Chapter 1 Introduction
1.1 History & Application of OTFTs
Chapter 1 Introduction
1.1 History & Application of OTFTs
Semiconductors have been widely used in various electronic devices. In addition, the invention of semiconductor transistor took replacement of traditional vacuum tube based electronics. Silicon is the dominate material in the use of high performance electronic chip from the end of 20th century and lead to the omnipresence of semiconductor microelectronics in our daily life. Organic semiconductor materials have been widely studied for the expectation of new applications, such as flexible light sources [1-5], flexible display [6-11], flexible electronics [12-14], and plastic solar cells [15-17]. In recent years, Flat panel displays (FPD) are growing quickly because flat panel displays are lighter, much thinner, and less power than traditional cathode ray tube (CRT). The development of flat panel display has made great progress in resolution, brightness, contrast ratio, viewing angle, response time, weight and so on. FPD are widely used in mobile phone, notebook, camera, and television and become more and more important for our life. Flexible displays are considered as the revolutionary product because it could be applied for e-paper, e-book, and large area screen which would decrease the use of woods, keep environment, and make our life colorful. Therefore, many national companies focus their investigation and development on the flexible display such as PVI, AUO, Fujitsu, Samsung Electronics, Sony, LG, and so forth. Figure 1-1 showed the pictures of the flexible display announced by LG. Recently, Sony Corporation announced that they developed a super-flexible 80 μm-thick, 4.1-in, 121 ppi, and OTFT-driven full color OLED display which can be wrapped around a thin cylinde at SID (Society for Information Display) 2010 International Symposium on May 27. Flexible display announced by Sony
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Corporation was shown in Figure 1-2.
Organic thin film transistors (OTFTs) play an important role of flexible display and have been widely studied by many researchers. Pixel of active matrix organic light-emitting displays (AM-OLED) was shown in Figure 1-3. OTFTs could be used to charge the capacitor and drive the OLED in the pixel of AM-OLED and as a key component. In addition, OTFTs were also applied for flexible electronics, including ratio frequency identification smart card (RFID) and sensors such as pressure sensor, gas sensor, bio-sensor, and electronic artificial skin [18-23], shown in the Figure 1-4.
In the following sections, we would introduce the operation theory, fabrication methods, and major issues of OTFTs. Also, atmospheric pressure plasma jet (APPJ) with many advantages, applied for OTFTs fabrication would be introduced in the following sections, simultaneously.
1-1 Operation Theory of OTFTs
The structures of OTFTs mainly included two types of top contact (TC) and bottom contact (BC) [24-27], respectively, shown in Figure 1-5 (a) and (b). The structures of OTFTs have several variants. In general, an OTFT is composed of four parts – a metal electrode, an insulator, a thin channel layer, and the sourc/drain metal contacts. Each of these structures has its advantages and drawbacks. In the BC structure, source-drain electrodes deposited on insulator could be patterned by lithographic technology to get higher device density. However, source-drain electrodes could not be defined by lithographic technology in TC structure. This is because the organic semiconductor materials are much easier influenced by oxygen, water, organic solvent [28-29], and so on. Therefore, most TC structures need to pattern the source-drain electrodes by shadow mask but then decrease the densities of OTFT devices. In addition, TC
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structures with lower contact resistance than BC structure could increase the mobility of OTFTs due to its channel material with better molecule ordering and larger grain size.
Transfer characteristics (ID-VG) and output characteristics (ID-VD) of OTFTs are shown in the Figure 1-6 (a) and (b). These curves were measured on the pentacene-based OTFTs made with gold source and drain electrodes. The chemical structure of pentacene was also inserted in Figure 1-6 (a). Figure 1-7 shows the energy scheme which indicates the respective positions of the Fermi level of gold and the frontier orbitals (highest occupied molecular orbital, HOMO, and lowest unoccupied molecular orbital, LUMO) of pentacene. The LUMO level of pentacene is quite far away from the Fermi level of gold, so there is a substantial energy barrier for electrons. Therefore, electron injection is very difficult when a positive voltage is applied to the gate and source is connected with ground. In contrast, holes can be injected to the semiconductor from the source by applying a reverse gate voltage, since the Fermi level of gold is close to the HOMO level of pentacene. With increasing reverse gate voltage, the hole conductive channel forms at the interface between insulator and semiconductor. The hole charge could be driven from source to drain by applying negative voltage to the drain. It is a reason that pentacene is call as a p-type semiconductor. However, this concept is obviously different to the common p-type or n-type materials doped by implanting. In another words, when the source and drain electrodes can inject electrons into its LUMO level, an organic semiconductor will be a n-type material.
Generally, the gate voltage of OTFTs could induce the equal charges at both sides of gate insulator. These injected charge carriers increase as applying a reverse gate voltage and then generate a conductive channel with high conductance.. At low drain voltage, the current almost follows Ohm’s law especially using high work function
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source-drain metal material. Therefore, OTFTs are mainly controlled by gate and drain voltages. As the drain current approaches to the gate voltage, the channel gradually becomes pinch off. Then, the channel current becomes independent to drain bias which is called saturation region. The mobility of OTFTs was extracted in saturation region from the following equation:
( )2 voltage that takes into account various potential drops through the gate-insulator- semiconductor structure. In the saturation region, μ can be calculated from the slope of the plot of |ID|1/2 versus VG.
1-2 Major Issues for OTFT Development
Organic thin film transistor are expected to be applied for flexible display. In recent years, although the performance of OTFTs have great improvement, there are still some issues influencing the development of OTFTs. These issues of OTFTs mainly include low mobility, stability, and high operation voltage.
The mobility of organic thin film transistors is only about 0.001~10 cm2/V-s, much lower than silicon-base MOSFET and polysilicon TFT. The mobility of OTFTs is dominated by organic material and ordering of organic material [30-32]. On the other hand, stability of organic semiconductor material is a serious problem. Electrical characteristics of OTFTs would be degraded by oxygen, moisture, and organic solvent
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and so on [28, 29]. Some researchers believed that oxygen would diffuse into active layer and formed impurities in the organic semiconductor layer but these studies resulting from environment influence are not very clear. These environment factors may lead to the degradations of on current, threshold voltage shift, and subthreshold swing. It is well known that lithography processes such as organic solvent, photoresistor, water, exposure, and baking may degrade the performance of OTFTs.
Therefore, there are many researchers focus on fabrication of capping layer [33, 34]
and some of studies endeavor to overcome the issue of active layer tied to lithography processes[35, 37], which are important for the improvement on device performance and circuit density.
However, another pending problem of OTFTs is high operation voltage [38-41].
The high operation voltage would result in high active power consumption, not suitable for the low-power protable electronics. Active power consumption is on current multiplied by operation voltage. However, we could not decrease the on current because higher on current could increase operation speed to overcome RC time delay. Therefore, decreasing operation voltage is urgent for OTFTs. In order to keep the magnitude of on current and decrease operation voltage, increasing the capacitance per unit area is the one of the ways to obtain the goal. Capacitance is positive proportional to dielectric constant and negative proportional to thickness of insulator. Therefore, many studies used high-k material or thinner insulator as the gate insulator to decrease the operational voltage [11-16]. Gate insulator materials mainly consist of polymer, metal oxide, and nanocomposite. Polymer is suitable for low cost processes such as spin, inject, and print. However, the drawbacks of polymer include lower dielectric constant, longer baking time, and more holes pin-holes. Metal oxide materials usually have high dielectric constant such as HfO2, Ta2O5, TiO2, Al2O3, AlN, Si3N4 [42-45] and so on but these material often fabricated by CVD or PVD which is
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high cost and low throughput processes for flexible electronics. Metal oxide material usually need a high temperature anneal to improve quality which is not suitable for plastic substrate. Plastic substrate usually could not sustain over 200 oC and the upper processing temperature of different kinds flexible substrate are shown in Figure 1-8.
1-4 Overview of Dissertation
Organic thin film transistors attracted many researchers and company to investigate and develop due to their future applications. Some main issues including low mobility, stability, and high operation voltage had been described above. In our work, we focus on developing and investigating low temperature and low cost processes to fabricate high quality silicon oxide as a gate insulator for OTFTs. Silicon dioxide is a kind of very cheap and rich resource in the world and used as main gate insulator for MOSFET. Band gap of silicon dioxide is about 9 eV which is good to be used as a gate insulator. In addition, good stability and reliability of silicon oxide were also proposed in many researches. The highest process temperature of OTFTs usually happened in gate insulator. So decreasing the processes temperature of gate insulator is very important for OTFTs due to the plastic substrate could not sustain high temperature. For the purpose of low cost and high throughput processes, atmospheric pressure plasma jet (APPJ) was utilized in this thesis. We controlled our processes temperature from room temperature to 200 oC. There are lot of advantages of APPJ such as low temperature, low cost, and high throughput.
Figure 1-9 shows the basic structure of atmospheric pressure plasma jet. Because APPJ could be operated in atmospheric pressure it is suitable for large area application which is very important for cost down.. Good quality of silicon oxide deposited by APPJ had been developed by us and leakage current density was
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suppressed below 1~2E-8 A/cm2 at 0.5 MV/cm in MIM structure. We controlled temperature, flow rate, gap distance, and main gas to improve the performance of silicon oxide which were all described in chapter 2. In addition, silicon oxide deposited with APPJ was successfully applied for the gate insulator of OTFTs. The thickness of our demonstrated silicon oxide is only 9 nm so the operation voltage of OTFTs was about -2 V. Threshold voltage is about 0.8 V, mobility is about 0.66 cm2/V-s, and subthreshold swing is about 0.6 V/decade in our proposed OTFTs. The detail fabrication and discussion of OTFTs were described in chapter 3. We found that higher molecular ordering of organic semiconductor would influence the leakage current of OTFTs. Many studies used surface treatment to improve the molecular ordering of channel layer and increase the mobility of OTFTs. However, leakage current of OTFTs was also increased after improving molecular ordering and this drawback was ignored. We used to contact structure of OTFTs to investigate and discuss this problem in this chapter 4.
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Figure 1-1 Flexible displays announced by LG.
Figure 1-2 Photo of OTFT-driven OLED display wrapped around a cylinder with 4 mm radius. (Sony Corp.)
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Figure 1-3 2T-1C OTFT driven OLED pixel design
Figure 1-4 Electronic artificial skin integrated pressure and temperature sensors.
Made by Prof. Takao Someya, University of Tokyo in Ref. [22].
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Source Drain
Active layer
Substrate Gate
Gate insulator
(a)
Substrate
Source Active layer Drain
Gate
Gate insulator
(b)
Figure 1-5 Top contact structure (a) and Bottom structure (b) of OTFTs
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(a)
(b)
Figure 1-6 Output (a) and transfer (b) characteristics of a typical OTFT. The inset shows the molecular structure of pentacene, which serves as semiconductor in the device.
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Figure 1-7 Schematic band diagram of gold and the energy of the frontier orbitals of pentacene. Data taken from Ref. 22.
Figure 1-8 Upper processing temperature of flexible substrate.
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AC Main Gas
Flow
Ar carrier gas
TEOS
Silicon wafer Hot
plate
Gap Distance
Figure 1-9 Schematic of the atmospheric-pressure plasma jet for the deposition of silicon oxide.
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