We
investigated the influence of the multilayered hybrid buffer
consisting of Al2O3/PA (polyacrylic) organic
layer/Al2O3 on the electrical and mechanical
properties of amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs).
The multilayered organic/inorganic hybrid buffer has multiple beneficial
effects on the flexible TFTs under repetitive bending stress. First,
compared to the PA or Al2O3 single-layered buffer,
the multilayered hybrid buffer showed an improved WVTR value of 1.1
× 10–4 g/m2 day. Even after 40,000
bending cycles, the WVTR value of the hybrid buffer increased only
by 17%, while the WVTR value of the Al2O3 layer
doubled after cyclical bending stress. We also confirmed that the
hybrid buffer has advantages in mechanical durability of the TFT layers
because of the change in the position of the neutral plane and the
strain reduction effect by the PA organic layer. When we fabricate
a top-gate a-IGZO TFT with the hybrid buffer layer (HB TFT), the device
shows V
th = 0.74 V, μFE = 14.4 cm2/V·s, a subthreshold slope of 0.27 V/dec,
and hysteresis of 0.21 V, which are superior to that of TFTs fabricated
on an Al2O3 single-layer buffer (IB TFT). From
the X-ray photoelectron spectroscopy and elastic recoil detection
analysis, the difference in the electrical performance of TFTs could
be explained by hydrogen-related molecules. After annealing at 270
°C, the amounts of hydrogen found in the a-IGZO layer for the
IB, HB, and OB TFTs were 3.57 × 1021, 5.77 ×
1021, and 7.34 × 1021 atoms/cm3, respectively. A top-gate bottom-contact structured a-IGZO TFT fabricated
on the PA layer (OB TFT) showed a gate dielectric breakdown because
of excessively high hydrogen content and high nonbonding oxygen content.
On the other hand, HB TFTs showed better positive bias stability because
of the higher hydrogen concentration, as hydrogen (when not excessive)
is beneficial in passivating electron traps. Finally, we conducted
60,000 repetitive bending cycles on IB TFTs and HB TFTs with various
bending radii down to 1.5 mm. The HB TFT shows improved mechanical
durability and exhibits less electrical degradation during and after
repetitive bending stress, compared to the IB TFT.
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