Nanotube Transistors as Direct Probes of the Trap Dynamics at

NANO LETTERS

Nanotube Transistors as Direct Probes of the Trap Dynamics at Dielectric-Organic Interfaces of Interest in Organic Electronics and Solar Cells

2008 Vol. 8, No. 11 3619-3625

Costin Anghel,†,‡ Vincent Derycke,*,† Arianna Filoramo,† Ste´phane Lenfant,§ Benoit Giffard,‡ Dominique Vuillaume,§ and Jean-Philippe Bourgoin† Laboratoire d’Electronique Mole´culaire, SerVice de Physique de L’Etat Condense´ (CNRS URA 2464), CEA, IRAMIS, F-91191 Gif-sur-YVette, France, CEA, LETI MINATEC, 17 rue des Martyrs, F-38054 Grenoble, France, and Molecular Nanostructures and DeVices Group, Institut d’Electronique Micro-e´lectronique et Nanotechnologie, CNRS, BP 60069, AVenue Poincare´, 59652 VilleneuVe d’Ascq, France Received May 30, 2008; Revised Manuscript Received September 4, 2008

ABSTRACT The high sensitivity of nanotube transistors is used for the first time as a probe to study charge dynamics at a dielectric/polymer (polythiophene) interface, an inorganic/organic junction of particular importance for organic solar cells, and organic field effect transistors (OFETs). A carbon nanotube field effect transistor is coated with a thin film of a photoconductive polymer and photoexcited so as to generate carriers in the structure. Comparison between devices using SiO2 and TiO2 as gate dielectric reveals the critical role of the dielectric and clearly elucidates the relative contributions of the polymer and the dielectric layers on the separation, trapping, and relaxation of photogenerated charges.

Charge trapping-detrapping at the interfaces between organic semiconductors and inorganic materials is of crucial importance in devices like organic field effects transistors,1-4 light emitting field effect devices,4 and organic photovoltaic cells,5 in which they strongly impact both the performances and stability. Though recent experiments showed significant advances in the direct investigation of trapping-detrapping processes and more generally in the electrostatics of hybrid organic-inorganic devices6-9 both remain difficult to probe directly. For instance, a debate still exists about the origin of time and voltage instabilities in organic FETs and the respective roles of water, dielectric, and organic bulk and surface traps under bias stress and/or illumination are still under intense investigation.6-15 Conventional electrical methods, and in particular conductance-voltage (G-V) and capacitance-voltage (C-V) measurements,3,9-17 are powerful tools to study these types of effects, in particular in combination with photoexcitation.9,11,12,14,17 However, they suffer from strong limitations. In particular the most com* Corresponding author, [email protected]. † Laboratoire d’Electronique Mole ´ culaire, Service de Physique de L’Etat Condense´ (CNRS URA 2464), CEA, IRAMIS. ‡ CEA, LETI MINATEC. § Molecular Nanostructures and Devices Group, Institut d’Electronique Micro-e´lectronique et Nanotechnologie, CNRS. 10.1021/nl801543k CCC: $40.75 Published on Web 10/24/2008

 2008 American Chemical Society

monly used C-V measurements require a top-contact on the organic semiconductor, which brings in an additional interface, prevents the use of very thin films, and may induce a degradation of the organic material. Investigating minority carriers trapping requires illumination, which sets some limitations on the type of usable (transparent) contacts. These methods are also relatively slow, making difficult the study of dynamical effects in the millisecond range. Finally, when multiple interfaces and/or multiple trapping/detrapping processes are involved, it proves very difficult to separate the different contributions without the help of modelization, and clear-cut conclusions are thus scarce. With the objective of elucidating the exact role played by the trapping of charges at polymer-dielectric interfaces under illumination, we designed an experiment where a carbon nanotube (CNT) transistor is used to probe the charge dynamics at a polymer-dielectric interface. One-dimensional nanoscale objects such as CNTs and semiconducting nanowires (NWs) are attracting increasing attention as building blocks for new types of electronic devices as they allow high levels of performances associated with additional advantages such as compatibility with a large range of substrates or charge/dipole sensing capabilities for example.18-21 In particular, the very high charge sensitivity of CNT-based field

effect transistors (CNTFETs) is known22 and has been used in relation with charge trapping in SiO2 to demonstrate nonvolatile memory elements.23,24 The optoelectronic properties of nanoscale transistors based on these objects have been recently intensively studied.25,26 Phototransistors composed of a CNT or NW channel functionalized with an organic chromophore layer (either grafted or deposited) are especially attracting as light sensors.27-35 Indeed, they combine the exceptional intrinsic electronic properties of CNTs33 or NWs20,21 and the large diversity of optical properties of organic materials. In a recent contribution, we showed that CNTFETs using SiO2 as gate dielectric and coated with a thin film of photoconductive polymer display remarkable optoelectronic properties, including the possibility to be used as a nonvolatile memory working on the “optical write/electrical erase” basis.30 In that work, the organic material improves the functionality of the CNT device. Conversely, in the present study, the CNTFET is used as a probe with the aim of understanding and improve organic devices. In this Letter, we perform the first detailed dynamical study of trapping/detrapping processes at organic semiconductor/ inorganic dielectric interfaces using CNTFETs as local probe, in combination with photoexcitation. As detailed below, the method has quite significant advantages when compared with conventional techniques. Applied to the thorough comparison of the SiO2/P3OT and TiO2/P3OT interfaces, it allows in particular clarifying the respective (bias dependent) roles and dynamics of electron traps at the SiO2 surface, bulk hole traps within TiO2, and accumulation and trapping of photogenerated electrons within the P3OT layer at the interface with both dielectrics. We choose to focus on SiO2 and TiO2 for several reasons. SiO2 is the most common and most studied dielectric and serves as a reference. The properties of TiO2 are important in the framework of organic FETs,7,37-39 for which its high-k permittivity allows low bias operation. TiO2 is also an important material for photovoltaic devices in which TiO2 thin films or nanoparticles are often included,40-42 in particular in combination with organic hole transporting layers,5,43-46 among which are polythiophene films.47-50 Moreover, TiO2 has intrinsic optoelectronic properties51 in the wavelength range relevant for this study (around 450 nm), which helps differentiating the effects. The nanotube transistors used in this study are fabricated on highly resistive Si wafers covered with 200 nm of thermal SiO2 using three steps of e-beam lithography. First a chromium/gold (5 nm/50 nm thickness) gate and two access electrodes are patterned. A thin layer of titanium (5 nm) is subsequently deposited to cover the gate electrode. The titanium is then thermally oxidized at 550 °C in air for 10 h to form the amorphous TiO2 gate dielectric layer.52 The thickness of the TiO2 layer is ∼10 nm as measured using atomic force microscopy (AFM), in good agreement with the expected value of 9 nm based on the expansion coefficient of Ti during thermal oxidation (which was measured at 1.8 in ref 52). At that stage, single-wall carbon nanotubes (SWNTs) are deposited above the gate stack by dielectrophoresis (DEP) in the same conditions as in ref 53 and 54 3620

with the ac-electric field applied between the access electrodes. After the CNTs deposition, a last lithographic step is used to form the source and drain electrodes (gold 50 nm thick) as extensions of the access electrodes. The final channel length is 750 nm. A schematic of the device structure is shown in Figure 1a. After a first set of optoelectrical characterizations, the devices are covered with a ∼5 nm thick film of commercial (Sigma-Aldrich) poly(3-octylthiophene) (P3OT) from a solution diluted in toluene (0.1% in weight), spin coated at 2000 rpm and dried in air at 80 °C for 10 min. Figure 1b presents the transfer characteristics, in the dark, of a representative device before and after P3OT deposition. We obtain a p-type characteristics typical of hole conduction in multinanotube devices. The drive currents reach several hundreds of microamperes (at VDS ) -400 mV) confirming the large number of CNTs involved, in agreement with the AFM image in Figure 1c. Noticeably, the device has no offstate due to the presence of a significant amount of metallic SWNTs. This is an expected result as the DEP process facilitates the deposition of metallic nanotubes. Nevertheless, we showed previously from the comparison of CNTFETs based on arrays and on a single CNTs that the results are not affected by the presence of the metallic SWNTs.30 The efficient current modulation is directly related to the dielectric permittivity of TiO2, which was estimated at εr_TiO2 ∼ 28 from capacitance measurements. The reduced value of the gate leakage current (also presented in Figure 1b) emphasizes the insulating quality of the TiO2 layer. Comparison of the characteristics with and without P3OT indicates a drastic increase in the device conductivity after deposition of the polymer. This has already been observed for devices fabricated on SiO2 even though the effect is more pronounced here. The difference is probably related to the use of gold contacts in the present work in place of palladium in ref 30. Indeed, gold-contacted semiconducting nanotubes produce rather high Schottky barriers which limit the injection of holes, while palladium leads to more ohmic contacts.55,56 The sensitivity of carrier injection at the nanotube-gold interface is a largely documented topic.57-60 It is known that chemical treatments can easily alter (in our case decrease) the Schottky barrier height and thus strongly impact performances. An additional p-type doping effect of the nanotubes,30 associated with this improvement of hole injection at the source contact globally improves both the current level and the transconductance. Note that the current flowing directly through the P3OT film is negligible and does not contribute at all to the observed increase in current.30 Before considering the optoelectrical response of the device, we checked the material’s optical properties. Figure 1d displays the absorption spectrum of a 10 nm TiO2 layer on quartz before and after deposition of the P3OT. Noticeably, at λ ) 457 nm, both the TiO2 and P3OT layers display approximately the same absorbance. This choice allows comparing the respective contributions of the two materials in the following experiments. Figure 2 presents the transitory response to light excitation of the same device as in Figure 1. A blue laser at λ ) 457 Nano Lett., Vol. 8, No. 11, 2008

Figure 1. (a) Schematic representation of a CNT transistor with TiO2 as gate dielectric. (b) Transfer characteristics and gate leakage current of a representative device (VDS ) -400 mV). (c) Atomic force microscope image showing the density of nanotubes in the 750 nm long channel. The channel width is 10 µm. (d) Absorption spectra of quartz samples covered with P3OT, TiO2, and both.

nm with an optical power of ∼70 W/cm2 is used. Note that we checked that this power level is well below the onset of oxygen photodesorption,61,62 a known effect that can affect uncoated CNTFETs upon illumination. We also verified, using a calibrated test resistor of comparable geometry, that light does not induce local heating of the electrodes (as also confirmed by the absence of effect for uncoated devices). We perform a comparison for three different gate biases, both before and after P3OT deposition. We first focus on the device without P3OT (Figure 2 a-c). At light turn-on, the current through the nanotubes sharply decreases regardless of the gate bias. The incident light produces electron-hole pairs in the TiO2. As proposed by Dittrich et al.,63 the photogenerated electrons have a significant mobility in TiO2 and can thus flow through the dielectric layer, as confirmed by an increase of the gate leakage current during illumination (not shown). Conversely, the photogenerated holes are easily trapped in the TiO2.63 The current decrease in the CNT transistor is thus explained by the influence of these trapped holes that apply a positive potential to the transistor channel, shifting its characteristics toward the off-state. At light turnoff and negative gate bias (Figure 2a), the uncoated device presents a very slow current relaxation. It can be explained by the progressive detrapping of holes in the TiO2 by electrons originating from the gate electrode. At positive gate bias, the detrapping of holes is much slower as electrons now originate from the nanotubes which do not cover the whole surface and present a much lower density of electrons (in particular, semiconducting nanotubes are depleted from electrons even at VGS ) 1 V as evidenced by the p-type transfer characteristic in Figure 1b). Note that this slow and Nano Lett., Vol. 8, No. 11, 2008

bias-dependent relaxation of the conductivity following a light pulse is comparable to the situation observed in uncoated crystalline silicon NWs on SiO2 except that in that case64 electron trapping in SiO2 replaces hole trapping in TiO2. Such an effect may well contribute to the results obtained in some of the coated Si-NW devices.34 We now turn to the P3OT-coated device. At negative gate bias (Figure 2d) the optoelectric response is qualitatively similar to the one of the uncoated devices. At zero and positive gate bias though (Figure 2e,f), the situation is very different. During the first ∼100 ms after light turn-on, the current abruptly drops. It then starts to increase and stabilizes at a level which depends on VGS (at VGS ) 0 V, the current level after 10 s of illumination is below the dark current while it is above at VGS ) 1 V). When the light is turned off, the current abruptly increases with a dynamics similar to the one of the initial fast drop ( 0, an electron accumulation layer is formed at the interface.

Figure 3. Comparison between the transient response of (a) the SiO2/P3OT system at VGS ) 3 V and (b) the TiO2/P3OT system at VGS ) 1 V. VDS ) -400 mV in both cases.

The situation on SiO2 is different for two reasons: SiO2 is transparent at λ ) 457 nm and the trap states of interest are electron acceptors. As proposed in our previous study, step (1) corresponds to the photogeneration of electron-hole pairs in the P3OT film, the separation of which is favored by the presence of a high density of electron trap states at the P3OT/ SiO2 interface. An important point here is that, while our explanation for step (1) is similar to our previous proposition, its differentiation from step (2) can only be deduced from the comparison with the TiO2 case. Once the SiO2 interface states are charged with electrons, they remained in that state even after light has been turned off, resulting in the nonvolatile memory state (4). This is confirmed by the value of the current in this state which is directly related to the height of step (1) (see dotted line in Figure 3a and Figure 7b in ref 31). Finally, we focus our attention on step (7), which is the most interesting case. At VGS > 0 and just before light is turned off, the situation of the device on TiO2 is as follows: the TiO2 traps are charged with holes, and the P3OT film is charged with electrons close to the interface. At light turnoff, the holes trapped in TiO2 recombine very fast with a fraction of these accumulated electrons leading to the abrupt increase in current (step (7)). The rise in current indicates that electrons at the interface are in large excess when compared with trapped holes in the dielectric. Following this rapid process, the remaining trapped electrons in the polymer decay slowly allowing the current to reach its initial (dark) value with absence of memory effect. At VGS < 0 the absence of an electron accumulation layer prevents the efficient detrapping of the holes in TiO2 (Figure 2d). A scheme of the band bending situation and processes involved is presented in Figure 4. Band alignment at organic/inorganic Nano Lett., Vol. 8, No. 11, 2008

interfaces is of critical importance for the performances of organic photovoltaic cells. The conjugated polymer/TiO2 system is studied in this context with the usual addition of a dye coating on the TiO2. The choice of the different materials forming this type of heterostructure is usually based on the electron affinities of the constituents and is supposed to favor the efficient transfer of electrons toward the TiO2 and of holes toward the polymer. In practice, barriers for transport are often formed at the interfaces.66-70 The presence of a barrier for holes at the P3OT/TiO2 interface is known and allows the fabrication of p-type P3HT OFETs using TiO2 as gate dielectric.7,37 A nanotube transistor directly embedded at the TiO2/P3OT interface provides direct evidence of the presence of a barrier for electrons and allows its impact on the device operating mode to be studied as a function of illumination and biasing conditions. In conclusion, we studied the optoelectronic properties of carbon nanotube field effect transistors coated with a thin film of P3OT using either SiO2 or TiO2 as gate dielectrics. Comparison of the respective photoinduced effects highlights the critical role of charge trapping in the gate dielectric. From their different dynamics, we clearly separated the respective contributions of hole trapping in TiO2, electron trapping at the SiO2 surface and electron accumulation in the polymer. Our measurements revealed the presence of significant offsets in the alignment of the TiO2 and P3OT bands resulting noticeably in a problematic barrier for efficient injection of electrons from the polymer to the TiO2. These results may have some consequence in the design of future stacks in OFETs or OPV devices. They were allowed by the use of carbon nanotubes as probes, a technique that presents many advantages. First, the high carrier mobility in CNTs allows amplifying the studied effect. In the presented study, the electrical characteristics of the P3OT thin film could not have been probed directly under illumination due to the dominant effect of the TiO2 photoconductivity but are clearly revealed through their impact on the CNTFET. CNTFETs are compatible with most substrates including organic ones, and their nanometer-size diameter is perfectly well suited to probe very thin films and interfaces. Compared with conventional C-V methods, it does not require a top contact, which makes easier the use of photoexcitation and very thin films. As the transport is probed through the nanotube, control experiments without the organic layers are possible. As shown here and in ref 8, the ability to study the dielectric alone is very useful. Finally, CNTFETs are known for their gigahertz frequency 3623

capabilities,53,54 so that the method can be extended to study fast processes. In addition to the above conclusions, the present study is of interest in the framework of CNT and NW optoelectronic devices. In contrast with recently published studies, we showed that processes at dielectric interfaces can dominate the global behavior of the devices upon illumination. Such strong effects need to be carefully taken into account before any claim of direct photoinduced charge transfer from molecules to nanotube can be made in transistor configurations. We have also provided routes to control the volatile or nonvolatile character of the memory effects obtained in optically gated CNTFETs. Indeed, the interplay between the (VGS-dependent) type of charges in the polymer at the dielectric-polymer interface and the type of majority trap states in the dielectric leads to the stabilization of the optical memory effect on SiO2 and the absence of such a permanent state on TiO2. As probes or as devices, CNTFETs have an interesting role to play in future organic optoelectronics at both the macro- and the nanoscales. Acknowledgment. This work was supported by the French National Research Agency (ANR) through the Carnot Project “Imageurs en rupture” and by the Re´gion Ile-de-France (Sesame project) for the nanofabrication facility at SPEC. The authors thank P. Chenevier for nanotube purification and scientific inputs, Samuel Saada for help with titanium oxidation, and P. Lavie for expert technical assistance References (1) Veres, J.; Ogier, S.; Lloyd, G.; De Leeuw, D. Chem. Mater. 2004, 16, 4543. (2) Facchetti, A.; Yoon, M.-H.; Marks, T. J. AdV. Mater. 2005, 17, 1705. (3) Taylor, D. M. IEEE Trans. Dielectr. Electr. Insul. 2006, 13, 1063. (4) Zaumseil, J.; Sirringhaus, H. Chem. ReV. 2007, 107, 1296. (5) Hoppe, H.; Sariciftci, N. S. J. Mater. Res. 2004, 19, 1924. (6) Chua, L.-L.; Zaumseil, J.; Chang, J.-F.; Ou, E. C.-W.; Ho, P. K.-H.; Sirringhaus, H.; Friend, R. H. Nature 2005, 434, 194. (7) Li, Z. Q.; Wang, G. M.; Sai, N.; Moses, D.; Martin, M. C.; Di Ventra, M.; Heeger, A. J.; Basov, D. N. Nano Lett. 2006, 6, 224. (8) Mathijssen, S. G. J.; Kemerink, M.; Sharma, A.; Co¨lle, M.; Bobbert, P. A.; Janssen, R. A. J.; de Leeuw, D. M. AdV. Mater. 2008, 20, 975. (9) Calhoun, M. F.; Hsieh, C.; Podzorov, V. Phys. ReV. Lett. 2007, 98, 096402. (10) Scheinert, S.; Pernstich, K. P.; Batlogg, B.; Paasch, G. J. Appl. Phys. 2007, 102, 104503. (11) Taylor, D. M.; Drysdale, J. A.; Torres, I.; Ferna´ndez, O. Appl. Phys. Lett. 2006, 89, 183512. (12) Gu, G.; Kane, M. G.; Mau, S.-C. J. Appl. Phys. 2007, 101, 014504. (13) Yun, M.; Gangopadhyay, S.; Bai, M.; Taub, H.; Arif, M.; Guha, S. Org. Electron. 2007, 8, 591. (14) Lancaster, J.; Taylor, D. M.; Sayers, P.; Gomes, H. L. Appl. Phys. Lett. 2007, 90, 103513. (15) Alves, N.; Taylor, D. M. Appl. Phys. Lett. 2008, 92, 103312. (16) Torres, I.; Taylor, D. M.; Itoh, E. Appl. Phys. Lett. 2004, 85, 314. (17) Meijer, E. J.; Mangnus, A. V. G.; Huisman, B.-H.; Hooft, G. W.; de Leeuw, D. M.; Klapwijk, T. M. Synth. Met. 2004, 142, 53. (18) Understanding Carbon Nanotubes: From Basics to Applications, ; Loiseau, A., Launois, P., Petit, P., Roche, S., Salvetat, J.-P., Eds.; Lecture Notes in Physics, 677; Springer-Verlag: Berlin, 2006. (19) Derycke, V.; Filoramo, A.; Bourgoin, J. P. Carbon Nanotube Electronics. In Electronic DeVices Architectures for the NANO-CMOS Era; Deleonibus, S., Ed.; Pan Stanford Publishing: Hackensack, NJ, 2008. (20) Li, Y.; Qian, F.; Xiang, J.; Lieber, C. M. Mater. Today 2006, 9, 18. (21) Thelander, C.; Agarwal, P.; Brongersma, S.; Eymery, J.; Feiner, L. F.; Forchel, A.; Scheffler, M.; Riess, W.; Ohlsson, B. J.; Go¨sele, U.; Samuelson, L. Mater. Today 2006, 9, 28. 3624

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