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.