Defect Passivation and Carrier Reduction Mechanisms in Hydrogen-Doped In-Ga-Zn-O (IGZO:H) Films upon Low-Temperature Annealing for Flexible Device Applications

Abstract

Low-temperature activation of oxide semiconductor materials such as In-Ga-Zn-O (IGZO) is a key approach for their utilization in flexible devices. We previously reported that the activation temperature can be reduced to 150 °C by hydrogen-doped IGZO (IGZO:H), demonstrating a strong potential of this approach. In this paper, we investigated the mechanism for reducing the activation temperature of the IGZO:H films. In situ Hall measurements revealed that oxygen diffusion from annealing ambient into the conventional Ar/O₂-sputtered IGZO film was observed at >240 °C. Moreover, the temperature at which the oxygen diffusion starts into the film significantly decreased to 100 °C for the IGZO:H film deposited at hydrogen gas flow ratio (R[H₂]) of 8%. Hard X-ray photoelectron spectroscopy indicated that the near Fermi level (E_F) defects in the IGZO:H film after the 150 °C annealing decreased in comparison to that in the conventional IGZO film after 300 °C annealing. The oxygen diffusion into the film during annealing plays an important role for reducing oxygen vacancies and subgap states especially for near E_F. X-ray reflectometry analysis revealed that the film density of the IGZO:H decreased with an increase in R[H₂] which would be the possible cause for facilitating the O diffusion at low temperature.

Keywords: defect passivation; flexible electronics; hydrogen in In–Ga–Zn–O; low-temperature activation; oxide semiconductors; oxygen diffusion.

Figure A1

Annealing time dependence of the carrier density for H-doped IGZO films after 1 h annealing in N₂ at 150 °C.

Figure 1

Properties of conventional IGZO film: (a) carrier density obtained during the in situ Hall measurement in air and vacuum; (b) O 1s spectra before and after 1 h annealing in air at 300 °C; (c) electronic structure around the band gap region for as-deposited and after annealing at various temperature.

Figure 2

Carrier density of H doped IGZO films obtained from the in situ Hall measurement performed in air and vacuum atmosphere: (a) R[H₂] = 2%; (b) R[H₂] = 5%; (c) R[H₂] = 8%; (d) Carrier density variation as a function of annealing time at fixed temperature of 150 °C performed in air.

Figure 3

O 1s spectra of IGZO and IGZO:H films: (a) and (c) as-deposited before and after deconvolution, respectively; (b) and (d) annealed films in air for 1 h before and after deconvolution, respectively.

Figure 4

HAXPES spectra around band gap of the IGZO films deposited at different R[H₂] and at fixed R[O₂] = 1%: (a) as-deposited; (b) before and after annealing for 1 h in air at different temperature.

Figure 5

XRR results of the IGZO films deposited at different R[H₂] and at fix R[O₂] of 1%: (a) as-deposited; (b) after annealing in air for 1 h at the annealing temperature of 150 °C. The insets show the reflectivity intensity near the critical angle representing the change in film density upon the hydrogen incorporation.