Carrier Generation Mechanism and Origin of Subgap States in Ar- and He-Plasma-Treated In–Ga–Zn–O Thin Films

Abstract

The electrical properties, carrier generation mechanism, and origin of subgap states in Ar- and He-plasma-treated In–Ga–Zn–O (IGZO) films were investigated. The incident ion energy was varied by applying a substrate bias power (PB). The sheet carrier concentration of plasma-treated IGZO films increased with increasing PB. The carrier generation mechanism and origin of subgap states were investigated by X-ray photoelectron spectroscopy (XPS) with CrKα (5415 eV) and AlKα (1487 eV) sources. The XPS analysis revealed that oxygen vacancies (VO) increased with increasing PB. As the VOincreased, subgap states were formed near valence band maximum (near-VBM) in the He-plasma-treated IGZO films. On the other hand, we found the formation of metallic In (In0) and Zn (Zn0) states at a surface of the Ar-plasma-treated IGZO, and the VO in the Ar-plasma-treated IGZO film was much lower than that in the He-plasma-treated film. The subgap states in Ar-plasma-treated IGZO film would be originated from In0 and Zn0 states. We also demonstrated a self-aligned IGZO thin-film transistor with the S/D regions formed by the plasma treatment with PB. An appropriate PB significantly reduced a series resistance of the S/D regions and improved the TFT properties after post-annealing at 350°C.