Coupling between gate-electrode plasmon and gate-dielectric phonon on carrier mobility of InGaZnO thin-film transistor

Abstract

The adoption of high-dielectric-constant (high-k) material as the gate dielectric of thinfilm transistor (TFT) introduces the remote phonon scattering (RPS) on the neighboring channel carriers, leading to severely degraded carrier mobility. However, the oscillation of free carriers about ions in the gate electrode can form plasmons to electrically couple with gate-dielectric phonons. The main objective of this research is to study the effect of the coupling between gate-electrode plasmon and gate-dielectric phonon on the carrier mobility of InGaZnO (IGZO) TFT. Firstly, the effect of gate hole concentration on the RPS and on the carrier mobility of IGZO TFT is studied. Experimental results show that the carrier mobility increases with increasing gate hole concentration, which indicates that like electrons in n-Si gate, holes in p-Si gate can also form plasmons to electrically couple with gate-dielectric phonons and to suppress the RPS. The higher carrier mobility achieved by the higher hole concentration in the gate electrode is attributed to more gate-electrode plasmons participating in suppressing the RPS (gate screening effect). Secondly, a comparison of gate screening effect between n-Si gate and p-Si gate in IGZO TFT is carried out. After correction for the depletion layer in the n-Si gate under a positive gate voltage, the carrier mobility of IGZO TFT with n-Si gate is higher than that with p-Si gate for the same doping concentration. This result suggests that electrons in n- Si gate have a stronger screening effect on the RPS than holes in p-Si gate possibly due to the smaller effective mass of electrons than holes. Thirdly, the effect of gate carrier mass on the RPS in IGZO TFT is investigated by adopting different semiconductors (i.e. p-Ge, p-GaAs and p-Si) with various doping concentrations as the gate electrode. For the same gate doping concentration, the highest carrier mobility is achieved by p-Ge gate, followed by p-GaAs gate. The higher carrier mobility achieved by the gate material with smaller carrier mass is attributed to the higher gate-plasmon frequency/energy for stronger coupling with the gate-dielectric phonons. Fourthly, besides the screening effect which suppresses the RPS, an anti-screening effect in IGZO TFT is demonstrated to enhance the RPS. When the gate-electrode plasmons have a close/equal frequency to gate-dielectric phonons, resonance between them enhances the atomic vibration of the gate dielectric and thus the RPS to produce significantly degraded carrier mobility. Finally, the effect of dopant redistribution in the gate electrode on the RPS is investigated. Both screening effect and anti-screening effect of the gate electrode on the RPS are observed in IGZO TFTs with the same gate doping concentration in the bulk but different doping concentrations near the gate-electrode surface. The result implies the dominant role of the plasmons near the gate electrode surface in interacting with the gate-dielectric phonons to produce both the screening and anti-screening effects on the RPS.