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Modeling the frequency response of organic metal-insulator-semiconductor capacitors
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 19 (2019) 53–57
www.materialstoday.com/proceedings
NANOTEXNOLOGY2018
Modeling the frequency response of organic metal-insulatorsemiconductor capacitors Prashant Kumar Manda, Logesh Karunakaran, Soumya Dutta Department of electrical engineering, Indian Institute of Technology madras, Chennai, 600036, India
Abstract Substantial improvement of organic field effect transistors (OTFTs) toward circuit applications certainly demands stable device performance upon varying frequency and temperature. Organic metal-insulator-semiconductor (MIS) capacitor has been considered to be an efficient model system to predict the device performance of OTFT under the influence of frequency and temperature. In this study we show that the capacitance dispersion with respect to frequency and temperature in organic MIS capacitor device is an inherent property, which arises due to the low conductivity of the semiconductor along with Schottky type contact at the semiconductor-metal junction. In addition we propose an equivalent circuit model to explain the dispersion of capacitance with respect to frequency and temperature. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of International Conferences & Exhibition on Nanosciences & Nanotechnologies and Flexible Organic Electronics 2018, June 30th - July 6th, 2018 Keywords: Organic MIS capacitors; Schotty contact; Frequency response
* Corresponding author. Tel.: 918754408225 E-mail address: [email protected]
2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of International Conferences & Exhibition on Nanosciences & Nanotechnologies and Flexible Organic Electronics 2018, June 30th - July 6th, 2018
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P.K. Manda et al. / Materials Today: Proceedings 19 (2019) 53–57
1. Introduction Implementation of organic field effect transistors (OTFT) in practical application has been improved over last decade. The OTFT with bottom gate top contact (BGTC) configuration has been studied widely due to its ease of fabrication. The BGTC OTFT has already been employed in several circuit applications [1, 2]. However, the stability of OTFT has been considered to be a major concern for circuit application. Several factors such as bias stress effect, hysteresis, contact resistance, current and capacitance variation with frequency and temperature directly or indirectly affect the stability of OTFTs. The capacitance variation with frequency is one of the important phenomena that needs to be understood and modeled to predict the instability of the circuits. To study the frequency response of OTFT, organic metal-insulator-semiconductor (MIS) capacitors are often used [3, 4, 5]. In case of organic MIS capacitors the previous studies have shown capacitance dispersion with frequency [6,7] and temperature [3,4]. The capacitance dispersion with frequency was attributed to the presence of traps [6], deterioration of quality of metal-semiconductor junction [7], change in the dielectric constant of insulator with frequency [8] and due to the low carrier response time [3, 7]. However, to the best of our knowledge, there is no report that can explain the capacitance dispersion with frequency and temperature over a wide range of applied gate bias consistently. In the present report, we propose physics based equivalent circuit model, which explains the capacitance dispersion with frequency and temperature over a wide range of applied gate bias. 2. Fabrication and characterization The devices (the device structure is shown in Fig. 1) were fabricated on cleaned glass substrates by evaporating aluminum (50 nm) as gate metal, followed by spin coating of cross linked Poly(4-vinylphenol) (PVP) at 6000 RPM as insulator. Subsequently, the samples were annealed at 200oC for 20 minutes. Poly(3-hexylthiophene-2,5-diyl) (P3HT) (15 mg/ml in 1,2-Dichlorobenzene) was spun at 4000 RPM, followed by annealing at 120 oC for 20 minutes. Finally, gold (50 nm) was thermally evaporated as the top metal contact. All the fabrication processes were carried out inside the glove box. The fabricated devices were characterized inside a vacuum probe station using Agilent E4980A precision LCR meter.
Fig. 1. Device structure of organic MIS capacitor, where ts and ti correspond to thickness of semiconductor and insulator respectively.
3. Results and discussion We consider P3HT, which is relatively a high band gap (2 eV) [11] intrinsic semiconductor. Hence the intrinsic free carrier (electron and hole) concentration is less, which turns the MIS capacitor to a parallel plate capacitor with two different dielectric materials stacked in between two electrodes. Accordingly a constant capacitance with respect to gate voltage (Vg) is expected. However, P3HT based organic MIS capacitor shows good capacitance variation with Vg as shown in Fig. 2(a). As the capacitance corresponds to the change in charge with Vg, C-V characteristics evokes the presence of charge within the organic semiconductor and its variation with Vg, as shown in Fig. 2(a). The undoped P3HT makes Schottky type contact with Au. Hence Au injects the charge into P3HT by thermionic emission process [9, 10]. The injected charge concentration profile and the magnitude of the charge are
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55
modulated by the applied gate field as shown in Fig. 2(d). The simulation study of MIS capacitors based on undoped semiconductor with Schottky contact suggests the same. The C-V characteristics obtained from the simulation, as shown in Fig. 2(b), captures the experimental trend. The capacitance characteristics as shown in Fig. 2(a) and (b) are divided into two regions, based on capacitance variation with Vg. In region-1 the capacitance varies with Vg and in region-2 capacitance is constant with Vg. From Fig. 2(d) one can understand that for Vg corresponds to region-1, the hole concentration is high, leading to a significant change in hole concentration with Vg . Whereas in region-2, the hole concentration is less, which makes P3HT to behave as an insulator, resulting a constant capacitance with respect to Vg, as explained before. To explain the capacitance nature in region-1, we propose a charge based capacitance model, where the capacitance is defines as
1 1 1 C Ci C g Cs
(1)
where Ci and Cg correspond to the geometrical capacitance of insulator and semiconductor respectively. The semiconductor capacitance Cs can be calculated using the total charge (Qs) present within the semiconductor and the potential drop across the semiconductor ψs as Cs
Qs s
(2)
where Qs is calculated as ts
Qs q p( x)dx
(3)
0
Fig. 2. C-V characteristics for with different frequency: (a) Experimental results; Comparison between simulation results (symbols) and model (lines) with mobility of (b) 10-4 cm2V-1s-1; (c) 1 cm2V-1s-1 inset shows the equivalent circuit; (d) p(x) variation with Vg.
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P.K. Manda et al. / Materials Today: Proceedings 19 (2019) 53–57
The capacitance, as estimated using Qs and ψs obtained from simulation (solid line), shows a good match with small signal capacitance (symbols) for low frequency as shown in Fig. 2(b) and (c). Eq. (1) is formulated based on the assumption that the total charge present within the semiconductor is contributing for the capacitance. This indeed valid only for low frequencies, where in the capacitance is invariant with frequency. However, in practical devices, the capacitance decreases with increase in frequency (capacitance dispersion) as shown in Fig. 2(a). The possible reasons for the capacitance dispersion are dielectric constant variation with frequency [8] and, low response time of carriers [3, 7]. The capacitance dispersion in region-2 is due to the dielectric constant variation of insulator with frequency. This report focuses on the capacitance dispersion with frequency in region-1. The capacitance dispersion in region-1 is due to poor response time of the charge carriers present within the organic semiconductor which in turn depends on the conductivity of the semiconductor. This is evident from simulation results, where MIS capacitor corresponds to mobility (μh) of 10-4 cm2V-1s-1 captures the capacitance dispersion with frequency. In contrast, for μh of 1 cm2V-1s-1, where no capacitance dispersion is observed up to frequency of 10 kHz as shown in Fig. 2(c). To explain the capacitance dispersion with frequency, we propose an equivalent circuit model using Ci, Cg, Cs and averaged resistance (Rs) as shown in inset of Fig. 3(c), where Rs is estimated as follows ts
Rs
1
qp( x) 0
dx
(4)
h
Fig.3 C-V characteristics with different temperature at 1 kHz frequency: Comparison between simulation results (symbols) and model (lines) with mobility of (a) 10-4 cm2V-1s-1; (b) 1 cm2V-1s-1; (c) Cs, Rs variation with temperature for different mobility,(d) p(x) variation with temperature for different mobility at Vg=-3 V.
The comparison between the model (lines) and TCAD results (symbols) for different frequencies are shown in Fig. 2(b) and (c) where the proposed model explains the capacitance dispersion with frequency. The model further supports the argument that the capacitance dispersion is due to the low conductivity of the semiconductor. We extend our study to investigate the capacitance variation with temperature. Fig. 3(a) and (b) show the capacitance variation with temperature for MIS capacitor with mobility of 10-4cm2V-1s-1and 1cm2V-1s-1 respectively.
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With change in temperature the charge profile varies within the semiconductor as shown in Fig. 3(d). It is due to the charge injection process which is governed by thermionic emission which in turn is a thermally activated process. Hence the capacitance dispersion with temperature can arise due to Cs and Rs collectively as they both change with temperature as shown in Fig. 3(c). However, in low mobility case the capacitance dispersion is more compared to high mobility, though Cs is same for both the cases. The predominant capacitance dispersion with temperature can be understood from Rs variation, where Rs is orders of magnitude high for low mobility case compared to high mobility case as shown in Fig. 3(c). The excellent agreement between model (lines) and simulation (symbols) further strengthens the arguments. 4. Conclusion The capacitance dispersion with respect to frequency is originated due to the relative dielectric constant variation of insulator with frequency and due to the low conductivity of the semiconductor. The predominant capacitance dispersion in accumulation region is mainly dominated by the low conductivity of the organic semiconductor. In weak accumulation region the capacitance dispersion is due to the dielectric constant variation with frequency. However, the capacitance dispersion with temperature is dominated by the low conductivity of the organic semiconductor. The proposed equivalent circuit model successfully explains the experimental trend in agreement with TCAD simulation results. Acknowledgements The authors would like to acknowledge Centre for NEMS and Nanophotonics (CNNP) for providing fabrication facility. References [1] T. Zaki, F. Ante, U. Zschieschang, J. Butschke, F. Letzkus, H. Richter, H. Klauk, J. N. Burghartz, IEEE JSSC 47 (2012) 292–300. [2] W. Xiong, Y. Guo, U. Zschieschang, H. Klauk, B. Murmann, , IEEE JSSC 45 (2010) 1380–1388. [3] E. J. Meijer, A. V. G. Mangnus, C. M. Hart, D. M. de Leeuw, and T. M. Klapwijk, Appl. Phys. Lett. 78 (2001) 3902-3904. [4] X. Lu, T. Minari, C. Liu, A. Kumatani, J.M. Liu, K. Tsukagoshi, Appl. Phys. Lett., 100 (2012) 183308 1-4. [5] P. Darmawan,T. Minari, A. Kumatani, Y. Li, C. Liu, K.Tsukagoshi, Appl. Phys. Lett. 100 (2012) 013303 1-3. [6] I. Torres and D. M. Taylor, J. Appl. Phys, vol. 98 (2005) 073710 1-9. [7] A. Nigam, P. Nair, M. Premaratne, and V. Rao Elec. Dev. Lett., IEEE, 35 (2013) 581–583. [8] Maddalena, F., C. de Falco, M. Caironi, and D. Natali, Org. Electron., 17 (2015), 304– 318. [9] P. K. Manda, S. Ranaswamy, S. Dutta, IEEE Trans.Electron Devices, 65 (2018) 184-190. [10] A. Nigam, M. Premaratne, and P. R. Nair Org. Electron., 14 (2013) 2902 – 2907. [11] M. Mohan, V. Nandal, S. Paramadam, K. P. Reddy, S. Ramkumar, S. Agarwal, C. S. Gopinath, P. R. Nair, M. A.G. Namboothiry, J. Phys. Chem.C 121 (2017) 5523-5530.
ScienceDirect Materials Today: Proceedings 19 (2019) 53–57
www.materialstoday.com/proceedings
NANOTEXNOLOGY2018
Modeling the frequency response of organic metal-insulatorsemiconductor capacitors Prashant Kumar Manda, Logesh Karunakaran, Soumya Dutta Department of electrical engineering, Indian Institute of Technology madras, Chennai, 600036, India
Abstract Substantial improvement of organic field effect transistors (OTFTs) toward circuit applications certainly demands stable device performance upon varying frequency and temperature. Organic metal-insulator-semiconductor (MIS) capacitor has been considered to be an efficient model system to predict the device performance of OTFT under the influence of frequency and temperature. In this study we show that the capacitance dispersion with respect to frequency and temperature in organic MIS capacitor device is an inherent property, which arises due to the low conductivity of the semiconductor along with Schottky type contact at the semiconductor-metal junction. In addition we propose an equivalent circuit model to explain the dispersion of capacitance with respect to frequency and temperature. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of International Conferences & Exhibition on Nanosciences & Nanotechnologies and Flexible Organic Electronics 2018, June 30th - July 6th, 2018 Keywords: Organic MIS capacitors; Schotty contact; Frequency response
* Corresponding author. Tel.: 918754408225 E-mail address: [email protected]
2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of International Conferences & Exhibition on Nanosciences & Nanotechnologies and Flexible Organic Electronics 2018, June 30th - July 6th, 2018
54
P.K. Manda et al. / Materials Today: Proceedings 19 (2019) 53–57
1. Introduction Implementation of organic field effect transistors (OTFT) in practical application has been improved over last decade. The OTFT with bottom gate top contact (BGTC) configuration has been studied widely due to its ease of fabrication. The BGTC OTFT has already been employed in several circuit applications [1, 2]. However, the stability of OTFT has been considered to be a major concern for circuit application. Several factors such as bias stress effect, hysteresis, contact resistance, current and capacitance variation with frequency and temperature directly or indirectly affect the stability of OTFTs. The capacitance variation with frequency is one of the important phenomena that needs to be understood and modeled to predict the instability of the circuits. To study the frequency response of OTFT, organic metal-insulator-semiconductor (MIS) capacitors are often used [3, 4, 5]. In case of organic MIS capacitors the previous studies have shown capacitance dispersion with frequency [6,7] and temperature [3,4]. The capacitance dispersion with frequency was attributed to the presence of traps [6], deterioration of quality of metal-semiconductor junction [7], change in the dielectric constant of insulator with frequency [8] and due to the low carrier response time [3, 7]. However, to the best of our knowledge, there is no report that can explain the capacitance dispersion with frequency and temperature over a wide range of applied gate bias consistently. In the present report, we propose physics based equivalent circuit model, which explains the capacitance dispersion with frequency and temperature over a wide range of applied gate bias. 2. Fabrication and characterization The devices (the device structure is shown in Fig. 1) were fabricated on cleaned glass substrates by evaporating aluminum (50 nm) as gate metal, followed by spin coating of cross linked Poly(4-vinylphenol) (PVP) at 6000 RPM as insulator. Subsequently, the samples were annealed at 200oC for 20 minutes. Poly(3-hexylthiophene-2,5-diyl) (P3HT) (15 mg/ml in 1,2-Dichlorobenzene) was spun at 4000 RPM, followed by annealing at 120 oC for 20 minutes. Finally, gold (50 nm) was thermally evaporated as the top metal contact. All the fabrication processes were carried out inside the glove box. The fabricated devices were characterized inside a vacuum probe station using Agilent E4980A precision LCR meter.
Fig. 1. Device structure of organic MIS capacitor, where ts and ti correspond to thickness of semiconductor and insulator respectively.
3. Results and discussion We consider P3HT, which is relatively a high band gap (2 eV) [11] intrinsic semiconductor. Hence the intrinsic free carrier (electron and hole) concentration is less, which turns the MIS capacitor to a parallel plate capacitor with two different dielectric materials stacked in between two electrodes. Accordingly a constant capacitance with respect to gate voltage (Vg) is expected. However, P3HT based organic MIS capacitor shows good capacitance variation with Vg as shown in Fig. 2(a). As the capacitance corresponds to the change in charge with Vg, C-V characteristics evokes the presence of charge within the organic semiconductor and its variation with Vg, as shown in Fig. 2(a). The undoped P3HT makes Schottky type contact with Au. Hence Au injects the charge into P3HT by thermionic emission process [9, 10]. The injected charge concentration profile and the magnitude of the charge are
P.K. Manda et al. / Materials Today: Proceedings 19 (2019) 53–57
55
modulated by the applied gate field as shown in Fig. 2(d). The simulation study of MIS capacitors based on undoped semiconductor with Schottky contact suggests the same. The C-V characteristics obtained from the simulation, as shown in Fig. 2(b), captures the experimental trend. The capacitance characteristics as shown in Fig. 2(a) and (b) are divided into two regions, based on capacitance variation with Vg. In region-1 the capacitance varies with Vg and in region-2 capacitance is constant with Vg. From Fig. 2(d) one can understand that for Vg corresponds to region-1, the hole concentration is high, leading to a significant change in hole concentration with Vg . Whereas in region-2, the hole concentration is less, which makes P3HT to behave as an insulator, resulting a constant capacitance with respect to Vg, as explained before. To explain the capacitance nature in region-1, we propose a charge based capacitance model, where the capacitance is defines as
1 1 1 C Ci C g Cs
(1)
where Ci and Cg correspond to the geometrical capacitance of insulator and semiconductor respectively. The semiconductor capacitance Cs can be calculated using the total charge (Qs) present within the semiconductor and the potential drop across the semiconductor ψs as Cs
Qs s
(2)
where Qs is calculated as ts
Qs q p( x)dx
(3)
0
Fig. 2. C-V characteristics for with different frequency: (a) Experimental results; Comparison between simulation results (symbols) and model (lines) with mobility of (b) 10-4 cm2V-1s-1; (c) 1 cm2V-1s-1 inset shows the equivalent circuit; (d) p(x) variation with Vg.
56
P.K. Manda et al. / Materials Today: Proceedings 19 (2019) 53–57
The capacitance, as estimated using Qs and ψs obtained from simulation (solid line), shows a good match with small signal capacitance (symbols) for low frequency as shown in Fig. 2(b) and (c). Eq. (1) is formulated based on the assumption that the total charge present within the semiconductor is contributing for the capacitance. This indeed valid only for low frequencies, where in the capacitance is invariant with frequency. However, in practical devices, the capacitance decreases with increase in frequency (capacitance dispersion) as shown in Fig. 2(a). The possible reasons for the capacitance dispersion are dielectric constant variation with frequency [8] and, low response time of carriers [3, 7]. The capacitance dispersion in region-2 is due to the dielectric constant variation of insulator with frequency. This report focuses on the capacitance dispersion with frequency in region-1. The capacitance dispersion in region-1 is due to poor response time of the charge carriers present within the organic semiconductor which in turn depends on the conductivity of the semiconductor. This is evident from simulation results, where MIS capacitor corresponds to mobility (μh) of 10-4 cm2V-1s-1 captures the capacitance dispersion with frequency. In contrast, for μh of 1 cm2V-1s-1, where no capacitance dispersion is observed up to frequency of 10 kHz as shown in Fig. 2(c). To explain the capacitance dispersion with frequency, we propose an equivalent circuit model using Ci, Cg, Cs and averaged resistance (Rs) as shown in inset of Fig. 3(c), where Rs is estimated as follows ts
Rs
1
qp( x) 0
dx
(4)
h
Fig.3 C-V characteristics with different temperature at 1 kHz frequency: Comparison between simulation results (symbols) and model (lines) with mobility of (a) 10-4 cm2V-1s-1; (b) 1 cm2V-1s-1; (c) Cs, Rs variation with temperature for different mobility,(d) p(x) variation with temperature for different mobility at Vg=-3 V.
The comparison between the model (lines) and TCAD results (symbols) for different frequencies are shown in Fig. 2(b) and (c) where the proposed model explains the capacitance dispersion with frequency. The model further supports the argument that the capacitance dispersion is due to the low conductivity of the semiconductor. We extend our study to investigate the capacitance variation with temperature. Fig. 3(a) and (b) show the capacitance variation with temperature for MIS capacitor with mobility of 10-4cm2V-1s-1and 1cm2V-1s-1 respectively.
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With change in temperature the charge profile varies within the semiconductor as shown in Fig. 3(d). It is due to the charge injection process which is governed by thermionic emission which in turn is a thermally activated process. Hence the capacitance dispersion with temperature can arise due to Cs and Rs collectively as they both change with temperature as shown in Fig. 3(c). However, in low mobility case the capacitance dispersion is more compared to high mobility, though Cs is same for both the cases. The predominant capacitance dispersion with temperature can be understood from Rs variation, where Rs is orders of magnitude high for low mobility case compared to high mobility case as shown in Fig. 3(c). The excellent agreement between model (lines) and simulation (symbols) further strengthens the arguments. 4. Conclusion The capacitance dispersion with respect to frequency is originated due to the relative dielectric constant variation of insulator with frequency and due to the low conductivity of the semiconductor. The predominant capacitance dispersion in accumulation region is mainly dominated by the low conductivity of the organic semiconductor. In weak accumulation region the capacitance dispersion is due to the dielectric constant variation with frequency. However, the capacitance dispersion with temperature is dominated by the low conductivity of the organic semiconductor. The proposed equivalent circuit model successfully explains the experimental trend in agreement with TCAD simulation results. Acknowledgements The authors would like to acknowledge Centre for NEMS and Nanophotonics (CNNP) for providing fabrication facility. References [1] T. Zaki, F. Ante, U. Zschieschang, J. Butschke, F. Letzkus, H. Richter, H. Klauk, J. N. Burghartz, IEEE JSSC 47 (2012) 292–300. [2] W. Xiong, Y. Guo, U. Zschieschang, H. Klauk, B. Murmann, , IEEE JSSC 45 (2010) 1380–1388. [3] E. J. Meijer, A. V. G. Mangnus, C. M. Hart, D. M. de Leeuw, and T. M. Klapwijk, Appl. Phys. Lett. 78 (2001) 3902-3904. [4] X. Lu, T. Minari, C. Liu, A. Kumatani, J.M. Liu, K. Tsukagoshi, Appl. Phys. Lett., 100 (2012) 183308 1-4. [5] P. Darmawan,T. Minari, A. Kumatani, Y. Li, C. Liu, K.Tsukagoshi, Appl. Phys. Lett. 100 (2012) 013303 1-3. [6] I. Torres and D. M. Taylor, J. Appl. Phys, vol. 98 (2005) 073710 1-9. [7] A. Nigam, P. Nair, M. Premaratne, and V. Rao Elec. Dev. Lett., IEEE, 35 (2013) 581–583. [8] Maddalena, F., C. de Falco, M. Caironi, and D. Natali, Org. Electron., 17 (2015), 304– 318. [9] P. K. Manda, S. Ranaswamy, S. Dutta, IEEE Trans.Electron Devices, 65 (2018) 184-190. [10] A. Nigam, M. Premaratne, and P. R. Nair Org. Electron., 14 (2013) 2902 – 2907. [11] M. Mohan, V. Nandal, S. Paramadam, K. P. Reddy, S. Ramkumar, S. Agarwal, C. S. Gopinath, P. R. Nair, M. A.G. Namboothiry, J. Phys. Chem.C 121 (2017) 5523-5530.