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Ion-dependent gate dielectric characteristics of ion-conducting SiO2 solid-electrolytes in oxide field-effect transistors Jia Sun,abc Chuan Qian,bc Wenlong Huang,bc Junliang Yangbc and Yongli Gao*abcd The effect of ions on the gate dielectric behavior of oxide field-effect transistors (FETs) was studied using lithium ion-incorporated porous SiO2. The frequency dependence of the impedance was observed to vary with the ion concentrations in the ion-conducting SiO2 solid-electrolyte. The microstructure of the porous SiO2 was tailored by changing the depositions and porous SiO2 with an ordered columnar microstructure was realized, which provides an unobstructed pathway for the transportation of electrolyte ions. An enhanced electric-double-layer (EDL) capacitance of 11.9 mF cm2 and an improved EDL
Received 1st December 2013, Accepted 29th January 2014 DOI: 10.1039/c3cp55056g
formation upper-limit-frequency of B105 Hz were obtained. Due to the enhanced EDL capacitance, oxide FETs gated by these solid-electrolytes showed a very low operating voltage of 0.6 V. A current on/off ratio of B106, a subthreshold swing of B82 mV per decade, a near-zero threshold voltage of B0.01 V, and an electron field-effect mobility of B27.1 cm2 V1 s1 were obtained. These ultra
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low-voltage FETs have potential applications in portable devices and biochemical sensors.
Introduction Ion-conducting electrolytes are of great interest due to their potential applications in lithium ion batteries, fuel cells, electrochromic devices and low-cost sensors.1–4 Recently, ion-conducting electrolyte dielectrics with large specific electric-double-layer (EDL) capacitance were used to fabricate field-effect transistors (FETs) and explore the charge transport of semiconducting materials.5–7 This type of device can be considered as a specific class of FET and the device structure is identical to that of traditional ones. However, the gate insulating material is replaced with an electrolyte as an ion conducting but electron insulating dielectric. For quite a few years, excellent progress has been achieved in the field of electrolyte-gated ionic/electronic devices. A representative electrolyte material is a lithium salt–poly(ethylene oxide) (PEO) material (LiBF4 or LiClO4 in PEO) and its ion conducting mechanism involves the segmental motion-assisted diffusion of lithium ions in the PEO matrix.5,8 a
School of Materials Science and Engineering, Central South University, Changsha, Hunan 410083, China. E-mail: [email protected] b Institute of Super-microstructure and Ultrafast Process in Advanced Materials (ISUPAM), School of Physics and Electronics, Central South University, Changsha, Hunan 410083, China c Hunan Key Laboratory for Super-microstructure and Ultrafast Process, School of Physics and Electronics, Central South University, Changsha, Hunan 410083, China d Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627, USA
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To further improve the gate dielectric behavior of electrolyte materials, other ion-conducting electrolytes, such as ionic liquids, ion gels and proton conductors, were prepared using different processing techniques and were successfully used in FETs.9–11 All of the electrolyte-gated devices already displayed low-voltage behavior (less than 3.0 V) and reasonable electrical performance.8–11 Inorganic solid sodium b-alumina (SBA) ionic conductors with high two-dimensional ionic conductivity were used for FET fabrication by Katz’s group.12 These are very interesting ion-conducting materials, due to their high processing temperature, and are complementary to recent electrolyte materials. Most recently, the same group extended the investigation of the dielectric properties of other oxides related to alumina incorporating other alkali metal ions. They demonstrated that the ions in the oxide matrix could move through continuous pores or channels, rather than through domains of the materials.13 Porous inorganic dielectrics can provide highdensity nanochannels for ion transportation, which is favorable for EDL formation.14,15 Such systems comprise a solid porous host material that is inert and the dissociated mobile ions serve as the ion conducting phase which are confined in the porous framework.15 In a previous study, some porous materials were used as gate dielectrics in FETs, such as, nanoporous SiO2,16 Al2O313,17 and zeolites.18 In this article, we report the effect of ions on the gate dielectric behavior of lithium ion-incorporated porous SiO2. The frequency dependence of the observed impedance of the ion-conducting SiO2 solid-electrolyte varied with the ion concentrations.
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Importantly, the microstructure of the porous SiO2 was tailored by changing the deposition and an ordered columnar SiO2 microstructure was realized which is favorable for ion transportation. An enhanced EDL capacitance of 11.9 mF cm2 and an improved EDL formation upper-limit-frequency of B105 Hz were obtained. Optimized SiO2 solid-electrolyte gated oxide FETs show high performance and very low operating voltages of B0.6 V. Oxide FETs gated by these ion-conducting electrolytes are very promising for invisible biochemical sensors and portable electronics.
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For the impedance measurements, the samples were sandwiched between two ITO electrodes to form a capacitor (ITO resistivity: 7 104 O cm, electrode area: 1.5 103 cm2). The oxide FETs with a channel width-to-length ratio of 1000/50 mm were fabricated on ion-conducting electrolyte/ITO glass by magnetron sputtering. The electrical characteristics of the porous solid-electrolyte and the oxide FETs were measured by an impedance analyzer (Wayne Kerr 6500B) and a semiconductor parameter analyzer (Keithley 4200 SCS) in air with a relative humidity of 60%.
Experimental section Inorganic porous solids are of scientific and technological interest because of their ability to interact with ions throughout the bulk material.19 In our study, porous SiO2 films with a thickness of B4.0 mm were deposited by a plasma-enhanced chemical vapor deposition (PECVD) method.16 The lithium ions were incorporated into the porous SiO2 by immersing the samples into 0%, 20%, and 40% LiCl solution for 2 hours. However, the ion guests in the nanochannels of the porous solid matrix showed unusual behavior compared with the bulk phase. This is because the behavior of the guests is heavily dependent on the nature of the porous nanochannels, such as their size and shape.14,20 Thus, tailoring the nanopores to possess an ordered columnar microstructure is highly favorable for ion transportation and EDL formation. As shown in Fig. 1, we propose a transport model for ion-incorporated porous SiO2 solid materials. If the unordered porous materials were used as dielectric films, it may dramatically slow down the ion transport to the interface of the electrode/electrolyte to form EDLs, presumably due to the presence of irregularly curved pores (Fig. 1(a)). Therefore, it is necessary to effectively control the microstructure of the void spaces in porous materials because it can lead to superior properties. As shown in Fig. 1(b), highly ordered porous materials are more convenient for ion transport because the formation of a vertically columnar microstructure can support the effective diffusion of ions perpendicular to the electrodes as they enable the unobstructed transportation of mobile ions. In order to tune the microstructure of porous SiO2, the deposition conditions of the films were changed. Structural characterization of the deposited porous SiO2 was carried out by field emission scanning electron microscopy (Hitachi S-4800 SEM).
Fig. 1 An ion transportation model of the ion-incorporated porous solid materials.
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Fig. 2
FTIR spectrum of the as-prepared porous SiO2.
Fig. 3 The low-magnification SEM images of the samples prepared at (a) room temperature, (c) 100 1C and (e) 150 1C. The high-magnification SEM images of the samples prepared at (b) room temperature, (d) 100 1C and (f) 150 1C.
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Results and discussions Fourier transform infrared spectroscopy (FTIR) was used to characterize chemical species and chemical bonding. As shown in Fig. 2, the Si–O rocking vibration mode at 479 cm1, the strong Si–O stretching vibration mode at 1089 cm1 and the bending vibration mode around 797 cm1 are observed in the spectrum of the as-prepared sample. The absorption peak at 622 cm1 originated from the Si–Si bond vibration. In our samples, the adsorbed water in the pores shows an intense characteristic absorption band. From 3000 to 3600 cm1, a broad peak was observed and it corresponds to the O–H stretching vibration mode. Also, this band can be verified through the band at 1638 cm1 from the bending vibration mode of molecular water. The peak at 1401 cm1 may result from the O–H bending vibration mode. Previous studies have demonstrated that the microstructure of porous oxide materials is dependent on deposition temperature.21–23 Through controlling the deposition temperature, the microstructure of the porous SiO2 films was optimized. The cross-sectional microstructure of the porous SiO2 films deposited at temperatures ranging from room temperature to 150 1C was examined by SEM (Fig. 3). The low-magnification SEM images of the samples prepared at room temperature, 100 1C and 150 1C are shown in Fig. 3(a), (c) and (e), respectively. The high-magnification SEM images of the same samples are shown in Fig. 3(b), (d) and (f). As shown in Fig. 3(a) and (b), porous SiO2 films with some branched structures are observed
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when deposited at room temperature. As shown in Fig. 3(e) and (f), if the temperature is increased to 150 1C, the diffusion of the species is enhanced and SiO2 films with a compact structures are observed. However, by performing the deposition at 100 1C well-defined ordered porous SiO2 films with vertical columnar-array microstructures are obtained, as shown in Fig. 3(c) and (d). These porous SiO2 thin films show an aligned columnar-array microstructure instead of a branched or compact SiO2 thin film structure. The control over the ordered columnar structures of the porous materials which creates an aligned nanopath is of importance, potentially realizing fast ion transportation in porous materials. According to the existing knowledge from the theories for thin film growth, the restriction of the diffusion of the species and the shadowing effects can be considered as the critical factors leading to the development of ordered porous films.21–23 In order to investigate the effect of the nature of the mobile ions on the porous SiO2 dielectric properties, the impedance spectra of ITO/solid-electrolyte/ITO (MIM) capacitors with different ion concentrations were measured with an impedance analyzer. The ionic conductivity and phase angle versus the frequency curves of the porous SiO2 deposited at room temperature, treated with 0%, 20% and 40% lithium ion concentrations, are shown in Fig. 4(a) and (b), respectively. The ionic conductivity of all of the samples shows frequencydependent behavior. This is universal ionic conductivity behavior in ion-conducting solid materials.24 Compared to porous SiO2 without lithium ions, ion-incorporated porous
Fig. 4 Lithium ion concentration dependence of the (a) ionic conductivity and (b) phase angle versus the frequency of the porous SiO2 films deposited at room temperature. Lithium ion concentration dependence of the (c) ionic conductivity and (d) phase angle versus the frequency of the porous SiO2 films deposited at 100 1C.
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SiO2 showed a high ionic conductivity which increases with lithium ion concentration, this is attributed to the lithium ion-migration in the ion-incorporated porous SiO2. As shown in Fig. 4(c) and (d), for the ion-incorporated columnar SiO2 films deposited at 100 1C, a higher ionic conductivity was observed. According to the value of the ionic conductivity and phase angle, the dipole relaxation, ion migration and EDL formation at different frequency regions are identified.25 As shown in Fig. 4, the upper-limit-frequency of the EDL formation of room temperature deposited porous SiO2 shifted from 330 Hz to 3.55 kHz when the lithium ion concentration increased from 0% to 40%. This behavior originates from the increase in the concentration of the mobile ions in the porous SiO2 matrix. At the same time, a significant improvement in the frequency response was observed in porous SiO2 deposited at 100 1C treated with LiCl solution. The upper-limit-frequencies of the EDL formation of the ordered columnar SiO2 with 0%, 20% and 40% LiCl treatments are estimated to be 2 kHz, 13 kHz and 200 kHz, respectively, which are significantly improved compared with the sample deposited at room temperature. The ion concentration-dependent capacitances of the room temperature deposited porous SiO2 in a frequency range from
20 Hz to 1 MHz are shown in Fig. 5(a). The high capacitance (41 mF cm2) achieved at a low frequency is explained by the formation of an EDL by the polarization of ions at the electrode/ solid-electrolyte interface. In the frequency ranges studied so far, there are hints of the expected leveling out of capacitances at low frequency. This is due to an interfacial double layer being established which has a dominant capacitance because the bulk capacitance of the 4 mm-thick porous SiO2 is negligible. From the capacitance-frequency curves, two distinct features are observed. Firstly, the specific capacitance value at low frequency (20 Hz) for the porous SiO2-based solid-electrolytes deposited at room temperature increases with the increase in ion concentration, from 2.06 mF cm2 to 9.2 mF cm2. Secondly, a dramatic frequency-dependent drop in the specific capacitance of the room temperature deposited porous SiO2 occurs at high frequencies of several kHz when the lithium ion concentration is increased from 0% to 40%. As shown in Fig. 5(b), the 100 1C deposited sample with 40% LiCl shows a large specific capacitance of 11.9 mF cm2 at 20 Hz and the value does not drop significantly even at 100 kHz, which means that the formation of the EDL occurs within B10 ms. In fact, all of the impedance spectroscopy measurements reflect the motion of ions under the influence of an alternating current. This can illustrate how many solvated ions have reached the solid-electrolyte/electrode interface at a specific frequency. Compared with the room temperature porous SiO2, columnar-array SiO2 films deposited at 100 1C treated with
Fig. 5 Lithium ion concentration dependence of the specific capacitance versus frequency of (a) room temperature deposited porous SiO2 films and (b) 100 1C deposited porous SiO2 films.
Fig. 6 (a) Output characteristics for EDL transistors gated by the optimized solid-electrolyte. (b) Transfer curve ((Ids)1/2–Vg with a logarithmic scale) of the same device at Vds = 0.6 V.
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lithium ions show a much better frequency response with a higher upper-limit-frequency of the EDL formation. This means that the ionic diffusion to the solid-electrolyte/electrode interface is more efficient in the case of columnar SiO2 films at higher frequencies. Since the room temperature and 100 1C deposited porous SiO2 have the same compositions, were treated with the same ion concentrations and were tested at the same ambient conditions, the better frequency behavior is possibly due to the presence of the ordered columnar microstructures, which favor more efficient ion transportation. As shown in the SEM results, porous SiO2 films deposited at 100 1C show an ordered columnar microstructure with high-density nanopaths. These observed superior impedance properties are mainly due to the absence of obstacles in the ion transportation. The vertical nanopaths of columnar SiO2 for electrolyte ions foster the achievement of the EDL formation at higher frequencies of larger than 100 kHz. The electrical transport measurements of the SiO2 solidelectrolyte deposited at 100 1C and treated with 40% LiCl solution, are shown in Fig. 6. Because of the large EDL capacitance, the operating voltage of the device is greatly reduced to 0.6 V. Typical output curves, drain current (Ids)– drain voltage (Vds), at different gate voltages (Vg) are shown in Fig. 6(a). The FET exhibited strong gate modulation of the drain current. The drain current was saturated at a very low voltage, showing effective pinch-off behavior. At a gate voltage of 0.6 V, the saturated drain current is 0.7 mA with a channel W/L ratio of B20. The transfer (Ids–Vg and (Ids)1/2–Vg) characteristics at Vds = 0.6 V, shown in Fig. 6(b), were measured by sweeping Vg from 0.6 V to +0.6 V and back. The hysteresis window is as small as B0.06 V due to the high EDL specific capacitance and the coupling efficiency.26 A high current on/off ratio (Ion/Ioff) of B106, a small subthreshold swing of B82 mV per decade and a near-zero threshold voltage (Vth) of B0.01 V are obtained. The electron field-effect mobility (m) of the device in the saturation operation regime is calculated using the relation 2 WCi ship, Ids ¼ m Vg Vth , where Ci is the capacitance per 2L unit area at 20 Hz of the gate dielectric. Using this model, the field-effect electron mobility is estimated to be as high as 27.1 cm2 V1 s1.
Conclusions In conclusion, the ion-dependent gate dielectric characteristics of ion-conducting SiO2 solid-electrolytes were studied. A tendency for the EDL capacitance to increase with increasing lithium ion concentrations was observed. Furthermore, the porous SiO2 with an ordered columnar microstructure was realized by controlling the depositions. The optimized microstructure of the samples fosters fast ion polarization and transportation. The enhanced EDL specific capacitance of the columnar-array SiO2 solid-electrolyte is found to be 11.9 mF cm2 at 20 Hz. This solid-electrolyte shows a fast polarization response and the upper-limit-frequency of the EDL formation is estimated
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to be higher than 100 kHz. The operating voltage of the oxide FETs gated by the optimized gate dielectric is reduced to 0.6 V. The mobility, current on/off ratio, subthreshold swing S and threshold voltage Vth were found to be 27.1 cm2 V1 s1, B106, B82 mV per decade and B0.01 V, respectively. These EDL transistors have potential for application in portable electronics and biochemical sensors.
Acknowledgements This project was supported in part by the National Natural Science Foundation of China (51173205, 61306085, 11334014), the China Postdoctoral Science Foundation (2013M530357), the Hunan Postdoctoral Scientific Program (2013RS4045), and The Postdoctoral Science Foundation of Central South University. J. Yang acknowledges support from the National Natural Science Foundation of China (51203192) and the Hunan Provincial Natural Science Foundation of China (13JJ4019).
References 1 S. K. Fullerton-Shirey and J. K. Maranas, Macromolecules, 2009, 42, 2142. 2 S.-Y. Lee, G. Scharfenberger, W. H. Meyer and G. Wegner, Adv. Mater., 2005, 17, 626. 3 C. A. Nguyen, S. Xiong, J. Ma, X. Lu and P. S. Lee, J. Phys. Chem. B, 2009, 113, 8006. ¨ller, C. Plog, A. Knezevic, H. Leye, E. Irion, 4 R. Moos, R. Mu T. Braun, K.-J. Maquardt and K. Binder, Sens. Actuators, B, 2002, 83, 181. 5 C. Lu, Q. Fu, S. Huang and J. Liu, Nano Lett., 2004, 4, 623. 6 M. J. Panzer and C. D. Frisble, Adv. Mater., 2008, 20, 3177. 7 H. Yuan, H. Shimotani, A. Tsukazaki, A. Ohtomo, M. Kawasaki and Y. Iwasa, Adv. Funct. Mater., 2009, 19, 1046. 8 M. Taniguchi and T. Kawai, Appl. Phys. Lett., 2004, 85, 3298. 9 T. Uemura, M. Yamagishi, S. Ono and J. Takeya, Appl. Phys. Lett., 2009, 95, 103301. 10 J. H. Cho, J. Lee, Y. Xia, B. Kim, Y. He, M. J. Renn, T. P. Lodge and C. D. Frisble, Nat. Mater., 2008, 7, 900. 11 L. Herlogsson, X. Crispin, N. D. Robinson, M. Sandberg, O.-J. Hagel, G. Gustafsson and M. Berggren, Adv. Mater., 2007, 19, 97. 12 B. N. Pal, B. M. Dhar, K. C. See and H. E. Katz, Nat. Mater., 2009, 8, 898. 13 Y. Liu, P. Guan, B. Zhang, M. L. Falk and H. E. Katz, Chem. Mater., 2013, 25, 3788. 14 S. Bureekaew, S. Horike, M. Higuchi, M. Mizuno, T. Kawamura, D. Tanaka, N. Yanai and S. Kitagawa, Nat. Mater., 2009, 8, 831. ¨llen, Chem. 15 T. Echelmeyer, H. W. Meyer and L. van Wu Mater., 2009, 21, 2280. 16 J. Sun, Q. Wan, A. Lu and J. Jiang, Appl. Phys. Lett., 2009, 95, 222108. 17 H. Zhang, L. Guo and Q. Wan, J. Mater. Chem. C, 2013, 1, 2781. 18 G. Wu, H. Zhang, J. Zhou, A. Huang and Q. Wan, J. Mater. Chem. C, 2013, 1, 5669. 19 M. E. Davis, Nature, 2002, 417, 813.
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´s, J. Cotrino and A. R. Gonza ´lez-Elipe, J. Electrochem. 23 A. Borra Soc., 2007, 154, P152. 24 J. C. Dyre and T. B. Schorder, Rev. Mod. Phys., 2000, 72, 873. 25 O. Larsson, E. Said, M. Berggren and X. Crispin, Adv. Funct. Mater., 2009, 19, 3334. 26 G. P. Siddons, D. Merchin, J. H. Back, J. K. Jeong and M. Shim, Nano Lett., 2004, 4, 927.
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20 T. J. Barton, L. M. Bull, W. G. Klemperer, D. A. Loy, B. McEnaney, M. Misono, P. A. Monson, G. Pez, G. W. Scherer, J. C. Vartuli and O. M. Yaghi, Chem. Mater., 1999, 11, 2633. 21 F. N. Dultsev, L. A. Nenasheva and L. L. Vasilyeva, Thin Solid Films, 1998, 315, 72. ´s, J. R. Sa ´nchez-Valencia, J. Garrido-Molinero, 22 A. Borra ´lez-Elipe, Microporous A. Barranco and A. R. Gonza Mesoporous Mater., 2009, 118, 314.
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Ion-dependent gate dielectric characteristics of ion-conducting SiO2 solid-electrolytes in oxide field-effect transistors Jia Sun,abc Chuan Qian,bc Wenlong Huang,bc Junliang Yangbc and Yongli Gao*abcd The effect of ions on the gate dielectric behavior of oxide field-effect transistors (FETs) was studied using lithium ion-incorporated porous SiO2. The frequency dependence of the impedance was observed to vary with the ion concentrations in the ion-conducting SiO2 solid-electrolyte. The microstructure of the porous SiO2 was tailored by changing the depositions and porous SiO2 with an ordered columnar microstructure was realized, which provides an unobstructed pathway for the transportation of electrolyte ions. An enhanced electric-double-layer (EDL) capacitance of 11.9 mF cm2 and an improved EDL
Received 1st December 2013, Accepted 29th January 2014 DOI: 10.1039/c3cp55056g
formation upper-limit-frequency of B105 Hz were obtained. Due to the enhanced EDL capacitance, oxide FETs gated by these solid-electrolytes showed a very low operating voltage of 0.6 V. A current on/off ratio of B106, a subthreshold swing of B82 mV per decade, a near-zero threshold voltage of B0.01 V, and an electron field-effect mobility of B27.1 cm2 V1 s1 were obtained. These ultra
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low-voltage FETs have potential applications in portable devices and biochemical sensors.
Introduction Ion-conducting electrolytes are of great interest due to their potential applications in lithium ion batteries, fuel cells, electrochromic devices and low-cost sensors.1–4 Recently, ion-conducting electrolyte dielectrics with large specific electric-double-layer (EDL) capacitance were used to fabricate field-effect transistors (FETs) and explore the charge transport of semiconducting materials.5–7 This type of device can be considered as a specific class of FET and the device structure is identical to that of traditional ones. However, the gate insulating material is replaced with an electrolyte as an ion conducting but electron insulating dielectric. For quite a few years, excellent progress has been achieved in the field of electrolyte-gated ionic/electronic devices. A representative electrolyte material is a lithium salt–poly(ethylene oxide) (PEO) material (LiBF4 or LiClO4 in PEO) and its ion conducting mechanism involves the segmental motion-assisted diffusion of lithium ions in the PEO matrix.5,8 a
School of Materials Science and Engineering, Central South University, Changsha, Hunan 410083, China. E-mail: [email protected] b Institute of Super-microstructure and Ultrafast Process in Advanced Materials (ISUPAM), School of Physics and Electronics, Central South University, Changsha, Hunan 410083, China c Hunan Key Laboratory for Super-microstructure and Ultrafast Process, School of Physics and Electronics, Central South University, Changsha, Hunan 410083, China d Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627, USA
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To further improve the gate dielectric behavior of electrolyte materials, other ion-conducting electrolytes, such as ionic liquids, ion gels and proton conductors, were prepared using different processing techniques and were successfully used in FETs.9–11 All of the electrolyte-gated devices already displayed low-voltage behavior (less than 3.0 V) and reasonable electrical performance.8–11 Inorganic solid sodium b-alumina (SBA) ionic conductors with high two-dimensional ionic conductivity were used for FET fabrication by Katz’s group.12 These are very interesting ion-conducting materials, due to their high processing temperature, and are complementary to recent electrolyte materials. Most recently, the same group extended the investigation of the dielectric properties of other oxides related to alumina incorporating other alkali metal ions. They demonstrated that the ions in the oxide matrix could move through continuous pores or channels, rather than through domains of the materials.13 Porous inorganic dielectrics can provide highdensity nanochannels for ion transportation, which is favorable for EDL formation.14,15 Such systems comprise a solid porous host material that is inert and the dissociated mobile ions serve as the ion conducting phase which are confined in the porous framework.15 In a previous study, some porous materials were used as gate dielectrics in FETs, such as, nanoporous SiO2,16 Al2O313,17 and zeolites.18 In this article, we report the effect of ions on the gate dielectric behavior of lithium ion-incorporated porous SiO2. The frequency dependence of the observed impedance of the ion-conducting SiO2 solid-electrolyte varied with the ion concentrations.
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Importantly, the microstructure of the porous SiO2 was tailored by changing the deposition and an ordered columnar SiO2 microstructure was realized which is favorable for ion transportation. An enhanced EDL capacitance of 11.9 mF cm2 and an improved EDL formation upper-limit-frequency of B105 Hz were obtained. Optimized SiO2 solid-electrolyte gated oxide FETs show high performance and very low operating voltages of B0.6 V. Oxide FETs gated by these ion-conducting electrolytes are very promising for invisible biochemical sensors and portable electronics.
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For the impedance measurements, the samples were sandwiched between two ITO electrodes to form a capacitor (ITO resistivity: 7 104 O cm, electrode area: 1.5 103 cm2). The oxide FETs with a channel width-to-length ratio of 1000/50 mm were fabricated on ion-conducting electrolyte/ITO glass by magnetron sputtering. The electrical characteristics of the porous solid-electrolyte and the oxide FETs were measured by an impedance analyzer (Wayne Kerr 6500B) and a semiconductor parameter analyzer (Keithley 4200 SCS) in air with a relative humidity of 60%.
Experimental section Inorganic porous solids are of scientific and technological interest because of their ability to interact with ions throughout the bulk material.19 In our study, porous SiO2 films with a thickness of B4.0 mm were deposited by a plasma-enhanced chemical vapor deposition (PECVD) method.16 The lithium ions were incorporated into the porous SiO2 by immersing the samples into 0%, 20%, and 40% LiCl solution for 2 hours. However, the ion guests in the nanochannels of the porous solid matrix showed unusual behavior compared with the bulk phase. This is because the behavior of the guests is heavily dependent on the nature of the porous nanochannels, such as their size and shape.14,20 Thus, tailoring the nanopores to possess an ordered columnar microstructure is highly favorable for ion transportation and EDL formation. As shown in Fig. 1, we propose a transport model for ion-incorporated porous SiO2 solid materials. If the unordered porous materials were used as dielectric films, it may dramatically slow down the ion transport to the interface of the electrode/electrolyte to form EDLs, presumably due to the presence of irregularly curved pores (Fig. 1(a)). Therefore, it is necessary to effectively control the microstructure of the void spaces in porous materials because it can lead to superior properties. As shown in Fig. 1(b), highly ordered porous materials are more convenient for ion transport because the formation of a vertically columnar microstructure can support the effective diffusion of ions perpendicular to the electrodes as they enable the unobstructed transportation of mobile ions. In order to tune the microstructure of porous SiO2, the deposition conditions of the films were changed. Structural characterization of the deposited porous SiO2 was carried out by field emission scanning electron microscopy (Hitachi S-4800 SEM).
Fig. 1 An ion transportation model of the ion-incorporated porous solid materials.
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Fig. 2
FTIR spectrum of the as-prepared porous SiO2.
Fig. 3 The low-magnification SEM images of the samples prepared at (a) room temperature, (c) 100 1C and (e) 150 1C. The high-magnification SEM images of the samples prepared at (b) room temperature, (d) 100 1C and (f) 150 1C.
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Results and discussions Fourier transform infrared spectroscopy (FTIR) was used to characterize chemical species and chemical bonding. As shown in Fig. 2, the Si–O rocking vibration mode at 479 cm1, the strong Si–O stretching vibration mode at 1089 cm1 and the bending vibration mode around 797 cm1 are observed in the spectrum of the as-prepared sample. The absorption peak at 622 cm1 originated from the Si–Si bond vibration. In our samples, the adsorbed water in the pores shows an intense characteristic absorption band. From 3000 to 3600 cm1, a broad peak was observed and it corresponds to the O–H stretching vibration mode. Also, this band can be verified through the band at 1638 cm1 from the bending vibration mode of molecular water. The peak at 1401 cm1 may result from the O–H bending vibration mode. Previous studies have demonstrated that the microstructure of porous oxide materials is dependent on deposition temperature.21–23 Through controlling the deposition temperature, the microstructure of the porous SiO2 films was optimized. The cross-sectional microstructure of the porous SiO2 films deposited at temperatures ranging from room temperature to 150 1C was examined by SEM (Fig. 3). The low-magnification SEM images of the samples prepared at room temperature, 100 1C and 150 1C are shown in Fig. 3(a), (c) and (e), respectively. The high-magnification SEM images of the same samples are shown in Fig. 3(b), (d) and (f). As shown in Fig. 3(a) and (b), porous SiO2 films with some branched structures are observed
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when deposited at room temperature. As shown in Fig. 3(e) and (f), if the temperature is increased to 150 1C, the diffusion of the species is enhanced and SiO2 films with a compact structures are observed. However, by performing the deposition at 100 1C well-defined ordered porous SiO2 films with vertical columnar-array microstructures are obtained, as shown in Fig. 3(c) and (d). These porous SiO2 thin films show an aligned columnar-array microstructure instead of a branched or compact SiO2 thin film structure. The control over the ordered columnar structures of the porous materials which creates an aligned nanopath is of importance, potentially realizing fast ion transportation in porous materials. According to the existing knowledge from the theories for thin film growth, the restriction of the diffusion of the species and the shadowing effects can be considered as the critical factors leading to the development of ordered porous films.21–23 In order to investigate the effect of the nature of the mobile ions on the porous SiO2 dielectric properties, the impedance spectra of ITO/solid-electrolyte/ITO (MIM) capacitors with different ion concentrations were measured with an impedance analyzer. The ionic conductivity and phase angle versus the frequency curves of the porous SiO2 deposited at room temperature, treated with 0%, 20% and 40% lithium ion concentrations, are shown in Fig. 4(a) and (b), respectively. The ionic conductivity of all of the samples shows frequencydependent behavior. This is universal ionic conductivity behavior in ion-conducting solid materials.24 Compared to porous SiO2 without lithium ions, ion-incorporated porous
Fig. 4 Lithium ion concentration dependence of the (a) ionic conductivity and (b) phase angle versus the frequency of the porous SiO2 films deposited at room temperature. Lithium ion concentration dependence of the (c) ionic conductivity and (d) phase angle versus the frequency of the porous SiO2 films deposited at 100 1C.
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SiO2 showed a high ionic conductivity which increases with lithium ion concentration, this is attributed to the lithium ion-migration in the ion-incorporated porous SiO2. As shown in Fig. 4(c) and (d), for the ion-incorporated columnar SiO2 films deposited at 100 1C, a higher ionic conductivity was observed. According to the value of the ionic conductivity and phase angle, the dipole relaxation, ion migration and EDL formation at different frequency regions are identified.25 As shown in Fig. 4, the upper-limit-frequency of the EDL formation of room temperature deposited porous SiO2 shifted from 330 Hz to 3.55 kHz when the lithium ion concentration increased from 0% to 40%. This behavior originates from the increase in the concentration of the mobile ions in the porous SiO2 matrix. At the same time, a significant improvement in the frequency response was observed in porous SiO2 deposited at 100 1C treated with LiCl solution. The upper-limit-frequencies of the EDL formation of the ordered columnar SiO2 with 0%, 20% and 40% LiCl treatments are estimated to be 2 kHz, 13 kHz and 200 kHz, respectively, which are significantly improved compared with the sample deposited at room temperature. The ion concentration-dependent capacitances of the room temperature deposited porous SiO2 in a frequency range from
20 Hz to 1 MHz are shown in Fig. 5(a). The high capacitance (41 mF cm2) achieved at a low frequency is explained by the formation of an EDL by the polarization of ions at the electrode/ solid-electrolyte interface. In the frequency ranges studied so far, there are hints of the expected leveling out of capacitances at low frequency. This is due to an interfacial double layer being established which has a dominant capacitance because the bulk capacitance of the 4 mm-thick porous SiO2 is negligible. From the capacitance-frequency curves, two distinct features are observed. Firstly, the specific capacitance value at low frequency (20 Hz) for the porous SiO2-based solid-electrolytes deposited at room temperature increases with the increase in ion concentration, from 2.06 mF cm2 to 9.2 mF cm2. Secondly, a dramatic frequency-dependent drop in the specific capacitance of the room temperature deposited porous SiO2 occurs at high frequencies of several kHz when the lithium ion concentration is increased from 0% to 40%. As shown in Fig. 5(b), the 100 1C deposited sample with 40% LiCl shows a large specific capacitance of 11.9 mF cm2 at 20 Hz and the value does not drop significantly even at 100 kHz, which means that the formation of the EDL occurs within B10 ms. In fact, all of the impedance spectroscopy measurements reflect the motion of ions under the influence of an alternating current. This can illustrate how many solvated ions have reached the solid-electrolyte/electrode interface at a specific frequency. Compared with the room temperature porous SiO2, columnar-array SiO2 films deposited at 100 1C treated with
Fig. 5 Lithium ion concentration dependence of the specific capacitance versus frequency of (a) room temperature deposited porous SiO2 films and (b) 100 1C deposited porous SiO2 films.
Fig. 6 (a) Output characteristics for EDL transistors gated by the optimized solid-electrolyte. (b) Transfer curve ((Ids)1/2–Vg with a logarithmic scale) of the same device at Vds = 0.6 V.
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lithium ions show a much better frequency response with a higher upper-limit-frequency of the EDL formation. This means that the ionic diffusion to the solid-electrolyte/electrode interface is more efficient in the case of columnar SiO2 films at higher frequencies. Since the room temperature and 100 1C deposited porous SiO2 have the same compositions, were treated with the same ion concentrations and were tested at the same ambient conditions, the better frequency behavior is possibly due to the presence of the ordered columnar microstructures, which favor more efficient ion transportation. As shown in the SEM results, porous SiO2 films deposited at 100 1C show an ordered columnar microstructure with high-density nanopaths. These observed superior impedance properties are mainly due to the absence of obstacles in the ion transportation. The vertical nanopaths of columnar SiO2 for electrolyte ions foster the achievement of the EDL formation at higher frequencies of larger than 100 kHz. The electrical transport measurements of the SiO2 solidelectrolyte deposited at 100 1C and treated with 40% LiCl solution, are shown in Fig. 6. Because of the large EDL capacitance, the operating voltage of the device is greatly reduced to 0.6 V. Typical output curves, drain current (Ids)– drain voltage (Vds), at different gate voltages (Vg) are shown in Fig. 6(a). The FET exhibited strong gate modulation of the drain current. The drain current was saturated at a very low voltage, showing effective pinch-off behavior. At a gate voltage of 0.6 V, the saturated drain current is 0.7 mA with a channel W/L ratio of B20. The transfer (Ids–Vg and (Ids)1/2–Vg) characteristics at Vds = 0.6 V, shown in Fig. 6(b), were measured by sweeping Vg from 0.6 V to +0.6 V and back. The hysteresis window is as small as B0.06 V due to the high EDL specific capacitance and the coupling efficiency.26 A high current on/off ratio (Ion/Ioff) of B106, a small subthreshold swing of B82 mV per decade and a near-zero threshold voltage (Vth) of B0.01 V are obtained. The electron field-effect mobility (m) of the device in the saturation operation regime is calculated using the relation 2 WCi ship, Ids ¼ m Vg Vth , where Ci is the capacitance per 2L unit area at 20 Hz of the gate dielectric. Using this model, the field-effect electron mobility is estimated to be as high as 27.1 cm2 V1 s1.
Conclusions In conclusion, the ion-dependent gate dielectric characteristics of ion-conducting SiO2 solid-electrolytes were studied. A tendency for the EDL capacitance to increase with increasing lithium ion concentrations was observed. Furthermore, the porous SiO2 with an ordered columnar microstructure was realized by controlling the depositions. The optimized microstructure of the samples fosters fast ion polarization and transportation. The enhanced EDL specific capacitance of the columnar-array SiO2 solid-electrolyte is found to be 11.9 mF cm2 at 20 Hz. This solid-electrolyte shows a fast polarization response and the upper-limit-frequency of the EDL formation is estimated
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to be higher than 100 kHz. The operating voltage of the oxide FETs gated by the optimized gate dielectric is reduced to 0.6 V. The mobility, current on/off ratio, subthreshold swing S and threshold voltage Vth were found to be 27.1 cm2 V1 s1, B106, B82 mV per decade and B0.01 V, respectively. These EDL transistors have potential for application in portable electronics and biochemical sensors.
Acknowledgements This project was supported in part by the National Natural Science Foundation of China (51173205, 61306085, 11334014), the China Postdoctoral Science Foundation (2013M530357), the Hunan Postdoctoral Scientific Program (2013RS4045), and The Postdoctoral Science Foundation of Central South University. J. Yang acknowledges support from the National Natural Science Foundation of China (51203192) and the Hunan Provincial Natural Science Foundation of China (13JJ4019).
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