Effect of ALD- and PEALD- Grown Al2O3 Gate Insulators on Electrical and Stability Properties for a-IGZO Thin-Film Transistor

Abstract

This study investigated the electrical and stability characteristics of Al2O3 as a gate insulator, which was deposited by various atomic layer deposition methods in top-gate staggered amorphous InGaZnO (a-IGZO) thin film transistors. A trimethylaluminum precursor was used as an Al source, and H2O gas (H2O device) and O2 plasma with a long plasma time (O2 LP device) and a short plasma time (O2 SP device) were used as oxidants. The initial electrical characteristics, including the hysteresis, on–off current ratio, and subthreshold swing, were superior in the H2O device compared to the O2 LP and O2 SP devices. In the positive bias stress (PBS) results, the degradation characteristics showed a tendency similar to the transfer properties. However, under the negative bias illumination stress (NBIS), the stability of the H2O device was significantly reduced compared to the O2 LP and O2 SP devices. In this paper, the mechanism of instability, which has opposite results in terms of the PBS and NBIS for the three devices, was identified using capacitance–voltage, three-terminal charge pumping as electrical analysis techniques and secondary ion mass spectroscopy (SIMS) as a physical analysis technique. It was confirmed that the surface oxidation of a-IGZO deteriorates the interfacial properties, causing the transfer characteristics to degrade. The carbon of the Al2O3 film identified via SIMS analysis acts as a trap layer, causing deterioration in the PBS. Alternatively, in the NBIS, it was observed that the carbon acts as a capture site for photo-excited holes, thereby promoting device stability.