Photonic I–V Technique With Photogating Effect for Extended Extraction of Subgap Density of States in Amorphous Oxide Semiconductor TFTs

The extraction of the subgap density of states (DoS, <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>) is crucial in modeling, characterization, and simulation of amorphous oxide semiconductor thin-film transistors (AOS TFTs). Previous photonic current-voltage (I–V) methods were primarily focused on the photoconductive effect (PCE), often leading to erroneous <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>. They are also limited to the subthreshold region with narrow energy range 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>. This study reports an enhanced photonic I–V technique expanding the energy range and refines the energy mapping with both the photovoltaic effect (PVE) and PCE. The method was experimentally validated using an amorphous In-Ga–Zn-O thin-film transistors (TFTs), yielding tail DoS parameters with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${N} _{\text {TA}} \,\, = 2.9\times 10^{{17}}$ </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}} \cdot $ </tex-math></inline-formula>cm-3 and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${E} _{\text {TA}} \,\, =0.06$ </tex-math></inline-formula> eV. These results are consistent with those of technology computer-aided design (TCAD) simulations and are further confirmed with those of a previous method, along with various wavelengths of photons.