Inﬂuence of Hydrogen Ions on the Performance of Thin-Film Transistors with Solution-Processed AlO x Gate Dielectrics

: Over the past decade, there have been many reports on solution-processed oxide thin-ﬁlm transistors (TFTs) with high mobility (even >100 cm 2 V − 1 s − 1 ). However, the capacitance uncertainty of the solution-processed oxide gate dielectrics leads to serious overestimation of the mobility. Here, solution-processed AlO x dielectrics are investigated systematically, and the effect of mobile ions on the frequency-dependent capacitance of the solution-processed AlO x dielectrics is also studied. It was found that the capacitance of the AlO x depends on the frequency seriously when the annealing temperature is lower than 300 ◦ C, and the water treatment causes more seriously frequency-dependent capacitance. The strong frequency-dependent capacitance of the AlO x annealed at 250 or 300 ◦ C is attributed to relaxation polarization of the weakly bound ions in the incompletely decomposed AlO x ﬁlms. The water treatment introduces a large number of protons (H + ) that would migrate to the ITO/AlO x interface under a certain electric ﬁeld and form an electric double layer (EDL) that has ultrahigh capacitance at low frequency.


Introduction
In the past decade, oxide thin-film transistors (TFTs) have drawn much attention for their potential applications in large-size, high-frequency, transparent, flexible, or energysaving displays due to the advantages of ultralow off-current, relatively high field-effect mobility, good uniformity in large size, etc. [1,2]. In oxide TFTs, the gate dielectric layer plays an important role; therefore, it is necessary to investigate the influence the of gate dielectric materials and their fabrication process on the performance of the oxide TFTs. A number of gate dielectrics such as HfO 2 [3,4], Al 2 O 3 [5][6][7][8][9][10], and ZrO 2 [11][12][13][14][15], and giant dielectric constant materials [16] have been investigated for oxide TFTs. However, the effect of the water-induced mobile ions of the gate dielectrics on the TFT performance has not been studied in detailed yet.
Compared to the traditional vacuum-processed method, the solution-processed method is more attractive for the advantages of low-cost, high-throughput, and easy chemical composition control [17]. Recently, solution-processed AlO x gate dielectric has drawn attention due to the high dielectric constant, low leakage current, and good compatibility with oxide semiconductors [7,18,19]. However, the capacitance of solution-processed AlO x dielectrics depends on the preparing processes strongly [20,21]. Therefore, it is necessary to investigate the mobile ions and residue groups in the solution-processed AlO x dielectrics to improve the insulating properties. In addition, there have been many reports on solution-processed oxide TFTs with high field-effect mobility (even >100 cm 2 V −1 s −1 ). These values are highly controversial, because the capacitance used to calculate the field-effect mobility is 1kHz or above, which is much lower than the actual capacitance during TFT measuring (the gate 2 of 8 sweeps at a certain V GS step, which means that the charging of the gate dielectric is stepby-step, meaning quasistatic capacitance is more appropriate for the mobility calculation). For this reason, it is indispensable to regulate the capacitance measurement for calculating the field-effect mobility of TFTs. In this paper, the properties of solution-processed AlO x dielectrics are investigated systematically, and the effect of mobile protons on the frequency-dependent capacitance and on the performance of the oxide TFTs is also studied.

Experiment
The AlO x precursor solution was prepared by dissolving 0.2 M Al(NO 3 ) 3 ·9H 2 O in 2-methoxyethanol, stirred at room temperature for 24 h, and aged for 6 h. The precursor materials, including solutes and solvents, were purchased from Aladdin. The AlO x precursor films were deposited by spin-coating at 3000 rpm for 30 s and then soft-baked on a hot plate at 150 • C for 10 min to remove the solvents. After that, the AlO x precursor films were annealed in the air at different temperature of 250, 300, and 350 • C for 1 h. Each sample was baked at 150 • C for 10 min immediately after spin-coating. Then, each sample was annealed separately at different temperature for 1 h. The thickness of AlO x is about 57 ± 3 nm.
TFTs with AlO x gate dielectric layer and InScO x (In 2 O 3 :Sc 2 O 3 = 98:2 wt%) semiconductor layer were constructed with a bottom-gate top-contact structure. A 200 nm indium tin oxide (ITO, In:Sn = 9:1) gate electrode was deposited onto the glass substrate by DC magnetron sputtering (70 W) under argon pressure of 0.5 Pa at room temperature through a shading mask. Then, a layer of AlO x dielectric film was deposited onto the ITO gate electrode using the process described above. After that, the InScO x semiconductor layer (20 nm) was deposited by RF magnetron sputtering (60 W) under argon pressure of 0.5 Pa at room temperature through a shading mask. The ITO source and drain electrodes (200 nm) were deposited onto the InScO x semiconductor layer by DC magnetron sputtering with the same conditions as that of the gate deposition. The channel width (W) and length (L) were defined by a shading mask to be 800 and 200 µm, respectively. Finally, the TFT devices were post-annealed at 250 • C for 1 h. The ITO/AlO x /ITO metal-insulator-metal (MIM) devices were prepared by depositing a circular ITO top electrode (200 nm) with a diameter of 0.04 mm onto the AlO x film by DC magnetron sputtering.
To investigate the effect of hydrogen ions (H + ) on the dielectric properties of the AlO x layer, water treatment was performed on the surface of the ITO gate electrode before spin-coating the AlO x film. Because the solution-processed AlO x film is very sensitive to moisture, water treatment is a simple way to increase the density of the mobile H + . The water treatment process was performed by spin-coating deionized water at a speed of 3000 rpm for 30 s onto the surface of the ITO gate electrode to introduce large amount of adsorbed H + and OH − groups at the ITO/AlO x interface. The ITO surface is treated by O 2 plasma before water treatment. The contact angles of the plasma treated and plasma + water treated ITO surfaces are 5.03 • and 3.93 • , respectively.
The thermal behavior of AlO x precursor was analyzed by thermogravimetric analyses (TG). The chemical composition, water adsorption, and proton quantity behavior of the AlO x films were characterized by X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific Inc, ESCALAB250Xi), infrared spectroscopy (FT-IR), and time-of-flight secondaryion mass spectrometry (TOF-SIMS), respectively. The electrical properties of MIM and TFTs were characterized by semiconductor parameter analyzer (Keysight B1500A). Figure 1a shows the TG curve of AlO x precursor. It reveals an initial mass loss event of~60%, occurring around 120-180 • C, which is assigned to the removal of solvent and organic residues and the dehydroxylation of AlO x precursor. Then, a slow mass loss is observed around 180-350 • C, with weight stabilizing at close to 20% by 350 • C, indicating complete conversion of precursors to form the dense metal oxide. Figure 1b shows the FT-IR spectra of AlO x films annealed at different temperatures. The peaks around 1700 cm −1 (C=C stretching) and 1500 cm −1 (N-O asymmetric stretching) are attributed to the residual organic elements and the undecomposed precursor metal salts, respectively [22,23]. As the annealing temperature increases from 250 to 350 • C, the intensity of the peaks decreases continuously, which is consistent with the TG analysis. The peaks in the range of 3500-3800 cm −1 is due to the peaks of O-H stretching plausibly resulting from surface hydroxylation [22,23].

Results and Discussion
Appl. Sci. 2021, 11, x FOR PEER REVIEW 3 of 9 organic residues and the dehydroxylation of AlOx precursor. Then, a slow mass loss is observed around 180-350 °C, with weight stabilizing at close to 20% by 350 °C, indicating complete conversion of precursors to form the dense metal oxide. Figure 1b shows the FT-IR spectra of AlOx films annealed at different temperatures. The peaks around 1700 cm −1 (C=C stretching) and 1500 cm −1 (N-O asymmetric stretching) are attributed to the residual organic elements and the undecomposed precursor metal salts, respectively [22,23]. As the annealing temperature increases from 250 to 350 °C, the intensity of the peaks decreases continuously, which is consistent with the TG analysis. The peaks in the range of 3500-3800 cm −1 is due to the peaks of O-H stretching plausibly resulting from surface hydroxylation [22,23].    The insulating properties of AlOx films were evaluated with a MIM structure of ITO/AlOx/ITO. Figure 3a shows the frequency dependence of capacitance of the MIM devices. Interestingly, the capacitance decreases as the annealing temperature increases at lower frequency regime (<10 kHz), while it increases as the annealing temperature in- The insulating properties of AlO x films were evaluated with a MIM structure of ITO/AlO x /ITO. Figure 3a shows the frequency dependence of capacitance of the MIM devices. Interestingly, the capacitance decreases as the annealing temperature increases at lower frequency regime (<10 kHz), while it increases as the annealing temperature increases at higher frequency regime (>1 MHz). In addition, the capacitance of the MIM devices annealed at 250 or 300 • C depends on the frequency greatly, while the AlO x film annealed at 350 • C is almost independent of the measuring frequency (when the measuring frequency is lower than 100 kHz). The areal capacitances at 1 kHz for the MIM devices annealed at 250, 300, and 350 • C are 179.1, 166.3, and 148.1 nF/cm 2 , respectively; and the areal quasistatic (QS) capacitances (see Figure 3b) for the devices annealed at 250, 300, and 350 • C are 288.9, 263.4, and 152.8 nF/cm 2 , respectively. It shows that the areal quasistatic capacitance of the MIM devices annealed at 250 or 300 • C is much higher than those measured at 1 kHz, while there is no much difference between the areal quasistatic capacitance and the areal capacitance at 1 kHz for the one annealed at 350 • C. The strong frequency-dependent capacitance of the MIM devices annealed at 250 or 300 • C is attributed to relaxation polarization of the weakly bound ions in the incompletely decomposed AlO x films. Especially, the protons (H + ), which can move in the whole AlO x film, move to the ITO/AlO x interface and form an electric double layer (EDL) that has ultrahigh capacitance at very low frequency [24,25].  To further investigate the effect of mobile hydrogen-related ions on the low-frequency capacitance of the AlOx films, the surface of the bottom electrode (ITO) was treated by water before spin-coating the AlOx precursor on it. The water treatment can introduce large amounts of the adsorbed water molecules at the ITO/AlOx interface that would further form hydrogen and OH groups. Figure 3c shows the frequency dependence of capacitance of the MIM devices with water treatment. It can be seen that the capacitance of all devices (even annealed at 350 °C) depends on the frequency seriously. The areal capacitances at 1 kHz of the MIM devices annealed at 250, 300, and 350 °C are 387.9, 295.3, and To further investigate the effect of mobile hydrogen-related ions on the low-frequency capacitance of the AlO x films, the surface of the bottom electrode (ITO) was treated by water before spin-coating the AlO x precursor on it. The water treatment can introduce large amounts of the adsorbed water molecules at the ITO/AlO x interface that would further form hydrogen and OH groups. Figure 3c shows the frequency dependence of capacitance of the MIM devices with water treatment. It can be seen that the capacitance of all devices (even annealed at 350 • C) depends on the frequency seriously. The areal capacitances at 1 kHz of the MIM devices annealed at 250, 300, and 350 • C are 387.9, 295.3, and 268.9 nF/cm 2 , respectively. However, the areal quasi-static capacitances of the MIM devices annealed at 250, 300, and 350 • C are as high as 1520, 846, and 647 nF/cm 2 (see Figure 3d). The extremely high quasistatic capacitance is most probably attributed to H + ions which are small and easy to be driven by the electric field. The H + ions in oxide films are generally associated with oxygen atoms to form a three-coordinate oxygen center (M-OH+-M), and the motion of H + ions is relied on "a sequence of hops" from one bridging oxygen atom to another [13,24,26]. When a voltage is applied to the MIM device, the H + ions migrate to the AlO x /ITO interface by a sequence of hops and form a very thin EDL with an extremely large capacitance. Under an electric field of 0.3 MV/cm (2 V), the leakage current density (J) of the MIM devices without water treatment annealed at 250, 300, and 350°C are 3.2 × 10 −8 , 2.4 × 10 −8 , and 1.4 × 10 −8 A/cm 2 , respectively, corresponding to much higher J of 5.5 × 10 −7 , 4.5 × 10 −7 , and 2.6 × 10 −7 A/cm 2 for the water-treated ones (not shown). The breakdown field of the MIM devices without water treatment annealed at 250, 300, and 350°C are 1.9, 2.9, and 3.2 MV/cm, respectively (not shown). Interestingly, the leakage current for the water-treated MIM devices increases greatly at~0.3 MV, but there are not apparent breakdown points. The difference may be attributed to the large amount of movable H + ions in the water-treated AlO x samples, which form leakage current paths. To verify the existence of H + ions in the AlO x films, TOF-SIMS experiments were carried out. Figure 4 shows the depth-profile element distribution of the water-treated AlO x /ITO sample annealed at 350 • C. The intensities for the carbon signal are very weak, revealing little carbon-related residuals. By contrast, there are a number of hydrogens in the whole AlO x dielectric. It is worth noting that the hydrogen distribution is not uniform with the density gradually decreasing from AlO x surface to the AlO x /ITO interface. The results confirm that a large amount of hydrogen elements is introduced during treatment. The obvious overlap of Al and In signals is mainly due to the diffusion of Al and In elements at the ITO/AlO x interface.  Figure 4 shows the depth-profile element distribution of the watertreated AlOx/ITO sample annealed at 350 °C. The intensities for the carbon signal are very weak, revealing little carbon-related residuals. By contrast, there are a number of hydrogens in the whole AlOx dielectric. It is worth noting that the hydrogen distribution is not uniform with the density gradually decreasing from AlOx surface to the AlOx/ITO interface. The results confirm that a large amount of hydrogen elements is introduced during treatment. The obvious overlap of Al and In signals is mainly due to the diffusion of Al and In elements at the ITO/AlOx interface. Finally, TFTs with AlOx gate insulator and InScOx channel layer were fabricated to verify the formation of EDL. InScOx semiconductor can effectively decrease the influence of water and oxygen in the environment on the stability of TFTs [27]. Figure 5a,b shows the transfer curves of the TFTs without and with water treatment, respectively. Interestingly, the TFT without water treatment exhibits clockwise hysteresis in the transfer curve between forward and reverse gate sweeps, while the one with water treatment exhibits anticlockwise hysteresis. The anticlockwise hysteresis of the water-treated TFT is ascribed to the low migration speed of the H + ions [28]. When the gate voltage increases, the H + ions migrate to the AlOx/InScOx interface slowly; after the gate voltage reaches the highest value and begin to decrease, some of the H + ions still move toward the AlOx/InScOx interface, causing further increase in the EDL capacitance. As a result, the current for the reverse sweep is higher than that for the forward sweep (due to the higher capacitance). The Finally, TFTs with AlO x gate insulator and InScO x channel layer were fabricated to verify the formation of EDL. InScO x semiconductor can effectively decrease the influence of water and oxygen in the environment on the stability of TFTs [27]. Figure 5a,b shows the transfer curves of the TFTs without and with water treatment, respectively. Interestingly, the TFT without water treatment exhibits clockwise hysteresis in the transfer curve between forward and reverse gate sweeps, while the one with water treatment exhibits anticlockwise hysteresis. The anticlockwise hysteresis of the water-treated TFT is ascribed to the low migration speed of the H + ions [28]. When the gate voltage increases, the H + ions migrate to the AlO x /InScO x interface slowly; after the gate voltage reaches the highest value and begin to decrease, some of the H + ions still move toward the AlO x /InScO x interface, causing further increase in the EDL capacitance. As a result, the current for the reverse sweep is higher than that for the forward sweep (due to the higher capacitance). The saturation mobility (µ sat ) was extracted by fitting a straight line to the plot of the square root of the I D versus V G and using the following equation: where C is the areal capacitance of the gate dielectric, and W and L are the channel width and length, respectively. The calculated mobilities for forward-and reversed-sweep curves of the TFTs without water treatment is 6.75 and 9.71 cm 2 V −1 s −1 , respectively, while those for forward-and reversed-sweep curves of water-treated ones are 6.72 and 5.02 cm 2 V −1 s −1 , respectively. Although the TFT with water treatment is high on-current, the mobility is lower than that of the TFT without water treatment. The results confirm that the mobility of the water-treated TFTs is overestimated if using the same capacitance (C i ) of the untreated TFTs for calculating mobility. The key properties of the TFTs with/without water treatment are summarized in Table 1.
Appl. Sci. 2021, 11, x FOR PEER REVIEW 7 of 9 the untreated TFTs for calculating mobility. The key properties of the TFTs with/without water treatment are summarized in Table 1.

Conclusions
In summary, MIM and TFT devices based on solution-processed AlOx dielectrics were fabricated, and the effect of mobile ions on the frequency-dependent capacitance of the solution-processed AlOx dielectrics is studied. It is found that the capacitance of the AlOx dielectrics annealed at 250 or 300 °C depends on the frequency greatly, while the AlOx film annealed at 350 °C is almost independent of the frequency (<100 kHz); and the water treatment causes more seriously frequency-dependent capacitance. The strong frequency-dependent capacitance of the AlOx annealed at 250 or 300 °C is attributed to relaxation polarization of the weakly bound ions in the incompletely decomposed AlOx films. The water treatment introduces a large number of protons (H + ) that would migrate to the ITO/AlOx interface under a certain electric field and form an electric double layer (EDL) that has ultrahigh capacitance at low frequency. The oxide TFTs based on water treated AlOx dielectrics exhibit anticlockwise hysteresis in the transfer curves that confirm existence of mobile ions in the AlOx films. The calculated mobilities for forward-and reversed-sweep curves of the TFTs without water treatment is 6.75 and 9.71 cm 2 V -1 s -1 , re-

Conclusions
In summary, MIM and TFT devices based on solution-processed AlO x dielectrics were fabricated, and the effect of mobile ions on the frequency-dependent capacitance of the solution-processed AlO x dielectrics is studied. It is found that the capacitance of the AlO x dielectrics annealed at 250 or 300 • C depends on the frequency greatly, while the AlO x film annealed at 350 • C is almost independent of the frequency (<100 kHz); and the water treatment causes more seriously frequency-dependent capacitance. The strong frequency-dependent capacitance of the AlO x annealed at 250 or 300 • C is attributed to relaxation polarization of the weakly bound ions in the incompletely decomposed AlO x films. The water treatment introduces a large number of protons (H + ) that would migrate to the ITO/AlO x interface under a certain electric field and form an electric double layer (EDL) that has ultrahigh capacitance at low frequency. The oxide TFTs based on water treated AlO x dielectrics exhibit anticlockwise hysteresis in the transfer curves that confirm existence of mobile ions in the AlO x films. The calculated mobilities for forward-and reversed-sweep curves of the TFTs without water treatment is 6.75 and 9.71 cm 2 V −1 s −1 , respectively, while those for forward-and reversed-sweep curves of water-treated ones are 6.72 and 5.02 cm 2 V −1 s −1 , respectively. Although the TFT with water treatment is high on-current, the mobility is lower than that of the TFT without water treatment. The results confirm that the mobility of the water-treated TFTs is overestimated if using the same capacitance (C i ) of the untreated TFTs for calculating mobility.