A solution-processed quaternary oxide system obtained at low-temperature using a vertical diffusion technique

We report a method for fabricating solution-processed quaternary In-Ga-Zn-O (IGZO) thin-film transistors (TFTs) at low annealing temperatures using a vertical diffusion technique (VDT). The VDT is a deposition process for spin-coating binary and ternary oxide layers consecutively and annealing at once. With the VDT, uniform and dense quaternary oxide layers were fabricated at lower temperatures (280 °C). Compared to conventional IGZO and ternary In-Zn-O (IZO) thin films, VDT IGZO thin film had higher density of the metal-oxide bonds and lower density of the oxygen vacancies. The field-effect mobility of VDT IGZO TFT increased three times with an improved stability under positive bias stress than IZO TFT due to the reduction in oxygen vacancies. Therefore, the VDT process is a simple method that reduces the processing temperature without any additional treatment for quaternary oxide semiconductors with uniform layers.

Scientific RepoRts | 7:43216 | DOI: 10.1038/srep43216 In this study, we introduce a simple method to reduce the processing temperature for a quaternary oxide: the vertical diffusion technique (VDT). The VDT is a process used to deposit two oxide layers successively and anneal them simultaneously in order to effectively facilitate diffusion between each layer. With the VDT, uniform IGZO TFTs were fabricated with lower processing temperatures and superior electrical performance, without any additional treatment. The VDT enables a significant reduction in processing temperatures to below 300 °C, while maintaining the electrical performance of the IGZO TFTs. Thus, this approach is expected to be useful in the fabrication of flexible oxide TFTs due to lower fabrication cost and simple process compared to aforementioned methods.

Experimental Procedure
Materials. Indium  To enhance the electrical properties, nitric acid (HNO 3 ) was added to the solution. All chemicals were purchased from Sigma-Aldrich and used without further purification. Solution preparation. IZO, GaO, and IGZO solutions were prepared. We controlled the molarity of each solution to achieve the desired total atomic composition. The mole ratios were 5:2:1 = In:Ga:Zn for IGZO and 5:1 = In:Zn for IZO. The molarities of the IGZO, IZO, and GaO solutions were 0.4, 0.3, and 0.1 M, respectively. The total atomic composition (In, Ga, and Zn) was the same in the IGZO and VDT IGZO thin films. The solutions were stirred at 60 °C for 1 h, and the precursors dissolved entirely. The solutions were filtered through a Whatman 0.2-μ m polytetrafluoroethylene (PTFE) syringe filter, and aged for at least than 24 h in ambient air.
TFT fabrication. The VDT TFTs exhibited an inverted staggered structure. The substrate was 120-nm-thick SiO 2 thermally oxidized on heavily p-doped Si. The solutions were spin-coated on the substrate. In this experiment, samples of IGZO, IZO, and VDT IGZO were prepared. The conventional IGZO and IZO thin films were pre-annealed for 5 min at 100 °C. For the VDT IGZO, GaO was first spin-coated and pre-annealed for 5 min at 100 °C. Then, IZO was spin-coated on the GaO-coated thin film, and pre-annealed for 5 min at 100 °C. After pre-annealing, all of the samples were post-annealed 280 °C for 4 h. Aluminum (Al) was used for the source and drain electrodes and was deposited on the IGZO thin film via a shadow mask by thermal evaporation. The channel length and width of the IGZO TFTs were 150 μ m and 1000 μ m, respectively. The HPA process was conducted under 1 MPa O 2 at 280 °C for 4 h, as a post-annealing process.
Characterization. An electrical measurement system with a probe station and a semiconductor parameter analyzer (HP 4156 C) was used to measure the transfer characteristics when V DS was 30 V and V GS was swept from − 30 V to + 30 V. For positive bias stress (PBS), stress conditions were applied (V GS = 20 V) for 1000 s. Depth-X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) were conducted using a TOF-SIMS 5 system (iONTOF, Germany), equipped with a Cs gun (Thermo Scientific, U.K.) operating at 1 keV with a monochromatic Al X-ray source (Al Kα line: 1486.6 eV). All of the XPS peaks were calibrated using the C 1 s peak, centered at 284.8 eV. AFM analysis (JPK instrument, Germany) was performed to measure RMS (root mean square) and peak-to-valley roughness. For the VDT, we spin-coated the substrate with GaO and IZO; each layer was pre-annealed. The IZO/GaO thin film, i.e., the VDT IGZO, was post-annealed at 280 °C. Figure 2(a) shows the transfer curves of the IGZO, IZO, and VDT IGZO TFTs; Table 1 summarizes their electrical properties, including the field-effect mobility (μ FET ), maximum on-current/minimum off-current (on/off ratio), subthreshold swing (S.S), equivalent maximum density of states between channel and gate insulator (N max ), threshold voltage (V TH ), and on-current maximum (I on,max ). The IGZO TFTs deposited using the conventional method did not have suitable transfer characteristics at 280 °C because 280 °C is insufficient to form an IGZO thin film. In contrast, the IZO and VDT IGZO TFTs had suitable transfer characteristics. Generally, the processing temperature of ternary oxide thin film is lower than that of a quaternary oxide thin film (IGZO) and IZO is the ternary oxide most commonly used for lower-temperature processes. For this reason, IZO TFTs showed appropriate transfer curves at 280 °C. The VDT IGZO TFTs also had suitable transfer characteristics despite being a quaternary oxide. With the GaO layer, the μ FET improved from 0.40 to 1.26 cm 2 /Vs without decreasing I on,max . Moreover, SS decreased from 1.92 to 1.16 V/dec. with the GaO layer. Therefore, Ga from the GaO layer successfully controlled the carrier concentration in the IZO thin film as a carrier suppressor by reducing oxygen vacancies in the oxide thin film 6,8,9,32,33 . Consequently, the mobility increased by 215% and the S.S improved by 40%. Moreover, as shown in Fig. 2(b) and (c), the V TH shift under PBS for 1000 s also improved from 15.47 to 6.32 V (by 59%) with the VDT process and Fig. 2(d) shows variation of V TH shift under PBS for 3600 s. This result was correlated with the reduced N max and reduction in oxygen vacancies, as oxygen vacancies give rise to PBS instability [34][35][36] .

Results and Discussion
To confirm the thickness and atomic composition, TOF-SIMS was performed for the conventional IGZO and VDT IGZO thin films, and the thickness of the samples was calculated by spectroscopic ellipsometry, as shown in Figs 3 and 4(a-c). Both IGZO and VDT IGZO had uniform atomic ratios with respect to depth, although some irregular peaks were observed at the interface due to matrix effects 37,38 . Although the first (GaO) and second (IZO) oxide layers were deposited separately, the atoms uniformly diffused into the thin film during the post-annealing process. Moreover, no matrix effects were observed in the VDT IGZO layer, which means that there was no interface in the thin film. In addition, densification occurred during the VDT process because the VDT IGZO was thinner than IGZO. Therefore, the VDT process resulted in a uniform, dense thin film without an interface. Furthermore, to investigate densification effect for VDT process, RMS roughness was measured for  conventional IGZO and VDT IGZO thin films using AFM analysis. Figure S1 show the results of AFM analysis for conventional IGZO and VDT IGZO thin films. As a result, the RMS and peak-to-valley roughness of conventional IGZO and VDT IGZO thin films were 0.348 nm and 0.279 nm, and 12.99 nm and 3.91 nm, respectively. Therefore, it should be noted that VDT process helps to recover pore sites caused by solvent evaporation resulting in densification of thin films. Figure 4(d-f) shows the O 1 s XPS peaks according to the depth of the conventional IGZO, IZO, and VDT IGZO thin films. The O 1 s peaks did not change significantly with the depth of the oxide thin films. First, to confirm uniformity, we compared the O 1 s peaks of the three samples at the surface and interface between the channel and gate insulator layer. If the In, Ga, and Zn did not diffuse entirely in the VDT IGZO thin films, there would be different O 1 s peaks in the middle and at the IZO interface, because the initial VDT IGZO layers were IZO (from the middle to the surface) and GaO (from the interface to the middle). Therefore, to confirm diffusion, we analyzed the middle of the IGZO, which is the entirely diffused layer, and VDT IGZO.
All three O 1 s peaks were deconvoluted and centered at 530, 531, and 532 ± 0.5 eV; the three peaks corresponded to lower (metal oxide bonds), intermediate (oxygen vacancy), and higher (metal hydroxide species) binding energies, and the relative areas of the three peaks corresponded to M-O, O vac , and M-OH, respectively 39,40 .   that the VDT process enabled a thin film to form at low temperature. Moreover, the VDT IGZO had lower O vac due to diffusion of Ga, which is a carrier suppressor, in the IZO. Generally, the instability origin under PBS was electron trap sites; i.e., oxygen vacancies in the oxide semiconductor [34][35][36] . Due to the Ga effect, the oxygen vacancy and Nmax decreased, leading to improved PBS results for VDT IGZO.
To enhance the electrical properties, the IGZO and VDT IGZO TFTs were subjected to HPA on flexible substrates as a post-treatment under 1 MPa O 2 . HPA effectively reduced the processing temperature and improved electrical performance, not only μ FET but also the electrical stability [18][19][20] . As the IZO TFTs were inferior to VDT IGZO TFTs, the IGZO and VDT IGZO TFTs were used in the experiments, with IGZO TFTs serving as a reference. Figure 5 shows the transfer curves for the HPA IGZO, VDT IGZO, and HPA VDT IGZO TFTs; Table 2 summarizes the electrical parameters. With HPA, the IGZO TFTs had suitable transfer characteristics at 280 °C, as shown in Fig. 5. Although the IGZO TFTs were subject to HPA, which requires additional equipment, the VDT IGZO TFTs had superior electrical performance, in particular, a three-fold difference in μ FET . Moreover, HPA also improved the electrical performance of VDT IGZO TFTs. The μ FET of the HPA VDT IGZO was 1.38 cm 2 /V·s, an improvement of 8.7%. Therefore, the VDT is a simple method that not only decreases the processing temperature but also improves the electrical properties; moreover, the VDT combined with HPA maximized the improvement in electrical performance.

Conclusion
In this paper, we suggest a strategy to reduce the processing temperature for IGZO TFTs using vertical diffusion. With the VDT, uniform quaternary oxide layers were fabricated at lower temperatures, and VDT resulted in quaternary oxide TFTs with suitable transfer characteristics at 280 °C, without the need for additional treatment. It can be explained that VDT process enables to form high quality quaternary oxide film by diffusing atoms between gel-state binary and ternary oxide system. Therefore, by post-annealed at once, atoms of binary and ternary oxide are effectively diffused each layer resulting in formation of quaternary oxide film at low temperature. In contrast, conventional IGZO TFTs did not show proper transfer characteristics at 280 o C because four kinds of atoms are difficult to make high metal-oxide-metal framework at this low temperature. The VDT IGZO TFTs had a higher μ FET with a lower S.S than IZO TFTs due to the reduction in oxygen vacancies. The PBS results for the VDT IGZO TFTs were also superior to those of the IZO TFTs due to the Ga component, which is a carrier suppressor. Moreover, HPA improved the μ FET of VDT IGZO TFTs by 8.7%, and the μ FET of VDT IGZO increased three times than HPA IGZO. Therefore, the VDT process is a simple method that reduces the processing temperature without any additional treatment for quaternary oxide semiconductors with uniform layers.