Effects of Oxygen Flows and Annealing Temperatures on Optical, Electrical, and Structural Properties of Co-Sputtered In 2 O 3 -Ga 2 O 3 -Zn Thin Films

: This study investigated the effects of oxygen (O 2 ) ﬂow rates and annealing temperatures on optical, electrical, and structural properties of indium–gallium–zinc oxide (IGZO) ﬁlm on glass substrates fabricated by using a co-sputtering system with two radio-frequency (RF) (In 2 O 3 and Ga 2 O 3 ) and one direct current (DC) (Zn) magnetron. The average transmittance and optical energy gap increased signiﬁcantly when the oxygen ﬂow rate was increased from 1 sccm to 3 sccm. An increased O 2 ﬂow during co-sputtering IGZO ﬁlms caused the crystallinity of the InGaZn 7 O 10 phase to increase, yielding a smoother and more uniform granular structure. The carrier mobility rose and the carrier concentration decreased with increasing O 2 ﬂow. The results of X-ray photoelectron spectra (XPS) analyses explained the impacts of the O 2 ﬂow rates and annealing temperatures on optical and electrical properties of the co-sputtered IGZO ﬁlms. The optimum process conditions of the co-sputtered In 2 O 3 -Ga 2 O 3 -Zn ﬁlms were revealed as an O 2 ﬂow rate of 3 sccm and an annealing temperature at 300 ◦ C, which showed the largest average transmittance of 82.48%, a larger optical bandgap of 3.21 eV, and a larger carrier mobility of 7.01 cm 2 V − 1 s − 1 . XPS results at various annealing temperatures indicated that the co-sputtered IGZO ﬁlms with an O 2 ﬂow rate of 3 sccm have more stable chemical compositions among different annealing temperatures.


Introduction
In recent years, transparent amorphous oxide semiconductors (TAOSs) have received significant attention owing to their technologically significant characteristics, including high electron mobility, exceptional transparency, and the capability for low-temperature processing. Several TAOSs have been identified as promising channel materials for thinfilm transistors (TFTs) in display applications [1,2]. The materials suitable for serving as the active layer in TFTs include Al-In-Zn-Sn-O [3], Al-Zn-Sn-O (AZTO) [4], amorphous In-Ga-Zn-O (α-IGZO) [5][6][7][8][9][10], In-Sn-O (ITO) [11], In-Zn-O (IZO) [12][13][14], ZnO [15], and Zn-Sn-O [16]. Among these, amorphous InGaZnO 4 (IGZO) has garnered significant attention in research and development since Hosono and colleagues first showcased its potential as an active layer for TFTs [5]. One crucial aspect of TFTs based on TAOSs is their multicomponent composition, which grants them a high degree of flexibility in tuning the properties for TFT applications. Iwasaki et al. [17] indicated a combinatorial approach has been applied to TFTs by co-sputtering with three targets of In 2 O 3 , Ga 2 O 3 , and ZnO. Koinuma et al. [18] the deposition methods, the working pressure, the substrate temperature, the final film thickness, and the O 2 /Ar flow ratio. Among these factors, the O 2 /Ar flow ratio is particularly significant as it profoundly impacts the film's structure, tunes the optical band gap, and influences the electrical properties of IGZO films [28]. However, few studies have investigated the effects of the oxygen flow rate and annealing temperature on the optical, electrical, and structural properties of IGZO films fabricated by magnetron co-sputtering using three targets of In 2 O 3 , Ga 2 O 3 , and Zn. The present work investigates the effects of various oxygen (O 2 ) flow rates and different annealing temperatures on the microstructure, phase formation, and the chemical composition, as well as the optical and electrical properties of co-sputtered IGZO films. The impact on the chemical composition and chemical bonding are analyzed using X-ray photoelectron spectroscopy (XPS). Finally, the most suitable O 2 flow rate and annealing temperature for fabricating co-sputtered IGZO films will be recommended.

Materials and Methods
IGZO films were deposited on 1 × 1 cm 2 square SCHOTT B270 glass substrates at room temperature using In 2 O 3 (purity, 99.99%), and Ga 2 O 3 (purity, 99.99%) ceramic targets and a Zn (purity, 99.99%) metallic target, all 7.62 cm (3 in diameter, as shown in Figure 1a. Details of the preparation of targets are found elsewhere [29]. IGZO films were deposited using the following co-sputtering parameters, 40 W of direct-current (DC) power was used on a Zn target, and 75 W radio-frequency (RF) power was applied to both an In 2 O 3 target and a Ga 2 O 3 target, respectively. The deposition chamber was initially evacuated to 5.0 × 10 −4 Pa, and then a fixed Ar gas flow of 30 sccm and an O 2 gas flow varying from 1, 3, 5, or 7 sccm were introduced into the chamber to maintain a working pressure of 0.67 Pa. After deposition, the samples were annealed at 200, 300, or 400 • C at a working pressure of 0.67 Pa for 1 h in an ambient environment of 40 sccm N 2 . The thicknesses of the IGZO films deposited under different O 2 flow rates were controlled within 200-215 nm. The film thicknesses were measured and fitted using a spectroscopic ellipsometer (SE) (M-2000U, J. A. Woollam Co., Inc., Lincoln County, NE, USA) and CompleteEASE software (version 5.30), respectively. The optical transmittance spectra of these films were examined in the wavelength range of 340-1200 nm using a UV-visible spectrophotometer (LAMBDA UV Lambda 900, PerkinElmer, Inc., Waltham, MA, USA). The film resistivity was performed with the four-point probe method using a 3S, MFP series (Swin). Hall measurements were performed with a HALL8800 system (Swin) using a magnetic field of 0.68 T and the van der Pauw method. The surface morphologies and roughness of the co-sputtered films were investigated using a scanning electron microscope (SEM) (JOEL JSM 6500-F, JEOL Ltd., Akishima, Tokyo, Japan) at an operating voltage of 15 kV and was observed using atomic force microscopy (AFM) (Veeco Dimension 3100, Plainview, NY, USA) at a scan area of 2 µm × 2 µm. The crystallinity of the co-sputtered IGZO films was also investigated using a grazing incidence X-ray diffraction (GIXRD) analysis (PANalytical X'Pert Pro, Malvern Panalytical Ltd., Malvern, UK) with a Ni-filtered Cu Kα (λ = 1.5418 Å) source at a glancing incident angle of 1 • . The scanning range was between 2θ = 20 • and 80 • . X-ray photoelectron spectroscopy (XPS, Thermo VG Scientific Microlab 310F Instruments, Thermo Fisher Scientific Inc. Waltham, MA, USA) was also employed to characterize the chemical composition and chemical bonding states of the co-sputtered IGZO films. The binding energy calibration was performed using the C 1s peak at 284.5 eV. A data analysis of the O 1s peaks was employed using XPS Peak 4.0 software to enable the identification of chemical bond variations.  Figure 1b shows the deposition rate of co-sputtered IGZO films decreased rapidly from 11.81 to 2.07 nm/min when the O2 flow rate increases from 1 sccm to 7 sccm. This trend can be explained as follows: the increase in oxygen flow rate results in the scattering of sputtered atoms. Furthermore, oxygen might dilute argon gas, and collisions between sputtered target atoms and oxygen atoms on the substrate are intensified [28]. Compared with a single ceramic target (the composition of the target is In:Ga:Zn:O = 1:1:1:4), the deposition rate of IGZO films decreased rapidly from 3.974 to 1.427 nm/min by changing the flow rate of O2 from 0 to 6 sccm [28]. Utilizing the co-sputtering method provides higher deposition rates. Figure 2a shows the transmittance spectra of the co-sputtered IGZO films deposited with various O2 flow rates and annealed at 300 °C. The transmittance spectrum of the glass substrate B270 is also indicated. In terms of optical transmittance, IGZO films deposited at a low O2 flow rate of 1 sccm exhibit low transparency in the wavelength range of 340-1200 nm Additionally, there is a broad absorption tail, which could be attributed to oxygen deficiency or the presence of metallic interstitials [30,31]. The transmitted wavelengths of these films exhibited a shorter shift of wavelength in the photo-generative absorption threshold and a larger optical band gap (Eg) with an increased O2 flow rate. The transmittance in the ultraviolet region displayed a slight blue shift, resulting in an increase in the optical bandgap. The average transmittance (Tavg%) in the visible spectrum (400-800 nm) of the respective co-sputtered IGZO films deposited with O2 flow rates of 1, 3, 5, and 7 sccm annealed at 300 °C were 4.24%, 82.48%, 81.13%, and 81.03%, respectively. The Tavg% increased significantly from 4.24% to 82.48% as the O2 flow rate increased from 1 sccm to 3 sccm.

Optical and Electrical Properties of Cosputtered IGZO Films at Different Oxygen Flow Rates
In a direct transition semiconductor, the absorption coefficient, α, and the optical band gap (Eg) are estimated using a standard Tauc plot method from the optical transmission spectra by [32,33], where B is a constant, λ is the wavelength of the incident photon, hν is the energy of the incident photon, and T and d are the transmittance and thickness of the thin films, respectively. Figure 2b illustrates the plots of α 2 versus the photon energy for co-sputtered IGZO films deposited at various O2 flow rates and annealed at 300 °C. Since the α of the IGZO thin films deposited at an O2 flow rate of 1 sccm is significantly greater than the α of the  Figure 1b shows the deposition rate of co-sputtered IGZO films decreased rapidly from 11.81 to 2.07 nm/min when the O 2 flow rate increases from 1 sccm to 7 sccm. This trend can be explained as follows: the increase in oxygen flow rate results in the scattering of sputtered atoms. Furthermore, oxygen might dilute argon gas, and collisions between sputtered target atoms and oxygen atoms on the substrate are intensified [28]. Compared with a single ceramic target (the composition of the target is In:Ga:Zn:O = 1:1:1:4), the deposition rate of IGZO films decreased rapidly from 3.974 to 1.427 nm/min by changing the flow rate of O 2 from 0 to 6 sccm [28]. Utilizing the co-sputtering method provides higher deposition rates. Figure 2a shows the transmittance spectra of the co-sputtered IGZO films deposited with various O 2 flow rates and annealed at 300 • C. The transmittance spectrum of the glass substrate B270 is also indicated. In terms of optical transmittance, IGZO films deposited at a low O 2 flow rate of 1 sccm exhibit low transparency in the wavelength range of 340-1200 nm Additionally, there is a broad absorption tail, which could be attributed to oxygen deficiency or the presence of metallic interstitials [30,31]. The transmitted wavelengths of these films exhibited a shorter shift of wavelength in the photo-generative absorption threshold and a larger optical band gap (E g ) with an increased O 2 flow rate. The transmittance in the ultraviolet region displayed a slight blue shift, resulting in an increase in the optical bandgap. The average transmittance (T avg %) in the visible spectrum (400-800 nm) of the respective co-sputtered IGZO films deposited with O 2 flow rates of 1, 3, 5, and 7 sccm annealed at 300 • C were 4.24%, 82.48%, 81.13%, and 81.03%, respectively. The T avg % increased significantly from 4.24% to 82.48% as the O 2 flow rate increased from 1 sccm to 3 sccm. °C were 2.53, 3.21, 3.26, and 3.30 eV, respectively. The Eg of the IGZO films annealed at 300 °C increased from 2.53 to 3.30 eV as the O2 flow rate increases from 1 to 7 sccm, which may be explained as the carrier concentration decreases for oxygen vacancies filled by oxygen atoms with increasing O2 introducing into the deposition process. The elevated oxygen partial pressure induces oxidation in a-IGZO and inhibits the presence of the oxygen vacancy defects that reside at the bottom of the conduction band [34,35]. Therefore, the inhibition of the formation of vacancy defects resulting from higher oxidation efficiency will conversely increase the Eg of thin films [36].  [27]. Reference [27] is the previous conference paper, of which the authors retain the copyright. Figure 3a depicts the co-sputtered IGZO films' resistivity as a function of O2 flow rate with different annealing temperatures, as measured by the four-point probe method. The results demonstrate that the resistivity increased as the oxygen flow rate increased for the films annealed at 200 °C and 300 °C. However, the resistivity remained relatively un- Reference [27] is the previous conference paper, of which the authors retain the copyright.

Optical and Electrical Properties of Cosputtered IGZO Films at Different Oxygen Flow Rates
In a direct transition semiconductor, the absorption coefficient, α, and the optical band gap (E g ) are estimated using a standard Tauc plot method from the optical transmission spectra by [32,33], where B is a constant, λ is the wavelength of the incident photon, hν is the energy of the incident photon, and T and d are the transmittance and thickness of the thin films, respectively. Figure 2b illustrates the plots of α 2 versus the photon energy for co-sputtered IGZO films deposited at various O 2 flow rates and annealed at 300 • C. Since the α of the IGZO thin films deposited at an O 2 flow rate of 1 sccm is significantly greater than the α of the IGZO films deposited at other flow rates, α 2 for the O 2 flow rate of 1 sccm is displayed on the right vertical axis of Figure 2b. The optical bandgap (E g ) was determined by extrapolating the straight region of the plot of α 2 versus the photon energy. The E g of the respective co-sputtered IGZO films deposited with O 2 flow rates of 1, 3, 5, and 7 sccm annealed at 300 • C were 2.53, 3.21, 3.26, and 3.30 eV, respectively. The E g of the IGZO films annealed at 300 • C increased from 2.53 to 3.30 eV as the O 2 flow rate increases from 1 to 7 sccm, which may be explained as the carrier concentration decreases for oxygen vacancies filled by oxygen atoms with increasing O 2 introducing into the deposition process. The elevated oxygen partial pressure induces oxidation in a-IGZO and inhibits the presence of the oxygen vacancy defects that reside at the bottom of the conduction band [34,35]. Therefore, the inhibition of the formation of vacancy defects resulting from higher oxidation efficiency will conversely increase the E g of thin films [36]. Figure 3a depicts the co-sputtered IGZO films' resistivity as a function of O 2 flow rate with different annealing temperatures, as measured by the four-point probe method. The results demonstrate that the resistivity increased as the oxygen flow rate increased for the films annealed at 200 • C and 300 • C. However, the resistivity remained relatively unchanged even with an increase in the O 2 flow rate for the films annealed at 400 • C. Figure 3b shows the Hall mobility of the co-sputtered IGZO films as a function of O 2 flow rate with different annealing temperatures. For the films annealed at 300 • C, the Hall mobility increased from 1.31 to 7.01 (cm 2 V −1 s −1 ) when the O 2 flow rate increased from 1 to 3 sccm and then decreased with increasing O 2 flow from 3 to 7 sccm. However, for the films annealed at 400 • C, as the O 2 flow rate increased from 3 to 5 sccm the carrier mobility remained in the range of 9.92 to 11.18 (cm 2 V −1 s −1 ) The films deposited with an O 2 flow rate of 5 sccm and an annealing temperature of 400 • C had the highest carrier mobility of all films tested, at 11.18 cm 2 V −1 s −1 , after which it gradually decreases with increasing O 2 flow. Figure 3c shows the carrier concentration of co-sputtered IGZO films as a function of O 2 flow rate with different annealing temperatures. These results indicate that the carrier concentration decreased as the oxygen flow rate increased, although there was a slight increase in carrier concentration when the O 2 flow rate was changed from 5 to 7 sccm for the films annealed at 400 • C. According to high resistance in co-sputtered IGZO films deposited with the O 2 flow rates of 5 and 7 sccm and annealed at 200 • C, it is hard to measure the mobility and carrier concentration of the films using Hall measurement instruments.

Structural Analysis of Co-Sputtered IGZO Thin Films at Different Oxygen Flow Rates
To further validate the impact of the O 2 flow rate on surface morphologies and roughness of the co-sputtered IGZO thin films, Figures 4 and 5 present the SEM and AFM images, respectively, of co-sputtered IGZO thin films deposited at various O 2 flow rates. Figure 4a-c show the SEM graphs of the co-sputtered IGZO films deposited under the respective O 2 gas flow rates of 1, 3, and 5 sccm and annealed at 300 • C. Other annealing temperatures exhibited similar results. Figure 5a-c depict the AFM images of the co-sputtered IGZO films deposited under the respective O 2 gas flow rates of 1, 3, and 5 sccm and annealed at 300 • C. The root-mean-square (RMS) roughness of the IGZO films deposited with O 2 flow rates of 1, 3, and 5 sccm and annealed at 300 • C were 0.472, 0.411, and 0.513 nm, respectively, and these values are given in Figure 5. In summary, it can be observed that the surface morphology of the co-sputtered IGZO films appears rougher in structure when the O 2 flow rate is 1 sccm. However, the surface morphology transitions to a smoother and more uniform granular structure when the O 2 flow rate is increased to 3 sccm. At an O 2 flow rate of 5 sccm, the grain size increases resulting in a rougher surface compared to that of 3 sccm. Thus, the AFM and SEM results reveal that the co-sputtered IGZO films deposited under an O 2 flow rate of 3 sccm displayed the smoothest surface and the smallest RMS roughness. The surface conditions observed in these images explain why the T avg % of co-sputtered IGZO films (shown in Figure 2a) increased significantly as the O 2 flow rate increased from 1 sccm to 3 sccm.  [27]. Reference [27] is the previous conference paper, of which the authors retain the copyright.

Structural Analysis of Co-Sputtered IGZO Thin Films at Different Oxygen Flow Rates
To further validate the impact of the O2 flow rate on surface morphologies and roughness of the co-sputtered IGZO thin films, Figures 4 and 5 present the SEM and AFM images, respectively, of co-sputtered IGZO thin films deposited at various O2 flow rates. Figure 4a-c show the SEM graphs of the co-sputtered IGZO films deposited under the respective O2 gas flow rates of 1, 3, and 5 sccm and annealed at 300 °C. Other annealing temperatures exhibited similar results. Figure 5a-c depict the AFM images of the co-sputtered IGZO films deposited under the respective O2 gas flow rates of 1, 3, and 5 sccm and annealed at 300 °C. The root-mean-square (RMS) roughness of the IGZO films deposited with O2 flow rates of 1, 3, and 5 sccm and annealed at 300 °C were 0.472, 0.411, and 0.513 nm, respectively, and these values are given in Figure 5. In summary, it can be observed that the surface morphology of the co-sputtered IGZO films appears rougher in structure when the O2 flow rate is 1 sccm. However, the surface morphology transitions to a smoother and more uniform granular structure when the O2 flow rate is increased to 3 sccm. At an O2 flow rate of 5 sccm, the grain size increases resulting in a rougher surface compared to that of 3 sccm. Thus, the AFM and SEM results reveal that the co-sputtered IGZO films deposited under an O2 flow rate of 3 sccm displayed the smoothest surface and the smallest RMS roughness. The surface conditions observed in these images explain why the Tavg% of co-sputtered IGZO films (shown in Figure 2a) increased significantly as the O2 flow rate increased from 1 sccm to 3 sccm.
In order to verify the effects of different oxygen flow rates and annealing temperatures on the crystalline phases of co-sputtered IGZO films, the crystallinities of the IGZO films were also analyzed using glancing angle X-ray diffraction. Figure 6 shows the XRD graphs of co-sputtered IGZO films deposited under the respective O2 gas flow rates of 1, 3, and 5 sccm and annealed at 300 °C. Other annealing temperatures exhibited similar results. The IGZO films co-sputtered using three targets of In2O3, Ga2O3, and Zn with an O2 flow rate of 1 sccm exhibited an amorphous structure. However, the XRD patterns reveal a polycrystalline oxide film when the O2 flow rates were 3 sccm and 5 sccm, and the crystallinity increased as the O2 flow rate increased.   [27]. Reference [27] is the previous conference paper, of which the authors retain the copyright.      In order to verify the effects of different oxygen flow rates and annealing temperatures on the crystalline phases of co-sputtered IGZO films, the crystallinities of the IGZO films were also analyzed using glancing angle X-ray diffraction. Figure 6 shows the XRD graphs of co-sputtered IGZO films deposited under the respective O 2 gas flow rates of 1, 3, and 5 sccm and annealed at 300 • C. Other annealing temperatures exhibited similar results. The IGZO films co-sputtered using three targets of In 2 O 3 , Ga 2 O 3 , and Zn with an O 2 flow rate of 1 sccm exhibited an amorphous structure. However, the XRD patterns reveal a polycrystalline oxide film when the O 2 flow rates were 3 sccm and 5 sccm, and the crystallinity increased as the O 2 flow rate increased. The peak positions of InGaZn 7 where λ is the wavelength of the X-ray (λ = 0.15406 nm), θ is Bragg's diffraction angle, and β is the FWHM of the diffraction peak (in radians). Using this method, the co-sputtered IGZO films deposited at the respective O 2 gas flow rates of 3 and 5 sccm yielded a grain size of about 9.9 nm and 11.5 nm, respectively. Therefore, the crystallinity and grain growth of the InGaZn 7 O 10 phase were enhanced by increasing the O 2 flow rate. A slight increase in the surface roughness was observed in the films with an O 2 flow rate of 5 sccm (shown in Figures 4 and 5) which is attributed to the creation and significant growth of crystalline IGZO grains in these films. Surface roughness plays a significant role in shaping the electron propagation within the IGZO films. The results of the structural analyses can explain why, with the O 2 flow rate of 5 sccm, the carrier mobility slightly decreases to 4.82 cm 2 V −1 s −1 at an annealing temperature of 300 • C. Additionally, at an annealing temperature of 400 • C, the carrier mobility reaches the highest value of 11.18 cm 2 V −1 s −1 , after which it gradually decreases.
growth of the InGaZn7O10 phase were enhanced by increasing the O2 flow rate. A slight increase in the surface roughness was observed in the films with an O2 flow rate of 5 sccm (shown in Figures 4 and 5) which is attributed to the creation and significant growth of crystalline IGZO grains in these films. Surface roughness plays a significant role in shaping the electron propagation within the IGZO films. The results of the structural analyses can explain why, with the O2 flow rate of 5 sccm, the carrier mobility slightly decreases to 4.82 cm 2 V −1 s −1 at an annealing temperature of 300 °C. Additionally, at an annealing temperature of 400 °C, the carrier mobility reaches the highest value of 11.18 cm 2 V −1 s −1 , after which it gradually decreases.    Figure 7 shows (a) In 3d, (b) Ga 2p, and (c) Zn 2p core levels of the XPS spectra for co-sputtered IGZO films deposited with O 2 gas flow rates of 1, 3, and 5 sccm and annealed at 300 • C. The In 3d 5/2 peak is centered at 444.5 eV, the Ga 2p 3/2 peak is centered at 1118 eV, and the Zn 2p 3/2 peak is centered at 1021.8 eV. According to previous research [38][39][40] the peak centers of In 3d 5/2 , Ga 2p 3/2 , and Zn 2p 3/2 were reported to be 444.6 eV, 1118.9 eV, Since the In 3d 5/2 and Zn 2p 3/2 core levels observed in the XPS spectra are consistent with those reported in previous studies [39,40], these findings suggest that indium and zinc are almost totally oxidized to In-O and Zn-O in the co-sputtered IGZO films. In addition, the Ga-O 2p 3/2 peak is centered at 1118 eV, which is close to Ga-Ga (1117.4 eV) [40], indicating the presence of metallic Ga bonding in the co-sputtered IGZO films. Furthermore, the area of the Ga 2p 3/2 peak gradually decreased with increasing O 2 gas flow rate. Figure 7d shows the In, Ga, Zn/(In + Ga + Zn) atomic ratios (at.%) of co-sputtered IGZO films deposited with the O 2 gas flow rates of 1, 3, and 5 sccm and annealed at 300 • C. The composition ratios were defined as the relative integrated intensity of In 3d 5/2 , Ga 2p 3/2 , and Zn 2p 3/2 XPS peaks, respectively. It can be clearly observed that the Zn/(In + Ga + Zn) ratio increases from 66.51% to 72.15% when the O 2 flow rate increases from 1 to 3 sccm, but after 5 sccm, the Zn/(In + Ga + Zn) ratio is reduced to a smaller ratio of 62.09%. The In/(In + Ga + Zn) ratio obviously increases from 14.24% to 29.93% when increasing the O 2 flow rate from 1 to 5 sccm. The Ga/(In + Ga + Zn) ratio decreases from 19.25% to 6.83% when the O 2 flow rate increases from 1 to 3 sccm, and at an O 2 flow rate of 5 sccm, the Ga at.% slightly increases to 7.98%. Some articles mentioned that increasing the at% of Zn [8,20,25,26] and In [8,24] can enhance the carrier mobility of IGZO thin films. Therefore, it is expected that as the O 2 flow rate increases, the carrier mobility will rise in tandem with the increase in Zn and In content. When the O 2 flow rate is 5 sccm, due to the decrease in Zn at% (shown in Figure 7d) and the increase in AFM RMS roughness (shown in Figure 5c), the carrier mobility decreases to 4.82 cm 2 V −1 s −1 , as indicated in Figure 3b. Because of the highest Zn at% and the lowest surface roughness is observed at an O 2 flow rate of 3 sccm, the IGZO film exhibits the highest mobility of 7.01 cm 2 V −1 s −1 during the 300 • C annealing process. The surface morphology has crucial effects on the carrier mobility of the IGZO films [26]. Figure 7 shows (a) In 3d, (b) Ga 2p, and (c) Zn 2p core levels of the XPS spectra for co-sputtered IGZO films deposited with O2 gas flow rates of 1, 3, and 5 sccm and annealed at 300 °C. The In 3d5/2 peak is centered at 444.5 eV, the Ga 2p3/2 peak is centered at 1118 eV, and the Zn 2p3/2 peak is centered at 1021.8 eV. According to previous research [38][39][40] the peak centers of In 3d5/2, Ga 2p3/2, and Zn 2p3/2 were reported to be 444.6 eV, 1118.9 eV, and 1021.8 eV, respectively, corresponding to In-O, Ga-O, and Zn-O bonds. However, if these peak positions shift to lower binding energies, it indicates that some of the bonding configurations In-In (443.8 eV), Ga-Ga (1117.4 eV), and Zn-Zn (1021.5 eV) are represented. Since the In 3d5/2 and Zn 2p3/2 core levels observed in the XPS spectra are consistent with those reported in previous studies [39,40], these findings suggest that indium and zinc are almost totally oxidized to In-O and Zn-O in the co-sputtered IGZO films. In addition, the Ga-O 2p3/2 peak is centered at 1118 eV, which is close to Ga-Ga (1117.4 eV) [40], indicating the presence of metallic Ga bonding in the co-sputtered IGZO films. Furthermore, the area of the Ga 2p3/2 peak gradually decreased with increasing O2 gas flow rate. Figure 7d shows the In, Ga, Zn/(In + Ga + Zn) atomic ratios (at.%) of co-sputtered IGZO films deposited with the O2 gas flow rates of 1, 3, and 5 sccm and annealed at 300 °C. The composition ratios were defined as the relative integrated intensity of In 3d5/2, Ga 2p3/2, and Zn 2p3/2 XPS peaks, respectively. It can be clearly observed that the Zn/(In + Ga + Zn) ratio increases from 66.51% to 72.15% when the O2 flow rate increases from 1 to 3 sccm, but after 5 sccm, the Zn/(In + Ga + Zn) ratio is reduced to a smaller ratio of 62.09%. The In/(In + Ga + Zn) ratio obviously increases from 14.24% to 29.93% when increasing the O2 flow rate from 1 to 5 sccm. The Ga/(In + Ga + Zn) ratio decreases from 19.25% to 6.83% when the O2 flow rate increases from 1 to 3 sccm, and at an O2 flow rate of 5 sccm, the Ga at.% slightly increases to 7.98%. Some articles mentioned that increasing the at% of Zn [8,20,25,26] and In [8,24] can enhance the carrier mobility of IGZO thin films. Therefore, it is expected that as the O2 flow rate increases, the carrier mobility will rise in tandem with the increase in Zn and In content. When the O2 flow rate is 5 sccm, due to the decrease in Zn at% (shown in Figure 7d) and the increase in AFM RMS roughness (shown in Figure 5c), the carrier mobility decreases to 4.82 cm 2 V −1 s −1 , as indicated in Figure 3b. Because of the highest Zn at% and the lowest surface roughness is observed at an O2 flow rate of 3 sccm, the IGZO film exhibits the highest mobility of 7.01 cm 2 V −1 s −1 during the 300 °C annealing process. The surface morphology has crucial effects on the carrier mobility of the IGZO films [26].   XPS spectra of (a) In3d, (b) Ga2p, (c) Zn2p core levels, and (d) In, Ga, Zn/(In + Ga + Zn) atomic ratios (at.%) of co-sputtered IGZO films deposited with O2 gas flow rates of 1, 3, 5 sccm and annealed at 300 °C. Figure 8a-c shows the O 1s peak spectra of co-sputtered IGZO films deposited with the respective O2 gas flow rates of 1, 3, and 5 sccm and annealed at 300 °C. The O 1s peak can be fitted by three nearly Gaussian distributions, centered at 530.3 ± 0.3, 531.3 ± 0.3, and 532.4 ± 0.3 eV, respectively [38,41]. To simplify the following descriptions, OI, OII, OIII, and Ototal refer to the areas of the components located at 530.3, 531.3, and 532.4 eV, and the total area of the O 1s peak, respectively. The black line represents the raw data, while the red line depicts the peak summation of the peaks OI, OII, and OIII. It is evident that the black lines are almost identical with the red lines. The component on the lower binding energy side of the O 1s spectrum at 530.3 eV (OI) can be attributed to O 2− ions surrounded by Ga, In, and Zn atoms in the IGZO compound system [41]. The OI/Ototal values increased from 35.67% to 54.68% as the O2 flow rate increased. Therefore, the increase in O 2− ions in the lattice leads to the increase in the average transmittance and optical bandgap, shown in Figure 2a,b, respectively. The binding energy component centered at 531.3 eV (OII) is associated with O 2− ions that are in oxygen-deficient regions within the IGZO matrix [41]. The results show that the OII/Ototal values decreased from 25.39% to 17.21% with increasing O2 flow rate, indicating a decrease in the concentration of oxygen vacancies. Generally, oxygen vacancies are the main source of free electron carriers in IGZO films [35] and the decrease in OII-related oxygen vacancies results in the decrease in carrier concentration and the increase in resistivity. The high binding energy component located at 532.4 eV (OIII) is usually attributed to the presence of oxygen on the surface of film belonging to a specific loosely bound specie, e.g., -CO3 or adsorbed H2O or adsorbed O2 [41]. The OIII/Ototal values decreased from 38.94% to 27.66% with increasing O2 flow rate, indicating a decrease in the specific chemisorbed oxygen.    The XPS results revealed that the co-sputtered IGZO films with an O 2 flow rate of 3 sccm have more stable chemical compositions among different annealing temperatures, whereas the co-sputtered IGZO films with an O 2 flow rate of 1 sccm exhibit a more diverse chemical composition. Figure 9c-f show the XPS spectra of In 3d, Ga 2p, Zn 2p, and O 1s core levels for co-sputtered IGZO films deposited with an O 2 gas flow rate of 3 sccm and subjected to different annealing temperatures. As the annealing temperature increased, the In 3d, Ga 2p, and Zn 2p binding energy shifted towards a higher energy by 0.3-0.4 eV, which is attributed to an increase in the oxidation state of In, Ga, and Zn. The results of the XPS spectra indicate that the In 3d and Zn 2s peaks were totally oxidized to In-O and Zn-O in the co-sputtered IGZO films, and the Ga-Ga bonds gradually transformed into Ga-O bonding configurations during annealing. Furthermore, the lower binding energy level of O 1s peaks shifted from 530.3 eV to 530.7 eV with an increase in the annealing temperature, and this was attributed to the increase in oxygen vacancies. In other words, the O atom is deficient on the surface of the IGZO film annealed at 400 • C in ambient N 2 .

X-ray Photoelectron Spectroscop (XPS) ic Analysis of Co-Sputtered IGZO Thin Films
Zn-O in the co-sputtered IGZO films, and the Ga-Ga bonds gradually transformed into Ga-O bonding configurations during annealing. Furthermore, the lower binding energy level of O 1s peaks shifted from 530.3 eV to 530.7 eV with an increase in the annealing temperature, and this was attributed to the increase in oxygen vacancies. In other words, the O atom is deficient on the surface of the IGZO film annealed at 400 °C in ambient N2. Figure 9. In, Ga, Zn/(In + Ga + Zn) at.% of co-sputtered films deposited at the O2 gas flow rates of (a) 1 and (b) 3 sccm with different annealing temperatures; XPS spectra of (c) In3 d, (d) Ga 2p, (e) Zn 2p, and (f) O 1s core levels for co-sputtered IGZO films deposited with an O2 gas flow rate of 3 sccm and subjected to different annealing temperatures.

Conclusions
The average transmittance (T avg %) and optical bandgap (E g ) of the co-sputtered IGZO films increased significantly when the oxygen flow rate was increased from 1 sccm to 3 sccm. The E g of the IGZO films increased and the carrier concentration decreased with increasing O 2 flow rates, which may be explained by oxygen vacancies being filled by oxygen atoms with increasing O 2 during the deposition process. When the IGZO films were annealed at temperatures of 200 • C and 300 • C, the resistivity obviously increased with an increased O 2 flow rate. With added O 2 flow, the carrier concentration decreased and the Hall mobility rose, the latter reaching its highest value of 11.18 cm 2 V −1 s −1 when the O 2 flow was 5 sccm. When the films were annealed at a temperature of 400 • C, the resistivity did not change with respect to an increased O 2 flow rate.
The co-sputtered IGZO films with an O 2 flow rate of 1 sccm exhibited an amorphous structure. The polycrystalline InGaZn 7 O 10 phase and the crystallinity gradually increased when the O 2 flow rate was increased from 3 sccm to 5 sccm. The surface morphology appears a rougher in structure when the O 2 flow rate is 1 sccm. However, the surface morphology transitions to a smoother and more uniform granular structure with an increased O 2 flow rate, which explains why the T avg % of the co-sputtered IGZO films significantly increased as the O 2 flow rate increased from 1 sccm to 3 sccm. The AFM results revealed that the co-sputtered IGZO film deposited with an O 2 flow rate of 3 sccm displayed the smoothest surface and the smallest RMS roughness.
Due to the maximum Zn at% and the lowest surface roughness observed at the O 2 flow rate of 3 sccm, the IGZO film demonstrates its highest mobility of 7.01 cm 2 V −1 s −1 during the 300 • C annealing process. The impacts of the O 2 flow rate on E g can be understood through the O 1s peak in the XPS spectra of the co-sputtered IGZO films annealed at 300 • C. A Gaussian fitting of the O 1s peaks revealed that increasing O 2 gas flows led to an increase in O I bonding (O 2− ions in the lattice), thereby yielding an increase in the T avg % and the E g , and a decrease in O II -related oxygen vacancies, thus resulting in a decreased carrier concentration and an increased resistivity. XPS results at various annealing temperatures indicated that the co-sputtered IGZO films with an O 2 flow rate of 1 sccm exhibited a diverse chemical composition, whereas the co-sputtered IGZO films with an O 2 flow rate of 3 sccm have more stable chemical compositions among different annealing temperatures.
A comprehensive assessment of the impacts of the O 2 flows and annealing temperature on the optical, electrical and structural properties of co-sputtered In 2 O 3 -Ga 2 O 3 -Zn films suggests the following optimal process conditions. An O 2 flow rate of 3 sccm and an annealing temperature at 300 • C showed the largest average transmittance of 82.48%, a larger optical bandgap of 3.21 eV, and a larger carrier mobility of 7.01 cm 2 V −1 s −1 . This study is valuable for the applications of co-sputtered IGZO films in transparent thin-film transistors.