大氣電漿技術對有機薄膜電晶體特性改善之研究

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(3) . Enhancing the properties of polymer thin-film transistors using a novel atmospheric-pressure plasma technology. . .

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(7) . Enhancing the properties of polymer thin-film transistors using a novel atmospheric-pressure plasma technology. .

(8). . StudentChih-Hsiang Lin . AdvisorKow-Ming Chang .

(9) . . . . . . . . . . . A Dissertation Submitted to Department of Electronics Engineering and Institute of Electronics College of Electrical and Computer Engineering National Chiao Tung University in partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in Electronics Engineering July 2008 Hsinchu, Taiwan, Republic of China. . . .

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(15) Enhancing the properties of polymer thin-film transistors using a novel atmospheric-pressure plasma technology. StudentChih-Hsiang Lin. AdvisorDr. Kow-Ming Chang. Department of Electronics Engineering and Institute of Electronics National Chiao Tung University. Abstract Organic thin film transistors made by spin-coating from solutionprocessable conjugated polymers have potential advantages in fabricating lowcost devices with large areas. Since OTFT performance depends strongly on the interface between the semiconductor and the dielectric layer, this study attempts to demonstrate that the characteristics of P3HT-based OTFT are improved by controlling the chemistry of the dielectric/polymer interface. Thermal SiO2 is adopted as the dielectric because of its well-characterized properties and ease of chemical modification. Regioregular P3HT (of which HT linkages represent more than 98.5% of the linkages) is utilized as the active layer, so it exhibits better ordering and crystallinity in the solid state, and substantially improved electroconductivities. The field-effect mobility was markedly improved by modifying the surface of SiO2 with using a hexamethyldisilazane (HMDS) selfassembled monolayer. Before the active layer was deposited, the surface of SiO2 was modified using atmospheric-pressure plasma technique (APPT). APPT is a new process that can be implemented at atmospheric pressure and at low temperature. The steps of APPT are performed below 120°C and at atmospheric iii.

(16) pressure, so the approach is very suited to use on a plastic substrate. After the SiO2 surface has been modified by the APPT process with hexamethyldisilazane (HMDS), it exhibits typical I-V characteristics of TFTs. Calculations reveal that its field effect mobility can reach 0.02-0.03 cm2/Vs, which is about 15 times that before the modification, and the threshold voltage is below -10V. The performance is even better than that obtained following the usual surface treatment of the SiO2 surface by spin-coating or evaporation. This work suggests an interesting direction for preparing high-performance OTFTs with high efficiency and low-temperature surface treatment by APPT.. iv.

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(21) Contents Abstract (in Chinese)……………………………………………….…………….i Abstract (in English)……………………………………………….…………...iii Acknowledgment……………………………………………………….………..v Contents………………………………………………………………………....vi Table Captions……………………………………………….………….………ix Figure Captions…………………………………….…………………….……..xi Chapter 1 Introduction 1.1 General background and motivation…………………………………...……1 1.2 Organic conjugated materials for OTFT…………………….........................3 1.3 Thesis organization………………………………………………………….5. Chapter 2 Property of P3HT and Spin-Coating Technique 2.1 Introduction of P3HT…………………………………………………..…...11 2-1-1 The molecular structure of P3HT…………………………………….11 2.1.2 Conduction mechanism……………………………………………....12 2.2 Solution processed deposition…………………………………………..….16 2.2.1 Methods of OTFT fabricates…………………………………..……..16 2.2.2 The motivation of spincoating………………………………….....…16 2.2.3 Effect of polymer morphology and solvents……………………..…..17 2.3 Contact resistance of P3HT OTFT……………………………………...….18 2.4 Operation of organic thin filmtransistors…………………………….…….19. Chapter 3 Surface Treatment by Atmospheric-Pressure Plasma Technology for vi.

(22) Electrical Properties of OTFT……………………………………...30 3.1 Modification of oxide surface……………………………….……………..30 3.2 Introduction of APPT……………………………………….……….….….31 3.2.1 Introduction of plasma……………………………….…….…….…...31 3.2.2 Applications of APPT……………………………………….….….…32 3.2.3 Plasma surface modification………………………………..……...…34 3.3 Fabrication of OTFT………………………………………………..………36 3.4 Determination of thershold voltage and mobility………………………..…38 3.5 Results and discussions………………………………………….…………39 3.5.1 The influence of spin speed for OTFT………………………..…..….39 3.5.2 Electrical properties of APP surface treatment…………………...…..40 3.5.3 The other methods of HMDS-treated SiO2………………………..….42. Chapter 4 Surface Treatment by Atmospheric-Pressure Plasma Technology for P3HT Alignment…………………………………………………83 4.1 Introduction of P3HT alignment…………………………..……………….83 4.2 The methods to provide P3HT alignment…………………………………..84 4.2.1 Crystallization behavior of P3HT……………………………...….….84 4.2.2 XRD and UV-vis for highly oriented crystals of P3HT………….......85 4.3 Hysteresis……………………………………………………………...…...87 4.4 Anomalous leakage current……………………………………..………….89. Chapter 5 Conclusions and Future Work…………………………….…….….104 5.1 Conclusions……………………………………………………………….104 5.2 Future work…………………………………………………................….106 vii.

(23) 5.2.1 In-situ passivation layer for protecting the P3HT film……………...106 5.2.2 Novel method for depositing P3HT thin films………………….......106 5.2.3 Thermal stability of P3HT OTFT……………………..................….107 5.2.4 New gate insulator materials for P3HT OTFT……………………...107. Reference……………………………………………………………………...108. viii.

(24) Table Captions. Chapter 1 Table 1-1 The differences of TFT and OTFT materials and processes……….....6 Table 1-2 Carrier mobility of inorganic and organic materials ………………....6 Table 1-3 Highest field-effect mobility(u) values measured from OTFT as reported in the literature annually from 1986 through 2000…………………………………………………………….……8 Table1-4 The chemical structures and reported mobilities of representative classes of organic materials compared to those of inorganic silicon materials……………………………………………………………….9. Chapter 2 Table 2-1 Field-effect mobility and ON/OFF ratio of samples prepared from different solvents and process condition………………………...…25. Chapter 3 Table 3-1 The types of atmospheric pressure plasma…………………..………45 Table 3-2 Threshold voltage, saturation mobility, and on/off ratio at different spin-speed…………………….………………..…53 Table 3-3 Electrical parameters of the OTFTs in this study……………………62 Table 3-4 Comparison of contact angle and surface roughness with different scanning times by APPT………………….…..…......63 Table 3-5 The different methods of surface treatment……………………....….74 ix.

(25) Chapter 4 Table 4-1: Tm and ∆H of P3HT in Run 1 and Run 2 with difference pre-heating temperature…………………………….….95. x.

(26) Figure Captions. Chapter 1 Fig. 1-1 Semilogarithmic plot of the highest field-effect mobility……………....7 Fig. 1-2 The chemical structures of P3HT and BBL (Polymer)………………..10 Fig. 1-3 The chemical structures of pentacene and oligothiophenes…………...10. Chapter 2 Fig. 2-1 The structure of the polymer chain of P3HT……………………….…21 Fig. 2-2 Two different orientations of ordered P3HT (a) Edge-on orientation, (b) Face-on orientation………...................…22 Fig. 2-3 A polaron in polythiophene. Top: Change in the chemical structure. Bottom: Corresponding energy diagram……………………………....23 Fig. 2-4 (a) Charge carrier transport in conjugated polymers, (b) Charge transport mechanisms in solid…………………………….24 Fig. 2-5 Schematic of operation of organic thin film transistor, showing a lightly doped semiconductor: (a) No-bias, (b) Depletion mode, (c) Accumulation mode, (d) Non-uniform charge density, (e) Pinch-off of channel, (f) and (g) Growth of the depletion zone……………………..29. xi.

(27) Chapter 3 Fig. 3-1 (a) The structure of APPT, (b) The diagram of plasma surface Treatment…………………………………………………………..….46 Fig.3-2 (a) APP system of ITRI, (b) APP systems show the cold temperature………………………………………………………....…47 Fig. 3-3 Two basic structures of OTFT………………………...…………...….48 Fig. 3-4 Process flow of top-contact OTFT……………………................……49 Fig. 3-5 ID-VG for different spin-speed (a) 800 rpm, (b) 1500 rpm, (c) 2000 rpm (All are no treatment) OTFT with W/L = 2000 um/500 um…………………………...………............….51 Fig. 3-6 ID-VD for different spin-speed (a) 800 rpm, (b) 1500 rpm, (c) 2000 rpm (All are no treatment) OTFT with W/L = 2000 um/500 um……………………………………............…52 Fig. 3-7 SEM photo: cross-section view of test sample (upper layer is P3HT) made at spin-speed (a) 800 rpm, (b) 1500 rpm, (c) 2000 rpm………………………...………………....55 Fig. 3-8 Surface treatment of APPT HMDS 1 for OTFT (a) ID-VG curve, (b) IDVD curve (W/L = 2000 um/500 um)………...........................…........…56 Fig. 3-9 Surface treatment of APPT HMDS 2 for OTFT (a) ID-VG curve, (b) IDVD curve (W/L = 2000 um/500 um)………..……………………….....57 Fig. 3-10 Surface treatment of APPT HMDS 4 for OTFT (a) ID-VG curve, (b) IDVD curve (W/L = 2000 um/500 um)……...……………………….....58 Fig.3-11 Surface treatment of APPT HMDS 8 for OTFT (a) ID-VG curve, (b) IDVD curve (W/L = 2000 um/500 um)……….………………………...59 Fig. 3-12 The comparison of (a), (b) ID-VG and (c), (d) ID-VD with different xii.

(28) scanning times by APPT……………….………………………….…61 Fig. 3-13 Comparison of threshold voltage and saturation mobility with different scanning times by APPT………………..…………………………....62 Fig. 3-14 Contact angle vs. different scanning times by APPT……………..….63 Fig. 3-15 Contact angle vs. exposure time for dielectric layer with different scanning times by APPT……………………...……………………...64 Fig. 3-16 Comparison of contact angle and surface roughness with different scanning times by APPT………………...…………………………...64 Fig. 3-17 Comparison of surface roughness and mobility with different scanning times by APPT……………………………………………………….65 Fig. 3-18 Contact angle of (a) No treatment (<10°), (b) APP 1(68.9°), (c) APP 2 (76.3°), (d) APP 4 (90.5°), (e) APP 8 (90.9°)..................…68 Fig. 3-19 AFM photographs of (a) No treatment, (b) APP 1, (c) APP 2, (d) APP 4, (e) APP 8…………………………..............….71 Fig. 3-20 (a) ID-VG curve, (b) ID-VD curve with spin-coating HMDS for OTFT………………………………………………….…72 Fig. 3-21 (a) ID-VG curve, (b) ID-VD curve with evaporated HMDS for OTFT………………………………..…………………...73 Fig. 3-22 Comparison of threshold voltage and saturation mobility with different methods of surface treatment……………………………………...…75 Fig. 3-23 Contact angle of (a) spin-coating HMDS (65.49°), (b) Evaporated HMDS (75.28°)…………….…………………..…....76 Fig. 3-24 AFM photography of (a) Spin-coating, (b) Evaporated……………...77 Fig. 3-25 The comparison of (a), (b) ID-VG and (c), (d) ID-VD with different methods of surface treatment……………..………………………….79 Fig. 3-26 Comparison of contact angle and surface roughness with different xiii.

(29) methods of surface treatment………..…………………………..…..80 Fig. 3-27 Comparison of surface roughness and mobility with different methods of surface treatment……………………………………………….…80 Fig. 3-28 Comparison of contact angle and mobility with different methods of surface treatment and different scanning times by APPT………..…..81 Fig. 3-29 Comparison of contact angle and roughness with different methods of surface treatment and different scanning times by APPT………...….81 Fig. 3-30 Comparison of roughness and mobility with different methods of surface treatment and different scanning times by APPT…………....82. Chapter 4 Fig. 4-1 The molecular structure of P3HTs for High RR…………………...….90 Fig. 4-2 TGA thermograph of P3HT. 5 % weight loss is about 500…………………………....………………………….......91 Fig. 4-3 DSC thermograph of P3HT was pre-heating at the temperature of 70 for 3 min………………….………………………………………...…91 Fig. 4-4 DSC thermograph of P3HT was pre-heating at the temperature of 90 for 3 min…………………….…………………………………..……92 Fig. 4-5 DSC thermograph of P3HT was pre-heating at the temperature of 110 for 3 min………………………....................................................……92 Fig. 4-6 DSC thermograph of P3HT was pre-heating at the temperature of 130 for 3 min………………..………………………………………….….93 Fig. 4-7 DSC thermograph of P3HT was pre-heating at the temperature of 150 for 3 min…………………………………………………………....…93 Fig. 4-8 Tm and ∆H of P3HT in Run 1 with difference xiv.

(30) pre-heating temperature…………………………………....…………94 Fig. 4-9 Tm and ∆H of P3HT in Run 2 with difference pre-heating temperature…………………….………………...……….94 Fig. 4-10 X-ray analysis of deposition of P3HT on SiO2 dielectric layer with various surface treatments…………………………………………...96 Fig. 4-11 UV-vis absorption spectra of P3HT films that are deposited on SiO2 dielectric layers following various surface treatments, normalized to the maxima of the spectra…………………………………………....97 Fig. 4-12 A typical hysteresis curve……………………..………………….….98 Fig. 4-13 Hysteresis of P3HT OTFTs with no surface treatment……………....99 Fig. 4-14 Hysteresis of P3HT OTFTs with spin-coating surface treatment………………………………………………….…99 Fig. 4-15 Hysteresis of P3HT OTFTs with evaporation surface treatment…………………………………………………...100 Fig. 4-16 Hysteresis of P3HT OTFTs with APP 1 surface treatment…………………………………………………...100 Fig. 4-17 Hysteresis of P3HT OTFTs with APP 2 surface treatment…………………………………………………...101 Fig. 4-18 Hysteresis of P3HT OTFTs with APP 4 surface treatment…………………………………………………...101 Fig. 4-19 Hysteresis of P3HT OTFTs with APP 8 surface treatment……………………………………………….…..102 Fig. 4-20 ID versus VG for various surface treatment processes and the gate leakage currents in VG approaches 0 V………….........103 Fig. 4-21 ID versus VG for different numbers of APP scans and the gate leakage currents in VG approaches 0 V……........................103 xv.

(31) Chapter 1 Introduction. 1.1 General background and motivation Over the last few decades, Today’s microelectronics is based on the use of highly pure and high performance semiconductors like Si, Ge, GaAs, InP, etc. These materials can provide carrier mobility (u) in the order 103 cm2/Vs at room temperature, offer long lifetime, can be precisely doped and patterned with accuracy better than 100 nm. In this way, they profit at best of the device speed and manufacture complex systems on chip that are capable to receive, memorize, elaborate and transmit enormous quantity of information, making possible like PCs, mobile phones and almost every commercial products around our daily life. Organic thin-film transistors (OTFT) using organic semiconductors have attracted a great deal of interest for use in lightweight, low-cost, large-area and flexible electronic products such as flat-panel displays, sensors, smart cards, and radio-frequency identification (RFID) tags. OTFT are more compatible with polymeric substrates than conventional silicon-based transistors because they can fabricated with a low-temperature process. Therefore, OTFT on polymeric substrates have been developed to construct organic integrated circuits [1,2,3], electric papers, active-matrix liquid crystal displays (AMLCDs) [4,5], and active matrix organic electroluminescent displays [6]. The differences of TFT and OTFT materials and processes are shown in Table 1-1.. 1.

(32) Organic thin-film transistors (OTFT) based on conjugated polymers, oligomers, or other molecules have been envisioned as a viable alternative to more traditional, mainstream thin-film transistors (TFT) based on inorganic materials. Because of the relatively low mobility of the organic semiconductor layers, OTFT can’t rival the performance of field-effect transistors based on single-crystalline inorganic semiconductors, such as Si, Ge, GaAs, InP, which have charge carrier mobility about three orders of magnitude higher, such as Table 1-2 [7]. The performance of OTFT has steadily improved in the last two decades as a result of the development of new organic semiconductors, the optimization of deposition conditions and gate dielectric surface treatments [8,9,10,11].We presented a semilogarithmic plot of the highest yearly reported field-effect mobility value measured from thin-film transistors based on specific organic semiconductors, beginning in 1986. An update of that plot is shown in Figure 1-1, which is based on Table 1-3. Solution-processable conjugated polymers are among the most promising candidates for a cheap electronic and optoelectronic technology on plastic substrates. The technology that is believed to have the potential to produce the highest impact on manufacturing costs is the use of soluble organic semiconductor, both polymers and oligomers, combined with large area coating employed in OTFT is the fact that can be deposited using very low cost procedures such as spin-coating. This is the case of soluble polymers such as regioregular polythiophenes we used in experiments. Spin-coating procedures are also thermally compatible with plastic substrates, because they are carried out at the room temperature. Therefore, here we employ the poly-3-hexylthiophene (P3HT), solution 2.

(33) processable conjugated materials, as active layers in OTFT. Atmosphericpressure plasma technology (APPT) will be adopted to treat the surface of dielectric of OTFT and discuss the influence.. 1.2 Organic conjugated materials for OTFT Depending on the molecular weigh, organic conjugated materials used in OTFTs can be sorted into polymers and small molecules [12]. Table 1-4 shows the chemical structures and reported mobilities of representative classes of organic materials compared to those of inorganic silicon materials.. A. Polymers Conjugated polymers present the advantage of being amenable to specific deposition techniques that have been developed for conventional polymers. Their main drawback is that their performance is still lower than that of small molecules. Two polymers share the majority of the papers dealing with polymerbased OTFTsPoly(3-hexyl thiophene) (P3HT) for p-type and Poly (benzobisimidazobenzophenanthroline) (BBL) for n-type. The chemical structures of P3HT and BBL are shown in Fig. 1-2. We are mainly deal with the latter, which offers the highest mobility. After the pioneering work by Sirringhaus [13], it is now well established that the performance of polymer OTFTs crucially depends on the chemical and structural ordering of the polymer. High order first depends on the regioregularity of the polymer, the percentage of regioregular head-to-tail attachment of the alkyl side chains to the beta position of the thiophene rings. However, high regioregularity is not enough. The work by the Cambridge group 3.

(34) showed that two orientations could be found in P3HT films, one with the polymer chains flat on the surface, and the other one with the chains edge on. Highest mobility was 0.1 cm2/Vs, and the on/off ratio greater than 106.. B. Small Molecules Encouraging performance has been reported with small molecules, which currently offers higher mobility than hydrogenated amorphous silicon. However, high performance requires high ordering, particularly in the vicinity of the insulator-semiconductor interface, a constraint that may be difficult to fulfill when specific deposition methods are used. At present, practically all devices made of small molecules use pentacene. or oligothiophenes and their. derivatives (Fig. 1-3). Highest mobility now reaches 6 cm2/Vs for pentacene and 1 for sexithiophenes [14]. The solubility of organic semiconductors is vital for their use in low-cost electronic devices since the desired processing techniques include solutionbased methods like spin coating, dip coating, or printing techniques. However, practically all the small molecules used in OTFTs are insoluble, and need to undergo vapor deposition to form thin films. Solution processing has been reported with oligothiophenes end-substituted with alkyl groups [15,16], but these compounds require high temperatures of both the solvent and the substrate. An alternative strategy consists of using a soluble precursor that would convert into the desired molecule through a thermal post-processing step. This strategy has been used with polymers such as polyacetylene and poly-p-phenylenevinylene, and more recently, to pentacene. Mobility ranging from 0.3 to 0.9 cm2/Vs has been measured on OTFTs using the soluble precursor technique.. 4.

(35) 1.3 Thesis organization In chapter 1, we describe our background and motivation of our study. In chapter 2, we will introduce the characteristic of P3HT and methods for OTFT fabrication. In chapter 3, we adopt a new process, APPT, which can be operated under low temperature and atmospheric ambient. And APPT will make use of modify surface of dielectric layer SiO2 for our experiment. In addition, the other methods of HMDS surface treatment will also be utilized in our experiment. Finally, we compare the various methods of surface treatment and discuss the results. In chapter 4, we used DSCXRD and UV-vis to demonstrate that high mobility requires an ordered structure. And we also explain that the phenomenon of the hysteresis behavior and the anomalous leakage current of OTFT device. In chapter 5, we will describe the conclusions and the future works.. 5.

(36) Table 1-1: The differences of TFT and OTFT materials and processes. Materials. Processes. Process temperature. Cost. TFT. AmorphousPolySilicon. Like semiconductor process. High (200~400). High. OTFT. Small Molecular PolymerComplex. Printing process. Low (100). Low. Table 1-2: Carrier mobility of inorganic and organic materials.. 6.

(37) Figure 1-1: Semilogarithmic plot of the highest field-effect mobility (u) Reported for OTFT fabricated from the most promising polymeric and oligomeric semiconductors versus year from 1986 to 2000 [17].. 7.

(38) Table 1-3: Highest field-effect mobility(u) values measured from OTFT as reported in the literature annually from 1986 through 2000 [17].. 8.

(39) Table1-4: The chemical structures and reported mobilities of representative classes of organic materials compared to those of inorganic silicon materials. Semiconductor. Representative chemical structure. 300-900. Polysilicon. 50-100. Amorphous silicon. ~1. Pentacene. ~1. S. S. S. S. S. S. Dihexylanthra-. S. dithiophene. Regioregular Poly(3-hexyl thiophene). Organic-inorganic hybrid. (cm2/Vs). Silicon crystal Silicon. Dihexyl-sexithiophene. Mobility. 10-1 10-1. S. S. S S. S S. Phenethylamine-tin iodide. 9. 10-1. ~1.

(40) Figure 1-2: The chemical structures of P3HT and BBL (Polymer).. Sexithiophenes. Figure 1-3: The chemical structures of pentacene and oligothiophenes (Small molecules). 10.

(41) Chapter 2 Property of P3HT and Spin-Coating Technique. 2.1 Introduction of P3HT. 2.1.1. The molecular structure of P3HT. The field-effect mobility of P(3-hexylthiophene) P3HT is strongly influenced by the structure of the polymer chain and the direction of intermolecular - stacking. The structure of the polymer chain of P3HT is shown in Fig. 2-1. The 3-alkylsubstituents can be incorporated into a polymer chain with two different regioregularities: head to tail (HT) and head to head (HH) [18,19]. R represents the alkyl side chain (C6H13 for P3HT), which allows P3HT to be dissolved in 1 like chloroform. This solution processability enables simple film deposition. A regiorandom P3HT consists of both HH and HT 3hexylthiophene in a random pattern while a regioregular has only one kind of 3hexylthiophene, either HH and HT. This type of order is known as regioregularity and has been shown to give much higher field-effect mobility values over regiorandom material [20]. In our experiments, regioregular P3HT (HT regioregularity of 98.5%) and high grade solvent, chloroform, were purchased from Aldrich Chemical Company. A dramatic increase in mobility was observed relative to regiorandom poly-3-alkylthiophenes [21] when regioregular P3HT consisting of 98.5% head to tail (HT) linkages, so we did not perform further purification to these chemicals in our experiments. After being 11.

(42) deposited on the substrate, P3HT backbones may form two different morphologies, edge-on or face-on of lamella structure as shown in Fig. 2-2. The higher mobility is given by edge-on structure since the carriers can move more efficiently through intra-chain transport along the direction of - stacking. Two different methods are applied to deposit the P3HT film, one is spin-coating and while the other is dip-casting. The mobility of dip-coated films is usually higher than that of the spin-coating that’s maybe due to the evaporation rate of solvents. Lower evaporation rate results in a slower crystal growth with better ordered polymer structure [22,23]. In spite of that method provide the higher field effect mobility, the dip-coating method can not be applied for coverage of a large area. Therefore, in all of our experiments, we used spin-coating technique as a key process of organic layer deposition.. 2.1.2 Conduction mechanism Compared to the tremendous progress that the field of organic thin-film transistors has known during the past years, the theory of charge transport has hardly evolved. Basically, one can distinguish several families of charge transport models.. A. Hopping [24] This family pertains to disordered materials, such as polymers. In metals and conventional semiconductors, charge transport occurs in delocalized states, and is limited by the scattering of the carriers, mainly on phonons, that is, thermally induced lattice deformations. Such a model is no longer valid in low conductivity materials such as amorphous or organic semiconductors, where a 12.

(43) simple estimate shows that the mean free path of carriers would become lower than the mean atomic distance. In these materials, transport occurs by hopping of charges between localized states. A main difference between the delocalized and localized transport is that, in the former, the transport is limited by phonon scattering, whereas in the latter, it is phonon assisted. Accordingly, the charge mobility decreases with temperature in conventional semiconductors, the reverse being true in most organic materials. Several models have been developed to rationalize the hopping transport. In most cases, the temperature dependence of the mobility follows a law of the form µ = µ0 exp[-(T0/T)1/α], where α is an integer ranging from 1 to 4. The boundary between the localized and delocalized processes is usually taken at a mobility between 0.1 and 1 cm2/Vs. The mobility in highly ordered molecular crystals is close to that limit, so that there is still controversy as to whether the conductivity in these materials should be described by localized or delocalized transport.. B. The Small Polaron [24] Localization in conjugated organic materials occurs via the formation of polarons. A polaron results from the deformation of the conjugated chain under the action of the charge. In other words, in a conjugated molecule, a charge is self-trapped by the deformation it induces in the chain. This mechanism of selftrapping is often described through the creation of localized states in the gap between the valence and the conduction bands, as shown in Fig. 2-3 in the case of polythiophene [25]. The existence of such levels in doped conjugated polymers and oligomers has indeed been identified by UV-visible spectroscopy. A useful model to describe the charge transport in organic materials is that of the small polaron, developed by Holstein [26]. 13.

(44) C. Multiple Trapping and Release (MTR) In the multiple trapping and release model [27], a narrow delocalized band is associated with a high concentration of localized levels that act as traps. During their transit through the delocalized levels, the charge carriers interact with the localized levels through trapping and thermal release. The following assumptions are usually made: First, the carriers arriving at a trap are instantaneously trapped with a probability close to one. Second, the release of trapped carriers is controlled by a thermally activated process. The resulting drift mobility µD is related to the mobility µ0 in the delocalized band by an expression of the form in Equation: µD = µ0 α exp (-Et/kT) In the case of a single trapping level, Et corresponds to the distance between the trap level and the delocalized band edge, and α is the ratio of the effective density of states at the delocalized band edge to the concentration of traps. In the case of energy-distributed trap, effective values of Nt and α have to be calculated. The MTR model is currently the one most widely used to account for charge transport in amorphous silicon.. The weak intermolecular interaction forces in organic semiconductors, most usually van der Waals interactions with energies smaller than 10 Kcal mol-1, may be responsible for such small carriers mobility. In contrast, in inorganic semiconductors such as Si and Ge, the atoms are tied together with very strong covalent bonds, which for the case of Si have energies as high as 76 Kcal mol-1. In these semiconductors, charge carrier flows like highly delocalized plane 14.

(45) waves in the wide bands and have very high mobility. On the other hand, inorganic semiconductors usually have high order lattice structures and there are fewer traps than organic ones. This is another reason to explain the poor electrical characteristics of organic electronics. However, for conjugated organic materials, the polymer chains are weakly bound by van der Waals force. These polymer typically have narrow energy bands, highest occupied molecular orbit(HOMO) and lowest occupied molecular orbit(LOMO), which can easily be disrupted by disorder. Due to disorder structures, band transport is not applicable to organic semiconductors; in which carrier transport take place by hopping [23] between localized state like Fig. 2-4. Transport from one molecular to another is much more difficult due to a small energetic coupling between molecules held by weak van der Waals force of~10 10 Kcal mol-1. Another characteristic of organic material is that most polymers conduct one kind of carrier only, either electron or hole (P3HT is p-type that majority carriers are holes). Because of the nature of large band gap(e.q. Eg of P3HT = 2.2 eV), the active layer cannot be inversed by thermal energy at room temperature(i.e. slow generation rate of inversion layer). Therefore, OTFTs operate in the accumulation mode at it’s ON state and depletion at it’s OFF state. P3HT are semi-crystalline in nature, and their conduction mechanism is complex. The crystalline portion can conduct through intra-chain and inter-chain transport, whereas the amorphous portion conducts current through hopping processes.. 15.

(46) 2.2 Solution processed deposition. 2.2.1 Methods of OTFT fabrication There are four methods to form organic semiconductor film: (1) Solutionprocessed deposition, (2) Electro-polymerization, (3) Vacuum evaporation, (4) Langmuir-Blodgett Technique [28]. Recently, many researchers extensively use solution-processed deposition to fabricate organic semiconductor film. For solution-deposited organic semiconductor film, one kind of the organic semiconductor material such as poly (3-hexylthiophene) is dissolved in solvent such as chloroform. In our experiment, we use P3HT as the semiconductor because P3HT has many potential advantages for use the semiconductor layer in field-effect transistors. (1) P3HT is a well-known polymer as an organic semiconductor and has shown the effect mobility from 10-4 cm2/Vs in 1988 to 0.2 cm2/Vs in 2003 [13,29]. (2) P3HT has high solvent selectiveness, can dissolve in toluene, xylene, chloroform and so on. (3) P3HT is solution processed, therefore can be processed by spin-coating .. 2.2.2 The motive of spin-coating The organic semiconductors that exhibit the best mobility, ON/OFF current ratio, uniformity over large areas, and devices reproducibility have been deposited by vacuum sublimation. However the need for expensive vacuum chambers and lengthy pump-down cycles is unavoidable. Since the organic semiconductors have the relativity low mobility of organic semiconductors as described in chapter 1, OTFT cannot rival the performance of based on single 16.

(47) crystalline inorganic semiconductors, such as Si, Ge, and GaAs. However, the unique processing characteristics and demonstrated performance of OTFT suggest that they can be competitive candidates for existing or novel thin film transistor applications requiring large area coverage, structural flexibility, low temperature processing, and especially low cost. Some recent efforts in the field have focused on processes for solution deposition of small molecule [30] and polymers, as well as integration of these processes with other non-lithographic device fabrication technique [31]. To realize truly the advantages (i.e., processability and low cost) of organic materials in device applications, liquid phase processing technique by spin-coating is strongly desired. In all of our experiments, we used spin-coating technique as a key process of organic layer fabrication.. 2.2.3 Effect of polymer morphology and solvents The molecular structure of the P3HT greatly influences the charge carrier mobility and related current-voltage (I-V) characteristics of OTFT. A comparison study of P3AT (A = hexyl, octyl, dodecyl, hexadecy) with side chains ranging from butyl to decyl showed that field-effect mobility decreases with increasing chain length [32]. Under different processing conditions, the field effect mobility of OTFT is highly anisotropic. For example, Karl et al [33] observed that the field effect mobility was highly anisotropic, with the larger mobility along the direction in which the polymer chain axis aligned. The molecular structure obtained by using spin-coating films is usually lower than that of the cast films [18]. This is perhaps because in the cast films, 17.

(48) the rate of solvent evaporation is slower and has slower crystal growth, and hence better ordering, and large grain size. The choice of solvents and polymers has a very significant impact on the electrical characteristics of OTFT. In a recent publication, Bao et al [16]. Observed that when chloroform was used as a solvent to make poly -(3hexylthiophene)-based transistors, the field-effect mobility was 0.1 cm2Vs-1. However when Tetra hydrofuran (THF) was used as the solvent, the value of field-effect mobility is only 0.0006 cm2Vs-1. Table 2-1 shows the performance of various devices made from casting poly(3-hexylthiophene) films using different solvents with different process conditions [16]. Sirringhaus et al, [20] observed that the mobility could differ by a factor of 100 depending on the direction of - stacking in which efficient inter-chain transport is happened . The polymer solution we used is regioregular P3HT in chloroform with high purity. From Table 2-1, the mobility is typically in the range 10-3 which matches the result obtained in our experiment. In chapter 4, we will discuss the relation between the orientation of P3HT and carrier mobility.. 2.3 Contact resistance of P3HT OTFT There are many parameters will impact the performance of OTFT. The contact resistance between the source/drain electrodes and the organic semiconductor is an important one of them [34,35,36]. The contact resistance between the source/drain electrodes and the semiconductor becomes increasingly important to device performance. The contact resistance dominates the overall device resistance. Material of source/drain electrodes and the structure both affect the contact 18.

(49) characteristics. between. the. source/drain. electrode. and. the. organic. semiconductors. Unlike the FET of single-crystalline silicon, polycrystalline silicon, or hydrogenated amorphous silicon, the P3HT material cannot be optimized easily by semiconductor doping or silicide formation. Such properties of organic semiconductors deteriorate the performance of devices; moreover, the chemical compound always increase the contact resistance between the source/drain electrode and the organic semiconductor [37,38]. It is a straightforward method to find a suitable electrode material which forms ohmic contact with the organic active layer and thus to improve the performance of OTFT. P3HT can form an ohmic contact with material for its work function larger than 4.5eV because the work function of P3HT is 4.5eV. Work functions of all materials we used are larger than 4.5eV; they include Ni(4.5eV), Pt(5.29eV), and Cr(4.5eV).. 2.4 Operation of organic thin film transistors Refer to [39], the operation of the P3HT which bases on OTFT is described below. Organic thin-film transistors are opposed to the usual inversion mode operation of silicon MOSFETs and primarily operated as a P-type accumulation-mode enhancement type transistor. There are four basic modes which will be described later. Mode (I): When zero bias is applied to three electrodes of OTFT. The schematic diagram is shown in Fig. 2-5(a), it is called cut-off. If applied a small drain bias, Vd, and the source-current, Ids, will be small and ohmic. Mode (II): When a positive bias applied, the bend bending will occur in the interface between dielectric layer and semiconductor layer. Negative charges 19.

(50) will locate at interface and form the depletion region. The schematic diagram is shown in Fig. 2-5(b). The channel resistance is so large that the current will smaller than that of mode (I). Because of the large band gap, inversion layer cannot be observed in the organic thin-film transistor. Mode (III): When gate bias is negative, the schematic diagram is shown in Fig. 2-5(c), the voltage is dropped over the insulator and over the semiconductor near the interface between dielectric layer and semiconductor layer. More positive charges will be accumulated in the accumulate region. When a small bias is applied to drain, the source-drain current will be larger than that of Mode (I), the schematic diagram is shown in Fig. 2-5(d). Mode (IV): When drain voltage is negative enough that the voltage difference of gate and drain, Vgd, which is lower than Vth (<0), therefore, the depletion region will form near drain and pitch-off (Fig. 2-5(e)). If drain voltage is more negative, the depletion region will grow and approach source. The schematic diagram is shown in Fig. 2-5(f), (g).. 20.

(51) Figure 2-1: The structure of the polymer chain of P3HT.. 21.

(52) (a). (b). Figure 2-2: Two different orientations of ordered P3HT (a) Edge-on orientation, (b) Face-on orientation.. 22.

(53) Figure 2-3: A polaron in polythiophene. Top: Change in the chemical structure. Bottom: Corresponding energy diagram.. 23.

(54) (a). (b) Figure 2-4: (a) Charge carrier transport in conjugated polymers, (b) Charge transport mechanisms in solid.. 24.

(55) Table 2-1: Field-effect mobility and ON/OFF ratio of samples prepared from different solvents and process condition [16]. Condition 1, casting , vacuum pumped for 24 h; condition 2, spin-coated; condition 3, treated with NH3 for 10 h; condition 4, heated to 100 °C under N2 for 5 min; condition 5, heated to 150 °C under N2 for 35 min.. .. 25.

(56) Vg = Vs = Vd = 0.

(57). . . . . . . . . . . . . . . . . . . . . . . . . . . . . (a). Vs = Vd = 0, Vg > 0. . . .

(58) . . . . . . . . . . (b). 26.

(59) Vs = Vd = 0, Vg < 0. .

(60) . . . . . . . . . . (c). Vs =0, Vg < Vd < 0. . .

(61) . . . . . . .

(62) (d). 27. . . . . . . . . . .

(63) Vs =0, Vd < Vg < 0. .

(64) . . . . . . . .

(65)

(66) . . . . (e). Vs = Vg = 0, Vd < 0. . . . . .

(67) . . . . . . (f). 28. .

(68) Vs = Vg = 0, Vd << 0. .

(69). . . . . . . . . . . . . . . . . . . . . (g). Figure 2-5: Schematic of operation of organic thin film transistor, showing a lightly doped semiconductor: + indicates a positive charge in semiconductor; indicates a negatively charge in semiconductor. (a) No-bias, (b) Depletion mode, (c) Accumulation mode, (d) Non-uniform charge density, (e) Pinch-off of channel, (f) and (g) Growth of the depletion zone. 29.

(70) Chapter 3 Surface Treatment by Atmospheric-Pressure Plasma Technology for Electrical Properties of OTFT. 3.1 Modification of oxide surface The interface between an organic material and dielectric layer is a critical factor for device performance. This is because the surface of the dielectric strongly influences the quality of the dielectric/channel interface and the crystalline organic channel. The quality of the interface and the organic channel, as well as the electrical properties of the gate dielectric itself, play a major role in determining the device performance of an OTFT [40,41,42]. Although several methods have been recently proposed to improve the condition of the interface states, only a few have been proved to be reliable and robust. One of the proposed methods is the use of a self-assembly monolayer (SAM), such as octadecyltrichlorosilane (OTS) [43] and hexamethyldisilazane (HMDS) [44], have been extensively studied. A dielectric surface treatment with OTS is found to improve the mobility of OTFTs. Another dielectric surface treatment technique is O2 plasma cleaning and subsequent HMDS deposition on dielectrics [44]. A problem owing to O2 plasma cleaning, which is applied to remove residues generated from previous photolithography processes, was found to be the generation of a large number of trap states during the cleaning process by assisting OH termination at the SiO2 surface [45]. Although a HMDS layer subsequently applied is expected to reduce the number of traps and act as a SAM, the time-consuming wet processes 30.

(71) used to apply a SAM on the interface are unreliable and can cause other undesirable contaminations of the device. Surface treatments using an ion beam have been widely studied in other research fields. It is well known that ion implantation techniques can change the surface conditions or thin-film properties [46]. In the LCD fabrication process, for example, Ar ion beam treatment has been considered as a viable option as a surface treatment method to replace conventional contact-based treatment such as rubbing [47]. One of the advantages of Ar ion beam treatment is that because argon is an inert gas, it can clean the surface effectively without affecting the chemical structure of the dielectric layer.. 3.2 Introduction of APPT. 3.2.1 Introduction of plasma Plasma can be defined as a partially or wholly ionized gas with a roughly equal number of positively and negatively charged particles. Some scientists have dubbed plasma the "fourth state of matter" because while plasma is neither gas nor liquid, its properties are similar to those of both gases and liquids. There are two types of plasma - high temperature and low temperature. A good example of naturally occurring high temperature plasma is lightning. This type of plasma can be artificially generated using a high voltage, high temperature arc, which is the basis for the corona discharge process and for the plasma torch used to vaporize and redeposit metals. Low temperature plasmas, used in surface modification and organic cleaning, are ionized gases generated at pressures between 0.1 and 2 torr. These types of plasmas work within a vacuum 31.

(72) chamber where atmospheric gases have been evacuated typically below 0.1 torr. Low pressure allows for a relatively long free path of accelerated electrons and ions. Since the ions and neutral particles are at or near ambient temperatures and the long free path of electrons, which are at high temperature or electron volt levels, have relatively few collisions with molecules at this pressure the reaction remains at low temperature.. 3.2.2 Applications of APPT Plasma processes allow realizing a multitude of surface modifications. However, since these processes usually operate under vacuum, this makes them unsuitable for many industrial applications e.g. for large-area low price products. Therefore this has been occupied for several years with developing atmospheric pressure plasma processes for surface coating and treatment. Table 3-1 shows the type of atmospheric pressure plasma. Atmospheric pressure plasma is particularly suited for the large-area surface treatment of flat substrates (Fig. 3-1). This forms between two electrodes on application of an alternating current if at least one dielectric barrier or insulator obstructs the current. Gases are activated in these micro discharges by electronic excitation, ionization and dissociation to form very chemically reactive species. Thus the average gas temperature in the discharge gap rises only a few degrees Kelvin. Since the discharge in effect remains "cold" even temperature-sensitive substrates can be treated. Despite the filamentation of the discharges, with appropriate process control it is normal to achieve a very uniform surface treatment. Atmospheric pressure plasma-processes are used extensively in industry for 32.

(73) the activation of surfaces, the generation of ozone and for the electrical charging of particles. These processes are additionally used for cleaning, coating and functionalizing a variety of surfaces. Often ultra-thin coatings are deposited, which impart the desired properties to the surface. Examples of this are very thin Si-oxide coatings on steel which then enable corrosion protection properties to be conferred e.g. by phosphatization, or the placing of epoxide groups onto the surface, which are required e.g. in biotechnology. APP can also be successfully applied to the fine cleaning of steel surfaces, since organic contaminants are removed in the plasma. The atmospheric-pressure plasma technology (APPT) is useful for treating and modifying the surface properties of organic and inorganic materials. The APPT apparatus does not require any vacuum systems, produces high density plasma, and provides treatment of various substrates at low temperatures while operating open to the atmosphere. The plasma system has used for a wide variety of applications including treatment of polymer films, paper, wood, and foils; plasma grafting and plasma polymerization; ash various materials in the microelectronics industry; barrier layer deposition for the packaging industry; and sterilizing biologically contaminated materials. For polymer films, the technique offers the following advantages: •. Uniform treatment and No backside treatment.. •. Improved surface energy with concomitant improved wettability, printability, and adhesion. •. No additional vacuum system and low cost. •. Continuous fabrication availably and high speed for production 33.

(74) •. High plasma density. As shown in Fig. 3-2(a), we exhibited the atmospheric-pressure plasma system which was used in our experiment, and also showed the cold temperature atmospheric-pressure plasma systems in Fig. 3-2(b).. 3.2.3 Plasma surface modification Fig. 3-2 shows the mechanisms of plasma surface modification, glow discharge plasma is created by evacuating a reaction chamber and then refilling it with a low-pressure gas. The gas is then energized by one of the following types of energy: radio frequency, microwaves, and alternating or direct current. The energetic species in gas plasma include ions, electrons, radicals, metastables, and photons in the short-wave ultraviolet (UV) range. Surfaces in contact with the gas plasma are bombarded by these energetic species and their energy is transferred from the plasma to the solid. These energy transfers are dissipated within the solid by a variety of chemical and physical processes to result in a unique type of surface modification that reacts with surfaces in depths from several hundred angstroms to 10µm without changing the bulk properties of the material. A wide variety of parameters can greatly affect the physical characteristics of plasma and subsequently affect the surface chemistry obtained by plasma modification. Processing parameters, such as gas types, treatment power, treatment time and operating pressure, can be varied by the user; however system parameters, such as electrode location, reactor design, gas inlets and vacuum are set by the design of the plasma equipment. This broad range of. 34.

(75) parameters offers greater control over the plasma process than that offered by most high-energy radiation processes. Plasma treatment is aiming for various goals as for example: •. Improved adhesion. •. Removal of the "water skin". •. Activation of the substrate surface. •. Modification of the substrate surface. •. Cleaning of substrate surfaces. Since the organic film of OTFT is fabricated on to the dielectric layer under the influence of the physical and chemical interactions between organic and dielectric. layer,. the. OTFT. performance. strongly. depends. on. the. semiconductor/dielectric interface. The purpose of this work is to show the improvement of OTFT performance by controlling the surface treatments of dielectric/polymer interface. The surface properties such as frictional or abrasion, permeability, insulating properties, wettability and chemical reactivity are strongly dependent on a molecular aggregation state of the surface [48,49]. Therefore, the control of a molecular aggregation state in the film is important to construct a highly functionalized surface. One of the most effective ways of studying surface properties is contact angle measurement. The contact angle is the angle between the tangent to the drop’s profile and the tangent to the surface at the interaction of the vapor, the liquid, and the solid. The contact angle is an index of the wettability of the solid surface. A low contact angle between solid surface water-drop indicates that the surface is hydrophilic and has a high surface energy. On the contrary, a high contact angle means that the surface is hydrophobic and has a low surface energy. The surface free energy was 35.

(76) traditionally quantified by contact angle measurements [50,51]. In our work, we investigated the electrical properties of the OTFT which surface treatment by APPT. Hexamethyldisilasane ((CH3)3-Si-O-Si-(CH3)3) (HMDS) have already been widely used for oxide-based dielectric [52]. Oxide surfaces were treated with hexamethyldilazane to improve the adhesion between polymer chain and oxide surfaces. Modification of the substrate surface prior to deposition of regioregular P3HT has also been found to influence film morphology. For example, treatment of SiO2 with hexamethyldilazane (HMDS) or an alkyltrichlorosilane replaces the hydroxyl groups at the SiO2 surface with methyl or alkyl groups. The apolar nature of these groups apparently attracts the hexyl side chains of P3HT, favoring lamellae with an edge-on orientation [23]. According to [23], the mobility of OTFT with an edge-on orientation P3HT film is higher than the one with a face-on orientation. And so HMDS would be adopted in our experiment. We will discuss and analyze the effects of APPT surface treatments latter, and find the optimum parameters in our experiments.. 3.3 Fabrication of OTFT There are two kinds of basic structures which are adopted generally, bottom-contact (BC) and top-contact (TC) were shown in Fig. 3-3. Top-contact device is favorable compared to deposition onto prefabricated source and drain electrodes bottom-contact device, yielding mobilities that are typically larger by a factor of 2 [53,54]. First, an n-type bare silicon wafer was cleaned by the standard RCA cleaning process. After that, phosphorous atoms were diffused into an n-type 36.

(77) silicon wafer by POCl3 to form a common gate electrode. We used dilute to remove SiO2 after diffusing. Before the insulating layer of silicon dioxide was deposited, the n+ silicon wafer must be cleaned by the standard RCA cleaning again. An insulating layer of silicon dioxide was grown by thermal oxidation 5hr at 1000. The thickness of silicon dioxide was 2000A measured by n&k system. The wafers were taken to remove silicon dioxide of backside, and gate dielectric layer was formed. The “top-contact” OTFT structures were treated by different surface treatments before deposition of the P3HT active layer. The surface treatments were to control chemical and physical characteristics of surface by different ways. In our experiments, we adopted three methods of surface treatments and compared the difference of them. (1) Hexamethyldisilasane (HMDS) was deposited by spin-coating at 800 rpm for 3 sec as step one, 2000 rpm for 35 sec as step two, and baking at 150 for 30 min. (2) Evaporated HMDS at 150 (3) Atmospheric pressure plasma technology (APPT) was operated at 50W of plasma power, 0.1 sccm of He-gas flow, below 120, and various scanning times which are one, two, four, and eight times.. After finishing surface treatments, active layer P3HT was spun-coated at 1500 rpm 35sec and baked 130 for 3 min on hot plate. The P3HT (with headto-tail linkages greater than 98.5 %) and the high purity solvent (chloroform) used in this study were obtained from the Aldrich Chemical Company. The solutions of P3HT in chloroform were made with weight concentration of 0.3 %, and filtered through a 0.2 µm pore-size PTFE filter. Finally, deposition of S/D contacts was formed by sputter system, Ion Tech Microvac 450CB, and 37.

(78) patterned through the shadow mask. The thickness of Pt contacts was 600A. W (2000 um) is the channel width, L (500um) is the channel length. The process flow is shown in Fig. 3-4.. 3.4 Determination of threshold voltage and mobility The linear regime field effect mobility can be obtained by the calculation described below. At low VD, ID increases linearly with VD (linear regime) and is approximately determined by the following equation:. ID =. Wµ n C ox 2(VG + VT )V D − V 2 D 2L. [. ]. (3-1). where L is the channel length, W is the channel width, Cox is the capacitance per unit area of the insulating layer, VT is the threshold voltage, and is the field effect mobility, which can be calculated in the linear regime from the transconductance,. Gm =. ∂I D Z = µ n C oxVD ∂VG L. (3-2). by plotting ID versus VG at a constant low VD, with –VD <<-(VG - VT), and equating the value of the slope of this plot to Gm, then find Gm,max which can gain the value of threshold voltage (VT) and linear mobility. For the known values included Cox, VT, and W/L, the value of saturation mobility can be obtained from equation (3-3). 38.

(79) I D ( sat ) =. Wµ n C ox (VG + VT )2 2L. (3-3). 3.5 Results and discussions. 3.5.1 The influence of spin speed for OTFT In our experiment, we try to test the different spin-speed of active layer P3HT and discuss their influence. There are three different spin-speed which 800, 1500, and 2000 rpm are used in our experiment. The other detail process will be not repeated in this section. The electrical characteristics of OTFT were measured immediately in atmospheric ambient by using an HP4156 semiconductor parameter analyzer. A typical plot of drain current ID versus gate voltage VG at various drain voltage VD with the different spin-speed shown in Fig. 3-5. Gate voltage VG was swept from 0 volt to -40 volt, and drain voltage VD was -10 volt as a step volt from -20 volt to -50 volt over all our measurements. The plot will show absolute values of X-axis and Y-axis. From the data of ID-VG, the values were taken into Eq. (3-1) ~ (3.3). The threshold voltage and mobility in the saturation region would be calculated. All values are shown in Table 3-2. In Table 3-2, threshold voltage increase with increasing spin-speed. But the field-effect mobility is not with increasing spinspeed, 1500 rpm has the largest field-effect mobility. This result may be due to the different evaporation rate of the chloroform. Additionally, the film thickness measured by SEM is shown in Fig. 3-7. It shows that thickness is 61nm, 49nm, and 24nm for 800 rpm, 1500 rpm, and 2000 rpm respectively. Different thickness of active layer P3HT results various electrical performance. 39.

(80) To summarize these electrical characteristics, we would adopt 1500 rpm as our optimal parameter. Plot of drain current ID versus gate voltage VD at various drain voltage VG is also shown in Fig. 3-6 with the different spin-speed.. 3.5.2 Electrical properties of APP surface treatment Here we focus on the influence of APPT under different conditions which have different scanning times of APPT. They are one, two, four, eight scanning times respectively. We define that one time as APP 1, two times as APP 2 and so on. An absence of treatment is denoted APP 0. As shown from Fig. 3-8 to Fig. 3-11, plot of drain current ID versus gate voltage VG at various drain voltage VD and drain current ID versus gate voltage VD at various drain voltage VG with different scanning times. In all figures of APP 0 to APP 8, we can observe that APP 4 has best electrical characteristic about ID-VG and ID-VD. Additionally, we plotted the comparison of ID-VG and IDVD in the same figure due to observe clearly, they were shown in Fig. 3-12. The magnitude of saturation current at the same operated voltage shows that APP 4 > APP 8 > APP 2 > APP 1 > APP 0. Furthermore, threshold voltage and mobility would be calculated by Eq. (3-1) ~ (3-3). Arrangement of threshold voltage and mobility is shown in Table 3-3 (labeled as APP 0, APP 1, APP 2, APP 4, and APP 8) and Fig. 3-13. In Fig. 3-13, when dielectric layer was modified by the APP, it is clear that the threshold voltage reduction. This can prove that the dielectric layer / semiconductor interface really have improved. As for the device without surface treatment, the mobility in the saturation region and the threshold voltage of the OTFT are 1.9×10-3 cm2/Vs and -21.7V, respectively. On the other hand, the values of field-induced current at the same gate voltage for 40.

(81) APP 4 has almost ten times higher than without treatment, as shown in Table 3-3 and Fig. 3-13. After surface treatment, threshold voltage reduce down to -8.3V and field-effect mobility (µsat= 2.6×10-2cm2/Vs) which is 15-fold improvement over the mobility on bare silicon oxide. In order to further analyze the phenomenon about surface treatment of APPT, we used atomic force microscope (AFM) to observe the surface morphology. Contact angle was measured to judge the surface state. The contact angle and surface roughness of SiO2 with different scanning times of APPT, as shown in Table 3-4. The bare SiO2 surface is hydrophilic (contact angle10°). After surface treatment of APPT, the surface of SiO2 approach hydrophobic state. With increasing times of surface treatment, contact angle will present an increasing trend. When scanning times are more than four times, the increasing trend of contact angle will be flattened gradually, as shown in Fig. 3-14. In addition, Fig. 3-15 shows the trend of contact angle with increasing exposure time after surface treatment of APPT. It is found that the contact angle is not changed significantly for longer exposure time. It also reveals good stability for HMDS after APPT surface treatment. In the AFM photographs, the rms values of surface roughness increases with increasing scanning times of APPT. (Fig. 316) The rms value of surface roughness changes from 4.32 nm for APP 1 to 10.42 nm for APP 8. The surface roughness will become smooth after spincoating P3HT, as shown in Table 3-4. In general, most inorganic oxide surface including SiO2 shows hydrophilic state while most of organic semiconductor (P3HT in this case) shows hydrophobic states. This mismatch has bad influence on crystalline formation of organic semiconductor fabricated on oxide substrates [55]. After surface 41.