Pentacene Organic Field Effect Transistors on Flexible substrateswith polymer dielectrics

S.P. Tiwaril, V. Ramgopal Raolt, Huei Shaun Tan2, E. B. Namdas2, Subodh G Mhaisalkar2

'Department of Electrical Engineering, Indian Institute of Technology Bombay, Mumbai - 400076, India2School of Materials Engineering, Nanyang Technological University, Nanyang Avenue-639798, Singapore

tEmail: rrao eejitb.acin, Phone: 91-22-25767456

ABSTRACT EXPERIMENTATION

Pentacene Organic Field Effect Transistors are fabricated using Figure 1 shows the device structure of pentacene OFET onpolymethyl methacrylate (PMMA) as a gate dielectric on flexible flexible Substrates. The ITO coated PET substrates (commerciallypolymeric substrates like the Indium tin oxide coated polyethylene available from Aldrich Chemicals) are cut into 2.5 cm square pieces,terephthalate (ITO coated PET), and polyethylene napthalate (PEN) cleaned by ultrasonic bath, DI water and IPA followed by a blow dryin top contact configuration. The ITO and aluminium (evaporated on process. The substrates are glued to glass pieces of the same sizePEN) act as the gate materials for the OFETs. The devices on ITO with the help of small water drops before dielectric spin coating.coated PET show electron mobility up to 0.07 cm2/V-s and the on/off PMMA (450K) 4wt% in anisole is spin coated at 800 rpm for 60ratios in the order of 103. seconds to get a thickness of about 500 nm. Pentacene (45 nm) is

evaporated as semiconductor. The gold S/D electrodes are depositedINTRODUCTION by shadow masks for top contact structure. For the pentacene OFETs

on PEN substrate, Al is evaporated to act as Bottom Gate. TheOrganic Field Effect Transistors (OFETs) are being investigated substrates are glued on glass pieces with the help of thermo tapes on

for a number of low-cost, large-area applications, particularly those the corners. The rest of the process details of PMMA spin coating,that are compatible with flexible plastic substrates [1, 2]. The organic pentacene and gold evaporation are kept identical.materials that have been used as active semiconductor materialsinclude both vacuum evaporated (such as pentacene) and solution Pentacene Au S/Dprocessed semiconductors. However, the vacuum evaporated (45nm) (50nm)pentacene is widely preferred because of its multi crystalline naturewhich results in higher mobilities and current on/off ratios [3,4].

Development of low temperature processes can make way for PMMA (-500nm)organic FETs to be integrated on low cost plastic substrates.Considerable progress has been made in this area in the recent years, ITO coated PET orthough achieving high carrier mobilities and On/Off ratios with Al deposited PEN Substrateorganic (polymeric) dielectric materials still remains a challenge.Though many polymeric dielectrics have been employed as gate FIGURE 1 Pentacene OFETs on flexible substrates (ITO coateddielectric, most of the studies are still made with highly doped silicon PET) with polymer (PMMA) dielectric and ITO as gate or aas the gate material. In Polymeric dielectrics, Poly Vinyl Alcohol flexible substrate PEN with Aluminum as gate.(PVA) and cross-linked PVA are studied as gate dielectric materialsfor pentacene OFETs on PET substrate. However, these dielectricsshow poor breakdown characteristics as well as very low immunity The freshly prepared devices are transferred into a nitrogen gloveagainst moisture. A PMMA buffer layer insertion noticeably box containing a micro-probe station, and connected to a HP 4155Cimproves the performance of these OFETs [5]. Pentacene thin-film system to perform the electrical measurements. The gate wastransistors with PMMA as a gate dielectric on a silicon substrate are probed through the dielectric for measurements. For eachreported to show good electrical performance, with mobilities around device, the output characteristics (IDS VS. VDS at multiple VGS values0.01 cm2 /V-s with threshold voltages around 15 volt [6]. It has also from OV to VDD) and transfer characteristics (IDS VS. VGS at multiplebeen shown that PMMA seems to influence the pentacene thin-film VDS from OV to VDD) were measured. For most of the devices VDDmicrostructure, increasing the crystallinity which results in a higher was kept constant at -20V. Field-effect mobility and thresholdfield effect mobility of OFETs [6]. Pentacene OFETs with a voltages were calculated in the saturation regime from the saturationPMMA/Ta2O5 bilayer gate insulator are reported to achieve both region current equation of standard MOSFET, using highest slope ofstable and low operating voltage devices. Ta2O5 operates at very low IDS1/2 VS. VGS plot. The current equation of a FET in saturation is:voltages but shows a lack of stability, whereas PMMA, although 1 W

_Vworking at high gate voltages, exhibits a remarkable stability [7]. IDS =1iC0X (VGSoxVT (2 L

However, in order to realize low cost and large area electronics where pt is the field-effect mobility, Cox is the capacitance perwith organic and polymeric materials, the silicon (Gate/Substrate) unit area of the gate dielectric, VT is the threshold voltage, and Wand 5i02 (Gate dielectric) need to be substituted with a polymeric (width) and L (length) are the dimensions of the semiconductorsubstrate and organic dielectric material respectively. Here, we channel defined by the source and drain electrodes of the transistor.present the study OFETs using PMMA as a gate dielectric material, The dielectric constant ofPMMA is 3.6.and thermally evaporated pentacene as an active layer. The flexiblesubstrates used for this study are substrates with ITO coated PET and RESULTS & DISCUSSIONAl evaporated PEN.

Figure 2 shows the output and transfer characteristics of a devicewith ITO coated PET (ITO as gate) at room temperature. The device

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shows a field effect mobility of 6.9x 10-2 cm2/V-s and the on/off ratiois 1.8x 103. The calculated threshold voltage for this device is -11Volts. These devices also show an average electron mobility of VG=OV to -40V6x10-2 cm2/V-s and the on/off ratios in the order of 103 The -1.5 in Vstepscalculated threshold voltages are around 10 Volts.

-1.00C

<2-35 1Oteo-40V -0.5-3.0' VGOV-4Vc)0.=L in -5V steps-2.5-

C -2.0 0 -10 -20 -30 -40t -1.5 a Drain Voltage VD (V)) -1.0 = 0.6

-0.5 1E7 VD=-2OV 0.5o 0.0 _ 0( -4 -8 -12 -16 -20 _:o°0.

C ~~~~~~~~~cca Drain Voltage VD (V) 1E-8i-c re 0.0.5

1E-78 VD - 7 l m1- g 1, , j -2I1 E-7 V.- -2OV/ , . b Gat0.4tVE-9

X 0.1,.C ~~~~0.3

D) 1E-10 ; .ttIEIC0.00 -5 --1 -15 -20

0~~~~~~~~~~~~02rIE-9 b Gate VoltageV (V).7v ~~~~~~~~0.1

0 -5 -10 -15 -20 -VG (half field circles) plots of pentacene OFETsb Gate Voltage VG (V) (W/L=4000/200) with Al as gate on a PEN substrate.

FIGURE 2. ID-VD (a), ID-VG (half filled rectangles) and IDS -VG (half filled circles) (b) plots of pentacene OFETs [1] B. Crone, A. Dodabalapur, Y. Y. Lin, R. W. Filas, Z. Bao, A.(W/L=1000/100) with ITO as gate on PET substrate. LaDuca, R. Sarpeshkar H.E. Katz, and W. Li, "Large-scale

complementary integrated circuits based on organictransistors" Nature, Vol. 403, pp. 521-523, 2000.Figure 3 shows the output and transfer characteristics of atrnisos Naue Vo.43 p 515300

Ftigulardeviswsthe outpu an ,tran charaterit ofm [2] Hagen Klauk, Marcus Halik, Ute Zschieschang, Florian Eder,PartiCUlar deViCe with Al as a gate on PEN substrate at roomtemperature. The device shows a field effect mobility of 2.5x 10-2 Dirk Rohde, Guinter Schmid, and Christine Dehm, "Flexible

cm2/V-s and an on/off ratio of 1.3x 1 The calculated threshold Organic Complementary Circuits", IEEE Trans. on Electron

voltage for this device is -9 volts. These devices show the on/off Devices, Vol. 52, No. 4, April 2005.ratios in the order of 10 and the threshold voltages around 10 volts, [3] T. W. Kelley, L.D. Boardman, T. D. Dunbar, D. V. Muyres,which are lower than those with ITO gate. M. J. Pellerite and T. Y. P. Smith, "High-Performance OTFTs

A significant contact resistance/offset voltage is seen in output Using Surface-Modified Alumina Dielectrics", J. Phys. Chem.characteristics of the devices with Al gate. Though a nice saturation B. Vol. 107, pp. 5877-5881, 2003.behavior can be seen in output characteristics, there is a positive [4] P. F. Baude, D. A. Ender, M. A. Haase, T. W. Kelley, D. V.drain current (not shown) at lower negative drain voltages (figure Muyres and S. D. Theiss, "Pentacene-based radio-frequency3a), which is due to the gate leakage through the PMMA dielectric. identification circuitry" Appl. Phys. Lett., Vol. 82, pp. 3964-One possibility is that the evaporated Al surface roughness is higher 3966.than the commercially available ITO surface which contributes to the [5] Sung Hun Jin, Jae Sung Yu, Chun An Lee, Jin Wook Kim,contact resistance for the Au S/D on the semiconductor layer. These Byung-Gook Park and Jong Duk Lee, "Pentacene OTFTs witheffects can however be reduced by gate layer patterning. PVA Gate Insulators on a Flexible Substrate", Journal of the

Korean Physical Society, Vol. 44, No. 1, pp. 18 1-184, JanuaryCONCLUSION 2004.

[6] J. Puigdollers, C. Voz, A. Orpella, R. Quidant, I. Martin, M.Vetter and R. Alcubilla, "Pentacene thin-film transistors withPentacene OFETs are successfullv fabricated on flexible

substates ithMMA a gatedielctric Thes devces o ITOpolymeric gate dielectric", Organic Electronics, Vol. 5, pp 67-coated PET have shown good electrical performances with mobilities 71204of 0.07 cm2/V-s and the current on/off ratio in the order of 10~, which [7 A.L ea,J ad,"tbliyo etcn rai ilare better than those with Al deposited on PEN substrate. effect transistors with a low-k polymer/high-k oxide two-layer

gate dielectric ", Materials Science and Engineering C, Vol.26, pp. 421 -426, 2006.

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