1. INTRODUCTION

Organic thin-film transistors (OTFTs) have been widely studied as promising candidates for next-generation electronics, including flexible displays and sensor arrays, owing to their mechanical flexibility, biocompatibility, light weight, and compatibility with low-cost large-area fabrication^[1-4]. Among the various organic semiconductors, pentacene has particularly attracted attention because of its relatively high field-effect mobility and stability^{[5, 6]}. However, to practically implement OTFTs, both the active layer and the gate dielectric play crucial roles, as the dielectric layer strongly affects the threshold voltage, mobility, and overall device reliability^[7].

Reducing the operating voltage is one of the major challenges to lower power consumption in OTFTs. Conventional silicon dioxide and low-k polymeric dielectrics often lead to operating voltages that exceed several tens of volts and thus are unsuitable for low-power applications^[8-9]. High-permittivity metal oxide (MO) dielectrics have been investigated as alternatives because their large dielectric constants can reduce the operating voltage. Among these dielectrics, zirconium oxide (ZrO_x) prepared by solution-based sol–gel processing is a promising candidate owing to its high-k properties and reasonable insulating performance ^[10-13]. Notably, the electrical properties of oxide thin films are strongly influenced by processing conditions such as dopant concentration, oxygen pressure, and annealing temperature^[14].

Sol–gel derived MO films typically require high annealing temperatures (>400 °C) to remove organic residues and achieve sufficient condensation and film densification^[15]. Such processing conditions are incompatible, however, with flexible plastic substrates and restrict their application in flexible organic electronics. To overcome this limitation, ultraviolet (UV)-assisted annealing has been explored. Deep-UV irradiation facilitates low-temperature activation of sol–gel MO semiconductors such as IGZO and IZO^[16]. More recently, pulsed UV (p-UV) irradiation using a xenon lamp has been reported as an efficient approach that enables rapid activation within tens of seconds at relatively low temperatures^[17]. In addition, p-UV-assisted annealing has been successfully applied to oxide thin films for TFT applications^[18]. This feature offers advantages in processing throughput and reduced substrate heating, which are critical for practical device manufacturing.

Despite the advantages offered by p-UV irradiation, its application to sol–gel MO gate dielectrics in OTFTs has not been extensively investigated. In this study, we investigate the use of p-UV-assisted thermal annealing for the low-temperature activation of sol–gel ZrO_x films employed as gate dielectrics in pentacene-based TFTs. The optical, chemical, and morphological properties of the ZrO_x films are systematically analyzed, and their influence on the electrical performance of OTFTs is evaluated to assess the potential of this approach for application to flexible organic electronics.

2. EXPERIMENTAL

The ZrO_x precursor solution was prepared by dissolving zirconium (IV) acetylacetonate [Zr(C₅H₇O₂)₄, Sigma-Aldrich] (4 wt%) in methanol [CH₃OH, Sigma-Aldrich]. Monoethanolamine [C₂H₇NO, Sigma-Aldrich] was added as a stabilizer to the zirconium precursor in a 1:1 molar ratio. The solution was stirred at 1200 rpm and 50 °C for 12 h using a magnetic stirrer, followed by an additional 2 h of stirring under the same conditions immediately prior to device fabrication.

Top-contact bottom-gate pentacene TFTs incorporating solution-processed ZrO_x gate dielectrics were fabricated on indium tin oxide (ITO)-patterned glass substrates. Before depositing the dielectric layer, the substrates were sequentially cleaned by ultrasonication in acetone, 2-propanol, and deionized water. To improve surface wettability, the ITO substrates were treated with oxygen plasma (40 W, 20 sccm, 1 min). The ZrO_x solution was filtered through a 0.2 µm poly(tetrafluoroethylene) syringe filter (Hyundai Micro, Seoul, Korea) and spin-coated at 3000 rpm for 30 s in air.

The thermal treatment of the ZrO_x films followed a previously reported procedure^[19]. The films were annealed in two steps: soft and hard baking. Soft baking was conducted on a hot plate (Corning, Seoul, Korea) at 65 °C for 10 min. Hard baking was performed either (i) at 200 °C for 60 min on a hot plate, or (ii) at 200 °C for 5 min under simultaneous p-UV irradiation (DTX Inc., Gyeonggi Province, Korea). p-UV irradiation was conducted at a frequency of 15 Hz with a lamp-to-sample distance of 10 cm.

A 60 nm pentacene active layer was deposited by thermal evaporation at a rate of 0.3 nm/s. Finally, gold source and drain electrodes were deposited through a finger-type mask by thermal evaporation at a rate of 0.15 nm/s, forming channels with a length of 80 µm and a width of 2000 µm. The device structure is schematically illustrated in Fig. 1.

Fig. 1. Device structure of the pentacene OTFT with a solution-processed ZrO_x dielectric layer.

3. RESULTS AND DISCUSSION

To investigate the influence of annealing methods on the optical properties of solution-processed ZrO_x films, four types of films were prepared on quartz substrates: a coated-only film, a film prebaked at 65 °C, a film prebaked at 65 °C and thermally annealed at 200 °C for 60 min on a hot plate, and a film prebaked at 65 °C and p-UV annealed at 200 °C for 5 min. The optical properties of these films were characterized using a UV-vis spectrophotometer over a wavelength range of 200–800 nm.

Figure 2(a) shows the transmittance spectra of the films. All the samples exhibited high transparency in the visible range (400–800 nm), with transmittance values exceeding 80%, while a decline in transparency was observed below 400 nm owing to intrinsic absorption. The absorption spectra (Fig. 2(b)) revealed that the intensity of the absorption peak in the 250–350 nm region progressively decreased from the coated-only film to the p-UV annealed film. This trend suggests that annealing, particularly p-UV treatment, modifies the electronic structure of the films and reduces the sub-bandgap absorption. The optical band gap (E_g) was estimated using Tauc’s relation (αhν)² ∝ (hν-E_g), where α is the absorption coefficient, h is Planck’s constant, and ν is the photon frequency. The absorption coefficient was calculated as α = (2.303 × log(1/T))/d, where T is the transmittance and d is the film thickness. The band gap was determined by extrapolating the linear portion of the (αhν)² versus hν plots (Fig. 2(c))^[20]. The hot plate annealed film exhibited an E_g of ~ 5.1 eV while the p-UV annealed film showed a slightly higher E_g (~5.2 eV), with both falling within the reported ZrO_x range of 4.5–5.8 eV^[21-22].

Fig. 2. Optical characterization of solution-processed ZrO_x films: (a) UV–vis transmittance spectra, (b) absorbance spectra, and (c) optical band gap values determined from Tauc’s plots.

Notably, p-UV annealing achieved a comparable band gap and slightly improved optical transparency in just 5 min compared to 60 min in the case of conventional thermal annealing. These results demonstrate that p-UV treatment is a rapid and effective method for tuning the optical properties of solution-processed ZrO_x films without compromising their transparency or electronic structure.

Figures 3(a, b) show the contact angles of distilled water on ZrO_x thin films prepared to analyze the surface properties of the insulator layer. The two samples consisted of a ZrO_x thin film annealed on a hot plate for 60 min and a ZrO_x thin film annealed for 5 min using p-UV. The measured contact angles were 36.1° and 31.8°, respectively. Based on these values, the surface energy (γ_p) of the two ZrO_x thin films was calculated using Eq. (1):

where γ_w is the surface free energy of water (73.0 mJ/m²) and θ₀ is the contact angle at equilibrium^[23]. From Eq. (1), the calculated surface energy values of the two ZrO_x thin films were 59.6 and 62.4 mJ/m², respectively. The surface energy of the ZrO_x film annealed with p-UV for 5 min was higher than that annealed on a hot plate for 60 min, demonstrating that the annealing method influenced the surface energy. Regarding changes in the surface properties of zirconia induced by UV treatment, previous studies have reported that UV irradiation reduced the amount of surface carbon species and significantly decreased carboxylic groups^[24-25]. Consequently, the hydrophilization observed in the contact angles of ZrO_x films can be attributed to the increased surface energy resulting from the reduction of relatively hydrophobic carboxylic groups under p-UV annealing. This increase in surface energy suggests that p-UV annealing induces surface-related chemical modification of the ZrO_x films rather than substantial changes in bulk dielectric properties.

Fig. 3. Water contact angles of ZrO_x thin films annealed on (a) a hot plate for 60 min and (b) by p-UV for 5 min; and (c) FT-IR spectra of ZrO_x films prepared under the two annealing conditions.

Figure 3(c) shows the Fourier transform infrared (FT-IR) spectra of ZrO_x thin films prepared under the two annealing conditions on Si substrates, measured in a range of 400–4000 cm^-1 using a vacuum FT-IR spectrometer (Bruker Corporation, Billerica, USA). An absorption peak centered at 867 cm^-1 was observed in both ZrO_x films and is attributed to the stretching vibration of the Zr–O bond. Additional Zr–O and Zr–OH vibrational modes appeared in the 500–800 cm^-1 region^[26-27]. The band at 1710–1780 cm^-1 is assigned to C=O stretching vibrations^[28]. The absorption bands in the 1300–1500 cm^-1 range, centered at 1406 cm^-1, are attributed to C–H deformation vibrations^{[27, 29-30]}. The 900–1280 cm^-1 region is associated with Zr–OC/Zr–O–C and C–O stretching vibrations^{[29, 31]}. This pronounced reduction in carbon-related absorption bands suggests that the high photon energy of pulsed-UV irradiation effectively decomposes residual organic species originating from the sol–gel precursor, which are less effectively removed by thermal annealing alone at 200 °C. These FT-IR results provide direct spectroscopic evidence supporting the surface chemical modification inferred from the contact angle and surface energy analysis.

Compared with the ZrO_x film annealed at 200 °C for 60 min on a hot plate, the film annealed using p-UV showed a pronounced reduction in absorption intensity in the carbon-related regions. This observation is consistent with the contact angle results, where p-UV treatment rendered the surface more hydrophilic. The higher photon energy of UV irradiation, compared with that of thermal annealing, more effectively decomposes surface carbon species and reduces carboxylic groups. Consequently, these FT-IR results support the conclusion that p-UV annealing increases the surface energy of ZrO_x thin films.

The surface energy change induced by p-UV treatment is expected to affect the growth of pentacene thin films deposited on ZrO_x. Figures 4(a, b) present representative atomic force microscopy (AFM) images (3 × 3 μm²) of thermally deposited pentacene films on ZrO_x annealed on a hot plate for 60 min and on ZrO_x annealed by p-UV for 5 min, respectively. The films exhibited surface roughness values of approximately 8.8 and 7.7 nm, respectively. The pentacene film grown on the p-UV annealed ZrO_x showed a larger grain size compared with that grown on the hot plate annealed film.

Fig. 4. AFM images (3 × 3 μm²) of pentacene thin films deposited on ZrO_x thin films annealed (a) on a hot plate for 60 min and (b) by p-UV for 5 min.

In the pentacene thin film growth process, nuclei initially formed on the substrate surface and subsequently grew into large islands. These islands coalesce as pentacene molecules aggregate, and their growth is influenced by the surface energy of the underlying dielectric. When the surface energy of the gate insulator is high, the monolayer coverage of pentacene grains increases. The enhanced coverage results from the promoted surface diffusion of adsorbed pentacene molecules, which improves the connectivity between individual pentacene islands^[32-33]. Therefore, the relatively large grains observed for the film grown on p-UV annealed ZrO_x can be attributed to the higher surface energy of the dielectric. Consistent with this growth mechanism, the increased surface energy of the p-UV-annealed ZrO_x provides a more energetically favorable surface for pentacene nucleation and lateral grain growth, resulting in improved inter-grain connectivity.

Based on the previous results, Figs. 5 and 6 present the electrical characteristics of the pentacene TFTs with solution-processed ZrO_x thin-film gate insulators. Figures 5(a, b) show the output characteristics of the pentacene TFTs with ZrO_x gate insulators, measured by sweeping the drain voltage from 0 to –4 V at a constant gate voltage of 0 V in –1 V steps. Because grain boundaries in pentacene limit charge transport in the film, larger pentacene grains enhance electrical conduction^[32]. As shown in Fig. 5, all devices exhibited a linear increase and saturation in the drain current in the low and high drain voltage region, respectively. The slight decrease in drain current observed in the high drain voltage region after saturation may be associated with self-heating effects under high electric field conditions, resulting in a reduced effective carrier mobility in pentacene-based TFTs^[34]. Therefore, the pentacene TFT with the ZrO_x film annealed at 200 °C for 5 min using p-UV exhibits a higher drain current than that annealed at 200 °C for 60 min. Figures 6(a, b) show the transfer characteristics of the pentacene TFTs with annealed ZrO_x films. The measurements were performed at a drain voltage of –4 V, while the gate voltage was swept from 0.5 to –4 V in –0.1 V steps. The improved transfer characteristics observed in Fig. 6, including the enhanced field-effect mobility and on/off current ratio without a significant threshold voltage shift, are consistent with the surface-related modifications of the ZrO_x gate insulator induced by p-UV annealing.

Fig. 5. Output characteristics of the fabricated OTFTs with ZrO_x gate insulator films annealed (a) on a hot plate for 60 min and (b) by p-UV for 5 min.

Fig. 6. Transfer characteristics of the fabricated OTFTs with ZrO_x films annealed (a) on a hot plate and (b) by p-UV.

To evaluate the performance of the OTFTs, important parameter values were obtained. The threshold voltage (V_TH) was obtained from a plot of |I_D|^1/2 versus the gate voltage (V_G) by extrapolation to a drain current of 0 A. The field effect mobility (µ_eff) was calculated using Eq. (2) in the saturation region:

where C_ox is the gate insulator capacitance, V_TH is the threshold voltage, and W and L are the channel width and length, respectively. The OTFT devices exhibited approximate µ_eff values of 3.10 and 4.13 cm²/V∙s.

As expected from the output characteristics, the pentacene TFT with a ZrO_x film annealed using p-UV exhibited slightly improved performance in terms of the on/off current ratio and field-effect mobility compared with the pentacene TFT with ZrO_x film annealed on a hot plate. However, the threshold voltage remained nearly unchanged. The key parameter values of the OTFTs are summarized in Table 1. The absence of a significant threshold voltage shift further suggests that the observed performance enhancement is mainly associated with surface-related effects rather than substantial changes in bulk dielectric trap states.

4. CONCLUSIONS

In this study, we demonstrated a low-temperature and rapid fabrication method to prepare high-k ZrO_x gate dielectrics in OTFTs using p-UV-assisted thermal annealing. Compared with conventional hot plate annealing at 200 °C for 60 min, p-UV annealing at 200 °C for 5 min effectively activated the sol–gel derived ZrO_x films without degrading their optical or structural properties. The p-UV-annealed ZrO_x films exhibited a slightly higher optical band gap (~5.2 eV), reduced water contact angles (31.8°), and increased surface energy (62.4 mJ/m²), consistent with FT-IR spectroscopic evidence of reduced carbon-related species. These changes promoted the growth of pentacene with larger grain size and a smoother morphology, which improved the charge transport pathways. These results highlight that p-UV-assisted annealing is a fast, energy-efficient, and substrate-compatible processing route to fabricate high-performance organic TFTs with oxide gate dielectrics, offering strong potential for next-generation low-power and flexible organic electronic devices.