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

Abstract

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 ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {O}}$ </tex-math></inline-formula> ) peak. It was found that the observed DOS variation was consistent with the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {O}}$ </tex-math></inline-formula> ionization model during NBIS. In addition, the threshold voltage shift by charge trapping ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\Delta } {V}_{\text {T,CT}}$ </tex-math></inline-formula> ) and the threshold voltage shift by defect creation, i.e., change of DOS, ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\Delta } {V}_{\text {T,DOS}}$ </tex-math></inline-formula> ) were separated and extracted through the subthreshold slope decomposition method. After NBIS, the threshold voltage shift ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\Delta } {V}_{\text {T}}$ </tex-math></inline-formula> ) was composed of a component by charge trapping and a component with the ionization of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {O}}$ </tex-math></inline-formula> . After PBS, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\Delta } {V}_{\text {T}}$ </tex-math></inline-formula> was dominated by a component arising due to charge trapping. Fitting through the stretched exponential function (SEF) was performed for each separately extracted <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\Delta } {V}_{\text {T}}$ </tex-math></inline-formula> , and the activation energy for each <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\Delta } {V}_{\text {T}}$ </tex-math></inline-formula> was extracted using the inverse Laplace transform method through the extracted <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\Delta } {V}_{\text {T0}}$ </tex-math></inline-formula> , <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\tau $ </tex-math></inline-formula> , and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\beta $ </tex-math></inline-formula> . 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 ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${I}$ </tex-math></inline-formula> – <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}$ </tex-math></inline-formula> ) 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.