Abnormal threshold voltage shift by the effect of H2O during negative bias stress in amorphous InGaZnO thin film transistors

• Published in: Solid-state electronics Vol. 174; p. 107916
• Main Authors: Ha, Tae-Kyoung, Kim, Yongjo, Yu, SangHee, Kim, GwangTae, Jeong, Hoon, Park, JeongKi, Kim, Ohyun
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.12.2020
• Subjects: a-IGZO TFTs; Charge neutralization; H2O dissociation: H+ and OH; Negative bias stress; pH; Charge neutralization; a-IGZO TFTs; H2O dissociation: H+ and OH; Negative bias stress
• ISSN: 0038-1101; 1879-2405
• DOI: 10.1016/j.sse.2020.107916

•Two-phase VT shift occurred under negative gate bias stress after soaking TFTs in H2O.•Abnormal VT shift was caused by dissociation of H2O: H+ and OH−.•VT decreased due to H+ trapping, then increased because of neutralization between H+ and OH−.•Recovery also occurred in two phase: it was recombination process.•TCAD simulation identified the mechanisms. We observed abnormal threshold voltage (VT) shift in amorphous InGaZnO (a-IGZO) thin-film transistors under negative gate bias stress (NBS) after soaking them in H2O (pH 8). Before NBS, we soaked a-IGZO TFTs in H2O. During application of NBS, VT decreased by −0.43 V, then increased to nearly the initial value. We hypothesize that the electrical field that was applied during NBS caused some dissociation of H2O to hydrogen ions (H+) and hydroxide ions (OH−); the effects between H+ and OH− are responsible for the changes of ΔVT. The initial decrease was a result of trapping of H+ at the front channel; the subsequent increase was caused by neutralization of the H+ and the OH−; the a-IGZO was very thin, so the front channel and the back channel could affect each other; therefore, mitigation of energy band bending was possible. Recovery after NBS also occurred in two-phases: VT first increased then decreased to its initial value. During the recovery process, accumulation of an OH− layer generated electric field that attracted H+ so that the two species recombined. Increase in ΔVT occurred due to desorption of H+ from the front-channel interface, and decrease in ΔVT occurred by recombination.