Unveiling the electronic landscape of defect and tail states in a-IGZO thin films through photoluminescence excitation spectroscopy

Abstract

Amorphous indium gallium zinc oxide (a-IGZO) has emerged as a promising semiconductor for next generation optoelectronic and display technologies due to its high carrier mobility, optical transparency, and process compatibility. It should be noted that the performance of a-IGZO devices strongly depends upon the defect states. While advanced techniques such as X-ray photoelectron spectroscopy and photoconduction spectroscopy are frequently employed to probe these defect and tail states, photoluminescence (PL) spectroscopy provides an inexpensive and powerful alternative for directly revealing the defect-related transitions. In this work, a-IGZO thin films were fabricated via the spin coating technique on transparent glass substrates. Structural and morphological analyses confirmed the amorphous phase and uniform surface morphology. The optical bandgap was estimated to be ∼3.3 eV from Tauc plots derived from UV-Vis transmission spectra, along with the Urbach energy of ∼0.4 eV. PL and photoluminescence excitation spectroscopy (PLE) were employed to resolve radiative transitions involving defect and tail-related sub-gap states. PL spectra revealed strong emission peaks centered in the range of 378–406 nm (3.28–3.05 eV) and a second set of emissions centered around 460 nm (∼2.70 eV). In addition, a large Stokes shift value (∼1 eV) was observed, providing further insights into the degree of localized states and structural disorder within the a-IGZO atomic network. Experimental studies, integrated with literature-based sub-gap assignments, enabled the construction of a detailed energy level diagram capturing contributions from oxygen and metal vacancies and under-coordinated cations. This work advances understanding of sub-gap transitions relevant for optimizing its performance in flexible optoelectronic devices.