One of the interesting oxide semiconductors for TFTs applications is a ternary material composed of In2O3、Ga2O3 and ZnO named IGZO. Hosono et. al. have proposed the high performance TFTs using a-IGZO deposited on plastic substrates by pulse laser deposition at room temperature as the active layer [10]. The performance of a-IGZO TFTs is also confirmed by using the sputter deposition [7], which demonstrates the possibility of the large area applications. In addition, the a-IGZO is transparent to visible light due to its optical energy
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band gap of about 3eV [13]. Fig. 1-5 shows the transmittance is greater than 80 percent from 400 nm to 850 nm wavelength [14]. Hence a-IGZO TFTs are useful for new applications such as transparent displays.
Fig. 1-5 Transmittance of a-IGZO in visible light. [14]
The a-IGZO has considered favorable material for practical TFTs such as liquid crystal displays (LCDs) and organic light-emitting diode (OLED) displays. Consequently, the a-IGZO TFTs technology is rapidly expanding to more practical prototypes such as 12.1 inch AMOLED and 15 inch AMLCD high-resolution displays [15,16]. In SID 2008, a full color 12.1 inch WXGA AMOLED display using a-IGZO TFTs as an active-matrix backplane was demonstrated by Samsung, as shown in Fig. 1-6. The a-IGZO TFTs exhibited the field-effect mobility (μ) of 8.2 cm2/Vs, threshold voltage (Vth) of 1.1 V, Ion/Ioff around 108, and subthreshold swing (S.S.) of 0.58 V/decade.
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Fig. 1-6 The display image of Samsung’s 12.1 inch WXGA AMOLED display. [15]
In SID 2010, a full-HD (1,920 x 1,080 pixels) 37-inch LCD panel based on a transparent a-IGZO semiconductor was demonstrated by AU Optronics Corporation, as shown in Fig. 1-7.
The coplanar type of a-IGZO TFTs with gate width and length are 22 and 5 μm, respectively, exhibited the field-effect mobility (μ) of 10 to 13 cm2/Vs and Ion/Ioff higher than 108.
Fig. 1-7 The photo of the lecture delivered by AUO.
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1.3.2 The In
2O
3-Ga
2O
3-ZnO Ternary System
For the In2O3-Ga2O3-ZnO ternary system, the mobility can be changed by choosing different chemical composition [17,18]. Fig. 1-8 shows the relationship among chemical composition, mobility and carrier concentration in a-IGZO thin film by pulse laser deposition at room temperature. It is clear that higher value of mobility is obtained around the samples containing more In2O3 fraction. In contrast, incorporation of Ga decreases carrier concentration and mobility.
The largest mobility of 39 cm2/Vs is obtained in a-IZO with the carrier concentration of 1×1020 cm-3. For good TFTs operation, a-IZO has a high carrier concentration. In practice, the effect of binary amorphous materials in the In2O3–ZnO system is employed in commercial flexible transparent conductive films by depositing on the plastic sheet. The carrier concentration of amorphous metal oxide materials is related to oxygen vacancies. Thus, the effect of partial oxygen pressure was studied during the deposition processes on the carrier concentration in a-IGZO and a-IZO, the results are shown in Fig. 1-9. The carrier concentration in the a-IGZO is distinctly reduced to below 1013 cm-3 by increasing PO2 to 8 Pa.
Nevertheless, the carrier concentration of a-IZO remains at 1018 cm-3 under the same condition. The result indicates the incorporation of Ga is effective to suppress the electron carrier generation. Ga3+ is supposed to attract the oxygen ions tightly due to its small ionic radius causing high ionic potential, and thereby suppressing electron injection which is caused by oxygen ion escaping from the thin film. This is why a-IGZO has been widely researched as active layer for TFTs instead of a-IZO which has the higher mobility.
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Fig. 1-8 The amorphous formation region (right) and the electron mobility and concentrations evaluated from the Hall effects for the amorphous thin films (left) in the In2O3–Ga2O3–ZnO system, respectively. The thin films were deposited on a glass substrate by pulse laser deposition under deposition atmosphere of PO2 = 1 Pa. Number in the parenthesis denotes carrier electron concentration (x1018 cm-3). [1]
Fig. 1-9 The carrier concentration as a function of O2 pressure during the deposition in a-IGZO and a-IZO. [1]
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1.3.3 The Issue and Prior Art of a-IGZO TFTs for Stability
Although a-IGZO has been demonstrated successfully as the active layer in high performance TFTs, an issue yet to be extensively explored is the electrical stability of TFTs.
Some experimental works have been reported on the degradation of electrical characteristics in a-IGZO TFTs under the ambient environments [19,20,21]. It is thought that the sensitivity to adsorption and desorption of oxygen-, hydrogen- and water-related molecules and the photosensitivity are the drawbacks of a-IGZO TFTs. To improve the stability of a-IGZO TFTs, the passivation layer is needed to protect the back channel from oxygen-, hydrogen- and water-related molecules and light illumination.
The reported methods of depositing a passivation layer as shown in Table 1-1 can be classified into two types. One is the coating method which is often used for organic materials such as polymide, acrylic, parylene and siloxane [22,23]. However, temperature-dependence and reliability need to be improved due to the contained moisture in the coating film. The other one is the plasma enhanced chemical vapor deposition (PECVD). The method is used to deposit silicon oxide (SiOx) and silicon nitride (SiNx) as the passivation layer [22]. Though, the deposition process of PECVD generates hydrogen to degrade the electrical characteristics of a-IGZO TFTs. This is due to SiH4 which decomposes to hydrogen is used as a reactive gas.
The decrease in resistivity of the a-IGZO films is attributed to formation of hydrogen donors when the passivation layer is deposited by PECVD [24]. To compare the above mentioned methods, a better deposition process is necessary to enhance both the electrical stability and performances of a-IGZO TFTs.
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Table 1-1 The prior arts of a-IGZO TFTs with passivation Active Layer Passivation Layer Drawback Ref.
a-IGZO
The typical TFT is composed of an amorphous silicon as channel layer, a silicon nitride as dielectric layer, phosphorous doped a-Si:H as contact layer, and a second silicon nitride layer to passivate the back of the channel. In current manufacturing, these films are deposited by PECVD using silane mixed with ammonia or phosphine. This method requires the use of flammable and toxic process gases, which increases the costs and hazards associated with production. The film deposited by PECVD becomes too rich in H at temperatures of less than 250 °C so as to affect the performance of TFT [25]. The sputtering offers the potential for the lower temperature deposition and the a-IGZO film is researched and deposited in the sputtering machine for room temperature deposition [7]. The gate insulator, gate electrode, and source/drain electrodes are also developed in the sputtering machine [26,27]. Therefore, the all-sputtered a-IGZO TFTs are possible at low temperature. To combine the fabrication