Defects in Amorphous Semiconductors: The Case of Amorphous Indium Gallium Zinc Oxide

Abstract

Based on a rational classification of defects in amorphous materials, we propose a simplified model to describe intrinsic defects and hydrogen impurities in amorphous indium gallium zinc oxide (a-IGZO). The proposed approach consists of organizing defects into two categories: point defects, generating structural anomalies such as metal-metal or oxygen-oxygen bonds, and defects emerging from changes in the material stoichiometry, such as vacancies and interstitial atoms. Based on first-principles simulations, it is argued that the defects originating from the second group always act as perfect donors or perfect acceptors. This classification simplifies and rationalizes the nature of defects in amorphous phases. In a-IGZO, the most important point defects are metal-metal bonds (or small metal clusters) and peroxides (O─O single bonds). Electrons are captured by metal-metal bonds and released by the formation of peroxides. The presence of hydrogen can lead to two additional types of defects: metal-hydrogen defects, acting as acceptors, and oxygen-hydrogen defects, acting as donors. The impact of these defects is linked to different instabilities observed in a-IGZO. Specifically, the diffusion of hydrogen and oxygen is connected to positive-and negative-bias stresses, while negative-bias illumination stress originates from the formation of peroxides.

Ultrabroadband density of states of amorphous In-Ga-Zn-O

“…Last, recent works suggest that the incorporation of hydrogen into a-IGZO can lead to increased subgap states near the valence-band maximum [29][30][31]56]. While this may be a potential alternative explanation for these near-VB states, it is important to note that the photoexcitation and subsequent relaxation of these hydrogen states do not exhibit the same slowed dynamics associated with electron recombination to an acceptorlike state and is, thus, insufficient to explain our observations [57].…”

“…TFT behavior under illumination has also been shown to depend strongly on photon energy (hν), especially upon photoexcitation of "deep states" near the valence band [25][26][27][28], which suggests the existence of multiple species of subgap states. However, the near-band-gap photoexcited TFT behavior has been multiply attributed to defects related to hydrogen [29][30][31], excess oxygen [32][33][34], and, contradictorily, the lack of oxygen [35]. Recently, Jia et al suggested the existence of cation vacancy (V M )-related clusters in IGZO as a vehicle for the inclusion of stable excess oxygen [36].…”

Defects in complex oxide thin films for electronics and energy applications: challenges and opportunities

“…298,[352][353][354][355] Defects have a significant influence on a-IGZO materials and TFTs, i.e., while most defects are detrimental to device performance, some defects actually play a positive role in improving carrier density. 304,356,357 V O , metal-metal bonds and H incorporation are the three main defect types present in channel region of a-IGZO TFTs. 358 V O could participate in several different local coordination structures: (i) 'Corner-share' structure formed by V O coordinated with small numbers of cations, (ii) 'Free space' structure formed by V O adjacent to a large open space and (iii) 'Edge/Face-sharing' structure formed by V O surrounded by many cations, as shown in Fig.…”

“…356,370,371 Reduction of metal-metal bonds generally follows the path with the decrease of V O , yet they are harder to eliminate for the additional energy required to break the bonds. 357 The incorporation of hydrogen could happen during annealing processes or by diffusion of other materials in TFT devices, i.e., gate dielectrics, passivation layers or buffer layers. [372][373][374] The role of hydrogen in a-IGZO varies under different H concentrations.…”

Defect Self‐Compensation for High‐Mobility Bilayer InGaZnO/In2O3 Thin‐Film Transistor

“…As it is well known, there is high density of intrinsic defects existing in the AOS materials, including the dangling bonds, atoms, impurities, etc . The intrinsic defects can be mainly divided into three categories: 1) the point defects including the peroxide (OO, acting as donor) and the metal–metal bond (or small metal cluster, MM, acting as acceptor), 2) the stoichiometric defects including the oxygen vacancy (V O , donor) and oxygen interstitial (O i , acceptor), and 3) the impurities like the metal–hydrogen and oxygen–hydrogen bonds, depending on the AOS material . While considering the bond dissociation energy between metal and oxygen atoms, e.g., InO, ZnO, and GaO, it is crucial to understand formation of these intrinsic defects (dangling bond, atom, or cluster) and their possible interactions (i.e., defect self‐compensation mechanism) in the stacked‐layer TFT …”