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THz-wave absorption by field-induced carriers in pentacene thin-film transistors for THz imaging sensors
Organic Electronics 14 (2013) 1157–1162
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
THz-wave absorption by field-induced carriers in pentacene thin-film transistors for THz imaging sensors Shi-Guang Li a,b,⇑, Ryosuke Matsubara b, Toshio Matsusue c, Masatoshi Sakai a, Kazuhiro Kudo a, Masakazu Nakamura b,⇑ a
Department of Electrical and Electronic Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0192, Japan c Graduate School of Advanced Integration Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan b
a r t i c l e
i n f o
Article history: Received 18 October 2012 Received in revised form 22 January 2013 Accepted 5 February 2013 Available online 24 February 2013 Keywords: THz wave Free carrier absorption Organic field-effect transistor Pentacene Time-domain spectroscopy Band-edge fluctuation
a b s t r a c t Innovative sensing systems based on THz electromagnetic waves have been attracting a great deal of attention. Although many THz detectors have been developed over the years, it is currently difficult to manufacture low-cost THz sensing/imaging devices. In the present study, we propose to use organic field-effect transistors (OFETs) and small potential fluctuation against the carriers within them (N. Ohashi, H. Tomii, R. Matsubara, M. Sakai, K. Kudo, M. Nakamura, Appl. Phys. Lett. 91 (2007) 162105). We use THz time-domain spectroscopy for OFETs in which the carrier density in the pentacene active layer is modulated by the gate bias. We found evidence that the accumulated free holes in pentacene films can be excited by THz photons to overcome the surrounding barriers in the fluctuating potential. The Drude–Lorentz model could not account for the shape of the absorption spectra, which suggests that the holes are weakly restricted by the potential fluctuation. The integrated absorption intensity was proportional to the transfer characteristics of the OFETs. The present findings represent an important step toward developing a new class of THzwave sensors. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction THz waves (frequency: 0.1–10 THz) lie between light and radio waves. Innovative sensing systems based on THz waves have been attracting a great deal of attention in recent years. The growth of research into THz waves is driven by scientific and industrial applications in areas such as defense, security, biology, and medicine [1–4]. Although many THz detectors have been developed over the years [5], it is currently difficult to manufacture low-
⇑ Corresponding authors. Address: Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0192, Japan. Tel./fax: +81 743 72 6125 (S.G. Li), +81 743 72 6031 (M. Nakamura). E-mail addresses: [email protected] (S.-G. Li), [email protected] (M. Nakamura). 1566-1199/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2013.02.003
cost THz sensing/imaging devices. Here, we therefore propose using organic field-effect transistors (OFETs) in THz detectors due to their low fabrication cost and their ability to realize flexible electronics. In previous studies, we found that the highest occupied molecular orbital (HOMO)-band edge of a pentacene thin film fluctuates randomly due to the boundaries of small crystallites [6]. These fluctuations have a root-meansquare amplitude of about 15 meVrms and the local barrier height against carrier transport is typically 1–10 meV [7,8], which is close to the photon energies of THz waves (0.41– 41 meV). It is thus highly probable that irradiating THz waves will enhance carrier transport in OFETs that have pentacene thin films (see Fig. 1). When OFETs are used in THz sensors, large-area THz sensor matrices such as a sheet imaging devices can be produced at a relatively low cost. The sensing mechanism of a THz sensor with an OFET structure consists of two steps (see Fig. 2): free carriers
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the Si gate/substrate was carefully separated from modulation by the entire OFET structure. This enabled us to obtain THz absorption spectra of the free carriers in organic thin films induced by the gate electric field. 2. Experimental
Fig. 1. Schematic 3D band diagram of THz sensor with OFET structure that has small band-edge fluctuation.
Fig. 2. Sensing mechanism of THz wave by the THz sensor with an OFET structure.
in the organic layer receive energy from THz photons (first stage) and they then overcome the barrier due to potential fluctuations and drift toward the drain electrode (second stage). The first stage can be demonstrated by showing that absorption of THz waves depends on the density of free carriers in organic layers; however, there have been no reports of this so far. To the best of our knowledge, only one study [9] has reported the THz modulation absorption spectrum of gate-bias-induced carriers in OFETs. In that study, poly[(9,9-dioctylfluorene-2,7-diyl)-co-(bithiophene)] (F8T2) was used in the active layers and the THz absorption was measured by varying the gate bias. However, the authors of that study concluded that modulation of THz absorption by the gate bias is not due to free carriers in the organic layer but to those in the Si gate/substrate. In the present study, a high-mobility organic material, pentacene, was used instead of F8T2, which has a low mobility. In addition, modulation of THz absorption in
Fig. 3 shows a schematic diagram of the bottom-contact OFET fabricated in this study. A lightly doped n-Si(1 0 0) wafer (resistivity: 3.78–4.20 X cm) was used as the substrate and the gate electrode, while thermally oxidized SiO2 (thickness: 300 nm) served as the gate dielectric. We used a comparatively high-resistivity Si wafer to reduce THz absorption by electrons in the substrate. Substrates with SiO2 were sequentially cleaned in acetone, methanol, and deionized (DI) water and blown dry by N2. Interdigital Au source/drain electrodes were utilized to give a wide channel surface area. Bottom-contact OFETs with the above structure was formed on SiO2/Si substrates by the conventional photolithography and lift-off technique. After stripping the photoresist, the partially completed device was cleaned using acetone and UV/O3 (200 °C, 30 min) to remove surface contamination. A 50-nm-thick pentacene layer was vacuum deposited (2 10 4 Pa) on the bare SiO2/Si substrates and hexamethyldisilazane (HMDS)-treated SiO2/Si substrates. The growth rate was fixed at 1.0 nm/ min and the substrate temperature was 60 °C. The OFETs had a channel width of 4.4 20 mm and a length of 100 lm. To check the reproducibility, we fabricated four OFETs with different field-effect mobilities and gate threshold voltages. The transistor characteristics were measured using a semiconductor parameter analyzer (Agilent Technologies, E5272A equipped with E5281A SMU). To investigate the influence of free carrier absorption in silicon substrates, we used a control sample in which a thin (6 nm) Au layer that can transmit THz waves was deposited instead of the pentacene active layer. The absorption spectra of both the OFET and control samples were measured by THz time-domain spectroscopy (THz-TDS) [10– 12] under the same measurement conditions by alternately applying on- and off-state gate biases. In the THzTDS system, THz-wave emitter and detector antennas with low-temperature-grown gallium arsenide (LT-GaAs) (Hamamatsu, G10620) [13,14] were excited by pump and probe femtosecond laser pulses (pulse width: 76–100 fs; wavelength: 800 nm; repetition rate: 75 MHz). The average power incident on each antenna was 10 mW. The sample and THz wave path were in an N2 atmosphere to
Fig. 3. (a) Plan and (b) cross-sectional views of OFET used in this study.
S.-G. Li et al. / Organic Electronics 14 (2013) 1157–1162
minimize absorption by atmospheric water vapor. To prevent diffraction by the source/drain electrodes from greatly affecting the absorption measurements [9], two lenses were placed in a wide parallel THz beam before and after the sample, resulting in a wide range of incident and detection angles of THz waves. The procedure used to measure the modulation absorption spectrum by THz-TDS is as follows: (1) an OFET-structure sample was inserted in the THz-TDS system with connections for transistor operation; (2) two time-domain profiles were accumulated by alternately switching the gate bias between the on ( 10 V to 30 V) and off (+30 V) states; (3) the time-domain profiles were converted to frequency-domain power spectra by taking the Fourier transform; and (4) the on-state spectrum was normalized by the off-state spectrum to obtain the gate-modulation spectrum. A resulting value of less than unity indicates that THz absorption is increased by carriers accumulated by the on-state gate electric field. 3. Results and discussion The red solid lines in Fig. 4a–d respectively show the transfer characteristics of OFET samples A–D, which are operated in the linear region. The four OFETs exhibit different field-effect mobility and gate-threshold voltage characteristics. The OFETs fabricated using HMDS treatment (samples C and D) have higher mobilities than those that fabricated without HMDS treatment (samples A and B).
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Fig. 5 shows some of the results of the modulation absorption measurements performed in this study. The four different markers in Fig. 5 represent the modulation absorption spectra of the four OFETs for an on-state gate bias of 30 V, while the thick red solid line indicates their average. In this measurement, the spectrum is reliable only within the frequency range of 0.2–2.0 THz due to absorption by residual water vapor and the sensitivity of the THz-TDS system. Within this range, the shapes of the spectra are approximately the same and the transmittance is always less than unity. These results indicate that carriers accumulated in the OFETs increase the THz absorption. One may notice that there are two sharp peaks at around 1.1 and 1.7 eV. Since the measurements were carried out at room temperature, these sharp peaks cannot originate from any electronic structure in the sample. Judging from the reproducibility of these peaks, which also depended on the sample azimuth angle in the measurement system, we provisionally concluded that these are due to weak resonances by the interdigital electrodes. The THz-wave absorption in Fig. 5 is considered to be increased by free holes in the pentacene layer and by free electrons in the Si substrate, as pointed out in Ref. [9]. It is thus necessary to remove the influence of the substrate to obtain the actual THz absorption spectrum of the holes in pentacene. To achieve this, we used a control sample that had a 6-nm-thick Au layer instead of a pentacene active layer; this gold layer is semi-transparent in the THz range, but is sufficiently conductive to act as an electrode of the
Fig. 4. Transfer characteristics of four OFETs (red solid lines). Panels (a–d) correspond to samples A–D, respectively. The SiO2 substrate surfaces of samples C and D were treated with HMDS, whereas they were not treated with HMDS in samples A and B. Blue squares indicate the THz absorption intensity integrated from 0.2 to 2.0 THz at each gate bias, which corresponds to the hatched area in Fig. 7 (sample B) and similar results (other samples). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 5. Modulation absorption spectra of the four OFETs for an on-state gate bias of 30 V. Four different markers indicate spectra of samples A– D, while the solid line indicates their average.
metal–oxide-semiconductor capacitor. Only a small proportion of the free electrons in a metal which is close to the Fermi level can absorb the photon energy of a THz wave since they have low energies ranging from a few meV to a few tens of meV. Although the thin Au layer has a high free electron density, the free electrons that contribute to THz absorption are much more insensitive to the typical gate voltage than the free electron density in Si because the density-of-state function of Au near the Fermi level is nearly constant [15]. We thus assume that the change in the free electron density in the Si substrate in the control sample is mainly responsible for modulation of THz absorption. Fig. 6 shows the modulation absorption spectra of sample B (see Fig. 4) and the control sample measured under the same conditions. These two spectra have different shapes and the control sample has a lower absorption than
Fig. 6. Modulation absorption spectra of OFET sample B (red thick line) and control sample (blue thin line) obtained under the same measurement conditions. Dashed line is a theoretical curve obtained using the Drude model; it is fitted to the control sample spectrum. Horizontally and vertically hatched areas indicate absorption by the accumulated electrons in Si and by the holes in pentacene, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
the OFET sample. The absorption spectrum of the control sample is well fitted by a theoretical curve obtained using the Drude model (indicated by the dashed line in Fig. 6), which is consistent with previously reported results [16]. We thus conclude that modulation of the absorption by the control sample is due to the change of the electron density in the Si. On the other hand, the OFET sample had a larger absorption than the control sample. The absorption difference between the OFET and control samples (emphasized by red vertical hatching in Fig. 6) is thus considered to be due to absorption by accumulated holes in the pentacene layer induced by the gate electric field. Fig. 7a–c shows the change in the modulation absorption spectra of sample B when the on-state gate bias is varied from 10 to 30 V. At all gate biases, the OFET sample has a higher absorption than the control sample. The difference in the absorption spectra between the OFET and control samples (red vertically hatched area in Fig. 7) increases as the on-stage gate bias is shifted to the higher accumulation (negative) side. In addition, at all biases, the difference is smaller at lower frequencies. The spectral shape in the range 0.2–2.0 THz differs significantly from that of the free electrons in Si and the simple Drude or Drude–Lorentz models [17] cannot reproduce it. We thus conclude that the gate-induced holes in the pentacene layer have quite a different environment from the free electrons in a Si single crystal, although both are free carriers in semiconducting bands. The holes in the pentacene are not completely free; rather, they are weakly restricted at the potential minima in the fluctuating HOMO band [6], as depicted in Fig. 2. By increasing the photon energy to a few meV ranges, restricted holes will absorb energy more efficiently from THz photons, allowing them to overcome the surrounding potential barriers. To compare THz absorption and the transistor characteristics, we calculated the absorption by accumulated holes in the pentacene layer. First, the absorption spectrum of the control sample was subtracted from the corresponding OFET spectrum. We then converted the spectrum from transmittance to absorbance and integrated it from 0.2 to 2.0 THz. The integrated intensity of THz absorption is plotted in the transfer curves in Fig. 4a–d using scaling factors to fit them to the drain current curves. The integrated THz absorption intensity is proportional to the drain current (or channel conductance) of each OFET. The OFETs fabricated by treating the SiO2 substrates with HMDS generally have higher mobilities than those fabricated without HMDS treatment. However, in a previous study [18], we found that both OFETs fabricated with and without HMDS treatment had quantitatively the same crystallite size and HOMO-band-edge fluctuation. The mobility difference observed in the present study is thus due to a difference in the average grain sizes of the pentacene films. Within the reliable frequency range (0.2– 2 THz), all four OFETs have similar spectral shapes, which implies that the HOMO-band-edge fluctuation is insensitive to the growth conditions of the pentacene film and to chemical modification of the substrate surfaces. To confirm the proportionality between the THz absorption intensity and the free carrier density, Fig. 8 compares the field-effect mobilities of the four OFETs with the ratio
S.-G. Li et al. / Organic Electronics 14 (2013) 1157–1162
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Fig. 7. Variation of modulation absorption spectra of the OFET sample B (red thick line) and the control sample (blue thin line) for on-state gate bias ( 10 to 30 V). Vertically hatched area denotes absorption by holes accumulated in pentacene. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 8. Plot of field-effect mobility as a function of ratio of the drain current to integrated THz absorbance. The vertical axis denotes the fieldeffect mobility calculated from the linear part of the transfer curve in Fig. 4. The horizontal axis denotes the value of [drain current]/[integrated THz absorbance] used to scale the left and right vertical axes in Fig. 4.
of the drain current to the integrated THz absorbance (which is plotted in Fig. 4). All the data points lie on a straight line that passes through the origin of the graph. The results of Figs. 4 and 8 suggest that the THz absorption intensity is proportional to the density of free holes in the pentacene layer that can contribute to channel conduction. The sensitivity of THz absorption to carrier density is constant, despite the ‘apparent’ field-effect mobility of the OFET varying by one order of magnitude. The results also suggest that the nonlinear increase in the drain current with increasing gate bias (see Fig. 4a–d) is not due to the gate voltage dependence of the mobility but to the nonlinear increase in free carriers. As mentioned above, all the free holes in the four OFETs are considered to be distributed over similarly fluctuating HOMO bands. In that case, most of the free holes will be located in the local minima of the band-edge fluctuation according to Fermi–Dirac statistics. This would explain why the modulation of absorption increases with increasing THz photon energy. At the same time, this distribution will reduce the effective carrier mobility because the weakly restricted holes in the fluctuation valley cannot evenly contribute to the steady-state drift current in the channel. This is why pentacene thin films have a much lower carrier mobility than single crystals [7,8,19]. The absorption intensity of the four OFETs used in this study had the same dependence on the density of holes that con-
tribute to the drain current (see Fig. 8), despite only a portion of THz holes being able to contribute to the steadystate drain current. This is also understandable if the four OFETs have quantitatively the same band-edge fluctuation [18]. Finally, we discuss the difference between this study and a previous study by Lloyd-Hughes et al. [9]. The main difference is the semiconductor materials used: we used high-carrier-mobility polycrystalline pentacene, whereas Lloyd-Hughes et al. used a low-carrier-mobility (<10 2 cm2/V s) disordered semiconducting material (F8T2). Even though the OFETs used in this study had an apparent field-effect mobility of less than 1 cm2/V s, the local carrier mobility in the small crystallites is expected to be as high as that in single crystals (>10 cm2/V s). This large difference of carrier mobility results in the difference of the sensitivity of THz absorption. The present study also used a control sample to separate the modulation of the absorption by the Si substrate from that of the pentacene layer. These are the reason why this work succeeded to obtain the THz absorption by the accumulated carriers in an organic semiconductor layer. 4. Conclusions We obtained THz-modulated adsorption spectra due to accumulated free holes in the pentacene layers of OFETs by separating the absorption due to free electrons in the Si substrate. The THz absorption by free holes in pentacene increased when the frequency was increased from 0.2 to 2.0 THz. The Drude–Lorentz model could not explain the spectral shape, despite agreeing well with the absorption spectra of free electrons in Si. The obtained unique absorption spectra are considered to be due to fluctuations in the HOMO band edge of the pentacene films, as found in previous studies [6–8]. The four OFETs tested in this study exhibited the same sensitivity of the integrated absorption intensity to the density of holes that contribute to the drain current. This suggests that the four OFETs have qualitatively the same band-edge fluctuations. The present results clearly indicate that holes accumulated in the pentacene channel can be excited by THz photons, allowing them to overcome the surrounding potential barriers in the fluctuating HOMO band.
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We consider that the results of the present study represent an important step toward developing a new class of THz-wave sensors with OFET structures. In the next step, THz photoconductivity measurements will be performed and the sensitivity will be maximized by optimizing the device design and eliminating higher barriers against carrier transport. The density of high barriers, >100 meV, at grain boundaries can be reduced by chemically modifying the substrate surface [18]. Using graphoepitaxy to align the grains in a single direction [20–22] will also reduce the density of high barriers. Acknowledgements This study was supported by a Grant-in-Aid for Scientific Research (B) (No. 21350099), the Chiba University Global COE Program ‘‘Advanced School for Organic Electronics’’, and a Grant-in-Aid for JSPS Fellows (No. 2356142). References [1] K. Kawase, Y. Ogawa, Y. Watanabe, Opt. Exp. 11 (2003) 2549. [2] Y.C. Shen, T. Lo, P.F. Taday, B.E. Cole, W.R. Tribe, M.C. Kemp, Appl. Phys. Lett. 86 (2005) 241116. [3] B. Ferquson, S. Wang, D. Gray, D. Abbot, X.C. Zhang, Opt. Lett. 27 (2002) 1312.
[4] J.B. Baxter, G.W. Guglietta, Anal. Chem. 83 (2011) 4342. [5] F. Sizov, Opto-Electron Rev. 18 (2010) 10. [6] N. Ohashi, H. Tomii, R. Matsubara, M. Sakai, K. Kudo, M. Nakamura, Appl. Phys. Lett. 91 (2007) 162105. [7] R. Matsubara, N. Ohashi, M. Sakai, K. Kudo, M. Nakamura, Appl. Phys. Lett. 92 (2008) 242108. [8] R. Matsubara, M. Sakai, K. Kudo, N. Yoshimoto, I. Hirosawa, M. Nakamura, Org. Electron. 12 (2011) 195. [9] J. Lloyd-Hughes, T. Richards, H. Sirringhaus, E. Castro-Camus, L.M. Herz, M.B. Johnston, Appl. Phys. Lett. 89 (2006) 112101. [10] D. Grischkowsky, S. Keiding, M. Van Exter, Ch. Fattinger, J. Opt. Soc. Am. B 7 (1990) 2006. [11] M.C. Nuss, J. Orenstein, Top. Appl. Phys. 74 (1998) 7. [12] M. Tani, Y. Hirota, C. Que, S. Tanaka, R. Hattori, M. Yamaguchi, S. Nishizawa, M. Hangyo, Int J. Infrared Milli. 27 (2006) 531. [13] M. Tani, S. Matsuura, K. Sakai, S.-I. Nakashima, Appl. Opt. 36 (1997) 7853. [14] K. Sakai, M. Tani, Top. Appl. Phys. 97 (2005) 1. [15] Z. Lin, L.V. Zhigilei, V. Celli, Phys. Rev. B 77 (2008) 075133. [16] T.-I. Jeon, D. Grischkowsky, Appl. Phys. Lett. 72 (1998) 3032. [17] S.R. Nagel, S.E. Schnatterly, Phys. Rev. B 9 (1974) 1299. [18] R. Matsubara, T. Nomura, Y. Sakai, M. Sakai, K. Kudo, Y. Majima, M. Nakamura, International Conference on Organic Electronics 2010, in: June 22–25, Paris, France, 2010, pp. 196P82. [19] M. Nakamura, H. Ohguri, N. Goto, H. Tomii, M.-S. Xu, T. Miyamoto, R. Matsubara, N. Ohashi, M. Sakai, K. Kudo, Appl. Phys. A 95 (2009) 73. [20] M.-S. Xu, M. Nakamura, M. Sakai, K. Kudo, Adv. Mater. 19 (2007) 371. [21] S. Ikeda, K. Saiki, Y. Wada, K. Inaba, Y. Ito, H. Kikuchi, K. Terashima, T. Shimada, J. Appl. Phys. 103 (2008) 084313. [22] S.-G. Li, N. Nakayma, M. Sakai, K. Kudo, R. Matsubara, M. Nakamura, Org. Electron. 13 (2012) 864.
Contents lists available at SciVerse ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
THz-wave absorption by field-induced carriers in pentacene thin-film transistors for THz imaging sensors Shi-Guang Li a,b,⇑, Ryosuke Matsubara b, Toshio Matsusue c, Masatoshi Sakai a, Kazuhiro Kudo a, Masakazu Nakamura b,⇑ a
Department of Electrical and Electronic Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0192, Japan c Graduate School of Advanced Integration Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan b
a r t i c l e
i n f o
Article history: Received 18 October 2012 Received in revised form 22 January 2013 Accepted 5 February 2013 Available online 24 February 2013 Keywords: THz wave Free carrier absorption Organic field-effect transistor Pentacene Time-domain spectroscopy Band-edge fluctuation
a b s t r a c t Innovative sensing systems based on THz electromagnetic waves have been attracting a great deal of attention. Although many THz detectors have been developed over the years, it is currently difficult to manufacture low-cost THz sensing/imaging devices. In the present study, we propose to use organic field-effect transistors (OFETs) and small potential fluctuation against the carriers within them (N. Ohashi, H. Tomii, R. Matsubara, M. Sakai, K. Kudo, M. Nakamura, Appl. Phys. Lett. 91 (2007) 162105). We use THz time-domain spectroscopy for OFETs in which the carrier density in the pentacene active layer is modulated by the gate bias. We found evidence that the accumulated free holes in pentacene films can be excited by THz photons to overcome the surrounding barriers in the fluctuating potential. The Drude–Lorentz model could not account for the shape of the absorption spectra, which suggests that the holes are weakly restricted by the potential fluctuation. The integrated absorption intensity was proportional to the transfer characteristics of the OFETs. The present findings represent an important step toward developing a new class of THzwave sensors. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction THz waves (frequency: 0.1–10 THz) lie between light and radio waves. Innovative sensing systems based on THz waves have been attracting a great deal of attention in recent years. The growth of research into THz waves is driven by scientific and industrial applications in areas such as defense, security, biology, and medicine [1–4]. Although many THz detectors have been developed over the years [5], it is currently difficult to manufacture low-
⇑ Corresponding authors. Address: Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0192, Japan. Tel./fax: +81 743 72 6125 (S.G. Li), +81 743 72 6031 (M. Nakamura). E-mail addresses: [email protected] (S.-G. Li), [email protected] (M. Nakamura). 1566-1199/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2013.02.003
cost THz sensing/imaging devices. Here, we therefore propose using organic field-effect transistors (OFETs) in THz detectors due to their low fabrication cost and their ability to realize flexible electronics. In previous studies, we found that the highest occupied molecular orbital (HOMO)-band edge of a pentacene thin film fluctuates randomly due to the boundaries of small crystallites [6]. These fluctuations have a root-meansquare amplitude of about 15 meVrms and the local barrier height against carrier transport is typically 1–10 meV [7,8], which is close to the photon energies of THz waves (0.41– 41 meV). It is thus highly probable that irradiating THz waves will enhance carrier transport in OFETs that have pentacene thin films (see Fig. 1). When OFETs are used in THz sensors, large-area THz sensor matrices such as a sheet imaging devices can be produced at a relatively low cost. The sensing mechanism of a THz sensor with an OFET structure consists of two steps (see Fig. 2): free carriers
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the Si gate/substrate was carefully separated from modulation by the entire OFET structure. This enabled us to obtain THz absorption spectra of the free carriers in organic thin films induced by the gate electric field. 2. Experimental
Fig. 1. Schematic 3D band diagram of THz sensor with OFET structure that has small band-edge fluctuation.
Fig. 2. Sensing mechanism of THz wave by the THz sensor with an OFET structure.
in the organic layer receive energy from THz photons (first stage) and they then overcome the barrier due to potential fluctuations and drift toward the drain electrode (second stage). The first stage can be demonstrated by showing that absorption of THz waves depends on the density of free carriers in organic layers; however, there have been no reports of this so far. To the best of our knowledge, only one study [9] has reported the THz modulation absorption spectrum of gate-bias-induced carriers in OFETs. In that study, poly[(9,9-dioctylfluorene-2,7-diyl)-co-(bithiophene)] (F8T2) was used in the active layers and the THz absorption was measured by varying the gate bias. However, the authors of that study concluded that modulation of THz absorption by the gate bias is not due to free carriers in the organic layer but to those in the Si gate/substrate. In the present study, a high-mobility organic material, pentacene, was used instead of F8T2, which has a low mobility. In addition, modulation of THz absorption in
Fig. 3 shows a schematic diagram of the bottom-contact OFET fabricated in this study. A lightly doped n-Si(1 0 0) wafer (resistivity: 3.78–4.20 X cm) was used as the substrate and the gate electrode, while thermally oxidized SiO2 (thickness: 300 nm) served as the gate dielectric. We used a comparatively high-resistivity Si wafer to reduce THz absorption by electrons in the substrate. Substrates with SiO2 were sequentially cleaned in acetone, methanol, and deionized (DI) water and blown dry by N2. Interdigital Au source/drain electrodes were utilized to give a wide channel surface area. Bottom-contact OFETs with the above structure was formed on SiO2/Si substrates by the conventional photolithography and lift-off technique. After stripping the photoresist, the partially completed device was cleaned using acetone and UV/O3 (200 °C, 30 min) to remove surface contamination. A 50-nm-thick pentacene layer was vacuum deposited (2 10 4 Pa) on the bare SiO2/Si substrates and hexamethyldisilazane (HMDS)-treated SiO2/Si substrates. The growth rate was fixed at 1.0 nm/ min and the substrate temperature was 60 °C. The OFETs had a channel width of 4.4 20 mm and a length of 100 lm. To check the reproducibility, we fabricated four OFETs with different field-effect mobilities and gate threshold voltages. The transistor characteristics were measured using a semiconductor parameter analyzer (Agilent Technologies, E5272A equipped with E5281A SMU). To investigate the influence of free carrier absorption in silicon substrates, we used a control sample in which a thin (6 nm) Au layer that can transmit THz waves was deposited instead of the pentacene active layer. The absorption spectra of both the OFET and control samples were measured by THz time-domain spectroscopy (THz-TDS) [10– 12] under the same measurement conditions by alternately applying on- and off-state gate biases. In the THzTDS system, THz-wave emitter and detector antennas with low-temperature-grown gallium arsenide (LT-GaAs) (Hamamatsu, G10620) [13,14] were excited by pump and probe femtosecond laser pulses (pulse width: 76–100 fs; wavelength: 800 nm; repetition rate: 75 MHz). The average power incident on each antenna was 10 mW. The sample and THz wave path were in an N2 atmosphere to
Fig. 3. (a) Plan and (b) cross-sectional views of OFET used in this study.
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minimize absorption by atmospheric water vapor. To prevent diffraction by the source/drain electrodes from greatly affecting the absorption measurements [9], two lenses were placed in a wide parallel THz beam before and after the sample, resulting in a wide range of incident and detection angles of THz waves. The procedure used to measure the modulation absorption spectrum by THz-TDS is as follows: (1) an OFET-structure sample was inserted in the THz-TDS system with connections for transistor operation; (2) two time-domain profiles were accumulated by alternately switching the gate bias between the on ( 10 V to 30 V) and off (+30 V) states; (3) the time-domain profiles were converted to frequency-domain power spectra by taking the Fourier transform; and (4) the on-state spectrum was normalized by the off-state spectrum to obtain the gate-modulation spectrum. A resulting value of less than unity indicates that THz absorption is increased by carriers accumulated by the on-state gate electric field. 3. Results and discussion The red solid lines in Fig. 4a–d respectively show the transfer characteristics of OFET samples A–D, which are operated in the linear region. The four OFETs exhibit different field-effect mobility and gate-threshold voltage characteristics. The OFETs fabricated using HMDS treatment (samples C and D) have higher mobilities than those that fabricated without HMDS treatment (samples A and B).
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Fig. 5 shows some of the results of the modulation absorption measurements performed in this study. The four different markers in Fig. 5 represent the modulation absorption spectra of the four OFETs for an on-state gate bias of 30 V, while the thick red solid line indicates their average. In this measurement, the spectrum is reliable only within the frequency range of 0.2–2.0 THz due to absorption by residual water vapor and the sensitivity of the THz-TDS system. Within this range, the shapes of the spectra are approximately the same and the transmittance is always less than unity. These results indicate that carriers accumulated in the OFETs increase the THz absorption. One may notice that there are two sharp peaks at around 1.1 and 1.7 eV. Since the measurements were carried out at room temperature, these sharp peaks cannot originate from any electronic structure in the sample. Judging from the reproducibility of these peaks, which also depended on the sample azimuth angle in the measurement system, we provisionally concluded that these are due to weak resonances by the interdigital electrodes. The THz-wave absorption in Fig. 5 is considered to be increased by free holes in the pentacene layer and by free electrons in the Si substrate, as pointed out in Ref. [9]. It is thus necessary to remove the influence of the substrate to obtain the actual THz absorption spectrum of the holes in pentacene. To achieve this, we used a control sample that had a 6-nm-thick Au layer instead of a pentacene active layer; this gold layer is semi-transparent in the THz range, but is sufficiently conductive to act as an electrode of the
Fig. 4. Transfer characteristics of four OFETs (red solid lines). Panels (a–d) correspond to samples A–D, respectively. The SiO2 substrate surfaces of samples C and D were treated with HMDS, whereas they were not treated with HMDS in samples A and B. Blue squares indicate the THz absorption intensity integrated from 0.2 to 2.0 THz at each gate bias, which corresponds to the hatched area in Fig. 7 (sample B) and similar results (other samples). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 5. Modulation absorption spectra of the four OFETs for an on-state gate bias of 30 V. Four different markers indicate spectra of samples A– D, while the solid line indicates their average.
metal–oxide-semiconductor capacitor. Only a small proportion of the free electrons in a metal which is close to the Fermi level can absorb the photon energy of a THz wave since they have low energies ranging from a few meV to a few tens of meV. Although the thin Au layer has a high free electron density, the free electrons that contribute to THz absorption are much more insensitive to the typical gate voltage than the free electron density in Si because the density-of-state function of Au near the Fermi level is nearly constant [15]. We thus assume that the change in the free electron density in the Si substrate in the control sample is mainly responsible for modulation of THz absorption. Fig. 6 shows the modulation absorption spectra of sample B (see Fig. 4) and the control sample measured under the same conditions. These two spectra have different shapes and the control sample has a lower absorption than
Fig. 6. Modulation absorption spectra of OFET sample B (red thick line) and control sample (blue thin line) obtained under the same measurement conditions. Dashed line is a theoretical curve obtained using the Drude model; it is fitted to the control sample spectrum. Horizontally and vertically hatched areas indicate absorption by the accumulated electrons in Si and by the holes in pentacene, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
the OFET sample. The absorption spectrum of the control sample is well fitted by a theoretical curve obtained using the Drude model (indicated by the dashed line in Fig. 6), which is consistent with previously reported results [16]. We thus conclude that modulation of the absorption by the control sample is due to the change of the electron density in the Si. On the other hand, the OFET sample had a larger absorption than the control sample. The absorption difference between the OFET and control samples (emphasized by red vertical hatching in Fig. 6) is thus considered to be due to absorption by accumulated holes in the pentacene layer induced by the gate electric field. Fig. 7a–c shows the change in the modulation absorption spectra of sample B when the on-state gate bias is varied from 10 to 30 V. At all gate biases, the OFET sample has a higher absorption than the control sample. The difference in the absorption spectra between the OFET and control samples (red vertically hatched area in Fig. 7) increases as the on-stage gate bias is shifted to the higher accumulation (negative) side. In addition, at all biases, the difference is smaller at lower frequencies. The spectral shape in the range 0.2–2.0 THz differs significantly from that of the free electrons in Si and the simple Drude or Drude–Lorentz models [17] cannot reproduce it. We thus conclude that the gate-induced holes in the pentacene layer have quite a different environment from the free electrons in a Si single crystal, although both are free carriers in semiconducting bands. The holes in the pentacene are not completely free; rather, they are weakly restricted at the potential minima in the fluctuating HOMO band [6], as depicted in Fig. 2. By increasing the photon energy to a few meV ranges, restricted holes will absorb energy more efficiently from THz photons, allowing them to overcome the surrounding potential barriers. To compare THz absorption and the transistor characteristics, we calculated the absorption by accumulated holes in the pentacene layer. First, the absorption spectrum of the control sample was subtracted from the corresponding OFET spectrum. We then converted the spectrum from transmittance to absorbance and integrated it from 0.2 to 2.0 THz. The integrated intensity of THz absorption is plotted in the transfer curves in Fig. 4a–d using scaling factors to fit them to the drain current curves. The integrated THz absorption intensity is proportional to the drain current (or channel conductance) of each OFET. The OFETs fabricated by treating the SiO2 substrates with HMDS generally have higher mobilities than those fabricated without HMDS treatment. However, in a previous study [18], we found that both OFETs fabricated with and without HMDS treatment had quantitatively the same crystallite size and HOMO-band-edge fluctuation. The mobility difference observed in the present study is thus due to a difference in the average grain sizes of the pentacene films. Within the reliable frequency range (0.2– 2 THz), all four OFETs have similar spectral shapes, which implies that the HOMO-band-edge fluctuation is insensitive to the growth conditions of the pentacene film and to chemical modification of the substrate surfaces. To confirm the proportionality between the THz absorption intensity and the free carrier density, Fig. 8 compares the field-effect mobilities of the four OFETs with the ratio
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Fig. 7. Variation of modulation absorption spectra of the OFET sample B (red thick line) and the control sample (blue thin line) for on-state gate bias ( 10 to 30 V). Vertically hatched area denotes absorption by holes accumulated in pentacene. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 8. Plot of field-effect mobility as a function of ratio of the drain current to integrated THz absorbance. The vertical axis denotes the fieldeffect mobility calculated from the linear part of the transfer curve in Fig. 4. The horizontal axis denotes the value of [drain current]/[integrated THz absorbance] used to scale the left and right vertical axes in Fig. 4.
of the drain current to the integrated THz absorbance (which is plotted in Fig. 4). All the data points lie on a straight line that passes through the origin of the graph. The results of Figs. 4 and 8 suggest that the THz absorption intensity is proportional to the density of free holes in the pentacene layer that can contribute to channel conduction. The sensitivity of THz absorption to carrier density is constant, despite the ‘apparent’ field-effect mobility of the OFET varying by one order of magnitude. The results also suggest that the nonlinear increase in the drain current with increasing gate bias (see Fig. 4a–d) is not due to the gate voltage dependence of the mobility but to the nonlinear increase in free carriers. As mentioned above, all the free holes in the four OFETs are considered to be distributed over similarly fluctuating HOMO bands. In that case, most of the free holes will be located in the local minima of the band-edge fluctuation according to Fermi–Dirac statistics. This would explain why the modulation of absorption increases with increasing THz photon energy. At the same time, this distribution will reduce the effective carrier mobility because the weakly restricted holes in the fluctuation valley cannot evenly contribute to the steady-state drift current in the channel. This is why pentacene thin films have a much lower carrier mobility than single crystals [7,8,19]. The absorption intensity of the four OFETs used in this study had the same dependence on the density of holes that con-
tribute to the drain current (see Fig. 8), despite only a portion of THz holes being able to contribute to the steadystate drain current. This is also understandable if the four OFETs have quantitatively the same band-edge fluctuation [18]. Finally, we discuss the difference between this study and a previous study by Lloyd-Hughes et al. [9]. The main difference is the semiconductor materials used: we used high-carrier-mobility polycrystalline pentacene, whereas Lloyd-Hughes et al. used a low-carrier-mobility (<10 2 cm2/V s) disordered semiconducting material (F8T2). Even though the OFETs used in this study had an apparent field-effect mobility of less than 1 cm2/V s, the local carrier mobility in the small crystallites is expected to be as high as that in single crystals (>10 cm2/V s). This large difference of carrier mobility results in the difference of the sensitivity of THz absorption. The present study also used a control sample to separate the modulation of the absorption by the Si substrate from that of the pentacene layer. These are the reason why this work succeeded to obtain the THz absorption by the accumulated carriers in an organic semiconductor layer. 4. Conclusions We obtained THz-modulated adsorption spectra due to accumulated free holes in the pentacene layers of OFETs by separating the absorption due to free electrons in the Si substrate. The THz absorption by free holes in pentacene increased when the frequency was increased from 0.2 to 2.0 THz. The Drude–Lorentz model could not explain the spectral shape, despite agreeing well with the absorption spectra of free electrons in Si. The obtained unique absorption spectra are considered to be due to fluctuations in the HOMO band edge of the pentacene films, as found in previous studies [6–8]. The four OFETs tested in this study exhibited the same sensitivity of the integrated absorption intensity to the density of holes that contribute to the drain current. This suggests that the four OFETs have qualitatively the same band-edge fluctuations. The present results clearly indicate that holes accumulated in the pentacene channel can be excited by THz photons, allowing them to overcome the surrounding potential barriers in the fluctuating HOMO band.
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We consider that the results of the present study represent an important step toward developing a new class of THz-wave sensors with OFET structures. In the next step, THz photoconductivity measurements will be performed and the sensitivity will be maximized by optimizing the device design and eliminating higher barriers against carrier transport. The density of high barriers, >100 meV, at grain boundaries can be reduced by chemically modifying the substrate surface [18]. Using graphoepitaxy to align the grains in a single direction [20–22] will also reduce the density of high barriers. Acknowledgements This study was supported by a Grant-in-Aid for Scientific Research (B) (No. 21350099), the Chiba University Global COE Program ‘‘Advanced School for Organic Electronics’’, and a Grant-in-Aid for JSPS Fellows (No. 2356142). References [1] K. Kawase, Y. Ogawa, Y. Watanabe, Opt. Exp. 11 (2003) 2549. [2] Y.C. Shen, T. Lo, P.F. Taday, B.E. Cole, W.R. Tribe, M.C. Kemp, Appl. Phys. Lett. 86 (2005) 241116. [3] B. Ferquson, S. Wang, D. Gray, D. Abbot, X.C. Zhang, Opt. Lett. 27 (2002) 1312.
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