Low power organic field effect transistors and inverters with high dielectric constant (high-k) amorphous metal oxide

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Low power organic field effect transistors and inverters with high dielectric constant (high-k) amorphous metal oxide


Author: Wang, Zongrong
Title: Low power organic field effect transistors and inverters with high dielectric constant (high-k) amorphous metal oxide
Degree: Ph.D.
Year: 2014
Subject: Organic field-effect transistors.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Mechanical Engineering
Pages: xxv, 150 leaves : ill. ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2747300
URI: http://theses.lib.polyu.edu.hk/handle/200/7494
Abstract: Organic field effect transistors (OFETs) are critical building blocks in many new generations of organic electronic devices, such as memories, bio-sensors, flexible organic light emitting diodes (OLEDs) displays. The main advantages of OFETs are lightweight, low processing temperature, compatibility with low cost flexible substrates and more. To make them suitable for portable and wearable electronics, the reduction of operating power in flexible of OFETs are becoming more important. For OFETs, low operating power means sufficient charges can be accumulated at the semiconductor/dielectric interface to generate a conductive channel at a low voltage at gate and it can be achieved by introducing high capacitance gate dielectric insulator into the device. This thesis summarized the existing methods to achieve low operating power in OFETs, including high dielectric constant (high-k) inorganic dielectric and polymer dielectric, ultrathin dielectric with self-assemlely monolayers (SAMs). Although these areas have been rapidly developed since the last decade, a number of problems remain unresolved, such as high processing temperature for inorganic dielectric, instability of polymer under air and moisture, and surface specific properties in applying SAMs on the ultrathin oxide. A new kind of metal oxide high-k dielectric with low processing temperature are proposed and developed by different methods have been detailed studied in this thesis. We separately fabricated Ba₀.₇Sr₀.₃TiO₃ thin film by pulsed laser deposition (PLD) and Ba₀.₆₂Sr₀.₂₈TiO₃.₀₃ thin film by UV-Ozone processing method. Ba₀.₇Sr₀.₃TiO₃ (BST-PLD) thin films of different thickness are deposited by the PLD at low temperature at 110°C under vacuum environment. This is the lowest temperature of PLD applications for depositing Ba₀.₇Sr₀.₃TiO₃ thin film for OFET application that has ever been reported. OFETs devices based on BST-PLD thin films with different thicknesses are fabricated. It is found that the carrier mobility of pentacene OFET increases with the thickneses of the BST-PLD thin film. The highest mobility of 1.24cm²V⁻¹s⁻¹ can be obtained on 770nm thick BST-PLD film. Besides, we also investigated the flexibility of both the BST-PLD and the effect on the overall performance of OFETs. The details of the PLD processed BST is discussed in Chapter 3 of the thesis.
To further meet the needs of low cost and large area applications, sol-gel processed and solidified by UV-Ozone Ba₀.₆₂Sr₀.₂₈TiO₃.₀₃ (BST-UVO) was developed. This thin film is deposited by spin coating with Ba-Sr-Ti sol solution and solidified by UV-Ozone at room temperature in ambient atmosphere. Pentacene OFETs on both Si and polyethylene naphthalate (PEN) substrates based on BST-UVO thin film are fabricated. The technical detials of this method and the performance of devices are discussed in the Chapter 4. After succeeding in obtaining high quality solution-processed dielectric by the low temperature UV-Ozone processing, we take a step further to investigate the surface properties of these high-k films under different storage conditions and their applications in both solution and vacuum processed transistors. The surface energy of BST-UVO thin film is found to be decrease with storage time in ambient air and decreases even faster when stored in evaporation chamber with diffusion pump. like 6,13-Bis (triisopropylsilylethynyl) pentacene (TIPS-pentacene) OFETs fabricated by DPC method, a new solution processing method, on the high surface energy fresh BST-UVO surface can achieve an averaged mobility of 0.11cm²V⁻¹s⁻¹ . The longer storage time in the evaporation chamber, the smaller surface energy would be obtained due to the surface contamination. By separately loading the BST-UVO thin films into the evaporation chamber for 5 hours and hours respectively, the work of adhesion between the BST and [2,3-b:2′,3′-f]thieno[3,2-b] thiophene (DNTT) are calculated to be 86.8 and 71.02 mJ/m², which gives different OFET device performance due to different growth mode of the semiconductor. The OFET devices based on 5h stored BST-UVO showed layer-island growth mode and higher average mobility of 1.51cm²V⁻¹s⁻¹ . The OFET devices based on 24h stored BST-UVO showed island growth mode and lower average mobility of that of 1.12cm²V⁻¹s-1. Two kinds of logic inverters consisting of two DNTT transistors are fabricated on PEN substrate. The saturated load one showed full-swing performance and shows a high gain as 25 and the depleted showed the gain of 12. The detailed surface energy calculation and the device performances are disussed in the Chpater 5.

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