- Email: [email protected]
Tuning on threshold voltage of organic field-effect transistor with a copper oxide layer
Organic Electronics 12 (2011) 429–434
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Letter
Tuning on threshold voltage of organic field-effect transistor with a copper oxide layer Guozheng Nie, Junbiao Peng ⇑, Linfeng Lan, Ruixia Xu, Jianhua Zou, Yong Cao Institute of Polymer Optoelectronic Materials and Devices, Key Laboratory of Special Functional Materials of the Ministry of Education, South China University of Technology, Guangzhou 510640, PR China
a r t i c l e
i n f o
Article history: Received 25 September 2010 Received in revised form 10 December 2010 Accepted 20 December 2010 Available online 4 January 2011 Keywords: Field-effect transistors CuO embedded layer Tuning of threshold voltage Electron trapping
a b s t r a c t Organic field-effect transistors (OFETs) based on the pentacene semiconductor with an embedded thin layer of copper oxide (CuO) were investigated. The drain current of OFETs with a thin CuO layer embedded in pentacene increases more than three times compared to that of traditional OFETs without the CuO layer, and the threshold voltage shifts from 17.5 V to 7.9 V. A possible mechanism for OFETs with the CuO layer was discussed via analysis of electron transfer near the contact between CuO and pentacene. OFETs with the CuO layer exhibit an interesting result of a systematic tuning of threshold voltage by controlling the initial voltage of gate-to-source upon scanning transfer curves. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Organic field-effect transistors (OFETs), as the critical component of organic integrated circuits (ICs), have attracted considerable attention due to their low cost, easy processability, flexibility, and applicability in large-area devices [1,2]. Nowadays, great efforts have been devoted to design and synthesize organic semiconductors as well as to fabricate OFETs with high performance, high stability and low cost [3–5]. However, there are still many challenges of restricting their practical applications in organic ICs, such as wide range tuning of the threshold voltage (VTH). So far, the works on tuning of VTH to control OFETs at low-power modes have made a progress [6–11]. For example, the VTH of OFETs can be tuned by using polar self-assembled monolayers [6], depositing an additional layer of organic acceptor on the top of channel [7], and using ferroelectric gate dielectrics [8] or ion-dispersed gate dielectrics [9,10]. In addition, the shift of threshold voltage ⇑ Corresponding author. Address: Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, 381 Wushan Road, Guangzhou 510640, PR China. Tel.: +86 20 87114535; fax: +86 20 87110606. E-mail address: [email protected] (J. Peng). 1566-1199/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2010.12.012
could be observed in a single OFET with molybdenum trioxide (MoO3) covered on the surface of p-bis [(p-styry1) styry1] benzene single crystal [11]. However, these methods are slightly complicated to systematically control VTH in a wide range. In inorganic FETs, doping in the active channel was used to accurately control the VTH of the devices [12]. For organic semiconductors, we found an efficient way to systematically tune VTH in OFETs by embedding a copper oxide acceptor layer in the semiconductor layer. Copper oxide was normally used for hole injection layer in the OFETs [13] or organic light-emitting diodes (OLEDs) [14,15]. In this paper, we significantly improved both VTH and mobility of the OFETs by embedding a thin CuO layer inside the pentacene semiconductor. Furthermore, a systematic tuning of threshold voltage of the OFETs can be realized by controlling the initial scanning voltage upon measurement of transfer curves. A possible mechanism for the results is discussed. 2. Experimental The OFETs were fabricated with a top contact configuration (Fig. 1) by using a heavily-doped n-type silicon wafer
430
G. Nie et al. / Organic Electronics 12 (2011) 429–434
Fig. 1. Cross section of the OFETs, (a) pentacene-based OFET (b) CuOembedded OFET.
where Ci is the capacitance of per unit area of the gate insulator, VGS is the gate-to-source voltage, and VTH is the threshold voltage. The mobility, VTH , and current on/off ratio of the pentacene-based OFET are obtained from Eq. (1) to be 0.06 cm2 V1 S1, 17.5 V, and 4.5 104, respectively, while the corresponding parameters of CuO-embedded OFET are 0.18 cm2 V1 S1, 7.9 V, and 1.4 104, respectively. The results show that both hole mobility and VTH are much improved in the CuO-embedded OFET. 3.1. Function of CuO embedded layer
with a 300 nm thick layer of thermally oxided SiO2 (Ci = 10nF), serving as the gate electrode and gate insulator, respectively. The wafers were cleaned with deionized water, acetone and isopropanol for 10 min in sequence. Pentacene and CuO were thermally deposited at rate of 0.5 Å/s and 0.3 Å/s, respectively, in a vacuum chamber under a pressure of 3 104 Pa. The 3 nm thick CuO is sandwiched in the pentacene layer with a distance of 30 nm apart from the interface of SiO2 and pentacene. The total thickness of the pentacene is 50 nm. The Cu source and drain electrodes (40 nm-thick) were then thermally evaporated through a shadow mask. The length (L) and width (W) of the channel were 100 lm and 10,000 lm, respectively. Fig. 1a and b shows the schematic structures of pure pentacene OFET (pentacene-based OFET) and CuO layer embedded pentacene OFET (CuO-embedded OFET). The electrical characteristics of OFETs were measured by using a semiconductor parameter analyzer (Agilent 4155C) at room temperature. The absorption spectra were recorded by using a UV–Vis spectrophotometer (HP 8453). The XPS spectra were measured by using Mutil-photoelectron spectroscopy (Kratos AXis Ultra (DLD)). 3. Results and discussion Output and transfer characteristics of pentacene-based OFET and CuO-embedded OFET, are shown in Fig. 2 and in Fig. 3, respectively. Obviously, a great increase in saturation current (ID) of CuO-embeded OFET was observed. The field-effect mobility of OFET in the saturation region can be calculated from the following equation [16].
ID ¼
W lC i ðV GS V TH Þ2 2L
ð1Þ
The VTH shift of OFET was usually originated from the following two reasons: (1) slow polarization of the gate dielectric [17]; (2) charge trapping at the interface between dielectric and semiconductor [18] or in the organic semiconductor [19]. Since the polarization of SiO2 dielectric was hardly occurred in this case, we prefer to the second reason as stated below. In order to clarify the VTH shift mechanism, time domain measurement was carried out in ambient air. This method was often used to determine the type of charge trapping in OFETs [20]. At the beginning, a gate-to-source voltage (denoted as VGS0) was constantly applied for 100 s (for gaining clear signal), and then abruptly stepped to a constant gate-to-source voltage of VGS1 (20 V) for another 100 s. The source-drain current (ID) was measured under source-to-drain voltage (VDS) at 10 V. Fig. 4a and b shows ID versus time (t) curves of applied voltages for both CuO-embedded and pentacenebased OFETs under different VGS0, respectively. For the p-type CuO-embedded OFET, when the VGS switched from VGS0 with negative values (30 V or 50 V) to VGS1 (20 V), the current (ID) was slightly decreased and then fixed in the value of about 2.0 105 A. However, when the VGS switched from VGS0 with positive values (30 V or 50 V) to VGS1 (20 V), the current (ID) was remarkably enhanced. The results of CuO-embedded OFETs suggest that positive VGS0 could induce extra hole accumulation in the OFET channel, resulting in current increase under polar change of VGS from positive VGS0 to negative VGS1. The production of extra holes was considered to be due to electron trapping in the active layer at a positive VGS0 [21]. However, the negative VGS0 could not induce any extra charged carriers in p-type OFET. So the ID was fixed to a constant value after VGS switched from negative VGS0 to
Fig. 2. Output characteristics (a) and transfer characteristics (b) of pentacene-based OFET.
G. Nie et al. / Organic Electronics 12 (2011) 429–434
431
Fig. 3. Output characteristics (a) and transfer characteristics (b) of CuO-embedded OFET.
Fig. 4. Time domain measurements of the OFETs (a) CuO-embedded OFET, (b) pentacene-based OFET, in ambient air at different VGS0 (VGS0 = 50,30,30,50 V), and VGS1 = 20 V, VDS = 10 V.
VGS1 (20 V). In addition, when VGS0 was applied at positive value, the larger the VGS0 value, the higher the value of ID was obtained after switching VGS from positive VGS0 to VGS1 (20 V). For example, ID = 2.5x105 A for VGS0 = 30 V, while ID = 4.5 105 A for VGS0 = 50 V (see Fig. 4a). The data suggests that electron trapping population could increase with increasing the positive value of VGS0. No such phenomenon was observed from pentacene-based OFETs (see Fig. 4b), hence it was further confirmed that the CuO layer could trap electrons and induce holes in pentacene. The detailed discussion for the roles of the embedded CuO layer is given below.
The capability of CuO accepting electrons could result in electron transfer from pentacene to CuO and formation of charge transfer (CT) complexes between CuO and pentacene, as confirmed by the absorption spectra [22] (see Fig. 5a). The absorption spectrum of the CuO-doped pentacene film shows the additional absorption peaks at around 423 nm and 1000 nm, indicating the electron transfer from pentacene to high work-function CuO (5.3 eV [23]). In addition, X-ray photoelectron spectra (XPS) can also confirm electron transfer at the interface between pentacene and CuO layers, as shown in Fig. 5b. The Cu 2p3/2 peak shifts to higher energies as shown in the XPS spectra of sample (E). Since a negatively charged atom has a higher binding energy of electrons [24], the peak shift further verifies electron transfer from pentacene to CuO and formation of CT complexes at their interface. The CT complexes could push the Fermi level of pentacene toward its highest occupied molecular orbital (HOMO), because it is p-type doping system [25,26]. The shift of Fermi level can reduce bulk hole trap density in pentacene [26,27] and enhance the bulk hole injection [27]. Therefore, the improvement of field-effect mobility of CuO-embedded OFETs could be attributed to the formation of CT complexes at the CuO/ pentacene interfaces. The holes supplied from the CT complexes enhance the drain current and result in the threshold voltage shift from 17.5 V to 7.9 V (see Figs. 2 and 3). Additionally, the field-effect mobility of the CuO-embedded OFET does not vary with VGS0 after time domain measurements, as shown from the almost unchangeable slopes of the (–ID) 1/2 versus VGS curves in Fig. 6. In this case, similar to the V2O5 [28] and MoO3 layers [29], the embedded CuO layer could act as a charge-generation layer when a positive VGS0 was applied. In CuO layer, electrons and holes were dissociated under positive VGS0, and the generated electrons were trapped at the interface between pentacene and CuO due to the poor electron-transporting ability of pentacene and electron traps of CuO surface. The trapped electrons can induce holes in pentacene semiconductor. The larger the VGS0 value, the more electron–hole pairs were generated in CuO layer, leading to the higher source-drain current (ID) (see Fig. 4a). Therefore, the trapped electron population in CuO could be controlled by adjusting of positive VGS0.
432
G. Nie et al. / Organic Electronics 12 (2011) 429–434
Fig. 5. (a) UV–Vis absorption spectra of samples of (A) pentacene, (B) CuO-doped pentacene and (C) pure CuO films on quartz substrate. (b) XPS spectra: Cu 2p3/2 peak observed in the samples of (D): glass substrate/CuO (50 nm), and (E): glass substrate/pentacene (50 nm)/CuO (1 nm).
where Q trap is the amount of trapped charge, DV TH the shift of VTH, and C i the capacitance of per unit area of the gate insulator. If the trapped electrons can be controlled, the VTH of OFETs can be strictly tuned. So the VTH of CuOembedded OFET depended on the trapped electron population in CuO, and could be changeable with the positive VGS0. The induced holes could act as a floating gate voltage that can influence the threshold voltages. When adjusting
Fig. 6. (ID)1/2 versus VGS transfer curves for the CuO-embedded OFET after the time domain measurements at different VGS0 (VGS0 = 50, 30,30,50 V), respectively.
3.2. Tuning of VTH The shift of VTH is related to trapped charges in OFETs according to the following equation [30].
Q trap ¼ DV TH C i
ð2Þ
Fig. 8. ID–VGS transfer curves at VDS = 40 V measured for five times. Each transfer curve is double-swept, VGS scans from 50 V to 80 V then back to 50 V.
Fig. 7. (a) Log plots of ID versus VGS and (b) (ID)1/2 versus VGS transfer curves for CuO-embedded OFET with different VGS0: 80, 70, 60, 50, 40, 20 and 0 V.
G. Nie et al. / Organic Electronics 12 (2011) 429–434
433
Fig. 9. (a) Drain current as a function of stress time over a period of 3 103 s for a constant voltage-bias stress testing, the time interval is 10 s, and the drain current was measured at VGS = 30 V and VDS = 30 V. (b) (ID)1/2–VGS transfer curves with VDS = 40 V before and after constant-voltage bias-stress testing over a period of 3 103 s, respectively.
positive VGS0, trapped electrons in CuO could be changed as discussed (see Fig. 4a). In the transfer curves, when changing the starting gate-to-source voltages (VGS0) in positive range, systematic tuning of the VTH of the CuO-embedded OFET can be realized. For example, when VGS0 varies from 0 V to 80 V, the transfer curves regularly shift from negative voltage to positive voltage, and the VTH shifts from 9.5 V to 32.5 V (as shown in Fig. 7a and b). The shift of threshold voltage was not observed in pentacene-based OFET (data is not shown here). Therefore, the systematic tunable VTH in our device can be ascribed to the fact that VGS0 influenced the trapped electron population at the interface between CuO and pentacene layers. 3.3. Stability of VTH To investigate the stability of the tuned VTH, the transfer curves with VGS0 at 50 V were repeatedly measured in five times, and exhibited a good repeatability (see Fig. 8). On the other hand, the drain current was measured with VGS = 30 V and VDS = 30 V at the mode of constant-voltage bias-stress testing for 3 103 s (as shown in Fig. 9a), exhibiting a very small decrease of the drain current. The (ID)1/2 versus VGS transfer curves were then measured at VGS0 of 50 V, exhibiting almost unchangeable VTH after constant-voltage bias-stress testing over a period of 3 103 s, as shown in Fig. 9b. The results suggest that the trapped electrons at the interface between CuO and pentacene layers are not easily released and the tuned VTH is stable. 4. Conclusion An organic field-effect transistor based on pentacene with an embedded CuO layer was investigated. With the thin CuO layer embedded in the pentacene semiconductor, the drain current was increased and the threshold voltage shifted from 17.5 V to 7.9 V. The interesting results are attributed to the formation of CT complexes at the interface of CuO and pentacene. Electrons trapped in CuO can induce holes in pentacene. The induced holes act as a floating gate voltage that influences the threshold voltages. The regular tuning on threshold voltages of CuO-embedded
OFET is achieved by scanning the transfer curves with different starting positive gate-to-source voltage (VGS0) because the population of trapped electrons can be changed by setting the VGS0. The threshold voltages are stable under stress of source-to-drain and gate-to-source voltages. Acknowledgements The authors are grateful to National ‘‘863’’ Project of China (Grant No. 2008AA03A335), National ‘‘973’’ Project of China (Grant No. 2009CB623604), the National Natural Science Foundation of China (under the Grant Nos. 610,36,007, 50573024 and 50433030), the project of Guangdong province (Grant No. 20081202) and Key Project of Chinese Ministry of Education (Grant No. 104208). Reference [1] C.D. Dimitrakopoulos, P.R.L. Malenfant, Adv. Mater. 14 (2002) 99. [2] B.K. Crone, A. Dodabalapur, R. Sarpeshkar, et al., J. Appl. Phys. 89 (2001) 5125. [3] Y.L. Wu, Y.N. Li, S. Gardner, B.S. Ong, J. Am. Chem. Soc. 127 (2005) 614. [4] A.L. Briseno, M. Roberts, M.M. Ling, H. Moon, E.J. Nemanick, Z. Bao, J. Am. Chem. Soc. 128 (2006) 3880. [5] C.A. Di, G. Yu, Y.Q. Liu, X.J. Xu, D.C. Wei, Y.B. Song, Y.M. Sun, Y. Wang, D.B. Zhu, J. Liu, X.Y. Liu, D.X. Wu, J. Am. Chem. Soc. 128 (2006) 16418. [6] K.P. Pernstich, S. Haas, D. Oberhoff, C. Goldmann, D.J. Gundlach, B. Batlogg, A.N. Rashid, G. Schitter, J. Appl. Phys. 96 (2004) 6431. [7] Y. Abe, T. Hasegawa, Y. Takahashi, T. Yamada, Y. Tokura, Appl. Phys. Lett. 87 (2005) 153506. [8] C.A. Nguyen, P.S. Lee, S.G. Mhaisalkar, Org. Electron. 8 (2007) 415. [9] H. Sakai, Y. Takahashi, H. Murata, Appl. Phys. Lett. 91 (2007) 113502. [10] H. Sakai, K. Konno, H. Murata, Appl. Phys. Lett. 94 (2009) 073304. [11] H. Nakanotani, M. Saito, H. Nakamura, C. Adachi1, Appl. Phys. Lett. 95 (2009) 103307. [12] S.M. Sze, K.K. Ng, Physics of Semiconductor Devices (Chapter 6), third ed., Wiley, Hoboken, 2007. pp. 346. [13] J.W. Park, K.J. Baeg, J. Ghim, S. Ju, K.J.H. Park, D.Y. Kim, Electrochem. Solid-State Lett. 10 (2007) H340. [14] G.B.M. Greiner, M.G. Helander, Z.B. Wang, Z.H. Lu, Appl. Phys. Lett. 93 (2008) 083309. [15] S. Kim, K. Hong, K. Kim, et al., J. Electrochem. Soc. 157 (2010) J147. [16] S.M. Sze, Physics of Semiconductor Devices, second ed., Wiley, New York, 1981. [17] G. Nunes Jr., S.G. Zane, J.S. Meth, J. Appl. Phys. 98 (2005) 104503. [18] J.W.H. Smith, I.G. Hill, J. Appl. Phys. 101 (2007) 044503. [19] T.P.I. Saragi, J. Londenberg, J. Salbeck, J. Appl. Phys. 102 (2007) 046104.
434
G. Nie et al. / Organic Electronics 12 (2011) 429–434
[20] G. Gu, M.G. Kane, S.C. Mau, J. Appl. Phys. 101 (2007) 014504. [21] G. Gu, M.G. Kane, Appl. Phys. Lett. 92 (2008) 053305. [22] J.H. Lee, D.S. Leem, H.J. Kim, J.J. Kim, Appl. Phys. Lett. 94 (2009) 123306. [23] F.P. Koffyberg, F.A. Benko, J. Appl. Phys. 53 (1982) 1173. [24] T. Matsushima, Y. Kinoshita, H. Murata, Appl. Phys. Lett. 91 (2007) 253504. [25] K. Walzer, B. Maennig, M. Pfeiffer, K. Leo, Chem. Rev. 107 (2007) 1233.
[26] B. Maennig, M. Pfeiffer, A. Nollau, X. Zhou, K. Leo, P. Simon, Phys. Rev. B 64 (2001) 195208. [27] T. Minari, T. Miyadera, K. Tsukagoshi, Y. Aoyagi, H. Ito, Appl. Phys. Lett. 91 (2007) 053508. [28] M. Terai, K. Fujita, T. Tsutsui, Jpn. J. Appl. Phys., Part 2 44 (2005) L1059. [29] X.F. Qi, N. Li, S. Forrest, J. Appl. Phys. 107 (2010) 014514. [30] S.M. Sze, K.K. Ng, Physics of Semiconductor Devices, Wiley, New York, 2007.
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Letter
Tuning on threshold voltage of organic field-effect transistor with a copper oxide layer Guozheng Nie, Junbiao Peng ⇑, Linfeng Lan, Ruixia Xu, Jianhua Zou, Yong Cao Institute of Polymer Optoelectronic Materials and Devices, Key Laboratory of Special Functional Materials of the Ministry of Education, South China University of Technology, Guangzhou 510640, PR China
a r t i c l e
i n f o
Article history: Received 25 September 2010 Received in revised form 10 December 2010 Accepted 20 December 2010 Available online 4 January 2011 Keywords: Field-effect transistors CuO embedded layer Tuning of threshold voltage Electron trapping
a b s t r a c t Organic field-effect transistors (OFETs) based on the pentacene semiconductor with an embedded thin layer of copper oxide (CuO) were investigated. The drain current of OFETs with a thin CuO layer embedded in pentacene increases more than three times compared to that of traditional OFETs without the CuO layer, and the threshold voltage shifts from 17.5 V to 7.9 V. A possible mechanism for OFETs with the CuO layer was discussed via analysis of electron transfer near the contact between CuO and pentacene. OFETs with the CuO layer exhibit an interesting result of a systematic tuning of threshold voltage by controlling the initial voltage of gate-to-source upon scanning transfer curves. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Organic field-effect transistors (OFETs), as the critical component of organic integrated circuits (ICs), have attracted considerable attention due to their low cost, easy processability, flexibility, and applicability in large-area devices [1,2]. Nowadays, great efforts have been devoted to design and synthesize organic semiconductors as well as to fabricate OFETs with high performance, high stability and low cost [3–5]. However, there are still many challenges of restricting their practical applications in organic ICs, such as wide range tuning of the threshold voltage (VTH). So far, the works on tuning of VTH to control OFETs at low-power modes have made a progress [6–11]. For example, the VTH of OFETs can be tuned by using polar self-assembled monolayers [6], depositing an additional layer of organic acceptor on the top of channel [7], and using ferroelectric gate dielectrics [8] or ion-dispersed gate dielectrics [9,10]. In addition, the shift of threshold voltage ⇑ Corresponding author. Address: Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, 381 Wushan Road, Guangzhou 510640, PR China. Tel.: +86 20 87114535; fax: +86 20 87110606. E-mail address: [email protected] (J. Peng). 1566-1199/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2010.12.012
could be observed in a single OFET with molybdenum trioxide (MoO3) covered on the surface of p-bis [(p-styry1) styry1] benzene single crystal [11]. However, these methods are slightly complicated to systematically control VTH in a wide range. In inorganic FETs, doping in the active channel was used to accurately control the VTH of the devices [12]. For organic semiconductors, we found an efficient way to systematically tune VTH in OFETs by embedding a copper oxide acceptor layer in the semiconductor layer. Copper oxide was normally used for hole injection layer in the OFETs [13] or organic light-emitting diodes (OLEDs) [14,15]. In this paper, we significantly improved both VTH and mobility of the OFETs by embedding a thin CuO layer inside the pentacene semiconductor. Furthermore, a systematic tuning of threshold voltage of the OFETs can be realized by controlling the initial scanning voltage upon measurement of transfer curves. A possible mechanism for the results is discussed. 2. Experimental The OFETs were fabricated with a top contact configuration (Fig. 1) by using a heavily-doped n-type silicon wafer
430
G. Nie et al. / Organic Electronics 12 (2011) 429–434
Fig. 1. Cross section of the OFETs, (a) pentacene-based OFET (b) CuOembedded OFET.
where Ci is the capacitance of per unit area of the gate insulator, VGS is the gate-to-source voltage, and VTH is the threshold voltage. The mobility, VTH , and current on/off ratio of the pentacene-based OFET are obtained from Eq. (1) to be 0.06 cm2 V1 S1, 17.5 V, and 4.5 104, respectively, while the corresponding parameters of CuO-embedded OFET are 0.18 cm2 V1 S1, 7.9 V, and 1.4 104, respectively. The results show that both hole mobility and VTH are much improved in the CuO-embedded OFET. 3.1. Function of CuO embedded layer
with a 300 nm thick layer of thermally oxided SiO2 (Ci = 10nF), serving as the gate electrode and gate insulator, respectively. The wafers were cleaned with deionized water, acetone and isopropanol for 10 min in sequence. Pentacene and CuO were thermally deposited at rate of 0.5 Å/s and 0.3 Å/s, respectively, in a vacuum chamber under a pressure of 3 104 Pa. The 3 nm thick CuO is sandwiched in the pentacene layer with a distance of 30 nm apart from the interface of SiO2 and pentacene. The total thickness of the pentacene is 50 nm. The Cu source and drain electrodes (40 nm-thick) were then thermally evaporated through a shadow mask. The length (L) and width (W) of the channel were 100 lm and 10,000 lm, respectively. Fig. 1a and b shows the schematic structures of pure pentacene OFET (pentacene-based OFET) and CuO layer embedded pentacene OFET (CuO-embedded OFET). The electrical characteristics of OFETs were measured by using a semiconductor parameter analyzer (Agilent 4155C) at room temperature. The absorption spectra were recorded by using a UV–Vis spectrophotometer (HP 8453). The XPS spectra were measured by using Mutil-photoelectron spectroscopy (Kratos AXis Ultra (DLD)). 3. Results and discussion Output and transfer characteristics of pentacene-based OFET and CuO-embedded OFET, are shown in Fig. 2 and in Fig. 3, respectively. Obviously, a great increase in saturation current (ID) of CuO-embeded OFET was observed. The field-effect mobility of OFET in the saturation region can be calculated from the following equation [16].
ID ¼
W lC i ðV GS V TH Þ2 2L
ð1Þ
The VTH shift of OFET was usually originated from the following two reasons: (1) slow polarization of the gate dielectric [17]; (2) charge trapping at the interface between dielectric and semiconductor [18] or in the organic semiconductor [19]. Since the polarization of SiO2 dielectric was hardly occurred in this case, we prefer to the second reason as stated below. In order to clarify the VTH shift mechanism, time domain measurement was carried out in ambient air. This method was often used to determine the type of charge trapping in OFETs [20]. At the beginning, a gate-to-source voltage (denoted as VGS0) was constantly applied for 100 s (for gaining clear signal), and then abruptly stepped to a constant gate-to-source voltage of VGS1 (20 V) for another 100 s. The source-drain current (ID) was measured under source-to-drain voltage (VDS) at 10 V. Fig. 4a and b shows ID versus time (t) curves of applied voltages for both CuO-embedded and pentacenebased OFETs under different VGS0, respectively. For the p-type CuO-embedded OFET, when the VGS switched from VGS0 with negative values (30 V or 50 V) to VGS1 (20 V), the current (ID) was slightly decreased and then fixed in the value of about 2.0 105 A. However, when the VGS switched from VGS0 with positive values (30 V or 50 V) to VGS1 (20 V), the current (ID) was remarkably enhanced. The results of CuO-embedded OFETs suggest that positive VGS0 could induce extra hole accumulation in the OFET channel, resulting in current increase under polar change of VGS from positive VGS0 to negative VGS1. The production of extra holes was considered to be due to electron trapping in the active layer at a positive VGS0 [21]. However, the negative VGS0 could not induce any extra charged carriers in p-type OFET. So the ID was fixed to a constant value after VGS switched from negative VGS0 to
Fig. 2. Output characteristics (a) and transfer characteristics (b) of pentacene-based OFET.
G. Nie et al. / Organic Electronics 12 (2011) 429–434
431
Fig. 3. Output characteristics (a) and transfer characteristics (b) of CuO-embedded OFET.
Fig. 4. Time domain measurements of the OFETs (a) CuO-embedded OFET, (b) pentacene-based OFET, in ambient air at different VGS0 (VGS0 = 50,30,30,50 V), and VGS1 = 20 V, VDS = 10 V.
VGS1 (20 V). In addition, when VGS0 was applied at positive value, the larger the VGS0 value, the higher the value of ID was obtained after switching VGS from positive VGS0 to VGS1 (20 V). For example, ID = 2.5x105 A for VGS0 = 30 V, while ID = 4.5 105 A for VGS0 = 50 V (see Fig. 4a). The data suggests that electron trapping population could increase with increasing the positive value of VGS0. No such phenomenon was observed from pentacene-based OFETs (see Fig. 4b), hence it was further confirmed that the CuO layer could trap electrons and induce holes in pentacene. The detailed discussion for the roles of the embedded CuO layer is given below.
The capability of CuO accepting electrons could result in electron transfer from pentacene to CuO and formation of charge transfer (CT) complexes between CuO and pentacene, as confirmed by the absorption spectra [22] (see Fig. 5a). The absorption spectrum of the CuO-doped pentacene film shows the additional absorption peaks at around 423 nm and 1000 nm, indicating the electron transfer from pentacene to high work-function CuO (5.3 eV [23]). In addition, X-ray photoelectron spectra (XPS) can also confirm electron transfer at the interface between pentacene and CuO layers, as shown in Fig. 5b. The Cu 2p3/2 peak shifts to higher energies as shown in the XPS spectra of sample (E). Since a negatively charged atom has a higher binding energy of electrons [24], the peak shift further verifies electron transfer from pentacene to CuO and formation of CT complexes at their interface. The CT complexes could push the Fermi level of pentacene toward its highest occupied molecular orbital (HOMO), because it is p-type doping system [25,26]. The shift of Fermi level can reduce bulk hole trap density in pentacene [26,27] and enhance the bulk hole injection [27]. Therefore, the improvement of field-effect mobility of CuO-embedded OFETs could be attributed to the formation of CT complexes at the CuO/ pentacene interfaces. The holes supplied from the CT complexes enhance the drain current and result in the threshold voltage shift from 17.5 V to 7.9 V (see Figs. 2 and 3). Additionally, the field-effect mobility of the CuO-embedded OFET does not vary with VGS0 after time domain measurements, as shown from the almost unchangeable slopes of the (–ID) 1/2 versus VGS curves in Fig. 6. In this case, similar to the V2O5 [28] and MoO3 layers [29], the embedded CuO layer could act as a charge-generation layer when a positive VGS0 was applied. In CuO layer, electrons and holes were dissociated under positive VGS0, and the generated electrons were trapped at the interface between pentacene and CuO due to the poor electron-transporting ability of pentacene and electron traps of CuO surface. The trapped electrons can induce holes in pentacene semiconductor. The larger the VGS0 value, the more electron–hole pairs were generated in CuO layer, leading to the higher source-drain current (ID) (see Fig. 4a). Therefore, the trapped electron population in CuO could be controlled by adjusting of positive VGS0.
432
G. Nie et al. / Organic Electronics 12 (2011) 429–434
Fig. 5. (a) UV–Vis absorption spectra of samples of (A) pentacene, (B) CuO-doped pentacene and (C) pure CuO films on quartz substrate. (b) XPS spectra: Cu 2p3/2 peak observed in the samples of (D): glass substrate/CuO (50 nm), and (E): glass substrate/pentacene (50 nm)/CuO (1 nm).
where Q trap is the amount of trapped charge, DV TH the shift of VTH, and C i the capacitance of per unit area of the gate insulator. If the trapped electrons can be controlled, the VTH of OFETs can be strictly tuned. So the VTH of CuOembedded OFET depended on the trapped electron population in CuO, and could be changeable with the positive VGS0. The induced holes could act as a floating gate voltage that can influence the threshold voltages. When adjusting
Fig. 6. (ID)1/2 versus VGS transfer curves for the CuO-embedded OFET after the time domain measurements at different VGS0 (VGS0 = 50, 30,30,50 V), respectively.
3.2. Tuning of VTH The shift of VTH is related to trapped charges in OFETs according to the following equation [30].
Q trap ¼ DV TH C i
ð2Þ
Fig. 8. ID–VGS transfer curves at VDS = 40 V measured for five times. Each transfer curve is double-swept, VGS scans from 50 V to 80 V then back to 50 V.
Fig. 7. (a) Log plots of ID versus VGS and (b) (ID)1/2 versus VGS transfer curves for CuO-embedded OFET with different VGS0: 80, 70, 60, 50, 40, 20 and 0 V.
G. Nie et al. / Organic Electronics 12 (2011) 429–434
433
Fig. 9. (a) Drain current as a function of stress time over a period of 3 103 s for a constant voltage-bias stress testing, the time interval is 10 s, and the drain current was measured at VGS = 30 V and VDS = 30 V. (b) (ID)1/2–VGS transfer curves with VDS = 40 V before and after constant-voltage bias-stress testing over a period of 3 103 s, respectively.
positive VGS0, trapped electrons in CuO could be changed as discussed (see Fig. 4a). In the transfer curves, when changing the starting gate-to-source voltages (VGS0) in positive range, systematic tuning of the VTH of the CuO-embedded OFET can be realized. For example, when VGS0 varies from 0 V to 80 V, the transfer curves regularly shift from negative voltage to positive voltage, and the VTH shifts from 9.5 V to 32.5 V (as shown in Fig. 7a and b). The shift of threshold voltage was not observed in pentacene-based OFET (data is not shown here). Therefore, the systematic tunable VTH in our device can be ascribed to the fact that VGS0 influenced the trapped electron population at the interface between CuO and pentacene layers. 3.3. Stability of VTH To investigate the stability of the tuned VTH, the transfer curves with VGS0 at 50 V were repeatedly measured in five times, and exhibited a good repeatability (see Fig. 8). On the other hand, the drain current was measured with VGS = 30 V and VDS = 30 V at the mode of constant-voltage bias-stress testing for 3 103 s (as shown in Fig. 9a), exhibiting a very small decrease of the drain current. The (ID)1/2 versus VGS transfer curves were then measured at VGS0 of 50 V, exhibiting almost unchangeable VTH after constant-voltage bias-stress testing over a period of 3 103 s, as shown in Fig. 9b. The results suggest that the trapped electrons at the interface between CuO and pentacene layers are not easily released and the tuned VTH is stable. 4. Conclusion An organic field-effect transistor based on pentacene with an embedded CuO layer was investigated. With the thin CuO layer embedded in the pentacene semiconductor, the drain current was increased and the threshold voltage shifted from 17.5 V to 7.9 V. The interesting results are attributed to the formation of CT complexes at the interface of CuO and pentacene. Electrons trapped in CuO can induce holes in pentacene. The induced holes act as a floating gate voltage that influences the threshold voltages. The regular tuning on threshold voltages of CuO-embedded
OFET is achieved by scanning the transfer curves with different starting positive gate-to-source voltage (VGS0) because the population of trapped electrons can be changed by setting the VGS0. The threshold voltages are stable under stress of source-to-drain and gate-to-source voltages. Acknowledgements The authors are grateful to National ‘‘863’’ Project of China (Grant No. 2008AA03A335), National ‘‘973’’ Project of China (Grant No. 2009CB623604), the National Natural Science Foundation of China (under the Grant Nos. 610,36,007, 50573024 and 50433030), the project of Guangdong province (Grant No. 20081202) and Key Project of Chinese Ministry of Education (Grant No. 104208). Reference [1] C.D. Dimitrakopoulos, P.R.L. Malenfant, Adv. Mater. 14 (2002) 99. [2] B.K. Crone, A. Dodabalapur, R. Sarpeshkar, et al., J. Appl. Phys. 89 (2001) 5125. [3] Y.L. Wu, Y.N. Li, S. Gardner, B.S. Ong, J. Am. Chem. Soc. 127 (2005) 614. [4] A.L. Briseno, M. Roberts, M.M. Ling, H. Moon, E.J. Nemanick, Z. Bao, J. Am. Chem. Soc. 128 (2006) 3880. [5] C.A. Di, G. Yu, Y.Q. Liu, X.J. Xu, D.C. Wei, Y.B. Song, Y.M. Sun, Y. Wang, D.B. Zhu, J. Liu, X.Y. Liu, D.X. Wu, J. Am. Chem. Soc. 128 (2006) 16418. [6] K.P. Pernstich, S. Haas, D. Oberhoff, C. Goldmann, D.J. Gundlach, B. Batlogg, A.N. Rashid, G. Schitter, J. Appl. Phys. 96 (2004) 6431. [7] Y. Abe, T. Hasegawa, Y. Takahashi, T. Yamada, Y. Tokura, Appl. Phys. Lett. 87 (2005) 153506. [8] C.A. Nguyen, P.S. Lee, S.G. Mhaisalkar, Org. Electron. 8 (2007) 415. [9] H. Sakai, Y. Takahashi, H. Murata, Appl. Phys. Lett. 91 (2007) 113502. [10] H. Sakai, K. Konno, H. Murata, Appl. Phys. Lett. 94 (2009) 073304. [11] H. Nakanotani, M. Saito, H. Nakamura, C. Adachi1, Appl. Phys. Lett. 95 (2009) 103307. [12] S.M. Sze, K.K. Ng, Physics of Semiconductor Devices (Chapter 6), third ed., Wiley, Hoboken, 2007. pp. 346. [13] J.W. Park, K.J. Baeg, J. Ghim, S. Ju, K.J.H. Park, D.Y. Kim, Electrochem. Solid-State Lett. 10 (2007) H340. [14] G.B.M. Greiner, M.G. Helander, Z.B. Wang, Z.H. Lu, Appl. Phys. Lett. 93 (2008) 083309. [15] S. Kim, K. Hong, K. Kim, et al., J. Electrochem. Soc. 157 (2010) J147. [16] S.M. Sze, Physics of Semiconductor Devices, second ed., Wiley, New York, 1981. [17] G. Nunes Jr., S.G. Zane, J.S. Meth, J. Appl. Phys. 98 (2005) 104503. [18] J.W.H. Smith, I.G. Hill, J. Appl. Phys. 101 (2007) 044503. [19] T.P.I. Saragi, J. Londenberg, J. Salbeck, J. Appl. Phys. 102 (2007) 046104.
434
G. Nie et al. / Organic Electronics 12 (2011) 429–434
[20] G. Gu, M.G. Kane, S.C. Mau, J. Appl. Phys. 101 (2007) 014504. [21] G. Gu, M.G. Kane, Appl. Phys. Lett. 92 (2008) 053305. [22] J.H. Lee, D.S. Leem, H.J. Kim, J.J. Kim, Appl. Phys. Lett. 94 (2009) 123306. [23] F.P. Koffyberg, F.A. Benko, J. Appl. Phys. 53 (1982) 1173. [24] T. Matsushima, Y. Kinoshita, H. Murata, Appl. Phys. Lett. 91 (2007) 253504. [25] K. Walzer, B. Maennig, M. Pfeiffer, K. Leo, Chem. Rev. 107 (2007) 1233.
[26] B. Maennig, M. Pfeiffer, A. Nollau, X. Zhou, K. Leo, P. Simon, Phys. Rev. B 64 (2001) 195208. [27] T. Minari, T. Miyadera, K. Tsukagoshi, Y. Aoyagi, H. Ito, Appl. Phys. Lett. 91 (2007) 053508. [28] M. Terai, K. Fujita, T. Tsutsui, Jpn. J. Appl. Phys., Part 2 44 (2005) L1059. [29] X.F. Qi, N. Li, S. Forrest, J. Appl. Phys. 107 (2010) 014514. [30] S.M. Sze, K.K. Ng, Physics of Semiconductor Devices, Wiley, New York, 2007.