Structural-relaxation-driven electron doping of amorphous oxide semiconductors by increasing the concentration of oxygen vacancies in shallow-donor states

The electronic states of oxygen vacancies (VO s) in amorphous oxide semiconductors are shallow donors, deep donors or electron traps; these are determined by the local atomic structure. Because the amorphous phase is metastable compared with the crystalline phase, the degree of structural disorder is likely to decrease, which is referred to as structural relaxation (SR). Thus SR can affect the VO electronic state by changing the local atomic conditions. In this study, we demonstrated that electron doping is possible through the SR of amorphous oxides without redox reactions using a novel device structure that prevents extrinsic reactions with electrodes and ambient atmosphere during annealing. The concentration of VO s in the shallow-donor state in amorphous In-Ga-Zn-O (a-IGZO) increases from ~1016 to ~1019 cm−3 with increasing annealing temperatures between 300 and 450 °C. The SR-driven doping effect is strongly dependent on the annealing temperature but not on the annealing time. The Arrhenius activation energy of the SR-driven doping effect is 1.76 eV, which is similar to the bonding energies in a-IGZO. Our findings suggest that the free volume in a-IGZO decreases during SR, and the VO s in either deep-donor or electron-trap states are consequently transformed into shallow-donor states. A chemical-free method for enhancing the conductivity of transparent transistor materials could benefit flat-panel and flexible displays. Amorphous indium–gallium–zinc oxide (a-IGZO) is a semiconductor that can withstand bending better than silicon technology thanks to its disordered internal structure. However, a process known as structural relaxation slowly reduces the randomness of a-IGZO atoms, which alters the electrical characteristics of a-IGZO. To initiate controlled structural relaxations, Young-Chang Joo from Seoul National University and colleagues annealed a stacked novel metal/a-IGZO/metal device at temperatures below the glass transition point. Electrical measurements revealed the relaxations increased the concentration of shallow-donor-state oxygen vacancies, enabling electron dopants to be added to a-IGZO without using redox chemistry. The temperature-dependent doping can be sustained for prolonged annealing times, making it a promising approach for enhancing the thermal stress resistance of flexible electronics. The effects of structural relaxation (SR) on the electronic state of oxygen vacancies (VO s) in amorphous oxide semiconductors is investigated. Without redox reactions, the concentration of VO s in the shallow-donor state (NDS) increases about 103 times with increases in the annealing temperature from 300 to 450 °C. The reduction in the free volume size and transformation of VO s in either deep-donor or electron-trap states into the shallow-donor state during SR is the primary mechanism responsible for the increase in NDS.