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

Abstract

The subgap density of states [ ${g}_{\text {DOS}}$ (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 ${g}_{\text {DOS}}$ (E) in AOS TFTs through the photonic capacitance-voltage (C–V) characterization. We focused on the gate voltage ( ${V}_{\text {G}}$ ) dependence of the photovoltaic effect (PVE), which has not been considered in previous studies. The PVE strongly depends on the amount of ${g}_{\text {DOS}}$ (E) reacting in each energy interval, requiring the consideration of ${V}_{\text {G}}$ -dependency. Furthermore, we incorporated the ${V}_{\text {G}}$ -dependency of the parasitic capacitance into the equivalent capacitance model, resulting in a more accurate extraction of ${g}_{\text {DOS}}$ (E). For validation, the proposed method was applied to amorphous indium-gallium–zinc-oxide (a-IGZO) TFTs with an optical source with $\lambda = 532$ nm and obtained ${N}_{\text {T}} = 6\times 10^{{15}}$ cm $^{-{3}} \cdot $ eV $^{-{1}}$ , ${N}_{\text {D}} = 7\times 10^{{13}}$ cm $^{-{3}} \cdot $ eV−1, ${kT}_{\text {T}} = 0.28$ eV, and ${kT}_{\text {D}} = 0.7$ eV of the exponential and gaussian superposed model of ${g}_{\text {DOS}}$ (E). The proposed method is expected to be a useful tool in the characterization of AOS TFTs.