A simple route to prepare (100) preferred orientation indium tin oxide film onto polyimide substrate by direct current pulsed magnetron sputtering

Accepted Manuscript A simple route to prepare (100) preferred orientation indium tin oxide film onto polyimide substrate by direct current pulsed magnetron sputtering

Zhixuan Lv, Jindong Liu, Dengyao Wang, Hualong Tao, Weichao Chen, Haoting Sun, Yanfei He, Xin Zhang, Zhiyu Qu, Zicheng Han, Xuelin Guo, Shiping Zhao, Yunxian Cui, Hualin Wang, Shimin Liu, Chaoqian Liu, Nan Wang, Weiwei Jiang, Weiping Chai, Wanyu Ding PII:

S0254-0584(18)30036-1

DOI:

10.1016/j.matchemphys.2018.01.036

Reference:

MAC 20308

To appear in:

Materials Chemistry and Physics

Received Date:

24 July 2017

Revised Date:

14 November 2017

Accepted Date:

12 January 2018

Please cite this article as: Zhixuan Lv, Jindong Liu, Dengyao Wang, Hualong Tao, Weichao Chen, Haoting Sun, Yanfei He, Xin Zhang, Zhiyu Qu, Zicheng Han, Xuelin Guo, Shiping Zhao, Yunxian Cui, Hualin Wang, Shimin Liu, Chaoqian Liu, Nan Wang, Weiwei Jiang, Weiping Chai, Wanyu Ding, A simple route to prepare (100) preferred orientation indium tin oxide film onto polyimide substrate by direct current pulsed magnetron sputtering, Materials Chemistry and Physics (2018), doi: 10.1016/j.matchemphys.2018.01.036

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT A simple route to prepare (100) preferred orientation indium tin oxide film onto polyimide substrate by direct current pulsed magnetron sputtering Zhixuan Lva, Jindong Liub, c, d, Dengyao Wanga, Hualong Taob, Weichao Chena, Haoting Suna, Yanfei Heb, c, d, Xin Zhangb, c, d, Zhiyu Qub, Zicheng Hanb, Xuelin Guob, Shiping Zhaob, Yunxian Cuia, Hualin Wangb, c, d, Shimin Liub, c, d, Chaoqian Liub, c, d, Nan Wangb, c, d, Weiwei Jiangb, c, d, Weiping Chaib, c, d, Wanyu Dinga, b, c, d* a

College of Mechanical Engineering, Dalian Jiaotong University, Dalian 116028, China.

b

College of Materials Science and Engineering, Dalian Jiaotong University, Dalian 116028, China

c

Engineering Research Center of Optoelectronic Materials and Devices, Education Department of Education of Liaoning Province, Dalian 116028, China

d

Engineering Research Center of Optoelectronic Materials and Devices, Bureau of Science and Technology of Dalian City, Dalian 116028, China

*

Corresponding author: Prof. Wanyu Ding

E-mail: [email protected], [email protected], Tel: +86-411-84106876, Fax: +86-411-84105118. Address: College of Materials Science and Engineering, Dalian Jiaotong University, No.

794

Huanghe

Road,

Shahekou

Province/116028, China.

1

District,

Dalian,

Liaoning

ACCEPTED MANUSCRIPT Abstract: The indium tin oxide (ITO) film was deposited onto the polyimide (PI) quartz, and Si (100) substrates by the traditional direct current (DC) pulsed magnetron sputtering technology, which no any heating treatment was carried out onto the substrates. With the increase of sputtering power density from 0.83-8.33 W/cm2, ITO films with different crystal structure, optical, and electrical properties were obtained. X-ray diffraction results showed that with the increase of sputtering power density, the crystal structure of ITO film changed from the polycrystal without preferred orientation to (100) preferred orientation. ITO film with (100) preferred orientation showed a satisfactory optical and electrical properties, which the band gap, resistivity, and carrier concentration was about 4.05 eV, 4.91×10-4 Ω·cm, and 5.10×1020 cm-3, respectively. Combining the increase rate of ITO film growth rate, sputtering voltage, and sputtering current density, as well as DC pulsed voltage waveform, the formation of ITO film with (100) preferred orientation was analyzed logically. Finally, the relationship between sputtering power density and preferred orientation, as well as optical and electrical properties of ITO film, was investigated systematically. With above results, the short time overload sputtering voltage of DC pulsed sputtering process could be an ideal choice to prepare ITO film with (100) preferred orientation, especially for substrate without high temperature resistance.

Keywords: Indium tin oxide; Direct current pulsed waveform; Sputtering power density; Preferred orientation; Optical and electrical property

2

ACCEPTED MANUSCRIPT 1. Introduction As the important transparent conductive material, the indium tin oxide (ITO) film has been widely used in fields of photo-electric and electro-optical conversions [1-3]. So, the optical and electrical properties were most important for ITO film, which was strongly influenced by the crystal structure and chemical composition [46]. In industrial application field, ITO film was mainly prepared by the sputtering technology, which was used ITO ceramics target with certain composition (generally SnO2:In2O3=1:9 in weight ratio). So it was easy to control the chemical composition of ITO film. While, ITO film prepared by sputtering could display the different crystal structures, such as amorphous and polycrystalline cubic ferrite with/without preferred orientation. In general, ITO film with preferred orientation consisted of the columnar grains, which the coherent grain boundary was the primary in film. In turn, the coherent grain boundary could effectively decrease the electron scattering between grains, which resulted in that ITO film with preferred orientation displayed the satisfactory optical and electrical properties [7-9]. Many researchers focused on the controlling of ITO film preferred orientation [10-12]. The most popular way was to heat the substrate during ITO film growth, at about 300 oC [10-13]. So, it was hard to prepare ITO film with preferred orientation onto substrate without high temperature resistance, such as organic polymer substrate. This factor restricted the application of ITO film in fields of flexible photo-electric and electro-optical conversions. Thus, in order to extend the application field, ITO film with preferred orientation should be prepared onto the substrate without any heating treatment. Actually, Wang et al. has deposited ITO film by the traditional direct current (DC) pulsed magnetron sputtering technology, which no any heating treatment was carried out onto the substrate [14, 15]. In our experiment, the traditional DC pulsed magnetron sputtering technology

3

ACCEPTED MANUSCRIPT was used to deposit ITO film onto polyimide (PI) substrate. The aim is to systematically investigate the relationship between preferred orientation, sputtering power density, and DC pulsed waveform, as well as optical and electrical properties of ITO film.

2. Experimental 2.1. Preparation In our experiments, Kapton type PI substrate was used as the substrate, which was 50 μm in thickness, 135 N/25 mm in stretching resistance, and elongation≥35%. PI substrate was fixed onto the general glass. For the optical and electrical properties measurement, as well as crystal structure measurement, the quartz and polished Si (100) substrates were also used. The above three kinds of substrates were firstly ultrasonically pre-cleaned in the acetone, ethanol, and deionized water respectively, and then dried with N2 gas. Finally the above three kinds of substrates were loaded into the vacuum chamber, which was faced to ITO target. ITO target consisted of 10 wt.% SnO2 and 90 wt.% In2O3 in composition, as well as 200×90 mm2 in area. After the pressure in chamber was less than 9.9×10-4 Pa, the high purity Ar (99.99%) was introduced into the chamber as sputtering gas. The sputtering power was supplied by Pinnacle™ Plus+ 5 kW DC pulsed power supply unit. By controlling the sputtering power density on ITO target, ITO film was deposited onto PI, quartz, and Si (100) substrates, simultaneously. Besides, during ITO target sputtering process, the sputtering voltage, sputtering current density, and pulsed waveform were recorded. The detailed deposition parameters are listed in the Table 1.

4

ACCEPTED MANUSCRIPT Table 1. The detailed parameters for ITO film deposition. Parameter Value Working gas Ar Flow rate 20 sccm Distance between target and substrate 12 cm Pulse frequency 100 kHz Reverse time 1 μs Sputtering power density 0.83-8.33 W/cm2 Working pressure 0.6 Pa 2.2. Characterization The analytical techniques used in this study included X-ray diffraction (XRD), Hall Effect measurement, ultraviolet-visible (UV) spectrophotometer, surface profiler, transmission electron microscopy (TEM), and oscilloscope. The crystal structure and preferred orientation of ITO film on PI, quartz, and Si (100) substrates was analyzed by a PANalytical Empyrean XRD system with Cu kα1 radiation (λ=1.54056 Å), where the step size and counting time was 0.02 º and 0.5 s, respectively. The electrical property of ITO film on quartz substrate was measured using a Hall 8800 system at room temperature with 0.68 T in magnetic flux density. The optical property of ITO film on quartz substrate was determined by U-3310 UV spectrophotometer, with 200900 nm in scale and 2 nm in step. Si (100) substrate was partly covered to prepare a step during ITO film deposition process. By this way, the film thickness was obtained by Dektak 6M surface profiler with 1 nm in resolution. In turn, the growth rate of ITO film was calculated. In order to detect the details of crystal structure and preferred orientation of ITO film, the cross section TEM measurement was carried out onto ITO film on Si (100) substrate by JEM-2100F field emission TEM system. During ITO target sputtering process, the home-made shunt circuit with non inductive resistances and Tektronix TDS 2022B oscilloscope were used to record the output waveform of Pinnacle™ Plus+ 5 kW DC pulsed power supply unit.

5

ACCEPTED MANUSCRIPT 3. Results and discussion 3.1 Crystal structure analysis The crystallographic texture was one of the most important properties of ITO film, which strongly influenced the optical and electrical properties. Fig. 1 showed the normalized XRD patterns of ITO films on PI substrate. From Fig. 1, it can be seen clearly that XRD pattern (a) showed the diffraction peaks attributed to the cubic ferrite In2O3 (222) and (440) phase, respectively [16-18]. With the sputtering power density over than 2.50 W/cm2, the diffraction peak for cubic ferrite In2O3 (400) phase appeared [16-18]. It can be found that all diffraction peaks shifted toward small angle. Such shift could be mainly caused by the following two reasons. On one hand, parts In3+ with ionic radius 0.73 Å was substitutionally doped by Sn2+ with ionic radius 0.93 Å partially, which larger ionic radius of Sn2+ could enlarge the lattice In2O3 and shift the diffraction peaks toward small angle. On the other hand, ITO films without substrate heating contained the internal stress, which displayed the tension stress along normal direction of film growth surface. Such tension stress along normal direction of film growth surface could shift the diffraction peaks toward small angle too. Combining above reasons, XRD patterns in Fig. 1 shifted toward small angle. While, the most important feature of Fig. 1 was that with the sputtering power density over than 4.16 W/cm2, the relative intensity of (400) peak had been stronger than those of (222) and (440) peaks. Especially for ITO film with sputtering power density in 8.33 W/cm, only (400) diffraction peak could be found, which meant ITO film with (100) preferred orientation. It was known that one Kapton type PI constitutional unit contained at least 4 C=O bonds [19]. Based on our previous work, these C=O bonds could be broken to form C-O-In(Sn) crosslink bonds at the initial stage of ITO film growth [20-22]. So ITO film was adhered well onto PI by these C-O-In(Sn) crosslink

6

ACCEPTED MANUSCRIPT bonds, satisfactorily. By controlling the preferred orientation, ITO film on PI substrate could be used in more fields. Actually, XRD measurement was carried out onto ITO film on all above three kinds of substrates. It can be found that XRD results of ITO on above three kinds of substrates displayed the same patterns and changing trend. So, one major premise could be deduced that ITO films on above three kinds of substrates were with same properties.

Fig. 1. XRD patterns of ITO film with different sputtering power density, which was deposited on PI substrate. In order to detect the detailed crystalline structure of ITO film deposited with different sputtering power density, the cross-section TEM measurement should be carried out for ITO films on PI substrate. But, it was very difficult to prepare the cross-section TEM measurement sample of ITO film on PI substrate. Based on the major premise that ITO films on above three kinds of substrates were with same properties, the cross-section TEM measurement was carried out for ITO films onto Si

7

ACCEPTED MANUSCRIPT (100) substrate, just as shown in Fig. 2. From Fig. 2 (a), it can be seen clearly that except a little columnar grains, ITO film consisted of small equiaxed grains, which was deposited with 0.83 W/cm2. On the contrary, ITO film consisted of large columnar grains, which was deposited with 8.33 W/cm2, just shown in Fig. 2 (b). Besides, for more details, the high resolution TEM measurement was carried out on cross-section ITO film, just as shown in Fig. 3. From Fig. 3 (a), it can be seen that for ITO film deposited with 0.83 W/cm2, (100) direction of each grain was random, which corresponded well with XRD pattern without preferred orientation. On the contrary, for ITO film deposited with 8.33 W/cm2, (100) direction of all grains were same, which corresponded well with XRD pattern with (100) preferred orientation, just as shown in Fig. 3 (b). Combining XRD and TEM results, one conclusion could be drown that with the increase of sputtering power density, the crystal structure of ITO film changed from no preferred orientation to (100) preferred orientation. Besides, it should be considered whether such change in preferred orientation influence the electrical and optical properties of ITO film.

8

ACCEPTED MANUSCRIPT Fig. 2. The cross-section TEM image of ITO films, which the red arrow was the film growth direction. (a). 0.83 W/cm2 in sputtering power density, (b). 8.33 W/cm2 in sputtering power density.

Fig. 3. The cross-section high resolution TEM image of ITO films, which the green arrow was the film growth direction, the blue lines were the grain boundary, and red arrows were (100) directions. (a). 0.83 W/cm2 in sputtering power density, (b). 8.33 W/cm2 in sputtering power density.

3.2 Electrical and optical properties analysis In order to detect the relationship between preferred orientation and electrical properties of ITO film, Hall Effect measurement should be carried out on ITO films on PI substrate. But it was very difficult to keep Ohmic contact for PI substrate with 50 μm in thickness. Based on the major premise that ITO films on above three kinds of substrates were with same properties, Hall Effect measurement was carried out for ITO films onto quartz substrate. The results showed that the carrier mobility was about 23.08±1.79 cm2/V·S for all ITO films. The resistivity and carrier concentration

9

ACCEPTED MANUSCRIPT for ITO films onto quartz substrate were shown in Fig. 4. From Fig. 4, it can be seen that for ITO film deposited with 0.83 W/cm2, the resistivity and carrier concentration was about 2.28×10-3 Ω·cm and 1.17×1020 cm-3, respectively. With the increase of sputtering power density, the resistivity and carrier concentration gradually decreased and increased, respectively. After the sputtering power density over than 4.16 W/cm2, the resistivity and carrier concentration kept at about 4.91×10-4 Ω·cm and 5.10×1020 cm-3, respectively. Combining Hall, XRD, and TEM results, another conclusion could be drown that ITO film with (100) preferred orientation showed the satisfactory electrical properties.

Fig. 4. The resistivity and carrier concentration of ITO films with different sputtering power density. Except the electrical properties, the optical properties of ITO film on PI substrate should also be investigated. But 50 μm thick PI substrate was almost opaque in visible range of light. Based on the major premise that ITO films on above three kinds of substrates were with same properties, the transmission spectra measurement was carried out for ITO films onto quartz substrate, just as shown in Fig. 5.

From Fig. 5,

it can be seen clearly that with the increase of sputtering power density, a blue-shift of optical absorption edge was observed. ITO was a typical direct band gap

10

ACCEPTED MANUSCRIPT semiconductor [23, 24]. Based on Tauc relationship for direct band gap semiconductor, the band gap could be deduced from the formula (1) [25, 26],

(αhν )2  A( hν  E g )

(1)

where α is the intrinsic absorption coefficient, h is Planck constant (6.62×10-34 J·s), ν is the frequency of photons, A is the direct transition absorption coefficient, and Eg is the band gap of film. Based on the formula (1), the band gap of ITO film could be deduced for the curve of transmission spectra, just as shown in the insert of Fig. 5. The calculated results showed that with the increase of sputtering power density from 0.83 to 8.33 W/cm2, the band gap of ITO film increased gradually from 3.71 to 4.05 eV.

Fig. 5. The transmission spectra of ITO films with different sputtering power density. 3.3 Sputtering process analysis From above analysis, it can be seen that XRD, TEM, Hall, and spectrophotometer measurements was carried out onto ITO film on PI, Si (100), and quartz substrates respectively. Based on the major premise that ITO films on above three kinds of substrates were with same properties, it can be found that the sputtering power density had a strong influence on the crystallographic texture, electrical, and optical properties of ITO film. In order to investigate the relationship between

11

ACCEPTED MANUSCRIPT sputtering power density and above properties of ITO film, as well as the physicochemical mechanism behind deposition process, the parameters about deposition process was recorded, which were automatically shown on the panel of power supply unit. The sputtering power density began at 0.83 W/cm2, which the sputtering voltage, sputtering current density, and ITO film growth rate was -242 V, 3.4 mA/cm2, and 0.27 nm/s, respectively. With the increase of sputtering power density to 8.33 W/cm2, the above parameters monotonously increase to -396 V, 22.5 mA/cm2, and 2.97 nm/s, respectively. Set 0.83 W/cm2, -242 V, 3.4 mA/cm2, and 0.27 nm/s as the starting point of above parameters. Then the increase rates of above parameters were shown in Fig. 6. From Fig. 6, it can be seen clearly that with the increase of sputtering power density to 900%, the sputtering voltage, sputtering current density, and ITO film growth rate increased 63%, 562%, and 1007%, respectively.

Fig. 6. The increase rate of sputtering voltage, sputtering current, and ITO film growth rate.

12

ACCEPTED MANUSCRIPT While, what does the parameters mean exactly, which are shown on the panel of power supply unit? For the detailed information about DC pulsed magnetron sputtering process, DC pulsed waveform was recorded by the home-made shunt circuit with non inductive resistances and oscilloscope, just as shown in Fig. 7. From Fig. 7 (a), it can be seen that the virtual DC pulsed waveform had some difference from the ideal one, especially for the beginning part of sputtering process after the reverse time, just as shown as the blue dash rectangle in Fig. 7 (a). For pulsed frequency in 100 kHz and reverse time in 1 μs, one complete pulsed circle contained the sputtering time in 9 μs. And, the sputtering time could be divided into two parts. One was 7 μs stable sputtering period, just as shown as Region 1 in Fig. 7 (a). The other one was 2 μs overload sputtering period, just as shown as Region 2 in Fig. 7 (b). The mean value of one complete pulsed circle, Region 1 in Fig. 7 (a), and Region 2 in Fig. 7 (b) was calculated respectively, just as shown in Fig. 8 (a). From Fig. 8 (a), it can be seen that the mean value of one complete pulsed circle corresponded well with that showed on the panel of sputtering power supply unit. The mean value of one complete pulsed circle consisted of the positive voltage in reverse time and negative voltage in sputtering time, which the former had no contribute to the sputtering process. So for one complete pulsed circle, more attention should be focused on the negative voltage in sputtering time, especially for V3 and V4 in Fig. 8 (a). After the reverse time, the negative sweep voltage was carried out onto ITO target. In general, when the negative sweep voltage was a little more over than the ignition voltage of ITO target, the plasma was generated and the negative sweep voltage automatically decreased to a certain stable value, just as shown as Region 2 in Fig. 7 (b). Then the sputtering process was steadily carried out onto ITO target surface, just as shown as Region 1 in Fig. 7 (a). With other parameters constant, the negative sputtering voltage

13

ACCEPTED MANUSCRIPT on target was only determined by the product of pressure in vacuum chamber and anode-cathode distance, which was well known as Paschen's law [27, 28]. In our experiment, the working pressure in vacuum chamber and anode-cathode distance was constant, respectively. So the negative sputtering voltage in Region 1 of Fig. 7 (a) should be unrelated to the sputtering power density. While, with the increase of sputtering power density, the mean value of Region 1 in Fig. 7 (a) increased toward negative voltage direction, just as shown as V3 in Fig. 8 (a). In this situation, the sputtering yield on ITO target should be considered. In general, with other parameters constant, the target sputtering yield was proportional to the sputtering current density. From Fig. 6, it can be found that with the increase of sputtering power density, the sputtering current density increased monotonously. Of cause the working pressure in vacuum chamber was kept at 0.6 Pa for all experiment. Actually, the pressure at target surface was a little higher than that at other position in vacuum chamber. Besides, with the increase of sputtering current density, more atoms/ions/groups were sputtered from the target, which increased the pressure at target surface. Combining Paschen’ law and the increase of pressure at target surface, the sputtering voltage on ITO target increased, just as shown as V3 in Fig. 8 (a). Except the change of Region 1 in Fig. 7 (a), more attention should be paid to Region 2 in Fig. 7 (b) for 2 μs overload sputtering. The mean voltage of Region 2 in Fig. 7 (b) was shown as V4 in Fig. 8. It can be found that V4 were more negative than V3. It was well known that the higher negative voltage on target could result in higher sputtering yield, as well as the sputtered atoms/ions/groups with higher kinetic energy. Based on V3 and V4 in Fig. 8 (a), the equivalent sputtering waveform was shown in Fig. 8 (b), which consisted of V4 for 2 μs and V3 for 7 μs.

14

ACCEPTED MANUSCRIPT

Fig. 7. (a) was DC pulse voltage waveform with different sputtering power density. (b) was the enlarged image of part closed by blue dash rectangle in (a).

Fig. 8. The curve V1, V2, V3, and V4 in (a) was the voltage value showed on the panel of sputtering power supply unit, mean voltage of one complete waveform circle, mean voltage of Region 1 in Fig. 7 (a), and mean voltage of Region 2 in Fig. 7 (b), respectively. (b) The equivalent sputtering waveform. Based on the analysis of DC pulse sputtering process in Fig. 6-8, the changing of crystallographic texture, optical, and electrical properties of ITO film could be logically discussed. Firstly, for one DC pulse sputtering circle, the sputtering process began with a higher sputtering voltage |V4|, then ended with a lower sputtering voltage |V3|, just as shown in Fig. 8 (b). For the sputtering with a higher sputtering voltage |V4|, the sputtered particles contained higher kinetic energy. These particles

15

ACCEPTED MANUSCRIPT arrived at ITO film growth surface with higher kinetic energy, which was lost as the heat through inelastoc collision and/or transformed as crystal energy. Then the sputtering voltage changed to the lower |V3|, and the sputtered particles contained lower kinetic energy. These particles arrived at ITO film growth surface with lower kinetic energy, which was totally lost as the heat through inelastoc collision. Besides, just as mentioned above, with other parameters constant, the target sputtering yield was proportional to the sputtering current density. And, the growth rate of ITO film was proportional to the target sputtering yield, which in turn was proportional to the sputtering current density on ITO target. In the range of sputtering current density lower than 4.16 W/cm2, the increase rate of ITO film growth rate proportionally corresponded well with that of target sputtering current density, just as shown as the green circle in Fig. 4. Based on the plasma theory [27, 29], the mean free path of monatomic particle for 0.6 Pa was about 100-101 cm, which was similar to the distance between target and substrate. In case of sputtering current density lower than 4.16 W/cm2, it was hard for sputtered In, Sn, and O atom/ion to collide each other. So sputtered In, Sn, and O atom/ion was incident to the growth surface of ITO film, directly. Besides, |V3| and |V4| were both lower value, which resulted in the incident In, Sn, and O atom/ion without kinetic energy higher enough and all kinetic energy was lost as the heat through inelastoc collision. Combining above reasons, the incident In, Sn, and O atom/ion was located at ITO film growth surface randomly, which resulted in the random chemical bond and random crystal growth direction. So in case of sputtering power density lower than 4.16 W/cm2, the growth rate of ITO film was proportional to the sputtering current density in Fig. 6, as well as XRD and TEM results displayed no preferred orientation in Fig. 1-3.

16

ACCEPTED MANUSCRIPT Secondly, a critical point appeared at 4.16 W/cm2 in sputtering power density. With the sputtering power density over the critical point, the growth rate of ITO film increased faster than the sputtering current density, obviously. Besides, with the increase of sputtering power density, the difference between above two increase rates became more and more obvious, just as shown as the brown circle in Fig. 6. In case of sputtering power density higher than critical point, the value of |V4| increased, which resulted in the sputtered In, Sn, and O atom/ion with higher kinetic energy. In this situation, these particles arrived at ITO film growth surface with kinetic energy higher enough, which couldn’t be totally lost as the heat through inelastoc collision. So parts of kinetic energy was transformed as the crystal energy in ITO crystal lattices. Then the sputtering voltage changed to the lower |V3|, and the sputtered particles contained lower kinetic energy. These particles arrived at ITO film growth surface with lower kinetic energy, which was totally lost as the heat through inelastoc collision. While, the crystal energy in ITO crystal lattices from V4 sputtering process worked for the growth of ITO grains along the certain direction. Besides, based on the film growth ratio, the sputtering yield had increased more than 3 times of original value, which in turn decreased the mean free path at ITO target surface to one third of original value. So the third factor should be considered, which was the collision effect between sputtered In, Sn, and O atom/ion from ITO target. Except the elastic collisions between sputtered In, Sn, and O atom/ion, one reasonable hypothesis was considered that the inelastic collision between sputtered In, Sn, and O ion was the key point in our experiment, which could chemically react as In-O and Sn-O groups. For the growth surface of ITO film, the incident In-O and Sn-O groups also arranged themselves at the positions with lowest energy. For In-O and Sn-O groups, the crystal growth model could effectively decrease the system energy. For free growth cubic

17

ACCEPTED MANUSCRIPT ferrite ITO grain, the growth ratio along three axes should be same, which resulted in the formation of ITO equiaxed grain in film. Actually, for ITO grain, the growth ratio parallel to film surface was restrained by the growth of adjacent ITO grains. On the contrary, no any restraint happened on the growth ratio along normal direction of ITO film growth surface. So, for cubic ferrite ITO grain, the growth ratio along normal direction of ITO film surface was much higher than those of parallel to ITO film surface, which resulted in the formation of ITO columnar crystalline structure. Besides, the lattice constants along three axes of cubic ferrite ITO grain were same, which resulted in the same preferred orientation along any axis. Combining above reasons, with the increase of sputtering power density, the crystal structure of ITO film changed from the polycrystalline without preferred orientation to columnar crystalline structure with (100) preferred orientation, just as XRD and TEM results in Fig. 1-3. Combining above analysis, one feature about DC pulsed sputtering process should be noticed that the short time overload sputtering voltage influenced the film growth mode, obviously. So in order to grow ITO film with preferred orientation onto substrate without high temperature resistance, the short time overload sputtering voltage of DC pulsed sputtering process could be an ideal choice. Based on the different ITO grain growth modes, the change of optical and electrical properties of ITO film in Fig. 4 and 5 could be well understood. With the increase of sputtering power density, ITO grain growth mode changed from the equiaxed grain with small size to columnar grain with large size. In general, the crystallinity of large columnar grain was much higher than that of small equiaxed grain, which resulted in more Sn4+ substituting In3+ in In2O3 lattice. In turn, more free electrons existed in ITO film, which resulted in the increase of carrier concentration. So, with the constant of carrier mobility, the resistivity decreased with the increase of

18

ACCEPTED MANUSCRIPT carrier concentration, just as shown in Fig. 4. Besides, ITO was the typical n type semiconductor with higher electron densities, which was effected by Burstein-Moss shift [30, 31]. With more Sn4+ substituting In3+ in In2O3 lattice, the free electron carrier concentration increased, which resulted in the electrons to populate the states within the conduction band. Such electron population states could push Fermi level to higher energy, even inside the conduction band. So the electron from top of valence band could only be excited into the conduction band above Fermi level since all the states below Fermi level were occupied states. Pauli's exclusion principle forbad the electron to be excited into these occupied states [32, 33]. Thus with the increase of sputtering power density, the absorption edge and the band gap gradually shifted toward blue side and higher energy respectively, just as shown in Fig. 5. The change of optical and electrical properties of ITO film corresponded well with the transformation of ITO grain growth mode, which pointed out that the sputtering power density was one of the most important parameters to control ITO grain growth mode, in turn to control the optical and electrical properties of ITO film. Based on the major premise that ITO films on PI, Si (100), and quartz substrates were with same properties, the growth modes, growth mechanism, as well as transform between different growth modes, could be applied to ITO films on above three kinds of substrates.

4. Conclusions In summary, the traditional direct current pulsed magnetron sputtering technology was used to prepare ITO film on polyimide tape without any heating treatment. Comparing the growth rate with sputtering power density on ITO target, a critical point appeared at 4.16 W/cm2. In case of the sputtering power density lower than critical point, the increase rate of ITO film growth rate proportionally

19

ACCEPTED MANUSCRIPT corresponded well with that of target sputtering current density. The kinetic energy of incident particles was lost as the heat and ITO grains grew along random crystal direction. So ITO film consisted of the equiaxed cubic ferrite ITO grains and displayed no preferred orientation. In this situation, the incoherent grain boundary was the primary in ITO film, which worsened the optical and electrical property of film. In case of the sputtering power density higher than critical point, parts kinetic energy of incident particles was transformed as the crystal energy in ITO crystal lattices, which worked for the growth of ITO grains along the certain direction. Besides, the incident In-O and Sn-O group should be considered, which resulted in the growth ratio along normal direction of film surface was much higher than those of parallel to film surface. For this reason, ITO columnar crystalline gains were formed and ITO film displayed (100) preferred orientation. Consideration the primary of coherent grain boundary between columnar grains, as well as Burstein-Moss effect, ITO film deposited with sputtering current density higher than critical point displayed the satisfactory optical and electrical properties, such as resistivity and carrier concentration in 4.91×10-4 Ω·cm and 5.10×1020 cm-3, respectively. Finally, for extending the application field of ITO film, it is interesting to prepare (100) preferred orientation ITO film onto organic polymer substrate. So the short time overload sputtering voltage of DC pulsed sputtering process could be an ideal choice. The present work reveals DC pulsed sputtering process to realize it, as well as the physicochemical mechanism happened behind the sputtering process.

Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 51472039, 51772038, 51575074, 61504017), Program for Liaoning Excellent Talents in University (No. LR2015010), Project of Dalian Youth Star of Science and

20

ACCEPTED MANUSCRIPT Technology (No. 2015R071), Natural Science Foundation of Liaoning Province, China (Nos. 2015020182, 2015020191, 2015020653, 201602120), Project Sponsored by the Scientific Research Foundation for Doctor, Liaoning Province, China (No. 201601248). References: [1] M. G. Helander, Z. B. Wang, J. Qiu, M. T. Greiner, D. P. Puzzo, Z. W. Liu, Z. H. Lu, Chlorinated indium tin oxide electrodes with high work function for organic device compatibility, Science 332 (2011) 944-947. [2] K. Ellmer, Past achievements and future challenges in the development of optically transparent electrodes, Nat. Photonics 6 (2012) 809-817. [3] C. G. Granqvist, Transparent conductors as solar energy materials: A panoramic review, Solar Energy Mater. Sol. Cells 91 (2007) 1529-1598. [4] N. M. Sangeetha, M. Gauvin, N. Decorde, F. Delpech, P. F. Fazzini, B. Viallet, G. Viau, J. Grisolia and

L. Ressier, A transparent flexible z-axis sensitive multi-

touch panel based on colloidal ITO nanocrystals, Nanoscale 7 (2015) 12631-12640. [5] M. Shakiba, A. Kosarian, E. Farshidi, Effects of processing parameters on crystalline structure and optoelectronic behavior of DC sputtered ITO thin film, J Mater Sci: Mater Electron 28 (2017) 787-797. [6] M. Usman, S. Khan, M. Khan, T. A. Abbas, Re-crystallization of ITO films after carbon irradiation, Appl. Surf. Sci. 392 (2017) 863-866. [7] M. K. Chong, K. Pita, S. T. H. Silalahi, Correlation between diffraction patterns and surface morphology to the model of oxygen diffusion into ITO films, Mater. Chem. Phys. 115 (2009) 154-157. [8] F. Latteyer, H. Peisert, U. Aygül, I. Biswas, F. Petraki, T. Basova, A. Vollmer, and T. Chassé, Laterally Resolved Orientation and Film Thickness of Polar Metal

21

ACCEPTED MANUSCRIPT Chlorine Phthalocyanines on Au and ITO, J. Phys. Chem. C, 115 (2011) 1165711665. [9] V. S. Vidhya, V. Malathy, T. Balasubramanian, V. Saaminathan, C. Sanjeeviraja, M. Jayachandran, Influence of RF power on the growth mechanism, preferential orientation and optoelectronic properties of nanocrystalline ITO films, Curr. Appl. Phys. 11 (2011) 286-294. [10] D. P. Pham, B. T. Phan, V. D. Hoang, H. T. Nguyen, T. K. H. Ta, S. Maenosono, C. V. Tran, Control of preferred (222) crystalline orientation of sputtered indium tin oxide thin films, Thin Solid Films 570 (2014) 16-19. [11] S. Ahn, A. H. T. Le, S. Kim, C. Park, C. Shin, Y. Lee, J. Lee, C. Jeong, V. A. Dao, J. Yi, The effects of orientation changes in indium tin oxide films on performance of crystalline silicon solar cell with shallow-emitter, Mater. Lett. 132 (2014) 322-326. [12] C. M. Ghimbeu, M. Lumbreras, M. Siadat, J. Schoonman, Detection of pollutant gases using electrostatic sprayed indium oxide and tin-doped indium oxide, Mater. Chem. Phys. 114 (2009) 933-938. [13] Y. Lan, Y. Chen, J. He, J. Chang, Microstructural characterization of highquality indium tin oxide films deposited by thermionically enhanced magnetron sputtering at low temperature, Vacuum 107 (2014) 56-61. [14] H. Wang, W. Ding, C. Liu, W. Chai, Influence of O2 flux on compositions and properties of ITO films deposited at room temperature by direct-current pulse magnetron sputtering, Chinese Phys. Lett. 27 (2010) 127302. [15] H. Wang, H. Liu, W. Ding, W. Chai, Study on the growth mechanism of tindoped indium oxide films deposited by direct current pulse magnetron sputtering, Thin Solid Films 542 (2013) 415-419.

22

ACCEPTED MANUSCRIPT [16] PCPDFWIN card number: 00-06-0416 (Version 2.1, Copyright 2000) [17] T. Sasabayashi, N. Ito, E. Nishimura, M. Kona, P. K. Song, K. Utsumi, A. Kaijo, Y. Shigesato, Comparative study on structure and internal stress in tin-doped indium oxide and indium-zinc oxide films deposited by r.f. magnetron sputtering, Thin Solid Films 445 (2003) 219-223. [18] D. A. Kirienko and O. Y. Berezina, On the detachment of thin ITO films from silicon substrate by microsecond laser irradiation, Semiconductors 51 (2017) 823827. [19] H. Suda and K. Haraya, Gas Permeation through Micropores of Carbon Molecular Sieve Membranes Derived from Kapton Polyimide, J. Phys. Chem. B 101 (1997) 3988-3994. [20] L. Li, H. Liu, W. Ding, D. Ju, and We. Chai, The effect of oxygen ion beam bombardment on the properties of tin indium oxide/polyethylene terephthalate complex, Thin Solid Films 545 (2013) 365-370. [21] W. Ding, D. Ju, and W. Chai, The effect of working pressure on the chemical bond structure and hydrophobic properties of PET surface treated by N ion beams bombardment, Appl. Surf. Sci. 256 (2010) 6876-6880. [22] L. Li, Master's Thesis: The influence of the performance of ITO/PET by O ion beaming PET surface, Dalian Jiaotong University, Dalian, 2014. (In Chinese). [23] H. Ma and J. Ma, The transparent oxide semiconductor, Science Press, Beijing, 2014 (in Chinese). [24] X. Jiang, C. Sun, R. Jiang, D. Dai, The transparent conductive oxide film, Higher Education Press, Beijing, 2008 (in Chinese). [25] O. Stenzel, The physics of thin film optical spectra: an introduction, SpringerVerlag, Berlin, 2005.

23

ACCEPTED MANUSCRIPT [26] K. W. Böer, Handbook of the physics of thin-film solar cells, Springer Berlin Heidelberg, New York, 2013. [27] M. A. Lieberman, A. J. Lichtenberg, Principles of plasma discharges and materials processing, Second Edition, Wiley-Interscience, Hoboken, 2005. [28] Y. Fu, S. Yang, X. Zou, H. Luo, and X. Wang, Intersection of Paschen's curves for argon, Phys. Plasmas 23 (2016) 093509. [29] T. Ma, X. Hu, Y. Chen, The principles of plasma physics, Revised Edition, University of Science and Technology of China Press, 2012 (in Chinese). [30] M. Grundmann, The Physics of Semiconductors, Springer Berlin Heidelberg, New York, 2006. [31] M. Feneberg, J. Nixdorf, C. Lidig, R. Goldhahn, Z. Galazka, O. Bierwagen, and J. S. Speck, Many-electron effects on the dielectric function of cubic In2O3: Effective electron mass, band nonparabolicity, band gap renormalization, and Burstein-Moss shift, Phys. Rev. B 93 (2016) 045203. [32] D. J. Griffiths, Introduction to Quantum Mechanics, Second Edition, Pearson Education Limited, London, 2004. [33] C. Kittel, Introduction to Solid State Physics, Eighth Edition, John Wiley & Sons, Inc., Danvers, 2005.

24

ACCEPTED MANUSCRIPT 

Increasing sputtering power density, ITO film change to (100) texture gradually.



ITO film with (100) texture displays better resistivity and carrier concentration.



Short time overload sputtering voltage improve kinetic energy of incident In/Sn/O atom/group.



Incident In/Sn/O with higher kinetic energy result in ITO film with (100) texture.