- Email: [email protected]
Bias-stress effects in diF-TES-ADT field-effect transistors
Solid State Electronics 153 (2019) 23–26
Contents lists available at ScienceDirect
Solid State Electronics journal homepage: www.elsevier.com/locate/sse
Bias-stress effects in diF-TES-ADT field-effect transistors
T
Chang-Hyun Kim Department of Electronic Engineering, Gachon University, Seongnam 13120, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
The review of this paper was arranged by Prof. S. Cristoloveanu
A systematic analysis of the bias-stress effects in solution-processed organic field-effect transistors is reported. Difluoro 5,11-bis(triethylsilylethynyl) anthradithiophene, a high-performance molecular semiconductor, forms a charge-transport channel and is coupled with injection contacts made of Au, Ag, or Cu. The electrode metal is found to not only greatly affect the switching performances but also drive the response of transistors to the extended applications of gate voltage. The observations are put into the framework of contact-limited transistor model, which holistically assesses the material, geometry, and stress-related contributions.
Keywords: Organic field-effect transistors DiF-TES-ADT Bias stress Contact resistance Device physics
1. Introduction The contact resistance (R c ) has long been a theoretically justifiable characteristic of organic field-effect transistors (OFETs) [1], while it has received renewed attention in light of its profound influence on mobility ( μ ) extraction [2]. At a fundamental level, R c exists because of the energy-level misalignment at a metal/organic junction as well as a low doping concentration that significantly extends the charge depletion to the core of a semiconducting channel [3–6]. In addition, the specific device geometry has major implications, as the current crowding and injection length is critical to R c in staggered devices, while the barrierinduced carrier re-distribution rather plays a dominant role in coplanar OFETs [7–9]. Now, there is a growing consensus that R c , which is therefore widely present and is especially critical in high- μ transistors, should be fully taken into account in analysis routines, to avoid possible misconception of actual performances [10–12]. Therefore, it is timely to revisit traditionally important phenomena of OFETs in the new parameterization context. We especially note the lack of such understanding for the bias-stress effect [13], which dictates how stably a device operates under repeated use, thus having a clear technological impact. This effect has been widely explored in organic transistors, but its origins and manifestations were rarely associated with contact properties [14]. Here, we report on the first direct correlation between the bias-stress and contact effects in OFETs based on difluoro 5,11-bis(triethylsilylethynyl) anthradithiophene (diF-TESADT), one of the most extensively adopted organic semiconductors for high-performance transistors (chemical structure in Fig. 1a) [15]. The study also builds upon our previous demonstration of chemically treated low-cost contacts for diF-TES-ADT devices [16]. By diversifying
the critical interfaces and introducing a holistic modeling approach, new insights into the OFET electrical stresses are proposed, and our results provide a useful guideline for the understanding and engineering of the bias-stress effects in devices with new materials. 2. Experimental methods OFETs with the structure in Fig. 1c were fabricated as follows. Heavily doped p-type Si wafers were used as a substrate and a gate electrode, with 300-nm-thick SiO2 serving as a gate dielectric. The wafers were cleaned with acetone/isopropanol rinsing and nitrogen blow dry. The source/drain electrodes were formed by vacuum-evaporating a 5-nm-thick Cr adhesion layer and 35-nm-thick contact metals of Au, Ag, or Cu through a shadow mask. To promote the semiconductor crystallinity and enhance the charge injection, selfassembled monolayers (SAMs) of pentafluorobenzenethiol (PFBT) were grown, by immersing the substrates into a 10 mM PFBT solution in isopropanol for 20 min. The samples were then rinsed with pure isopropanol and dried with nitrogen. A diF-TES-ADT solution was prepared at a concentration of 15 mg/mL in 1,2,3,4-tetrahydronaphthalene. The organic channels were created by spinning this solution onto the substrates at 1000 rpm for 30 s, followed by annealing on a hot plate at 100 °C for 30 min. The completed OFET devices were electrically characterized in the dark and at room temperature, using a Keithley model 4200 semiconductor parameter analyzer with embedded pre-amplifiers. Note that OFETs with SAM-treated electrodes are exclusively examined here, because our previous study confirmed that those without SAMs do not exhibit technologically relevant performances [16]. We
E-mail address: [email protected]. https://doi.org/10.1016/j.sse.2018.12.014 Received 28 September 2018; Received in revised form 12 December 2018; Accepted 13 December 2018 Available online 14 December 2018 0038-1101/ © 2018 Elsevier Ltd. All rights reserved.
Solid State Electronics 153 (2019) 23–26
C.-H. Kim
Fig. 1. (a) Chemical structure of diF-TES-ADT. (b) Photograph of a semiconductor ‘ink’ that underlines the versatile processability. (c) OFET device structure. The electrical circuit is drawn together to visualize the measurement conventions. All devices had W = 5000 μm and L = 250 μm.
Fig. 2. (a) Time-varying ID at constant on-state bias conditions. The symbols correspond to the experimental data (with the colors assigned to different metals) and the solid lines are fits to the stretched exponential decay function. (b) The decay parameters for OFETs with the three contact materials.
also refer the readers to the same reference to see the significant difference in microstructure and crystallinity of diF-TES-ADT films grown on different metals, which will help interpret the following results.
trap density Nt was extracted from the subthreshold swing of the un9.3 × stressed devices [18]. The was Nt 1011 cm−2 eV−1, 8.2 × 1011 cm−2 eV−1, and 1.3 ×1012 cm−2 eV−1 for the Au-, Ag-, and Cu-contact OFETs, respectively. Note that these values well correlate with the trend in τ , confirming a substantial influence of initial trap densities on stress behaviors. It is worth mentioning that SiO2 generally increases surface trapping due to its hydroxyl groups [19]. Nonetheless, Nt of our devices compares favorably with that of other small-molecule OFETs [18,20], and it is likely to be a benefit of hydrophobic fluorinated semiconductor. Recent studies meanwhile suggest that the stress endurance of diF-TESADT devices can be further enhanced by employing an inert dielectric [21].
3. Results and discussion 3.1. Gate-induced current decay To investigate the direction and degree of change in drain current (ID ) upon prolonged driving, the time-varying ID of all devices were recorded at a gate voltage (VG ) of −40 V and a drain voltage (VD ) of −5 V. As shown in the normalized graph of Fig. 2a, the OFETs commonly experienced a monotonous decrease in the magnitude of their ID . However, the OFETs with Au or Ag electrodes exhibited relatively stable currents while that with Cu contacts showed a rapid decrease. Therefore, Fig. 2a confirms a substantial influence of contact metal on the bias-stability of diF-TES-ADT transistors, adding to its reported role in manipulating charge-injection and transport properties [16]. For more quantitative understanding, the measured ID versus time (t) curves were fitted to the stretched exponential decay function
t β ID (t ) = ID0exp ⎡−⎛ ⎞ ⎤, ⎢ ⎝τ ⎠ ⎥ ⎣ ⎦
3.2. Model-based approach In many cases, the threshold voltage (VT ) shift concept may suffice to illustrate the stress instabilities of OFETs [22]. However, we hypothesized that other relevant device parameters, including R c , may experience changes during stress. Therefore, the overall effect is broken down into the variation of relevant parameters in the linear-regime transistor model
(1)
ID =
where ID0 is the initial drain current, τ is the time constant that reflects the trap diffusion/retention kinetics, and β is the stretch parameter associated with the energetic width of trap distribution [17]. The optimized mathematical fit for each device is overlaid with the corresponding experimental data in Fig. 2a, showing a good overall agreement. The extracted τ and β are compared among different OFETs in Fig. 2b. Here, the Cu-contact device was found to be strongly affected by the extremely small τ in accelerating the ID decay despite a large β that in principle relaxes the decay speed. The Au- and Ag-contact OFETs’ slow decay can then be equally understood as a result of a large τ although their exponentials were less stretched than that of the Cucounterpart. In the trapping scenario, this indicates that the dynamic nature of traps have a stronger effect on our devices than their energetic distribution. As an independent gauge of charge traps, the total deep
VD Rc −
1 W μC (VG − VT ) L
, (2)
where W is the channel width, L is the channel length, and C is the gate capacitance per unit area [9,23]. Global fitting was performed to each transfer curve to verify the validity of this model and to access μ, R c , and VT self-consistently. Here, it is important to note that while a general OFET theory may include VG -dependent μ and R c [24–26], we regarded these two as constant based on the observation of minimal VG -dependence in these OFETs [16]. As shown in Fig. 3a, the model elucidates the overall curve shape for OFETs with any contact, especially the nonlinearities that evidence R c contributions [27]. More importantly, the extracted parameters reveal that it is not VT alone that accounts for the stress response. As depicted in Fig. 3b, VT indeed is a dramatically affected parameter, while the 24
Solid State Electronics 153 (2019) 23–26
C.-H. Kim
Fig. 3. (a) Linear-scale transfer curves of the diF-TES-ADT OFETs with Au (left), Ag (middle), and Cu (right) source/drain contacts. The single legend on the righthand side of the graphs applies to all three panels. (b) The bias-stress effects as changes in model parameters. The voltage-dependence trends of (c) R ch and (d) R c / Rtot for a Au-contact device.
the Au-contact OFET in Fig. 3c, R ch decreases with increasing VG by normal transconduction, and the stress is manifested in a significant upward shift of the curve. In Fig. 3d, the calculated ratio R c /Rtot , a critical indicator of OFET mechanism [2], shows that the stressed devices are actually less contact-limited (and counter-intuitively more ideal in terms of channel-contact interplay). Although with different exact values for the resistive components, the same conclusions were drawn from the Ag- and Cu-contact transistors. It is worth clarifying that even if μ and R c are VG -independent in our model, R ch and R c /Rtot in turn are VG -dependent because of the explicit gate control in (4).
other parameters underwent non-negligible changes due to the stress, to different extents. The physical interpretation can be summarized as follows. (i) The VT in all OFETs shifted to the negative direction, meaning the existence of positively charged traps (with respect to their initial state) after the devices were stressed. Those traps can have either a bulk or interface origin [28]. While it is subtle to distinguish between the Au- and Ag-contact devices, a remarkable VT shift in the Cu-contact OFET is in line with the less crystallinity (and therefore more traps) developed from this metal [16]. (ii) The ‘contact-corrected’ μ [24] near and above 0.1 cm2 V−1 s−1 is comparable to that of state-of-the-art devices [29], and reaffirms the excellent charge-transport property of diF-TES-ADT. The stress induced a decrease in μ , for which trappedcharge-induced scattering might be responsible. (iii) It is interesting to realize that R c slightly decreases in all cases despite the decrease in μ due to the stress. According to our former investigation [25,30], R c in coplanar OFETs would rather increase when μ drops, because of reduced local conductivity of injection-limited regions. Therefore, the improvement in injection, or the decrease in R c , in Fig. 3b is most likely to support the stress-induced barrier-height reduction. Having validated the model, we can further physical analyses. A major assumption behind (2) is the definition of the total (source-todrain) resistance
Rtot = R c + R ch ,
3.3. Recovery characteristics After measuring the electrical properties at a stressed state of OFETs, these devices were left for natural relaxation. As shown in Fig. 4, it was found that the same stress and relaxation conditions led to the different degrees of recovery for the transistors with different contact metals. It is clear that the performance of the Au- or Ag-contact devices seemed to be mostly recovered, while at the same time a small reduction in their on-state ID is identifiable. Interestingly, a large VT shift in the Cu-contact OFETs (Fig. 3b) was completely reversed, but a substantially decreased on-off current ratio was observed. This simple comparison addresses another significant effect of source/drain contacts on the coupled stress/recovery cycle, which is of practical importance in consumer electronics. However, further studies would be desirable to fully conceptualize the reversibility of the stress effects and to pinpoint dedicated methods to compensate for the corresponding instabilities in diF-TES-ADT devices.
(3)
with R ch being the channel resistance that reads
R ch = −
W μC (VG − VT ), L
(4)
where the first minus sign is due to the sign conventions for p-type transistors (VG − VT , VD , and ID all negative in on-state). As traced for 25
Solid State Electronics 153 (2019) 23–26
C.-H. Kim
[9] [10] [11] [12]
[13]
[14]
[15]
[16]
[17]
Fig. 4. Measured semi-log scale transfer curves for the OFETs showing differences in their stress response and recovery. The axis labels and the legend apply to all three panels.
[18]
[19]
4. Conclusion [20]
The bias-stress phenomena in solution-processed diF-TES-ADT OFETs have been investigated. An important implication is the confirmed strong contact effects on both the initial and stressed behaviors, of which the assessment was made possible by matching the semiconductor with metals with different work functions and templating strengths. Reflecting the multifaceted physics of OFETs, a model-based approach was employed, which proved effective in decoupling stressinduced outcomes. Adding to VT shift, μ and R c were found to exhibit measurable changes when stressed. Finally, the different degrees of stress response were related to the dissimilar recovery characteristics. By providing deeper understanding of a critical issue in the OFET technology, our results will serve to raise the applicability of highperformance diF-TES-ADT devices.
[21] [22]
[23] [24]
[25]
[26] [27]
Acknowledgments [28]
This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education (NRF-2015R1D1A4A01018560).
[29]
[30]
References [1] Horowitz G. Organic thin film transistors: from theory to real devices. J Mater Res 2004;19:1946–62. [2] Bittle EG, Basham JI, Jackson TN, Jurchescu OD, Gundlach DJ. Mobility overestimation due to gated contacts in organic field-effect transistors. Nat Commun 2016;7:10908. [3] Koch N. Organic electronic devices and their functional interfaces. ChemPhysChem 2007;8:1438–55. [4] Hwang J, Wan A, Kahn A. Energetics of metal-organic interfaces: new experiments and assessment of the field. Mater Sci Eng R-Rep 2009;64:1–31. [5] Natali D, Caironi M. Charge injection in solution-processed organic field-effect transistors: physics, models and characterization methods. Adv Mater 2012;24:1357–87. [6] Kim C-H, Bonnassieux Y, Horowitz G. Compact DC modeling of organic field-effect transistors: review and perspectives. IEEE Trans Electron Devices 2014;61:278–87. [7] Vinciguerra V, Rosa ML, Nicolosi D, Sicurella G, Occhipinti L. Modeling the gate bias dependence of contact resistance in staggered polycrystalline organic thin film transistors. Org Electron 2009;10:1074–81. [8] Ante F, Kälblein D, Zaki T, Zschieschang U, Takimiya K, Ikeda M, et al. Contact
resistance and megahertz operation of aggressively scaled organic transistors. Small 2012;8:73–9. Kim CH, Bonnassieux Y, Horowitz G. Fundamental benefits of the staggered geometry for organic field-effect transistors. IEEE Electon Device Lett 2011;32:1302–4. McCulloch I, Salleo A, Chabinyc M. Avoid the kinks when measuring mobility. Science 2016;352:1521–2. Choi HH, Cho K, Frisbie CD, Sirringhaus H, Podzorov V. Critical assessment of charge mobility extraction in FETs. Nat Mater 2017;17:2–7. Paterson AF, Singh S, Fallon KJ, Hodsden T, Han Y, Schroeder BC, et al. Recent progress in high-mobility organic transistors: a reality check. Adv Mater 2018;30:1801079. Gomes HL, Stallinga P, Dinelli F, Murgia M, Biscarini F, de Leeuw DM, et al. Electrical characterization of organic based transistors: stability issues. Polym Adv Technol 2005;16:227–31. Wang SD, Minari T, Miyadera T, Aoyagi Y, Tsukagoshi K. Bias stress instability in pentacene thin film transistors: contact resistance change and channel threshold voltage shift. Appl Phys Lett 2008;92:063305. Niazi MR, Li R, Li EQ, Kirmani AR, Abdelsamie M, Wang Q, et al. Solution-printed organic semiconductor blends exhibiting transport properties on par with single crystals. Nat Commun 2015;6:8598. Kim C-H, Hlaing H, Hong J-A, Kim J-H, Park Y, Payne MM, et al. Decoupling the effects of self-assembled monolayers on gold, silver, and copper organic transistor contacts. Adv Mater Interfaces 2015;2:1400384. Chen Y, Podzorov V. Bias stress effect in “air-gap” organic field-effect transistors. Adv Mater 2012;24:2679–84. Kim CH, Castro-Carranza A, Estrada M, Cerdeira A, Bonnassieux Y, Horowitz G, et al. A compact model for organic field-effect transistors with improved output asymptotic behaviors. IEEE Trans Electron Devices 2013;60:1136–41. Diemer PJ, Lamport ZA, Mei Y, Ward JW, Goetz KP, Li W, et al. Quantitative analysis of the density of trap states at the semiconductor-dielectric interface in organic field-effect transistors. Appl Phys Lett 2015;107:103303. Kalb WL, Batlogg B. Calculating the trap density of states in organic field-effect transistors from experiment: a comparison of different methods. Phys Rev B 2010;81:035327. Jia X, Fuentes-Hernandez C, Wang C-Y, Park Y, Kippelen B. Stable organic thin-film transistors. Sci Adv 2018;4:eaao1705. Mathijssen S, Cölle M, Gomes H, Smits E, de Boer B, McCulloch I, et al. Dynamics of threshold voltage shifts in organic and amorphous silicon field-effect transistors. Adv Mater 2007;19:2785–9. Braga D, Horowitz G. Subthreshold regime in rubrene single-crystal organic transistors. Appl Phys A-Mater Sci Process 2009;95:193–201. Bourguiga R, Mahdouani M, Mansouri S, Horowitz G. Extracting parameters from the current-voltage characteristics of polycrystalline octithiophene thin film fieldeffect transistors. Eur Phys J-Appl Phys 2007;39:7–16. Kim CH, Bonnassieux Y, Horowitz G. Charge distribution and contact resistance model for coplanar organic field-effect transistors. IEEE Trans Electron Devices 2013;60:280–7. Zschieschang U, Weitz RT, Kern K, Klauk H. Bias stress effect in low-voltage organic thin-film transistors. Appl Phys A-Mater Sci Process 2009;95:139–45. Liu C, Li G, Di Pietro R, Huang J, Noh Y-Y, Liu X, et al. Device physics of contact issues for the overestimation and underestimation of carrier mobility in field-effect transistors. Phys Rev Appl 2017;8:034020. Haüsermann R, Batlogg B. Gate bias stress in pentacene field-effect-transistors: charge trapping in the dielectric or semiconductor. Appl Phys Lett 2011;99:083303. Ward JW, Li R, Obaid A, Payne MM, Smilgies D-M, Anthony JE, et al. Rational design of organic semiconductors for texture control and self-patterning on halogenated surfaces. Adv Funct Mater 2014;24:5052–8. Kim C-H, Thomas S, Kim JH, Elliott M, Macdonald JE, Yoon M-H. Potentiometric parameterization of dinaphtho[2,3-b:2’,3’-f]thieno[3,2-b]thiophene field-effect transistors with a varying degree of nonidealities. Adv Electron Mater 2018;4:1700514.
Chang-Hyun Kim is an Assistant Professor of Electronic Engineering at Gachon University. His research focuses on the development of novel electronic devices based on organic semiconductors and hybrid nanoarchitectures. After receiving Ph.D. in physics from the Ecole Polytechnique, France, he worked at Columbia University, the French National Centre for Scientific Research (CNRS), and Gwangju Institute of Science and Technology. Prof. Kim was a recipient of the Alliance Doctoral Mobility Award, the Prix de thèse (Ph.D. thesis award) of the Ecole Polytechnique, the Korean Ministry of Education Research Fellowship, and the IEEE International Symposium on NextGeneration Electronics (ISNE) Best Paper Award.
26
Contents lists available at ScienceDirect
Solid State Electronics journal homepage: www.elsevier.com/locate/sse
Bias-stress effects in diF-TES-ADT field-effect transistors
T
Chang-Hyun Kim Department of Electronic Engineering, Gachon University, Seongnam 13120, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
The review of this paper was arranged by Prof. S. Cristoloveanu
A systematic analysis of the bias-stress effects in solution-processed organic field-effect transistors is reported. Difluoro 5,11-bis(triethylsilylethynyl) anthradithiophene, a high-performance molecular semiconductor, forms a charge-transport channel and is coupled with injection contacts made of Au, Ag, or Cu. The electrode metal is found to not only greatly affect the switching performances but also drive the response of transistors to the extended applications of gate voltage. The observations are put into the framework of contact-limited transistor model, which holistically assesses the material, geometry, and stress-related contributions.
Keywords: Organic field-effect transistors DiF-TES-ADT Bias stress Contact resistance Device physics
1. Introduction The contact resistance (R c ) has long been a theoretically justifiable characteristic of organic field-effect transistors (OFETs) [1], while it has received renewed attention in light of its profound influence on mobility ( μ ) extraction [2]. At a fundamental level, R c exists because of the energy-level misalignment at a metal/organic junction as well as a low doping concentration that significantly extends the charge depletion to the core of a semiconducting channel [3–6]. In addition, the specific device geometry has major implications, as the current crowding and injection length is critical to R c in staggered devices, while the barrierinduced carrier re-distribution rather plays a dominant role in coplanar OFETs [7–9]. Now, there is a growing consensus that R c , which is therefore widely present and is especially critical in high- μ transistors, should be fully taken into account in analysis routines, to avoid possible misconception of actual performances [10–12]. Therefore, it is timely to revisit traditionally important phenomena of OFETs in the new parameterization context. We especially note the lack of such understanding for the bias-stress effect [13], which dictates how stably a device operates under repeated use, thus having a clear technological impact. This effect has been widely explored in organic transistors, but its origins and manifestations were rarely associated with contact properties [14]. Here, we report on the first direct correlation between the bias-stress and contact effects in OFETs based on difluoro 5,11-bis(triethylsilylethynyl) anthradithiophene (diF-TESADT), one of the most extensively adopted organic semiconductors for high-performance transistors (chemical structure in Fig. 1a) [15]. The study also builds upon our previous demonstration of chemically treated low-cost contacts for diF-TES-ADT devices [16]. By diversifying
the critical interfaces and introducing a holistic modeling approach, new insights into the OFET electrical stresses are proposed, and our results provide a useful guideline for the understanding and engineering of the bias-stress effects in devices with new materials. 2. Experimental methods OFETs with the structure in Fig. 1c were fabricated as follows. Heavily doped p-type Si wafers were used as a substrate and a gate electrode, with 300-nm-thick SiO2 serving as a gate dielectric. The wafers were cleaned with acetone/isopropanol rinsing and nitrogen blow dry. The source/drain electrodes were formed by vacuum-evaporating a 5-nm-thick Cr adhesion layer and 35-nm-thick contact metals of Au, Ag, or Cu through a shadow mask. To promote the semiconductor crystallinity and enhance the charge injection, selfassembled monolayers (SAMs) of pentafluorobenzenethiol (PFBT) were grown, by immersing the substrates into a 10 mM PFBT solution in isopropanol for 20 min. The samples were then rinsed with pure isopropanol and dried with nitrogen. A diF-TES-ADT solution was prepared at a concentration of 15 mg/mL in 1,2,3,4-tetrahydronaphthalene. The organic channels were created by spinning this solution onto the substrates at 1000 rpm for 30 s, followed by annealing on a hot plate at 100 °C for 30 min. The completed OFET devices were electrically characterized in the dark and at room temperature, using a Keithley model 4200 semiconductor parameter analyzer with embedded pre-amplifiers. Note that OFETs with SAM-treated electrodes are exclusively examined here, because our previous study confirmed that those without SAMs do not exhibit technologically relevant performances [16]. We
E-mail address: [email protected]. https://doi.org/10.1016/j.sse.2018.12.014 Received 28 September 2018; Received in revised form 12 December 2018; Accepted 13 December 2018 Available online 14 December 2018 0038-1101/ © 2018 Elsevier Ltd. All rights reserved.
Solid State Electronics 153 (2019) 23–26
C.-H. Kim
Fig. 1. (a) Chemical structure of diF-TES-ADT. (b) Photograph of a semiconductor ‘ink’ that underlines the versatile processability. (c) OFET device structure. The electrical circuit is drawn together to visualize the measurement conventions. All devices had W = 5000 μm and L = 250 μm.
Fig. 2. (a) Time-varying ID at constant on-state bias conditions. The symbols correspond to the experimental data (with the colors assigned to different metals) and the solid lines are fits to the stretched exponential decay function. (b) The decay parameters for OFETs with the three contact materials.
also refer the readers to the same reference to see the significant difference in microstructure and crystallinity of diF-TES-ADT films grown on different metals, which will help interpret the following results.
trap density Nt was extracted from the subthreshold swing of the un9.3 × stressed devices [18]. The was Nt 1011 cm−2 eV−1, 8.2 × 1011 cm−2 eV−1, and 1.3 ×1012 cm−2 eV−1 for the Au-, Ag-, and Cu-contact OFETs, respectively. Note that these values well correlate with the trend in τ , confirming a substantial influence of initial trap densities on stress behaviors. It is worth mentioning that SiO2 generally increases surface trapping due to its hydroxyl groups [19]. Nonetheless, Nt of our devices compares favorably with that of other small-molecule OFETs [18,20], and it is likely to be a benefit of hydrophobic fluorinated semiconductor. Recent studies meanwhile suggest that the stress endurance of diF-TESADT devices can be further enhanced by employing an inert dielectric [21].
3. Results and discussion 3.1. Gate-induced current decay To investigate the direction and degree of change in drain current (ID ) upon prolonged driving, the time-varying ID of all devices were recorded at a gate voltage (VG ) of −40 V and a drain voltage (VD ) of −5 V. As shown in the normalized graph of Fig. 2a, the OFETs commonly experienced a monotonous decrease in the magnitude of their ID . However, the OFETs with Au or Ag electrodes exhibited relatively stable currents while that with Cu contacts showed a rapid decrease. Therefore, Fig. 2a confirms a substantial influence of contact metal on the bias-stability of diF-TES-ADT transistors, adding to its reported role in manipulating charge-injection and transport properties [16]. For more quantitative understanding, the measured ID versus time (t) curves were fitted to the stretched exponential decay function
t β ID (t ) = ID0exp ⎡−⎛ ⎞ ⎤, ⎢ ⎝τ ⎠ ⎥ ⎣ ⎦
3.2. Model-based approach In many cases, the threshold voltage (VT ) shift concept may suffice to illustrate the stress instabilities of OFETs [22]. However, we hypothesized that other relevant device parameters, including R c , may experience changes during stress. Therefore, the overall effect is broken down into the variation of relevant parameters in the linear-regime transistor model
(1)
ID =
where ID0 is the initial drain current, τ is the time constant that reflects the trap diffusion/retention kinetics, and β is the stretch parameter associated with the energetic width of trap distribution [17]. The optimized mathematical fit for each device is overlaid with the corresponding experimental data in Fig. 2a, showing a good overall agreement. The extracted τ and β are compared among different OFETs in Fig. 2b. Here, the Cu-contact device was found to be strongly affected by the extremely small τ in accelerating the ID decay despite a large β that in principle relaxes the decay speed. The Au- and Ag-contact OFETs’ slow decay can then be equally understood as a result of a large τ although their exponentials were less stretched than that of the Cucounterpart. In the trapping scenario, this indicates that the dynamic nature of traps have a stronger effect on our devices than their energetic distribution. As an independent gauge of charge traps, the total deep
VD Rc −
1 W μC (VG − VT ) L
, (2)
where W is the channel width, L is the channel length, and C is the gate capacitance per unit area [9,23]. Global fitting was performed to each transfer curve to verify the validity of this model and to access μ, R c , and VT self-consistently. Here, it is important to note that while a general OFET theory may include VG -dependent μ and R c [24–26], we regarded these two as constant based on the observation of minimal VG -dependence in these OFETs [16]. As shown in Fig. 3a, the model elucidates the overall curve shape for OFETs with any contact, especially the nonlinearities that evidence R c contributions [27]. More importantly, the extracted parameters reveal that it is not VT alone that accounts for the stress response. As depicted in Fig. 3b, VT indeed is a dramatically affected parameter, while the 24
Solid State Electronics 153 (2019) 23–26
C.-H. Kim
Fig. 3. (a) Linear-scale transfer curves of the diF-TES-ADT OFETs with Au (left), Ag (middle), and Cu (right) source/drain contacts. The single legend on the righthand side of the graphs applies to all three panels. (b) The bias-stress effects as changes in model parameters. The voltage-dependence trends of (c) R ch and (d) R c / Rtot for a Au-contact device.
the Au-contact OFET in Fig. 3c, R ch decreases with increasing VG by normal transconduction, and the stress is manifested in a significant upward shift of the curve. In Fig. 3d, the calculated ratio R c /Rtot , a critical indicator of OFET mechanism [2], shows that the stressed devices are actually less contact-limited (and counter-intuitively more ideal in terms of channel-contact interplay). Although with different exact values for the resistive components, the same conclusions were drawn from the Ag- and Cu-contact transistors. It is worth clarifying that even if μ and R c are VG -independent in our model, R ch and R c /Rtot in turn are VG -dependent because of the explicit gate control in (4).
other parameters underwent non-negligible changes due to the stress, to different extents. The physical interpretation can be summarized as follows. (i) The VT in all OFETs shifted to the negative direction, meaning the existence of positively charged traps (with respect to their initial state) after the devices were stressed. Those traps can have either a bulk or interface origin [28]. While it is subtle to distinguish between the Au- and Ag-contact devices, a remarkable VT shift in the Cu-contact OFET is in line with the less crystallinity (and therefore more traps) developed from this metal [16]. (ii) The ‘contact-corrected’ μ [24] near and above 0.1 cm2 V−1 s−1 is comparable to that of state-of-the-art devices [29], and reaffirms the excellent charge-transport property of diF-TES-ADT. The stress induced a decrease in μ , for which trappedcharge-induced scattering might be responsible. (iii) It is interesting to realize that R c slightly decreases in all cases despite the decrease in μ due to the stress. According to our former investigation [25,30], R c in coplanar OFETs would rather increase when μ drops, because of reduced local conductivity of injection-limited regions. Therefore, the improvement in injection, or the decrease in R c , in Fig. 3b is most likely to support the stress-induced barrier-height reduction. Having validated the model, we can further physical analyses. A major assumption behind (2) is the definition of the total (source-todrain) resistance
Rtot = R c + R ch ,
3.3. Recovery characteristics After measuring the electrical properties at a stressed state of OFETs, these devices were left for natural relaxation. As shown in Fig. 4, it was found that the same stress and relaxation conditions led to the different degrees of recovery for the transistors with different contact metals. It is clear that the performance of the Au- or Ag-contact devices seemed to be mostly recovered, while at the same time a small reduction in their on-state ID is identifiable. Interestingly, a large VT shift in the Cu-contact OFETs (Fig. 3b) was completely reversed, but a substantially decreased on-off current ratio was observed. This simple comparison addresses another significant effect of source/drain contacts on the coupled stress/recovery cycle, which is of practical importance in consumer electronics. However, further studies would be desirable to fully conceptualize the reversibility of the stress effects and to pinpoint dedicated methods to compensate for the corresponding instabilities in diF-TES-ADT devices.
(3)
with R ch being the channel resistance that reads
R ch = −
W μC (VG − VT ), L
(4)
where the first minus sign is due to the sign conventions for p-type transistors (VG − VT , VD , and ID all negative in on-state). As traced for 25
Solid State Electronics 153 (2019) 23–26
C.-H. Kim
[9] [10] [11] [12]
[13]
[14]
[15]
[16]
[17]
Fig. 4. Measured semi-log scale transfer curves for the OFETs showing differences in their stress response and recovery. The axis labels and the legend apply to all three panels.
[18]
[19]
4. Conclusion [20]
The bias-stress phenomena in solution-processed diF-TES-ADT OFETs have been investigated. An important implication is the confirmed strong contact effects on both the initial and stressed behaviors, of which the assessment was made possible by matching the semiconductor with metals with different work functions and templating strengths. Reflecting the multifaceted physics of OFETs, a model-based approach was employed, which proved effective in decoupling stressinduced outcomes. Adding to VT shift, μ and R c were found to exhibit measurable changes when stressed. Finally, the different degrees of stress response were related to the dissimilar recovery characteristics. By providing deeper understanding of a critical issue in the OFET technology, our results will serve to raise the applicability of highperformance diF-TES-ADT devices.
[21] [22]
[23] [24]
[25]
[26] [27]
Acknowledgments [28]
This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education (NRF-2015R1D1A4A01018560).
[29]
[30]
References [1] Horowitz G. Organic thin film transistors: from theory to real devices. J Mater Res 2004;19:1946–62. [2] Bittle EG, Basham JI, Jackson TN, Jurchescu OD, Gundlach DJ. Mobility overestimation due to gated contacts in organic field-effect transistors. Nat Commun 2016;7:10908. [3] Koch N. Organic electronic devices and their functional interfaces. ChemPhysChem 2007;8:1438–55. [4] Hwang J, Wan A, Kahn A. Energetics of metal-organic interfaces: new experiments and assessment of the field. Mater Sci Eng R-Rep 2009;64:1–31. [5] Natali D, Caironi M. Charge injection in solution-processed organic field-effect transistors: physics, models and characterization methods. Adv Mater 2012;24:1357–87. [6] Kim C-H, Bonnassieux Y, Horowitz G. Compact DC modeling of organic field-effect transistors: review and perspectives. IEEE Trans Electron Devices 2014;61:278–87. [7] Vinciguerra V, Rosa ML, Nicolosi D, Sicurella G, Occhipinti L. Modeling the gate bias dependence of contact resistance in staggered polycrystalline organic thin film transistors. Org Electron 2009;10:1074–81. [8] Ante F, Kälblein D, Zaki T, Zschieschang U, Takimiya K, Ikeda M, et al. Contact
resistance and megahertz operation of aggressively scaled organic transistors. Small 2012;8:73–9. Kim CH, Bonnassieux Y, Horowitz G. Fundamental benefits of the staggered geometry for organic field-effect transistors. IEEE Electon Device Lett 2011;32:1302–4. McCulloch I, Salleo A, Chabinyc M. Avoid the kinks when measuring mobility. Science 2016;352:1521–2. Choi HH, Cho K, Frisbie CD, Sirringhaus H, Podzorov V. Critical assessment of charge mobility extraction in FETs. Nat Mater 2017;17:2–7. Paterson AF, Singh S, Fallon KJ, Hodsden T, Han Y, Schroeder BC, et al. Recent progress in high-mobility organic transistors: a reality check. Adv Mater 2018;30:1801079. Gomes HL, Stallinga P, Dinelli F, Murgia M, Biscarini F, de Leeuw DM, et al. Electrical characterization of organic based transistors: stability issues. Polym Adv Technol 2005;16:227–31. Wang SD, Minari T, Miyadera T, Aoyagi Y, Tsukagoshi K. Bias stress instability in pentacene thin film transistors: contact resistance change and channel threshold voltage shift. Appl Phys Lett 2008;92:063305. Niazi MR, Li R, Li EQ, Kirmani AR, Abdelsamie M, Wang Q, et al. Solution-printed organic semiconductor blends exhibiting transport properties on par with single crystals. Nat Commun 2015;6:8598. Kim C-H, Hlaing H, Hong J-A, Kim J-H, Park Y, Payne MM, et al. Decoupling the effects of self-assembled monolayers on gold, silver, and copper organic transistor contacts. Adv Mater Interfaces 2015;2:1400384. Chen Y, Podzorov V. Bias stress effect in “air-gap” organic field-effect transistors. Adv Mater 2012;24:2679–84. Kim CH, Castro-Carranza A, Estrada M, Cerdeira A, Bonnassieux Y, Horowitz G, et al. A compact model for organic field-effect transistors with improved output asymptotic behaviors. IEEE Trans Electron Devices 2013;60:1136–41. Diemer PJ, Lamport ZA, Mei Y, Ward JW, Goetz KP, Li W, et al. Quantitative analysis of the density of trap states at the semiconductor-dielectric interface in organic field-effect transistors. Appl Phys Lett 2015;107:103303. Kalb WL, Batlogg B. Calculating the trap density of states in organic field-effect transistors from experiment: a comparison of different methods. Phys Rev B 2010;81:035327. Jia X, Fuentes-Hernandez C, Wang C-Y, Park Y, Kippelen B. Stable organic thin-film transistors. Sci Adv 2018;4:eaao1705. Mathijssen S, Cölle M, Gomes H, Smits E, de Boer B, McCulloch I, et al. Dynamics of threshold voltage shifts in organic and amorphous silicon field-effect transistors. Adv Mater 2007;19:2785–9. Braga D, Horowitz G. Subthreshold regime in rubrene single-crystal organic transistors. Appl Phys A-Mater Sci Process 2009;95:193–201. Bourguiga R, Mahdouani M, Mansouri S, Horowitz G. Extracting parameters from the current-voltage characteristics of polycrystalline octithiophene thin film fieldeffect transistors. Eur Phys J-Appl Phys 2007;39:7–16. Kim CH, Bonnassieux Y, Horowitz G. Charge distribution and contact resistance model for coplanar organic field-effect transistors. IEEE Trans Electron Devices 2013;60:280–7. Zschieschang U, Weitz RT, Kern K, Klauk H. Bias stress effect in low-voltage organic thin-film transistors. Appl Phys A-Mater Sci Process 2009;95:139–45. Liu C, Li G, Di Pietro R, Huang J, Noh Y-Y, Liu X, et al. Device physics of contact issues for the overestimation and underestimation of carrier mobility in field-effect transistors. Phys Rev Appl 2017;8:034020. Haüsermann R, Batlogg B. Gate bias stress in pentacene field-effect-transistors: charge trapping in the dielectric or semiconductor. Appl Phys Lett 2011;99:083303. Ward JW, Li R, Obaid A, Payne MM, Smilgies D-M, Anthony JE, et al. Rational design of organic semiconductors for texture control and self-patterning on halogenated surfaces. Adv Funct Mater 2014;24:5052–8. Kim C-H, Thomas S, Kim JH, Elliott M, Macdonald JE, Yoon M-H. Potentiometric parameterization of dinaphtho[2,3-b:2’,3’-f]thieno[3,2-b]thiophene field-effect transistors with a varying degree of nonidealities. Adv Electron Mater 2018;4:1700514.
Chang-Hyun Kim is an Assistant Professor of Electronic Engineering at Gachon University. His research focuses on the development of novel electronic devices based on organic semiconductors and hybrid nanoarchitectures. After receiving Ph.D. in physics from the Ecole Polytechnique, France, he worked at Columbia University, the French National Centre for Scientific Research (CNRS), and Gwangju Institute of Science and Technology. Prof. Kim was a recipient of the Alliance Doctoral Mobility Award, the Prix de thèse (Ph.D. thesis award) of the Ecole Polytechnique, the Korean Ministry of Education Research Fellowship, and the IEEE International Symposium on NextGeneration Electronics (ISNE) Best Paper Award.
26