Solution-Processable Zirconium Oxide Gate ... - ACS Publications

ACS eBooks; C&EN Global Enterprise ..... Low temperature solution processed high-κ zirconium oxide gate insulator by Broadband-UV annealing ... Digit...
1 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/cm

Solution-Processable Zirconium Oxide Gate Dielectrics for Flexible Organic Field Effect Transistors Operated at Low Voltages Young Min Park, Amit Desai, and Alberto Salleo* Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States

Leslie Jimison Department of Bioelectronics, Ecole Nationale Superieure des Mines, CMP-EMSE, MOC Gardanne 13541, France S Supporting Information *

ABSTRACT: We investigate solution based fabrication of high-k ZrO2 thin films for low-voltage-operated organic field effect transistors (OFETs). An alternative UV curing method for the densification of Zr-based gel films, which allows for lowtemperature processing, is compared to the conventional thermal annealing method. Elemental and microstructural analysis shows that UV-curing induces the decomposition of organic-metal bonds and causes the densification of the metal oxide film, just as the conventional thermal annealing of gel films does, resulting in a high-k dielectric layer from Zr-based solutions. Furthermore, we found that the low temperature associated with UV-curing prevents the interface layer from intermixing with the substrate. Fabricated ZrO2 films (5−6 nm in thickness) treated with an octadecylphosphonic acid self-assembled monolayer exhibit low leakage current density (below 10−6 to 10−7 A/cm2) at 3 V and high dielectric breakdown strength (V > 4 V). Using this dielectric layer, solution processable polymer OFETs with PBTTTC-14 as the organic semiconductor function well at low voltage (below −3 V.) The effect of self-assembled monolayers (SAMs) on the morphology and microstructure of the organic semiconductor deposited on the ZrO2 dielectrics are investigated. Finally, we demonstrate solution-processable, low-temperature fabrication of OFETs on a flexible substrate. KEYWORDS: solution processed zirconium oxide, low temperature process, high-k dielectric, low-voltage organic field effect transistor, hybrid materials

1. INTRODUCTION Solution processable organic field effect transistors (OFETs) have attracted considerable attention due to their potential for applications in low-cost flexible electronics such as sensors, radio frequency identification tags, and backplane circuitry for active matrix displays.1−6 To realize these applications, several reports have addressed the correlation of microstructure to charge transport and new molecular design for optimized intermolecular stacking and electronic structure of the organic semiconductor.7−10 Equally significant for practical applications, however, is the development of reliable gate dielectrics. In the context of printed electronics, dielectrics deposited via a solution-based process are of particular relevance.11−13 It is known that the electrical performance of an OFET is strongly dependent on the interface between the gate dielectric and the semiconductor as well as on the intermolecular stacking of organic semiconductor.13−15 Desirable requirements for gate dielectrics include not only good dielectric properties, such as large capacitance and low leakage current density for low voltage operation, but also the possibility of low temperature processing and compatibly with flexible substrates. High performance polymer gate dielectrics have some advantages, © XXXX American Chemical Society

such as solution processing and good compatibility with organic semiconductors in terms of forming a favorable dielectric/ semiconductor interface.16,17 However, the use of these materials is limited by the need for solvents orthogonal to those dissolving the semiconductor. Furthermore, the operation voltage and charge transport of organic semiconductors are restricted by the low capacitance of polymer dielectrics.18 Hybrid dielectrics composed of self-assembled monolayers (SAMs) on inorganic high-κ metal oxides have been proposed as promising alternative candidates.12,19 This dielectric stack displays high areal capacitance, allowing the operation of OFETs at low voltage (below −3 V). The hybrid nature of the stack takes advantage of the high-κ metal oxide while simultaneously using the SAMs to improve the interface compatibility between the organic semiconductor and the dielectric layer and to reduce the gate leakage current. Furthermore, the addition of the inorganic layer helps overcome some disadvantages of SAM-based multilayer Received: November 2, 2012 Revised: May 8, 2013

A

dx.doi.org/10.1021/cm303547a | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

Article

dielectrics, such as the reliability and their dependence on the surface chemistry of the substrate.20,21 Inorganic/SAM dielectric stacks offer many advantages; however, most of the common techniques to deposit high-κ metal oxides may require expensive vacuum-deposition steps22 and/or high-temperature processing that is incompatible with flexible substrates19,23,24 or impose limitations on the choices of gate electrode.12 Here, a systematic study of solution processed high-κ ZrO2 dielectrics is reported in order to enable low-voltage operation of OFETs compatible with a solution-based organic semiconductor. We investigate two different approaches for dielectric fabrication: conventional thermal annealing and UV curing in ambient condition to form high-κ ZrO2 with high areal capacitance and low leakage current when capped with an octadecylphosphonic acid (ODPA) SAM. The microstructure and chemical composition of the solution-processed ZrO2 as well as the dielectric/semiconductor interface obtained using both approaches are compared in detail. We demonstrate that the hybrid dielectrics enable solution-based fabrication of OFETs with high performance (μ> 0.2 cm2/(V s)), operated with low onset voltage (ca. −1 V) on a metal-gated flexible substrate as well as on a rigid substrate, using poly(2,5-bis(3tetradecylthiophen-2yl(thieno[3,2-b]thiophene) (PBTTTC14) as the semiconductor.

2. RESULTS AND DISCUSSION High-κ metal oxides have been used as gate dielectric layers in conventional metal oxide semiconductor technology. Among them, ZrO2 has been investigated due to its high dielectric constant (25) and wide band gap (5.8 eV), allowing a low onset voltage and low leakage current.25 However, most techniques for the fabrication of ZrO2, such as atomic layer deposition,26 sputtering,27,28 and e-beam evaporation,29 require vacuum processing and high-temperature annealing, raising both the cost and complexity of OFETs. The fabrication of ZrO2 layers reported here is based on solution-deposition using zirconium(IV) acetylacetonate as a precursor in a N, N-dimethylformamide solvent. To achieve high-κ functional ZrO2 layers, two methods for film densification are investigated: high temperature annealing and UV-curing at ambient temperature. We demonstrate that high power UV light can be used to prepare ZrO2 films that are as functional as those prepared by conventional high temperature annealing with respect to working as gate dielectrics in OFETs. Furthermore, UV-curing improves the dielectric-substrate interface compared to thermal annealing. The UV-curing process is compatible with flexible substrates as the maximum temperature measured during the process by placing a thermocouple on the film is approximately 65 °C. The dielectric properties of the ZrO2 film fabricated via thermal annealing and UV irradiation were assessed by fabricating metal−insulator−metal capacitors. The areal capacitance vs frequency characteristics of bare ZrO2 and ODPA/ZrO2 hybrid dielectrics on a heavily doped Si substrate were measured with an Al top metal electrode (Figure 1a). The capacitance of bare ZrO2 processed by thermal annealing and UV curing was ∼760 (661) nF/cm2 and ∼520 (413) nF/cm2, respectively, at 50 Hz (50 kHz). The difference in areal capacitance between the annealed and UV cured ZrO2 can be attributed to the thickness of the dielectric stacks, which will be discussed in detail later, as the UV-cured ZrO2 is overall thicker than the thermally cured ZrO2. After treatment with ODPA, the capacitance is reduced to 624 (521) nF/cm2 for the annealed

Figure 1. (a) Areal capacitance vs frequency and (d) leakage current vs applied voltage for thermal annealed ZrO2, UV cured ZrO2, and ZrO2 binded with ODPA on Si.

and 320 (284) nF/cm2 for the UV-cured ZrO2 at 50 Hz (50 kHz), due to the introduction of low permittivity alkyl chains. UV-cured bare ZrO2 also exhibited a lower leakage current density (10−6 A/cm2) compared to thermally annealed ZrO2 (10−4 A/cm2) when measured at an applied voltage of −3 V. This difference in current density is consistent with the thickness difference between the two samples (Figure 1b). The slow decrease of the measured capacitance with frequency can be attributed to series resistance rather than interface trap states, which would exhibit a characteristic emission frequency.30 Chemical passivation by ODPA dramatically reduced the leakage current density (measured at an applied voltage of −3 V) to approximately 10−7 A/cm2 for the UV cured and 10−6 A/cm2 for the thermally annealed ZrO2. The ODPA also raises the breakdown voltage to a level compatible with low-voltage OFET operation, below −3 V. Furthermore, the thickness of the ZrO2 layer, as measured by ellipsometery, decreases with UV irradiation time (Figure 2a). The change in capacitance with irradiation time is in line with the thickness change (Figure 2b). This observation is a consequence of the previously proposed mechanism of slow densification of the gel film by UV irradiation and oxidation, following a relatively fast photolysis.31 Over 50% of the total thickness decrease occurs within the first 15 min while the entire densification process takes approximately 90 min to complete. Reactive oxygen species generated from ambient O2 by light in the UVV wavelength range present in the UV lamp spectrum are absorbed into the ZrO2 film and remove oxygen vacancies. As a result, the ZrO2 is formed under high UV power. Assuming that the interface layer between Si and ZrO2 is mostly SiO2 and the thickness of this layer is fixed regardless of irradiation time, the dielectric constant estimated using a series capacitor model (1/ CSiO2+1/CZrO2 = 1/Ctotal) increases from ∼6 for 15 min of B

dx.doi.org/10.1021/cm303547a | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

Article

ODPA layer has a thickness of 3.1 nm and is well stacked on the ZrO2 after treatment. XTEM micrographs indicate the existence of an interface layer between the ZrO2 and the Si substrate. The interface layer is thicker (4.2 nm) in the UVprocessed ZrO2 stack than in the thermally annealed one (2.2 nm). Assuming that there exists initially a layer of native oxide (∼2 nm) of Si, UV curing increases the thickness of the interface layer, suggesting that the reactive oxygen generated by UV irradiation and photolysis of Zr-based sol−gel layer form a reactive oxygen-rich environment at the interface between the porous Zr based film and the Si substrate, therefore oxidizing the Si substrate. Bare, heavily doped Si samples under the same UV irradiation conditions, on the other hand, do not show any significant change in the native oxide thickness (∼2.2 nm) compared to the Zr based sol−gel deposited ones as measured by ellipsometry, indicating that Zr-based sol−gel layer act as a source for reactive oxygen to oxidize the substrate under UV irradiation. Although the Gibbs free energy of formation of ZrO2 (−1042 kJ/mol) is more negative than that of SiO2 (−856 kJ/mol) at 298 K,33 a reactive oxygen-rich environment induces additional oxidation of Si substrate, hence forming a thicker Si-rich oxide interface during UV irradiation. In addition, the ZrO2 layer in the UV-cured sample was thicker than that in the annealed sample (ca. 6.5 nm compared to 5.4 nm). Assuming that same initial amount of Zr sol−gel was deposited by spin-coating in both the UV-cured and the thermally annealed films, this result shows that the UV-cured ZrO2 is less dense than the thermally annealed ZrO2, in agreement with the areal capacitance measurements. XTEM analysis further shows that the microstructure of ZrO2 obtained from the Zr based sol−gel is not a single tetragonal phase but a mixture of amorphous and crystalline ZrO2, regardless of the processing method. This mixed structure induces a lower dielectric constant than that of previously reported vacuum-processed crystalline ZrO2.25,34 Atomic force microscopy (AFM) shows that the ZrO2 surface is pinhole free and smooth after UV curing and thermal annealing (Figure 3c and d). The roughness of the thermally annealed and UV-cured ZrO2 is 0.32 and 0.34 nm, respectively, which is comparable to that of the thermally grown SiO2 substrate. The presence of a smooth dielectric surface eliminates any effect of roughness on the morphology and charge transport in the organic semiconductor.35 X-ray photoelectron spectroscopy (XPS) reveals the elemental composition in the film spin-coated from the Zr(acac)4 precursor solution on the Si substrate. We studied the as-deposited, thermally annealed at 400 °C for 1 h, and UVcured for 90 min films (Figure 4a). In the XPS survey spectra, while carbon and nitrogen from the precursor and solvent are detected in the as-spun film, thermally annealed and UV-cured films do not show any carbon or nitrogen. Since the solution from the precursor is expected to form organometallic structures comprising Zr, O, C, and N, the absence of C and N clearly indicates that UV curing induces a photolysis reaction, similar in final result to the pyrolysis occurring during thermal annealing. Ultimately, C and N are removed from the film as volatile gases, followed by densification and reaction with the reactive oxygen generated by UV light or the oxygen present in the deposited gel film. Furthermore, the atomic ratio of Zr to O is almost 1 to 2 for both thermally annealed and UV-cured films, indicating that the deposited film is oxidized to form the functional ZrO2 dielectric layer. A 90° takeoff angle does not show any Si peak from the interface due to the thickness of

Figure 2. (a) Change in thickness and (b) change in areal capacitance (error bars derive from the capacitance change with frequency) of deposited Zr based gel film as a function of UV irradiation time.

irradiation to ∼10 for 90 min of irradiation which is similar to the dielectric constant of thermally annealed ZrO2, ∼10. (Figure 2c). The lower κ value observed at shorter UV irradiation time is consistent with a lower atomic density and corresponding smaller ionic polarization. Although shorter UV irradiation times produce functional ZrO2 films, the variation of capacitance with measured frequency, shown as a range in Figure 2b, decreases with UV irradiation time. This large variation with frequency is qualitatively believed to be due to the larger density of defects in a film irradiated with UV for only a short time.32 Furthermore, it was observed that low power UV irradiation, ∼ 15 mW/cm2, including the UVV wavelength range, enabled the Zr-based sol−gel to undergo decomposition but did not result in functional ZrO2 films with a high dielectric constant. These results indicate that densification by high UV power can produce a functional gate dielectric ZrO2 film from a Zrbased sol−gel solution. In addition, the estimated dielectric constant of UV irradiated ZrO2 was not significantly different from that of thermally annealed ZrO2, suggesting that UV curing is a method for the densification of ZrO2 film from a metal based sol−gel solution, comparable and able to replace conventional thermal annealing. In addition to the advantages of low temperature processing, UV-curing enables selective patterning of the dielectric layer using a conventional photomask (see the Supporting Information, Figure S1). Masked regions of the initial Zr-based film are easily removed with acetone or the same solvent used for the initial deposition, while the exposed areas are insoluble. Cross-sectional transmission electron microscopy (XTEM) was used to investigate the microstructure of the metal−insulator−metal (MIM) capacitor Al/ODPA/ZrO2 on Si for both the thermally annealed and the UV-cured oxide (Figure 3). A lowmagnification XTEM micrograph (Figure 3a) shows that the C

dx.doi.org/10.1021/cm303547a | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

Article

Figure 3. High-resolution cross sectional TEM micrograph of (a) MIM capacitor on thermally annealed ZrO2 and (b) UV irradiated ZrO2 during 90 min. AFM noncontact mode height image for (c) UV cured ZrO2, irradiation time of 90 min (roughness: 0.34 nm) and (d) thermally annealed ZrO2, annealing time of 60 min at 400 °C (roughness: 0.32 nm). The scale is 2 μm × 2 μm, height color scale is 0 to 2 nm.

the interface layer between the ZrO2 and SiO2 is different chemically depending on the densification method. As Ar+ sputters the sample, the Zr-silicide peak (179.1 eV) appears and decreases successively with increasing sputtering time in the thermally annealed sample. The UV-cured sample, on the other hand, shows a Zr-silicide feature (179.1 eV) of lower intensity compared to that measured in the annealed sample, regardless of Ar+ sputtering time, supporting that additional oxidation of the substrate occurs due to a reaction with the oxygen generated by UV radiation. This observation is in agreement with a previous report that SiO2 is decomposed and forms Zrsilicide at the SiO2/ZrO2 interface.37 Although it is reported that the Zr metallic peak may appear as a result of a sputtering artifact to some extent,38 the lower intensity of the Zr_Si peak in the UV-cured sample indicates that it is less favorable to form a Zr-sillicide at the interface with the substrate during UV curing, compared to thermally annealing, due to the low temperatures. Hence, processing with UV light reduces the interdiffusion of species between the dielectric and the substrate, hence minimizing the effect of the substrate on the properties of the dielectric stack. OFETs operating at low voltages are obtained using the semiconducting polymer PBTTT deposited on thermally

both dielectrics. To further investigate the interface between ZrO2 and the Si substrate. XPS spectra during depth profiling were taken. Chemical state differences between thermally annealed and UV cured Zr films Zr were observed, as evidenced by the Zr 3d feature (Figure4b and c). These differences are observed both at the substrate interface and throughout the film. No metallic peak in the Zr 3d spectra is detected on the surface, indicating fully oxidized ZrO2. While the thermally annealed sample has a Zr 3d5/2 feature at 183.2 eV, the UVcured sample displays a lower binding energy of Zr 3d5/2 (182.4 eV), suggesting that the Zr−O bond in the thermally annealed film has a more ionic character than in the UV-cured film.36 In addition to the Zr 3d feature, we fitted the oxygen 1s peak to a superposition of two peak components. (Figure 4d and e) The band around 530 eV can be attributed to lattice oxygen atom in a fully coordinated environment (M−O−M), whereas the band around 532 eV is due to M−OH species in both films. Although both samples contain an appreciable concentration of M−OH defects, the UV irradiated sample exhibits a lower binding energy (530 eV) of the M−O−M peak than the thermally annealed ZrO2 (530.6 eV), consistently with our observations of the Zr 3d feature, indicating a more ionic characteristic in the thermally annealed sample. Furthermore, D

dx.doi.org/10.1021/cm303547a | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

Article

Figure 4. (a) XPS survey scan of Zr based sol−gel as-deposited, after UV irradiation and after thermal annealing at 400 °C indicating the change of elemental composition. Depth profile high resolution XPS spectra of Zr 3d feature for (b) thermal annealed ZrO2 at 400 °C and (c) UV irradiated ZrO2. High resolution XPS spectra of O 1s feature for (d) the thermally annealed ZrO2 and (e) UV-irradiated ZrO2.

mobility by an order or magnitude, form ∼0.02 to ∼0.2 cm2/(V s), regardless of fabrication method of ZrO2. AFM characterization shows that the PBTTT film on ODPA-treated ZrO2 exhibits large domain sizes compared to those observed on bare ZrO2 (Figure 6). Hence, the PBTTT surface structure evolution on ZrO2 as a function of substrate functionalization is similar to that observed on SiO2.41 It has been reported, however, that these surface domains are not single crystalline units but granular regions composed of microstructural subunits with smaller size.42 Two-dimensional grazing incident X-ray scattering (2D-GIXS) patterns of as-spun PBTTT on bare ZrO2 and ODPA/ZrO2 reveal a strong family of (h00) peaks along the vertical axis (∼qz), corresponding to the lamellar spacing perpendicular to the sample surface (Figure 7). These 2D diffraction patterns are useful for qualitative analysis of film texture. In the images shown, the (h00) peaks display significant arcing, indicating imperfect fiber texture of the crystalline domains on both surfaces. The similarity in the

annealed and UV cured ZrO2 films fabricated on a heavily doped Si substrate (Figure 5). The OFET characteristics are summarized in Table 1. OFETs on the bare thermally annealed ZrO2 did not function properly due to high gate leakage currents. Gate currents, on the order of 10−8 A at −3 V of gate voltage for the device on the hybrid ODPA/annealed ZrO2 dielectric are consistent with the I−V characteristics measured in the capacitor. OFETs fabricated on bare UV-cured ZrO2 show a threshold voltage of −1.1 V, on/off current ratio of 104 and subthreshold swing of ∼150 mV/decade. ODPA passivation, on the other hand, enhances the device performance, lowering the threshold voltage to −0.7 V, raising the on/ off current ratio of 105, and lowering the subthreshold swing to ∼110 mV/decade. These results demonstrate that the introduction of a hydrophobic surface reduces the trap states at the interface between the dielectric and organic semiconductor.39,40 Furthermore, the ODPA treatment enhances the field effect E

dx.doi.org/10.1021/cm303547a | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

Article

Figure 5. (a) Output curve and (b) transfer curve of PBTTT C-14 on a ODPA/thermal annealed ZrO2 hybrid dielectric. (c) Output curve and (d) transfer curve of PBTTT C-14 on UV irradiated ZrO2 with (black) and without (red) ODPA.

Table 1. Summary of PBTTT-C14 Device Performances on UV-Cured ZrO2 and Hybrid Dielectrics of ZrO2/ODPA ODPA/Annealed ZrO2 ODPA/UV cured ZrO2 UV cured ZrO2

areal capacitance (nF/cm2) at 50 hz

mobility (cm2/(V s))

threshold voltage (V)

subthreshold swing (mv/decade)

Ion/Ioff

620 320 522

0.18 ± 0.03 0.17 ± 0.02 0.016 ± 0.006

−0.75 −0.7 −1.1

115 110 150

105 105 104

Figure 6. Noncontact mode AFM image of (a) PBTTT on ODPA/ZrO2 and (b) PBTTT on bare ZrO2. The scale is 2 μm × 2 μm, height color scale is 0 to 20 nm.

Finally, OFETs operating at low voltage on flexible substrates, poly(ethylene terephthalate) (PET), are demonstrated using the hybrid ODPA/UV-cured ZrO2 dielectric. The dielectric stack deposited on an e-beam evaporated Al gate electrode exhibits a higher areal capacitance of 548 nF/cm2 at 50 Hz compared to that measured on a Si substrate (322 nF/ cm2). The improvement is due to the thin high-κ AlOx interface layer between ZrO2 and Al (Figure 8). The leakage current, 10−6 A/cm2 at −3 V, is similar to that measured in a device on a rigid substrate, indicating that UV-curing does not transfer considerable heat to the substrate and is therefore compatible with processing on flexible substrates. A PBTTT-C14 OTFT

diffraction patterns between bare ZrO2 and ODPA/ZrO2 suggests that the presence of ODPA does not have a significant effect on the bulk crystalline microstructure of PBTTT films, as observed with other SAMs such as OTS.43 Microstructural effects at the semiconductor/dielectric interface cannot, however, be excluded on the basis of the 2D-GIXS patterns. Another possible beneficial effect of the introduction of ODPA is the fact that it generates a low-κ alkyl chain spacer between the semiconductor and the dielectric. The high polarizability of high-κ dielectrics has been shown to reduce mobility; hence, the introduction of a spacer may alleviate this effect.44 F

dx.doi.org/10.1021/cm303547a | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

Article

Figure 7. Two-dimensional grazing incidence X-ray scattering patterns of PBTTT C-14 on (a) bare ZrO2 (b) ODPA/ZrO2.

Figure 8. (a) Areal capacitance as a function of frequency and (b) leakage current vs applied voltage for UV irradiated ZrO2 on Al deposited flexible substrate. (c) Output curve and (d) transfer curve of PBTTT C-14 on UV irradiated ZrO2 on Al deposited PET. (inset: photograph of devices on the flexible substrate).

was fabricated on the flexible substrate displaying a carrier mobility of 0.09 (±0.01) cm2/(V s), a ∼105 on/off ratio, and an onset voltage of −1.1 V. The degradation of device performance parameters, such as charge carrier mobility and on/off ratio, was expected on the flexible substrate due to higher roughness of Al on PET compared to the Si substrate.

not only allows low gate leakage current and high areal capacitance to be compatible to a low voltage operation of OFETs but also improves the interface between high-κ ZrO2 and organic semiconductor. The use of SAMs does not significantly affect the crystal structure of the polymer semiconductor. With PBTTT-C14 as a polymer semiconductor, OFETs operate at gate voltages lower than −3 V with a high on−off current ratio of 105, a low subthreshold swing of ∼110 mV/decade, and a field effect mobility of ∼0.2 cm2/(V s). The advantage of this low-temperature UV irradiation process is directly demonstrated by fabricating low-voltage solution processable OFETs on flexible substrates. Hence, this work is evidence that solution-based ZrO2 films processed using low or high temperatures are an attractive candidate as solutionprocessable gate dielectrics while organic transistors advance toward application.

3. CONCLUSIONS In conclusion, we have fabricated solution-processed ZrO2 high-κ dielectric for low-voltage operation of OFETs. Two methods of densification, thermal annealing and UV curing, were compared. TEM and XPS studies shown that UV irradiation can be an alternative low-temperature method to form films from the Zr based sol−gel solution. Furthermore, the UV-curing process minimizes the interfacial intermixing of ZrO2 with the Si substrate. Treatment of the ZrO2 with ODPA G

dx.doi.org/10.1021/cm303547a | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

Article

avoid thermal effect on the substrate. The field effect mobility was calculated in the saturation regime using the equation, ID = μsatCiW(2L)−1(VG − Vth)2, where Ci is capacitance of gate dielectrics and μ is field effect mobility. The threshold voltage was extracted from the intercept of the (−Id)0.5 vs Vg curve with the Vg axis.

4. EXPERIMENTAL SECTION Dielectric Film Fabrication. Zirconium based sol−gel solution was prepared by dissolving Zr(IV) acetylacetonate (Zr(C5H7O2)4) (98% Sigma Aldrich) (0.1M) in 5 mL of N, N-dimethylformamide(C3H7NO) (Sigma Aldrich) under a nitrogen glovebox with the addition of an equimolar concentration of ethanolamine (C2H7NO) (Sigma Aldrich). The solution was stirred and kept at 70 °C for 3 h to enhance hydrolysis. For the zirconium oxide film deposition, the solution was spin-coated onto substrates (heavily doped p-type Si or ebeam evaporated Al(Pt) on the PET flexible substrate) at 5000 rpm for 60 s to form a 30 nm gel film and briefly baked on a hot plate, as reported previously. As-deposited films were annealed at 400 °C for 1 h under nitrogen atmosphere, then cooled to the room temperature. For UV assisted method, the sample was cured under a high pressure metal halide UV lamp (Dymax Corp., Model No. 38560) including the wavelength of UVA (340−400 nm), UVB (290−315 nm), and UVV (180−200 nm) range at 270 mW/cm2 in atmosphere (see the Supporting Information, Figure S2) and at room temperature during different times of 15 min, 20 min, 30 min, 60 min, and 90 min. A phosphonic acid self-assembled monolayer was passivated on the ZrO2 dielectric by immersing substrates in a 5 mM n-octadecylphosphonic acid solution in 2-propanol for about 4 h. Substrates were successively sonicated in pure 2-propanol for 10 min, blown dry with nitrogen, and briefly baked on a hot plate at 60 °C. The contact angle of the surface after phosphonic acid treatment is 102°. Materials Characterization. Cross sectional TEM samples were prepared using conventional methods (polishing and ion milling). TEM images were obtained using an FEI Tecnai G2 F20 operating at 200 kV. Characteristics of the films are measured with Gatan digital micrograph software using contrast profile. The AFM experiment was conducted with a Park Systems XE-70 scanning probe microscope in noncontact mode with a scan rate of 0.8 Hz. Chemical element analysis was performed using a PHI Versa Probe Scanning XPS Microprobe with Al Kα radiation. To obtain depth profile spectra, 1 keV Ar+ sputtering was conducted on the both thermal annealed and UV cured film and high resolution spectra was obtained with 1 min interval. Two-dimensional grazing incidence X-ray scattering was performed at Stanford Synchrotron Radiation Lightsource, beamline 11−3, equipped with a MAR345 image plate detector. Beamline 11−3 uses an X-ray energy of 12.7 keV. An incident angle of ∼0.11° was chosen to ensure that the X-ray beam penetrates the entire thickness of the polymer sample. 2D-GIXD images were corrected for polarization and the nonlinear relationship between scattering vector q and pixel position using WxDiff software. Samples were kept in an helium chamber during irradiation to minimize beam damage. Exposure time was approximately 10 min. Device Fabrication and Characterization. Prime grade heavily p-doped silicon wafers (100) with a resistivity of 0.001 Ω-cm and PET were used as rigid and flexible substrates, respectively. Before the deposition of the sol−gel solution, Si substrates were cleaned with sonication in acetone, methanol, and isopropanol, and treatment in a UV ozone cleaner for 20 min each. The PET substrate was cleaned with sonication in isopropanol. To define capacitors, a square pattern of Al with an area of 0.012 cm2 was thermally evaporated on the dielectric. The capacitance measurements were conducted with an HP 4284 LCR meter at frequencies ranging from 50 Hz to 50 kHz. The capacitance value measured at 50 Hz was used to extract the mobility. Based on measured capacitances and microstrucutre, the dielectric constant of ZrO2 was evaluated with series capacitance model, 1/ CInterface + 1/CZrO2 = 1/Ctotal. For the OFETs, poly(2,5-bis(3tetradecylthiophen-2yl(thieno[3,2-b]thiophene) (PBTTT-C14; Mn 22 000; Mw 44 000 g/mol) was deposited by spin-coating with 1000 rpm from 0.5 wt % solutions in 1,2-dichlorobenzene on the phosphonic acid treated ZrO2 or bare ZrO2. The top-contact Au source and the drain electrodes (thickness = 100 nm) were evaporated thermally through a shadow mask. The dimensions of device were defined as 500 μm channel width × 60 μm channel length by isolating single devices. For the device on the flexible substrate, the source and the drain electrode were depoisted via e-beam evaportation in order to

■

ASSOCIATED CONTENT

S Supporting Information *

Optical image of ZrO2 exposed to UV for 90 min through a patterned quartz photomask and optical power configuration of UV light. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Y.M.P. is partially supported by Thomas V. Jones Stanford Graduate Fellowship. A.S. thanks Toshiba Corporation for financial support. The authors thank Professor P. C. McIntyre for use of the LCR meter.

■

REFERENCES

(1) Forrest, S. R. Nature 2004, 428, 911. (2) Dimitrakopoulos, C.; Malenfant, P. Adv. Mater. 2002, 14, 99. (3) Roberts, M. E.; Mannsfeld, S. C. B.; Queraltó, N.; Reese, C.; Locklin, J.; Knoll, W.; Bao, Z. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 12134. (4) Sirringhaus, H. Adv. Mater. 2005, 17, 2411. (5) Someya, T.; Sekitani, T.; Iba, S.; Kato, Y.; Kawaguchi, H.; Sakurai, T. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 9966. (6) Park, Y. M.; Salleo, A. Appl. Phys. Lett. 2009, 95, 133307. (7) Jimison, L. H.; Toney, M. F.; McCulloch, I.; Heeney, M.; Salleo, A. Adv. Mater. 2009, 21, 1568. (8) Rivnay, J.; Jimison, L. H.; Northrup, J. E.; Toney, M. F.; Noriega, R.; Lu, S.; Marks, T. J.; Facchetti, A.; Salleo, A. Nat. Mater. 2009, 8, 952. (9) McCulloch, I.; Heeney, M.; Bailey, C.; Genevicius, K.; MacDonald, I.; Shkunov, M.; Sparrowe, D.; Tierney, S.; Wagner, R.; Zhang, W. Nat. Mater. 2006, 5, 328. (10) Yan, H.; Chen, Z.; Zheng, Y.; Newman, C.; Quinn, J. R.; Dötz, F.; Kastler, M.; Facchetti, A. Nature 2009, 457, 679. (11) Halik, M.; Klauk, H.; Zschieschang, U.; Schmid, G.; Dehm, C.; Schutz, M.; Maisch, S.; Effenberger, F.; Brunnbauer, M.; Stellacci, F. Nature 2004, 431, 963. (12) Klauk, H.; Zschieschang, U.; Pflaum, J.; Halik, M. Nature 2007, 445, 745. (13) Facchetti, A.; Yoon, M.-H.; Marks, T. J. Adv. Mater. 2005, 17, 1705. (14) Salleo, A.; Chabinyc, M. L.; Yang, M. S.; Street, R. A. Appl. Phys. Lett. 2002, 81, 4383. (15) Yang, S. Y.; Shin, K.; Park, C. E. Adv. Funct. Mater. 2005, 15, 1806. (16) Cheng, X.; Caironi, M.; Noh, Y. Y.; Wang, J.; Newman, C.; Yan, H.; Facchetti, A.; Sirringhaus, H. Chem. Mater. 2010, 22, 1559. (17) Roberts, M. E.; Queraltó, N.; Mannsfeld, S. C. B.; Reinecke, B. N.; Knoll, W.; Bao, Z. Chem. Mater. 2009, 21, 2292. (18) Halik, M.; Klauk, H.; Zschieschang, U.; Kriem, T.; Schmid, G.; Radlik, W.; Wussow, K. Appl. Phys. Lett. 2002, 81, 289. (19) Acton, O.; Ting, G.; Ma, H.; Ka, J. W.; Yip, H.-L.; Tucker, N. M.; Jen, A. K.-Y. Adv. Mater. 2008, 20, 3697. (20) Yoon, M.-H.; Facchetti, A.; Marks, T. J. Proc. Natl. Acad. Sci. U. S. A. 2005, 102, 4678. H

dx.doi.org/10.1021/cm303547a | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

Article

(21) Ha, Y.; Emery, J. D.; Bedzyk, M. J.; Usta, H.; Facchetti, A.; Marks, T. J. J. Am. Chem. Soc. 2011, 133, 10239. (22) Kim, C. S.; Jo, S. J.; Lee, S. W.; Kim, W. J.; Baik, H. K.; Lee, S. J. Adv. Funct. Mater. 2007, 17, 958. (23) Wei, Q.; You, E.; Hendricks, N. R.; Briseno, A. L.; Watkins, J. J. ACS Appl. Mater Interfaces 2012, 4, 2322. (24) Ha, Y.; Jeong, S.; Wu, J.; Kim, M. G.; Dravid, V. P.; Facchetti, A.; Marks, T. J. J. Am. Chem. Soc. 2010, 132, 17426. (25) Robertson, J. Rep. Prog. Phys. 2006, 69, 327. (26) Javey, A.; Kim, H.; Brink, M.; Wang, Q.; Ural, A.; Guo, J.; McIntyre, P.; McEuen, P.; Lundstrom, M.; Dai, H. Nat. Mater. 2002, 1, 241. (27) Zirkl, M.; Haase, A.; Fian, A.; Schön, H.; Sommer, C.; Jakopic, G.; Leising, G.; Stadlober, B.; Graz, I.; Gaar, N. Adv. Mater. 2007, 19, 2241. (28) Rothländer, T.; Palfinger, U.; Stadlober, B.; Haase, A.; Gold, H.; Palfinger, C.; Domann, G.; Kraxner, J.; Jakopic, G.; Hartmann, P. J. Mater. Res. 2011, 26, 2470. (29) Kim, J.-M.; Lee, J.-W.; Kim, J.-K.; Ju, B.-K.; Kim, J.-S.; Lee, Y.H.; Oh, M.-H. Appl. Phys. Lett. 2004, 85, 6368. (30) Nicolian, E. H. and Brews, J. R. MOS (Metal Oxide Semiconductor) Physics and Technology; Wiley: New York, 1982. (31) Park, Y. M.; Daniel, J.; Heeney, M.; Salleo, A. Adv. Mater. 2011, 23, 971. (32) Ramanathan, S.; Muller, D. A.; Wilk, G. D.; Park, C. M.; McIntyre, P. C. Appl. Phys. Lett. 2001, 79, 3311. (33) Dean, J. A. Lange’s Handbook of Chemistry; McGraw-Hill: New Yok, 1999. (34) Lee, S.-Y.; Kim, H.; McIntyre, P. C.; Saraswat, K. C.; Byun, J.-S. Appl. Phys. Lett. 2003, 82, 2874. (35) Jung, Y.; Kline, R. J.; Fischer, D. A.; Lin, E. K.; Heeney, M.; McCulloch, I.; DeLongchamp, D. M. Adv. Funct. Mater. 2008, 18, 742. (36) Guittet, M. J.; Crocombette, J. P.; Gautier-Soyer, M. Phys. Rev. B 2001, 63, 125117. (37) Seo, K.; McIntyre, P. C.; Kim, H.; Saraswat, K. C. Appl. Phys. Lett. 2005, 86, 082904. (38) Perkins, C. M.; Triplett, B. B.; McIntyre, P. C.; Saraswat, K. C.; Haukka, S.; Tuominen, M. Appl. Phys. Lett. 2001, 78, 2357. (39) 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. (40) Salleo, A.; Chabinyc, M.; Yang, M.; Street, R. Appl. Phys. Lett. 2002, 81, 4383. (41) Kline, R. J.; DeLongchamp, D. M.; Fischer, D. A.; Lin, E. K.; Heeney, M.; McCulloch, I.; Toney, M. F. Appl. Phys. Lett. 2007, 90, 062117. (42) Wang, C.; Jimison, L. H.; Goris, L.; McCulloch, I.; Heeney, M.; Ziegler, A.; Salleo, A. Adv. Mater. 2010, 22, 697. (43) Chabinyc, M. L.; Toney, M. F.; Kline, R. J.; McCulloch, I.; Heeney, M. J. Am. Chem. Soc. 2007, 129, 3226. (44) Hulea, I. N.; Fratini, S.; Xie, H.; Mulder, C. L.; Iossad, N. N.; Rastelli, G.; Ciuchi, S.; Morpurgo, A. F. Nat. Mater. 2006, 5, 982.

I

dx.doi.org/10.1021/cm303547a | Chem. Mater. XXXX, XXX, XXX−XXX