The Optoelectronic Properties Modification of Amorphous IZTO Thin Films by Oxygen Partial Pressure and Oxidation Annealing

Abstract

Transparent conductive oxide (TCO) thin films are crucial for modern electronic devices. Among them, amorphous indium-zinc-tin oxide (IZTO) films have attracted considerable interest due to their potential for flexible applications. This work systematically investigates the influence of oxygen partial pressure and oxidative annealing on the properties of IZTO films. IZTO films deposited at 3% oxygen partial pressure exhibited a low sheet resistance of 19.5 Ω/□, high visible (T_VIS = 83.6%) and near-infrared (T_NIR = 90.0%) transmittance. After oxidative annealing, IZTO films showed an increased sheet resistance of 29.2 Ω/□, and a high Hall mobility of 51.4 cm²·V⁻¹·s⁻¹, while retaining excellent optical transparency. All the films showed amorphous phase and smooth surface structure. These results are valuable for developing high-performance, transparent amorphous electrodes in flexible optoelectronic applications.

Keywords:
IZTO; TCO film; Amorphous; Mobility; Transmittance

1. Introduction

Transparent conductive oxide (TCO) thin films serve as an essential functional layer in modern optoelectronic devices, including touchscreens, liquid crystal displays, photovoltaic cells, and light-emitting diodes. Ideal TCOs necessitate a trade-off between maximizing visible light transmittance and ensuring excellent electrical conductivity, imposing stringent demands on material design and deposition processes¹. Tin doped indium oxide (ITO) has long been the benchmark TCO due to its outstanding optical and electrical properties and widespread industrial adoption. However, conventional ITO films typically require high-temperature annealing (300~450 °C) to achieve optimal performance²,³. Such high-temperature processes not only increase fabrication costs but also limit the applications in flexible electronics and low-temperature device integration.

To address these limitations, amorphous TCOs such as ZnO:Mg:Ga (MGZO)⁴, AlF₃:GaF₃:ZnO (FAGZO)⁵, Zn₂SnO₄ (ZTO)⁶, and ZnO:In₂O₃ (IZO)⁷ have attracted significant attention. Compared with crystalline counterparts, amorphous films exhibit superior mechanical flexibility, thickness uniformity, and thermal stability, enabling low-temperature deposition and compatibility with flexible substrates. Among these materials In₂O₃:ZnO:SnO₂ (IZTO) has emerged as a promising amorphous TCO. Synergistic Zn and Sn doping enables low-temperature deposition while maintaining high visible-light transmittance (>80%), tunable work function, favorable electrical properties, and excellent corrosion and thermal stability⁸. Recent studies have found that amorphous In-Zn-Sn-O films can achieve competitive conductivity without the need for high-temperature crystallization, provided that oxygen incorporation and structural relaxation are carefully controlled⁹,¹⁰. These studies suggest that room-temperature deposition is feasible when the balance between oxygen-related defects and carrier activation is optimized. This kind of study strengthens the motivation to explore IZTO fabrication strategies capable of simultaneously delivering high transparency and low resistivity under low-temperature conditions.

Nevertheless, simultaneously achieving high optical transmittance, low sheet resistance, and uniform thickness presents significant challenges, as optimizing one property often compromises others. Previous studies have attempted to reduce sheet resistance and enhance carrier mobility by adjusting sputtering process parameters, optimizing target composition, or introducing post-deposition annealing¹¹-¹⁴. For example, various reports indicated that high-temperature annealing can effectively lower sheet resistance; however, such treatments frequently led to either reduced visible light transmittance or insufficient thickness uniformity¹⁵,¹⁶. Recent findings emphasize that both post-annealing and oxygen partial pressure play critical roles in regulating defect states, carrier activation mechanisms, and amorphous network stability in multi-component In-Zn-Sn-O systems¹⁷,¹⁸. These findings highlight the importance of controlling oxygen stoichiometry and structural disorder to achieve high carrier mobility and low resistivity under low-temperature processing conditions.

In this study, we develop and fabricate IZTO thin films with low sheet resistance (<15 Ω/□) at room temperature using a DC pulsed magnetron sputtering system. To achieve an optimal balance between high optical transmittance and low sheet resistance, we systematically investigate the effects of film thickness on carrier mobility and sheet resistance, revealing the synergistic influence of thickness on electrical performance. Focusing on 300 nm-thick films, we further examine the roles of oxygen partial pressure during deposition and subsequent oxidative annealing in modulating morphological, structural, optical, and electrical characteristics. Through careful optimization of deposition and annealing conditions, the resulting IZTO films exhibit simultaneously high visible and near-infrared transmittance, low sheet resistance, and enhanced carrier mobility along with a superior overall performance. These findings not only provide practical guidelines for the fabrication of high-performance IZTO transparent electrodes but also will lay an experimental foundation for their application in next-generation flexible optoelectronic devices.

2. Experiment Procedures

The IZTO thin films were deposited on glass substrates (20 mm × 20 mm × 0.5 mm) via pulsed DC magnetron sputtering. The effects of oxygen partial pressures (P(O₂) = 0%, 2%, 3%, 4% and 5%) on the film properties were investigated. The as-deposited IZTO films were annealed in a tube furnace at 200°C with an O₂ flow rate of 2 L/min for 30 min. The IZTO target with dimensions of 399 mm × 70 mm × 7 mm was manufactured by Shenzhen APG Material Company Limited. A pulsed DC power supply was employed for depositing films. The chamber was evacuated to a base pressure below 3 × 10⁻³ Pa, and the working pressure was stabilized at 0.26~0.31 Pa. The substrate temperature was kept at room temperature.

All measurements were performed at room temperature. The sheet resistance of the IZTO films was measured using a four-point probe system (Jingge Electronics, S2558B-F01). Carrier concentration and Hall mobility were determined by the Van der Pauw method via a fast Hall effect measurement system (Yihong Scientific Instruments, M91). Optical transmittance in the wavelength range of 200~1200 nm was acquired using an ultraviolet-visible spectrophotometer (Yuanxi Instruments, X-8). Film thickness was characterized with a surface profilometer (Bruker, DektaXT). The morphology of the IZTO films was observed using field emission scanning electron microscopy (FE-SEM, Zeiss Sigma 300) and scanning probe microscopy (SPM, Veeco DiInnova). Phase structure was analyzed by X-ray diffraction (XRD, Rigaku SmartLab-9kW).

3. Results and Discussion

3.1. Electrical properties

Figure 1 shows the effects of oxygen partial pressure on the sheet resistance (R_sq) and Hall mobility of the IZTO films with thicknesses of 100, 200, and 300 nm. As shown in Figure 1a, R_sq decreases with increasing oxygen pressure, reaching a minimum at P(O₂) = 3%, and then gradually increases at higher pressures, a trend consistent across all thicknesses. The 300 nm thick film deposited at P(O₂) = 3% exhibits the lowest sheet resistance (R_sq = 19.54 Ω/□), highlighting the positive effect of increased thickness on carrier transport. Figure 1b shows that Hall mobility generally increases with increasing oxygen pressure, reaching a maximum of 42.09 cm²·V⁻¹·s⁻¹ for the 300 nm thick film at P(O₂) = 5%, indicating that an appropriate combination of thickness and oxygen partial pressure can optimize its electrical performance.

Figure 1
(a) Sheet resistance and (b) Hall mobility of IZTO films (100~300 nm) as a function of deposition oxygen partial pressure.

Figure 2 shows the resistivity of the IZTO films with different thicknesses as a function of oxygen partial pressure. As illustrated in Figure 2a, the resistivity decreases with increasing P(O₂), reaching a minimum at 3%, and goes up thereafter. The corresponding carrier concentration trends are presented in Figure 2b. For films with a thickness of ≤200 nm, carrier concentration increases with P(O₂) below 3%, which can be attributed not only to improved film continuity that enhances dopant (Sn⁴⁺) activation, but also to moderate suppression of deep-level oxygen vacancies (V_o) that reduces carrier trapping¹⁹-²². In this regime, increased oxygen incorporation promotes a more stable M-O (M = In, Zn, Sn) coordination, leading to a partial structural relaxation and reduced disorder-related scattering, which collectively contribute to an increase in effective carrier concentration²³,²⁴. The carrier concentration drops when the oxygen partial pressure is beyond 3%. Whereas, for the 300 nm thick films, oxygen vacancy suppression becomes the dominant factor because thicker films possess a denser amorphous network and incorporate oxygen more efficiently during deposition. Higher P(O₂) increasingly suppresses donor-like V_o, lowers the In/O sub-stoichiometry, and reduces the number of available shallow donor states, resulting in a monotonic decrease in carrier concentration with increasing oxygen partial pressure²²,²⁵. These results indicate that the carrier concentration evolution with P(O₂) is governed by a competition among dopant activation, V_o suppression, and oxygen-induced stoichiometric rearrangement, rather than a single mechanism²¹,²⁴,²⁶. This integrated mechanism explains why thinner films (≤200 nm) exhibit a non-monotonic carrier concentration response, whereas thicker films (300 nm) show a monotonic decrease due to more effective oxygen incorporation and stronger vacancy suppression²²,²³. Based on these observations, subsequent investigations focus on 300 nm films to systematically examine the effects of oxidative annealing on their optical and electrical properties, morphology, and phase structure.

Figure 2
(a) Resistivity and (b) Carrier concentration of IZTO thin films as a function of deposition oxygen partial pressure.

Figure 3a shows the sheet resistance (R_sq) of the IZTO films after annealing as a function of oxygen partial pressure. A minimum R_sq of 21.52 Ω/□ is obtained at P(O₂) = 2%. Relative to the as-deposited state, annealing reduces R_sq for films prepared at P(O₂) < 3%, with the film at P(O₂) = 2% exhibiting a decrease of 2.28 Ω/□. In contrast, films deposited at P(O₂) ≥ 3% show a pronounced increase in R_sq, with relative increments of 51~150%. This data suggests that outcome of the annealing process is highly sensitive to the initial P(O₂). Figure 3b presents the Hall mobility and carrier concentration of the annealed IZTO films as a function of oxygen partial pressure. Hall mobility increases with increasing P(O₂), reaching a maximum of 51.40 cm²·V⁻¹·s⁻¹ at 3%, before decreasing at higher pressures. Compared with the as-deposited films, all samples prepared at P(O₂) > 0% show enhanced mobility, with increases of 12~46%. Importantly, the oxygen partial pressure corresponding to the maximum Hall mobility shifts from 5% before annealing to 3% after annealing.

Figure 3
(a) Sheet resistance and (b) Hall mobility, resistivity, and carrier concentration of IZTO thin films as a function of oxygen partial pressure.

The observed increase in Hall mobility can be attributed to two mechanisms. First, annealing relieves deposition-induced residual stress, reducing phonon scattering and enhancing carrier transport. Second, oxidative annealing decreases oxygen vacancy concentration, lowering carrier density and suppressing ionized impurity scattering²⁷,²⁸. Below an oxygen partial pressure threshold of 2%, the carrier concentration remains relatively stable. In stark contrast, deposition above this threshold yields a 54~63% reduction in carrier concentration, thereby confirming the dominant role of vacancy reduction dynamics in modulating electronic properties. Changes in mobility and carrier concentration directly govern sheet resistance, given by ρ =1/(e·n·μ_n), where e is the elementary charge, n is the carrier concentration, and μ_n is Hall mobility. For P(O₂) > 3%, both μ_n and n decrease after annealing, reducing the product factor μ_n·n and improving sheet resistance. In contrast, for P(O₂) ≤ 3%, μ_n increases substantially, and despite a slight decrease in n, the overall μ_n·n increases, leading to lower sheet resistance. This indicates that mobility predominantly governs the variation in this regime.

3.2. Optical properties

The transmittance in the 300~1150 nm range before and after annealing was comparatively analyzed, and the transmittance spectra of the IZTO films deposited under different oxygen partial pressures are shown in Figure 4. The clear interference fringes confirm smooth surfaces and minimal scattering. As shown in the inset of Figure 4a, the absorption edge in the 300~400 nm region shifts slightly to longer wavelengths with increasing oxygen pressure. This behavior, together with carrier concentration trends, reflects the Burstein-Moss effect, where higher carrier concentrations cause a blue shift of the absorption edge²⁹. Visible transmittance in 380~780 nm also improved with increasing oxygen pressure, reaching 85.34% at P(O₂) = 4%. This enhancement is attributed to the reduction of oxygen vacancies and carrier concentration, which suppresses carrier absorption and defect scattering. A similar trend appears in the near-infrared light (780~1150 nm). Figure 4b presents the spectra after annealing, where interference fringes persist, confirming the preservation of surface smoothness and film uniformity. Annealing increases overall visible transmittance as evident from the upward spectral shift. Notably, the film deposited at P(O₂) = 2% shows a blue shift in the near-UV region after annealing, attributed to a Burstein-Moss effect caused by increased carrier concentration. Additionally, the film deposited at P(O₂) = 3% shows high optical transmittance (T_VIS = 83.62%, T_NIR = 89.96%), which is well preserved after oxidative annealing (T_VIS = 82.8%, T_NIR = 89.96%), confirming that annealing improves electrical performance without sacrificing transparency.

Figure 4
Transmittance of IZTO thin films as a function of oxygen partial pressure: (a) as-deposited, (b) after annealing.

Figure 5 shows the Tauc plots used to extract optical band gaps (E_g) for direct allowed transitions. For the as-deposited films, E_g increases from 3.45 eV (0%) to 3.61 eV (2%), corresponding to a blue shift of the absorption edge. This shift results from the Burstein-Moss effect, where higher carrier concentration fills conduction-band states and shifts the apparent band edge to higher energies. As the P(O₂) is increased beyond the 2~3%, the E_g exhibits a gradual reduction from 3.55 eV to 3.42 eV. This narrowing reflects the effective suppression of donor-like oxygen vacancies and a corresponding decrease in the occupation of conduction-band tail states. After annealing, the overall E_g values remain within a similar range, but their evolution with P(O₂) becomes more pronounced. The sample deposited at P(O₂) = 2% shows an increase in E_g (i.e., 3.55 eV), consistent with increased carrier concentration after annealing. In contrast, films deposited at higher oxygen pressures (3~5%) show slightly reduced E_g values, mainly due to the further elimination of sub-band-tail states associated with oxygen vacancies. These trends are consistent with the transmittance spectra in Figure 4 and confirm that the optical band gap evolution results from the interplay of carrier concentration, oxygen vacancy suppression, and structural relaxation in the amorphous IZTO network.

Figure 5
The Tauc's plots of IZTO thin films as a function of oxygen partial pressure: (a) as-deposited, (b) after annealing.

Overall, the observed trends in the optical and electrical properties of the IZTO films are consistent with those reported for most amorphous In₂O₃-based thin films, where the electrical conductivity is predominantly governed by oxygen vacancies³⁰,³¹. By precisely controlling the oxygen partial pressure during deposition within the range of 0~5%, the oxygen vacancy concentration in IZTO films was effectively reduced, enabling fine-tuning of both the electrical and optical properties. Furthermore, the annealing response of the IZTO films strongly depends on the oxygen partial pressure during deposition. For the films deposited at P(O₂) ≤ 2%, only minor changes in electrical properties are observed after annealing, but the films deposited at P(O₂) ≥ 3% exhibit significant modifications in their electrical characteristics following the same annealing process. This differential sensitivity is likely attributable to variations in initial film morphology, surface roughness, and defect structures resulting from the specific deposition conditions.

3.3. Amorphous structure and microstructure

Figure 6 presents the XRD patterns of the IZTO films as a function of oxygen partial pressure and annealing. The prominent diffraction peaks located at 21.4° and 31.5° remain invariant across all processing conditions. This stability confirms that the microstructure and fundamental phase composition of the IZTO films are independent of variations in oxygen partial pressure. Their broad, hump-like profiles of these peaks, with calculated full widths at half maximum (FWHM) values of 3.76° and 3.51°, confirm the amorphous nature of the films. After annealing, the peaks exhibit slightly reduced peak broadening while maintaining their positions, indicating that annealing relieves deposition-induced stress without altering the amorphous structure, thus demonstrating the structural stability of IZTO film.

Figure 6
XRD patterns of IZTO thin films as-deposited (top) and after annealing (bottom).

Figure 7 shows FE-SEM images of the IZTO films as a function of oxygen partial pressure and annealing. The as-deposited films consist of densely packed, island-like clusters formed from ultrafine particles. With increasing oxygen partial pressure, the islands grow larger and their boundaries sharpen, while the internal fine particles remain unresolved, consistent with the amorphous structure confirmed by XRD. At P(O₂) = 4%, the islands exhibit high uniformity without oversized grains. After annealing (Figure 7df), island-like morphology is preserved, but larger islands and stronger interconnections appear. These islands increased in size, and eventually coalesced, leading to a film-island growth mode consistent with the Stranski-Krastanov mechanism³². This improved connectivity likely facilitates carrier transport, providing a structural basis for enhanced Hall mobility.

Figure 7
Surface morphologies of IZTO films deposited at different oxygen partial pressures: (a-c) as-deposited, (d-f) after annealing.

Figure 8 compares the AFM surface topographies of the IZTO films as a function of oxygen partial pressures and annealing. Each subfigure includes annotations indicating the scan size, the maximum and minimum peak heights, the specific oxygen partial pressure, and the corresponding root mean square (RMS) roughness. The as-deposited films (Figure 8ae) show 5 × 5 µm 3D morphologies dominated by island-like peaks. For P(O₂) > 3%, localized nano-island protrusions appear, consistent with the FE-SEM observations. The RMS roughness exhibits a non-linear variation, decreasing to 2.04 nm at 4% P(O₂) before increasing again at 5% P(O₂), indicating that surface roughness can be tuned by oxygen partial pressure, though excessive oxygen induces agglomeration. After annealing (Figure 8fj), RMS roughness shows two distinct behaviors: films with localized protrusions (2%, 3%, 5% P(O₂)) exhibit roughness increases, whereas films with initially uniform surfaces (0% and 4% P(O₂)) become smoother. These results demonstrate that annealing simultaneously enhances uniformity and amplifies pre-existing agglomerations, optimizing surface morphology for improved electrical performance.

Figure 8
AFM images of IZTO films deposited at different oxygen partial pressures: (a-e) as-deposited, (f-j) after annealing.

4. Conclusions

IZTO films were fabricated at room temperature via pulsed DC magnetron sputtering, and the effects of film thickness and oxygen partial pressure on their electrical and optical properties were systematically investigated. Among the samples, the 300 nm-thick film exhibited the best overall performance, featuring an amorphous island-like structure, low sheet resistance, and high Hall mobility. Sheet resistance, carrier concentration, and mobility displayed a non-linear dependence on oxygen partial pressure: for P(O₂) ≤ 3%, sheet resistance is dominated by mobility, whereas for P(O₂) > 3%, it is governed by carrier concentration. Oxidative annealing effectively relieved residual stress, enhanced the mobility, and preserved the amorphous structure. The 300 nm-thick film deposited at 3% P(O₂) achieved optimal post-annealing performance with R_sq = 29.2 Ω/□, μ_n = 51.4 cm²·V⁻¹·s⁻¹, n = 1.4×10²⁰cm⁻³, and T_VIS = 82.8%. These findings demonstrate that room-temperature-fabricated amorphous IZTO films exhibit exceptional optoelectronic properties, and hold strong potential for advanced low-temperature optoelectronic devices.

Data Availability

The full data set supporting the findings of this study is available upon request to the corresponding author: [e-mail].

5. Acknowledgments

This work was financially supported by the Joint Fund of NSFC-Guangxi (U21A2065), Shenzhen Science and Technology Program (KJZD20230923114412026).

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Edited by

• Associate Editor:
  Jose Eiras.

• Editor-in-Chief:
  Luiz Antonio Pessan.

Publication Dates

• Publication in this collection
  20 Mar 2026
• Date of issue
  2026

History

• Received
  21 Sept 2025
• Reviewed
  14 Dec 2025
• Accepted
  28 Jan 2026