Photovoltaic Effect De-Embedded Photonic <i>C</i>–<i>V</i> Characterization of Subgap Density of States in Amorphous Oxide Semiconductor Thin-Film Transistors

The subgap density of states [<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${g}_{\text {DOS}}$ </tex-math></inline-formula>(E)] is a critical parameter governing the electrical characteristics and short-/long-term reliability of amorphous oxide semiconductor thin-film transistors (AOS TFTs). In this study, we propose an advanced technique for <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${g}_{\text {DOS}}$ </tex-math></inline-formula>(E) in AOS TFTs through the photonic capacitance-voltage (C–V) characterization. We focused on the gate voltage (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {G}}$ </tex-math></inline-formula>) dependence of the photovoltaic effect (PVE), which has not been considered in previous studies. The PVE strongly depends on the amount of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${g}_{\text {DOS}}$ </tex-math></inline-formula>(E) reacting in each energy interval, requiring the consideration of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {G}}$ </tex-math></inline-formula>-dependency. Furthermore, we incorporated the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {G}}$ </tex-math></inline-formula>-dependency of the parasitic capacitance into the equivalent capacitance model, resulting in a more accurate extraction of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${g}_{\text {DOS}}$ </tex-math></inline-formula>(E). For validation, the proposed method was applied to amorphous indium-gallium–zinc-oxide (a-IGZO) TFTs with an optical source with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda = 532$ </tex-math></inline-formula> nm and obtained <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${N}_{\text {T}} = 6\times 10^{{15}}$ </tex-math></inline-formula> cm<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{-{3}} \cdot $ </tex-math></inline-formula>eV<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{-{1}}$ </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">${N}_{\text {D}} = 7\times 10^{{13}}$ </tex-math></inline-formula> cm<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{-{3}} \cdot $ </tex-math></inline-formula>eV−1, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${kT}_{\text {T}} = 0.28$ </tex-math></inline-formula> eV, and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${kT}_{\text {D}} = 0.7$ </tex-math></inline-formula> eV of the exponential and gaussian superposed model of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${g}_{\text {DOS}}$ </tex-math></inline-formula>(E). The proposed method is expected to be a useful tool in the characterization of AOS TFTs.