Hydrogen anion and subgap states in amorphous In–Ga–Zn–O thin films for TFT applications

Abstract

Hydrogen is an impurity species having an important role in the physical properties of semiconductors. Despite numerous studies, the role of hydrogen in oxide semiconductors remains an unsolved puzzle. This situation arises from insufficient information about the chemical state of the impurity hydrogen. Here, we report direct evidence for anionic hydrogens bonding to metal cations in amorphous In–Ga–Zn–O (a-IGZO) thin films for thin-film transistors (TFT) applications and discuss how the hydrogen impurities affect the electronic structure of a-IGZO. Infrared absorption spectra of self-standing a-IGZO thin films prepared by sputtering reveal the presence of hydrogen anions as a main hydrogen species (concentration is ∼1020 cm−3) along with the hydrogens in the form of the hydroxyl groups (∼1020 cm−3). Density functional theory calculations show that bonds between these hydride ions with metal centers give rise to subgap states above the top of the valence band, implying a crucial role of anionic hydrogen in the negative bias illumination stress instability commonly observed in a-IGZO TFTs.

Control of O-H bonds at a-IGZO/SiO2 interface by long time thermal annealing for highly stable oxide TFT

“…The H -can be placed at the oxygen vacancy site and then form the metal-hydrogen bond (M-H) which is thermally stable. 26 Therefore, this does not change after annealing at 300 o C. On the other hand, H + in a-IGZO reacts with O 2-and donates one electron by the following reaction;…”

The Nature of the Oxygen Vacancy in Amorphous Oxide Semiconductors: Shallow Versus Deep

“…[4,5] In particular, the instability under negative-bias-illumination-stress (NBIS) is known to be most serious. [6,7] Among the suggested mechanisms of the NBIS instability, the oxygen vacancy (V O ) has been receiving intensive attention because AOS films are oxygen deficient. [8][9][10][11][12] The role of V O in the device instability critically depends on its nature, specifically, whether defect levels associated with V O is shallow or deep.…”

Enhancement of Electrical Characteristics and Stability of Amorphous Si-Sn-O Thin Film Transistors with SiOx Passivation Layer

“…Thus, the increase of I off in the passivated device was attributed to the byproduct hydrogen during the deposition of SiO x . Because neutral H 0 could migrate in the channel layer and further react with O 2− to form OH − and generate excessive electrons (${\mathrm{H}}^0+{\mathrm{O}}^{2-}\to {\mathrm{OH}}^{-}+{\mathrm{e}}^{-}$), which would result in the formation of back-channel current (I back ), as shown in Figure 2b [19]. …”