Mobility Enhancement of Back-Channel-Etch Amorphous InGaZnO TFT by Double Layers With Quantum Well Structures

The back-channel-etch amorphous InGaZnO (a-IGZO) double-layer thin-film transistor (DL-TFT) consists of an a-IGZO layer with no oxygen flow (NOF) as a top layer and an a-IGZO layer with oxygen flow (OF) as a bottom layer. The DL-TFT demonstrates the field-effect mobility of 19 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V-s, which is 1.6× of the NOF and the OF single-layer TFTs (SL-TFTs) at the overdrive voltage of 18 V and the drain voltage of 0.1 V. Several measurements are performed to obtain the band alignment between the NOF and the OF layer for further understandings of the mechanism of the improvement in the field-effect mobility. Tauc method, X-ray photoelectron spectroscopy, and Kelvin probe force microscopy measurements were used to extract the optical bandgaps, the Fermi level position, and the work function of the NOF and the OF, respectively. The conduction band difference of 0.28 eV between the NOF and the OF layer is obtained. The carriers in the DL-TFT are confined in the NOF layer by quantum confinement, where the OF layer serves as the barrier to reduce the Coulomb scattering and the surface roughness scattering.