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Transparent conducting indium tin oxide (ITO) thin films are deposited on polyethylene terephthalate (PET) and silicon (Si) substrates by DC magnetron sputtering at room temperature. The electrical and optical properties of the ITO films are then investigated as a function of the cumulative sputtering gas; a parameter newly proposed in this study and defined as the product of the gas (argon) flow rate and the deposition time. The results show that the ITO films deposited on the PET substrates have an amorphous structure, while those deposited on the Si substrates have a microcrystalline structure. For both ITO films, a critical value of the cumulative sputtering gas parameter exists at which the minimum resistivity occurs due to a corresponding increase in the carrier density. For the ITO films deposited on the Si substrates, the carrier mobility is insensitive to the cumulative sputtering gas. However, for the ITO films deposited on the PET substrates, the carrier mobility reduces as the cumulative sputtering gas increases. For the ITO film on the PET substrate, the average transmittance in the visible range increases with an increasing argon flow rate given a constant deposition time. The optical band gap of the ITO films on the PET substrates located in the visible range reduces the transparency of these samples. Finally, for the ITO films deposited on PET substrates, a low resistivity can be obtained without any significant reduction in the transmittance of the ITO film by using the critical cumulative sputtering gas value as a deposition guideline in determining suitable values of the gas flow rate and deposition time.
© 2014 Optical Society of America
1. Introduction
Indium tin oxide (ITO) films have good electrical conductivity and high optical transparency. As a result, they are extremely attractive for such optoelectronic applications as flat panel display devices [1], flexible organic light-emitting diodes (OLED), and flexible touch sensors [2]. Contemporary applications utilizing ITO films are generally based on light polymer substrates such as PET (polyethylene terephthalate) or polyethylene naphthalate (PEN) since, compared with other substrate materials (e.g., PC and PEN), such materials have high optical transmittance, excellent mechanical properties and good manufacturability [3]. Various techniques are available for the deposition of ITO thin films, including RF sputtering [2,4–6], DC sputtering [7–9], ion beam deposition [10–12], pulse laser deposition [13], and chemical vapor deposition [14–16]. Among these various methods, DC magnetron sputtering is particularly attractive for industrial applications due to its good reproducibility, relatively low cost, and potential for large area coverage [1].
The characterization of ITO thin films deposited on polymer substrates at low temperatures using reactive coating techniques has attracted significant attention in the literature [7,11–14]. In addition, several studies have investigated the properties of ITO films deposited on PET substrates by DC magnetron sputtering at room temperature without oxygen gas [8,9]. The reason of the deposition at room temperature in this study is the plastic substrates used in flexible electronics applications have a low thermal resistance, and thus sputtering must be performed at a low temperature in order to avoid thermal damage to the substrate surface. Actually, a R2R sputtering for ITO/PET is performed on a chilled drum in many industrial applications. The literature contains many investigations into the effects of various processing parameters on the properties of sputtered ITO thin films, including the substrate temperature [2,9,13], the oxygen partial pressure [13,14], the sputtering power [2,4,14], the bias voltage [6], and the interface material [12,16]. In general, these studies have shown that the electrical and optical properties of ITO thin films are significantly dependent on the sputtering conditions.
Thin film deposition by sputtering techniques involves the bombardment of the substrate surface by the metal ions ejected from a target in the presence of a sputtering gas. A review of the literature reveals that the effects of the cumulative ion bombardment quantity on the properties of ITO thin films have not yet been investigated. In practice, the total quantity of metal ions ejected by the target cannot be accurately determined. Thus, the present study proposes a cumulative sputtering gas (CSG) parameter (defined as the product of the sputtering gas flow rate and the deposition time) to approximate the total cumulative ion bombardment. Utilizing the CSG parameter, a series of experiments is performed to investigate the effects of the cumulative ion bombardment quantity on the electrical and optical parameters of ITO films deposited on PET and Si substrates by means of DC magnetron sputtering at room temperature in an argon atmosphere.
2. Experiments
2.1. Sample preparation
The ITO thin films were prepared by DC magnetron sputtering using indium tin oxide targets (SnO2 10 wt.%, purity 99.9%) with dimensions of 406 × 109 × 10 mm. The ITO films were deposited on two different substrate materials, namely polyethylene terephthalate (PET) and Si . In preparing the former substrates, a 0.12-mm thick PET film was rolled under tension over a polished metal roller at room temperature. Substrate specimens with dimensions of 20 × 15 cm were then sliced from the rolled PET film using a cutter. Meanwhile, Si substrate specimens with dimensions of 2 × 2 cm were cut from a commercial 2 inch wafer. The sputtering process was performed at room temperature using a DC power of 350 W and a chamber base pressure of 1.5 × 10−5 torr. Before sputtering, the PET and Si substrates were immersed in an alkaline solution for 20 minutes, cleaned ultrasonically in de-ionized (DI) water, and then dried in nitrogen gas. In performing the sputtering process, the substrate specimens were placed on a rotating holder (10 rpm) in order to ensure a homogeneous film thickness. Moreover, the PET and Si substrates were sputtered simultaneously in order to more accurately compare the effects of the cumulative ion bombardment quantity on the two different ITO thin film samples. The sputtering process was performed in a pure argon (99.99%) environment with no oxygen content. To examine the effects of the cumulative ion bombardment quantity on the electrical and optical properties of the ITO thin films, the argon flow rate was varied from 25 to 35 sccm in steps of 5 sccm and the deposition time was set as either 10 or 15 minutes. Finally, the distance between the target and the substrates was fixed at 22 cm. All ITO films were deposited at room temperature and the properties of the ITO films were examined in the as-sputtered (i.e., non-annealed) condition. The sputtering conditions are summarized in Table 1.
Table 1. DC magnetron sputtering conditions for ITO films prepared on PET and Si substrates
2.2. Thin film characterization
The microstructures of the ITO films were examined via X-ray diffraction (XRD) analysis (D8 DISCOVER, Bruker AXS). The resistivity, carrier mobility and carrier concentration of the ITO films were determined using a Hall Effect measurement system (Ecopia HMS-3000) with a magnetic flux of 0.51 T and a Van der Pauw configuration. It is noted that for the samples with high resistivity (higher than 20 mΩ cm), which were un-measureable by the Hall measurement system, the resistivity was calculated as ρ = Rsq/d, where Rsq is the sheet resistance and d is the film thickness. The sheet resistance was determined using a four-point probe station (Keithley 2182A with 6220 power supply). The thickness of the ITO films was measured by spectroscopic ellipsometry (SE) (J.A. Woollam, VASE M2000U) with a wavelength range of 245 ~1000 nm and an incident angle of 65°. In performing the SE analysis, the ellipsometry parameters were calculated using commercial WVASE32 software (Woollam Co.) and the dielectric function of the various ITO films were approached using the conventional Tauc-Lorentz model [17,18]. For a comparison, a α-step surface profilometer (Veeco, Dektak 150) is also used to measure the thickness of ITO films on Si substrate. A small tape was pasted on surface of Si substrate before deposition and then taken off after deposition was finished. Thus, the face jump between substrate and film representing the film thickness can be measured by a α-step surface profilometer. Finally, the optical transmission properties of the ITO films were measured in the wavelength range of 250 ~1000 nm using a UV-visible–near-IR spectrophotometer (Jasco, V-670) with normal light incidence.
3. Results and discussions
3.1. Structural properties
Figures 1(a) and 1(b) show the XRD spectra of the ITO films deposited on PET and Si substrates, respectively, given a cumulative sputtering gas (CSG) value of 350 cc. It can be seen that the XRD spectrum of the ITO/PET sample has no distinct peaks and exhibits only a broad halo pattern near 2θ = 31°. In other words, the ITO film has an amorphous structure. By contrast, the XRD spectrum of the ITO/Si sample has a distinct broad peak near 20 = 31° Thus, it is inferred that the ITO film has a (222) microcrystalline structure. It is noted that the present results for an amorphous ITO/PET structure and a microcrystalline ITO/Si structure are consistent with the results presented in [6, 8, 11] and [19, 20], respectively. Moreover, the preferred (222) crystallite orientation of the ITO/Si sample is consistent with the observations of many previous studies of ITO films deposited on various substrates at low temperature using different sputtering methods [9,10,20,21].
Figure 2 shows variations of the film thickness of the ITO/PET and ITO/Si samples for various CSG value. Because the ITO/PET and ITO/Si samples at the same CSG are deposited concurrently, the thickness of the ITO/PET and ITO/Si samples obtained by SE can be comparable. For ITO/Si and ITO/PET samples, the minimum thickness around 45 nm occurs at the CSG value of 350 cc, also the deposition time is the main reason for increasing of thickness. It is observed that ITO film thickness on PET is different from that on Si at the same CSG except at the value of 350 cc. It appears that the ITO growth mechanism on a Si surface is different from that on a PET surface because XRD spectra of ITO films, as illustrated in Fig. 1, deposited on the PET and Si substrates are quite different.
Fig. 2 Variations of film thickness of ITO/Si and ITO/PET measured by profilometer (dSi,α-step) and ellipsometry (dSi,SE, dPET,SE) for CSG values in the range of 250 ~525 cc.
3.2. Electrical properties
The electrical properties (i.e., resistivity, carrier density and carrier mobility) of the various ITO films were characterized using a Hall Effect measurement system. Figure 3(a) shows the variation of the resistivity of the ITO/PET and ITO/Si samples with the cumulative sputtering gas (CSG) parameter. It is seen that for both samples, the resistivity reduces as the CSG is increased to 350 cc, but then increases as the CSG is increased further to 525 cc. However, it is noted that for the film deposited on the PET substrate, the maximum variation in the resistivity over the considered CSG range is equal to approximately 37 × 10−3 Ω cm, while for the film deposited on the Si substrate, the variation in the resistivity is equal to just 3 × 10−3 Ω cm. In other words, the resistivity of the ITO film deposited on the PET substrate is particularly sensitive to the accumulated ion bombardment quantity. The results presented in Fig. 3(a) suggest that for the ITO/PET sample, the CSG should be carefully controlled within the range of 350 ~375 cc if a low resistivity (<5 × 10−3 Ω cm) is to be obtained.
Fig. 3 Variation of (a) resistivity, (b) carrier density and (c) carrier mobility of ITO/Si and ITO/PET samples for CSG values in the range of 250 ~525 cc.
It is noted that for all values of the CSG, the resistivity of the ITO/Si sample is lower than that of the ITO/PET sample. However, the difference in the resistivity of the two samples reduces significantly as the CSG approaches the value of 350 cc. From inspection, the minimum resistivities of the ITO films deposited on the Si and PET substrates are found to be 3.7 × 10−4 Ω cm and 6.8 × 10−4 Ω cm, respectively, and occur at the same critical CSG value of 350 cc. It is noted that the thickness around 45 nm for ITO films on Si and PET substrates is with the lowest resistivity. The lowest resistivity obtained in our study is very close to the high-quality ITO/PET samples reported by using other deposition techniques [6, 13, 16]. However, the film thicknesses in aforementioned reports are more than 200nm, which is much higher than 45 nm thickness of the optimized samples at the CSG value of 350 cc. In general, the results show that the sensitivity of the ITO resistivity to the substrate material can be reduced by performing the deposition process under the optimal CSG conditions. Theoretically, the resistivity (ρ) of the ITO films depends on both the carrier density (N) and the carrier mobility (μ), i.e., ρ = 1/(Neμ) [18], where e is the electron charge. In other words, as discussed below, the resistivity reduces as the carrier density or carrier mobility increases.
Figure 3(b) shows the variation of the carrier density with the CSG for the ITO/Si and ITO/PET samples. It is noted that due to the resistivity of ITO film is un-measurable by the Hall measurement system, the carrier concentration data for the ITO/PET samples prepared on CSG 250 and 525 cc are absent. It is seen that for both films, the carrier density increases as the CSG is increased to 350 cc, but then reduces as the CSG is further increased to 525 cc. The same tendency of change implies a similar effect of CSG on carrier density of ITO films on these two different substrate materials. It is observed that the maximum carrier densities of the ITO/Si and ITO/PET samples are around 5.3 × 1020 cm−3 and 3.4 × 1020 cm−3, respectively, and occur at the critical CSG value. Furthermore, as expected, the variation in the carrier density curve is inversely correlated with that in the resistivity curve (see Fig. 3(a)). Therefore, as implied by the relationship ρ = 1/(Neμ), it is inferred that the minimum resistivities of the ITO/Si and ITO/PET samples at a CSG of 350 cc are a result primarily of the maximum carrier density at the same value of the CSG. Many former studies have shown that the resistivity of ITO thin films is strongly dependent on the oxidation state during deposition [21–23]. Furthermore, the free carriers in ITO film stem from two shallow donors, namely oxygen vacancies with double charge and substitutional four valent tin in the indium sub-lattice [24, 25]. It is thus reasonable to infer that an appropriate oxidation state, obtained by a well-controlled oxygen deficiency or the availability of sufficient tin dopant, increases the number of free carriers in the ITO film and therefore enhances the electrical conductivity. In the present study, the oxidation state is governed by the argon flow rate and the deposition time. In other words, the optimization of the carrier density is dominated by the cumulative ion bombardment amount (i.e., the CSG) rather than by any individual control factor. Specifically, the optimum CSG condition (i.e., 350 cc for the present ITO samples) induces in turn the optimum oxidation state, and therefore results in the maximum carrier density. Finally, it is noted that the maximum variation in the carrier density over the considered CSG range has a value of 48 x1019 cm−3 for the ITO/Si sample and 31 x 1019 cm−3 for the ITO/PET sample. This relatively high variation in the carrier density implies that the oxidation states of both samples are highly sensitive to the CSG.
Figure 3(c) shows the variation of the carrier mobility of the ITO/Si and ITO/PET samples with the CSG. Again, due to the resistivity of ITO film is un-measurable by the Hall measurement system, the carrier mobility data for the ITO/PET samples prepared on CSG 250 and 525 cc are absent. It is observed that the carrier mobility of the ITO/Si sample varies randomly in the range of 22 ~30 cm2V−1S−1 as the CSG is increased over the considered range. By contrast, the carrier mobility of the ITO/PET sample reduces continuously as the CSG is increased from 300 to 450 cc. The different tendencies in the carrier mobility are thought to be the result of the different substrate materials of the two samples since both samples were sputtered simultaneously under the same conditions. In other words, it appears that the ITO growth mechanism on a crystalline Si surface is very different from that on a semi-crystalline PET surface [3]. Previous studies have shown that the carrier mobility in ITO thin films is determined by various scattering mechanisms, including grain boundary scattering, impurity scattering, and defect scattering [8, 22, 25]. Similarly, Bellingham [25] reported that the resistivity of amorphous ITO is dominated by ionized impurity scattering. The more the quantity of ionized impurity within an ITO film, the lower the mobility of the carrier in such ITO film. The thickness difference, illustrated in Fig. 2, between ITO/Si and ITO/PET samples can imply a nucleation difference between the silicon substrate and the PET substrate. Moreover, the PET substrate material possibly transfers into the growing film and then affects the structure over a large depth. Then, the amorphous nature of the ITO/PET sample (see Fig. 1(a)) is considered to be resulted by a non-well defined nucleation of the ITO seeds on the semi-crystalline PET surface [3] or the PET material transferred into the growing film during the early stage of film growth, which inhibits further grain growth in thin film. Moreover, the ionic impurities within the ITO film hard to change their state by diffusion since the current deposition process is performed under low temperature conditions. Thus, the number of ionic impurities increases with an increasing ion bombardment (i.e., an increasing CSG). In contrast, the nucleation of the ITO seeds on the crystalline Si surface is well defined and grown as to the result of microcrystalline (refer to Fig. 1(b)) instead of amorphous in ITO films. Tahar et al. [26] reported that the carrier mobility of a microcrystalline ITO is dominated by the grain boundary scattering. Thus, the reason for the random distribution of the carrier mobility in Fig. 3(c) is the grain boundary scattering induced by the microcrystal structure.
3.3. Transmittance
Figures 4(a) and 4(b) show the transmittance spectrum and average transmittance, respectively, of the ITO/PET sample in the visible range (i.e., 250 ~1000 nm). Figure 4(a) also shows the transmittance spectrum of the bare PET substrate for comparison purposes, while Fig. 4(b) also shows the film thickness. It is seen in Fig. 4(a) that the transmittance of the ITO/PET samples varies in the range of 70~80% and is non-linearly related to the CSG. In addition, it is observed that the transmittance of the ITO/PET samples is lower than that of the bare PET substrate for all wavelengths in the range of 400~800 nm. It has been reported that the optical band gap of amorphous ITO, which represents the photon energy at which interband absorption begins, has a key effect on the light absorption properties of ITO films [17,18]. In general, films with an optical band gap located in the visible range have a strong absorption of visible light and therefore a reduced transparency. In this work, the optical band gap of ITO films obtained by SE measurement has a value of approximately 2.8 eV (420 nm), which corresponds with the turning point of the transmittance curve in the near-UV range shown in the inset of Fig. 4(a). This value falls within the visible spectrum (400~800 nm), and hence the absorption of visible light is enhanced (and the transmittance correspondingly reduced).
Fig. 4 (a) Transmittance spectra of bare PET substrate and ITO/PET samples in visible range as function of CSG. (b) Average transmittance in visible range (400~800 nm) and thickness of ITO/PET samples as function of CSG.
The results presented in Fig. 4(b) show that the average transmittance of the ITO/PET samples in the visible range increases with increasing CSG over the two distinct CSG ranges of 250 ~350 cc and 375 ~525 cc, respectively. It is noted that these two ranges correspond to the same argon flow rate range (i.e., 25 ~35 sccm), but different deposition times (i.e., 10 and 15 minutes). It is therefore inferred that the average transmittance increases with an increasing argon flow rate given a constant deposition time. However, it is observed that for the same argon flow rate range (i.e., 25~35 sccm), the transmittance varies irregularly with the film thickness given a deposition time of 10 minutes, but increases continuously with a decreasing film thickness given a constant deposition time of 15 minutes. In other words, the mechanism responsible for the enhanced transmittance of the ITO film as the CSG is increased from 250 ~350 cc is different from that for the case where the CSG is increased from 375 ~525 cc. The transmittance (T) of a thin film is defined as T = (1-R)exp(-α.d) [21], where α is the absorption coefficient, d is the film thickness, and R is the reflection of the incident light at the interface between the environment air and the thin film. In other words, the transmittance increases with a reducing absorption coefficient, film thickness or reflection. The absorption coefficient is given by α = (4πkt)/λ, where λ is the wavelength of the incident light and kt is the extinction coefficient of the thin film [18]. Thus, the transmittance of the ITO film increases with an increasing CSG in the range of 250~350 cc, and it is a compound result of the absorption coefficient, film thickness and reflection. However, the improved transmittance of the ITO film with an increasing CSG in the range of 375~525 cc is primarily attributed to the reduced thickness of the sputtered film with an increasing argon flow rate. From Figs. 3(a) and 4(b), it is seen that the minimum resistivity and maximum transmittance are both obtained at the critical CSG value of 350 cc. It is therefore inferred that for amorphous ITO films prepared on PET substrates, the electrical and optical properties can both be optimized by controlling the gas flow rate and deposition time in such a way as to achieve the critical CSG value.
Figure 5 illustrates the wavelength dependent refractive index and extinction coefficient of ITO films on PET substrate obtained by ellipsometry measurement. The average refractive index and the extinction coefficient in visible range vary in range of 2.00~2.06 and 0.06~0.036, respectively. However, no obvious correlation between CSG and spectrum of n and k is observed. The obtained n and k spectrum of ITO films on PET substrate is close to the results reported in former studies [10, 19].
Fig. 5 The wavelength dependent refractive index and extinction coefficient for ITO films on PET substrate as function of CSG.
3.4. Repeatability of minimum resistivity
In order to confirm the repeatability of the minimum resistivity obtained at the critical CSG of 350 cc, a series of deposition experiments were performed using PET substrates in which various combinations of the argon flow rate and deposition time were specified so as to achieve a CSG of 350 cc in every case. For example, the argon flow rate (Ar) was assigned values of 28 and 33 sccm, respectively, and the deposition time (td) was set as 12.5 and 10.6 minutes accordingly. Figure 6 shows the variation of the film resistivity as a function of the selected argon flow rates and deposition times. It can be seen that the resistivity varies randomly between 5.6 × 10−4 and 4.2 × 10−4 Ω cm as the argon flow rate is increased from 25 to 35 sccm and the deposition time is reduced from 14 to 10 minutes. It is noted that the resistivity values are close to the minimal resistivity value of 6.8 × 10−4 Ω cm (Ar = 35 sccm and td = 10 min) shown in Fig. 3(a). In other words, the validity of the critical CSG rule defined at the end of the previous section is confirmed. The results presented in Fig. 6 show that the average transmittance of the ITO/PET samples varies randomly between 78% and 76% as the argon flow rate is increased from 25 ~35 sccm. This result is consistent with the average transmittance value of 77.5% given in Fig. 4(b) for a CSG of 350 cc. Thus, it is inferred that the critical CSG rule enables a low ITO film resistivity to be obtained without any significant reduction in the transparency.
Fig. 6 ITO/PET film resistivity and average transmittance given use of critical CSG rule in which argon flow rate and deposition time are controlled so as to achieve CSG of 350 cc.
4. Conclusions
ITO thin films have been deposited on PET and Si substrates by DC magnetron sputtering at room temperature. The electrical and optical properties of the ITO films have been examined as a function of the cumulative sputtering gas (CSG) parameter, defined as the product of the argon flow rate and the deposition time.
The results have shown that the ITO films grown on PET substrates have an amorphous structure, while those grown on Si substrates have a microcrystalline structure. Either the amorphous structure or the microcrystalline structure of the ITO films is attributed to the thin film thickness prompted by the use of a low deposition temperature. The resistivity of the ITO thin films is governed by both the CSG and the substrate material. However, the resistivity of the ITO/PET films is more sensitive to changes in the CSG than that of the ITO/Si films. For both ITO films, the minimum resistivity occurs at the same critical value of the CSG (i.e., 350 cc). The minimum resistivity at the critical CSG value is attributed to the maximum carrier density, which in turn implies that the critical CSG value results in the optimum oxidation state of the ITO film.
The transmittance of the ITO/PET samples is significantly lower than that of the pure PET substrate in the visible range due to the optical band gap of the ITO films are located in the visible range. The average transmittance in the visible range increases with an increasing CSG value over two distinct CSG ranges, namely 250 ~350 cc and 375 ~525 cc, corresponding to deposition times of 10 minutes and 15 minutes, respectively. Specifically, for the shorter deposition time of 10 minutes, the enhancement in the transmittance is due to a compound effect of absorption coefficient, film thickness and reflection, while for a longer deposition time of 15 minutes, the improved transmittance is primarily due to a reducing film thickness as the argon flow rate is increased.
Finally, it has been shown that the ITO/PET samples deposited using various combinations of the argon flow rate and deposition time carefully chosen in such a way as to achieve a constant CSG of 350 cc have a similar transmittance and resistivity to those of the original sample deposited using an argon flow rate of 35 sccm and a deposition time of 10 minutes. In other words, it is inferred that the resistivity of the ITO/PET thin films can be minimized without any significant reduction in the transmittance provided that the argon flow rate and deposition time are chosen in such a way as to achieve the critical CSG value.
Acknowledgments
The partial support provided to this study by the National Science Council of Taiwan under Grant NSC No. 101-2221-E-006- 028-MY3 is greatly appreciated. In addition, the authors wish to thank Mr. Cha-Hong Hwang of the Metal Industry Research and Development Centre (Taiwan) and Dr. Kemo Lin of Southern Taiwan University of Science and Technology for their assistance in preparing and characterizing the ITO thin films used in this study.
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