Thickness of accumulation layer in amorphous indium-gallium-zinc-oxide thin-film transistors by Kelvin Probe Force Microscopy

Abstract

In this letter, we measured the thickness of an accumulation layer (dacc) in amorphous Indium-Gallium-Zinc-Oxide thin-film transistors (TFTs) using Kelvin Probe Force Microscopy (KPFM). By scanning the active layer surface from the interface to the back channel, we obtained potential from different thickness profiles, which show the variation of the carrier concentration. It was found that potential followed an exponential decay function from the interface to the back channel. Furthermore, there was a transition point after which the potential changed little. From this potential map, the thickness of the accumulation layer could be considered as the height difference between the transition point and the interface. Meanwhile, by controlling gate voltage (VG) during the KPFM scanning process, we obtained a relationship between dacc and VG. The results indicated that when VG was smaller than threshold voltage (Vth), dacc increased drastically with the increase in VG; after that, dacc was almost independent of VG, indicating that dacc reached a saturation value around 15 nm. This finding gave us a clear physical image about charge distribution in TFT and facilitated the understanding of device physics.