Improvement in Short-Channel Effects and Bias-Stress Stability of Vertical Thin-Film Transistors Using Atomic-Layer-Deposited In–Ga–Sn–O Channels

Vertical-channel thin-film transistors (VTFTs), featured to uniquely employ the In–Ga–Sn–O (IGTO)-active channel layers prepared by atomic layer deposition, were fabricated with a channel length (L_ch) as short as 150 nm and characterized in terms of the short-channel effect (SCE) and operational reliability. All the fabricated devices exhibited sound and stable operations, including durability against SCEs, such as negligible DIBL effects, even for the nanoscale regime in L_ch. The IGTO VTFT fabricated with a channel cationic composition (In/Ga/Sn) of 3.8:3.9:1.0 was examined to exhibit excellent device characteristics such as a current drivability (C_DR) of 32.2 μA/μm and an on/off current ratio as high as 10⁸. The threshold voltage shift (ΔV_TH) representing the positive bias-stress (PBS) stability was secured as low as +0.87 V at a bias stress field of 2 MV/cm for 10,000 s. The ΔV_TH values of the IGTO VTFTs were noticeably found to decrease with increasing In compositions in the IGTO channels, showing more robust PBS stability. Feasible scenario for the improvement in the PBS instability for the IGTO VTFT was elucidated by analysis on the correlations between the bulk-trap density and the cationic composition in the ALD IGTO channel. The composition-dependent PBS instabilities were also suggested to be markedly influenced by the magnitude of the bias-stress field owing to the generation of Sn–O weak bonds in the IGTO channel and donation of free carriers during PBS. These obtained insights could provide a beneficial design scheme for improving the PBS instability determined by the structural disadvantages, leading to balancing C_DR with the bias-stress stability of the VTFTs, even when L_ch was vertically scaled down to 150 nm.