PMMA–Ta2O5 bilayer gate dielectric for low operating voltage organic FETs

doi:10.1016/j.orgel.2005.03.002

Organic Electronics

Volume 6, Issue 2, April 2005, Pages 78-84

https://doi.org/10.1016/j.orgel.2005.03.002 Get rights and content

Abstract

This paper reports on a study on pentacene field effect transistors (OFETs) with a gate dielectric made of a bilayer PMMA/Ta₂O₅ where PMMA (poly(methyl methacrylate)) is spin-coated on top of an evaporated layer of Ta₂O₅. A comparison with devices with only Ta₂O₅ is presented. These latter exhibit very low operating voltage associated to the high dielectric constant of this oxide but also show some surface trapping and gate leakage. These two drawbacks can be overcome by depositing a PMMA layer on Ta₂O₅. With such a bilayer gate dielectric, gate leakage current is considerably reduced and the quality of the interface between pentacene and PMMA was much improved compared to that with Ta₂O₅ as evidenced from the much higher output drain current. The influence of PMMA thickness in the range 15–250 nm is presented. OFETs with field effect mobility, on/off current ratio, and sub-threshold slope of respectively 0.4 cm² V⁻¹ s⁻¹, 3 × 10⁵ and 1.2 V/decade were obtained.

Introduction

This last decade, organic field effect transistors have gained much interest mainly due to their potential applications in what is commonly called large area electronics. These include driving circuits for future all-organic OLEDs flat panel displays [1], plastic RF–ID circuits [2], gas sensors [3], chemical species sensors [4]. The continuous improvement in the quality of organic semiconductors, their compatibility with plastic substrates [5], [6], and their low cost and low temperature process make them good candidates for all these applications. Nevertheless OFETs performances do not rely only on the semiconductor material quality as intrinsic mobility. OFETs are working in accumulation regime and most of the modulated charge lies within the first 10 nm. That does mean that the interface properties between the semiconductor and the gate dielectric are of tremendous importance on the field effect mobility. Improvement of the gate dielectric material and of its interface with the organic semiconductor would be highly beneficial to the performances of OFETs. Roughness [7], density of surface traps [8], dielectric constant [9] are crucial parameters. Among inorganic dielectrics, the most often used was SiO₂ grown on highly doped Si gate. However its weak dielectric constant (ε = 3.9) remains a serious limitation for low power and nomad applications since operating voltage largely above 50 V are necessary for sufficient charge injection in the channel. Replacement with silicon nitride does not lead to significant improvement [7]. High-K materials such as Al₂O₃ [9], [10] and Ta₂O₅ [11], [12] were also proposed for gate oxide in OFETs. This latter reference reports on an investigation on the influence of electric field and temperature on mobility of pentacene transistors with Ta₂O₅ grown by anodic oxidation on plastic substrates. The mobility and on/off ratio obtained in this study are competitive with typical results for this materials combination (0.36 cm² V⁻¹ s⁻¹ and 10⁴, respectively). It is commonly observed that the operating voltage was noticeably reduced with Ta₂O₅, but this dielectric remains somewhat leaky. Other studies proposed to realise all-organic transistors with the use of polymer dielectrics as polyvinyl phenol (PVP) based polymers [13], [14] or PMMA [15], [16]. One major advantage of organic–organic interface is the noninteracting nature of this interface in most cases [17]. That means an abrupt interface without reactive interlayer or dipoles as observed in metal/organic interfaces [18]. The organic/oxide interface is also highly dependent on the surface preparation of the oxide as the numerous studies on the influence of ITO surface treatment on OLEDs performances shown in the past [19]. In spite of the potential improvement of device operation with organic/organic interface, organic dielectrics also show the drawback of a small dielectric constant. In order to reconcile the respective advantages of an high-K oxide for low operating voltage and polymer dielectric for a better interface with the organic semiconductor, a bilayer gate dielectric consisting of a Ta₂O₅ film covered with a PMMA film is reported in this study.

Section snippets

Experimental

Ta₂O₅ (purchased from Cerac Inc.) was deposited by e-beam evaporation in presence of an oxygen partial pressure (8 × 10⁻⁵–2 × 10⁻⁴ mbar) on highly doped Si substrate kept at room temperature. The deposition rate and the film thickness were respectively 5 Å/s and 120 nm. Some films have been annealed at 600 °C for 1 h in O₂ atmosphere. For the bilayer gate dielectric, a solution of PMMA in anisole was spin-coated onto the Ta₂O₅ and allowed to dry at 120 °C for 1 min. The thickness of PMMA after drying is

Electrical properties of evaporated Ta₂O₅

It is well known that the performances of a MOSFET strongly rely on the structural and electronic qualities of the gate insulator. Ta₂O₅ deposited by reactive evaporation at room temperature may contain several defects which could degrade the electrical characteristics of the OFET: multiphase material, local variation of composition, inhomogeneity, roughness, bulk and surface traps. We then investigated different deposition conditions by varying the oxygen pressure, the substrate temperature

Conclusions

In this paper we reported a new gate dielectric for OFETs made of a PMMA/Ta₂O₅ bilayer. This dielectric design combines the respective advantages of the two materials, say, the high dielectric constant of Ta₂O₅ and an improved PMMA/pentacene interface. Devices with operating voltages in the range 0–2.5 V were obtained with Ta₂O₅ dielectric constant (ε_r ≅ 20). Devices showing a high mobility, a high on/off ratio and very low leakage current were obtained with PMMA spun on Ta₂O₅. Optimum PMMA

Acknowledgements

This study benefited from a financial support of French Research Ministry under the program Nanosciences 2003. The technical assistance of P. Cremillieu is warmly acknowledged.

References (20)

H. Sirringhaus et al.
Synth. Met.
(1999)
J.H. Lee et al.
Synth. Met.
(2003)
L.A. Majewski et al.
Organic Electronics
(2003)
C. Bartic et al.
Organic Electronics
(2002)
S. Uemura et al.
Thin Solid Films
(2003)
L.S. Hung et al.
Mater. Sci. Eng. Rep.
(2002)
P.F. Baude et al.
Appl. Phys. Lett.
(2003)
B. Crone et al.
Appl. Phys. Lett.
(2001)
C. Bartic et al.
Appl. Phys. Lett.
(2003)
A. Bonfiglio et al.
Appl. Phys. Lett.
(2003)

There are more references available in the full text version of this article.

Cited by (89)

Highly sensitive NO<inf>2</inf> sensors based on organic field effect transistors with Al<inf>2</inf>O<inf>3</inf>/PMMA bilayer dielectrics by sol-spin coating
2019, Organic Electronics
NO₂ gas sensors based on organic field effect transistors (OFETs) with alumina/poly(methyl methacrylate) (Al₂O₃/PMMA) bilayer dielectrics were prepared by sol-spin coating method. The sensors with 0.8 mol/L Al₂O₃-sol dielectric layer show a high NO₂ response, low baseline drift, stable sensing performance, and fast response/recovery time at room temperature. The sensitivity is over 129%/ppm and the limit of detection is below 1 ppm (134%). The results are comparable to most organic devices. The high sensor properties were attributed to the more ordered films on the dielectric layer and better electrical properties with bilayer dielectric devices than those with single dielectric devices. Compared to OFETs with PMMA single dielectric, the mobility was improved above 23 times, threshold voltage was reduced by 32.47 V, and both on-off current radio and saturation current were improved above an order of magnitude for OFETs with Al₂O₃/PMMA bilayer dielectrics at 0.8 mol/L Al₂O₃-sol. Moreover, the mechanism of organic film growth and the effect of bilayer dielectrics on sensing properties were analyzed. It is revealed that the highly electrical and NO₂ sensing properties are closely relate. A simple and effective method, the sol-spin coating method, was provided to obtain high performance NO₂ gas sensors based on OFETs.
Use of bilayer gate insulator to increase the electrical performance of pentacene based transistor
2017, Synthetic Metals
In this study, bottom-gate top-contact pentacene-based organic field effect transistors (OFET) with various spin-coated ultrathin organic dielectrics on anodized aluminum oxide (Al₂O₃) bilayer gate dielectrics were fabricated. We have investigated the influence of the bilayer gate insulator having different combinations on the OFETs performance. Polystyrene (PS), poly-4-methylstyrene (P4MS), Poly-4-vinylphenol (PVP), poly-methylmethacrylate (PMMA) and Poly(4-vinylphenol-co-methyl methacrylate) (PVP_co_PMMA) were used as an organic dielectric. The results indicate that Al₂O₃ gate dielectric with Poly (4-vinylphenol) shows the optimum electrical performance with carrier mobility as large as 0.65 cm²/Vs, on/off current ratio of 10⁶, and threshold voltage as −3.8 V.
Modification of gate dielectric on the performance of copper (II) phthalocyanine based on organic field effect transistors
2017, Optik
Top contact Copper (II) Phthalocyanine (CuPc) based on Organic Field Effect Transistor (OFET) using various dielectrics were fabricated. The bare SiO₂ and bilayer Polyvinyl alcohol (PVA)/SiO₂ gate insulators with different thickness (50 and 100 nm) onto thin SiO₂ layers were used as gate dielectrics. It is found that using bilayer gate dielectrics can improve device performance. The PVA/SiO₂ bilayer gate insulator architecture increases the field-effect mobility by 20 times and improves the on/off ratio from 10³–10⁴. Also the photosensitivity of device was examined under different light intensities. The photosensitivity CuPc-OFET was found as 10 at a light intensity of 100 mW/cm² at the off state. This proposes that the CuPc-OFET behaves as a phototransistor.
Progress in flexible organic thin-film transistors and integrated circuits
2016, Science Bulletin
Organic thin-film transistor constructs the headstone of flexible electronic world such as conformable sensor arrays and flexible active-matrix displays. With solution-processed methods, it forges ahead toward large-area, low-cost manufacturing goals. As an indispensable complement to traditional silicon-based transistors, organic thin-film field-effect transistors have made great progress in materials, performance, bending capacity, and integrated circuits in recent few years. Flexible transistors and circuitry have extremely promising application prospects and possess irreplaceable status in foldable displays, artificial skins and bendable smart cards. In this review, we will discuss the evolution of flexible organic transistors and integrated circuits in terms of material, fabrication as well as application.
Nanowires of indigo and isoindigo-based molecules with thermally removable groups
2016, Dyes and Pigments
Citation Excerpt :
We initially tried to fabricate the nanowires films on the Si/SiO2 wafer that was treated with a self-assemble monolayer of octadecyltrichlorosilane (OTS); however, uniform films cannot be formed on the surface of such OTS-treated substrates. Hence, PMMA was used to modify the gate dielectric layer since uniform nanowires can be obtained [35,36]. Semiconducting layer was spin-coated from nanowires ODCB/pyridine (volume ratio of 10/90) solution with concentration of 10 mg mL−1.
In this manuscript, indigo and isoindigo-based π-conjugated molecules with thermal removable tert-butoxycarbonyl (t-Boc) side groups were designed and synthesized. It was noted that the t-Boc side groups can be eliminated in nearly quantitative yields after thermal treatment at 200 °C for 15 min, as confirmed by thermogravimetric analysis and Fourier transform infrared spectroscopy. From the thermal treated solution of isoindigo-based molecule DTIIC8C12 in the co-solvent of 1,2-dichlorobenzene/pyridine with volume ratio of 10/90, one-dimensional nanowires can be formed due to the hydrogen bonding assisted self-assembly. The afforded nanowires exhibited a moderate hole mobility of 1.3 × 10⁻³ cm² V⁻¹ s⁻¹, as estimated from the organic field effect transistors. These observations illustrated that the utilization of thermal removable side chain functionalized conjugated polymers can be an effective strategy for developing conjugated polymers with impressive charge carrier transport.
Low operating voltage n-channel organic field effect transistors using lithium fluoride/PMMA bilayer gate dielectric
2015, Materials Research Bulletin
We report low temperature processed, low voltage operable n-channel organic field effect transistors (OFETs) using N,N′-Dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C₈) organic semiconductor and poly(methylmethacrylate) (PMMA)/lithium fluoride (LiF) bilayer gate dielectric. We have studied the role of LiF buffer dielectric in effectively reducing the gate leakage through the device and thus obtaining superior performance in contrast to the single layer PMMA dielectric devices. The bilayer OFET devices had a low threshold voltage (V_t) of the order of 5.3 V. The typical values of saturation electron mobility (μ_s), on/off ratio and inverse sub-threshold slope (S) for the range of devices made were estimated to be 2.8 × 10⁻³ cm²/V s, 385, and 3.8 V/decade respectively. Our work thus provides a potential substitution for much complicated process of chemically crosslinking PMMA to achieve low leakage, high capacitance, and thus low operating voltage OFETs.

View all citing articles on Scopus

View full text

PMMA–Ta2O5 bilayer gate dielectric for low operating voltage organic FETs

Abstract

Introduction

Section snippets

Experimental

Electrical properties of evaporated Ta2O5

Conclusions

Acknowledgements

Synth. Met.

Synth. Met.

Organic Electronics

Organic Electronics

Thin Solid Films

Mater. Sci. Eng. Rep.

Appl. Phys. Lett.

Appl. Phys. Lett.

Appl. Phys. Lett.

Appl. Phys. Lett.

PMMA–Ta₂O₅ bilayer gate dielectric for low operating voltage organic FETs

Electrical properties of evaporated Ta₂O₅