Stability of Amorphous in-Ga-Zn-O Thin Film Transistors with Silicon-Doped Back-Channel Layer under Temperature Humidity Stresses

In recent years, a-In-Ga-Zn-O thin film transistor has gained wide attention due to their good uniformity, high field-effect mobility and low cost process.^[1-2] Although a -IGZO TFT has high device performance, it is certainly necessary to obtain the device stability under high temperature-relative humidity. The improvement of device stability under external environments is considered to be an important subject for the development of a-IGZO TFTs.

To improve the a-IGZO TFTs characteristic and stability, the double active layer with a-IGZO/a-SIGZO was invested. The TFTs have an inverted staggered bottom gate structure with a channel width (W) of 500 µm and a channel length (L) of 50 µm. A 60 nm thick double active layer is grown by sputtering at room temperature, which contains 40 nm thick front-channel of a-IGZO and 20 nm thick back-channel with a-SIGZO by using cosputtering a-IGZO target and SiOx target at room temperature. Si concentration is changed with the rf power which split into 30, 40, 50, and 60 W.

Figure 1 shows the transfer curves of the TFTs with various Si concentrations in a-SIGZO channel layer. As increasing Si concentration in back-channel, the I_DS is lowered and the V_th shifted in the negative direction. The electrical properties reveal that Si is acting as a carrier suppressor due to reducing V_O in the a-IGZO system. The bonding-strengths of silicon (775KJ/mol) is higher than those of zinc (385KJ/mol), and indium (<470KJ/mol).^[3]

Figure 2(a) shows that the V_th of the conventional a-IGZO TFT was negatively shifted from 26.7 to 21.4 V. The H₂O molecules was adsorbed on the a-IGZO from the high humidity conditions lead to the decrease of the V_th. It may be attributed by the increase of carrier density in the a-IGZO bulk-channel due to the role of H₂O molecules as donor. Compared to the conventional a-IGZO TFT, there are almost no difference on the V_th value between as-deposited sample and that of after 15 days, as shown in Fig. 2(b). The maximum and minimum of V_th are 30.7 and 29.6 V, respectively. It is reasonable that Si-metal cation suppress the formation of V_O during the stability test. Additionally, SiO₂ distributed on the a-IGZO surface reduce H₂O adsorption site on a-IGZO back-channel surface.^[4] As a result, the TFTs with a Si-doped a-IGZO back-channel improve the electrical stability under temperature and relative humidity stresses

In our presentation, we will review the mechanism why the co-deposition of SiOx enhance the stability of a-IGZO TFT by analyzing the standard electric potential (SEP) and bonding strengths of Si.

* This work was financially supported by the Industrial Strategic Technology Development Program (10039191, The Next Generation MLC PRAM, 3D ReRAM, Device, Materials and Micro Fabrication Technology Development) funded by the Ministry of Trade, Industry and Energy (MOTIE), Korea.

Reference

[1] K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano and H. Hosono: Nature 432 (2004) 488.

[2] M. K. Ryu, S. H. Yang, S. H. Park, C. S. Hwang: Appl. Phys. Lett. 95 (2009) 0173508.

[3] J. K. Jeong, H. W. Yang, J. H. Jeong, Y. G. Mo, and H. D. Kim: Appl. Phys. Lett. 93 (2008) 123508.

[4] S. H. Yang, K. H. Ji, U. K. Kim, C. S. Hwang, S. H. K. Park, C. S. Hwang, J. Jang, and J. K. Jeong: Appl. Phys. Lett. 99 (2011) 102103.

Fig. 1. (a) The representative transfer characteristics of various sputtering powers of TFTs and (b) extracted subthreshold swing (S.S.), mobility and threshold voltage with various Si concentrations.

Fig. 2. Evolution of the transfer characteristics for (a) conventional a-IGZO TFT, (b) a-SIGZO / a-IGZO TFT, and (c) variation in V_th for different active layered TFTs as a function of high temperature and relative humidity exposure.