The Relation between Mobility and Microstructure in OTFT

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1 1. SID 25 2.OLED LED 6. 7.LCD-TV 8. The Relation between Mobility and Microstructure in OTFT T.C. Tien J.F. Wen Y.J. Chio L.C. Lin M.W. Lin MRL / ITRI T.S. Hu J.C. Ho ERSO / ITRI (OTFT) (Mobility) (Pentacene) OTFT OTFT Abstract So far, the mobility of organic thin film transistor (OTFT) is relatively low in flexible substrate, the working frequency of flexible display only can show the static image properly. As the requirement of higher level flexible display and low cost consideration, enhanced the mobility of organic thin film transistor are more and more important today, and also is one of the most popular topics in worldwide research area. In this paper, we focus on the material- pentacene which was use as channel material and with highest mobility value in OTFT today, studied on the relation between molecular arrangement, energy band structure, crystal structure, thin film, OTFT structure and mobility. In order to guiding the methods of improving working frequency in flexible display, it is necessary to adjust OTFT's processing conditions by understanding the microstructural mechanism of carriers transportation in OTFT

2 /Key words (Pentacene-based Organic Thin Film Transistor) (OTFT) (OST) (Mobility)(Flexible Display) Hall Effect Measurement µ Hall 2~1 7 cm 2 /Vs (2) Time of Flight Measurement (ToF) µ ToF 1 cm 2 /Vs (3) 1nm OTFT (Mobility) µ OTFT Field- (Pentacene) Channel Material Effect Transistor (FET) 3.2 cm 2 /Vs (1) OTFT (Amorphous Silicon) (b) OTFT I d =1/2 µc(w/l)(v G -V T ) 2 (Hybrids) I D Drain Current µ Mobility (Organics) 1 3 Today's Si Wafers Processors 1 2 Hybrids Poly-Si Pentacene Doping 1 1 Organics Low-cost ICs 1 Metal Atom a-si:h Smart Cards, displays 1-1 Band Structure 1-3 Polythiophenes E-paper 1-2 Thiophene Oligomers 1-4 Pentacene Organic/Inorganic Hybrid P N Time (Years) (2) IBM OTFT Road Map Mobility (cm 2 V -1 s -1 )

3 C Capacity V G Gate Voltage V T Thresold Voltage W L Channel Width Channel Length OTFT /.2µm 4µm 5 (3) 4% Thin-Film 97% Thin-Film (a) Pentacene C 22H 14 H H H H H H H Mobility 1 (3) Mobility OTFT (a) (b) (c) (d) OTFT OTFT H H H H H H (b) Bottom Contact OTFT Channel Source Drain H Gate Substrate Insulator (a)pentacene (b)bottom Contact OTFT (4) Pentacene a b c a z z b b a Pentacene Monolayer (4) d e f Unit Cell a= Å, b=7.588å, and γ= a z z 89.95, c=14.4å, 15.Å and 15.4Å a=6.266 Å, b b b a =7.775Å, and γ= , c= 14.1 (a) (b) (4) Å

Z Z (a-b Plane) 9 (5) 5nm SiO 2 AFM TXRD SiO 2 5nm PVP AFM TXRD PVP PVP SiO 2 RT (25 C) 65 C 7 C 75 C Intensity (Counts) 8 6 4 2 1µm 1µm 1µm 1µm < 2% 1 11 12 13 14 6.

4 Z Z (a-b Plane) 9 (5) 5nm SiO 2 AFM TXRD SiO 2 5nm PVP AFM TXRD PVP PVP SiO 2 RT (25 C) 65 C 7 C 75 C Intensity (Counts) µm 1µm 1µm 1µm < 2% % % % SiO 2 AFM TXRD (5) RT (25 C) 6 C 65 C 75 C Intensity (Counts) (2) 1µm 1µm 1µm 1µm < 2% (2) < 2% (2) < 2% (2) < 2% PVP AFM TXRD (5)

Grain Size Grain Size 1K Grain Size Grain Size Barrier Height J. H. Schon (6) Energy (ev) 1.2.8.4. -.

5 Grain Size Grain Size 1K Grain Size Grain Size Barrier Height J. H. Schon (6) Energy (ev) Γ X M Y Γ M Γ ZN H OMH Band Structure (6) Band Structure (6) Γ= (,, ), X= (a/2,, ), M= (a/2, b/2, ), Y=(, b/ 2, ), Z=(,, c/2), N= (, b/2, c/2), H=(a/2, b/2, c/2), O=(a/2,, c/2) ev M H (a/2, b/2, ) (a/2, b/2, c/2) Miller-Abraham (Energy Barrier) (Conductance) (Resistance) 3-D X-Z Arrange Y-Z Arrange X-Y Arrange 3-D

6 Pentacene-Hybrid Band Structure (7) Lattice Position I Rb Pentacene-Hybrid Band Structure Γ= (,, ), T= (a/2,, ), R= (a/2, b_/2, ), Y= (, b/2, ) ev R (a/2, b/2, ) (a/2, b/2, c/2) Hopping Type (7) Band Structure Molar Ratio 1.5 Iodine 1 11 P-type Molar Ratio.5 Rubidium 1 9 N-type Miller-Abraham 3-D Pentacene-Hybrids Grain Size (6) Grain Size (5) Thres Grain Size (Grain Boundary) Thres Grain Size Channel Length Energy (ev) Energy (ev) Energy (ev) Energy (ev) Γ T R ZΓ Y Γ T R ZΓ Y Γ T R ZΓ Y Γ T R ZΓ Y (a) (b) (c) (d) Pentacene-hybrid Band Structure (7) (a) Undoped (b) I-doped, type1 (c) I3-doped, type2 and (d) Rb-doped Pentacene Crystals

2 Unit Cell Enlargement Pentacene-Hybrid 3-D Mobility (cm 2 /Vs).5.45.4.35.3 Threshold Grain Size is a Lower.25 Boundary to the Mean Free Path of.2 Carriers Within the grains.15

7 2 Unit Cell Enlargement Pentacene-Hybrid 3-D Mobility (cm 2 /Vs) Threshold Grain Size is a Lower.25 Boundary to the Mean Free Path of.2 Carriers Within the grains SiO Grain Size (µm) Grain Size (5) Thermo Oxide SiO 2 OTS J.H. Schon (6) 1K Pentacene Grain Size Grain Size Grain Size (8) (a) 1nm.5nm/S.35nm/S 3nm 2nm (b)

8 Mobility (cm 2 /Vs) Thermal Oxide + OTS Thermal Oxide PECVD Silicon Nitride PECVD Silicon Oxide PECVD Silicon Oxide + OTS OTFT.4.2 Pentacene-base OTFT Channel Length Crystal Size (µm) Insulator Thickness (8) Channel Length (a) (b) Grain Size Channel 1.1 2nm Length Pt Au 3.5A/s.5A/s 3nm.1 PVP Average Crystal Size (µm) SiO 2 (a) 1nm (b) OTFT Stress OTFT / / / Ratio of x-ray Diffraction (1)/(1')

OTFT IEE OTFT Source Over Etching (Under Cut) Over Etching Channel Length Source Under Source

9 (DB-FIB) FEI Nova 2 NanoLab Xe Ion Etch Enhancement (IEE) OTFT OTFT 1nm OTFT Source Gate Pentacene Pt Source FIB OTFT 1nm Source Charge Trap Charge Trap Vth Gate 1nm Pt OTFT FIB IEE OTFT IEE OTFT Source Over Etching (Under Cut) Over Etching Channel Length Source Under Source Pentacene Insulator Gate Cut OTFT IEE OTFT IEE OTFT Source Over Etching

10 OTFT IEE-FIB OTFT 1. J.H. Schan. et. al. "On the Intrinsic Limits of Pantacene Field-effet Transistors", Organic Electronic, Vol. 1, No. 1, p Yukatsu SHICHIBU1 and Kazuyuki WATANABE1; Jpn. J. Appl. Phys. Vol. 42 (23) pp , Part 1, No. 9A, September 23, #23 The Japan Society of Applied Physics. 3. C.C. Hsieh, W.K. Hwang et al, "Pentacene Organic Thin- Film Transistor Integrated with Color Twisted Nematic Liquid Crystals Display (CTNLCD)" SID Sandra E. Fritz, Stephen M. Martin et al, Journal of the American Chemical Society on February 17, Ursula Haas, "Growth process control of pentacene thin films and its application in full organic thin film transistors" Polytronic 24, Institute of Nanostructured Materials and Photonics (NMP), Joanneum Research, Austria. 6. Gilles A. de Wijs, Christine C. Mattheus et al, Cond-mat/ 3178 v1, 7 Jan Yukatsu SHICHIBU, Kazuyuki WATANABE, Jpn. J. Appl. Phys. Vol. 42 (23) pp D. Knipp, a) R. A. Street et al, JOURNAL OF APPLIED PHYSICS VOLUME 93, NUMBER 1 1 JANUARY (Acryl) Micro Channel Microreactor 2 (Monomer) 5µm 1µm (3)

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