Enhancement of electrical properties of solution-processed oxide thin film transistors using ZrO₂ gate dielectrics deposited by an oxygen-doped solution

Due to their numerous advantages of high carrier mobility, excellent visible light transparency, low off-current and high compatibility, metal oxide (MO) semiconductors (e.g. In₂O₃, ZnO, InZnO and InGaZnO) have recently emerged as promising alternatives to amorphous silicon (a-Si) and low-temperature poly-silicon for use in the channel layer of thin film transistors (TFTs) [1–3]. Some MO semiconductors made by vacuum-based technologies (e.g. magnetron sputtering, atomic layer deposition, pulsed layer deposition, etc) have been commercially applied in transparent electronics and displays such as in mobile phones, monitors and tablets. Nevertheless, the expensive equipment and complicated vacuum process increase the fabrication cost and hinder large-scale production, which limits their future potential commercial applications. Accordingly, solution-processed MO-TFTs have recently attracted extensive attention because of their processing simplicity, low cost, highly controllable composition, high throughput and flexible compatibility [4–7]. Unfortunately, issues like poor electrical performance and stability still exist with solution processing owing to various physical and chemical defects generated by organic residues, pinholes or pores during the curing process [8–12]. A high annealing temperature (>400 °C) and/or long annealing hours are normally required in solution processing, but this method is energy intensive and destructive to most soft substrates.

To improve the electrical performance and reduce the processing temperature of solution-processed MO-TFTs, the MO channel layers have been investigated from the aspects of materials, processes and structures [13–16]. For example, solution-processed In₂O₃ has commonly been selected as the semiconductor channel layer for its high electron mobility due to the ns orbital of indium [17]. On the other hand, attention has been also paid to the solution deposition of high-quality gate dielectric layers, since the gate dielectric is critical for regulating carrier transport and conduction in field effect devices [18, 19]. In particualr, the solution deposition of high dielectric constant (κ) MO dielectrics can reduce not only the manufacturing cost but also the operational voltage, which also plays a crucial role in determining the electrical performance of MO-TFTs [20]. Generally, an ideal high-κ MO dielectric requires a smooth pinhole-free surface and a dense network obtained at a relatively low preparation temperature to ensure a low leakage current density and high breakdown field strength [21]. Accordingly, various studies (materials engineering, structural engineering, etc) have also been carried out to explore the deposition of high-quality, high-κ MO-based gate dielectrics in MO-TFTs. For instance, Jo et al fabricated a photochemically activated Al₂O₃/ZrO₂ bilayer gate dielectric with a dielectric constant of 8.5 and a leakage current density of ∼10⁻⁹ A cm⁻² at 1 MV cm⁻¹. Based on this Al₂O₃/ZrO₂ bilayer dielectric, the solution-prepared InGaZnO TFT exhibited a carrier mobility of 13.5 cm² V⁻¹ s⁻¹ [22]. Part et al fabricated a bi-layer solution-processed In₂O₃ TFT with boron-doped peroxo-zirconium oxide dielectric (ZrO₂:B) that had leakage current density of ∼10⁻⁷ A cm⁻² at 1 MV cm⁻¹, a breakdown field strength of 3.94 MV cm⁻¹ and a carrier mobility of 39.3 cm² V⁻¹ s⁻¹ [23]. In addition, Carlos et al reported aluminum oxide (AlO_x ) by combustion synthesis and far-ultraviolet irradiation, and the bi-layer solution-based AlO_x /In₂O₃ TFTs exhibited a saturation mobility of 5.57 cm² V⁻¹ s⁻¹ [24]. Most of these previous studies were devoted to improving the quality of the dielectric by fabricating multilayer structures [22, 25], element doping [23, 26] or using external energy [24, 27]. These methods or ideas to some extent increase the processing complexity and deteriorate the compositional uniformity and, even worse, the electrical performance of MO-TFTs. It is still challenging to find a simple alternative approach to obtain a high-quality uniform gate dielectric to enhance the electrical performance of bi-layer solution-processed MO-TFTs and to avoid these aforementioned disadvantages.

In our previous work, we found that an oxygen-doped precursor solution (ODS) can significantly enhance the electrical properties of solution-deposited La₂O₃ dielectric film even at processing temperatures as low as 160 °C [28]. It should be noted that in our earlier work for La₂O₃ deposition by ODS, oxygen did not act as a doping element into La₂O₃ thin film. The role of oxygen in the ODS process is to promote the cross-linking of ligands, increase the polarity of the solution and improve the quality and uniformity of the solution-deposited film. Although ODS is a novel method for fabricating high-quality, high-κ dielectric film by solution deposition we only demonstrated the excellent electrical properties of a flexible TFT with a vacuum-evaporated organic semiconductor as the channel layer. In this work, we deposited both the semiconductor layer and the gate dielectric layer using the solution process, and constructed so-called bi-layer solution-processed TFTs with ODS-ZrO₂ as the gate dielectric and solution-deposited In₂O₃ as the channel layer. The solution-deposited ODS-ZrO₂/In₂O₃ bi-layer showed no clear intermixing and the ODS-ZrO₂/In₂O₃ TFTs demonstrated excellent electrical properties. Our work shows the critical role of the dielectric film in the electrical performance of MO-TFTs. More importantly, we reveal that high-κ dielectric film deposited with ODS should be an effective way to significantly improve the electrical properties of MO-TFTs for future low-cost, high-performance applications.

2.1. Solution synthesis and ODS process

2.2. Device fabrication

2.3. Characterization of microstructure and electrical properties

Figure 1(a) shows the setup for preparation of the oxygen-doped ZrO₂ precursor solution (left figure). After stirring, the color of the solution gradually becomes deeper (crimson) with increasing oxygen doping time; the solution without any oxygen doping is transparent (middle figure). A small amount of oxygen doping can significantly change the solution characteristics. The spreading ability of the solution on the substrate surface can be greatly enhanced. The changes of precursor solutions and the corresponding mechanisms were discussed in detail in our previous work [28]. Figure 1(b) presents a schematic diagram of the device with BGTC architecture. ZrO₂ and In₂O₃ were deposited sequentially by the spin-coating process as the gate dielectric and oxide channel.

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AFM and HRTEM measurements were used to investigate the microstructural properties of the dielectric films deposited with or without ODS, and the corresponding results are shown in figures 1(c)–(g). ZrO₂ film deposited without ODS shows relatively poor surface smoothness with a root-mean-square (RMS) value of 0.562 nm (figure 1(c)). ZrO₂ film deposited with 30 min ODS shows a smoother surface with a RMS value of 0.288 nm (figure 1(e)). Figures 1(d) and (f) show the AFM images of In₂O₃ semiconductor films. It can be seen that the In₂O₃ semiconductor layer deposited on ODS-ZrO₂ exhibits a lower RMS roughness of 0.204 nm (figure 1(f)) than the In₂O₃ film deposited on non-ODS-ZrO₂ (figure 1(d)), suggesting that the underlying ODS-ZrO₂ film is beneficial for obtaining a smooth surface of the overlying In₂O₃ film.

Figure 1(g) shows the cross-sectional HRTEM images of the ODS-ZrO₂/In₂O₃ bilayer structure. It can be seen that there is a distinct separation between the ODS-ZrO₂ and In₂O₃ layers, and their thicknesses can be determined to be 26.9 and 4.3 nm, respectively. The clear separation of the bi-layer solution-processed ZrO₂/In₂O₃ gate stack means that there is no noticeable inter-diffusion between the ZrO₂ gate dielectric and In₂O₃ semiconductor layer. One of the challenges to the solution deposition of the ZrO₂/In₂O₃ bilayer structure is that the solvent in the In₂O₃ precursor solution may dissolve or deteriorate the deposited ZrO₂ dielectric layer below, resulting in degraded dielectric and interface quality. The present HRTEM image suggests that a high interface quality can be expected between ODS-ZrO₂ and In₂O₃, which is favorable for providing fast electron transport and few charge traps [29]. We also note the existence of an interfacial SiO₂ layer with a thickness of 2.2 nm, which is most probably formed during the post-annealing of the ZrO₂ and In₂O₃ films. The interfacial SiO₂ layer was also observed in MIM samples (figure S2). The ultrathin SiO₂ insertion layer can prevent electron tunneling, inhibiting the formation of dangling bonds and defect states at the gate/dielectric interface, which has been commonly used to increase the interface quality between high-κ dielectric and silicon substrates [30, 31].

It is well known that the properties of the precursor solution will significantly affect the density and thickness of sol–gel derived dielectric films after the densification process. To obtain an optimal doping time for the ZrO₂ precursor solution we deposited ZrO₂ thin films using precursors with different oxygen doping times from 0 to 60 min. The thickness of the ZrO₂ films shows a clear dependence on the oxygen doping time (figure 2(a)). A longer oxygen doping time results in a thinner film, indicating that ODS promotes densification of the ZrO₂ gate dielectric. The density values of ZrO₂ thin films have been also characterized by XRR measurements. As shown in figure 2(a), they increase with increase in the oxygen doping time (from 3.66 g cm⁻³ for 0 min ODS to 4.06 g cm⁻³ for 60 min ODS). The electrical properties of the ZrO₂ films treated for three representative oxygen doping times (0, 10 and 30 min) were examined using a P⁺⁺Si/ODS-ZrO₂/Cu MIM structure (as shown in the inset to figure 2(a)). Figure 2(b) shows the leakage current characteristics of the ZrO₂ films as functions of the electric field. When the oxygen doping time increases from 0 to 30 min, the leakage current density and breakdown electric field markedly changed from 6.3 × 10⁻⁴ to 6.2 × 10⁻⁷ A cm⁻² (at 2 MV cm⁻¹) and 4.5 to 7.0 MV cm⁻¹, respectively.

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The improvement of electrical characteristics might be due to the promotion of chemical reactions forming M–OH bonds in the precursor solution after ODS, making it easier to generate a M–O–M structure in ZrO₂ dielectric film after post-treatment [32]. The C–ƒ measurements exhibit the higher capacitance of the ZrO₂ dielectric film with longer oxygen doping time, and typical capacitance densities at 1 MHz were 450 (0 min), 488 (10 min) and 640 (30 min) nF cm⁻² (figure 2(c)). Figure 2(d) presents the frequency-dependent dielectric constant of the ZrO₂ films. The permittivity of ZrO₂ film increases with increasing oxygen doping time. When the oxygen doping time reaches 30 min, a high permittivity of 19.5 was observed at a frequency of 1 MHz for the 30 min ODS-ZrO₂ film. The optical transmittance spectra of the ZrO₂ thin films deposited on fused silica substrate have also been investigated (figure S3). Unlike the electrical properties, the transmittance of all these films deposited with or without ODS is almost the same, being higher than 90% in all cases. In a word, the ODS process can greatly enhance the electrical properties of solution-deposited ZrO₂ films, and has great potential for applications in the fabrication of high-quality gate dielectrics in future high-performance oxide TFTs.

Figure 3 shows the electrical properties of TFTs with solution-deposited ZrO₂ as the gate dielectric and solution-deposited In₂O₃ film as the semiconductor channel. The transfer characteristics of the bi-layer solution-processed ZrO₂/In₂O₃ TFTs with different oxygen doping times (0, 10 and 30 min) are shown in figure 3(a). It can be seen that ODS can slightly decrease the hysteresis loops. The reduced hysteresis may be attributed to the smooth surface of the ODS-ZrO₂ film, which contributes to reducing the trap sites in the ZrO₂–In₂O₃ interface [33]. More importantly, it can be seen clearly that ODS can effectively reduce the off drain current and increase the on drain current of the transistor, resulting in a significant increase in the drain current I_on/I_off ratio from 10⁴ for the non-ODS-ZrO₂ TFT to 10⁶ for the 30 min ODS-ZrO₂ TFT. The increase in the drain current for the ODS-ZrO₂/In₂O₃ TFT may be attributed to the enhanced carrier mobility since a better ZrO₂–In₂O₃ interface can reduce the interface Coulomb scattering from impurities and traps [34].

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The channel carrier mobility values as a function of V_GS are shown in figure 3(b) for TFTs with and without ODS. It was clear that TFTs with 10 min and 30 min ODS-ZrO₂ show much larger carrier mobility than the TFT without ODS, which is assumed to be the dominant reason for the higher drain current in the ODS-ZrO₂ TFTs. The decrease in the off drain current of the TFT may be due to the decrease in leakage current of the ZrO₂ film deposited with ODS. Figures 3(c) and (d) show the typical transfer and output characteristics of the ZrO₂-TFT with 30 min ODS. The carrier mobility for the TFT was calculated using the drain current equation in the saturation region, ${I_{\textrm{DS}}} = \frac{W}{{2L}}\mu {C_i}{\left( {{V_{\textrm{GS}}} - {V_{\textrm{th}}}} \right)^2}$, where I_DS is the source–drain current, W and L are the channel width and length, respectively, C_i is the dielectric capacitance per unit area at 1 MHz, μ is the carrier mobility, V_GS is the gate–source voltage and V_th is the threshold voltage. The 30 min ODS-ZrO₂/In₂O₃ TFT exhibits negligible hysteresis and good current switching and saturation characteristics. Excellent electrical performance was obtained, including high carrier mobility (62.02 cm² V s⁻¹), low threshold voltage (0.24 V), high on–off drain current ratio (I_on /I_off, 3.0 × 10⁶) and small subthreshold swing (SS, 0.14 V decade⁻¹). The carrier mobility values of previously reported bi-layer solution-processed ZrO₂/In₂O₃ TFTs mostly range from ~4.0 to ~55.0 cm² V s⁻¹ [17, 20, 35, 36]. Compared with these previous results, our 30 min ODS-ZrO₂/In₂O₃ TFT exhibits a higher carrier mobility. The aforementioned results indicate that ODS-ZrO₂ contributes to the formation of a good dielectric/semiconductor interface for a MO-TFT.

The bias stability of TFT devices is an essential element for their future commercial applications. Most previous reports on the bias stability of vacuum-based oxide TFTs have noted great improvements showing negligible or no significant degradation in electrical performance such as ΔV_th and field-effect mobility (μ) under gate bias stress. However, the bias stability of solution-processed oxide TFTs (especially bi-layer solution-processed) is much worse than that of vacuum-based oxide TFTs, and needs to be further improved. The transfer characteristics of ZrO₂/In₂O₃ TFTs have been investigated as a function of negative/positive bias stressing time, as shown in figure 4. The device bias stability was tested under a PGBS of 1 V and a NGBS of −1 V in a dark environment without any encapsulation. Figures 4(a) and (d) show the transfer characteristics of non-ODS-ZrO₂/In₂O₃ TFTs under gate bias stress. After a bias stress of 3600 s, the off-current for a non-ODS-ZrO₂ TFT increases significantly from ~10⁻⁸ A to ~10⁻⁵ A under NGBS and the threshold voltage shows a positive shift of 0.7 V under PGBS, indicating very poor bias stress stability. The poor bias stability of the non-ODS-ZrO₂ TFT can be attributed to the high leakage current and inhomogeneity of the ZrO₂ gate dielectric. Figures 4(b) and (c) show the transfer characteristics of 30 min ODS-ZrO₂/In₂O₃ TFTs after various bias stress times. The device parameters such as μ_sat, ΔV_th and I_on were extracted from figures 4(b) and (c), where the I_on and μ_sat were normalized to their initial values (t = 0 s). Their bias stability as a function of bias stress time is shown in figures 4(e) and (f). After a stress time of 7200 s, only a slight shift in V_th and the relatively stable I_on and μ_sat is observed, showing clearly enhanced stability of the ODS-ZrO₂/In₂O₃ TFT. The excellent electrical properties and enhanced bias stability of ODS-ZrO₂/In₂O₃ TFT devices can be attributed to the high-quality ODS-ZrO₂ gate dielectric layer. The defect or trap density in the gate dielectric should be very small, as speculated from the very small leakage current of the ODS-ZrO₂ thin films. Charge injection is one of the main mechanisms leading to the instability of V_th during the bias testing of TFT devices [37]. Therefore, charge injection is assumed to be very small, resulting in the excellent bias stability.

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Figure 5(a) shows comparisons of the leakage current at 2 MV cm⁻¹ and the breakdown electric field of ZrO₂ films deposited with and without ODS. A total of 20 MIM devices were measured for each type of ZrO₂ film. It can be observed that the non-ODS-ZrO₂ film exhibits a high J_i ranging from 10⁻⁴ to 10⁻² A cm⁻² and a low E_b of 2.5–5 MV cm⁻¹. In contrast, the ODS-ZrO₂ film exhibits J_i values of 10⁻⁷ to 10⁻⁵ A cm⁻² and E_b values ranging from 5.0 to 7.0 MV cm⁻¹, indicating much better electrical performance than that of non-ODS-ZrO₂ film. Furthermore, the narrower distribution range of J_i and E_b values of ODS-ZrO₂ films implies a better film uniformity than for the non-ODS-ZrO₂ films. Figure 5(b) shows comparisons of μ, SS and I_on/I_off among 30 ODS-ZrO₂/In₂O₃ TFTs and 30 non-ODS-ZrO₂/In₂O₃ TFTs. It can be seen that the ODS-ZrO₂/In₂O₃ TFTs show higher carrier mobility, larger I_on/I_off and smaller SS than non-ODS-ZrO₂/In₂O₃ TFTs. Table 1 shows a comparison of the device parameters of ZrO₂ TFTs with different ODS times, in which the standard deviations of these device parameters are also calculated from five transistors with the same channel length of 200 μm. Non-ODS-ZrO₂/In₂O₃ TFTs show relatively poor transistor performance including μ (10.68 cm² Vs⁻¹), I_on/I_off (~10⁴), V_th (0.24 V) and SS (0.19 V decade⁻¹). However, the 30 min ODS-ZrO₂/In₂O₃ TFTs exhibit significantly enhanced transistor performance, with a high μ of 60.42 cm² Vs⁻¹, high I_on/I_off (~10⁶), low V_th of 0.32 V and SS of 0.14 V decade⁻¹. The above results indicate that the overall electrical performance of In₂O₃ TFTs has been greatly improved due to the adoption of the ODS-ZrO₂ gate dielectric, which provides the In₂O₃ TFTs with a good dielectric–semiconductor interface, high gate dielectric capacitance density and small gate leakage current, resulting in improved electrical properties. It is worth noting that a low average SS value of 0.14 V decade⁻¹ in 30 min ODS-ZrO₂/In₂O₃ TFT is achieved. We know that SS is closely related to interfacial trap density according to the equation ${N_{\textrm{t}}} = \left( {\frac{{\textrm{SS}\,{\textrm{log}}\left( e \right)}}{{kT/q}} - 1} \right)\frac{{{C_{\textrm{ox}}}}}{q}$, where N_t is the total number of interfacial trap states, k is Boltzmann's constant and C_ox is capacitance density per unit area of dielectric layer [38]. Such a low SS value of 0.14 V decade⁻¹ implies a high ODS-ZrO₂/In₂O₃ interface quality with few defects and trap states. To explore the mechanism responsible for the enhancement of the electrical properties of ODS-ZrO₂ TFTs, we also carried out Fourier transform infrared spectroscopy measurements of the ZrO₂ precursor solutions (figure S4). It was found that the ZrO₂ precursor solution with 30 min ODS shows a much stronger –OH vibration. The enriched OH groups in the precursor solution can efficiently improve the affinity between the liquid and the solid substrate, leading to fewer physical defects and improved film uniformity. Therefore, enhanced electrical properties can be observed for ODS-ZrO₂ dielectric films and corresponding TFT devices.

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By adopting a ZrO₂ gate dielectric layer deposited by ODS, high-performance bi-layer solution-processed In₂O₃ TFTs have been fabricated. AFM measurements demonstrated that enhanced surface smoothness can be observed for both the ODS-ZrO₂ dielectric film and In₂O₃ films deposited on it. HRTEM measurements indicate that the solution-deposited ODS-ZrO₂/In₂O₃ bilayer shows no clear intermixing. Excellent electrical properties, including low leakage current density, high breakdown electric field and high permittivity, were observed for ODS-ZrO₂ films. Correspondingly, the ODS-ZrO₂/In₂O₃ TFTs also show excellent electrical performance such as high carrier mobility of 62.02 cm² V s⁻¹, a large on/off drain current ratio of 3.0 × 10⁶ and a small SS value of 0.14 V decade⁻¹. These ODS-ZrO₂/In₂O₃ TFTs also show excellent bias stress stability, whose threshold voltage shift, carrier mobility and on drain current can remain stable even after a long stress time of 7200 s. These results suggest that gate dielectrics deposited with ODS show great potential for application in the future low-cost, high-performance MO-TFTs.

This work is supported by National Natural Science Foundation of China (grant no. 51872099), Science and Technology Program of Guangzhou (grant no. 2019050001), Guangdong Provincial Key Laboratory of Optical Information Materials and Technology (grant no. 2017B030301007). X B L acknowledges support from the Project for Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2016). R Q T acknowledges support from the Research and Cultivation Foundation for Young Teachers in South China Normal University (grant no. 19KJ14) and the Joint Funds of Basic and Applied Basic Research Foundation of Guangdong Province (grant no. 2019A1515110605). J W G acknowledges support from the NSFC-Guangdong Joint Fund (grant no. U1801256).