Volume 4 Issue 4 - May 9, 2008
The Study of Organic Thin Film Transistor with Polymethylmethacrylate as a Dielectric Layer
Yan-Kuin Su*, Tsung-Syun Huang, Po-Cheng Wang

Institute of Microelectronics, Department of Electrical Engineering, Advanced Optoelectronic Technology Center

Applied Physics Letters 91, 092116 (2007)

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Abstract

The properties of pentacene-based organic thin film transistors (OTFTs) with polymethylmethacrylate (PMMA) as a dielectric layer have been investigated. The concentration of PMMA was about 8 wt% with toluene as solvent. The pentacene film on PMMA dielectric layer displays high thin-film quality according to results of x-ray diffraction scan and atomic force microscopy. The crystalline size was about 33.96 nm and the grain size was about 1000~1500 nm. The pentacene-based OTFTs with PMMA as a dielectric layer exhibited excellent electric characteristics, including high mobility of 0.241 cm2/V-s or larger, on/off current ratio of 104 or larger, and the threshold voltage of less than -6.3V

Keyword: OTFTs, PMMA, pentacene, SiO2



Organic materials were generally used in many electronic devices such as light emitting diodes,1 thin film transistors,2,3 and photo-detectors.4 Due to varieties of materials and fabrication methods used in making organic electron devices and due to their other amazing characteristics, many researchers are attracted to jump into this field. For organic thin film transistors, many different materials, structures, and special modification methods were used to improve the characteristics of devices. Nowadays, the performance characteristics of organic thin film transistors (OTFTs) were almost the same as amorphous silicon (A-Si) which was commonly adopted in the modern liquid crystal display. Pentacene-based OTFTs with a field-effect mobility greater than 1 cm2/(V-s) and an on/off current ratio over 108 have been demonstrated by several groups.5-7

Surface properties of a dielectric layer, i.e., surface energy (γs),8 crystallite size, grain size and surface roughness (Rrms), were the distinctive factors that determine potential improvements in electric characteristics of OTFTs. Polymeric insulators have been considered as preferable gate dielectric materials due to their numerous advantages over inorganic materials, i.e., flexible, hydrophilic, easy process, and low cost. Hence, in addition to studying surface properties, it is also interesting to explore the influence of polar groups of polymer insulators on the structures of organic semiconductors and the corresponding performance of OTFTs.

In this letter, polymethylmethacrylate (PMMA) was used as a dielectric layer to substitute SiO2 in order to confer the transfer characteristics of OTFTs with PMMA as a dielectric layer. We studied the quality of pentacene thin film on PMMA by atomic force microscopy (AFM) and X-ray diffraction (XRD). Comparison was made with other OTFTs fabricated with SiO2 as a dielectric layer. The resultant surface free energy between pentacene and insulator was also deduced.
Figure 1         A schematic of molecular structures of PMMA and pentacene showing alongside with (a) pentacene-base OTFTs with PMMA as a dielectric layer, and (b) pentacene-base OTFTs with SiO2 as a dielectric layer.

The PMMA-insulator OTFTs structure was shown in Fig. 1. The OTFTs were fabricated on highly doped n-type silicon substrates. PMMA (molecular weight = 996,000) was dissolved in toluene with a concentration of 8 wt% and then spin-coated onto the highly doped n-type silicon substrate to form a 300-nm-thick thin film to serve as polymer insulator. The PMMA layer was baked for 1 hour at the temperature of 100˚C. For comparison, SiO2 was thermally grown on the highly doped n-type silicon substrate with the thickness of SiO2 kept also at 300 nm. The molecular structures of PMMA and pentacene were also shown in Fig. 1. Pentacene thin films were grown onto SiO2 and PMMA by vacuum evaporation method each with a thickness of 60 nm. The deposition rate of approximately 0.5 Å/s and a base pressure of approximately 5 ×10-6 torr were applied. After depositing pentacene thin film, a 200-nm-thick Au layer was deposited through a shadow mask to respectively pattern the drain, source, and gate electrodes, using a thermal evaporator. The channel length and width of these devices were 50 μm and 1000 μm, respectively. The pentacene film was studied by XRD (X-ray diffraction) in the symmetric reflection coupled θ-2θ arrangement. XRD patterns were obtained using Cu Kα radiation (λKα1 = 1.5406Å) and a wide-angle graphite monochromator. The grain size and Rrms (roughness) were measured by AFM (atomic force microscopy). For the surface free energy, the contact angles (FACE contact-angle meter, Kyowa Kaimenkagaku Co.) of two test liquids were measured. The measurement was conducted by placing water and di-iodomethane of two test liquids on the surfaces of PMMA and SiO2. Electrical characteristics of OTFTs were measured by a Keithley 4200-SCS system.
Figure 2        X-Ray diffraction spectra of pentacene thin films on PMMA and SiO2.

The morphology and the structural properties of pentacene were shown in Fig. 2 and 3. Fig. 2 shows the X-ray diffraction spectra of pentacene thin films deposited onto PMMA and SiO2, respectively. From this figure, it can be found that the peak intensity of Bragg reflection of pentacene thin film on PMMA was more conspicuous than that on SiO2. It means that the crystallite quality of pentacene thin film on PMMA was greater than that of pentacene thin film on SiO2. Both films have two major diffraction peaks, which can be attributed to“thin film phase”(00l’) and “triclinic bulk phase”(00l), respectively.9,10 We observed the same 2θ of (00l’) peak, indicating a thin film phase with the same interlayer spacing of 15.4Å, for both pentacene films. The minimum crystalline size perpendicular to the (00l’) planes was calculated using the paracrystal theory.11 Since the (00l’) planes are oriented parallel to the substrate, this calculation provides a measurement of the crystal size and quality perpendicular to the film plane. The following formula was used:
(1)


where
(2)


Figure 3        Atomic force microscope images of pentacene thin-films on (a) SiO2 and (b) PMMA.
was the overall broadening, λ was the x-ray wavelength, θ was the diffraction angle and δθ was expressed in radians, (δs)c was broadening due to crystallite size, (δs)II was broadening due to lattice distortions of the second kind, m was the diffraction order, dnkl was the average (hkl) plane spacing, Lhkl was the average crystalline size perpendicular to the (hkl) planes, and gII was the mean distance fluctuation between successive (hkl) planes due to distortion of the second kind. Our measurement were based on the full width at half maxima (FWHM) of the diffraction peaks, thus repeating the methodology used in previous reports.8,9 The estimated L00L value of pentacene film grown on PMMA and SiO2 were equal to 33.96 nm and 12.8nm, respectively. XRD analysis shows that pentacene films on PMMA layer have a better crystal quality than that on SiO2 layer. The grain sizes and morphologies of pentacene films were observed by AFM to assess the crystalline qualities as shown in Fig. 3. According to Fig. 3, it can be found that the grain sizes of pentacene deposited onto PMMA layer was 1000-1500 nm and the grain sizes of pentacene deposited onto SiO2 layer was 100-300 nm. The Rrms (roughness) of pentacene on PMMA layer and SiO2 were 11.376 nm and 12.3 nm, respectively. Hence, the pentacene film on the PMMA layer apparently had significantly larger grain size as compared to the similar film on the SiO2 layer. The quality of pentacene on PMMA layer was better than that of pentacene on SiO2.

The surface free energy of PMMA and SiO2 was measured by the contact angle of water and di-iodomethane. The value of surface free energy of PMMA and SiO2 were 47.5 mJ/m2 and 45.6 mJ/m2, respectively. The surface free energy of pentacene was 47.4 mJ/m2. Obviously, the surface free energy of PMMA matches with that of the pentacene thin film. A matching surface free energy significantly contributes to the enhancement of mobility in OTFTs which results in a decrease in the threshold voltage.8

Figure 4        Electrical transfer characteristics of pentacene-based OTFTs with PMMA as a dielectric layer (dotted line) and with SiO2 as a dielectric layer (solid line) when VGS was increased from 0 to 50V.
Fig. 4 shows the transfer characteristics (ID-VDS) of the PMMA-insulator pentacene OTFT and SiO2-insulator pentacene OTFT fabricated on the high doping n-type silicon substrate. From this figure, it can be found that the magnitude of the drain saturation current ID produced by the OTFT with PMMA insulator layer was significantly larger than that of the similar OTFT with SiO2 insulator layer at the same gate voltage VG. Figs. 5(a) and 5(b) present plots of logID and the square root of as a function of VGS for OTFTs with PMMA and SiO2 insulator layers, respectively, when VDS was set at -50 V. Saturation field-effect mobility μsat, on/off current ratio, and the threshold voltage Vth were all measured for OTFTs with the PMMA and SiO2 insulator layers by a curve fitting applied to the linear region between -20 and -40 V of VGS and results are listed in Table I.
TABLE I. The crystalline quality of pentacene films and the corresponding electrical performance of the pentacene-based OTFTs.
Figure 5  The semi-log plots of both the drain current and the square root of drain current versus gate voltage showing the transfer characteristics of (a) pentacene OTFT with PMMA as a dielectric layer, and (b) pentacene OTFT with SiO2 as a dielectric layer, when VDS was set at -50 V

In conclusion, we have fabricated and characterized pentacene-based OTFTs with PMMA as a dielectric layer for this paper. This study gave clear experimental evidence that the quality of pentacene grown on the PMMA dielectric layer was better than that of the similar film grown on SiO2 dielectric layer. XRD was used to measure the diffraction intensity in order to study the crystalline quality of pentacene thin film on PMMA and SiO2. We also used AFM to measure the grain size and roughness of pentacene grown on PMMA and SiO2 and subsequently deduce a match in surface free energy between pentacene and PMMA. The maximum saturation field-effect mobility was above 0.241 cm2/V s. It was also found that the transfer characteristics of OTFT with PMMA dielectric layer were greater than those of OTFT with SiO2 dielectric layer. Giving the merits of PMMA because of its polymeric nature, for the purpose of fabricating OTFTs on flexible substrates, there is a possibility that SiO2 can be substituted by PMMA as a dielectric layer. It is foreseeable that the high performance flexible OTFTs will be expected in the future


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