Hydrogen incorporation into amorphous indium gallium zinc oxide thin-film transistors

TL;DR

This study investigates how hydrogen incorporation affects the electronic states and operation of amorphous indium gallium zinc oxide thin-film transistors, revealing anomalous donor behavior and defect complex formation.

Contribution

It uncovers the formation of a specific hydrogen-related defect complex in a-IGZO and its impact on subgap states, challenging conventional donor models.

Findings

Hydrogen acts as an anomalous donor in a-IGZO.

Formation of a defect complex [{O_O^{2-}}{H^+}]^{1-} creates subgap states.

Subgap states are centered at 0.4 eV above the valence band edge.

Abstract

Within the subgap of amorphous oxide semiconductors like amorphous indium gallium zinc oxide (a-IGZO) are donor-like and acceptor-like states that control the operational physics of optically transparent thin-film transistors (TFTs). Hydrogen incorporation into the channel layer of a top-gate a-IGZO TFT exists as an electron donor that causes an observed negative shift in the drain current-gate voltage ($\mathrm{I_{D} - V_{G}}$) transfer curve turn-on voltage. Normally, hydrogen is thought to create shallow electronic states just below the conduction band mobility edge, with the donor ionization state controlled by equilibrium thermodynamics involving the position of the Fermi level with respect to the donor ionization energy. However, hydrogen does not behave as a normal donor as revealed by the subgap density of states (DoS) measured by the photoconduction response of top-gate a-IGZO TFTs to within 0.3 eV of the CBM edge. Specifically, the DoS shows a subgap peak above the valence band mobility edge growing at the same rate that $\mathrm{I_{D} - V_{G}}$ transfer curve measurements suggest that hydrogen was incorporated into the channel layer. Such hydrogen donor behavior in a-IGZO is anomalous and can be understood as follows: Non-bonded hydrogen ionization precedes its incorporation into the a-IGZO network as a bonded species. Ionized hydrogen bonds to a charged oxygen-on-an-oxygen-site anion, resulting in the formation of a defect complex denoted herein as, $\mathrm{{[{O_{O}^{2-}}{H^+}]}^{1-}}$. Formation of an $\mathrm{{[{O_{O}^{2-}}{H^+}]}^{1-}}$ defect complex creates a spectrally-broad ($\sim$0.3 eV FWHM) distribution of electronic states observed in the bandgap centered at 0.4 eV above the valence band mobility edge.