Low-Temperature Enhancement-Mode Amorphous Oxide Thin-Film Transistors in Solution Process Using a Low-Pressure Annealing

The interest in low processing temperature for printable transistors is rapidly increasing with the introduction of a new form factor in electronics and the growing importance of high throughput. This paper reports the fabrication of low-temperature-processable enhancement-mode amorphous oxide thin-film transistors (TFTs) using the solution process. A facile low-pressure annealing (LPA) method is proposed for the activation of indium oxide (InOx) semiconductors at a significantly low processing temperature of 200 °C. Thermal annealing at a pressure of about ~10 Torr induces effective condensation in InOx even at a low temperature. As a result, the fabricated LPA InOx TFTs not only functioned in enhancement mode but also exhibited outstanding switching characteristics with a high on/off current ratio of 4.91 × 109. Furthermore, the LPA InOx TFTs exhibit stable operation under bias stress compared to the control device due to the low concentration of hydroxyl defects.


Introduction
Oxide thin-film transistors (TFTs) are regarded as one of the most important building blocks for applications such as displays, optoelectronics, and back-end-of-line transistors [1][2][3][4]. Recently, the solution process for oxide TFTs has attracted a lot of attention because it enables large-area manufacturing and high throughput [5][6][7]. However, compared to conventional sputtering-based oxide TFTs, solution-processed oxide TFTs not only exhibit relatively low field effect mobility (µ FE ) and electrical stability but also require a high processing temperature (>400 • C) [8]. In particular, high processing temperature leads to limitations in the application of flexible polymer substrates, making it difficult to utilize in next-generation electronics with new form factors [6]. Unfortunately, the solutionprocessed oxide TFTs fabricated at a low temperature of 200-300 • C exhibit extremely low µ FE and electrical stabilities.
Binary oxides, such as tin oxide and indium oxide (InO x ), were studied to address these issues [9,10]. Although these semiconducting materials can be used to obtain high mobility, a high electron concentration of these materials cause the negative threshold voltage (V T ), thereby resulting in depletion-mode TFTs. However, the positive V T is suitable for the pixel and gate driver circuits because enhancement-mode characteristic simplifies the circuit design and reduces power consumption [11,12]. Moreover, binary oxides typically have poly-or nanocrystalline structures [9,13,14]. Fabricating enhancement-mode oxide TFTs through low processing temperatures in the solution process is urgently required.
This work seeks to address the aforementioned issues by reporting the fabrication of enhancement-mode amorphous oxide TFTs at a low processing temperature of 200 • C using a solution process. A low-pressure annealing (LPA) method is introduced to achieve the activation and densification of solution-processed InO x semiconductors at a significantly lower processing temperature of 200 • C. Thermal annealing at a low pressure of about~10 Torr helps in diffusing out hydrogen even at low temperatures, resulting in an effective condensation reaction. Considering that the vacuum of thermal evaporation used for organic light-emitting diode deposition has a pressure of~10 −6 Torr, an efficient semiconductor activation can be achieved with a relatively weak vacuum level. As a result, the LPA InO x TFTs not only functioned in the enhancement mode but also exhibited excellent switching characteristics with a high on/off current ratio of 4.9 × 10 9 . Furthermore, compared with the control devices, the LPA InO x TFTs exhibited stable operation under positive and negative gate bias stress compared with the control devices.

Materials and Methods
A 0.1 M InO x solution was prepared by dissolving In(NO 3 ) 3 ·xH 2 O (Sigma-Aldrich, St. Louis, MO, USA) in 2-methoxyethanol to fabricate InO x semiconductor thin films. The solution was stirred at 50 • C for 3 h to obtain a clear and homogeneous solution. A 100 nm thick Si/SiO 2 wafer substrate was prepared for the gate electrode and dielectric through cleaning with sonication in acetone, isopropyl alcohol, and deionized (DI water) for 10 min each. The residual moisture present on the wafers was fully removed by N 2 and annealed at 350 • C. Figure 1 shows a schematic of the fabrication process of oxide TFTs. For the patterning of InO x semiconductors on SiO 2 dielectric, the water etchant-based photopatterning method was used, which was reported previously [15]. First, a 0.1 M InO x solution was spin-coated on the SiO 2 dielectric at 3000 rpm for 20 s to create a film thickness of 7 nm. The precursor solution-deposited substrate was soft-baked at 100 • C for 1 min. Afterward, to avoid ozone generation, the sample covered with a fine metal mask was exposed to ultraviolet (UV) radiation (25 mW cm −2 ) from a low-pressure mercury lamp with two main wavelengths of 253.7 nm (90%) and 184.9 nm (10%) in N 2 ambient for 150 s. In this stage, the photochemical cleavage of alkoxy groups and the decomposition of nitrate ligands were achieved in InO x [15,16]. Consecutively, the substrate was developed in DI water etchant for 1 min. Etching through water supplies additional water molecules to InO x and promotes the condensation reaction in the subsequent annealing process [15,17]. Preannealing was conducted at 100 • C in atmospheric conditions for 10 min. To fabricate the control device, which was annealed in an NPA (normal pressure annealing) environment, it was post-annealed in an oven at 200 • C for 2 h without vacuum (760 Torr). To fabricate the low-pressure annealed InO x based TFTs, InO x was post-annealed in an oven at a lowpressure environment of 10 Torr for 2 h at 200 • C. The InO x films annealed under normaland low-pressure are called NPA and LPA InO x . Finally, the 50 nm-thick Al was deposited using thermal evaporation for source and drain electrodes. The channel length (L) and width (W) were 100 and 1000 µm, respectively. X-ray photoelectron spectroscopy (XPS) (NEXSA, Thermo Fisher Scientific, Waltham, MA, USA) was used to analyze the chemical composition of the NPA and LPA InO x thin films. Grazing incidence X-ray diffraction (GIXRD) (DMAX-2500-PC, Rigaku, Tokyo, Japan) was performed to characterize the crystal structure of InO x . X-ray reflectance (XRR) (ATX-G, Rigaku) measurements were performed to investigate the film density of NPA and LPA InO x . The dielectric properties of NPA InO x and LPA InO x were analyzed using a probe station (Model 4000, MS Tech, Arlington, VA, USA) equipped with a precision LCR meter (Keysight, Santa Rosa, CA, USA) and a semiconductor parameter analyzer (Keithley 2636 B, Tektronix Inc., Beaverton, OR, USA).

Effect of LPA on Atomic Bonding State of InOx
XPS analysis was conducted to illustrate the impact of LPA on the atomic composition in the InOx semiconductor. Figure 2a,b presents the XPS C 1s and In 3d spectra of NPA and LPA InOx thin films, respectively. Both NPA and LPA InOx thin films show clear peaks at 284.6 and 445.5 eV for C 1s and In 3d, respectively, and there is negligible difference in peak intensity. In addition, NPA and LPA InOx thin films show a similar atomic ratio of C, N, O, and In, as shown in Figure 2c. This result indicates that the LPA process does not induce the diffusing out of byproducts, such as nitrogen or carbon. Figure 2d and e shows the deconvoluted XPS O 1s spectra of NPA InOx and LPA InOx, respectively. Three main peaks were observed at the ~529.6, ~530.5, and ~531.5 eV positions, corresponding to metal-oxygen (M-O), oxygen atoms near oxygen vacancies and in M-OC bonds (Vo + C-O), and metal hydroxide (M-OH), respectively [16,[18][19][20]. Compared to NPA InOx, LPA InOx showed a relatively increased ratio of M-O bonding to Vo + C-O, and in particular, M-OH bonding was significantly reduced, as shown in Figure 2d,e. NPA InOx and LPA InOx have M-OH bonding of 33% and 12.6%, respectively ( Figure 2f). In the process of etching with pure water, a large amount of M-OH was formed in InOx due to the additional supply of water molecules by the water etchant. The annealing temperature is not high enough for the condensation reaction. As a result, M-OH still remains in InOx [21]. On the other hand, the low M-OH ratio in LPA InOx results from the diffusing out of hydrogen via thermal annealing in a vacuum environment [22]. Although annealing temperature is 200 °C the same as NPA InOx, the vacuum environment assists in the effective removal of hydrogen, thereby leading to an increase in the M-O bond. The M-O increases from 38.0% to 45.2% through LPA. In general, in oxide semiconductors, hydroxyl acts as an acceptor-like trap and hinders electron conduction, while the M-O network provides an electron transport path through the overlap of metal s orbitals [23]. Therefore, it is necessary to minimize the M-OH and enhance the M-O network in the oxide semiconductor for high-performance electronic devices. Meanwhile, vacuum induces not only the diffusing out of hydrogen but also the desorption of oxygen from the surface [24]. This desorption of oxygen consequently promotes the formation of Vo + C-O and improves the conductivity of the semiconductor. The introduction of LPA led to an increase in Vo + C-O from 30.0% to 42.2%. ). In the process of etching with pure water, a large amount of M-OH was formed in InO x due to the additional supply of water molecules by the water etchant. The annealing temperature is not high enough for the condensation reaction. As a result, M-OH still remains in InO x [21]. On the other hand, the low M-OH ratio in LPA InO x results from the diffusing out of hydrogen via thermal annealing in a vacuum environment [22]. Although annealing temperature is 200 • C the same as NPA InO x , the vacuum environment assists in the effective removal of hydrogen, thereby leading to an increase in the M-O bond. The M-O increases from 38.0% to 45.2% through LPA. In general, in oxide semiconductors, hydroxyl acts as an acceptor-like trap and hinders electron conduction, while the M-O network provides an electron transport path through the overlap of metal s orbitals [23]. Therefore, it is necessary to minimize the M-OH and enhance the M-O network in the oxide semiconductor for high-performance electronic devices. Meanwhile, vacuum induces not only the diffusing out of hydrogen but also the desorption of oxygen from the surface [24]. This desorption of oxygen consequently promotes the formation of V o + C-O and improves the conductivity of the semiconductor. The introduction of LPA led to an increase in V o + C-O from 30.0% to 42.2%. Nanomaterials 2023, 13, x FOR PEER REVIEW 4 of 10

Physical Properties of InOx Semiconductor Thin Films
XRR analysis was performed to investigate the effect of LPA on the density of InOx semiconductor films. Figure 3a shows the XRR spectra of NPA and LPA InOx. The mass density of a thin film is related to the critical angle (θc). Equation (1) shows that there is a proportional relationship between the density and θc [25]: where NA is Avogadro's number, λ is the X-ray wavelength, Z is the average number of electrons per atom, A is the average atomic mass, and D is the mass density of the film. As shown in Figure 3a, the critical angle of LPA InOx is positively shifted. The extracted film densities of NPA InOx and LPA InOx are 5.19 and 5.45, respectively, indicating that the LPA treatment increases the film density. In other words, the LPA treatment induces dense M-O bonds by diffusing out of hydrogen ions and suppresses physical defects, such as pinholes and pores [26,27]. This result agrees with the XPS analysis. Figure 3b shows the GIXRD spectra for NPA InOx and LPA InOx thin films. No clear peaks were observed for both NPA InOx and LPA InOx, indicating that the thin films have an amorphous phase [28]. Binary oxide semiconductors, such as InOx, typically have polyor nanocrystalline structures, even at low processing temperatures [9]. Furthermore, since the poly-or nanocrystalline structure of semiconductors has grain boundaries, the yield and uniformity of devices in mass production are decreased. Therefore, the amorphous phase could be preferred in the industry. The amorphous phase of NPA InOx and LPA InOx might be attributed to the fact that additional water molecules are supplied in the initial stage of thin film formation through the water etching process to hinder InOx crystallization [15,29].

Physical Properties of InO x Semiconductor Thin Films
XRR analysis was performed to investigate the effect of LPA on the density of InO x semiconductor films. Figure 3a shows the XRR spectra of NPA and LPA InO x . The mass density of a thin film is related to the critical angle (θ c ). Equation (1) shows that there is a proportional relationship between the density and θ c [25]: where N A is Avogadro's number, λ is the X-ray wavelength, Z is the average number of electrons per atom, A is the average atomic mass, and D is the mass density of the film. As shown in Figure 3a, the critical angle of LPA InO x is positively shifted. The extracted film densities of NPA InO x and LPA InO x are 5.19 and 5.45, respectively, indicating that the LPA treatment increases the film density. In other words, the LPA treatment induces dense M-O bonds by diffusing out of hydrogen ions and suppresses physical defects, such as pinholes and pores [26,27]. This result agrees with the XPS analysis. The proposed chemical mechanism of LPA effect on InOx is illustrated in Figure 4. NPA InOx contains hydrogen-related defects as well as the M-O network due to insufficient thermal energy (Figure 4a). These OH-related defects not only lead to an increase in the electron concentration in the semiconductor but also act as acceptor-like traps [11,30]. In addition, the high ratio of M-OH weak bonds and the low mass density is induced due to insufficient condensation and densification. Such an imperfect atomic structure of NPA  an amorphous phase [28]. Binary oxide semiconductors, such as InO x , typically have polyor nanocrystalline structures, even at low processing temperatures [9]. Furthermore, since the poly-or nanocrystalline structure of semiconductors has grain boundaries, the yield and uniformity of devices in mass production are decreased. Therefore, the amorphous phase could be preferred in the industry. The amorphous phase of NPA InO x and LPA InO x might be attributed to the fact that additional water molecules are supplied in the initial stage of thin film formation through the water etching process to hinder InO x crystallization [15,29].
The proposed chemical mechanism of LPA effect on InO x is illustrated in Figure 4. NPA InO x contains hydrogen-related defects as well as the M-O network due to insufficient thermal energy (Figure 4a). These OH-related defects not only lead to an increase in the electron concentration in the semiconductor but also act as acceptor-like traps [11,30]. In addition, the high ratio of M-OH weak bonds and the low mass density is induced due to insufficient condensation and densification. Such an imperfect atomic structure of NPA InO x can be understood through the results of XPS and XRR, which were discussed above. On the other hand, in LPA InOx, film activation based on the condensation reaction is effectively achieved due to the diffusing out of hydrogen in the weakly bonded O-H ( Figure 4b). As a result, M-O bonding increases and defects decrease, thereby forming a favorable semiconducting film. The following Equation (2)   The proposed chemical mechanism of LPA effect on InOx is illustrated in Figure 4. NPA InOx contains hydrogen-related defects as well as the M-O network due to insufficient thermal energy (Figure 4a). These OH-related defects not only lead to an increase in the electron concentration in the semiconductor but also act as acceptor-like traps [11,30]. In addition, the high ratio of M-OH weak bonds and the low mass density is induced due to insufficient condensation and densification. Such an imperfect atomic structure of NPA InOx can be understood through the results of XPS and XRR, which were discussed above. On the other hand, in LPA InOx, film activation based on the condensation reaction is effectively achieved due to the diffusing out of hydrogen in the weakly bonded O-H (Figure 4b). As a result, M-O bonding increases and defects decrease, thereby forming a favorable semiconducting film. The following Equation (2)

Electrical Characteristics of LPA InOx TFTs
The transfer and output characteristics of the device were investigated to examine the effect of LPA on the electrical properties of InOx TFTs. Figure 5a,b shows the transfer characteristics of NPA and LPA InOx TFTs, respectively. To obtain the transfer curves, the gate voltage was increased from −10 to 30 V, while the drain voltage was maintained at 30 V. Surprisingly, the μFE of the LPA InOx TFTs showed a high value of 0.81 cm 2 V −1 s −1 despite the low processing temperature and amorphous phase. This value is about six times higher than the μFE of NPA InOx TFT, 0.13 cm 2 V −1 s −1 . Note that there is a trade-off relationship between annealing temperature and mobility, and Vth and mobility in oxide semiconductor system due to the charge transport mechanism [15,31]. Thus, the μFE of oxide TFTs in this study could be dramatically increased by increasing the annealing temperature or semiconductor thickness, although this study is focused on the low processing

Electrical Characteristics of LPA InO x TFTs
The transfer and output characteristics of the device were investigated to examine the effect of LPA on the electrical properties of InO x TFTs. Figure 5a,b shows the transfer characteristics of NPA and LPA InO x TFTs, respectively. To obtain the transfer curves, the gate voltage was increased from −10 to 30 V, while the drain voltage was maintained at 30 V. Surprisingly, the µ FE of the LPA InO x TFTs showed a high value of 0.81 cm 2 V −1 s −1 despite the low processing temperature and amorphous phase. This value is about six times higher than the µ FE of NPA InO x TFT, 0.13 cm 2 V −1 s −1 . Note that there is a trade-off relationship between annealing temperature and mobility, and V th and mobility in oxide semiconductor system due to the charge transport mechanism [15,31]. Thus, the µ FE of oxide TFTs in this study could be dramatically increased by increasing the annealing temperature or semiconductor thickness, although this study is focused on the low processing temperature and enhancement-mode operation. Furthermore, the introduction of LPA leads to an increase in the on/off current ratio from 6.83 × 10 7 to 4.91 × 10 9 . The subthreshold slope (SS), which is related to the interface trap density, also greatly decreased from 0.71 V dec −1 to 0.28 V dec −1 . Considering the above chemical and physical analysis results, the increase in M-O bonds acting as a charge transport path and the decrease in M-OH bonds serving as trap sites through the LPA treatment are the dominant factors that cause the changes in the electrical properties. Figure 5c,d shows the output characteristics of NPA and LPA InO x TFTs, respectively. The output curves were measured by applying the gate voltage from −10 to 30 V in 10 V steps. Drain voltage was swept from 0 to 30 V. LPA InO x TFT showed significantly higher drain current than NPA InO x TFT under the same voltage conditions, and both devices showed clear linear and saturation regions. The electrical parameters of LPA oxide TFTs and recently reported oxide TFTs are summarized in Table 1.

Bias Stress-Induced Instability of LPA InOx TFTs
Since the bias stress-induced instability directly affects the lifetime of the device, ensuring bias stability is one of the most important factors in determining its commercialization. The effects of LPA on the stability of TFTs under positive bias stress (PBS) and negative bias stress (NBS) are investigated. Figure 6a,b shows the PBS time-dependence of transfer curves of NPA and LPA InOx TFTs, respectively. Gate and drain voltages of 10 V and 0 V, respectively, were applied to the PBS test. The ΔVth of NPA and LPA InOx TFTs

Bias Stress-Induced Instability of LPA InO x TFTs
Since the bias stress-induced instability directly affects the lifetime of the device, ensuring bias stability is one of the most important factors in determining its commercialization. The effects of LPA on the stability of TFTs under positive bias stress (PBS) and negative bias stress (NBS) are investigated. Figure 6a,b shows the PBS time-dependence of transfer curves of NPA and LPA InO x TFTs, respectively. Gate and drain voltages of 10 V and 0 V, respectively, were applied to the PBS test. The ∆V th of NPA and LPA InO x TFTs were 6.20 and 4.40 V, respectively (Figure 6c). LPA InO x TFTs exhibited superior PBS stability compared to NPA InO x TFTs. In both devices, SS does not change as the stress time increases, and only a positive V th shift was confirmed. This suggests that the degradation caused by PBS is due to electron trapping at gate dielectricthe presence of InO x semiconductor interfaces and electron injection into gate dielectric rather than defect creation [38,39]. Since the thermally grown high-quality SiO 2 was used for the gate dielectric of NPA and LPA InO x TFTs, the improvement in ∆V th through LPA implies the reduction in electron trapping at the interface. Recently, it has been revealed that excess hydrogen peroxide could be the origin of PBS-induced V th degradation [4]. However, since solution-processed InO x is Nanomaterials 2023, 13, 2231 7 of 9 characterized by a relatively oxygen-poor configuration, unlike IGZO sputtered at high oxygen flow rates, oxygen-related defects cannot be considered the origin of PBS-induced ∆V TH . Therefore, it is reasonable that the reduction in hydrogen-related defects through LPA leads to the improvement of PBS in oxide TFTs based on the XRR and XPS results. Figure 6c,d shows the NBS time-dependent transfer curves of NPA and LPA InO x TFTs, respectively. The gate and drain voltages of −10 V and 0 V, respectively, were utilized for the NBS test. After 3000 s of NBS, the ∆V th of NPA and LPA InO x TFTs were −1.8 and −0.91 V, respectively, and LPA InO x TFTs showed superior NBS stability compared to NPA InO x TFTs (Figure 6f). Hole trapping, oxygen vacancy, and excess hydrogen peroxide models have been considered mechanisms of V th deterioration via NBS [4,38,39]. It is known that SS deterioration generally occurs based on oxygen vacancy or peroxide model under a strong negative gate bias or illumination stress [4,38,40]. Since the negative shift of V th is observed without changing SS in this study, the shift of V th in NPA and LPA InO x TFTs might be attributed to hole trapping and injection rather than the ionization of oxygen vacancy. Therefore, the superior NBS stability of LPA InO x TFTs originates from the reduction in organic chemical-and hydrogen-related defects, which could act as holetrapping states [26,41]. Although the PBS and NBS were analyzed at room temperature in this study, the stress temperature also highly affected the V th shift by accelerating the activation of defects. Thus, the PBS-and NBS-induced instability could deteriorate as the stress temperature increases based on the stretched exponential function [42,43].
caused by PBS is due to electron trapping at gate dielectricthe presence of InOx semiconductor interfaces and electron injection into gate dielectric rather than defect creation [38,39]. Since the thermally grown high-quality SiO2 was used for the gate dielectric of NPA and LPA InOx TFTs, the improvement in ΔVth through LPA implies the reduction in electron trapping at the interface. Recently, it has been revealed that excess hydrogen peroxide could be the origin of PBS-induced Vth degradation [4]. However, since solutionprocessed InOx is characterized by a relatively oxygen-poor configuration, unlike IGZO sputtered at high oxygen flow rates, oxygen-related defects cannot be considered the origin of PBS-induced ΔVTH. Therefore, it is reasonable that the reduction in hydrogenrelated defects through LPA leads to the improvement of PBS in oxide TFTs based on the XRR and XPS results. Figure 6c,d shows the NBS time-dependent transfer curves of NPA and LPA InOx TFTs, respectively. The gate and drain voltages of −10 V and 0 V, respectively, were utilized for the NBS test. After 3000 s of NBS, the ΔVth of NPA and LPA InOx TFTs were −1.8 and −0.91 V, respectively, and LPA InOx TFTs showed superior NBS stability compared to NPA InOx TFTs (Figure 6f). Hole trapping, oxygen vacancy, and excess hydrogen peroxide models have been considered mechanisms of Vth deterioration via NBS [4,38,39]. It is known that SS deterioration generally occurs based on oxygen vacancy or peroxide model under a strong negative gate bias or illumination stress [4,38,40]. Since the negative shift of Vth is observed without changing SS in this study, the shift of Vth in NPA and LPA InOx TFTs might be attributed to hole trapping and injection rather than the ionization of oxygen vacancy. Therefore, the superior NBS stability of LPA InOx TFTs originates from the reduction in organic chemical-and hydrogen-related defects, which could act as hole-trapping states [26,41]. Although the PBS and NBS were analyzed at room temperature in this study, the stress temperature also highly affected the Vth shift by accelerating the activation of defects. Thus, the PBS-and NBS-induced instability could deteriorate as the stress temperature increases based on the stretched exponential function [42,43].

Conclusions
In conclusion, the LPA method is suggested for the low-temperature annealing of the InO x semiconductor. The enhancement-mode amorphous InO x TFTs were successfully fabricated using a solution process at a low process temperature of 200 • C. Through XPS analysis, LPA was able to greatly reduce the M-OH ratio, and the defective bonding states, in InO x from 33% to 12.6%. As a result, the introduction of LPA leads to the significant improvement of µ FE and SS from 0.13 to 0.81 cm 2 V −1 s −1 and from 0.71 to 0.28 V dec −1 , respectively. In addition, the LPA InO x TFTs exhibited stable operation under PBS and NBS compared to the control device.