Real-time optical discrimination of competing defect pathways in a-IGZO heterostructures by in situ spectroscopic ellipsometry

We demonstrate that in situ spectroscopic ellipsometry can discriminate, in real time and nondestructively, buffer-dependent defect-evolution trajectories in amorphous indium–gallium–zinc–oxide (a-IGZO) thin films during thermal annealing at 300 °C. By continuously monitoring the imaginary dielectric function ε2—together with the optical bandgap Eg and Urbach energy EU—throughout the heating, isothermal-hold, and cooling stages, and by separating the reversible thermal response from the irreversible, annealing-induced change via the heating–cooling hysteresis, we show that IGZO on Si3N4 and IGZO on SiO2 buffer layers follow qualitatively distinct trajectories under identical isothermal conditions. These distinct trajectories indicate that thermal annealing does not act on a universal as-deposited IGZO state, but instead drives the evolution of buffer-imprinted defect landscapes formed during film deposition. The Urbach disorder relaxes progressively on Si3N4, whereas it reaches a steady plateau on SiO2, suggesting different thermal susceptibilities of the pre-existing defect configurations. Notably, the two systems converge to nearly identical end-states, so that these contrasting kinetics are invisible to any before/after comparison and emerge only from time-resolved monitoring. The distinct optical signatures are consistent with relaxation or reconfiguration of H-related/weakly coordinated defect states in IGZO on Si3N4 and thermally activated oxygen-defect equilibration in IGZO on SiO2. By resolving defect kinetics inaccessible to conventional ex situ characterization, this work establishes in situ optical-constant analysis as a sensitive diagnostic of buffer-imprinted defect chemistry and its thermal evolution in a-IGZO device stacks.