The high filed effect mobility and transparent thin films of a-IGZO can be fabricated at low temperature on several substrates by variety of physical and chemical method, such as DC/ or RF magnetron sputtering [14, 15], pulse laser deposition (PLD) [16], molecular beam epitaxy economical, easy-to-handle, large area deposition feasibility and controlled quality [26, 27].
The high quality thin films can be deposited in the atmosphere ambient by APPJ, but can’t use the PECVD based on the plasma used methods [28]. And deposit thin films by other plasma CVD processes also in low pressure conditions. The APPJ system is mainly used in high temperature arc plasma, for example, the plasma jet used for diamond synthesis [29] and the inductively coupled plasma (ICP) flash evaporation
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process for producing ferrite films formation [30] is possible to generate atmospheric pressure cold glow plasma. The atmospheric pressure cold glow plasma which is used the nitrogen gas as a plasma gas, employed a high frequency power source over one kilohertz, and stetted an insulating between two electrodes [31].
Therefore, the APPJ would be quite advantageous than other plasma enhanced CVD from the viewpoint of process cost, and particularly in the application of thin films deposition on ready prepare glass substrates.
Numerous films were fabricates by the APPJ system, such as SiOx films [32], TiOx films [33], ZnO films and Transparent Conductive Oxide (TCO). Consequently, we can mix two or more solutions that choose appropriate source material to be the precursor of deposition films for an APPJ system that it would be a convenient process for fabrication a variety of thin films, even though a-IGZO thin films.
In recent years, many researchers have been extensively studied targeting for transparent TFTs for the next display generation. Nowadays, various materials have been investigated as candidates of the channel semiconductors, such as hydrogenated amorphous silicon (a-Si:H) [34-36], polysilicon, organic semiconductor [37-41], metal-oxide semiconductors such as zinc oxide (ZnO) [42-47], and so on. The material of ZnO for channel TFT has attracted much attention with its notable advantages over the other semiconductors including high field effect mobility, wide bend gap of 3.37 eV, high transparency in the visible range, and good thermal stability. But the ZnO films examined are polycrystalline [48] even if formed at room temperature. These channels
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have grain boundaries that deteriorate the TFT performance, stability, and uniformity of the TFT characteristics. However the a-IGZO films also has the similar properties to ZnO films and it can be formed amorphous at room temperature that has better stability and uniformity of TFT characteristics than ZnO TFT.
For the indium-gallium-zinc-oxide ternary system, the IGZO films ratio of indium, gallium, and zinc are very important to determinate the carrier concentration and mobility. The carrier concentration of IGZO is primary determined by the incorporation of cations with large ionic valance such as indium and zinc that are effective to control the carrier concentration due to their strong metal-oxygen bonds. Besides the mobility of IGZO is determined by the indium content fraction because only In3+ meets the electron configuration criterion (n-1)d10ns0(n ≥ 5) of heavy post transition metal cation for ionic AOS among the three cations [49]. In the other hand the gallium ions can effectively suppress the formation of oxygen vacancy and this reduce the carrier concentration in the film owing to the fact that gallium ion forms a stronger chemical bond with oxygen than zinc and indium ions [50]. Figure 1-2 shows the carrier concentration and electron mobility that evaluated by the Hall effect measurement for amorphous thin films deposition by PLD.
The continuous decrease of the thickness of gate oxides used in modern electronics has to confront the limit of material itself. In order to make a breakthrough on what is aforementioned, it is necessary to increase the physical thickness as well as the dielectric constant of the gate dielectrics. Unfortunately, most popular transparent TFT structure
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employing SiO2 as dielectric material that has low dielectric constant (k) resulting in low channel modulation effect. Therefore SiO2-based structures should require higher driving voltage to achieve the same operating efficiency than the other structures incorporating high-k dielectrics. Moreover, in order to have the equivalent insulation efficiency, SiO2 dielectric layer should be thicker than high-k materials.
However, it has a critical drawback, high charge trap density mainly being concentrated into the interfaces between gate electrode and gate insulator, as well as gate insulator and channel layer [52, 53].
In this dissertation, for the purpose of high mobility, optical transparency, stability, wide band gap, room temperature process, and good uniformity over large area, we choose the IGZO thin film for TFTs active layer. Then, we prepared the precursor solution for IGZO film to blend the molarity ratio by dissolving indium, gallium, and zinc nitrate-based that was deposited by APPJ system. Because of the kind of APPJ does not require a complicated vacuum system which would reduce the cost of processing and enlarge the size limit [54]. Moreover, the deposition process is at low temperature which reducing the thermal damage of substrate and make sure the amorphous state of thin film of IGZO. After the thin film deposition, we discuss the influence of TFTs electric characteristic by different post annealing temperature. In the end, for lower driving voltage, lower leakage current, and higher capacitance we are going to substitute high-k material for SiO2 in the TFTs of gate dielectric. We explore high-k dielectric materials to find that HfO2 and Al2O3 are two of the most promising materials due to the properties of
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high dielectric constant, relatively low leakage current, low synthesis temperature, wide band gap sufficient to yield a positive band offset with respect to IGZO, and high transparency in visible range [55, 56].
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Figure 1-1 Schematic orbital drawing of electron pathway (conduction band bottom) in conventional silicon-base semiconductor and ionic oxide semiconductor [4].
Figure 1-2 Room temperature Hall mobility and carrier concentration as function of chemical composition. Values outside and inside parentheses show Hall mobility in and carrier concentration in 1018 cm-3, respectively [51].
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