Performance Improvement of Self-Aligned Coplanar Amorphous Indium–Gallium–Zinc Oxide Thin-Film Transistors by Boron Implantation
Seung Hee Kang, I. Sak Lee, Kyungmoon Kwak, Kyeong Take Min, Nack Bong Choi, Han Wook Hwang, Hyun Chul Choi, Hyun Jae Kim
Abstract
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.