From Oxide Surface to Organic Transistor Properties: The Nature and

J. Phys. Chem. C 2010, 114, 7153–7160

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From Oxide Surface to Organic Transistor Properties: The Nature and the Role of Oxide Gate Surface Defects Ste´phane Sua´rez,* Franziska D. Fleischli, Michel Schaer, and Libero Zuppiroli Laboratoire d’Optoe´lectronique des Mate´riaux Mole´culaires, STI-IMX-LOMM, Station 3, Ecole Polytechnique Fe´de´rale de Lausanne, CH-1015 Lausanne, Switzerland ReceiVed: NoVember 24, 2009; ReVised Manuscript ReceiVed: February 8, 2010

By applying various surface treatments to the oxide gate of pentacene transistors, both in vacuum (H2, O2 and Ar plasma treatments) and in aqueous solution, we were able to vary the balance between oxygen-free radicals (Si•), oxygen-centered radicals (Si-O• and Si-O-O•), and other defect types such as silanol (Si-OH), silicon hydride (Si-H), and silicate groups (Si-O-). The consequences of these modifications on the microstructure of pentacene deposited onto the gate have been studied by contact angle measurements and atomic force microscopy. The changes in the transport properties of the ultrathin devices were deduced from their electrical characteristics and from independent four-probe measurements. While the wetting properties of the oxide and in turn the pentacene growth process is mainly controlled by the Si-OH/Si-O- balance, it has been evidenced that oxygen-centered radicals act as charge transfer centers and determine to a large extent the transport properties of many thin-film transistors. Introduction A thin-film organic field-effect transistor is a device consisting of an organic semiconductor thin film deposited onto an oxide or a polymer gate dielectric, two metal contacts (source and drain) that collect the output current and a gate that controls the conductance of the channel. In principle, the transport properties of the channel are largely determined by the characteristics of the oxide gate dielectric interface and the microstructure of the organic semiconductor. In particular, the role of electroactive surface defects acting as traps or charge transfer centers is of paramount importance.1-3 An overview of the abundant literature concerned with the parameters that govern the microstructure and transport properties of organic thin-film transistors shows that a clear and general understanding of the most important parameters and the particular role they play is still missing. As an example, we can mention the confusion that appears from the literature to correlate apparent mobilities with the microstructure of pentacene films.4-12 Nevertheless, reports of the apparent mobility for typical pentacene thin-film transistors, whether measured in the linear or in the saturation regime, are in general between 0.1 and 1 cm2/(V · s).4,13,14 Contrary to the mobility, the threshold voltage is always sensitive to the details of the fabrication process.7,15 Most organic semiconductors used in organic field-effect transistors (OFETs) have holes as a majority carrier and part of the devices have a positive threshold voltage, an indication that carriers are present in the channel, even at zero gate voltage.16 They result from charge transfer reactions between the organic semiconductor and defects at the oxide surface acting as electron acceptors (Figure 1). These holes that are present at zero bias, which we call residual carriers, play an important role in determining the characteristics of the transistor by decreasing the on/off ratio and the contact resistance and by pushing the threshold voltage to positive values.16 To improve the on/off ratio, self-assembled * To whom correspondence should be addressed: e-mail, stephane. [email protected]; tel, (41) 21-693-3708.

Figure 1. Representation of the mechanism that produces charges (residual carriers) in the transistor channel by electron transfer from the pentacene to specific defects on the oxide surface.

monolayers of organic molecules were grafted on the oxide surface by several authors to inhibit the charge transfer reactions and decrease the threshold voltage of the devices.17 Silane and phosphonate molecules grafted on a silicone oxide gate or carboxylates grafted on an aluminum oxide gate were used for this purpose.7,18,19 The main purpose of the present paper is to investigate the role of the most common defects that are present at the oxide-semiconductor interface. We identify those that influence the pentacene film growth and those that are responsible for charge transfer reactions. For this purpose, we present a comprehensive series of experiments performed on ultrathin transistors fabricated with 10-20 nm pentacene films. We used atomic force microscopy (AFM) observations to characterize their microstructure and two-probe and four-probe conductance measurements to independently determine the contact resistance and the resistance of the channel. From the characteristics of the transistors we deduce the apparent charge mobility in the linear regime and estimate the density of residual carriers. The different transistors presented here differ by the surface treatment of the oxide gate prior to pentacene thin-film deposition. The novelty of these results lies in the fact that we were able to

10.1021/jp911167h  2010 American Chemical Society Published on Web 03/26/2010

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distinguish between specific defects on the oxide surface. We have used oxygen, hydrogen, and argon plasma treatments as a means of varying the balance of specific radicals and defects. We were then able to discriminate between stable and unstable species with time. The equilibrium between Si-OH and Si-Ospecies was varied by treating the oxide surface with aqueous solutions at different pH levels.

Sua´rez et al. Ω). Since there is no current flowing trough the contacts, the voltage drop across them is negligible and the voltage value measured along the sample reflects only the film resistance. The contact resistance is the difference between the resistance measured with the classical two-probe measurements and the film resistance.20 Results and Discussion

Experimental Section 1. OFET Device Fabrication. Top-contact ultrathin (10-20 nm thick active layers) pentacene transistors were fabricated by standard high-vacuum deposition techniques, as we reported previously.3 Thermal SiO2 (200 nm thick) on heavily borondoped silicon wafers and Al2O3 (6 nm thick) deposited on top of SiO2 via radio frequency magnetron sputtering at 100 W under pure argon at a pressure of 8 × 10-3 mbar were used as the device gate dielectrics. We also explored interfacial effects associated with the surface treatment of the dielectric layer under various plasma exposure conditions, prior to pentacene deposition. Plasma treatment on the oxides was done at a bias voltage ranging from -50 to -70 V, at a radio frequency of 10 W for 2-5 min at a constant gas pressure of 0.1 mbar with argon, oxygen, or formier gas (8% of H2 in N2). Without exposing the substrates to air, they were placed under high vacuum (10-7 mbar) overnight prior to pentacene deposition. Temperature of the substrates during pentacene deposition was varied from ambient (23 °C) to 55 °C and the pentacene deposition rate was varied from 0.4 up to 1.2 nm/min. Aqueous solution pH-treatment was done by immersion of the substrate for 2 s followed by drying with a nitrogen gas flow. Variation of the pH leads to a variation of the balance between SiOH (obtained with acidic solution) and SiO(obtained with basic solution) groups. The pH of the aqueous solutions was previously adjusted with a sodium hydroxide solution (32%) for the basic solution and with hydrobromic acid (48%) for the acidic solutions. The pH of the solutions was measured with pH test paper. pH-treated substrates were placed under high vacuum (10-7 mbar) overnight prior to pentacene deposition. The top source and drain gold contacts were evaporated through a shadow mask at the temperature of liquid nitrogen. The channels were fabricated with a width of 6 mm and a length of either 100 or 200 µm. The four-probe contact geometry consisted of four 6 mm wide gold strips spaced 600 µm apart. The morphology of the deposited films was studied by atomic force microscopy (NT-MDT Solver Pro in tapping mode). 2. Characterization. Dark current-voltage (I-V) characteristics (output characteristics) were obtained under nitrogen at ambient temperature using a Keithley 4200 semiconductor characterization system. Two-probe and four-probe measurements were carried out using a Keithley 236 source and measurement unit. The apparent field effect mobility is calculated, in the linear regime, from the slope of the conductance [gD ) (∂IDS)/(∂VDS)]VG)cst as a function of the gate voltage VG. The threshold voltage Vth is determined by the interception of the linear fit of the transconductance (transfer characteristics) with the x axis. The density of residual carriers Pres can be deduced from the following equation: Pres ) σ2D/(eµapp), where σ2D is the conductivity of the channel extracted from the fourprobe measurements, e the elementary charge, and µapp the apparent mobility in the linear regime. In the four-probe measurements, the inner contacts are connected to a voltage measurement unit of very high input impedance (typically 1014

There are two advantages to grow ultrathin film transistors on silicon dioxide gates: (i) by keeping the film thickness below 20 nm and the deposition rate below 0.6 nm/min it is possible to achieve layer-by-layer growth; (ii) top contacts are always less resistive on ultrathin films than on thicker ones. 1. Oxide Surface Defects. Gate surface defects play an important role in OFET performance since they are electroactive centers that interact with carriers in the channel.3,16,21,22 Some of these defects act as shallow traps that can capture and release the carriers. From an electrical point of view, they can be depicted as dipoles with strengths of about 2 or 3 D.23-25 Other kinds of defects act as recombination centers (or doping centers as in the doping of conducting polymers). They capture electrons from the organic semiconductor in a permanent charge transfer process that leaves residual holes in the channel.16,26 The presence of traps is generally detected by a negative threshold voltage and a characteristic curvature in the transconductance characteristics close to the threshold, while the presence of a large concentration of recombination centers results in a positive threshold voltage, much larger currents, and rather straight transconductance characteristics in p-type transistors.16,3 So far, to our knowledge, only silanol groups have been identified as doping centers at the SiO2 surface, in polymeric transistors.27-29 Chua et al. demonstrated that silanol groups hinder n-type transistors and that the suppression of the -OH groups can therefore lead to an interesting ambipolar behavior of the devices. Taking advantage of a combination of current-voltage characterization and four-probe conductivity measurements, we present here an original study that concerns specific oxidesurface defects related to charge transfer reactions at the oxide/ pentacene interface. 1.1. The Nature of the Defects. Despite the difficulties to understand the role of the different parameters controlling the properties of OFET, it is well admitted that the nature of the oxide surface plays an important role. It has been shown, for example, that self-assembled monolayers can be used between the organic semiconductor and the dielectric to tune properties such as threshold voltage, on/off ratio, and number of residual carriers.7,18,19 Treating a silicon dioxide gate dielectric with oxygen plasma prior to pentacene deposition has also been shown to drastically modify OFET performance.16 Despite the importance of this interface, to our knowledge no description of the chemistry at the interface between pentacene and oxides has been developed. Taking advantage of the abundant literature on the chemistry of silicone dioxide in the bulk30,31 and at the surface,32,33 we investigate the role of SiO2 defects on the transistor performance and then extend this study to an alumina gate. An oxide surface contains many different electroactive centers such as radicals and charged and neutral diamagnetic groups that can interact with pentacene molecules and become electron acceptors. The most well-known electroactive defects that exist on the SiO2 surface and are able to trap an electron are tSisOH, tSi•, tSisO•, tSisOsO•, tSisSit, tSis OsOsSit, and tSisH.30,34-38 The goal is to determine which of these species are prone to a charge transfer reaction with pentacene.

Oxide Gate Surface Defects

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Figure 2. Distinction of defects on oxide surface versus plasma treatment. From left to right: silanol, nonbridging oxygen center, peroxyl radical, E′ center, and silicon hydride. Exposition to air prior to pentacene deposition leads to the annihilation of the unstable radical species.

1.2. Defects and Recombination Centers. The results presented in this section characterize the role of specific defects on the microstructure of pentacene thin films and on charge transfer reactions at the interface between silicon dioxide and pentacene. As depicted in Figure 2, argon, oxygen, and hydrogen plasma treatments were used as a means of controlling the concentration of radicals and other defects on the oxide surface. Since each defect involved in a charge transfer reaction will result in an additional hole in the channel, i.e., a residual carrier, the density of residual carriers was determined for each interface to probe recombination centers. We were then able to distinguish between stable and unstable species with time or by exposing or not exposing the plasma-treated substrates to air prior to pentacene deposition. As a reference, the density of residual carriers for the untreated oxide gate surface was determined to be 8.4 × 1011 cm-2 (Table 1). Hydrogen plasma, which mainly creates tSi• and tSisH defects, does not produce additional residual carriers (Table 1) indicating that these species are not involved in a charge transfer reaction with pentacene. In contrast, oxygen plasma, which produces tSisO•, tSisOO•, and tSisOH, and argon plasma, which mainly creates tSisO• and tSisOH, drastically increase Pres up to 1013 cm-2. In order to differentiate between the roles of oxygen-centered radicals and silanol groups, two sets of samples (transistors and four probe devices) were fabricated on oxygen plasma treated wafers. When pentacene was deposited a few minutes after plasma treatment, oxygen radicals were still present on the surface leading to 3.6 × 1012 cm-2 residual carriers (Table 2). When the plasma-treated oxide was maintained under vacuum 24 h (or exposed to air) prior to pentacene deposition, oxygen radicals were annihilated, giving rise to an increase in the concentration of silanol groups on the surface. A low residual carrier density of 3.0 × 1011 cm-2 indicates that silanol groups do not contribute to charge transfer reactions. These results show the contribution of specific oxide defects on charge transfer reactions and demonstrate that oxygencentered radicals play the role of recombination or doping

Figure 3. Transfer characteristics in the linear regime of transistors fabricated with pentacene deposited a few minutes (scares) or 24 h (circles) after oxygen plasma on SiO2.

centers with pentacene. The impact of recombination centers on OFETs properties is evidenced by the drastic changes of the electrical characteristics observed in the transconductance curves (Figure 3). The transcondutance remains linear with the gate voltage and the threshold voltage is positive due to the high concentration of residual carriers. A non-negligible zero current is observed at VG ) 0 V leading to a miserable on/off ratio (see Table 2). In the case of transistors fabricated in the absence of oxygen-centered radicals, the current at VG ) 0 V is 2 orders of magnitude lower, the threshold voltage is negative, and the on/off ratio is larger than 100. 1.3. Energy LeWel Considerations. In order to estimate the likelihood of a charge transfer reaction occurring at a given defect, one must consider the energetic difference between the acceptor and the donor levels. In other words, the ionization potential (IP) of the donor has to be close to the electron affinity (EA) of the acceptor. The IP of pentacene in the gas phase is approximately 6.6 eV.39 The choice of the IP in the gas phase is used here due to the fact that the ultraviolet photoelectron spectroscopy (UPS) measurements generally used for determining the binding energy of the electrons gives information of the atom (or the molecule) in the excited state.40 The excitation induces a hole that polarizes the lattice in the solid state and shifts the energies (polarization energies), relative to the gas phase. This phenomenon is well understood for highly polarizable molecules such as pentacene, which has a polarization energy of 1.5 eV.41 As a consequence, taking into account the little influence in the solid state of the van der Waals intermolecular interactions on the energy levels, the ionization potential of pentacene determined by UPS in the gas phase is more relevant for our study of charge transfer reactions. Reported values of the EA for the defects considered in this work are listed in Table 3. The EA values of tSisOH, tSisH, tSisSi≡, and tSi• are approximately 4.0, 3.0, 2.2, and 4.5 eV, respectively, indicating a low probability of charge transfer

TABLE 1: Density of Residual Carriers (Pres) Depending on the Oxide Surface Treatments

Pres (cm-2)

SiO2 untreated

SiO2 pH ) 0.5

SiO2 pH ) 5

SiO2 pH ) 9

SiO2 plasma H2

SiO2 plasma Ar

SiO2 plasma O2

Al2O3 plasma H2

Al2O3 plasma O2

8.4 × 1011

6.7 × 1011

2.6 × 1011

7.2 × 1011

3.4 × 1011

1.0 × 1013

1.2 × 1013

3.5 × 1011

1.0 × 1013

TABLE 2: Experimental Conditions of Pentacene Deposition, Species Produced on SiO2 Surface, Pentacene Film Thickness, Conductivity, Mobility, Density of Residual Carriers (Pres), Threshold Voltage (Vth), and Contact Resistance (RC) of the Transistors Fabricated for Varying the Balance between Oxygen-Centered Radicals and Silanol Groups at the Interface substrate SiO2 SiO2

time between O2 plasma and PEN deposition 50 >50 -21 >70

RF (Ω)

RC (Ω)

2.4 × 10 3.1 × 106 3.6 × 106 3.6 × 106 3.3 × 106 1.9 × 105 2.4 × 105 1.4 × 107 4.2 × 105 6

ION/IOFF

1.2 × 10 1.1 × 107 9.0 × 106 1.8 × 107 1.3 × 107 9.5 × 104 1.5 × 105 1.5 × 107 7.0 × 104 7

6 × 102 1 × 102 7 × 101 3 × 103 4 × 102 2 2 7 × 102 2

is an important issue.52,53 Meijer et al. showed that the contact resistance of pentacene field-effect transistors has a large spread in value of several orders of magnitude for identically prepared devices.54 In our case, as illustrated in Figure 9, such deviations were not observed. For samples immersed at the same pH, the contact resistance varies by less than a factor of 2 and its typical values are in the order of 10 MΩ. The high contact resistance values obtained for a large part of the studied transistors (see Table 4) reflect the presence of a non-negligible contact barrier at the gold/pentacene interface, However a strong relation was found between the contact resistance and the density of the residual carriers as depicted in Figure 10. We considered 20 ultra-thin-film transistors with different film morphologies and a wide range of electrical characteristics obtained after various plasma- and pH-treatments of the oxide surface. We attributed this behavior to the screening of the contact Schottky barrier. This is an usual phenomenon in metal contacts made on doped semiconductor where, as it is well-known, the presence of extrinsic carriers from the dopants decreases the contact resistance.55 Thus, present in moderate concentration, recombination (or doping) centers at the gate surface can have a positive effect on the transistor characteristics. Of course, in the case of transistors fabricated on severe oxygen or argon plasma activated substrates, the very high density of residual carriers does overcome all other parameters. These results emphasize the role of recombination centers in OFETs properties and the control of some important electrical

Figure 9. Contact resistance when no gate bias is applied as a function of the pH used for aqueous solution treatment of SiO2 surface before the pentacene deposition. Aqueous pH treatments lead to different ratios of SiO- specie versus SiOH groups on the oxide surface. Width of the contacts: 0.6 mm.

Oxide Gate Surface Defects

Figure 10. Contact resistance as a function of the density of residual carriers. Until 1012 cm-2 of residual carriers the contact resistance remains constant. Over this value, a severe decrease of the contact resistance is observed indicating the strong dependence with the concentration of residual carriers. Contact width: W ) 6 mm.

characteristics such as the contact resistance and the threshold voltage by the density of residual carriers.

J. Phys. Chem. C, Vol. 114, No. 15, 2010 7159 surface was treated with an aqueous solution prior to deposition, two distinct types of microstructure were observed depending on the pH of the solutions. Growth was layer-by-layer for acidic pH and three-dimensional for basic pH. Water contact angle measurements on these solution-treated surfaces confirmed this distinction and showed a greater hydrophilicity of the substrate treated at basic pH, an indication of the presence of the charged Si-O- silicate on the surface. The hydrophobic nature of pentacene is in contrast with the charged hydrophilic surface, which inhibits layer-by-layer growth and forces the molecules of pentacene to a three-dimensional growing mode. We can thus conclude that the defects responsible for the growth process of pentacene films differ from those that create hole traps states and also differ from those involved in the charge transfer reaction. Acknowledgment. We thank Dr. Steven Konezny for scientific discussions and his critical reading of the manuscript. This work was supported by the Swiss National Science Foundation, Switzerland, through Grant SNSF 200020121715/1.

Conclusion The focus of this work was on the chemistry of oxide gate dielectric surfaces in pentacene field-effect transistors and their role on both transistor performance and pentacene film microstructure. The balance of the different dielectric surface defects was tuned by a variety of vacuum-plasma and solution-process surface treatments. Specific species such as Si-OH, Si-H, Si-O-, silicon-centered and oxygen-centered radicals were selectively controlled before the pentacene deposition. Characterization of pentacene film microstructure and the electrical properties of the transistors indicated that the microstructure does not play a major role on device performance. In particular, we measured apparent mobilities that were not dependent on the microstructure of the pentacene films. These results and the results presented in this paper argue that traps, which play a major role in governing charge transport, are localized at the dielectric/pentacene interface rather than at the grain boundaries. We have also demonstrated that the interface is important in its role in determining the density of residual carriers in the transistor channel, which in turn affects important electrical characteristics including the on/off ratio, threshold voltage, and contact resistance. An abrupt decrease of the contact resistance was observed when the number of residual carriers exceeds 1012 cm-2, an illustration of how the residual carriers can effectively reduce the contact barriers. These residual carriers come from an electron transfer between pentacene and defects on the dielectric surfaces. Among the defects present on the surfaces, we were able to identify the oxygen-centered radicals Si-O-O• and Si-O• as being responsible for the charge transfer reaction. The acceptor-donor levels involved in the charge transfer reaction at the pentacene/SiO2 interface is in agreement with our experimental results. It is conceivable that other oxygen species such as tSisOsOsSit might also be involved in charge transfer reactions, but their role could not be deduced based on the surface treatments utilized in this work. The microstructure of the deposited pentacene films varied greatly depending on properties of the different dielectric surfaces studied in this work. Layer-by-layer growth up to 20 nm of pentacene was obtained at room temperature on bare SiO2 and on SiO2 treated by oxygen, hydrogen, and argon plasma when the deposition rate was maintained below 0.6 nm · min-1, indicating that tSi•, tSisO•, tSisOH, and SisH play a role in the growth mode of pentacene. When the silicon dioxide

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