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

TL;DR

This work tackles how shallow subgap defect states near the conduction-band edge govern the electrical performance of amorphous IGZO TFTs. By directly measuring the on-chip subgap DoS with ultrabroadband photoconduction DoS microscopy and then feeding this DoS into Fermi–Dirac–statistical simulations, the authors reproduce TFT transfer curves with no adjustable parameters and a single conduction-band tail parameter $W_{TA}$. They identify a dominant shallow trap at about $0.32$ eV below the CBM, assigned to a Ga-Ga-In oxygen vacancy, and use DFT+U to map subgap peaks to specific vacancy coordinations; indium enrichment further reveals additional In-rich traps near the CBM, including a shallower trap around $0.12$ eV. The approach enables extraction of total shallow-trap density from transfer curves and offers a path to optimize a-IGZO TFTs by linking processing conditions to defect landscapes and device metrics such as subthreshold swing, threshold voltage, and drift mobility. This on-chip DoS–driven framework provides a concrete route to predict and tailor oxide semiconductor transistor performance for display, DRAM, and neuromorphic applications.

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.

Figures (8)

• Figure 1: (a) Transfer curves (solid lines) for four a-IGZO TFTs with different processing conditions. Overlaid is the independently measured shallow trap density of states (filled dashed) for each TFT. (b) Corresponding second derivative of the transfer curve (solid lines) overlaid with the first derivative of the experimental density of states (filled dashed) showing vertical peak alignment. In each TFT, the point of largest curvature in the transfer curve corresponds to when the subgap DoS is changing most rapidly. (inset) Zoom-out of transfer curves with the different operating regions shaded.
• Figure 2: (a) The ultrabroadband photoconduction spectrum (black dots) and the corresponding DoS (first derivative, dashed line) for a representative a-IGZO TFT is characterized by three distinct regions: i) direct bandgap (gray), (ii) non-direct bandgap (gold), (iii) subgap defects (blue and yellow) composed of an Urbach tail and Gaussian peaks numbered 0-10. (inset) Diagram of the UP-DoS experimental method. (b) Mobility, $\mu_{EKV}$, as a function of experimentally measured shallow trap density (see inset) for 15 different a-IGZO TFTs. (c) Subthreshold Swing plotted against the experimentally measured shallow trap density for TFTs designed for both display (red) and DRAM (black) applications.
• Figure 3: (a) Transfer curves of 7 different nanoscale DRAM a-IGZO TFTs processed under different growth and annealing conditions. (b) The ultrabroadband photoconduction DoS spectra near the conduction band mobility edge for the 7 different TFTs showing distinct steplike features. (lower panel) The corresponding experimentally measured subgap density of states for each TFT. The shallow trap density, highlighted by the red shaded region, is observed to be directly correlated to the TFT performance.
• Figure 4: (a) TFT simulated mobility ($\mu_{SIM}$, red line) as a function of quasi-Fermi level energy. $\mu_{SIM}$ is obtained by modeling trapping in experimentally measured Gaussian-like subgap states (filled dashed) and conduction band tail states (filled green). (b) a-IGZO TFT transfer curve (black, left y-axis) plotted with the experimentally measured subgap defect DoS (filled dashed, right y-axis) converted to a gate voltage axis, showing the dominant electron traps for different TFT operating regions. (c) Experimentally extracted (thick lines) and DoS simulated (thin lines) free and trapped electron density as a function of gate voltage. (d) DoS simulated transfer and mobility curves (red) plotted with experimentally measured data (black). Red dashed line is simulated transfer curve if Gaussian trap contributions are excluded.
• Figure 5: DoS simulated (a) TFT gate voltage induced trap density, (b) mobility, and (c) transfer curve as a function of gate voltage for different conduction band tail state Urbach energies. The slope of the transfer curve in the linear regime increases with Urbach energy, while the subthreshold behavior is unaffected. DoS simulated TFT (d) gate voltage induced trap density, (e) mobility, and (f) transfer curve for different shallow V$_O$ trap densities. With increasing shallow V$_O$ trap density, the threshold voltage and subthreshold swing increase as more trap states need to be filled before the TFT can enter the linear regime.