Total Subgap Range Density of States-Based Analysis of the Effect of Oxygen Flow Rate on the Bias Stress Instabilities in a-IGZO TFTs

In this study, the oxygen flow rate (OFR) dependence of negative bias illumination stress (NBIS) and positive bias stress (PBS) in amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistor (TFT) devices was investigated. Unlike in previous studies, our study simultaneously considered and analyzed the bias stress-induced degradation mechanisms by charge trapping and defect creation. NBIS and PBS instability were measured according to the OFR splits of bottom-gate (BG) IGZO TFTs and the behavior of full subgap range density of states (DOS) was experimentally tracked with especial emphasis on deep-level oxygen vacancy (
${V}_{\text {O}}$
) peak. It was found that the observed DOS variation was consistent with the
${V}_{\text {O}}$
ionization model during NBIS. In addition, the threshold voltage shift by charge trapping (
${\Delta } {V}_{\text {T,CT}}$
) and the threshold voltage shift by defect creation, i.e., change of DOS, (
${\Delta } {V}_{\text {T,DOS}}$
) were separated and extracted through the subthreshold slope decomposition method. After NBIS, the threshold voltage shift (
${\Delta } {V}_{\text {T}}$
) was composed of a component by charge trapping and a component with the ionization of
${V}_{\text {O}}$
. After PBS,
${\Delta } {V}_{\text {T}}$
was dominated by a component arising due to charge trapping. Fitting through the stretched exponential function (SEF) was performed for each separately extracted
${\Delta } {V}_{\text {T}}$
, and the activation energy for each
${\Delta } {V}_{\text {T}}$
was extracted using the inverse Laplace transform method through the extracted
${\Delta } {V}_{\text {T0}}$
,
$\tau $
, and
$\beta $
. Activation energy by charge trapping and DOS was extracted from NBIS and PBS, respectively. As a result of simultaneously considering the two degradation mechanisms, the extracted activation energy was located between the previous values. Experimentally extracted sets of DOS-specific parameters before and after bias stress were applied to technology computer-aided design (TCAD) simulations. Accurate reproduction of TFT current–voltage (
${I}$
–
${V}$
) curves before and after bias stress and recovery suggests that the methodology and parameter set used in this study are reasonable and potentially useful for a more robust and systematic analysis of the bias stress-induced instability in a-IGZO BG TFTs.