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Design Principle of Channel Material for Oxide-Semiconductor Field-Effect Transistor with High Thermal Stability and High On-current by Fluorine Doping

With the arrival of 5G era, further downsizing and lower-power consumption of semiconductor devices have been strongly required. Because of the background, oxide-semiconductor field-effect transistor (OS-FET) using InGaZnO (IGZO) have attracted much attention due to its advantages: (1) extremely low off-state current caused by a large band gap (~3 eV), (2)high mobility (~10 cm2/Vs) in an amorphous state, and (3)low temperature (<400 ℃) BEOL compatible process[1]. However, thermal stability especially in forming gas (N2/H2) atmosphere have been a critical issue of OS-FET for co-integration with Si-CMOS LSIs.

In order to overcome this issue, we have successfully improved thermal stability of OS-FET, i.e., suppression of negative shift of threshold voltage after 400℃ forming gas annealing, by using new channel material, Fluorine-doped IGZO (IGZO:F) (Fig.1). In order to get atomistic insights for F-doping effect and propose the material design principles, we have investigated properties of IGZO:F and IGZO by means of first-principles calculation. As a result, we have clarified that fluorine atoms suppress formation of oxygen vacancy leading to the high thermal stability (Fig.2). On the other hand, it was also revealed that overdose of F atoms largely decreases on-current of OS-FET due to formation of electron traps of metal-metal bonds. We have shown, however, that OS-FET with both high thermal stability and high on-current can be realized by optimizing F amount in IGZO:F (Fig. 3). These results are fundamental technologies to realize new memory devices with large amounts of storage, low latency, and ultralow-power consumption, which cannot be achieved by silicon-based FETs.

This achievement was presented in the 2020 IEEE IEDM (International Electron Devices Meeting)[2].

Fig.1 : Transfer characteristics of IGZO and IGZO:F after annealing at 400℃

Fig.1 : Transfer characteristics of IGZO and IGZO:F after annealing at 400℃

Fig.2 : Formation energies of oxygen vacancy in IGZO and IGZO:F

Fig.2 : Formation energies of oxygen vacancy in IGZO and IGZO:F

Fig.3 : Optimization of thermal stability and on-current by F doping

Fig.3 : Optimization of thermal stability and on-current by F doping

[1] K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, and H. Hosono, “Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors”, Nature, 432(7016), pp. 488-492, (2004)

[2] H. Kawai, H. Fujiwara, J. Kataoka, N. Saito, T. Ueda, T. Enda, T. Ishihara, and K. Ikeda, “2 Design Principle of Channel Material for Oxide-Semiconductor Field-Effect Transistor with High Thermal Stability and High On-current by Fluorine Doping”, 2020 IEEE IEDM, 22.2.

This material is a partial excerpt and a reconstruction of the reference [2] © 2020 IEEE.

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