Effect of Metal Capping Layer in Achieving Record High p-Type SnO Thin Film Transistor Mobility

Recently p-type tin monoxide (SnO) thin film transistors (TFTs) have drawn significant interest for use in all-oxide CMOS circuits for back-end-of-line (BEOL) integration with Si and flexible electronics. However, most SnO TFTs demonstrated so far have a limited field effect mobility and poor ON– OFF current ratio. Here, we used a floating metal cap layer to create a bilayer TFT structure and boost the device’s effective mobility. We achieved a record high p-type SnO TFT mobility of 19.1 cm
$^{{2}}\text{V}^{\text {-1}}\text{s}^{\text {-1}}$
using a Ni/Au capping layer to cover a portion of the active channel. By comparing the experiment and TCAD simulation, we explained the working mechanism of the SnO bilayer TFT. We also investigated the effect of layer thickness on TFT electrical performance. A thin SnO active layer is desired to minimize the vertical resistance in the ON-state and thereby increase the effective TFT mobility. Finally, by applying appropriate surface passivation we demonstrated an
$\sim 8\times $
improvement in the ON– OFF current ratio of our SnO bilayer TFTs. By combining the use of a floating metal capping layer and surface passivation, we can significantly and simultaneously improve both the effective mobility and the ON– OFF current ratio of SnO TFTs. In addition, we showed that SnO bilayer TFTs exhibit more robust behavior to gate-bias stress compared to traditional SnO TFTs.