Organic Thin Film Transistor - Case Study Example

Organic Thin Film Transistor: Fabrication and Characterization Organic electronic devices, especially the organic thin-film transistors, have presented rapid progress and promise within the past few years. Organic thin-film transistors (OTFTs) now present characteristics comparable to devices made using hydrogenated amorphous silicon (a-Si: H). Pentacene, which is a fused-ring polycyclic aromatic hydrocarbon, presents relatively high carrier mobility when used for fabricating the organic thin-film transistors and this means that pentacene is widely used as the active material in such transistors. It is possible to use a variety of substrates and fabrication processes when using pentacene and the thin-film transistors made using pentacene present excellent electrical characteristics. In this work, we used different structures to fabricate pentacene–based OTFTs. Insulating layer for thin-film transistors fabricated for this work is spin-coated thin film of PMMA. Contents 1- Introduction 1 2- Background 3 3- Experimental details 20 4- Results and discussion 21 5- Conclusion 22 Appendix A – Details of Experiment 23 6-Bibliography / References 26 Introduction Thin-film transistors, or TFTs, are widely used in the electronic flat panel industry and every flat panel display contains millions of such transistors (Kagan, 2003, Chapter 1). The world is now using more flat panel displays than ever before and less of the conventional cathode ray display technologies, with many giant factories around the world churning out millions of TFT displays annually. In addition to flat panel displays, TFTs are find application in active matrix liquid crystal flat panel displays, active matrix all organic flat panel displays, active matrix imagers, large sensor arrays and smart tags for price and inventory applications (Klauk, 2000, Pp. 63 – 64). It has been more than six decades since the conceptualisation of the TFT and over time new materials, structures and fabrication techniques have emerged for these transistors. However, the monocrystalline materials, such as gallium arsenide (GaAs), that showed the greatest promise for fabrication of high-speed TFTs are expensive to source in the required purity and costly to process, rendering manufacturing of large-area devices manufactured using such materials prohibitively expensive (Kagan, 2003, Chapter 6). Thus, efforts persisted to try to use organic materials, such as polythiophene, oligothiophenes, pentacene and other small molecule organic materials, which were low cost, cheaper to process at room temperature and lightweight despite the fact that such materials could not offer fast switching and presented a large number of defects for a large area. Fortunately, the response time requirements for the human eye did not require the fastest switching times for TFTs for flat panel display applications. Most organic materials are electrical insulators with a relatively low conductivity at room temperature in the range of 10−9 - 10−14 S cm−1 (Kolb, 2005, Pp.1-6). However, certain organic solids, such as bromine/perylene complex exhibit conductivity in the range of 10−1 S cm−1 and this makes it possible to use such organic solids for fabricating TFTs. The performance of TFTs made using organic materials has improved over time with better fabrication techniques and design (Klauk, 2000, Pp. 63 – 64). Pentacene, (C22H14), which belongs to a family of polyacenes, presents an interesting option for use in the fabrication of organic TFTs because it presents a strong tendency for packing into parallel layers, with a capacity for standing on many substrates while presenting high career mobility (Kagan, 2003, Chapter 6, Section 6.4.2.3). In addition, this compound presents the highest hole and electron mobility of small organic molecules (Knipp, 2002, Pp. 347 – 348).Vacuum deposition techniques involving thin films of pentacene deposited on a variety of substrates presents high-performance TFT devices, with processing at relatively low temperatures being possible. Pentacene is a polycyclic aromatic hydrocarbon with five fused benzene rings, with its structure presented in the figure below (Pichierri, 2006, “Pentacene”). Researchers state that TFTs fabricated using pentacene present excellent electrical characteristics, with field effect mobility in excess of 1.5 cm /V-s, on/off current ratio larger than one hundred million, and subthreshold slope as low as1.6 V/dec (Klauk, 1999, Pp. 1258). Figure 1: Molecular Structure of Pentacene (Kagan, 2003, Section 6.4.2.3) Background Prior to discussing fabrication techniques for pentacene TFTs, it is probably right to examine the structure of a typical pentacene organic TFT, because this helps with the understanding for fabrication methods. The figure below presents a cross-section of a pentacene TFT on a glass substrate to illustrate the essential elements for such a transistor (Klauk, 1999, Pp. 1258). The drain, source, gate dielectric and electrodes as well as the active layer on a substrate are essential elements of all TFTs (Kagan, 2003, Chapter 1). In a TFT, the negative channel of an electron channel device, or the positive channel of a hole channel device is the source, with the opposite channel being the drain. Voltage applied at the gate controls conduction with a “pinch off” voltage shutting off conduction. Figure 2: Schematic cross-section of Pentacene Thin Film Transistor on a Glass Substrate (Klauk, 1999, Pp. 1258) Figure 3: An Idealised Thin Film Transistor (Kolb, 2005, Pp. 6) Kolb (2005, Pp. 14) suggests that it is possible to fabricate pentacene-based TFTs by a combination of thermal evaporation of pentacene to form the semiconductor layer on silicon substrate, deposition of dielectric by ion-beam sputtering and use of evaporation or electron – beam evaporation. Puigdollers (2003, Pp. 367 – 370) states that it is possible to grow pentacene films on substrates at room temperature under conditions of high vacuum in a chamber, with a base pressure of mbar. High deposition rates of greater than 20 Å / sec are possible using this technique. According to the previously mentioned author, it is possible to increase the conductivity of pentacine films substantially by dipping these films in a solution of iodine in acetonitrile (5 mgy300 ml) for short times (-1 h). They also carried out further research into the fabrication of pentacene TFTs with polymeric gate dielectrics with polymethyl methacrylate (PMMA) for gate dielectric using thermal evaporation at a maximum process of 170 ºC (Puigdollers, 2004, Pp. 67-71). Pentacene deposition at room temperature in a high vacuum chamber with base pressure of mbar produced TFTs that present the following characteristics (Puigdollers, 2004, Pp. 67 – 71): Figure 4: Drain – Source Current as a Function of Drain – Source Voltage, with Gate-Source Voltage as a Parameter (Puigdollers, 2004, Pp. 69) Figure 5: Drain – Source Current as a Function of Gate – Source Voltage, with Drain-Source Voltage set at -40V and ION=IOFF ratio from -20 to 20 V of over a thousand (Puigdollers, 2004, Pp. 69) Figure 6: Square Root of Drain – Source Current as a Function of Drain – Source Voltage, with Gate-Source Voltage in the Saturation Region (Puigdollers, 2004, Pp. 69) As mentioned earlier, it is important to note that for higher carrier mobility and superior TFT performance, a need exists for larger crystals with larger crystal boundaries (Puigdollers, 2004, Pp. 70 - 71). Deposition of pentacene at moderate temperatures of 60º C and at low deposition rates of 1 Å / sec are stated by the previously mentioned author as being capable of producing large crystal boundaries. In addition, because pentacene crystal grains grow around core of impurities on the surface exposed to deposition of pentacene, it helps to expose the dielectric surface for pentacene deposition. Thus, coating the gate dielectric for deposition of pentacene with monolayer of organic cyclohexene helps with larger crystals and higher carrier mobility. However, a need exists for a process optimisation phase that determines the optimal evaporation and process parameters needed for optimal pentacene grain size ( Puigdollers,2004). Hu (2005, Pp. 2260 – 2266) presents a discussion about deposition of pentacene films on different surfaces and the orientation and shape of the molecules deposited. He suggest that pentacene films deposited on different surfaces exhibit different morphologies, with pentacene grains deposited on Au being smaller and rod-shaped, while deposits on self-assembled monolayers (SAMs) are larger and more island like. Thus, it is clear that the choice of material for deposition of pentacene for making the TFTs is important because as mentioned previously, a requirement exists for large crystals and large crystal boundaries for high carrier mobility. Ordered molecular packing in pentacene crystals is crucial for high mobility because of the fewer trap sites present in the crystals. The degree of π – orbital overlap that exists between adjacent molecules facilitates the charge hopping process and the self-assembled monolayers of various terminal functional groups exert similar effects in aligning pentacene molecules perpendicular to the surface. Thus, microcontact printing techniques help control molecular orientation packing to form a template for area selective alignment. The previously mentioned technique is useful for fabrication of the vertical organic field effect transistor. Thus, the choice of the material for deposition of pentacene film is important. Kwon (2007, Pp. 615 – 617) presents a method for fabricating pentacene TFTs that uses solution based dielectrics. Transfer of pentacene deposited on an ultraviolet curable poly (urethaneacrylate) mould (PUA) to a substrate after exposure to ultra violet light of specific wavelength of 250 – 400 nm for a few tens of seconds with a dose of 100 mJ / square cm transfers the pentacene structure, that is the gate formation for the TFT, on to the substrate. After peeling off the mould without the pentacene deposited gate structure the TFT structure is ready, with the cured mould available for more transfer operations involving transfer of pentacene gate structures to substrates. Knipp (2003, Pp. 347 – 355) presents an investigation about the structural and transport properties of evaporated pentacene organic thin film transistors (TFTs) for deposition conditions and the effect of different organic dielectrics. The previously mentioned authors noted that the roughness of the dielectric used for deposition influences the structure and morphology of the deposited pentacene film. Films on specially prepared smooth silicon nitride present improved structural properties for the pentacene film. Changing surface wetting to present a more hydrophobic surface, results in mobility improvements by factors of 2 – 3. It is important to have low-cost substrates for low-cost electronic TFTs and the previously mentioned authors state that sputtered Al2O3, sputtered silicon oxide or plasma-enhanced chemical vapour deposited (PECVD), silicon nitride, and silicon oxide are examples of dielectrics that demonstrate a capacity for large area fabrication with high carrier mobility. The fabrication process for PECVD silicon nitride is already established and well known, with TFTs produced using PECVD silicon nitride presenting 0.4 cm2/Vs and on/off ratios of more than eight orders of magnitude. It is clear from the previous discussion that a number of fabrication techniques exist for manufacturing pentacene TFTs. Research has already established that organic TFTs present advantages over TFTs based on inorganic semiconductors because it is possible to fabricate organic TFTs at lower temperatures and significantly lower costs compared to TFTs based on inorganic semiconductors (Klauk, 2000, Pp. 63). The literature review demonstrates that by judiciously selecting the material on which deposition will take place, the fabrication process and the fabrication parameters, it is possible to fabricate a wide variety of TFTs and ICs using the TFTs. Because pentacene film TFTs are cheaper and less complicated to fabricate and the TFT technology has the capacity for contributing to the manufacturing of complicated electronic devices, it is very likely that the organic and pentacene thin film technology will continue to play an important role in the future of electronics and technology. In nanofabrication and nanoprocessing, the types of resists and materials used decide about the success or failure (Wiederrecht, 2010, Pp. 131). Poly(methyl methacrylate) (PMMA) is one of the longest used resists for nanofabrication and it is still amongst the highest resolution materials in use for electron beam lithography. In addition, PMMA is widely used as a gate dielectric in organic thin film transistors. The thermal and mechanical stability presented by PMMA together with its high resistivity (> Ω cm), suitable dielectric constant and a capacity for thin film processing on large areas by spin coating make it an ideal candidate as a dielectric layer in organic electronics (Na, 2006, Pp. 205 – 206). The molecular structure of PMMA is in the figure below. Figure 7: Molecular Structure of PMMA, from (Na, 2006, Pp. 206) Researchers have reported that TFTs fabricated using PMMA as gate dielectric showed better electrical characteristics than those fabricated using silicon dioxide (Na, 2006, Pp. 206). According to the previously mentioned author, a PMMA surface favours the formation of larger crystalline grains and this leads to better field effect mobility and PMMA films obtained by spin coating showed no pits and pinholes on the surface. The polymer PMMA contains hydrophobic methyl radical groups that act as moisture inhibitors to prolong the life of the TFTs fabricated using PMMA as a gate dielectric (Yun, 2009, Pp. 105). According to the previously mentioned author, TFTs produced using spin coating of PMMA presented field effect mobility values lower than 0.1-cm2 V−1 s−1 with low on / off current ratios ( Read More