Page 188 - Read Online

Page 18 of 39 Jeon et al. Soft Sci. 2025, 5, 1 https://dx.doi.org/10.20517/ss.2024.35

an acceptor and as a defect binder due to its ionic radius being similar to that of oxygen. This similarity
allows nitrogen to effectively substitute for O (NO) and reduce oxygen vacancy in ZnO-based thin
films [161,174] . Consequently, nitrogen doping process can enhance the bias stability of a-IGZO TFTs [158,175] .
However, this process has also been associated with a decrease of mobility and a reduction of the optical
band gap energy in nitrogen-doped a-IGZO TFTs . Additionally, Xie et al. introduce nitrogen-doped
[158]
AOS TFTs with a double-stacked channel layer structure combined with amorphous InZnO (a-IZO) and
nitrogen-doped a-IGZO films. This TFT with a double-stacked channel layer exhibits superior field-effect
2
mobility (up to 49.6 cm /V·s) and stability under both bias and light stress tests compared to conventional
single-layer TFTs. The enhanced performance is primarily due to the heterojunction within the channel
layers and the lower interface and bulk trap densities achieved through nitrogen doping, which effectively
reduces oxygen vacancies and trap sites in the channel. Additionally, the a-IGZO layer functions as a
passivation layer, shielding the channel from ambient gases and UV light, further improving device stability
[159]
as shown in Figure 6C . Han et al. investigated the impact of nitrogen and IZO composition on electrical
performance and stability in IZO TFTs . Park et al. investigated nitrogen-doped ZnON TFTs on PEN
[176]
substrates by employing dual gates for high performance. The effects of applying a dual gate structure are
attributed to the formation of a high electron density channel away from the gate dielectric/semiconductor
2
-1 -1 [113]
interface (field-effect mobility = 65.8 to 147 cm ·V ·s ) .
The metal cation doping process is also widely used, and extensive research has been conducted on indium-
based materials. In O is a widely studied material for both conductive and semiconductive applications due
3
2
to its high electron mobility, which results from the extensive overlap of 5s orbitals. To enhance carrier
transport, In O is typically doped with secondary ions. Sn-doped In O (ITO) is commonly used as a
3
2
3
2
3+
transparent conductor because the difference in oxidation states between In and Sn leads to high carrier
4+
concentrations . When doped with Ga and/or Zn (forming materials such as IGO, IZO, and IGZO), In O
[95]
2
3
achieves better semiconductor properties. Pure In O TFTs often have high carrier concentrations, making
3
2
it difficult to control off-current and V , but Ga and Zn help mitigate this by acting as a carrier suppressor
TH
and stabilizing the amorphous state, respectively [97,177] . Other dopants are also used to enhance or stabilize
the electrical characteristics of oxide semiconductors. Important factors in choosing dopants include energy
bandgap, standard electrode potential (SEP), Gibbs Energy of oxidation, and electronegativity [55,60,95] . Larger
bandgaps shift donor states to deeper levels, lower SEP enhances carrier suppression, and higher differences
in electronegativity and Gibbs Energy of oxidation reduce V . Metals such as lanthanum (La), magnesium
O
(Mg), yttrium (Y), hafnium (Hf), strontium (Sr), and barium (Ba) are effective carrier suppressors due to
these properties [Figure 6D] [55-57,60] . Here, electrical properties of the hydrogen, nitrogen, fluorine and other
metal-doped MO semiconductors are summarized in Table 5.
Deposition method
The fabrication of high-performance flexible MO TFTs employs various deposition techniques to ensure
precise dimensions and consistent performance. These methods are chosen for their ability to produce
uniform, dense films at low temperatures compatible with flexible substrates. Deposition methods include
sputtering, ALD, PLD, spin coating, inkjet printing, and spray coating. In addition to the standard criteria
for thin-film deposition on flexible substrates, there are specific requirements critical for fabrication of
flexible devices. These include maintaining low deposition temperatures compatible with the thermal limits
of the flexible substrates, ensuring sufficient adhesion of the deposited materials to substrate to prevent
delamination during bending, and minimizing the strain within the deposited layers to preserve the
mechanical properties, such as bendability, of the final devices [10,12,15,95] .