Emmanuel S. Yakubu, MS December, 2020 PHYSICS
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Emmanuel S. Yakubu, M.S. December, 2020 PHYSICS MODELING AND FABRICATION OF AN ACTIVE MATRIX DISPLAY (0 pp.) Director of Dissertation: In this thesis, the use of a specific type of Thin-Film Transistor (TFT), namely Organic Field Effect Transistor (OFET), in a single-pixel circuit to produce a flexible display is studied. The goal is to characterize and optimize organic thin-film tran- sistors in an Active-Matrix Organic Light-Emitting Diode (AM-OLED) single-pixel circuit in order to get an optimized setup of a Flat-Panel Display (FPD). We use LT Simulation Program with Integrated Circuit Emphasis (LT-Spice) to simulate the driving circuit of an individual pixel to find target values of mobility and resistance. Finally, a small 2x2 pixel array based on the data produced from the simulation is produced as first step toward a full-size flat panel operation. MODELING AND FABRICATION OF AN ACTIVE MATRIX DISPLAY A dissertation submitted to Kent State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy by Emmanuel S. Yakubu December, 2020 c Copyright All rights reserved Except for previously published materials Dissertation written by Emmanuel S. Yakubu Approved by Dr. Bj¨orn L¨ussem , Advisor Dr. James T. Gleeson , Chair, Department of Physics Dr. Mandy Munro-Stasiuk , Interim Dean, College of Arts and Sciences TABLE OF CONTENTS TABLE OF CONTENTS . iv LIST OF FIGURES . vi LIST OF TABLES . viii 1 Introduction . 1 2 Background . 4 2.1 Organic Field Effect Transistor (OFET) . 4 2.2 Organic Light Emitting Diode (OLED) . 8 2.3 Influence of Mobility µ .......................... 9 2.4 Acknowledging Contact Resistance Rc . 13 2.5 Flat-Panel Display . 15 2.5.1 Passive Matrix . 15 2.5.2 Active Matrix . 18 3 SPICE Simulations . 22 3.1 Description of Simulation Model . 22 3.2 Influence of the Mobility, Resistance, and Capacitance . 23 iv 4 Standard OFET . 30 4.1 Structure of Device . 30 4.2 Results . 31 5 OFET in a Backplane . 36 5.1 Modelling . 36 5.2 Results . 36 6 Conclusion . 42 6.1 Outline . 42 v LIST OF FIGURES 2.1 Illustration of an electrolyte gated organic field-effect transistor with pdEthAA deposited on P3HT layer [3]. 5 2.2 Standard Structure of OFET [17]. 5 2.3 Output Characteristic denoting the Linear and Saturation regions of a Transistor . 7 2.4 Structure of a typical OLED . 9 2.5 Operation framework of OLED . 10 2.6 Active Matrix (left) and Passive Matrix (right) Addressing [18]. 16 2.7 Example of an AM-OLED Single-Pixel Circuit [9] . 21 3.1 AM-OLED Single-pixel Simulation Schematic . 24 3.2 LT-Spice syntax for AM-OLED FPD simulation . 24 3.3 Voltage Pulse at Gate of Switching Transistor (Top, V(vg1)), Current driven into OLED (mid, I(D1)), and Voltage Pulse at Gate of Driving Transistor (bottom, V(vg2)) . 25 3.4 Voltage Pulse at Gate of Driving Transistor with varying Resistance . 27 3.5 Voltage Pulse at Gate of Driving Transistor with varying Mobility . 28 3.6 Voltage Pulse at Gate of Driving Transistor with varying Capacitance 29 4.1 Lateral view of OFET . 30 4.2 Fabricated OFET with the highlighted Drain-Source-Gate electrodes 32 vi 4.3 Voltage Transfer Characteristic of OFET . 33 4.4 Error Plots for Mobility between Au and doped P5 . 34 4.5 Error Plots for Threshold Voltage between Au and doped C60 . 35 4.6 Error Plots for Hysteresis between Au and Al . 35 5.1 Shadow Masks . 37 5.2 Resulting Substrate . 38 5.3 Voltage Characteristic of Backplane OTFT . 39 5.4 Linear fit of The Square Root of the Drain Current . 40 5.5 Fabricated transistor vs. Mask plan . 41 vii LIST OF TABLES 3.1 Varied Resistance and KP and Their Respective Time Constants . 26 viii CHAPTER 1 Introduction Semiconductors and their devices have served as integral components of circuitry found in almost all electrical devices today. Ever since 1874, when Ferdinand Braun firstly discovered the rectification properties of a point contact on the semiconductor lead sulfide, our reliance on semiconductors and its devices has grown exponentially. Semiconductors originally existed as solely inorganic. The semiconductor layer is typically silicon (crystalline silicon being the most prominent form of silicon in tran- sistors) with transparent oxides due to their bandgaps being greater than 3eV [25]. Mobility, being the most important factor to focus on, can be improved by control- ling the morphology of the semiconductor from either amorphous, nanocrystalline, microcrystalline, polycrystalline, to single crystalline Silicon [25]. Inorganic semiconductors show a very high performance caused by their high charge carrier mobility. Organic semiconductors, however, outperform inorganic de- vices when criteria such as low-cost production and mechanical flexibility are re- quired. Organic semiconductors are currently used in solar cells that pave a way towards renewable energy, Organic Light-Emitting Diodes (OLEDs) illuminating the screens of our mobile phones, Television sets and so on. The focus of this thesis shall be the Organic Field-Effect Transistor (OFET) which has a strong footing in imaging sensor and display electronics [16]. 1 The OFET provides benefits such as low-cost manufacturing, flexibility, and tun- ability. Research in this field is very widespread and more advantages of this type of device are being studied. The application of an OFET for this paper is the operating of flexible displays in backplane electronics. Flexible displays have come a long way too. There are two main types of backplane architecture, namely; direct addressing, which is usually applied in traffic lights, and matrix addressing which is also divided into two sub- sets named passive matrix and active matrix addressing [9]. Passive matrix (PM) examples include pagers, classic printers, calculators, et cetera. This type of matrix involves the pixels in all rows and columns turning on at once while aligned in an orthogonal orientation (cf. Figure 2.6). PM displays are currently being replaced by active matrix addressing with examples such as: smart watches, television sets, or even the media interface in modern vehicles [9]. The pixels for AM addressing are in a standard row and column arrangement. In contrast to PM addressing, every pixel is addressed by one or more Thin-film Transistors (TFT). Unlike the PM, where all pixels are switched on, AM activates the pixels one row at a time in a cyclic sequence (see figure 2.6)[4, 12, 5, 9]. Thin-film transistors have been quintessential to the operation and improvement of flexible displays. Traditionally, the dielectric and semiconductor layers are de- posited on glass substrates, but to achieve flexibility, plastic substrates are usually the way to go [7]. Metal-foil substrates are also recommended for high temperature 2 fabrication processes, which are typically needed to fabricate high performing tran- sistors. Nowadays, flexible displays have more than proven themselves in aspects of low cost manufacturing, mechanical flexibility, and high performance [7, 12]. Contact Resistance, a plague that hinders the performance of semiconductors can arise from numerous instances. Examples of causes may be mishandling of device during fabrication, exposure of sensitive layers to air, misalignment of the metal work function and the transport layer of the semiconductor, or even just incompatibility between the material layers of the device [20]. The aim of this thesis is to evaluate the use of organic field-effect transistors for active matrix displays. Target performance parameters for OFETs will be derived by a circuit-level SPICE simulations. High-performance OFETs will be processed and the influence of contact resistance will be evaluated to arrive at device properties that will enable an OLED/liquid crystal to produce a lucid display. 3 CHAPTER 2 Background The OFETs in this thesis are fabricated and characterized to switch-on (or off) and drive the OLEDs in the active matrix configuration to produce a low cost, flexible, display. 2.1 Organic Field Effect Transistor (OFET) OFETs are a fascinating candidate of the organic device family. The intramolec- ular bonds of organic semiconductors are van der Waals weak forces, which allow for low energy and low temperature processing [8]. A metal-oxide-semiconductor capacitor consisting of polyacetylene, polysiloxane, aluminum, and gold as the semiconductor, gate dielectric, gate, and source/drain electrodes respectively, as shown in figure 2.1, was first assembled by Ebisawa et al. in 1982[8]. The corresponding transistor operated in depletion mode with a low modulation and transconductance. Regardless, Ebisawa and Nara, working at NTT in 1982 at the time, deemed this new transistor "promising" [8]. This first report on OFETs was followed up in 1986 by Ando, Tsumura, and Koezuka at Mitsubishi Chemical [8], which achieved a substantial current boost by an in-situ poylmerized polythiophene transistor. The source and drain contacts are integral layers of an OFET. The junction 4 Figure 2.1: Illustration of an electrolyte gated organic field-effect transistor with pdEthAA deposited on P3HT layer [3]. Figure 2.2: Standard Structure of OFET [17]. 5 between them and the channel can cause a parasitic impedance due to the formation of an energy barrier [8]. This barrier, however, can be minimized when proper materials are chosen that balance the energy levels of the semiconductor with the work function of the metal of the source/drain electrodes [8]. The operational basis of FETs illustrated in 2.2 involve channelling a current from source to drain. The charge transport mechanism dictate if the OFET is a n-type (electron charge transport) or p-type (holes) device. Given that it is p-type, a negative voltage bias will be applied to the gate, hence creating a depletion region across the channel enabling charge transport from source to drain via the organic layer [8].