Performance Improvement of Self-Aligned Coplanar Amorphous Indium-Gallium-Zinc Oxide Thin-Film Transistors by Boron Implantation

The electrical properties and device stability of a self-aligned (SA) coplanar amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistor (TFT) were investigated by implanting boron (B) into the source/drain (SD) n+ region. To evaluate the effect according to the depth profile of B in the a-IGZO film, various implantation energies were applied. The electrical properties were optimized when the projection range of B was in the central vertical region of the a-IGZO film. B implantation decreased the resistivity of the a-IGZO film from 3.1 × 102 to 2.1 × 10-3 ω·cm compared to an untreated a-IGZO film, while the field-effect mobility (μfe) improved from 2.96 to 17.22 cm2/(V·s). Moreover, the fabricated SA coplanar a-IGZO TFTs with a B-doped n+ region exhibited excellent stability, with a threshold voltage shift (ΔVth) of <0.2 V during a 3000 s thermal stability test performed at 200 °C and a bias stress test under a gate voltage of ±20 V. During the implantation process, B ions with high kinetic energy collide with IGZO atoms, resulting in the formation of an oxygen vacancy (VO) and an oxygen interstitial (Oi) simultaneously. The implanted B ions and Oi are bonded such that the VO sites are maintained by the B-O reaction and can contribute to an increase in the carrier concentration in a-IGZO films, thereby increasing the conductivity of the n+ region.