Shallow Trap States Control Electrical Performance of Amorphous Oxide Semiconductor Thin-Film Transistors

TL;DR

This study measures and models the subgap density of states in amorphous InGaZnO TFTs, revealing how shallow trap states influence electrical performance and enabling precise simulation and defect identification.

Contribution

The paper introduces ultrabroadband photoconduction microscopy to accurately measure subgap DoS and links it to TFT performance, providing a new method for defect analysis and device simulation.

Findings

Shallow trap states are key to tuning TFT electrical characteristics.

Full transfer curves can be simulated from measured DoS without adjustable parameters.

The dominant defect is identified as a Ga-Ga-In oxygen vacancy at ~0.32 eV below the conduction band.

Abstract

The performance of n-type amorphous oxide semiconductor thin-film transistors (TFTs) is largely controlled by the density of states (DoS) near the conduction band mobility edge. Here, the full subgap DoS of amorphous InGaZnO (a-IGZO) TFTs, used in display panels and dynamic random-access memory (DRAM) development, is measured by ultrabroadband photoconduction (UP-DoS) microscopy to within 0.1 eV of the mobility edge. The measured subgap DoS for 25 TFT processing conditions accurately predicts each transfer curve, showing how shallow defect states are electron traps that rigidly tune subthreshold swing, threshold voltage and drift mobility. For a set of TFTs, the subthreshold transfer characteristics can be independently simulated from the experimental shallow defect DoS, with no adjustable parameters. The full transfer curve is simulated by introducing a single parameter: the conduction band tail energy. Additionally, the simulation reveals that the shallow trap density controlling subthreshold behavior can be directly extracted from transfer curves. Finally, a systematic In-enrichment study, combined with DFT+U DoS simulations, enables identification of vacancy cation coordination environments for all experimentally observed subgap peaks. The dominant trap controlling conventional a-IGZO TFT performance is centered at ~0.32 eV below the conduction band mobility edge and is assigned to a Ga-Ga-In oxygen vacancy defect.