The influence of annealing atmosphere on sputtered indium oxide thin-film transistors

Indium oxide (In2O3) thin films sputtered at room temperature were annealed under different atmospheres and examined for thin-film transistor (TFT) active channel applications. The annealing process was performed in a rapid thermal annealing system at 350 °C under O2, Ar, forming gas (FG, 96% N2/4% H2), and N2. It was found that the annealed In2O3 TFTs exhibited high field-effect mobility (μ FE > 40 cm2 V−1s−1), high on/off current ratio (I on/off∼ 108), and controlled threshold voltage (V TH) for the enhancement- and depletion-mode operations. Note that the annealing atmosphere has a significant effect on the electrical performance of the In2O3 TFTs by inducing changes in oxygen-related species, particularly oxygen vacancies (VO) and hydroxyl/carbonate species (O–H/C–O). For the O2-, Ar-, FG-, and N2-annealed TFTs, μ FE was in increasing order accompanied by a negative shift in V TH, which is a result attributed to the larger VO in the In2O3 thin films. Furthermore, the ΔV TH of the FG-, and N2-annealed TFTs in a positive bias stress test was greater than that of the O2-, Ar-annealed devices, attributing to their lower density of O–H/C–O groups in the In2O3 thin films. Our results suggest that the annealing atmosphere contributes to the internal modifications of the In2O3 structure and in turn altered the electrical characteristics of TFTs. These annealed In2O3 TFTs with high performance are promising candidates for realizing large-area, transparent, and high-resolution displays.

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To meet the requirements of ultra-high-resolution and high frame rate next-generation displays, various OS TFTs with higher mobility than IGZO TFTs have been extensively studied for next-generation display applications [12][13][14]. Among these transistors, In-rich OS TFTs are the most promising candidates because of their excellent high-mobility performance. The In 5s orbital having a large spatial spread and a large overlap provides a facile electron transport path, which, in addition to their low electron effective mass, endows In-rich OS TFTs with high electron mobility [14].
For most OS TFTs, the presence of oxygen-related species, such as oxygen vacancies (V O ) or hydroxyl/carbonate species (O-H/C-O), has a considerable impact on their electrical performance [15,16]. Generally, V O readily creates shallow or deep donor states in the oxide channel layer. These shallow donor states supply electrons to the conduction band (CB), leading to an increase in electron concentration. In contrast, the deep donor states in the subgap region act as trap states, degrading device performance [17][18][19]. In addition, oxygen bonds in O-H/C-O groups are considered to generate acceptor-like states near the CB edge and enhance electron trapping during positive bias stress (PBS) applications [20]. Therefore, multiple attempts have been made to modulate oxygen-related species in the active channel layer such as multi-cation doping [21,22], changing the sputtering gas flow [23], annealing treatment [24], plasma treatment [25], or their combination [26,27]. One of the effective routines for realizing TFTs with good performance is the deposition of OS channel layers with high resistivity, followed by a postdeposition annealing treatment. Annealing is one of the most commonly used methods for optimizing active channel layers because of its effectiveness in alleviating deep traps formed by ion bombardment or unintended defects during film deposition or the synthesis process [28]. Most previous studies demonstrated that the performance of TFTs under multiple annealing conditions varies depending on the channel material and process conditions chosen [29,30]. Different annealing conditions, especially the annealing atmosphere, have a significant effect on the performance of OS TFTs. However, most studies on the effects of annealing conditions have focused on IGZO [31], indium-gallium-tin oxide (IGTO) [29], indium-zinctin oxide (IZTO) [32], indium-gallium oxide [33], and other In-based multi-cation composition OSs [34,35]. Few studies have been conducted on the effect of the annealing atmosphere on the performance of In 2 O 3 TFTs. Yuan et al reported that the sputtered In 2 O 3 TFTs followed by a post-deposition annealing in vacuum or air showed significantly different mobilities; however, the fundamental mechanism behind the carrier mobility enhancement is unclear [36]. Si et al reported that In 2 O 3 TFTs deposited by atomic layer deposition (ALD) and annealed in O 2 , H 2 , or N 2 respectively, produced similar results indicating that the annealing atmosphere had no effect on the performance of In 2 O 3 TFTs [37]. However, ALDprocessed and sputtered In 2 O 3 thin films significantly differ in terms of aspects such as chemical composition and bonding state [38,39]. Therefore, this conclusion cannot be inferred to be applicable to sputtered In 2 O 3 TFTs. For OS TFTs, a complete understanding of the effects of the annealing atmospheres on their electrical properties and stability is required. In particular, stability under gate bias stress is important for OS TFT applications. This suggests the need to determine the most suitable annealing atmosphere for In 2 O 3 thin films to improve the electrical properties of In 2 O 3 TFTs. In this study, we deposited In 2 O 3 thin films by sputtering at room temperature and investigated the effect of the annealing atmosphere on the electrical characteristics and bias stability of In 2 O 3 TFTs.

Experimental details
The fabrication process of the devices is as follows. First, heavily doped p-type silicon (p ++ -Si) substrates with 2 µm thermally grown SiO 2 were cleaned with acetone, isopropyl alcohol, and deionized water for 10 min each. Next, 30 nm Al was deposited using radio frequency (RF) magnetron sputtering as the gate metal. Then, 30 nm Al 2 O 3 was grown as the gate dielectric at 250 • C using (CH 3 ) 3 Al (TMA) and O 2 plasma as Al and O precursors. Next, ∼10 nm In 2 O 3 films were deposited by RF magnetron sputtering at room temperature as the active channel layer. In 2 O 3 films with a thickness of ∼40 nm were then deposited on the SiO 2 /Si substrates and sapphire substrates to examine their structural and optical properties. The sputtering power was 100 W, the O 2 and Ar gas flow ratios were set to 50%, and the working pressure was 10 mTorr. About 5 min pre-sputtering was performed to eliminate the contamination of the In 2 O 3 target. Subsequently, 20 nm Ti and 100 nm Au were deposited by RF magnetron sputtering as the source and drain metal contacts, respectively. Finally, the Al 2 O 3 was dry-etched using BCl 3 as the reactive gas to expose the underlying gate. The bottom gate, channel, and source/drain metals were fabricated by lithography, sputtering and lift-off processes. The channel length and width were 40 and 240 µm, respectively. The annealing process was performed in a rapid thermal annealing system under four different annealing atmospheres of O 2 , Ar, forming gas (FG) and N 2 . The annealing temperature and time were fixed at 350 • C and 1 min, respectively.
The surface morphology was characterized using an atomic force microscopy (AFM, Bruker Dimension Icon scanning probe microscope) in tapping mode. Film thickness was jointly determined by transmission electron microscope (TEM), AFM and ellipsometry. TEM lamella samples were prepared by focused ion beam (FIB) using a FEI Helios G4 scanning electron microscope. Before FIB milling, a platinum (Pt) layer was deposited to protect the sample surface from ion damage. The FEI Titan ST microscope system with an acceleration voltage of 300 kV was equipped with a Super-X energy-dispersive x-ray spectroscopy (EDX) for high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging. Crystal structures were studied using a Bruker D2 PHASER x-ray diffraction (XRD) system with a Cu tube source (λ = 1.541 84 Å) at 30 kV. O 1s spectra were examined by x-ray photoelectron spectroscopy (XPS) was performed in a high vacuum using a Kratos Amicus XPS system equipped with a monochromatic Al Kα x-ray source operating at 10 kV. The electrical properties and gate bias stability of the fabricated TFTs were measured using a Keithley 4200 system and a Cascade Summit probe station at room temperature and ambient atmosphere.   (100) substrate. The crystallinity of the In 2 O 3 thin films was further confirmed by TEM analysis. Figure 2(a) shows the UV-Vis transmission spectra of all annealed In 2 O 3 thin films. These films were deposited on a sapphire substrate with a thickness of ∼40 nm. The result indicates that the transmittance of the films does not depend on the annealing atmosphere, and all films demonstrate almost full transmittance in the visible region. Figure 2(b) shows the corresponding Tauc plots extracted from the transmission spectra. The optical bandgap (Eg) is determined using the Tauc's relation [40,41]: (ɑhʋ) 2 = A(hʋ − Eg), where ɑ is the absorption coefficient, A is a constant, h is the Planck constant, and ʋ is the frequency. Eg is extracted by extrapolating the linear part to the energy at (ɑhʋ) 2 = 0. Results indicate that the annealing atmosphere has no effect on the bandgap of the films because all annealed films exhibit the same Eg value of 3.71 eV.

Results and discussion
To investigate the oxygen-related species in the In 2 O 3 thin films annealed under different annealing atmosphere, XPS measurements were performed. As shown in figure 3, the O 1s     figure S1. Compared to the annealed samples, the as-deposited In 2 O 3 TFT shows undesirable electrical performance with a larger V TH (6.8 V) and lower I D (0.011 mA) at V GS of 10 V under the same V DS sweeping condition. Similar results have been reported for IGZO [47], IZTO [32], and InSnO [48] TFTs. It is known that the OS thin films deposited at room temperature typically have a loose structure that exhibits carrier scattering caused by poor film quality, resulting in few free carriers in the films and low conductivity and mobility. The electrical performance of the films can be greatly improved by promoting oxygen diffusion from the OS channel layer and rearrangement of molecular bonds through post-deposition annealing [32,47,48]. Thus, a high number of V O indicates high carrier concentration, resulting in low V TH , a high conductivity and mobility [49,50]. However, a higher electron concentration promotes the formation of percolation conduction paths in OSs, making it a problem for TFTs to reach complete depletion. The O 2annealed In 2 O 3 TFT exhibits lower conductivity and mobility because O 2 fills certain V O , resulting in a lower carrier concentration than those of the other films. The N 2 -annealed In 2 O 3 TFT only reached partial depletion because of an excessive carrier concentration, which is mainly attributed to the desorption of oxygen atoms during the N 2 annealing process resulting in a large amount of V O in the In 2 O 3 thin film [51]. These results are consistent with previous reports on IGZO and IGTO TFTs [29,45]. The SS of the N 2 -annealed In 2 O 3 TFT is not provided because V TH is so negative that SS cannot be meaningfully measured. The higher SS of N 2 -and FG-annealed TFTs than that of the O 2 -and Ar-annealed TFTs indicates their poorer gate control, which is mainly attributed to their higher carrier concentrations.
The dependence of gate bias stability on the annealing atmosphere was investigated with a positive bias value of 10 V and a stress time of 3000 s. During bias stress, the gate was biased at a fixed voltage value of 10 V, while the source and drain were grounded. Bias stability is evaluated by the variation of V TH , which is determined using the linear extrapolation of the transfer curve at the maximum g m point. Figures 6(a)-(d) show the evolution of the transfer characteristics of the In 2 O 3 TFTs annealed under O 2 , Ar, FG, and N 2 , respectively. From these figures, the transfer curves shift almost parallel toward the positive direction as the stress time increases. The long-term rate of V TH shift is significantly reduced with a clear directional difference, thus suggesting the rebalance of the rates of different degradation mechanisms [52]. From the V TH -stress time curves in figure 6(e), the corresponding ∆V TH of the O 2 -, Ar-, FG-, and N 2 -annealed TFTs are 4.8 V, 3.9 V, 3.3 V and 3.4 V, respectively, at a stress time of 3000 s. This ∆V TH difference showed the same trend as the difference of relative area of the O III component observed in the XPS analysis. Previous studies on OS thin films and TFTs have demonstrated that that O III mainly originates from the oxygen in absorbed hydroxyl and carbonate species (O-H/C-O), which typically generate acceptor-like states near the CB edge and enhance electron trapping during PBS application because of their polar nature in OSs [29]. Therefore, the larger It is known that there are two factors that contribute to V TH instability: (1) defect creation in the channel and (2) charge trapping in the dielectric and/or at the channel/insulator interface [53]. Defect creation typically results in the persistent degradation of sub-threshold slope and device mobility, whereas charge trapping does not [54]. The main difference between charge trapping at the interface and the injection into the dielectric is the amount of energy needed to remove the injected charge. Releasing the charge injected into a dielectric requires high energy and typically requires thermal annealing or the application of bias [55]. We observed that our devices can automatically relax to the original states within 4 h at room temperature after bias testing. The spontaneous recovery of V TH after relaxation without any high energy and the negligible changes in SS shown in figure 6(f) suggest that charge trapping at the channel/insulator interface is the main reason for the instability of the In 2 O 3 TFT PBS condition. The SS of the N 2 -annealed In 2 O 3 TFT is not provided in figure 6(f) because its large negative V TH so that SS cannot be meaningfully measured.
The time-dependent V TH observed during the PBS tests for the TFTs annealed under different atmospheres can be fitted using the stretched exponential equation: where ∆V TH0 is ∆V TH at infinite time, t is the stress time, β is the stretched-exponential exponent, and τ is the characteristic trapping time of the carriers, which correlates  with the average effective energy barrier [56,57]. Table 2 lists the exacted fitting parameters from curve fitting for all TFTs. The In 2 O 3 TFTs annealed under different atmospheres have different ∆V TH0 , τ and β, indicating different device degradation mechanisms, attributed to the generation of undesirable trap centers [52]. Furthermore, certain studies confirmed that the dynamic interaction between the exposed backchannel and the ambient atmosphere affects the V TH stability of TFTs in PBS tests. The adsorbed oxygen can capture electrons from the CB, thus resulting in different oxygen species such as O 2− and O − . As a result of charge transfer, a depletion layer is formed beneath the oxide surface, thus leading to a positive shift in the V TH of the transistor. Because our devices use a back-channel structure without a passivation layer covering it, the influence of the ambient atmosphere cannot be excluded [58][59][60].

Conclusion
In this study, the effects of annealing atmospheres (O 2 , Ar, FG, and N 2 ) on the structural and electrical properties of roomtemperature sputtered In 2 O 3 thin films as active channel layers were investigated. All annealed In 2 O 3 thin films exhibit polycrystalline properties with high transparency and optical band gap of 3.71 eV. The annealing atmosphere has a significant effect on the electrical properties and bias stability of In addition, the ∆V TH values observed in the PBS tests for the FG-, and N 2 -annealed TFTs were smaller than those for the O 2 -, Ar-annealed TFTs, which is attributed to the lower density of O-H/C-O groups in the In 2 O 3 active channel layer. Charge trapping at the channel/insulator interface was considered to be the main factor contributing to PBS instability, and the ∆V TH observed during the PBS tests agreed well with the stretched exponential equation. Our study suggests that the annealing of sputtered In 2 O 3 thin films is an effective approach to achieve high-performance TFTs with high mobility and controllable V TH . Considering the strong influence of the annealing atmosphere on the electrical properties of In 2 O 3 TFTs, careful selection of the annealing atmosphere is necessary.

Data availability statements
All data that support the findings of this study are included within the article (and any supplementary files).