Optimization of Atomic-Layer-Deposited Zinc-Tin-Oxide Thin-Film Transistors via Controlled Aluminum Doping

This study investigated the physical and electrical properties of atomic-layer-deposited amorphous aluminum-doped zinc-tin-oxide (a-AZTO) thin films and thin-film transistors (TFTs) using the a-AZTO channel. Al doping levels were controlled by adjusting the Al2O3 subcycle within the a-AZTO supercycle. The as-deposited a-AZTO films exhibited layered structures consisting of an a-ZTO matrix and amorphous Al2O3 layers, some of which substituted Zn atoms within the a-ZTO layer. As the number of Al2O3 layers increased, the band gap energy and crystallization temperature of the a-AZTO films increased from 3.37 eV and 670 degrees C (undoped) to 3.56 eV and 740 degrees C (18% Al cationic fraction), respectively. The optimized a-AZTO (a-A3ZTO, 6.5% Al cationic fraction) TFT, incorporating a 10-nm-thick HfO2/30-nm-thick SiO2 stacked gate insulator, exhibited a subthreshold swing of 81 mV/decade, a threshold voltage of -0.30 V, and a saturation mobility of 6.07 cm2/(V s). Additionally, the a-A3ZTO TFT showed a lower contact resistance (11.7 k Omega) with TiN source/drain electrodes compared to the undoped a-ZTO TFT (12.1 k Omega). Moreover, the a-A3ZTO TFT demonstrated improved bias stress stability compared to the undoped a-ZTO TFT, with threshold voltage shifts of -0.20 and 0.60 V under negative bias stress and positive bias stress, respectively, after a stress time of 1000 s. This enhanced stability is attributed to the decreased oxygen vacancy concentration in the a-A3ZTO channel and the presence of a thin Al2O3 layer at the channel's back surface.