Low-voltage organic transistors and inverters using HfOx dielectrics

Organic Electronics 30 (2016) 131e135

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Low-voltage organic transistors and inverters using HfOx dielectrics Jeong-Do Oh, Jang-Woon Kim, Dae-Kyu Kim, Jong-Ho Choi* Department of Chemistry, Research Institute for Natural Sciences, Korea University, 145 Anam-ro, Seongbuk-Gu, Seoul 02841, South Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Received 26 October 2015 Received in revised form 23 November 2015 Accepted 8 December 2015 Available online xxx

Based on the p-type pentacene and n-type N,N0 -ditridecylperylene-3,4,9,10-tetracarboxylic diimide (PTCDI-C13), low-voltage organic field-effect transistors (OFETs) and inverters using hafnium (Hf)-based dielectrics were produced and characterized. All the pristine and cyclic olefin copolymer (COC)-passivated HfOx gate dielectrics were deposited by the solution-processed solegel chemistry, and organic thin films were deposited on the dielectrics by the neutral cluster beam deposition method. In comparison to the pristine HfOx-based OFETs, the COC-passivated transistors showed better device performance: higher hole and electron mobilities, reduced hysteresis, decreased trap densities, and particularly improved operational stability of n-type transistors. The inverters composed of the optimized p- and n-type OFETs with the asymmetric Au and LiF/Al electrodes using COC-passivated HfOx dielectrics exhibited high gains and good noise margins under ambient conditions. © 2015 Elsevier B.V. All rights reserved.

Keywords: Organic field-effect transistors (OFETs) HfOx dielectrics Cyclic olefin copolymer (COC) Complementary metal oxide semiconductor (CMOS) inverters

1. Introduction Organic field-effect transistors (OFETs) and logic circuits are attractive owing to their easy processibility, cost effectiveness and flexible electronic devices such as displays, radio-frequency identification tags, sensors, e-papers, etc. [1e5]. For developing complex organic-based integrated circuits, low power consumption, high voltage gain, and large noise margin with little hysteresis are considered as the important prerequisites. To fulfill such challenging requirements, conventional low dielectric-constant (low-k) SiO2 gate dielectrics turned out to be unsuitable because of relatively high operation voltages exceeding 20 V. Instead, high-k metal oxides (for example, Al2O3, TiO2, HfO2, Ta2O5 and ZrO2) were expected to utilize p- and n-type OFETs and complementary metal-oxide semiconductor (CMOS) inverters to operate at low voltages [6e12]. From a practical standpoint, however, proper dielectric properties can be obtained using vacuum technologies such as atomic layer deposition, e-beam evaporation, and sputtering [12e16]. Besides, the unfavorable interfaces formed between the organic active layers and high-k oxide dielectrics degraded the performance in the p- and particularly n-channel OFETs: unwanted hysteresis, poor operational stability, low mobilities, etc. To overcome the incompatibility problems, one worthwhile scheme would be the passivation of the high-k dielectrics with

* Corresponding author. E-mail address: [email protected] (J.-H. Choi). http://dx.doi.org/10.1016/j.orgel.2015.12.006 1566-1199/© 2015 Elsevier B.V. All rights reserved.

hydroxyl-free polymer insulators such as cyclic olefin copolymer (COC) and polymethylmethacrylate. Such polymeric dielectrics can be handled using solution chemistry under ambient conditions and have attracted considerable attention as promising insulators for future all-organic electronics. In particular, the distinctive structures of hydroxyl-free COC contrast well with the charge-trapping silanol functional group in SiO2 dielectrics at the semiconductor/dielectric interface [17e19]. Herein, we report a comparative analysis of the low-voltage OFETs and CMOS inverters fabricated using various HfOx dielectrics and metal electrodes. First, the dielectric properties of the pristine (unpassivated) and COC-passivated HfOx gate dielectrics were examined. Second, based on p-type pentacene and n-type PTCDI-C13 grown on the HfOx dielectrics low-voltage OFET performance and characteristics were systematically optimized using Au and LiF/Al electrodes. Finally, high-gain CMOS inverters were produced by the integration of two optimized OFETs, and the device performance closely correlated to the HfOx dielectrics and metal electrodes was investigated.

2. Experimental Fig. 1 shows the molecular structures of pentacene (TCI Co.), PTCDI-C13 (SigmaeAldrich), and COC (Polysciences Inc.), and a schematic of the CMOS inverter with the energyelevel diagram. A series of sequential ultrasonic treatments were applied to clean heavily n-doped Si substrates acting as the gate electrodes for

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Fig. 1. Molecular structures of (a) pentacene, (b) PTCDI-C13, and (c) COC. (d) A 3D schematic of the CMOS inverter and (e) The corresponding energyelevel diagram. The widths and lengths of the CMOS inverters were 3400 and 150 mm, respectively, for p-type OFETs, and 6800 mm and 150 mm for n-type OFETs, respectively.

transistors and as the input electrodes for inverters. Approximately 10 nm-thick HfOx films were prepared by spin coating with a HfCl4 precursor solution (nitric acid added to HfCl4 dissolved in absolute ethanol) on the substrates at 5000 rpm for 30 s and then annealed at 500  C for 1 h. For the COC passivation the HfOx surface was spincoated with 0.5 wt% COC in toluene with an average thickness of 15 nm. The semiconducting pentacene (50 nm at a rate of ~0.8e1.0 Å/s) and PTCDI-C13 (50 nm at ~1.0e2.0 Å/s) were sequentially deposited onto the dielectric layers using a lab-built NCBD apparatus [20]. Finally, the Au (50 nm at ~2e4 Å/s) or LiF (0.7 nm at ~0.1e0.2 Å/s)/Al (70 nm at ~1e2 Å/s) electrodes were deposited on the active layer using a properly shaped shadow mask. The thicknesses of pristine and COC-passivated HfOx dielectrics were measured using ellipsometry (Rudolph Co.) and an alpha-step surface profile monitor (Tencor Co.). The contact angle and surface roughness were examined using a contact angle goniometer (Kyowa Interface Science Co.) and atomic force microscopy (AFM: PSI Co.), respectively. The capacitance and leakage current density of the dielectrics, the currentevoltage (IeV) characteristics of the transistors, and voltage transfer characteristics (VTCs) of the CMOS inverters were measured using an HP4284A system and an optical microscope probe station attached to an HP4145A semiconductor parameter analyzer under ambient conditions. 3. Results and discussion 3.1. Dielectric properties of the pristine and COC-passivated HfOX thin films A comparative examination of the surface morphology and dielectric properties of the pristine and COC-passivated HfOx layers was carried out. The 2D AFM micrographs were obtained by conducting section analyses on an area of 5  5 mm2 in the noncontact mode. The root-mean-square roughness (Rrms) values of the pristine and COC-passivated HfOx dielectrics were measured to be 0.20 and 0.17 nm, respectively. The smooth and pin-hole free dielectric layers acted as suitable substrates for the subsequent crystalline growth of pentacene and PTCDI-C13 films. The contact angles with water were also measured, as shown in Fig. 2(a). Compared to the contact angle of 18 on the hydrophilic HfOx layer, the COC

passivation induced a contact-angle increase to 105 , indicating that during the initial deposition of nonpolar organic molecules, the non-polar COC surface significantly decreased the structural mismatch at the interface. The capacitance and leakage current density of the pristine and COC-passivated HfOx layers were measured using a metaleinsulatoresemiconductor (MIS) configuration shown in the inset of Fig. 2(b). The typical plots of the capacitance versus frequency (Cef) and the leakage current density versus voltage (JeV) are shown in Fig. 2(b) and (c). Under the same bias conditions (4 V) the capacitance of the pristine HfOX layer (10 nm) was measured to be 756 nF/cm2 at 100 kHz, whereas that for the COC-passivated HfOx layer (25 nm) decreased to 111 nF/cm2 because of the low permittivity and increased thickness of the COC layer. The effective dielectric constant k for the Hf-based dielectrics could be derived using a model of two or three capacitors in a series because of the unavoidable passivation of the native SiO2 layer on the Si substrates in the air: 1/Ci ¼ 1/CHfOx þ 1/CSiO2 or 1/Ci ¼ 1/CHfOx þ 1/CCOC þ 1/ CSiO2. Here, CSiO2 and kSiO2 for ~3-nm thick SiO2 layer were found to be 1150 nF/cm2 and 3.9, respectively [21]. The kHfOx and kCOC can be determined by the following equation

Ci ¼ ε0

k d

where ε0 is the vacuum permittivity and d the thickness of the dielectric layer, the kHfOx and kCOC were determined to be 25 and 2.2, respectively. Moreover, the values of leakage current density for the pristine and COC-passivated HfOx dielectrics were very low, ~5  107 and ~9  108 A/cm2, respectively. The observed good dielectric properties and low level of leakage currents of the Hfbased layers were expected to enable OFETs and inverters to operate at low voltage levels. 3.2. Effects of COC-passivated on the performance of OFETs A comparative analysis of device performance was performed to investigate the effect of HfOx dielectrics. In this study, all the thinfilm OFETs grown on the pristine and COC-passivated HfOx dielectrics displayed reproducible characteristics in the low-voltage regime of
J.-D. Oh et al. / Organic Electronics 30 (2016) 131e135

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Fig. 2. (a) Comparison of the contact angles with water for the pristine (left) and COC-passivated (right) HfOx layers. Comparative plots of (b) the capacitance versus frequency (Cef) and (c) the leakage current density versus voltage (JeV). The inset represents a schematic of the MIS sandwich structure.

-3

-2

-1

-3

10

-7

1.2x10 8.0x10

-4

10

-8

4.0x10

-4

10

-9

0.0

10

-10

1/2

1/2

10

-6

-3

(c)

3

VGS (V)

4

8.0x10

-4

4.0x10

-4

-5

-4

5

6

-3

-2

-1

0

-6

10

-7

10

-8

10

-9

10

-10

10

-11

VGS (V)

(b)

1.6x10

2

-3

-6

-3

1

1.2x10

0.0

2.0x10

0

-3

0

VGS (V)

(a)

1.6x10

10

IDS (A)

-10

1/2

10

(A )

10

-9

1/2

10

-8

-3

pristine HfOX

10

Mobility (cm /Vs)

-4

(A )

10

-7

-3

0.0

(IDS)

10

-6

2.0x10

2

1.0x10

-3

-5

IDS (A)

(IDS)

1/2

2.0x10

10

IDS (A)

-3

1/2

(A )

3.0x10

electron mobilities (me,avg eff ) in a saturation regime were derived from the linear fits of (IDS)1/2 versus VGS with the standard deviation (s). Table 1 summarizes the device parameters including Ion/Ioff, average threshold voltages, the subthreshold swing [SS ¼ VGS/log(IDS)]. In the case of the OFETs with symmetric Au source and drain electrodes compared to the OFETs using the pristine HfOx

(IDS)

described by the standard field-effect transistor equations working in the accumulation mode. The typical transfer characteristics (drain current versus gate voltage and IDS versus VGS) curves of ptype pentacene and n-type PTCDI-C13 OFETs using Au or LiF/Al metal electrodes are shown in Fig. 3. For the transfer characteristics of more than 10 OFETs, the average field-effect hole (mh,avg eff ) and

0

10

-1

10

-2

10

-3

COC-passivated HfOX

0

(d)

5

10

15

20

Number of measurements (times)

Fig. 3. Transfer characteristics of p-type pentacene on (a) pristine and (b) COC-passivated HfOx dielectrics with Au source-drain electrodes. (c) Transfer characteristics of n-type PTCDI-C13 on COC-passivated HfOx dielectrics with LiF/Al electrodes. (d) Electron mobilities of n-type PTCDI-C13 OFETs on the pristine and COC-passivated HfOx dielectrics monitored as a function of the number of measurements.

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Table 1 Various device parameters deduced from the OFETs. Active layers

Dielectricsa

pentacene

Pristine HfOx COC-passivated HfOx Pristine HfOx COC-passivated HfOx COC-passivated HfOx

PTCDI-C13

Au Au Au Au LiF/Al

2 m,avg eff (cm /Vs)

0.36 0.47 0.016 0.19 0.27

± ± ± ± ±

0.06 0.04 0.003 0.01 0.01

Ion/Ioff 4 6 4 3 4

    

104 104 103 103 103

VT (V)

VTO (V)

1.1 2.3 1.6 2.6 2.3

0.6 1.2 0.7 1.3 1.0

SS (V/dec)

Ntrap (  1012/cm2)

0.37 0.45 0.28 0.42 0.31

8.0 0.8 4.3 0.9 0.9

The capacitance values pristine and COC-passivated HfOx dielectrics were measured to be 756 and 111 nF/cm2, respectively.

Ntrap

C jV  VTO j ¼ i T q

where Ci is the gate dielectric capacitance per unit area, VT is the threshold voltage, VTO is the turn-on voltage, and q is the elementary charge [26]. As listed in Table 1, the Ntrap values drastically decreased after the COC-passivation. Theoretically, the Ntrap value can represent the extent of structural disorders and/or defects in the thin films and show strong correlation with the transistor performance [27,28]. Such drastic reduction clearly represents that the hydroxyl-free COC passivation induces the growth of organic active layers with fewer traps, particularly in the n-type PTCDI-C13 films. The observed small hysteresis was also found to be closely related to both the trapping of charge carriers at the interface between organic semiconductors and dielectrics, and polarization of the gate dielectrics [29,30]. The aforementioned Ntrap reduction at the COC-passivated interfaces indicates that a considerable fraction of the trap sites was removed, because of the surface modification with hydroxyl-free COC, thus decreasing the gap between the forward and backward sweeps, as shown in Fig. 3. Another desirable passivation effect is the substantial improvement in the operational stability of the PTCDI-C13-based OFETs. In general, most organic-based n-type transistors are known to lack the stability in air when exposed to oxygen and moisture. To investigate the reproducibility of electrical characteristics, the me,avg values of PTCDI-C13 transistors using the pristine eff and COC-passivated HfOx dielectrics were investigated as a function of the number of measurements. As shown in Fig. 3(d), compared to the OFETs using the pristine HfOx dielectrics, the me,avg of the COCeff passivated transistors was well sustained with minor degradation. The improved operational stability resulted from the COC layer preventing the penetration of oxygen and moisture responsible for the charge trapping at the semiconductor/dielectric interfaces.

Such enhancement suggests that the passivation scheme can be suitable for the production of high-performance devices that include air-sensitive components under ambient conditions.

3.3. Voltage transfer characteristics of CMOS inverters The CMOS inverters comprising the optimized p- and n-type OFETs with the asymmetric electrodes were produced and characterized. In Fig. 4 the VTCs at the supply voltages (VDD) of 4, 5, and 6 V show high switching speeds with sharp inversions. The key parameters including the voltage inversion gain (G, defined as dVOUT/dVIN), switching voltage (VM, defined as VIN ¼ VOUT), output max min voltage swing (OVS, defined as VOUT  VOUT ), and noise margin for low and high levels (NML and NMH) were extracted from the VTCs. To examine the coupling of the p-type transistors to the n-type transistors during the operation, the VM values were calculated. In theory, VM is represented by the following equation:

VM

qffiffiffiffiffiffiffiffiffiffiffiffiffi    bn bp VTn þ VDD  VTp qffiffiffiffiffiffiffiffiffiffiffiffiffi ¼  1 þ bn bp

where VTp and VTn are the threshold voltages for p- and n-type transistors, respectively, and b is the transconductance parameter defined as b ¼ mCiW/L [31]. The calculated VM was 3.0 V at a VDD of 6 V and agreed well with the experimental values (3.2 V), indicating that a good balance between p- and n-type OFETs exists in our topcontact configuration. In addition, the wide NML (2.5 V, 83% of VDD/

6

12 9 6 3 0

5

Gain

dielectrics, the COC-passivation clearly improved device performance: higher mobilities and smaller hysteresis. In particular, when low work-function LiF/Al electrodes were adopted to induce more efficient electron injection for the n-type OFETs, very high me,avg value of 0.27 cm2/V was obtained with very slight hysteresis, eff as shown in Fig. 3(c). The observed me,avg eff values was among the best for the low-voltage n-type PTCDI-C13 OFETs reported so far [22e25]. The excellent device parameters were attributed to the combined effects of the acquisition of high-quality films and COC passivation. The neutral cluster beams used in this study enhanced the growth of pin-hole free, closely packed grain crystallites on the room-temperature HfOx dielectrics without any annealing process. The deposition of the crystalline films on the pristine and COCpassivated HfOx dielectrics induces efficient majority-carrier conduction with high mobilities through face-to-face pep stacks of organic active layers. The effect of the COC passivation of the gate dielectric surfaces was also clearly reflected in the trap density (Ntrap) at the interfaces. The Ntrap values could be extracted by the following equation:

4

VOUT

a

Electrodes

0 1 2 3 4 5 6 VIN

3 2

VDD= 6 V VDD= 5 V

1 0 (a)

VDD= 4 V

0

1

2

3

4

5

6

VIN

Fig. 4. The voltage-transfer characteristics of the CMOS inverter at the supply voltages (VDD) of 4, 5, and 6 V. The inset shows the corresponding gain curves.

J.-D. Oh et al. / Organic Electronics 30 (2016) 131e135

2) and NMH (1.6 V, 53% of VDD/2) derived using the maximum equal criteria were because of the closeness of VM of VDD/2, and the high G (11.5) and OVS (5.5 V), as demonstrated by good symmetry of the VTCs [32]. 4. Conclusion In summary, we demonstrated the fabrication and characterization of low-voltage pentacene and PTCDI-C13-based OFETs and inverters using the solution-processed HfOx dielectrics and metal electrodes. The COC-passivated OFETs and inverters showed better performance under ambient conditions. We believe that the HfOxbased transistors and inverters are promising potential candidates for the construction of high-performance, organic thin film-based ICs. Acknowledgments This study was supported by the grant from the National Research Foundation (NRF) of Korea funded by the Ministry of Science, ICT and Future Planning (NRF2014R1A2A2A01005719) and the Basic Science Research Program through the NRF funded by the Ministry of Education (NRF20100020209). References [1] D. Braga, G. Horowitz, Adv. Mater 21 (2009) 1473e1486. [2] M.A. McCarthy, B. Liu, E.P. Donoghue, I. Kravchenko, D.Y. Kim, F. So, A.G. Rinzler, Science 332 (2011) 570e573. [3] R. Rotzoll, S. Mohapatra, V. Olariu, R. Wenz, M. Grigas, K. Dimmler, O. Shchekin, A. Dodabalapur, Appl. Phys. Lett. 88 (2006) 123502. [4] Y.L. Guo, G. Yu, Y.Q. Liu, Adv. Mater 22 (2010) 4427e4447. [5] P. Andersson, D. Nilsson, P.-O. Svensson, M. Chen, A. Malmstrom, T. Remonen, T. Kugler, M. Berggren, Adv. Mater 14 (2002) 1460e1464. [6] R.P. Ortiz, A. Facchetti, T.J. Marks, Chem. Rev. 110 (2010) 205e239. [7] X.-H. Zhang, B. Domercq, X. Wang, S. Yoo, T. Kondo, Z.L. Wang, B. Kippelen,

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