang, ““All air-plasma” terahertz spectroscopy,” Opt. Lett. 36(13), 2399–2401 (2011). [CrossRef] [PubMed]

], and liquid crystals [37

37. N. Vieweg, B. M. Fischer, M. Reuter, P. Kula, R. Dabrowski, M. A. Celik, G. Frenking, M. Koch, and P. U. Jepsen, “Ultrabroadband terahertz spectroscopy of a liquid crystal,” Opt. Express 20(27), 28249–28256 (2012). [CrossRef] [PubMed]

], with features at frequencies above 2 THz. However, the signal-to-noise ratio (SNR) of this approach is often inferior to that of the photoconductive (PC) antenna-based THz-TDS in the frequency range below the 0.5 THz [35

35. I.-C. Ho, X. Guo, and X.-C. Zhang, “Design and performance of reflective terahertz air-biased-coherent-detection for time-domain spectroscopy,” Opt. Express 18(3), 2872–2883 (2010). [CrossRef] [PubMed]

37

37. N. Vieweg, B. M. Fischer, M. Reuter, P. Kula, R. Dabrowski, M. A. Celik, G. Frenking, M. Koch, and P. U. Jepsen, “Ultrabroadband terahertz spectroscopy of a liquid crystal,” Opt. Express 20(27), 28249–28256 (2012). [CrossRef] [PubMed]

].

In this paper, ITO nanomaterials (NRs and NWhs), were prepared by an electron-beam glancing-angle deposition (GLAD) system [13

13. C. H. Chiu, P. Yu, C. H. Chang, C. S. Yang, M. H. Hsu, H. C. Kuo, and M. A. Tsai, “Oblique electron-beam evaporation of distinctive indium-tin-oxide nanorods for enhanced light extraction from InGaN/GaN light emitting diodes,” Opt. Express 17(23), 21250–21256 (2009). [CrossRef] [PubMed]

]. Two terahertz (THz) time-domain spectrometers (TDS) based respectively on dual-color laser-induced gaseous plasma and photoconductive (PC) antenna were applied to study the optical and electrical properties of such ITO nanomaterials in the frequency range from 0.20 to 4.00 THz. Together with Fourier transform infrared spectroscopic (FTIR) measurements, we determined both the THz and far-infrared transmittance of these nanomaterials The complex conductivities of ITO NRs, NWhs as well as films are determined by fitting with the Drude-Smith model. Taking into account that the volume filling factors of both type of nanomaterials, mobilities and DC conductivities of ITO NWhs and NRs were retrieved to acertain mechanisms of carrier localization. Finally, we show that new features of complex conductivities in non-Drude-like materials from relatively broadband THz measurements reported in this work.

2. Experimental methods

2.1. Sample preparation and characterization

2.2. THz-TDS systems

2.3. Extraction of optical parameters and complex conductivities with Drude-Smith model approach

The method we employed for extraction of optical parameters of the nanomaterials has been described previously [17

17. C.-S. Yang, M.-H. Lin, C.-H. Chang, P. Yu, J.-M. Shieh, C.-H. Shen, O. Wada, and C.-L. Pan, “Non-Drude behavior in indium-tin-oxide nanowhiskers and thin films by transmission and reflection THz time-domain spectroscopy,” IEEE J. Quantum Electron. , accepted (2013).

, 33

33. X. Zou, J. Luo, D. Lee, C. Cheng, D. Springer, S. K. Nair, S. A. Cheong, H. J. Fan, and E. E. M. Chia, “Temperature-dependent terahertz conductivity of tin oxide nanowire films,” J. Phys. D Appl. Phys. 45(46), 465101 (2012). [CrossRef]

]. Specifically, the complex transmission coefficient of nanomaterials (or thin films) is described as
TSam*(ω)=ESam*(ω)ERef*(ω)=t12t23exp[i(n21)dω/c]t13[1r21r23exp(i2n2dω/c)]exp[i(n31)Δdω/c].
(1)
Here, ESam*(ω) and ERef*(ω)are electric fields of the THz wave transmitted through the nanomaterials (or thin films) and the bare substrate, respectively. The parameters,t12*, t23* and t13* are the transmission coefficients of the THz signal from air to the nanomaterials (or thin films), from nanomaterials (or thin films) to the substrate, and from air to the substrate, respectively. Similarly, r23* and r21* are the reflection coefficients of the THz signal from the nanomaterials (or thin films) to the substrate, and from nanomaterials (or thin films) to air, respectively. Here, n2* and n3* are the equivalent refractive indices of the nanomaterials (or thin films) and the substrate, respectively. Additionally, d is thickness of the nanomaterials (or thin films); ω and c are the angular frequency and speed of light in vacuum, respectively. Further, the slight difference of the thickness between the sample and reference substrates is taken into account by Δd. After numerically solving Eq. (1), one can extract the refractive indices n and κ (n2*=n+iκ) of the nanomaterials (or thin films) under investigation.

ITO nanomaterials are viewed as a composite of ITO and air. In the EMA, their effective dielectric constant can be written as, εeffective*=f×εm*+(1f)×εh* [17

17. C.-S. Yang, M.-H. Lin, C.-H. Chang, P. Yu, J.-M. Shieh, C.-H. Shen, O. Wada, and C.-L. Pan, “Non-Drude behavior in indium-tin-oxide nanowhiskers and thin films by transmission and reflection THz time-domain spectroscopy,” IEEE J. Quantum Electron. , accepted (2013).

, 33

33. X. Zou, J. Luo, D. Lee, C. Cheng, D. Springer, S. K. Nair, S. A. Cheong, H. J. Fan, and E. E. M. Chia, “Temperature-dependent terahertz conductivity of tin oxide nanowire films,” J. Phys. D Appl. Phys. 45(46), 465101 (2012). [CrossRef]

]. Here, f is the filling factor which defines the volume fraction of the nanomaterials. The parameters εh* and εm* are the dielectric function of air (host medium) and pure nanomaterials, respectively. In this approximation, the real (Re{σ}) and imaginary (Im{σ}) parts of pure ITO nanomaterials can be derived from the measured optical constants as Re{σ}=ωε0(2nκ)/f and Im{σ}=ωε0[ε[n2κ2(1f)]/f], respectively. Here, ε = 4 [17

17. C.-S. Yang, M.-H. Lin, C.-H. Chang, P. Yu, J.-M. Shieh, C.-H. Shen, O. Wada, and C.-L. Pan, “Non-Drude behavior in indium-tin-oxide nanowhiskers and thin films by transmission and reflection THz time-domain spectroscopy,” IEEE J. Quantum Electron. , accepted (2013).

, 27

27. C.-S. Yang, C.-H. Chang, M.-H. Lin, P. Yu, O. Wada, and C.-L. Pan, “THz conductivities of indium-tin-oxide nanowhiskers as a graded-refractive-index structure,” Opt. Express 20(S4Suppl 4), A441–A451 (2012). [CrossRef] [PubMed]

, 28

28. C.-W. Chen, Y.-C. Lin, C.-H. Chang, P. Yu, J.-M. Shieh, and C.-L. Pan, “Frequency-dependent complex conductivities and electric responses of indium tin oxide thin films from the visible to the far-infrared,” IEEE J. Quantum Electron. 46(12), 1746–1754 (2010). [CrossRef]

], is the high-frequency dielectric constant; ε0 = 8.854 × 10−12 (F/m) is the free-space permittivity.

In order to describe the material system with long-range transport which is suppressed by disorder, the Drude-Smith model, an extension of the Drude model proposed by N. V. Smith [42

42. N. V. Smith, “Classical generalization of the Drude formula for the optical conductivity,” Phys. Rev. B 64(15), 155106 (2001). [CrossRef]

], is applied to fit the experimental results. The complex conductivity of the nanomaterial can be written as
σ*(ω)=ε0ωp2τ1iωτ(1+γ1iωτ).
(2)
Here, ωp and τ are the plasma frequency and the carrier scattering time, respectively. The parameter γ is the carrier’s persistence of velocity after experiencing one collision, in other words, it is associated with the degree of carrier localization. The value of γ can vary from 0 to −1, corresponding to the limit of isotropic scattering to full carrier localization. The Drude-Smith model has been a common choice for describing the complex conductivities of nanostructured materials, e. g., ITO NWhs, TiO2 nanomaterials, ZnO nanomaterials, and SnO2 nanowires in the THz frequency range [17

17. C.-S. Yang, M.-H. Lin, C.-H. Chang, P. Yu, J.-M. Shieh, C.-H. Shen, O. Wada, and C.-L. Pan, “Non-Drude behavior in indium-tin-oxide nanowhiskers and thin films by transmission and reflection THz time-domain spectroscopy,” IEEE J. Quantum Electron. , accepted (2013).

, 27

27. C.-S. Yang, C.-H. Chang, M.-H. Lin, P. Yu, O. Wada, and C.-L. Pan, “THz conductivities of indium-tin-oxide nanowhiskers as a graded-refractive-index structure,” Opt. Express 20(S4Suppl 4), A441–A451 (2012). [CrossRef] [PubMed]

, 29

29. G. M. Turner, M. C. Beard, and C. A. Schmuttenmaer, “Carrier localization and cooling in dye-sensitized nanocrystalline titanium dioxide,” J. Phys. Chem. B 106(45), 11716–11719 (2002). [CrossRef]

34

34. D. Tsokkou, A. Othonos, and M. Zervos, “Carrier dynamics and conductivity of SnO2 nanowires investigated by time-resolved terahertz spectroscopy,” Appl. Phys. Lett. 100(13), 133101 (2012). [CrossRef]

, 43

43. L. V. Titova, T. L. Cocker, D. G. Cooke, X. Wang, A. Meldrum, and F. A. Hegmann, “Ultrafast percolative transport dynamics in silicon nanocrystal films,” Phys. Rev. B 83(8), 085403 (2011). [CrossRef]

].

3. Results and discussions

For the frequency range below 0.50 THz and above 1.40 THz, we used data taken from the PC antenna and laser-induced gaseous plasma based systems, respectively. Between 0.50 and 1.40 THz, the data from the two systems were identical within experimental error. In order to discuss the effect of morphology on electrical properties of the nanomaterials, the volume filling factors, f, of ITO short NRs, long NRs, short and long NWhs were estimated to be 17.9%, 9.8%, 20.0%, and 10.3% by examining Fig. 1.

The frequency-dependent transmittance of ITO NRs, NWhs and thin films, shown in the Fig. 3
Fig. 3 The transmittance of ITO nanomaterials and thin films are plotted as a function of frequency. The red circles, green squares, black stars, blue triangles, orange hexagon, and olive inverted triangles are experimental data from THz-TDS measurements. The red-dash, green-dot, black-dash dot, blue-solid, orange-dash dot, and olive-short dash curves are experimental data from FTIR measurements.
are plotted in the THz frequency range 0.20~15.00 THz. In addition to THz-TDS measurement, we have also conducted Fourier transform infrared spectroscopic (FTIR) using a (Bruker, Vertex 70V) from 0.50 to ~15.00 THz. Combining the results of both type of experiments, we find that the transmittance of shorter and longer NRs are around 65% and 73%. On the other hand, the transmittances of short and long NWhs are ~19% and ~65%, respectively. In general, by increasing the height of ITO nanomaterials, one can substantially increase their transmittance. Significantly, ITO NWhs exhibited conspicuous morphology-dependent transmittance. Plausibly, the longitudinal change of refractive indices of shorter NWhs is stronger than that of the longer one. As a result, the AR function of shorter ITO NWhs is not so apparent. For comparison, the transmittance of sputtered ITO thin films with thicknesses of 345.0 nm and 1062.0 nm in the same frequency range are much smaller, 9% and 4%. These are also plotted in Fig. 3 for comparison. Clearly, because of the broadband GRIN characteristic, the morphologies of NRs and long NWhs have outstanding transmission of electric field in the THz and far-infrared frequency range.

The complex conductivities of ITO nanomaterials with different heights and morphologies, are shown in Figs. 5(a)
Fig. 5 (a) Re{σ}, (b) Im{σ} of ITO nanomaterials and thin films are plotted as a function of frequency. The black circles, blue squares, red diamonds, green triangles, cyan stars, and olive pentagons are experimental data. The black-solid line, blue-dash line, red-dash dot line, green-dot line, cyan-short dash line, and olive-short dash dot line are fitting curves based on the Drude-Smith model.
and 5(b), respectively. The real conductivities or Re{σ} of both types of nanomaterials are suppressed at lower frequencies. Meanwhile, negative values of Im{σ} are observed. Such phenomena are typically associated with carrier localization in the materials [17

17. C.-S. Yang, M.-H. Lin, C.-H. Chang, P. Yu, J.-M. Shieh, C.-H. Shen, O. Wada, and C.-L. Pan, “Non-Drude behavior in indium-tin-oxide nanowhiskers and thin films by transmission and reflection THz time-domain spectroscopy,” IEEE J. Quantum Electron. , accepted (2013).

, 27

27. C.-S. Yang, C.-H. Chang, M.-H. Lin, P. Yu, O. Wada, and C.-L. Pan, “THz conductivities of indium-tin-oxide nanowhiskers as a graded-refractive-index structure,” Opt. Express 20(S4Suppl 4), A441–A451 (2012). [CrossRef] [PubMed]

, 29

29. G. M. Turner, M. C. Beard, and C. A. Schmuttenmaer, “Carrier localization and cooling in dye-sensitized nanocrystalline titanium dioxide,” J. Phys. Chem. B 106(45), 11716–11719 (2002). [CrossRef]

, 30

30. H. Němec, P. Kužel, and V. Sundström, “Far-infrared response of free charge carriers localized in semiconductor nanoparticles,” Phys. Rev. B 79(11), 115309 (2009). [CrossRef]

, 33

33. X. Zou, J. Luo, D. Lee, C. Cheng, D. Springer, S. K. Nair, S. A. Cheong, H. J. Fan, and E. E. M. Chia, “Temperature-dependent terahertz conductivity of tin oxide nanowire films,” J. Phys. D Appl. Phys. 45(46), 465101 (2012). [CrossRef]

, 34

34. D. Tsokkou, A. Othonos, and M. Zervos, “Carrier dynamics and conductivity of SnO2 nanowires investigated by time-resolved terahertz spectroscopy,” Appl. Phys. Lett. 100(13), 133101 (2012). [CrossRef]

, 43

43. L. V. Titova, T. L. Cocker, D. G. Cooke, X. Wang, A. Meldrum, and F. A. Hegmann, “Ultrafast percolative transport dynamics in silicon nanocrystal films,” Phys. Rev. B 83(8), 085403 (2011). [CrossRef]

]. Based on these observations, the Drude-Smith model, rather than the Drude model, should be applied to fit the experimentally extracted complex conductivity [42

42. N. V. Smith, “Classical generalization of the Drude formula for the optical conductivity,” Phys. Rev. B 64(15), 155106 (2001). [CrossRef]

]. As shown in Figs. 5(a) and 5(b), Re{σ} and Im{σ} of ITO materials can be fit excellently this way. All of the fitting parameters are summarized in Table 2

Table 2. Extracted parameters of ITO nanomaterials and thin films based on the Drude-Smith Model

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. Regarding NRs with heights of 621.1 and 991.6 nm, the plasma frequencies are 561 versus 1006 rad⋅THz; carrier scattering times are 31.7 versus 13.5 fs; the parameters γ are −0.71 versus −0.88. For the NWhs with heights of 802.6 and 1169.5 nm, the plasma frequencies are 751 versus 853 rad⋅THz; carrier scattering times are 39.6 versus 13.2 fs; the parameters γ are −0.60 versus −0.74. All γ values of the nanomaterials are smaller than −0.50 (approaching −1.00). Such behavior indicates the presence of significant carrier localization effects in ITO NRs and NWhs.

Even though Drude-Smith model was applied successfully to describe the experimental data, the trend of THz complex conductivities seems also conform to the model of free-electron plasma with a plasmon resonance. This is implied by nearly the same values of ωRe, Max/2π, and ωIm, Zero/2π for each sample (see Table 3). The plasmon-like model, as discussed by V. Sundström et al. [45

45. H.-K. Nienhuys and V. Sundström, “Influence of plasmons on terahertz conductivity measurements,” Appl. Phys. Lett. 87(1), 02101 (2005). [CrossRef]

], determines the plasmon frequency of the material from the values of ωRe, Max/2π, and ωIm, Zero/2π. In 2007, P. Parkinson et al. applied the plasmon-like model to describe complex conductivities of GaAs nanowires [46

46. P. Parkinson, L.-H. James, Q. Gao, H. H. Tan, C. Jagadish, M. B. Johnston, and L. M. Herz, “Transient terahertz conductivity of GaAs nanowires,” Nano Lett. 7(7), 2162–2165 (2007). [CrossRef]

]. In their study, a redshift of the plasmon frequency with decreasing the charge carrier density was also observed [46

46. P. Parkinson, L.-H. James, Q. Gao, H. H. Tan, C. Jagadish, M. B. Johnston, and L. M. Herz, “Transient terahertz conductivity of GaAs nanowires,” Nano Lett. 7(7), 2162–2165 (2007). [CrossRef]

]. In the present work, the decrease in carrier density corresponding to reducing heights of ITO nanomaterials and thin films is reflected by a redshift of ωRe, Max/2π, and ωIm, Zero/2π, respectively. For instance, ωRe, Max/2π of NRs shifts from 11.01 THz to 4.09 THz as the height decreases from 991.6 nm to 621.1 nm, while ωIm, Zero/2π also decreases from 10.28 THz to 3.26 THz. This behavior is indicated by the blue-dot and green dash-dot arrows Figs. 6(a)-6(f), respectively.

4. Conclusions

Indium-tin-oxide (ITO) nanorods (NRs) and nanowhiskers (NWhs) were fabricated by an electron-beam glancing-angle deposition GLAD system. Two terahertz (THz) time-domain spectrometers (TDS), based respectively on dual-color laser-induced gaseous plasma and photoconductive antenna, were applied to study the optical and electrical properties of such ITO nanomaterials in the frequency range from 0.20 to 4.00 THz. Together with Fourier transform infrared spectroscopic (FTIR) measurements, we found that both the THz and far-infrared transmittance of these nanomaterials can be as high as 70% as opposed to about 9% for sputtered ITO thin films. The complex conductivities of ITO NRs, NWhs as well as films are well fitted by the Drude-Smith model. Taking into account that the volume filling factors of both type of nanomaterials are nearly same, mobilities of short and long ITO NWhs (92.0 and 20.3 cm2V−1s−1) are higher than those of NRs (53.6 and 9.1 cm2V−1s−1) of nearly the same heights due to less severe carrier localization effects in the former. On the other hand, mobilities of sputtered ITO thin films are poorer than ITO nanomaterials because of larger (~10 times) concentration of dopant ions in films, which causes stronger carrier scattering. We note further that consideration of the extreme values of Re{σ} and Im{σ} as well as the inflection points, which are functions of the carrier scattering time (τ) and the expectation value of cosine of the scattering angle (γ), provide additional criteria for assessing the accuracy of the extraction of electrical parameters of non-Drude-like materials using THz-TDS. Our studies so far indicate ITO NWs with heights of ~1000 nm show outstanding transmittance and good electrical characteristics for applications such as transparent conducting electrodes of THz Devices.

Acknowledgments

This work was funded by the grant of the National Science Council 101-2221-E-007-103-MY3 and the Academic Top University Program of the Ministry of Education. The authors would like to thank Professor Hao-Chung Kuo in National Chiao Tung University for the use of the electron-beam GLAD system. They would also like to thank Dr. Jia-Min Shieh for providing the ITO thin films. Finally, they would like to thank Professor Ta-Jen Yen for use of the FTIR spectrometer.

References and links

1.

J. W. Shim, H. Cheun, J. Meyer, C. Fuentes-Hernandez, A. Dindar, Y. H. Zhou, D. K. Hwang, A. Kahn, and B. Kippelen, “Polyvinylpyrrolidone-modified indium tin oxide as an electron-collecting electrode for inverted polymer solar cells,” Appl. Phys. Lett. 101(7), 073303 (2012). [CrossRef]

2.

Ö. Şenlik, H. Y. Cheong, and T. Yoshie, “Design of subwavelength-size, indium tin oxide (ITO)-clad optical disk cavities with quality-factors exceeding 10⁴,” Opt. Express 19(23), 23469–23474 (2011). [CrossRef] [PubMed]

3.

Y.-J. Liu, C.-C. Huang, T.-Y. Chen, C.-S. Hsu, J.-K. Liou, T.-Y. Tsai, and W.-C. Liu, “Implementation of an indium-tin-oxide (ITO) direct-Ohmic contact structure on a GaN-based light emitting diode,” Opt. Express 19(15), 14662–14670 (2011). [CrossRef] [PubMed]

4.

S.-Y. Liu, Y.-C. Lin, J.-C. Ye, S. J. Tu, F. W. Huang, M. L. Lee, W. C. Lai, and J. K. Sheu, “Hydrogen gas generation using n-GaN photoelectrodes with immersed Indium Tin oxide Ohmic contacts,” Opt. Express 19(S6Suppl 6), A1196–A1201 (2011). [CrossRef] [PubMed]

5.

C. K. Choi, K. D. Kihm, and A. E. English, “Optoelectric biosensor using indium-tin-oxide electrodes,” Opt. Lett. 32(11), 1405–1407 (2007). [CrossRef] [PubMed]

6.

P. Yu, C.-H. Chang, C.-H. Chiu, C.-S. Yang, J.-C. Yu, H.-C. Kuo, S.-H. Hsu, and Y.-C. Chang, “Efficiency enhancement of GaAs photovoltaics employing antireflective indium tin oxide nanocolumns,” Adv. Mater. 21(16), 1618–1621 (2009). [CrossRef]

7.

P. Yu, C.-H. Chang, M.-S. Su, M.-H. Hsu, and K.-H. Wei, “Embedded indium-tin-oxide nanoelectrodes for efficiency and lifetime enhancement of polymer-based solar cells,” Appl. Phys. Lett. 96(15), 153307 (2010). [CrossRef]

8.

J. W. Leem and J. S. Yu, “Glancing angle deposited ITO films for efficiency enhancement of a-Si:H/μc-Si:H tandem thin film solar cells,” Opt. Express 19(S3Suppl 3), A258–A268 (2011). [CrossRef] [PubMed]

9.

C.-H. Chang, M.-H. Hsu, P.-C. Tseng, P. Yu, W.-L. Chang, W.-C. Sun, and W.-C. Hsu, “Enhanced angular characteristics of indium tin oxide nanowhisker-coated silicon solar cells,” Opt. Express 19(S3Suppl 3), A219–A224 (2011). [CrossRef] [PubMed]

10.

D.-J. Seo, J.-P. Shim, S.-B. Choi, T. H. Seo, E.-K. Suh, and D.-S. Lee, “Efficiency improvement in InGaN-based solar cell s by indium tin oxide nano dots covered with ITO films,” Opt. Express 20(S6), A991–A996 (2012). [CrossRef]

11.

J. W. Leem and J. S. Yu, “Indium tin oxide subwavelength nanostructures with surface antireflection and superhydrophilicity for high-efficiency Si-based thin film solar cells,” Opt. Express 20(S3), A431–A440 (2012). [CrossRef] [PubMed]

12.

C.-H. Chang, P. Yu, M.-H. Hsu, P.-C. Tseng, W.-L. Chang, W.-C. Sun, W.-C. Hsu, S.-H. Hsu, and Y.-C. Chang, “Combined micro- and nano-scale surface textures for enhanced near-infrared light harvesting in silicon photovoltaics,” Nanotechnology 22(9), 095201 (2011). [CrossRef] [PubMed]

13.

C. H. Chiu, P. Yu, C. H. Chang, C. S. Yang, M. H. Hsu, H. C. Kuo, and M. A. Tsai, “Oblique electron-beam evaporation of distinctive indium-tin-oxide nanorods for enhanced light extraction from InGaN/GaN light emitting diodes,” Opt. Express 17(23), 21250–21256 (2009). [CrossRef] [PubMed]

14.

T. H. Seo, K. J. Lee, A. H. Park, C.-H. Hong, E.-K. Suh, S. J. Chae, Y. H. Lee, T. V. Cuong, V. H. Pham, J. S. Chung, E. J. Kim, and S.-R. Jeon, “Enhanced light output power of near UV light emitting diodes with graphene / indium tin oxide nanodot nodes for transparent and current spreading electrode,” Opt. Express 19(23), 23111–23117 (2011). [CrossRef] [PubMed]

15.

Y. Y. Kee, S. S. Tan, T. K. Yong, C. H. Nee, S. S. Yap, T. Y. Tou, G. Sáfrán, Z. E. Horváth, J. P. Moscatello, and Y. K. Yap, “Low-temperature synthesis of indium tin oxide nanowires as the transparent electrodes for organic light emitting devices,” Nanotechnology 23(2), 025706 (2012). [CrossRef] [PubMed]

16.

S. H. Lee and N. Y. Ha, “Nanostructured indium-tin-oxide films fabricated by all-solution processing for functional transparent electrodes,” Opt. Express 19(22), 21803–21808 (2011). [CrossRef] [PubMed]

17.

C.-S. Yang, M.-H. Lin, C.-H. Chang, P. Yu, J.-M. Shieh, C.-H. Shen, O. Wada, and C.-L. Pan, “Non-Drude behavior in indium-tin-oxide nanowhiskers and thin films by transmission and reflection THz time-domain spectroscopy,” IEEE J. Quantum Electron. , accepted (2013).

18.

T. Bauer, J. S. Kolb, T. Löffler, E. Mohler, U. C. Pernisz, and H. G. Roskos, “Indium-tin-oxide-coated glass as dichroic mirror for far-infrared electromagnetic radiation,” J. Appl. Phys. 92(4), 2210–2212 (2002). [CrossRef]

19.

J. Kröll, J. Darmo, and K. Unterrainer, “Metallic wave-impedance matching layers for broadband terahertz optical systems,” Opt. Express 15(11), 6552–6560 (2007). [CrossRef] [PubMed]

20.

S. A. Jewell, E. Hendry, T. H. Isaac, and J. R. Sambles, “Tuneable Fabry-Perot etalon for terahertz radiation,” New J. Phys. 10(3), 033012 (2008). [CrossRef]

21.

D. G. Cooke and P. U. Jepsen, “Optical modulation of terahertz pulses in a parallel plate waveguide,” Opt. Express 16(19), 15123–15129 (2008). [CrossRef] [PubMed]

22.

C. H. Chang, P. Yu, and C. S. Yang, “Broadband and omnidirectional antireflection from conductive indium-tin-oxide nanocolumns prepared by glancing-angle deposition with nitrogen,” Appl. Phys. Lett. 94(5), 051114 (2009). [CrossRef]

23.

I. Hamberg, A. Hjortsberg, and C. G. Granqvist, “High quality transparent heat reflectors of reactively evaporated indium tin oxide,” Appl. Phys. Lett. 40(5), 362–364 (1982). [CrossRef]

24.

Q. Wan, Z. T. Song, S. L. Feng, and T. H. Wang, “Single-crystalline tin-doped indium oxide whiskers: synthesis and characterization,” Appl. Phys. Lett. 85(20), 4759–4761 (2004). [CrossRef]

25.

S.-P. Chiu, H.-F. Chung, Y.-H. Lin, J.-J. Kai, F.-R. Chen, and J.-J. Lin, “Four-probe electrical-transport measurements on single indium tin oxide nanowires between 1.5 and 300 K,” Nanotechnology 20(10), 105203 (2009). [CrossRef] [PubMed]

26.

J. Gao, R. Chen, D. H. Li, L. Jiang, J. C. Ye, X. C. Ma, X. D. Chen, Q. H. Xiong, H. D. Sun, and T. Wu, “UV light emitting transparent conducting tin-doped indium oxide (ITO) nanowires,” Nanotechnology 22(19), 195706 (2011). [CrossRef] [PubMed]

27.

C.-S. Yang, C.-H. Chang, M.-H. Lin, P. Yu, O. Wada, and C.-L. Pan, “THz conductivities of indium-tin-oxide nanowhiskers as a graded-refractive-index structure,” Opt. Express 20(S4Suppl 4), A441–A451 (2012). [CrossRef] [PubMed]

28.

C.-W. Chen, Y.-C. Lin, C.-H. Chang, P. Yu, J.-M. Shieh, and C.-L. Pan, “Frequency-dependent complex conductivities and electric responses of indium tin oxide thin films from the visible to the far-infrared,” IEEE J. Quantum Electron. 46(12), 1746–1754 (2010). [CrossRef]

29.

G. M. Turner, M. C. Beard, and C. A. Schmuttenmaer, “Carrier localization and cooling in dye-sensitized nanocrystalline titanium dioxide,” J. Phys. Chem. B 106(45), 11716–11719 (2002). [CrossRef]

30.

H. Němec, P. Kužel, and V. Sundström, “Far-infrared response of free charge carriers localized in semiconductor nanoparticles,” Phys. Rev. B 79(11), 115309 (2009). [CrossRef]

31.

J. B. Baxter and C. A. Schmuttenmaer, “Conductivity of ZnO nanowires, nanoparticles, and thin films using time-resolved terahertz spectroscopy,” J. Phys. Chem. B 110(50), 25229–25239 (2006). [CrossRef] [PubMed]

32.

G. Ma, D. Li, H. Ma, J. Shen, C. Wu, J. Ge, S. Hu, and N. Dai, “Carrier concentration dependence of terahertz transmission on conducting ZnO films,” Appl. Phys. Lett. 93(21), 211101 (2008). [CrossRef]

33.

X. Zou, J. Luo, D. Lee, C. Cheng, D. Springer, S. K. Nair, S. A. Cheong, H. J. Fan, and E. E. M. Chia, “Temperature-dependent terahertz conductivity of tin oxide nanowire films,” J. Phys. D Appl. Phys. 45(46), 465101 (2012). [CrossRef]

34.

D. Tsokkou, A. Othonos, and M. Zervos, “Carrier dynamics and conductivity of SnO2 nanowires investigated by time-resolved terahertz spectroscopy,” Appl. Phys. Lett. 100(13), 133101 (2012). [CrossRef]

35.

I.-C. Ho, X. Guo, and X.-C. Zhang, “Design and performance of reflective terahertz air-biased-coherent-detection for time-domain spectroscopy,” Opt. Express 18(3), 2872–2883 (2010). [CrossRef] [PubMed]

36.

B. Clough, J. Liu, and X.-C. Zhang, ““All air-plasma” terahertz spectroscopy,” Opt. Lett. 36(13), 2399–2401 (2011). [CrossRef] [PubMed]

37.

N. Vieweg, B. M. Fischer, M. Reuter, P. Kula, R. Dabrowski, M. A. Celik, G. Frenking, M. Koch, and P. U. Jepsen, “Ultrabroadband terahertz spectroscopy of a liquid crystal,” Opt. Express 20(27), 28249–28256 (2012). [CrossRef] [PubMed]

38.

G. Gallot and D. Grischkowsky, “Electro-optic detection of terahertz radiation,” J. Opt. Soc. Am. B 16(8), 1204–1212 (1999). [CrossRef]

39.

C.-L. Pan, C.-F. Hsieh, R.-P. Pan, M. Tanaka, F. Miyamaru, M. Tani, and M. Hangyo, “Control of enhanced THz transmission through metallic hole arrays using nematic liquid crystal,” Opt. Express 13(11), 3921–3930 (2005). [CrossRef] [PubMed]

40.

C.-S. Yang, C.-J. Lin, R.-P. Pan, C. T. Que, K. Yamamoto, M. Tani, and C.-L. Pan, “The complex refractive indices of the liquid crystal mixture E7 in the terahertz frequency range,” J. Opt. Soc. Am. B 27(9), 1866–1873 (2010). [CrossRef]

41.

C.-K. Lee, C.-S. Yang, S.-H. Lin, S.-H. Huang, O. Wada, and C.-L. Pan, “Effects of two-photon absorption on terahertz radiation generated by femtosecond-laser excited photoconductive antennas,” Opt. Express 19(24), 23689–23697 (2011). [CrossRef] [PubMed]

42.

N. V. Smith, “Classical generalization of the Drude formula for the optical conductivity,” Phys. Rev. B 64(15), 155106 (2001). [CrossRef]

43.

L. V. Titova, T. L. Cocker, D. G. Cooke, X. Wang, A. Meldrum, and F. A. Hegmann, “Ultrafast percolative transport dynamics in silicon nanocrystal films,” Phys. Rev. B 83(8), 085403 (2011). [CrossRef]

44.

J. Ederth, “Electrical transport in nanoparticle thin films off gold and indium tin oxide,” Ph.D. dissertation, Dept. Mat. Science, Uppsala Univ., Uppsala, Sweden, (2003).

45.

H.-K. Nienhuys and V. Sundström, “Influence of plasmons on terahertz conductivity measurements,” Appl. Phys. Lett. 87(1), 02101 (2005). [CrossRef]

46.

P. Parkinson, L.-H. James, Q. Gao, H. H. Tan, C. Jagadish, M. B. Johnston, and L. M. Herz, “Transient terahertz conductivity of GaAs nanowires,” Nano Lett. 7(7), 2162–2165 (2007). [CrossRef]

OCIS Codes
(120.4290) Instrumentation, measurement, and metrology : Nondestructive testing
(120.4530) Instrumentation, measurement, and metrology : Optical constants
(290.1350) Scattering : Backscattering
(300.6270) Spectroscopy : Spectroscopy, far infrared
(350.5400) Other areas of optics : Plasmas
(260.2065) Physical optics : Effective medium theory
(040.2235) Detectors : Far infrared or terahertz
(160.4236) Materials : Nanomaterials
(220.4241) Optical design and fabrication : Nanostructure fabrication
(300.6495) Spectroscopy : Spectroscopy, teraherz

ToC Category:
Materials

History
Original Manuscript: May 28, 2013
Manuscript Accepted: June 21, 2013
Published: July 3, 2013

Citation
Chan-Shan Yang, Chan-Ming Chang, Po-Han Chen, Peichen Yu, and Ci-Ling Pan, "Broadband terahertz conductivity and optical transmission of indium-tin-oxide (ITO) nanomaterials," Opt. Express 21, 16670-16682 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-14-16670


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References

  1. J. W. Shim, H. Cheun, J. Meyer, C. Fuentes-Hernandez, A. Dindar, Y. H. Zhou, D. K. Hwang, A. Kahn, and B. Kippelen, “Polyvinylpyrrolidone-modified indium tin oxide as an electron-collecting electrode for inverted polymer solar cells,” Appl. Phys. Lett.101(7), 073303 (2012). [CrossRef]
  2. Ö. Şenlik, H. Y. Cheong, and T. Yoshie, “Design of subwavelength-size, indium tin oxide (ITO)-clad optical disk cavities with quality-factors exceeding 10⁴,” Opt. Express19(23), 23469–23474 (2011). [CrossRef] [PubMed]
  3. Y.-J. Liu, C.-C. Huang, T.-Y. Chen, C.-S. Hsu, J.-K. Liou, T.-Y. Tsai, and W.-C. Liu, “Implementation of an indium-tin-oxide (ITO) direct-Ohmic contact structure on a GaN-based light emitting diode,” Opt. Express19(15), 14662–14670 (2011). [CrossRef] [PubMed]
  4. S.-Y. Liu, Y.-C. Lin, J.-C. Ye, S. J. Tu, F. W. Huang, M. L. Lee, W. C. Lai, and J. K. Sheu, “Hydrogen gas generation using n-GaN photoelectrodes with immersed Indium Tin oxide Ohmic contacts,” Opt. Express19(S6Suppl 6), A1196–A1201 (2011). [CrossRef] [PubMed]
  5. C. K. Choi, K. D. Kihm, and A. E. English, “Optoelectric biosensor using indium-tin-oxide electrodes,” Opt. Lett.32(11), 1405–1407 (2007). [CrossRef] [PubMed]
  6. P. Yu, C.-H. Chang, C.-H. Chiu, C.-S. Yang, J.-C. Yu, H.-C. Kuo, S.-H. Hsu, and Y.-C. Chang, “Efficiency enhancement of GaAs photovoltaics employing antireflective indium tin oxide nanocolumns,” Adv. Mater.21(16), 1618–1621 (2009). [CrossRef]
  7. P. Yu, C.-H. Chang, M.-S. Su, M.-H. Hsu, and K.-H. Wei, “Embedded indium-tin-oxide nanoelectrodes for efficiency and lifetime enhancement of polymer-based solar cells,” Appl. Phys. Lett.96(15), 153307 (2010). [CrossRef]
  8. J. W. Leem and J. S. Yu, “Glancing angle deposited ITO films for efficiency enhancement of a-Si:H/μc-Si:H tandem thin film solar cells,” Opt. Express19(S3Suppl 3), A258–A268 (2011). [CrossRef] [PubMed]
  9. C.-H. Chang, M.-H. Hsu, P.-C. Tseng, P. Yu, W.-L. Chang, W.-C. Sun, and W.-C. Hsu, “Enhanced angular characteristics of indium tin oxide nanowhisker-coated silicon solar cells,” Opt. Express19(S3Suppl 3), A219–A224 (2011). [CrossRef] [PubMed]
  10. D.-J. Seo, J.-P. Shim, S.-B. Choi, T. H. Seo, E.-K. Suh, and D.-S. Lee, “Efficiency improvement in InGaN-based solar cell s by indium tin oxide nano dots covered with ITO films,” Opt. Express20(S6), A991–A996 (2012). [CrossRef]
  11. J. W. Leem and J. S. Yu, “Indium tin oxide subwavelength nanostructures with surface antireflection and superhydrophilicity for high-efficiency Si-based thin film solar cells,” Opt. Express20(S3), A431–A440 (2012). [CrossRef] [PubMed]
  12. C.-H. Chang, P. Yu, M.-H. Hsu, P.-C. Tseng, W.-L. Chang, W.-C. Sun, W.-C. Hsu, S.-H. Hsu, and Y.-C. Chang, “Combined micro- and nano-scale surface textures for enhanced near-infrared light harvesting in silicon photovoltaics,” Nanotechnology22(9), 095201 (2011). [CrossRef] [PubMed]
  13. C. H. Chiu, P. Yu, C. H. Chang, C. S. Yang, M. H. Hsu, H. C. Kuo, and M. A. Tsai, “Oblique electron-beam evaporation of distinctive indium-tin-oxide nanorods for enhanced light extraction from InGaN/GaN light emitting diodes,” Opt. Express17(23), 21250–21256 (2009). [CrossRef] [PubMed]
  14. T. H. Seo, K. J. Lee, A. H. Park, C.-H. Hong, E.-K. Suh, S. J. Chae, Y. H. Lee, T. V. Cuong, V. H. Pham, J. S. Chung, E. J. Kim, and S.-R. Jeon, “Enhanced light output power of near UV light emitting diodes with graphene / indium tin oxide nanodot nodes for transparent and current spreading electrode,” Opt. Express19(23), 23111–23117 (2011). [CrossRef] [PubMed]
  15. Y. Y. Kee, S. S. Tan, T. K. Yong, C. H. Nee, S. S. Yap, T. Y. Tou, G. Sáfrán, Z. E. Horváth, J. P. Moscatello, and Y. K. Yap, “Low-temperature synthesis of indium tin oxide nanowires as the transparent electrodes for organic light emitting devices,” Nanotechnology23(2), 025706 (2012). [CrossRef] [PubMed]
  16. S. H. Lee and N. Y. Ha, “Nanostructured indium-tin-oxide films fabricated by all-solution processing for functional transparent electrodes,” Opt. Express19(22), 21803–21808 (2011). [CrossRef] [PubMed]
  17. C.-S. Yang, M.-H. Lin, C.-H. Chang, P. Yu, J.-M. Shieh, C.-H. Shen, O. Wada, and C.-L. Pan, “Non-Drude behavior in indium-tin-oxide nanowhiskers and thin films by transmission and reflection THz time-domain spectroscopy,” IEEE J. Quantum Electron., accepted (2013).
  18. T. Bauer, J. S. Kolb, T. Löffler, E. Mohler, U. C. Pernisz, and H. G. Roskos, “Indium-tin-oxide-coated glass as dichroic mirror for far-infrared electromagnetic radiation,” J. Appl. Phys.92(4), 2210–2212 (2002). [CrossRef]
  19. J. Kröll, J. Darmo, and K. Unterrainer, “Metallic wave-impedance matching layers for broadband terahertz optical systems,” Opt. Express15(11), 6552–6560 (2007). [CrossRef] [PubMed]
  20. S. A. Jewell, E. Hendry, T. H. Isaac, and J. R. Sambles, “Tuneable Fabry-Perot etalon for terahertz radiation,” New J. Phys.10(3), 033012 (2008). [CrossRef]
  21. D. G. Cooke and P. U. Jepsen, “Optical modulation of terahertz pulses in a parallel plate waveguide,” Opt. Express16(19), 15123–15129 (2008). [CrossRef] [PubMed]
  22. C. H. Chang, P. Yu, and C. S. Yang, “Broadband and omnidirectional antireflection from conductive indium-tin-oxide nanocolumns prepared by glancing-angle deposition with nitrogen,” Appl. Phys. Lett.94(5), 051114 (2009). [CrossRef]
  23. I. Hamberg, A. Hjortsberg, and C. G. Granqvist, “High quality transparent heat reflectors of reactively evaporated indium tin oxide,” Appl. Phys. Lett.40(5), 362–364 (1982). [CrossRef]
  24. Q. Wan, Z. T. Song, S. L. Feng, and T. H. Wang, “Single-crystalline tin-doped indium oxide whiskers: synthesis and characterization,” Appl. Phys. Lett.85(20), 4759–4761 (2004). [CrossRef]
  25. S.-P. Chiu, H.-F. Chung, Y.-H. Lin, J.-J. Kai, F.-R. Chen, and J.-J. Lin, “Four-probe electrical-transport measurements on single indium tin oxide nanowires between 1.5 and 300 K,” Nanotechnology20(10), 105203 (2009). [CrossRef] [PubMed]
  26. J. Gao, R. Chen, D. H. Li, L. Jiang, J. C. Ye, X. C. Ma, X. D. Chen, Q. H. Xiong, H. D. Sun, and T. Wu, “UV light emitting transparent conducting tin-doped indium oxide (ITO) nanowires,” Nanotechnology22(19), 195706 (2011). [CrossRef] [PubMed]
  27. C.-S. Yang, C.-H. Chang, M.-H. Lin, P. Yu, O. Wada, and C.-L. Pan, “THz conductivities of indium-tin-oxide nanowhiskers as a graded-refractive-index structure,” Opt. Express20(S4Suppl 4), A441–A451 (2012). [CrossRef] [PubMed]
  28. C.-W. Chen, Y.-C. Lin, C.-H. Chang, P. Yu, J.-M. Shieh, and C.-L. Pan, “Frequency-dependent complex conductivities and electric responses of indium tin oxide thin films from the visible to the far-infrared,” IEEE J. Quantum Electron.46(12), 1746–1754 (2010). [CrossRef]
  29. G. M. Turner, M. C. Beard, and C. A. Schmuttenmaer, “Carrier localization and cooling in dye-sensitized nanocrystalline titanium dioxide,” J. Phys. Chem. B106(45), 11716–11719 (2002). [CrossRef]
  30. H. Němec, P. Kužel, and V. Sundström, “Far-infrared response of free charge carriers localized in semiconductor nanoparticles,” Phys. Rev. B79(11), 115309 (2009). [CrossRef]
  31. J. B. Baxter and C. A. Schmuttenmaer, “Conductivity of ZnO nanowires, nanoparticles, and thin films using time-resolved terahertz spectroscopy,” J. Phys. Chem. B110(50), 25229–25239 (2006). [CrossRef] [PubMed]
  32. G. Ma, D. Li, H. Ma, J. Shen, C. Wu, J. Ge, S. Hu, and N. Dai, “Carrier concentration dependence of terahertz transmission on conducting ZnO films,” Appl. Phys. Lett.93(21), 211101 (2008). [CrossRef]
  33. X. Zou, J. Luo, D. Lee, C. Cheng, D. Springer, S. K. Nair, S. A. Cheong, H. J. Fan, and E. E. M. Chia, “Temperature-dependent terahertz conductivity of tin oxide nanowire films,” J. Phys. D Appl. Phys.45(46), 465101 (2012). [CrossRef]
  34. D. Tsokkou, A. Othonos, and M. Zervos, “Carrier dynamics and conductivity of SnO2 nanowires investigated by time-resolved terahertz spectroscopy,” Appl. Phys. Lett.100(13), 133101 (2012). [CrossRef]
  35. I.-C. Ho, X. Guo, and X.-C. Zhang, “Design and performance of reflective terahertz air-biased-coherent-detection for time-domain spectroscopy,” Opt. Express18(3), 2872–2883 (2010). [CrossRef] [PubMed]
  36. B. Clough, J. Liu, and X.-C. Zhang, ““All air-plasma” terahertz spectroscopy,” Opt. Lett.36(13), 2399–2401 (2011). [CrossRef] [PubMed]
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