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Jeon et al. Soft Sci. 2025, 5, 1 https://dx.doi.org/10.20517/ss.2024.35 Page 7 of 39

MO semiconductor
Semiconductor layer for high-performance
For high-performance flexible device applications such as wearable devices and AR/VR devices, the
semiconductor layer must exhibit both high electron mobility and exceptional stability. Achieving these
properties is critical for the development of advanced flexible and wearable electronics, which require
materials that can maintain superior electrical performance under mechanical stress. In O is extensively
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researched for its applications in both conductive and semiconductive roles, owing to its high electron
mobility derived from the extensive overlap of its 5s orbitals. To improve carrier transport, modulate
threshold voltage, and enhance stability, In O is doped with various secondary ions . Among the
[107]
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innovative n-type materials for this purpose are ITZO, nitrogen-doped zinc oxide (ZnON), indium
tungsten oxide (IWO), hafnium indium zinc oxide (HIZO) and lithium zinc oxide (LiZnO) [108-114] . These
materials improved electrical performance and stability, which ensures consistent performance over time.
Shi et al. exhibited the fabrication and performance of bottom gate TFTs with a channel layer of ITZO and a
passivation layer of GaO . The resulting ITZO TFTs demonstrate remarkable electrical properties, including
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a high field-effect mobility exceeding 58.3 cm /V·s, a low subthreshold gate swing of 0.087 V/decade, a
threshold voltage of -0.7 V, and impressive stability [Figure 2A] . The Sn ion contributes to the carrier
[115]
path in MO, similar to the In ion in the a-ITZO channel layer, due to the similarity in the 5s orbital
structures of Sn and In. Consequently, the higher content of In and Sn in the ITZO film promotes the
effective intercalation of the Sn or In 5s orbitals, or increases the percolation conductivity resulting in the
high mobility observed in the a-ITZO TFTs [116,117] . Recently, Li et al. demonstrated high-performance ITZO
TFT employing water vapor during the sputtering process. These devices exhibited a high field-effect
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mobility of 122.10 cm /V·s, a low threshold voltage of -2.30 V, and excellent stability under bias stress .
[118]
Similarly, Tiwari et al. fabricated flexible IWO TFTs with linear field effect mobility = 25.86 cm ·V ·s ,
-1 -1
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subthreshold swing = 0.30 V/decade, and threshold voltage = -1.5 V [Figure 2B] . IWO was an
[111]
appropriate material for flexible high-performance TFTs. The IWO thin films were deposited using radio
frequency (RF) sputtering, followed by post-annealing at 270 °C. These films exhibited smooth morphology
and high carrier density, resulting in excellent TFT characteristics. Additionally, the IWO TFTs
demonstrated stable performance during bending tests. As shown in Figure 2C, Ok et al. investigate high-
[119]
performance ZnON TFTs fabricated on PEN substrates . Utilizing thermal photochemical reaction, the
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field-effect mobility of these TFTs is significantly enhanced from 60 cm /V·s. Unusual variations in
nanostructures were evident during the phase transformation and densification processes. In amorphous
ZnO N , the separated Zn N and ZnO nanocrystalline lattice was remarkably stabilized by reducing oxygen
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y
defects and by interfacial atomic rearrangement without breaking the nitrogen bonding. Photochemical
reaction also improves device stability under bias and illumination stress, with electrical performance
remaining stable after 10,000 bending cycles. ZnON has recently attracted significant interest due to its
exceptional charge transport properties compared to conventional amorphous MOs. High electrical
performance has been attributed to low electron effective mass and the difference in transport mechanisms
*
compared to conventional amorphous MOs. The electron effective mass of ZnON (m /m = 0.19) is lower
e
e
than that of IGZO (0.34 m ) due to the presence of the Zn N (0.10 m ) structure [120-122] .
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The employing of multiple layers in flexible electronics offers several advantages. First, it allows for the
enhancement of electronic properties, enabling the fabrication of devices with high performance. Second,
multiple layers mitigate issues related to charge carrier recombination and trapping, which are common in
single-material systems. By providing pathways for efficient charge carrier movement, multiple layers
contribute to the stability and robustness of the device. Because of these advantages, many studies have been
conducted [98,100,123,124] . Hsu et al. investigate multiple layers of titanium oxide (TiO ) and IGZO TFTs to
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improve flexibility and electrical properties at a low fabrication temperature of 100 °C. The high bond
enthalpy of Ti–O can be optimized by adjusting the number of oxygen vacancies to satisfy the necessary