In-Sn-Zn Oxide Nanocomposite Films with Enhanced Electrical Properties Deposited by High-Power Impulse Magnetron Sputtering

In-Sn-Zn oxide (ITZO) nanocomposite films have been investigated extensively as a potential material in thin-film transistors due to their good electrical properties. In this work, ITZO thin films were deposited on glass substrates by high-power impulse magnetron sputtering (HiPIMS) at room temperature. The influence of the duty cycle (pulse off-time) on the microstructures and electrical performance of the films was investigated. The results showed that ITZO thin films prepared by HiPIMS were dense and smooth compared to thin films prepared by direct-current magnetron sputtering (DCMS). With the pulse off-time increasing from 0 μs (DCMS) to 2000 μs, the films’ crystallinity enhanced. When the pulse off-time was longer than 1000 μs, In2O3 structure could be detected in the films. The films’ electrical resistivity reduced as the pulse off-time extended. Most notably, the optimal resistivity of as low as 4.07 × 10−3 Ω·cm could be achieved when the pulse off-time was 2000 μs. Its corresponding carrier mobility and carrier concentration were 12.88 cm2V−1s−1 and 1.25 × 1020 cm−3, respectively.


Introduction
Si-based thin-film transistors (TFTs) are widely used in liquid crystal displays, sensors, logic integrated circuits, etc. [1][2][3]. However, the high temperatures required for the formation of Si materials severely limits their applications in novel optoelectronic devices. For instance, flexible electrical devices and wearable devices are being developed rapidly nowadays but the flexible substrates used in these devices possess poor heat resistance, which makes Si-based TFTs unusable in these fields. Conversely, amorphous oxide semiconductors can be fabricated at room temperature [4][5][6]. Such materials combine good light transmittance and conductivity. Therefore, TFTs based on such materials are gradually replacing Si-based TFTs in certain areas. To date, the amorphous oxide semiconductor that has been most widely studied and has achieved commercial applications is In-Ga-Zn-O (IGZO) [7,8]. Its good uniformity and high carrier mobility (~10 cm 2 /Vs) have led to it attracting much attention in recent years [9]. Unfortunately, during the traditional back-channel-etching process used to manufacture amorphous IGZO TFTs, IGZO reacts easily with weak acids [10,11]. Furthermore, the field-effect mobility of IGZO TFT is still inadequate to drive high-frame-rate displays [12]. Therefore, it is necessary to explore other amorphous oxide semiconductors.
In-Sn-Zn-O (ITZO) is a novel transparent conductive material that replaces Ga 2 O 3 in IGZO with more chemically stable SnO 2 , which helps to endow ITZO with better etching-resistance ability [13]. Meanwhile, the direct spatial overlap of the orbitals between Sn 5s orbital and In 5s can enhance the mobility of the electrons within the conduction band minimum, leading to a higher carrier mobility [14]. In addition, compared with the substitution of Zn 2+ by trivalent Ga 3+ , the substitution of Zn 2+ by tetravalent Sn 4+ will release more free electrons and improve the electrical properties of the films [15]. As a result, ITZO-based TFTs have high potential for the development of next generation displays due to their good etching-resistance during the back-channel-etching process.
Currently, magnetron sputtering and the sol-gel method are most commonly used to prepare ZnO-based thin films [16][17][18]. In particular, magnetron sputtering has attracted much attention due to its low deposition temperature, fast sputtering speed, uniform film formation, and good repeatability [19][20][21]. However, the films deposited by traditional magnetron sputtering method present loose structure with many defects, greatly affecting the films' performance [22]. The relatively recently developed high-power impulse magnetron sputtering (HiPIMS) technology has an important advantage in its high target ionization rate, which can improve the activity of the various species during the sputtering process [23][24][25][26]. In addition, due to the high instantaneous power density applied on the target, the energy of the incident species to the substrate is effectively increased, resulting in the formation of a denser and more uniform film, thereby reducing the carrier scattering and enhancing the carrier mobility [27,28]. To the best of our knowledge, no other groups have prepared ITZO films using HiPIMS technology. In the current work, the optoelectronic properties of ITZO films prepared by HiPIMS technology under different duty cycles were investigated.

Experimental Details
ITZO thin films with a thickness of 100 nm were deposited through HiPIMS technology on glass and silicon substrates from an ITZO target (99.9% purity, In 2 O 3 :SnO 2 :ZnO = 30:35:35 at.%, Φ = 76.2 mm) at room temperature. The working pressure was 0.7 Pa with the Ar flow rate maintained at 20 sccm. The sputtering power of the HiPIMS power supply was 300 W, while the pulse on-time (t on ) remained at 50 µs. The pulse off-time (t off ) varied from 0 to 2000 µs during the deposition process. The duty cycle is defined as the ratio between the t on and the sum of t on plus t off , and therefore reduces with an increase in t off . The deposition parameters maintained during the deposition are summarized in Table 1. The sputtering voltage and current variation of HiPIMS power output were monitored by oscilloscope (Rigol DS5202CA, Rigol Technologies. Inc., Beijing, China). The films' thickness was detected by step profiler (Kosaka Surfcoder, Kosaka Laboratory Ltd., Tokyo, Japan). The films' composition was characterized by electron probe X-ray microanalyzer (EPMA, JEOL JXA-8200, JEOL, Tokyo, Japan). The structural properties of ITZO films were analyzed through X-ray diffractometer (XRD, Rigaku Ultima IV, Tokyo, Japan). The surface roughness of the films was measured by atomic force microscope (AFM, DI-Dimension 3100, Digital instruments, Bresso, Italy). The microstructure of the specimens prepared by focused ion beam (FIB) milling was observed on cross-sections by high resolution transmission electron microscopy (HR-TEM, JEOL JEM-2100, JEOL, Tokyo, Japan). The films' electrical properties were obtained by the Hall effect measurement system (AHM-800B, Agilent Technologies, Santa Clare, CA, USA). Figure 1 shows the variation of the sputtering voltage and current on the target with the pulse off-time prolongation during the deposition process. Both of them increased with the extension of the pulse off-time. Consequently, the peak power density on the target also rises gradually. The variation of the duty cycle and the calculated peak power density as a function of t off are given in Table 2. As the pulse off-time extended from 0 µs to 2000 µs, the target peak power density rises greatly from 6.42 to 531.97 W·cm −2 . However, the deposition rate monotonically decreased with the extension of pulse off-time ( Figure 2). With increasing pulse off-time, the reduction in the effective sputtering period resulted in fewer target atoms being sputtered, which in turn reduced the deposition rate.

Results
The sputtering voltage and current variation of HiPIMS power output were monitored by oscilloscope (Rigol DS5202CA, Rigol Technologies. Inc., Beijing, China). The films' thickness was detected by step profiler (Kosaka Surfcoder, Kosaka Laboratory Ltd., Tokyo, Japan). The films' composition was characterized by electron probe X-ray microanalyzer (EPMA, JEOL JXA-8200, JEOL, Tokyo, Japan). The structural properties of ITZO films were analyzed through X-ray diffractometer (XRD, Rigaku Ultima IV, Tokyo, Japan). The surface roughness of the films was measured by atomic force microscope (AFM, DI-Dimension 3100, Digital instruments, Bresso, Italy). The microstructure of the specimens prepared by focused ion beam (FIB) milling was observed on cross-sections by high resolution transmission electron microscopy (HR-TEM, JEOL JEM-2100, JEOL, Tokyo, Japan). The films' electrical properties were obtained by the Hall effect measurement system (AHM-800B, Agilent Technologies, Santa Clare, CA, USA). Figure 1 shows the variation of the sputtering voltage and current on the target with the pulse off-time prolongation during the deposition process. Both of them increased with the extension of the pulse off-time. Consequently, the peak power density on the target also rises gradually. The variation of the duty cycle and the calculated peak power density as a function of toff are given in Table 2. As the pulse off-time extended from 0 μs to 2000 μs, the target peak power density rises greatly from 6.42 to 531.97 W·cm −2 . However, the deposition rate monotonically decreased with the extension of pulse offtime ( Figure 2). With increasing pulse off-time, the reduction in the effective sputtering period resulted in fewer target atoms being sputtered, which in turn reduced the deposition rate.    Table 3 shows the relationship between the pulse off-time and the film's composition. The content of In, Sn, Zn, and O in the film changed slightly as the pulse off-time was extended. When the pulse off-time was 0 µs, the sputtering mode was equivalent to conventional DCMS, where the target ionization rate is limited. Upon extension of the pulse off-time, the peak power density applied on the target surface increased markedly, and the instantaneous energy released on the target rises considerably, resulting in a significant increase in the ionization rate of the target species. The ionized target species possess higher activity and react more easily with the reactive O atoms. Therefore, O content in the films deposited using HiPIMS mode was higher than that in the films deposited by DCMS mode. Nevertheless, the films' composition remained almost unchanged and they were always oxygen-deficient, resulting in the formation of donor defects such as oxygen vacancies, thereby improving their conductivity.   Table 3 shows the relationship between the pulse off-time and the film's composition. The content of In, Sn, Zn, and O in the film changed slightly as the pulse off-time was extended. When the pulse off-time was 0 μs, the sputtering mode was equivalent to conventional DCMS, where the target ionization rate is limited. Upon extension of the pulse off-time, the peak power density applied on the target surface increased markedly, and the instantaneous energy released on the target rises considerably, resulting in a significant increase in the ionization rate of the target species. The ionized target species possess higher activity and react more easily with the reactive O atoms. Therefore, O content in the films deposited using HiPIMS mode was higher than that in the films deposited by DCMS mode. Nevertheless, the films' composition remained almost unchanged and they were always oxygen-deficient, resulting in the formation of donor defects such as oxygen vacancies, thereby improving their conductivity.    Figure 3 shows the XRD spectra of the ITZO thin films deposited with various pulse off-times. Amorphous-like structures were obtained when the pulse off-times were 0 µs and 500 µs. No obvious diffraction peak could be detected in either of these films. As the pulse off-time extended to 1000 µs and 1500 µs, ITZO films began to crystallize and an In 2 O 3 (222) diffraction peak emerged. Upon further extending the pulse off-time to 2000 µs, the films' crystallinity increased significantly. Additional diffraction peaks of In 2 O 3 (222), In 2 O 3 (400), In 2 O 3 (440), and In 2 O 3 (622) were also identified. This behavior was related to higher instantaneous energy being bombarded on the target with the extension of pulse off-time; therefore, the sputtering species consequently possessed more kinetic energy. This promoted the nucleation and orderly growth of the ITZO films, thus improving the films' crystallinity. The films' crystalline features can be analyzed by TEM, as shown in Figure 4. The close-up lattice images in this figure are taken from the areas marked by red squares and produced through inverse Fourier transformation. In Figure 4a, many amorphous areas can be found in the ITZO film deposited with the pulse off-time of 0 µs. In contrast, In 2 O 3 (222) planes with interplanar lattice spacing of about 2.9 Å can be clearly observed in Figure 4b, indicating that the films' crystallinity increased with the extension of pulse off-time.

Results
improving the films' crystallinity. The films' crystalline features can be analyzed by TEM, as shown in Figure 4. The close-up lattice images in this figure are taken from the areas marked by red squares and produced through inverse Fourier transformation. In Figure  4a, many amorphous areas can be found in the ITZO film deposited with the pulse offtime of 0 μs. In contrast, In2O3 (222) planes with interplanar lattice spacing of about 2.9 Å can be clearly observed in Figure 4b, indicating that the films' crystallinity increased with the extension of pulse off-time.   4a, many amorphous areas can be found in the ITZO film deposited with the pulse offtime of 0 μs. In contrast, In2O3 (222) planes with interplanar lattice spacing of about 2.9 Å can be clearly observed in Figure 4b, indicating that the films' crystallinity increased with the extension of pulse off-time.   The films' surface morphology was characterized through AFM analysis ( Figure 5). The roughness of the films decreased from 2.17 nm to 0.85 nm and further to 0.70 nm, as the pulse off-time extended from 0 µs to 1000 µs and on to 2000 µs. Due to the high-energy sputtering species bombardment of the substrate during the deposition process, the films became much denser and smoother under extended pulse off-time.
The films' electrical properties as analyzed by Hall measurement are shown in Figure 6. The variation of the carrier concentration and the carrier mobility are summarized in Figure 6a. As the pulse off-time was raised from 0 to 500 µs, the deposition mode changed from DC mode to HiPIMS mode. Due to the higher kinetic energy of the sputtering species bombarding the substrate during the HiPIMS deposition mode, a denser film with a low amount of defects was obtained. As a result, the carrier concentration decreased from 3.92 × 10 19 cm −3 to 9.11 × 10 18 cm −3 ; while the carrier scattering reduced, and the carrier mobility increased greatly from 3.99 cm 2 V −1 s −1 to 31.25 cm 2 V −1 s −1 . Upon further extending the pulse off-time, the films changed from an isotropic amorphous structure to an anisotropic polycrystalline structure, with more grain boundaries introduced. This increased the probability of grain boundary scattering and hindered the carrier migration, resulting in a decrease in the carrier mobility. Similar behavior has also been found in nitrogen-doped ITZO films [29]. In addition, the target ionization rate enhanced with the extension of the pulse off-time, thereby raising the activity of the doping species during the sputtering process. Thus, the substitution of In 3+ ions by Sn 4+ occurred more readily. This substitution leads to lattice distortion and aids in the formation of V O (oxygen vacancies) and Sn In (the substitution of In 3+ by Sn 4+ ) donor defects, which both improve the carrier concentration.
Nanomaterials 2021, 11, x FOR PEER REVIEW 6 of 8 The films' surface morphology was characterized through AFM analysis ( Figure 5). The roughness of the films decreased from 2.17 nm to 0.85 nm and further to 0.70 nm, as the pulse off-time extended from 0 μs to 1000 μs and on to 2000 μs. Due to the high-energy sputtering species bombardment of the substrate during the deposition process, the films became much denser and smoother under extended pulse off-time. The films' electrical properties as analyzed by Hall measurement are shown in Figure  6. The variation of the carrier concentration and the carrier mobility are summarized in Figure 6a. As the pulse off-time was raised from 0 to 500 μs, the deposition mode changed from DC mode to HiPIMS mode. Due to the higher kinetic energy of the sputtering species bombarding the substrate during the HiPIMS deposition mode, a denser film with a low amount of defects was obtained. As a result, the carrier concentration decreased from 3.92 × 10 19 cm −3 to 9.11 × 10 18 cm −3 ; while the carrier scattering reduced, and the carrier mobility increased greatly from 3.99 cm 2 V −1 s −1 to 31.25 cm 2 V −1 s −1 . Upon further extending the pulse off-time, the films changed from an isotropic amorphous structure to an anisotropic polycrystalline structure, with more grain boundaries introduced. This increased the probability of grain boundary scattering and hindered the carrier migration, resulting in a decrease in the carrier mobility. Similar behavior has also been found in nitrogen-doped ITZO films [29]. In addition, the target ionization rate enhanced with the extension of the pulse off-time, thereby raising the activity of the doping species during the sputtering process. Thus, the substitution of In 3+ ions by Sn 4+ occurred more readily. This substitution leads to lattice distortion and aids in the formation of VO (oxygen vacancies) and SnIn (the substitution of In 3+ by Sn 4+ ) donor defects, which both improve the carrier concentration.
The film's resistivity is related to the carrier mobility and carrier concentration. Their relationship is calculated using the following equation [30]: where ρ is the film's resistivity, e is the electron charge, µ is the carrier mobility, and N is the carrier concentration. Through the combined effects of carrier mobility and carrier concentration, the variation of the films' resistivity as a function of pulse off-time is shown in Figure 6b. It decreased from 3.98 × 10 −2 Ω·cm to 4.07 × 10 −3 Ω·cm as the pulse off time rises from 0 μs to 2000 μs.

Conclusions
In this work, ITZO thin films were deposited on glass substrates at room temperature through HiPIMS technology with various pulse off-times. The microstructures and electrical properties of the films were investigated. The results show that compared with the ITZO film deposited under DCMS mode, ITZO films deposited using HiPIMS mode are denser and possess smoother surface morphology. As the pulse off-time was extended, the crystallinity of ITZO films enhanced, and the film's resistivity effectively reduced. The optimal resistivity of about 4.07 × 10 −3 Ω·cm was achieved when the pulse off-time was 2000 μs. This result indicates that through utilizing HiPIMS technology, ITZO films with controllable carrier concentration and carrier mobility in addition to controllable resistivity can be produced, which is desirable in the production of films for applications in various optoelectronic devices.  The film's resistivity is related to the carrier mobility and carrier concentration. Their relationship is calculated using the following equation [30]: where ρ is the film's resistivity, e is the electron charge, µ is the carrier mobility, and N is the carrier concentration. Through the combined effects of carrier mobility and carrier concentration, the variation of the films' resistivity as a function of pulse off-time is shown in Figure 6b. It decreased from 3.98 × 10 −2 Ω·cm to 4.07 × 10 −3 Ω·cm as the pulse off time rises from 0 µs to 2000 µs.

Conclusions
In this work, ITZO thin films were deposited on glass substrates at room temperature through HiPIMS technology with various pulse off-times. The microstructures and electrical properties of the films were investigated. The results show that compared with the ITZO film deposited under DCMS mode, ITZO films deposited using HiPIMS mode are denser and possess smoother surface morphology. As the pulse off-time was extended, the crystallinity of ITZO films enhanced, and the film's resistivity effectively reduced. The optimal resistivity of about 4.07 × 10 −3 Ω·cm was achieved when the pulse off-time was 2000 µs. This result indicates that through utilizing HiPIMS technology, ITZO films with controllable carrier concentration and carrier mobility in addition to controllable resistivity can be produced, which is desirable in the production of films for applications in various optoelectronic devices.