【International Papers】Enhanced mobility and bias stability of SnO₂ thin-film transistors enabled by Ga₂O₃ passivation-induced two-dimensional electron gas

      Researchers from the Kyungpook National University have published a dissertation titled "Enhanced mobility and bias stability of SnO₂ thin-film transistors enabled by Ga₂O₃ passivation-induced two-dimensional electron gas" in Materials Today Advances.

Background

      Thin-film transistors (TFTs) are essential components that control each pixel in active-matrix displays, such as organic light-emitting diode (OLED) displays and liquid crystal displays (LCDs). Amorphous-phase hydrogenated silicon (a-Si:H) TFTs and polycrystalline silicon (poly-Si) TFTs, fabricated at low temperatures, have been widely employed. However, in recent decades, metal oxide-based TFTs have emerged as fundamental components in active-matrix displays and have replaced their predecessors because they provide higher field-effect mobility than amorphous-phase hydrogenated silicon and exhibit lower leakage currents than low-temperature polycrystalline silicon (poly-Si). In addition, their larger optical bandgap results in transparency, which makes them promising for transparent electronics. As a result, metal oxide-based TFTs are being explored for next-generation display, rollable, and flexible electronic applications.

      Historically, ZnO and In₂O₃ have been investigated as active channel layers in TFTs. Compared with these, SnO₂ offers distinct advantages. The intrinsic carrier mobility and transparency of SnO₂ are higher than those of ZnO or In₂O₃. In addition, SnO₂ has a lower melting point, which means that the sintering temperature required for high crystallinity is lower than that of ZnO and In₂O₃. This represents a critical advantage for achieving low-temperature processing with SnO₂. According to the United States Geological Survey, the global refined production of indium (In), zinc (Zn), and tin (Sn) in 2024 was approximately 990 tons, 12,000 tons, and 290,000 tons, respectively. This supply difference further strengthens the push to replace In and Zn in device applications.

Abstract

      This study presents a method to achieve both high performance and electrical stability in SnO₂ thin-film transistors (TFTs) fabricated by a sol–gel process. Sol–gel–processed Ga₂O₃ passivation layers are introduced onto the SnO₂ active channel layers to address bias instabilities. The Ga₂O₃ films passivate the back channel by compensating dangling bonds and isolating the channel from H₂O and O₂, which improves bias stability. Structural, optical, chemical, and device-level properties were systematically analyzed. When Ga₂O₃ is deposited on SnO₂, electrons redistribute to maintain thermal equilibrium and become confined in the potential well of SnO₂, forming a quasi two-dimensional electron gas that increases the free carrier concentration. This effect results in a 1.5-fold enhancement in field-effect mobility (from ∼10 cm²/Vs to ∼15 cm²/Vs) together with improved bias stability. The proposed SnO₂–Ga₂O₃ passivation method offers a promising route toward bias-stable, high-performance metal oxide films for applications in TFTs, solar cells, transparent conducting electrodes, and resistive random-access memory (ReRAM).

Highlights

      ● Sol-gel processed SnO₂/G₂O₃ hetero structure were introduced for metal oxide thin film transistors.

      ● The deposited Ga₂O₃ films effectively passivate the back channel of the TFTs, leading to bias stability of TFTs.

      ● A-quasi two-dimensional electron gas was formed in the potential well of SnO₂ due to lower work function of Ga₂O₃.

      ● The increased carrier concentration results in enhancement in field effect mobility simultaneously.

Conclusion

      This work demonstrates a strategy to achieve both high performance and electrical stability in metal oxide thin-film transistors through the introduction of Ga₂O₃ passivation layers into sol–gel–processed SnO₂ TFTs. The Ga₂O₃ layers effectively suppress oxygen vacancies at the SnO₂ surface because Ga-O bonds are stronger than Sn-O bonds. They also minimize unwanted surface reactions such as oxygen adsorption and desorption caused by ambient gases, both of which are critical factors in bias stability.

      Unlike conventional passivation approaches for metal oxide TFTs, the Ga₂O₃ passivation layers not only improve stability but also enhance field-effect mobility. Because Ga₂O₃ has a lower work function than SnO₂, electrons are confined within the SnO₂ potential well, forming a quasi-two-dimensional electron gas that increases the free carrier concentration. This results in a 1.5-fold improvement in field-effect mobility (from ∼10 cm²/Vs to ∼15 cm²/Vs) while maintaining strong bias stability. Overall, the proposed SnO₂–G_a2O₃ passivation scheme provides a practical route toward bias-stable, high-performance metal oxide films for next-generation electronics, including TFTs, solar cells, transparent electrodes, and resistive random-access memory (reRAM) devices.

Fig. 1. GIXRD spectra of the (a) single SnO₂ films and (b) single Ga₂O₃ films. The inset shows high-resolution XPS spectra of Ga 2p.

Fig. 2. Cross-sectional SEM images of SnO₂ films (a) without Ga₂O₃ films, (b) with a Ga₂O₃ (0.01M) passivation layer, and (c) with a Ga₂O₃ (0.05 M) passivation layer.

Fig. 3. High-resolution XPS depth-profile spectra of SnO₂ films without Ga₂O₃, with Ga₂O₃ passivation layer, and with a Ga₂O₃ (0.05 M) passivation layer. Panels (a)–(c) show Ga 2p spectra, (d)–(f) show O1s spectra, and (g) summarizes the O 1s depth-profile results.

Fig. 4. Representative I_D–V_D characteristics of SnO₂ TFTs (a) without Ga₂O₃ passivation, (b) with Ga₂O₃ (0.01M) passivation layer, and (c) with a Ga₂O₃ (0.05 M) passivation layer. (d) Representative transfer curves of the fabricated TFTs without and with Ga₂O₃ passivation layers. The inset shows a schematic of a SnO₂ TFT with a Ga₂O₃ passivation layer.

Fig. 5. Representative transfer characteristics of SnO₂ TFTs (a) without Ga₂O₃ passivation, (b) with a Ga₂O₃ (0.01M) passivation layer, and (c) with a Ga₂O₃ (0.05 M) passivation layer during the NBS test. (d) Extracted saturation field-effect mobility and threshold voltage V_th during the NBS test.

Fig. 6. Representative transfer characteristics of SnO₂ TFTs (a) without Ga₂O₃ passivation, (b) with a Ga₂O₃ (0.01M) passivation layer, and (c) with a Ga₂O₃ (0.05 M) passivation layer during the PBS test, (d) Extracted saturation field-effect mobility and V_th during the PBS test.

Fig. 7. Tauc plots ((ahv)² versus photon energy) of (a) SnO₂ and (b) Ga₂O₃ (0.05 M) films. (c, d) Low- and high-binding-energy slopes of the UPS spectra for SnO₂ films. (e, f) Low- and high-binding-energy slopes of the UPS spectra for Ga₂O₃ (0.05 M) films. (g, h) Energy-level alignment of Ga₂O₃ (0.05 M) and SnO₂ before and after contact, respectively.

 

DOI：

doi.org/10.1016/j.mtadv.2025.100619