Transparent Conductive Indium Zinc Oxide Films: Temperature and Oxygen Dependences of the Electrical and Optical Properties

: Achieving high-efﬁciency optoelectronic devices often requires the development of high transparency in the extended range and high-conductivity materials, which can be ensured by the high mobility of charge carriers being used as the electrode. Among the candidate materials, transparent conductive indium zinc oxide (IZO) has attracted signiﬁcant interest because of its superior electron mobility (5 − 60 cm 2 /V · s) and the thermal stability of its structure. In this study, the IZO ﬁlms were deposited by the radio frequency magnetron sputtering of the IZO ceramic target (containing 10 wt.% ZnO) by varying the two variables of the substrate temperature and the oxygen content in the working gas. Here, the importance of the deposition of the IZO ﬁlms at a low substrate temperature, not exceeding 100 ◦ C, in order to get the minimum values of the ﬁlm resistivity is revealed. At a substrate temperature of 100 ◦ C, the ﬁlm deposited in pure argon demonstrated a minimum resistance of 3.4 × 10 − 4 Ω · cm. Despite the fact that, with the addition of O 2 in the working gas, an increase in resistivity was observed, the IZO ﬁlm that deposited under 0.4% O 2 content demonstrated the highest mobility ( µ = 35 cm 2 /V · s at ρ = 6.0 × 10 − 4 Ω · cm) and enhanced transparency in the visible (VIS, 400 − 800 nm) and near-infrared (NIR, 800 − 1250 nm) ranges (T VIS ≥ 77% and T NIR ≥ 76%). At an oxygen content above 0.4%, a signiﬁcant deterioration in electrical properties and a decrease in optical characteristics were observed. SEM and XRD studies of the microstructure of the IZO ﬁlms allowed the clariﬁcation of the effect of both the substrate temperature and the oxygen content on the functional characteristics of the transparent conducting IZO ﬁlms.


Introduction
Transparent conductive oxide films (TCO films), due to their high transmittance and low resistivity, have been extensively used as electrodes in various optoelectrical devices, including photovoltaic solar cell [1,2], flat panel display [3], light-emitting diodes [4][5][6], sensors [7], etc. Conventional TCO materials, such as In 2 O 3 :Sn (ITO), highly doped ZnO (with Ga, Al, In, B, etc.) and SnO 2 :F, are steadily under study and have been widely utilized in the device industry [1,2,6,[8][9][10]. However, a critical problem in achieving good electrical performance for these materials is an unavoidable high-temperature process in order to activate dopants by substrate heating up to 300 • C directly during film deposition or during subsequent postannealing, which severely limits their application in the dynamically developing area of optoelectronic devices on organic flexible substrates, also called flexible and stretchable optoelectronics [7,11]. A boom in interest in recently emerging fields, such as flexible display using organic light-emitting diodes (OLED) and tandem solar cells stacked with perovskite and Cu(In,Ga)Se2 (CIGS), has led to new requirements for transparent electrodes. For OLED application, the control of the film microstructure and the film surface roughness associated with it, as well as the adjustment of the electrode work function, come to the fore [12,13]. In turn, the widening of the transparency window of the TCO electrode to the NIR region, while maintaining its good electrical performance, is one of the most important tasks applied to CIGS-based solar cells [14][15][16].
Currently, an indium zinc oxide system is one of the more promising materials for TCO films due to its high transmittance, good electrical properties, thermal stability and lowest deposition temperature. Moreover, a tunable work function, high etching rate, smoother surface and resistance to moist heat are the prominent features of IZO thin films, which are very important aspects for OLED display applications [13,17]. Regarding the use of IZO films as transparent electrodes in tandem solar cells, a very important fact is the opportunity to achieve a much lower NIR absorption due to their low charge-carrier density, which is compensated by their exceptionally high electron mobility that simultaneously allows for a high conductivity [16,18,19].
Among them, sputtering is the most established deposition technique due to its great advantages, such as a high deposition rate, high-quality film and simple scalability to large deposition areas [8,24], despite the fact that it can lead to plasma damage, resulting in degradation of the device's performance. Sufficient studies have been devoted to achieving good electrical properties while maintaining high optical transmittance through optimizing magnetron sputtering conditions [17,19,[25][26][27][28][29][30]. Initially, studies focused on the effects of the specific doping concentration [25,30], oxygen partial pressure [19,29] and temperature regime [17,28] on the functional properties of the IZO films. Often, the results obtained differed from each other due to the peculiarities of the growth conditions in a particular experiment.
In this study, we are going to explore the effects of the two key important variables of the substrate temperature and the oxygen content in the working gas on the final functional properties of the IZO (In 2 O 3 with 10 wt.% ZnO) films by using a magnetron sputtering setup, which had been equipped with a specially designed multi-position drumtype substrate holder [31]. The substrate holder allows for quick growth temperature optimization for different materials due to the simultaneous growth of films on substrates at different controlled temperatures in a single vacuum run [31,32], i.e., for each value of the oxygen content, a set of IZO thin-film samples differing from each other only in the deposition temperature will be obtained simultaneously.

IZO Thin Films Synthesis
A series of IZO films were fabricated by the radio frequency (rf) magnetron sputtering of an IZO ceramic target (90 wt.% In 2 O 3 and 10 wt.% ZnO) on soda-lime glass slides (25 × 25 × 1.1 mm) at substrate temperatures of room temperature (RT), 100, 200 and 300 • C under different contents of oxygen c(O 2 ) in an Ar-based working gas mix (from 0.0 to 2.0%).
The IZO ceramic target in the form of a 2-inch disk was synthesized as follows. Submicron powders of In 2 O 3 (99.99% purity) and ZnO (99.95% purity) under a weight ratio of 9:1 were used as a starting powder mix. The powder mixture was successively subjected to 5 h of dry mixing in a ball mill, 8 h of annealing in air at 900 • C and repeated 5 h of processing in a ball mill. Then, the prepared powder mix was used to form a ceramic disc with a diameter of 51 mm and a thickness of 4 mm using the spark plasma sintering (SPS) technique [33] under the SPS temperature of 950 • C and SPS duration of 5 min. After mechanical grinding, the ceramic disk was finally annealed in air at 900 • C for 2 h. A home-made rf magnetron sputtering setup provided pumping down to a base pressure of lower than 2 × 10 −4 Pa and was equipped with a multiposition drum-type heated substrate holder, which ensured the simultaneous formation of four films at different temperatures (from room temperature to 300 • C) in one vacuum cycle [31]. To study the effects of the oxygen, pure argon (c(O 2 ) = 0.0%) and preliminarily prepared Ar/O 2 working gas mixes (with varying c(O 2 ) from 0.3 to 2.0%) were used. For each c(O 2 ), four thin-film samples were deposited in a single vacuum cycle at four values of substrate temperatures under the following deposition conditions: the sputtering pressure of 0.5 Pa at a flow rate of 20 sccm, the rf power of 100 W, the rotation rate of the drum substrate holder of 10 rpm (the minimum distance from the target to the passing substrate was 100 mm), and a sputtering duration of 200 min.

IZO Film Characterization
The sheet resistance of the IZO thin films was measured by using a four-point technique (IUS-3, Moscow, Russia), while electrical transport properties were characterized from Hall effect measurements at room temperature by using the van der Pauw geometry. The optical transmittance of the IZO thin films coated on glass substrates was recorded by an optical spectrophotometer (Shimadzu UV-3600, Tokio, Japan) in the wavelength range of 300-1250 nm.
The films' thickness was evaluated from transmission spectra by using the PUMA approach and code described by Birgin et al. [34] and confirmed by scanning electron microscopy measurements (SEM, Leo-1450, Carl Zeiss, Oberkochen, Germany).
A scanning electron microscopy method was also used to observe both the cross-sectional and surface morphology of the deposited films, while an XRD method in θ-2θ scan mode was used to determine their structural characterization (Shimadzu XRD-7000, Tokio, Japan). Figure 1 shows the deposition rate of the IZO thin films prepared under varying oxygen concentrations c(O 2 ). In this figure, each node of the curve corresponds to the average value measured on the four films deposited at substrate temperatures ranging from room temperature to 300 • C. The deposition rate increased until an c(O 2 ) of 0.3% in the working gas mix was reached, while it noticeably decreased with the further increasing of oxygen concentration in the O 2 /Ar gas mix. The increase in the growth rate observed at a low oxygen content may be due to the formation of a metal-rich condition in the IZO target during its preparation by the SPS method [35]. The addition of a small amount of oxygen should promote the oxidation of both surplus Zn and hemioxide In 2 O, which, if sputtered in an inert atmosphere, can relatively easily resputter from the exposed surface due to high vapor pressure. Full oxidation of sputtered species results in a higher deposition rate. In turn, the decrease in the deposition rate with an increase in the oxygen content above 0.3% resulted in both the formation of a thin oxidized layer on the surface of the IZO target under the oxygen ambient in the chamber and the decrease of Ar ions bombarding the target surface [29].  Figure 2 shows the changes in sheet resistance RS of the IZO films deposited at an O2 content up to 0.5% as a function of the substrate temperature during deposition. In Figure  2, there are no data corresponding to the IZO films deposited at an O2 content of 1 and 2%, due to the fact that these films had high values of sheet resistance (RS > 10 5 Ω/sq) that   Figure 2 shows the changes in sheet resistance R S of the IZO films deposited at an O 2 content up to 0.5% as a function of the substrate temperature during deposition. In Figure 2, there are no data corresponding to the IZO films deposited at an O 2 content of 1 and 2%, due to the fact that these films had high values of sheet resistance (R S > 10 5 Ω/sq) that are far from the acceptable TCO resistance requirement. Figure 2 shows that, for each c(O 2 ), the sheet resistance of the IZO films decreased quite a bit with the increasing of the substrate temperature from RT to 100 • C, while a further increase in the substrate temperature leads to an increase in resistance by several times. This result indicates that IZO, unlike more traditional TCO materials (ITO, doped ZnO, etc.) [31,32], does not require high temperature substrate heating, as has been noted previously by many groups [13,16,19].  Figure 2 shows the changes in sheet resistance RS of the IZO films deposited at an O2 content up to 0.5% as a function of the substrate temperature during deposition. In Figure  2, there are no data corresponding to the IZO films deposited at an O2 content of 1 and 2%, due to the fact that these films had high values of sheet resistance (RS > 10 5 Ω/sq) that are far from the acceptable TCO resistance requirement. Figure 2 shows that, for each c(O2), the sheet resistance of the IZO films decreased quite a bit with the increasing of the substrate temperature from RT to 100 °C, while a further increase in the substrate temperature leads to an increase in resistance by several times. This result indicates that IZO, unlike more traditional TCO materials (ITO, doped ZnO, etc.) [31,32], does not require high temperature substrate heating, as has been noted previously by many groups [13,16,19].  In order to gain more information about the aspects leading to this IZO behavior, direct measurements of the electrical properties of the IZO thin films deposited at the various substrate temperatures were performed using the Hall Effect technique. Figure 3 shows the dependences of the resistivity ρ, the Hall mobility µ and the carrier concentration n as a function of the oxygen concentration c(O 2 ). It can be seen that, for the range of O 2 content studied, the minimum values of resistivity ρ were observed in the IZO films deposited at the substrate temperature of 100 • C (Figure 3a), as in the case of the dependence of sheet resistance on substrate temperatures (Figure 2). At the same time, as observed in Figure 3a, for each substrate temperature, there is a tendency for ρ to rise with the increase in the c(O 2 )-a minor change in the ρ at an c(O 2 ) of less than 0.4% and a sharp increase above an c(O 2 ) of 0.4%. tration n as a function of the oxygen concentration c(O2). It can be seen that, for the range of O2 content studied, the minimum values of resistivity ρ were observed in the IZO films deposited at the substrate temperature of 100 °C (Figure 3a), as in the case of the dependence of sheet resistance on substrate temperatures (Figure 2). At the same time, as observed in Figure 3a, for each substrate temperature, there is a tendency for ρ to rise with the increase in the c(O2)-a minor change in the ρ at an c(O2) of less than 0.4% and a sharp increase above an c(O2) of 0.4%. Thus, in the course of a comparative study of the electrical characteristics of the IZO films as functions of the substrate temperature and the O2 concentration, the following was found:

Results and Discussion
• In order to achieve the minimal values of resistivity in the IZO films, it was necessary to carry out sputtering at a substrate temperature not exceeding 100 °C; • At this substrate temperature, the mobility could be increased from 25 to more than 37 cm 2 /V·s by adding up to 0.4% O2 to the working gas Ar. At the same time, despite the concomitant slight decrease in the carrier concentration from 6.7×10 20 to 2.7×10 20 cm −3 , the deposited films were characterized by resistivity values ρ ≤ 6×10 −4 Ω·cm (which corresponds to a sheet resistance RS less than 15 Ω/sq at the films' thickness of 400 nm), meeting conventional TCO requirements; • When using substrate temperatures and oxygen concentrations above optimal values (100 °C and 0.4%O2, respectively), a noticeable downgrade of electrical performance in IZO is observed due to a decrease in both the concentration and mobility. The normal-incidence transmission spectra of a glass substrate with the IZO thin films were measured by the optical spectrophotometry in the 300-1250 nm wavelength range. Figure 4a displays the optical transmittance of the IZO films deposited in the pure Ar working gas with varying substrate temperatures. Thus, in the course of a comparative study of the electrical characteristics of the IZO films as functions of the substrate temperature and the O 2 concentration, the following was found: • In order to achieve the minimal values of resistivity in the IZO films, it was necessary to carry out sputtering at a substrate temperature not exceeding 100 • C; • At this substrate temperature, the mobility could be increased from 25 to more than 37 cm 2 /V·s by adding up to 0.4% O 2 to the working gas Ar. At the same time, despite the concomitant slight decrease in the carrier concentration from 6.7 × 10 20 to 2.7 × 10 20 cm −3 , the deposited films were characterized by resistivity values ρ ≤ 6 × 10 −4 Ω·cm (which corresponds to a sheet resistance R S less than 15 Ω/sq at the films' thickness of 400 nm), meeting conventional TCO requirements; • When using substrate temperatures and oxygen concentrations above optimal values (100 • C and 0.4%O 2 , respectively), a noticeable downgrade of electrical performance in IZO is observed due to a decrease in both the concentration and mobility.
The normal-incidence transmission spectra of a glass substrate with the IZO thin films were measured by the optical spectrophotometry in the 300-1250 nm wavelength range. Figure 4a displays the optical transmittance of the IZO films deposited in the pure Ar working gas with varying substrate temperatures.
The data showed distinct interference fringes indicating that the films were smooth with little scatter. Figure 4a shows that the transparency of the IZO films deposited in pure Ar was increased in the visible range (400−800 nm) with increased substrate temperatures. This may be due to a decrease in the photon absorbance and scattering on defects caused by the incorporation of surplus metals into the film. At the same time, the transmittance of the films in the NIR region (800−1250 nm), as a function of substrate temperatures, repeats the trend of the concentration n in the films with varying substrate temperatures. The minimum transparency in this region was shown by the film deposited at 100 • C, which had the maximum concentration n = 6.9 × 10 20 cm −3 among the films deposited at an c(O 2 ) of 0.0%. Figure 4b illustrates the optical transmittance spectra of the IZO films deposited at the substrate temperature of 100 • C with varying oxygen concentrations in the working gas. The films deposited at an c(O 2 ) of 0.3-0.4% present maximum transparency (the average transmittance T VIS calculated in the visible range of 400−800 nm for these films was more than 77%), while below and above the c(O 2 ) the average transmittance T VIS slightly drops (73.5 and 75.8%, respectively). The decrease in the T VIS with an increase in the c(O 2 ) to 0.5% is mainly due to a dip in the spectrum in the violet-blue region (380-470 nm). Regarding the NIR region, all three films deposited in the presence of O 2 are characterized by a high transparency in this range compared to the film deposited at an c(O 2 ) = 0%. Note that a clearer picture of the absorption edge red shift with an increasing c(O 2 ) is observed in the spectral vicinity of 350 nm for the IZO films deposited at 100 • C. That can be attributed to the well-known Burstein-Moss shift due to the observed increase in the concentration n by almost one order of magnitude with a decreasing c(O 2 ) from 0.5 to 0.0%.  The data showed distinct interference fringes indicating that the films were smooth with little scatter. Figure 4a shows that the transparency of the IZO films deposited in pure Ar was increased in the visible range (400−800 nm) with increased substrate temperatures. This may be due to a decrease in the photon absorbance and scattering on defects caused by the incorporation of surplus metals into the film. At the same time, the transmittance of the films in the NIR region (800−1250 nm), as a function of substrate temperatures, repeats the trend of the concentration n in the films with varying substrate temperatures. The minimum transparency in this region was shown by the film deposited at 100 °C, which had the maximum concentration n = 6.9 × 10 20 cm −3 among the films deposited at an c(O2) of 0.0%. Figure 4b illustrates the optical transmittance spectra of the IZO films deposited at the substrate temperature of 100 °C with varying oxygen concentrations in the working gas. The films deposited at an c(O2) of 0.3-0.4% present maximum transparency (the average transmittance VIS calculated in the visible range of 400−800 nm for these films was more than 77%), while below and above the c(O2) the average transmittance VIS slightly drops (73.5 and 75.8%, respectively). The decrease in the VIS with an increase in the c(O2) to 0.5% is mainly due to a dip in the spectrum in the violet-blue region (380-470 nm). Regarding the NIR region, all three films deposited in the presence of O2 are characterized by a high transparency in this range compared to the film deposited at an c(O2) = 0%. Note that a clearer picture of the absorption edge red shift with an increasing c(O2) is observed in the spectral vicinity of 350 nm for the IZO films deposited at 100 °C. That can be attributed to the well-known Burstein-Moss shift due to the observed increase in the concentration n by almost one order of magnitude with a decreasing c(O2) from 0.5 to 0.0%.
The average transmittances VIS and NIR calculated for the visible (400−800 nm) and NIR (800−1250 nm) regions, depending on the substrate temperature and c(O2), are also summarized in Figure 5a,b, respectively. It can be seen from Figure 5a   Taking into account the slightly larger film thickness obtained by an c(O 2 ) = 0.3%, we can conclude that the IZO thin film deposited at 100 • C and an O 2 content of 0.3% demonstrates maximum transparency in the visible area. In turn, the IZO film obtained at 100 • C and an c(O 2 ) = 0.4%, in which the maximum mobility was observed, is characterized by the maximum transmittance in the NIR region (T NIR = 76.4%) among all the films (Figure 5b).
Thus, the transmittance for the glass substrate with the IZO thin films showed that the adding of O 2 in the working gas clearly leads to a red shift in the optical response for the UV-violet region. However, for wavelengths greater than 400 nm, the transmittance values increased, reaching values greater than 76% of transparency in the wide visible-NIR range for the IZO films deposited at the optimal substrate temperature (100 • C) and the c(O 2 ) values up to 0.4%. A further increase in the c(O 2 ), especially in combination with an increase in the substrate temperature, leads to a substantial decrease in the average transmittance.
On the whole, based on the regularities observed in the study of the electrical and optical properties of the IZO films, it can be concluded that this behavior is characteristic of mostly amorphous In 2 O 3 -based thin films, in which the conductivity is governed mainly by oxygen vacancies [36][37][38]. In this regard, the resistance of the IZO films could be controlled and easily changed by nine orders of magnitude [39]. Furthermore, we also noted that, in our experiment, the IZO films that deposited at an c(O 2 ) ≥ 1% had high values of resistivity, beyond the measurement limits of our measurement setup. In turn, it has been demonstrated that the deposition of the IZO film under a controlled low oxygen content in a chamber allows the fine-tuning of both the electrical and optical properties of these thin films by enhancing the mobility of the IZO film due to the decrease in concentration of V O in the film [19,29]. Kamiya et al. suggested, on the basis of first-principles calculations, that oxygen vacancies (V O ) act as shallow donors and suppliers of conduction electrons in amorphous In 2 O 3 -containing thin films, and that defect sites, meanwhile, can also act as scattering centers, reducing charge-carrier conduction [40]. Thus, the transmittance for the glass substrate with the IZO thin films showed that the adding of O2 in the working gas clearly leads to a red shift in the optical response for the UV-violet region. However, for wavelengths greater than 400 nm, the transmittance values increased, reaching values greater than 76% of transparency in the wide visible-NIR range for the IZO films deposited at the optimal substrate temperature (100 °C) and the c(O2) values up to 0.4%. A further increase in the c(O2), especially in combination with an increase in the substrate temperature, leads to a substantial decrease in the average transmittance.
On the whole, based on the regularities observed in the study of the electrical and optical properties of the IZO films, it can be concluded that this behavior is characteristic of mostly amorphous In2O3-based thin films, in which the conductivity is governed mainly by oxygen vacancies [36][37][38]. In this regard, the resistance of the IZO films could be controlled and easily changed by nine orders of magnitude [39]. Furthermore, we also noted that, in our experiment, the IZO films that deposited at an c(O2) ≥ 1% had high values of resistivity, beyond the measurement limits of our measurement setup. In turn, it has been demonstrated that the deposition of the IZO film under a controlled low oxygen content in a chamber allows the fine-tuning of both the electrical and optical properties of these thin films by enhancing the mobility of the IZO film due to the decrease in concentration of VO in the film [19,29]. Kamiya et al. suggested, on the basis of first-principles calculations, that oxygen vacancies (VO) act as shallow donors and suppliers of conduction electrons in amorphous In2O3-containing thin films, and that defect sites, meanwhile, can also act as scattering centers, reducing charge-carrier conduction [40].
Based on our measurement data on the electrical characteristics of the IZO films, we plotted the dependence of μ with n ( Figure 6). The ramp and subsequent decrease of μ with n could be explained by the presence of different scattering mechanisms. According to Leenheer et al., the maximum μ in the IZO films is reached when carrier transport is mainly limited by phonon scattering, i.e., metal-like transport [41]. The change in trend observed on the curves in Figure 6 may indicate that ionized impurity scattering becomes Based on our measurement data on the electrical characteristics of the IZO films, we plotted the dependence of µ with n ( Figure 6). The ramp and subsequent decrease of µ with n could be explained by the presence of different scattering mechanisms. According to Leenheer et al., the maximum µ in the IZO films is reached when carrier transport is mainly limited by phonon scattering, i.e., metal-like transport [41]. The change in trend observed on the curves in Figure 6 may indicate that ionized impurity scattering becomes the main limiting factor of µ at a high concentration of carriers supplied by Vo. The values of the n, at which the scattering mechanism switches, are in agreement with [19,41]. Here, it is worth noting that a change in the location of the maximum of the curves depending on the substrate temperature may indicate the presence of some additional mechanisms of chargecarrier scattering. In particular, due to the nonstoichiometric nature of the sputtered target, the formation of metal pairs (e.g., In-In, In-Zn), and even subnanometer metal inclusions, is possible inside the film at RT [42,43]. These defects can be additional scattering centers for both conduction electrons and transmitted light. With increasing substrate temperatures, the formation of metal inclusions is reduced and the mobility of the films improves. This can explain the improvement in both the electrical and optical properties of the IZO films observed in our experiment with an increase in the substrate temperature from RT to 100 • C.
Furthermore, since the IZO films deposited at an optimal substrate temperature of 100 • C under the oxygen ambient have a higher transparence than the IZO films deposited at 300 • C under the same ambient (which is not in agreement with others [28]), we carried out additional comparative studies of the morphology and microstructure of the corresponding IZO films. Figure 7 shows the SEM micrographs of the IZO thin films deposited at a substrate temperature of 100 and 300 • C by using the various oxygen contents in the working gas. It is shown that a very flat surface was obtained at a low substrate temperature regardless of the c(O 2 ). Such morphology is typical in the case of the kinetic regime of the deposition of a multicomponent oxide film, the individual components of which have minimal solubilities in each other [13,36]. In this case, the absence of any significant external heating of the growing film usually results in the formation of a closely packed homogeneous amorphous/nanocrystalline structure. ues of the n, at which the scattering mechanism switches, are in agreement with [19,41]. Here, it is worth noting that a change in the location of the maximum of the curves depending on the substrate temperature may indicate the presence of some additional mechanisms of charge-carrier scattering. In particular, due to the nonstoichiometric nature of the sputtered target, the formation of metal pairs (e.g., In-In, In-Zn), and even subnanometer metal inclusions, is possible inside the film at RT [42,43]. These defects can be additional scattering centers for both conduction electrons and transmitted light. With increasing substrate temperatures, the formation of metal inclusions is reduced and the mobility of the films improves. This can explain the improvement in both the electrical and optical properties of the IZO films observed in our experiment with an increase in the substrate temperature from RT to 100 °C. Furthermore, since the IZO films deposited at an optimal substrate temperature of 100 °C under the oxygen ambient have a higher transparence than the IZO films deposited at 300 °C under the same ambient (which is not in agreement with others [28]), we carried out additional comparative studies of the morphology and microstructure of the corresponding IZO films. Figure 7 shows the SEM micrographs of the IZO thin films deposited at a substrate temperature of 100 and 300 °C by using the various oxygen contents in the working gas. It is shown that a very flat surface was obtained at a low substrate temperature regardless of the c(O2). Such morphology is typical in the case of the kinetic regime of the deposition of a multicomponent oxide film, the individual components of which have minimal solubilities in each other [13,36]. In this case, the absence of any significant external heating of the growing film usually results in the formation of a closely packed homogeneous amorphous/nanocrystalline structure. In contrast, the surface roughness of the IZO films deposited at 300 °C and an c(O2) range up to 0.5% is greatly increased in comparison with that of the IZO films deposited at the optimal 100 °C. The film deposited in pure argon had signs of columnarity. The formation of a columnar structure is typical, for example, for polycrystalline ITO films grown on a heated substrate [44]. When the c(O2) increased up to 0.5%, the surface became more rough and highly disordered grains appeared locally. Note that, when the c(O2) became greater than 0.5%, the surface of the film became very smooth, like that of the films deposited at a lower substrate temperature. The complex behavior of the morphology in the IZO films deposited at 300 °C is related, apparently, to the oppositional nature of the effect of oxygen on the growth of ZnO and In2O3, whose nanocrystals In contrast, the surface roughness of the IZO films deposited at 300 • C and an c(O 2 ) range up to 0.5% is greatly increased in comparison with that of the IZO films deposited at the optimal 100 • C. The film deposited in pure argon had signs of columnarity. The formation of a columnar structure is typical, for example, for polycrystalline ITO films grown on a heated substrate [44]. When the c(O 2 ) increased up to 0.5%, the surface became more rough and highly disordered grains appeared locally. Note that, when the c(O 2 ) became greater than 0.5%, the surface of the film became very smooth, like that of the films deposited at a lower substrate temperature. The complex behavior of the morphology in the IZO films deposited at 300 • C is related, apparently, to the oppositional nature of the effect of oxygen on the growth of ZnO and In 2 O 3 , whose nanocrystals (nanoclusters) can nucleate inside the film at a given temperature [35,45,46].
Thus, SEM studies have revealed the relationship between the deterioration in light transmission observed for films deposited at temperatures above 100 • C (Figure 4) and the development in their surface morphology. It is well-known that, during optical transmittance measurement, the incident light can be more scattered over a rough surface than over a uniform surface [47]. Figure 8 shows the x-ray diffraction spectra obtained from the IZO thin films deposited at 100 and 300 • C and various c(O 2 ). XRD studies confirmed the most common amorphous nature of the films of the indium zinc oxide system, except the IZO film deposited in pure Ar at 300 • C. All the samples prepared at a low substrate temperature show no clear or broad diffraction peaks, which indicate that these films were fully disordered due to the immiscibility of Zn in the In 2 O 3 matrix. A somewhat different picture was observed in the case of substrate temperatures of 300 • C. For the film deposited at 300 • C in pure Ar, a clear peak indicates the presence of polycrystalline IZO. Its position is very close to the position of the diffraction (222) line for In 2 O 3 (PDF card #00-006-0416). At the same time, with the addition of oxygen, ordering transformation occurs on the patterns of the IZO films deposited at 300 • C. For the film deposited at an c(O 2 ) = 0.4%, on the pattern there is only a weak and broad peak centered at 2θ = 30 • , indicating that the film is an amorphous structure with some short-range ordering. However, this broad peak does not exclude the possibility of the presence of ultrafine crystallites of oxide phases (In-doped ZnO, Zn-doped In 2 O 3 , and other compounds from the homologous series In 2 Zn k O k+3 ) into the film, the size of which does not exceed a few nanometers [48,49]. In the end, more complete amorphization is observed with an increasing c(O 2 ) up to 1%. is very close to the position of the diffraction (222) line for In2O3 (PDF card #00-006-0416). At the same time, with the addition of oxygen, ordering transformation occurs on the patterns of the IZO films deposited at 300 °C. For the film deposited at an c(O2) = 0.4%, on the pattern there is only a weak and broad peak centered at 2θ = 30°, indicating that the film is an amorphous structure with some short-range ordering. However, this broad peak does not exclude the possibility of the presence of ultrafine crystallites of oxide phases (In-doped ZnO, Zn-doped In2O3, and other compounds from the homologous series In2ZnkOk+3) into the film, the size of which does not exceed a few nanometers [48,49].
In the end, more complete amorphization is observed with an increasing c(O2) up to 1%.

Conclusions
The functional characteristics of the IZO TCO thin films prepared by the rfmagnetron sputtering of the SPS synthesized ceramic IZO target (90 wt.% In2O3 and 10 wt.% ZnO) were well investigated in this study. We have shown the influence on the carrier-transport properties and optical transmittance of the oxygen partial pressure and substrate temperature applied during the sputtering deposition of the IZO thin films.
It is shown that the electrical properties behaved as if dominant donors are mainly

Conclusions
The functional characteristics of the IZO TCO thin films prepared by the rfmagnetron sputtering of the SPS synthesized ceramic IZO target (90 wt.% In 2 O 3 and 10 wt.% ZnO) were well investigated in this study. We have shown the influence on the carrier-transport properties and optical transmittance of the oxygen partial pressure and substrate temperature applied during the sputtering deposition of the IZO thin films.
It is shown that the electrical properties behaved as if dominant donors are mainly generated by oxygen vacancies. Enhanced electrical property of the IZO films was achieved by decreasing the substrate temperature up to 100 • C, which showed a 9 Ω/sq of sheet resistance for the film with a thickness of about 380 nm obtained in pure Ar.
Despite the fact that, with an increase in the oxygen content, an increase in resistivity was observed, it was found that the addition of oxygen during sputtering differently affects the carrier concentration and its mobility. While the concentration decreased monotonically over the entire range of the oxygen content, the mobility had a maximum (µ = 39.2 cm 2 /V·s) at an oxygen content of 0.4%. It has been demonstrated that, in films deposited at the optimal temperature, the addition of oxygen up to 0.4% also leads to an increase in transparency in both the visible and near-infrared regions of the spectrum (T VIS ≥ 77% and T NIR ≥ 76%). Additional studies of the morphology and microstructure of the deposited films made it possible to identify the reasons for the observed deterioration of the optical properties of the IZO films deposited at non-optimal conditions.
In conclusion, the strategy to grow IZO film, based on the optimization of oxygen vacancies into IZO by O 2 introduced during sputtering, is a useful guideline for the tuning of the functional properties of low-temperature IZO TCO films and their adaptation for some practical applications, including a wide area of temperature-sensitive and flexible devices.