1.1 Background
Indium-tin-oxide (ITO) has been widely used as transparent electrodes in many optoelectronic applications, especially displays due to its high electrical conductivity combined with high transmission in the visible spectra. The use of ITO in display has quickly grown to become the primary end user of the world’s indium production as illustrated in Figure 1.1 [1].
ITO thin films that commonly were deposited on glass substrates have been used as the transparent conducting electrodes in many optoelectronic devices, such as flat panel displays and photovoltaic devices. This is because of their superior conductivity and high transparency [2-6]. However, glass substrates are unsuitable for certain applications where flexibility, light weight and toughness are needed because glass is brittle, heavy and not deformable. In order to overcome these disadvantages, flexible substrates, which are made of plastic, have been used instead of glass [7-11].
Recently, flexible microelectronic devices such as flexible organic emitting diode (FOLED), e-paper, flexible solar cell and liquid crystal display (LCD) have
Figure 1.1 The world-wide indium consumption. The majority of indium used in thin film application is the display industry [1]
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emerged as popular personal devices [12,13] for its light weight, toughness, and mobility. ITO films on polymer substrates have been widely used as transparent electrodes in these devices [12,13].
Figure 1.2 Example of flexible photovoltaic display [reference]
However, polymers pose three distinct challenges. First, ITO is often annealed at 200-250 oC to achieve both low resistivity and high transparency, but low Tg (glass transition temperature) of common polymer substrates, e.g. polyethylene terephthalate (PET) constrains the maximum processing temperature in the fabrication of flexible devices. Second, the properties of ITO films on polymer tend to be worse due to low processing temperature or poor nucleation [14]. Thirdly, ITO thin films on polymer substrate also show mechanical limitation. Currently, there are several polymer substrate candidates suitable for ITO deposition at or near desired high temperature, 300 oC. However, along with the temperature capabilities of the polymer substrate, its shrinkage or expansion with temperature, i.e. coefficient of thermal expansion (CTE) must be considered for its suitability and compatibility of use with ITO [15] and other transparent oxide in optoelectronic devices. When deposited onto flexible polymer substrates, cracking, delamination, or buckling of ITO have been reported.
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Thus, it is necessary to develop a low thermal budget annealing technique to improve the properties of ITO films deposited onto the polymeric substrate. It has been shown that excimer laser annealing (ELA) technique is powerful technique as a post treatment method for flexible substrate with several advantages [15]. Firstly, ELA technique takes only 25 ns to render ITO phase transformation, whose time scale is ten orders of magnitude quicker than the conventional furnace annealing. Secondly, ELA offers much shorter thermal diffusion length and thus enables post-anneal of flexible devices at room temperature.
In hope to address the challenges of ITO on polymers substrates from the point view of ELA method, the objectives of this thesis are the following:
1. To establish low temperature laser annealing technology of transparent conducting oxide for flexible device applications
2. To investigate the effect of ELA on the properties of ITO onto polymer substrates
3. To study the effect of polymer substrate on the optical bandgap of ITO films In specific, a KrF excimer laser was used to treat the ITO films sputtered onto polyethylene terephthalate (PET) and polyimide (PI) substrates. The microstructure, electrical, optical, and mechanical properties of ELA-treated ITO films were examined. Excimer laser parameters were carefully optimized to ensure the resistivity, carrier concentration as well as transmittance of ITO films are comparable to those deposited at high temperature (>200oC) on the glass substrate. Besides that, the mismatch of CTE and elastic modules between ITO film and polymers may induce thermal stress at the interface. We plan to explore the impact of the thermal stress from the mismatch of CTE and E on the ITO properties such as optical bandgap and
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xxx. Moreover, the substrate effect on ITO films with different thickness and deposited on different polymeric substrates will be examined.
1.1 Outline of this thesis
The aim of this thesis is fabrication and treatment ITO on polymeric substrates without deteriorating the high electrical and optical performance commonly achieved, and to create an understanding of the fundamental aspects in order to optimize and develop these materials. This thesis was organized into five chapters. Following a concise introduction in Chapter 1, Chapter 2 will review the general properties of ITO and introduce the ELA method as well as its application for flexible devices. Chapter 3 describes the sample preparation and instrumentation. Chapter 4 describes the properties of ITO on polymeric substrate under KrF excimer laser irradiation. Then the results and conclusion of this thesis are summarized in Chapter 5.
5 concentration (N) and mobility of the relevant free carrier
where e is the electronic charge and µ is the mobility which can be expressed as
where , , and are the mobilities corresponding t lattice vibration scattering [16], ion impurity scattering [16,17], neutral impurity scattering [16,17], and grain boundary scattering [18,19], respectively.
The electron carriers of ITO film can be generated by the tin dopant [15] and by oxygen vacancies [15,16]. Tin acts as a cationic dopant when it substitutes indium and is surrounded by In2O3. It provides an electron to the conduction band. Using the Kroger-Vink notation this reaction can be described as:
Theoretically, the maximum carrier density by tin doping is , where
is the tin concentration (at %) [15]. Consequently, at low Sn weight ratio the resistivity decreases with Sn concentration in accordance with Eq (2-1) [20]. It is well known, however, that increasing the Sn weight ratio above 5% leads to a saturation in resistivity and then to a slight increase in resistivity [17,20]. Above this Sn concentration the carrier concentration of ITO thin film is found to be essentially