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Indium tin oxide films deposited on polyethylene naphthalate substrates by radio frequency magnetron sputtering
Thin Solid Films 517 (2009) 2596–2601
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / t s f
Indium tin oxide films deposited on polyethylene naphthalate substrates by radio frequency magnetron sputtering M.G. Sandoval-Paz, R. Ramírez-Bon ⁎ Centro de Investigación y Estudios Avanzados del IPN-Unidad Querétaro, Apdo. postal 1-798, Querétaro, Qro., 76001, México
a r t i c l e
i n f o
Article history: Received 20 June 2007 Received in revised form 25 August 2008 Accepted 7 October 2008 Available online 17 October 2008 Keywords: Indium tin oxide Structural properties Optical properties Electrical properties and measurements Sputtering X-ray diffraction
a b s t r a c t Indium tin oxide (ITO) thin films were deposited on unheated polyethylene naphthalate substrates by radiofrequency (rf) magnetron sputtering from an In2O3 (90 wt.%) containing SnO2 (10 wt.%) target. We report the structural, electrical and optical properties of the ITO films as a function of rf power and deposition time. Low rf power values, in the range of 100–130 W, were employed in the deposition process to avoid damage to the plastic substrates by heating caused by the plasma. The films were analyzed by X-ray diffraction and optical transmission measurements. A Hall measurement system was used to measure the carrier concentration and electrical resistivity of the films by the Van der Pauw method. The X-ray diffraction measurements analysis showed that the ITO films are polycrystalline with the bixbite cubic crystalline phase. It is observed a change in the preferential crystalline orientation of the films from the (222) to the (400) crystalline orientation with increasing rf power or deposition time in the sputtering process. The optical transmission of the films was around 80% with electrical resistivity and sheet resistance down to 4.9 × 10- 4 Ωcm and 14 Ω/sq, respectively. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Indium tin oxide (ITO) thin films have been used as transparent conductive electrodes in many types of optoelectronic devices, because their exceptional properties of high optical transmittance in the visible range and low electrical resistivity. For most of those current applications, ITO films are deposited on rigid substrates, such as glass. However, the deposition of semiconductor thin films on plastic substrates is a research area of increasing interest because the advantages of light weight, flexibility and low cost which represent these substrates. For those reasons in the last few years many research works have been focused to the semiconductor deposition on plastic substrates for the development of flexible devices [1–23]. The low thermal resistance of plastics is one of the major drawbacks to obtain good quality semiconductor thin films, because the deposition process must be restricted to low substrate temperatures. Several papers in literature report about the properties of ITO films deposited on glass substrates at low temperature [4–8]. The relative high electrical resistivity of those as-deposited ITO films was improved after annealing at temperatures above 200 °C. The deposition of ITO films at room temperature on plastic substrates such as acrylic (AC), polycarbonate (PC), polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) has also been reported in several papers. In the paper by H. Shin, et al. [9] ITO films were deposited on PC, AC and PET substrates. The lowest value of resistivity of 9.3 × 10− 4 Ωcm was attained for the films deposited on PC substrates. On the other hand, values of electrical ⁎ Corresponding author. Tel.: +52 (442) 211 99 06; fax: +52 (422) 211 99 39. E-mail address: [email protected] (R. Ramírez-Bon). 0040-6090/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.10.016
resistivity as low as 5.0 × 10− 4 Ωcm [10], 6.6 × 10− 4 Ωcm [11], 7 × 10− 4 Ωcm [12] and 3 × 10− 4 Ωcm [13], have also been reported for ITO films on PET substrates. The techniques that have been utilized for the deposition of good quality ITO films at low temperature include oxygen assisted ion beam evaporation [11], pulsed laser deposition [12] and direct current (dc) or radio frequency (rf) magnetron sputtering [4–10,13]. PEN is a semicrystalline transparent polymeric material quite appropriated to be used in flexible electronic devices because it is easy to process and has good optical and mechanical properties. Its good optical transparence makes it a potential material to be used as substrate or superstrate in solar cells heterostructures. The glass transition temperature of this polymeric material is around 125 °C, however, its working temperature is up to 155 °C. It is resistant to many diluted acid and solvents. Several types of transparent conductive oxides, metallic and semiconductor films have been deposited on PEN substrates such as ITO [13,14], ZnO:Al [15], Ag [16] and CdSe [17]. In references [13] and [14] were reported the conditions for the deposition by rf sputtering of ITO films on PEN substrates with optical transmission higher than 80% and electrical resistivity between 1 × 10− 3 and 3 × 10− 4 Ωcm. Although the abundance of scientific literature about ITO and that it is currently a commercial available material, there are still some important issues which are worthy of investigation. In fact, that is why the interest on this material has remained for years. Since the importance of ITO as transparent conductive oxide in conventional optoelectronic devices is being translated to flexible devices, the deposition of ITO on plastic substrates is a current interesting research area. In this paper we report on the structural, optical and electrical
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chemical composition of the films was determined from energy dispersive X-ray analysis using a Philips XL 30 ESEM microscope. A Hall measurement system operating on a magnetic field of 0.5 T, perpendicular to the sample surface, was used to measure the carrier concentration, Hall mobility and electrical resistivity by the Van der Pauw method. 3. Results and discussion 3.1. Structural properties
Fig. 1. XRD patterns of ITO films deposited on PEN substrates at different rf powers during 10 min.
properties of the ITO films deposited by rf magnetron sputtering on PEN substrates at room temperature. We studied the influence of the rf power and deposition time on the properties of the films. We provide a set of favorable conditions such as non-heating substrate (low deposition temperature), pure argon plasma which yield to the sputtering deposition of good quality polycrystalline ITO films on PEN which is a plastic substrate of current interest in optoelectronic flexible devices applications. From our results it is possible to determine the most appropriated conditions to deposit (222) or (400) oriented ITO films. 2. Experimental details The ITO films were deposited by the rf magnetron sputtering technique on PEN substrates at room temperature. A sinterized ITO target of 5.08 cm diameter containing In2O3 90 wt.% and SnO2 10 wt.% was utilized. The distance between target and substrate was 5 cm. Before deposition, the deposition chamber was evacuated to a base pressure of 1.3 × 10− 4 Pa. The sputtering gas was only argon, flowing to the chamber at a rate of 50 cc/min. Two sets of films were deposited at different rf powers and deposition times, respectively. For the first set of films the rf power was varied from 100 to 130 W and the deposition time was 10 min for all the films. The rf power was set at these lower values to avoid substrate heating by the plasma during film deposition, however the film deposited at 130 W showed some damage by this effect. The second set of films was deposited at 100 W for deposition times from 10 to 30 min. The thickness of the films was determined from profilometry measurements using a contact probe Dektar II profilometer. The crystalline structure was investigated by X-ray diffraction (XRD) using a Rigaku D-Max/2100 diffractometer equipped with a thin film attachment in order to measure in the glancing angle mode equivalent to the asymmetric Bragg-Brentano geometry. The experimental conditions for the XRD measurements were: step size = 0.04 degrees, scan speed = 5 degrees/min, and scan time = 10 min. The optical transmission spectra of the films were measured with a Film Tek™ 3000 spectrometer in the wavelength range from 240 to 840 nm. The
In Fig. 1 are shown the XRD patterns of the ITO films deposited on PEN substrates at different rf powers, during 10 min of deposition time. Two broad and intense diffraction bands centered at about 15 and 26° are observed in all the patterns evidencing the semicrystalline structure of the PEN substrates. The three most intense diffraction peaks at about 30, 35 and 51° in these patterns coincide with those produced by the (222), (400) and (440) crystalline planes of the bixbite cubic structure of indium oxide [18]. In order to estimate the structural parameters of the ITO films deposited on PEN substrates, the (222) and (400) diffraction peaks in the XRD patterns of all the films were fit to Gaussian functions to determine the intensity (height), Bragg angle (center) and full width at half maximum (FWHM) , as well as the area of the peaks. In Table 1 are shown the values of the ratio of the intensity of (222) diffraction peak to that of the (400) one, (I222/I400), calculated like the quotient of the heights of the corresponding peaks. The ratio of the areas of such diffraction peaks, A222/A400, are also shown in Table 1. The observed decrease in the values of both intensity and area ratios indicates a change in the preferred crystalline orientation of the films produced by the increase of rf power. The films deposited at 100 W show a preferred crystalline orientation along the [222] or equivalent [111] direction, which gradually changes to the [400] or equivalent [100] direction for the film deposited with rf power of 130 W. This change of the preferred crystalline orientation in rf sputtered ITO films has also been observed and explained in previous papers [19-21]. The grain orientation in specific crystallographic directions is related with minimization of free surface energy density of the film and the energy density of the interface between the film and substrate [22]. The mobility of adatoms at the substrate surface depends on their energy when they reach the surface and on the substrate temperature. Thus, the adatoms can move on the substrate surface to equilibrium positions according to energy minimization principles and promote preferred crystalline orientation growth. The (222) crystalline orientation is often observed in sputtered ITO films and it has been related with a crystalline growth by the incidence on the substrate of sputtered thermalized atoms, that is, atoms which reduce their energy after successive collisions to the thermal energy kT [19]. In contrast, the (400) crystalline orientation is related with crystalline growth by the incidence on the substrate of sputtered atoms with higher energy. There are at least two observations to support these results. The first
Table 1 Structural parameters of ITO films deposited on PEN substrates as a function of rf power and deposition time t (min)
P (W)
d222 (Å)
d400 (Å)
Δd/do (%) (222)
(400)
Δa/ao (%)
L (nm)
I(222)/ I(400
A(222)/ A(400)
10 10 10 10 15 25 30
100 110 120 130 100 100 100
2.946 2.945 2.943 2.942 2.943 2.942 2.941
2.545 2.545 2.545 2.544 2.545 2.543 2.543
0.86 0.82 0.75 0.72 0.76 0.73 0.69
0.63 0.63 0.63 0.59 0.61 0.53 0.53
0.70 0.67 0.65 0.59 0.64 0.58 0.58
19.3 20.0 21.4 21.5 20.2 23.1 22.8
4.35 1.68 1.42 0.64 1.03 0.20 0.17
3.95 1.68 1.25 0.57 1.20 0.22 0.17
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one deals with the fact that evaporated ITO films present typically the (222) crystalline orientation, meanwhile many sputtered ITO films have the (400) crystalline orientation [20,23]. The energy of the sputtered atoms is about 4–10 eV higher than that of evaporated atoms [23,24], thus for the same target-substrate or source-substrate distance, the energy of the atoms reaching the substrate will be higher for the case of sputtered atoms. On the other hand, there are reports comparing the properties of dc and rf sputtered ITO films, where the (400) and (222) crystalline orientations observed in dc and rf sputtered films, respectively, are explained as a consequence of the higher energy of the dc sputtered particles reaching the growing film [20]. For the case of the sputtering deposition process, the thermalization of the energetic sputtered atoms from the target occurs in the region nearest to the target to a maximum thermalization distance where the atoms decrease their energy by successive collisions to kT. The thermalization distance depends on the mass and initial energy of the sputtered atoms from the target. The initial energy of the sputtered atoms depends on the growth parameters such as the rf power, in such a way that adjusting the growth parameters it is possible to modify the thermalization distance to values larger than the target-substrate distance. In this case, the substrate lies inside the thermalization region and then the atoms arrive to the substrate surface before being thermalized. As stated above, it has been found, for the case of ITO sputtered films, that the (400) crystalline orientation is promoted when the sputtered particles arrive to the substrate surface with higher energy. In our case the target-substrate distance was 5 cm and based in the previous discussion, the evolution of the XRD patterns of the ITO films with the rf power could be explained as follows. The (222) crystalline orientation of the films deposited at rf power of 100 W indicates that the sputtered particles arriving the growing film are thermalized and therefore the thermalization distance for these growth conditions is shorter than 5 cm. The gradual modification in the (222) crystalline orientation to the (400) one is due to the gradual increase of the thermalization distance with the increase of the rf power. The (400) crystalline orientation of the ITO films deposited at 130 W of rf power indicates that for these growth conditions the thermalization distance is larger than 5 cm. Additional analysis of the XRD measurements was done to obtain the interplanar distances between the (222) and (400) crystalline planes, d222 and d400, the percentage of variation of these distances, Δd/do, the percentage of variation of the lattice constant, Δa/ao, and the grain size, L, of the ITO films on PEN substrates as a function of the rf power, P. The results are shown in Table 1. The interplanar distances d222 and d400 were calculated using the Bragg formula. The grain size of the films was calculated from the FWHM of the peaks by means of the Debye-Scherrer formula [25]. The values of the lattice constant of ITO films were taken as the average of the values obtained from d222 and d400 and the corresponding Miller indexes. The lattice constants and interplanar distances were compared to those of bixbite cubic structure of indium oxide, which lattice constant is 10.118 Å [18] and it can be seen that the crystalline lattice of the ITO films is expanded in both, [222] and [400] crystalline directions. The expansion of the crystalline lattice of In2O3:Sn in comparison to that of undoped In2O3 can be explained in terms of the stoichiometry of the films. The oxygen deficiency, the substitution of Sn ions into In ions sites and/or the inclusion of Sn ions in interstitial sites produce an expansion of the crystalline lattice in ITO films [20,21,26]. It has been found that the lattice constant of ITO ceramics expands up to 10.125 Å when the content of Sn in the In2O3 matrix is 6 at.% [27], corresponding to a percentage of variation respect to the lattice constant of the undoped matrix of 0.069%. Residual strain effects due to the difference in the thermal expansion coefficient between the film and substrate can also yield to an expansion of the crystalline lattice of the ITO films [28]. In our case, as shown in Table 1, the
expansion along the [222] direction is larger than that for the [400] direction. It can be observed a decrease in the values of d222 distance and consequently a decrease in its percentage of variation with increasing rf power. On the other hand, the d400 distance is not modified by the increase of the rf power in the sputtered ITO films, there is only a slight decrease when the power increase from 120 to 130 W. The lattice constant of ITO films relaxes with rf power as consequence of the decreasing of the d222 distance. Taking into account that the lattice constant expansion for the ITO films due to the content of Sn ions, which is about 2.3 at.%, should be of the order of 0.049% at. [27], the experimental results in Table 1 show that there is an additional compressive stress producing larger expansion in the crystalline lattice of the ITO films. The compressive stress decreases with rf power and the crystalline lattice expansion decreases from 0.70% at 100 W to 0.59% at 130 W. A similar effect produced by the increasing of substrate temperature has been observed in sputtered ITO films deposited on glass substrates [28]. The values of percentage of lattice constant expansion shown in Table 1 are smaller than the corresponding values reported for rf sputtered ITO films on glass substrates, which are between 1 and 2.2% [29]. Even in unheated substrates, values of lattice expansion larger than 2% have been reported for ITO films deposited at low pressure [30,31]. Thus, although the large thermal expansion coefficient of PEN substrates (2 × 10− 5 °C− 1) the crystalline lattice distortion observed in ITO films was not as large as could be expected. This is a desirable characteristic of ITO films because larger lattice distortion can result in generation of structural defects which degrades the electrical properties [32]. In Fig. 2 are shown the XRD patterns of the ITO films deposited during different times at 100 W. As expected, the films get thicker at larger deposition times as can be seen by the increase in the intensity of the diffraction peaks. It is also observed a gradual change in the crystalline orientation of the films, from the (222) orientation for the film deposited for 10 min to the (400) for the film deposited for 30 min. In Table 1 are also shown the values of both intensity I222/I400
Fig. 2. XRD patterns of ITO films deposited on PEN substrates at 100 W during 10, 15, 25 and 30 min.
M.G. Sandoval-Paz, R. Ramírez-Bon / Thin Solid Films 517 (2009) 2596–2601
Fig. 3. Optical transmission spectra of ITO films deposited on PEN substrates at different rf powers during 10 min.
and area A222/A400 ratios for this set of films. As discussed above, the (400) crystalline orientation in ITO films is promoted by high energy adatoms with enough mobility to incorporate in the higher energy configuration of (400) crystalline oriented grains. Since it was concluded that the thermalization distance for 100 W of rf power was shorter than 5 cm, which is also the target-substrate distance for this set of films, the change of crystalline orientation from (222) to (400) observed in Fig. 2 with the increase of film thickness, keeping the other growth parameters unmodified, can not be explained by the energy of the sputtered atoms arriving at the substrate surface. This effect has also been reported in previous papers about ITO films [23,26]. There is not still a full explanation about this effect. It has been argued that there is always a competition between the film growth along the (222) and (400) orientations; at the beginning of the growing process the film would be metastable with predominating (222) orientation and at larger thickness, over the time, the film would be rather stable with preferred (400) orientation [23]. In this explanation, nothing is said about the reason for the change of preferred orientation during the film growth. A plausible, general description of the observed change from the (222) crystalline orientation to the (400) one, which considers the evolution of the configuration of the substrate surface-growing film system during the film deposition, could be as follows: As stated above, the grain orientation in specific crystallographic directions is defined by minimum energy configurations of the film-substrate system. The contributions to the energy of the system are the free surface energy of the film and the surface energy of the interface between the film and substrate. It could be expected that at the beginning of the deposition process the main contribution to the system energy comes from the surface energy, because it is inversely proportional to film thickness. At this deposition stage, the grains grow in the crystalline orientation which minimizes the surface energy. As the thickness of the film increases and the heating due to the plasma rises its temperature, the contribution of the surface energy decreases and that of the film free surface energy increases. When the main contribution to the system energy is that of the film free surface the minimum energy configuration changes and there is a transition, where the grains grow in a different crystalline orientation. Thus, the predominating (222) crystalline orientation, observed in our ITO films deposited at shorter deposition times, and the transition to the (400) orientation at larger deposition times could be assigned to minimum energy configurations of the surface energy and film free surface energy, respectively.
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Fig. 4. Optical transmission spectra of ITO films deposited on PEN substrates at 100 W during 10, 15, 25 and 30 min.
The additional structural information obtained from XRD measurements, displayed in Table 1, shows that the increase of deposition time produces a similar effect than the increase of rf power on the properties of ITO films. The expansion of the crystalline lattice of ITO films is also larger in the [222] crystalline direction than that in the [400] one and both interplanar distances relaxes as the deposition time (thickness) increases. The percentage of variation of the lattice constant also reduces with the time deposition from 0.70 to 0.58% for the films deposited for 10 min and 30 min respectively. The deformation of the crystalline lattice is also due mainly to the effect of strain by a compressive stress. The values of grain size of the ITO films (see Table 1) are similar to those reported in literature for sputtered ITO films [33,34]. It is also observed that the grain size of the ITO films slightly increases with film thickness and rf power as reported in other papers [12,34]. 3.2. Optical properties Optical transmission (T) spectra measurements taken at normal incidence were performed in the UV–Vis range to analyze the optical properties of the ITO films on PEN substrates. In Fig. 3 are shown the T spectra of the films deposited at different rf powers during 10 min. In this graph it was also included the T spectrum of the bare PEN substrate as reference. It can be observed that the optical transmission of PEN is about 83%, with absorption edge around 390 nm. The values of T for the ITO films attain values up to 80% for rf power of 100 and 110 W. The film deposited at 130 W had a whitish appearance due to the damage produced to the substrate by the heating of the plasma at this rf power, that is why the optical transmission for this film drops to values between 50 and 60%. The T spectra of all the ITO films show the absorption edge of the PEN substrate. Fig. 4 shows the T spectra for the
Table 2 Thickness, electrical resistivity, sheet resistance, average optical transmission in the visible region and figure of merit of ITO films deposited on PEN substrates at 100 W as a function of deposition time t (min) 10 15 20 25 30
d (nm) 154 236 347 431 523
ρ (Ωcm) −3
2.8 × 10 3.6 × 10− 3 4.9 × 10− 4 2.3 × 10− 3 1.9 × 10− 3
Rs (Ω/sq)
Tvis(%)
ϕ × 10− 3 (Ω− 1)
183 152.7 14.19 53.2 37.5
93.1 90.9 91.9 91.6 93.5
2.64 2.51 30.54 7.87 13.55
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films deposited for 5, 15 and 30 min at 100 W. It was also included the spectra of the PEN substrate. The values of T for the ITO films are in the range of 75–80% in the visible spectra region. The spectra of the films deposited for 15 and 30 min display the characteristic interference fringes produced by their larger thickness. H. Han et al. [14] have reported the optical transmission in the UV–Vis spectral region of ITO films deposited on PEN substrates. They observed a decreasing of T (at wavelength of 550 nm) from 85% to 64% when the thickness of the films increased from 50 to 400 nm. In our case, the decrease of T in the whole measured wavelength range due to the increase of film thickness is not so drastic, in fact, the value of T(550 nm) is about 80% for the three ITO films as can be seen in Fig. 4, although the thickness of the films increased from 154 to 431 nm as shown in Table 2. Neither the experimental results in Fig. 3 nor those in Fig. 4 show a clear evidence of the influence of the preferred crystalline orientation of the ITO films on their optical transparency. 3.3. Electrical properties Since the films deposited at 100 W had the higher values of T in the visible range, we focused the study of the electrical properties on these ITO films. The dependence of the electrical resistivity, ρ, Hall carrier mobility, μH, and carrier concentration, n, as a function of deposition time for ITO films grown on PEN substrates are shown in Fig. 5. It is observed an opposite behavior with deposition time between electrical resistivity and carrier concentration as expected. The carrier concentration in ITO films is controlled by electrons from oxygen ionized vacancies and by electrons from Sn4+ donors replacing In3+ in the In2O3 crystalline lattice [14,35]. The increasing of carrier concentration with deposition time has been related with the higher crystallinity of the thicker ITO films [12,36]. As the crystalline lattice becomes higher ordered, oxygen vacancies and Sn4+ ions have a higher probability to be located at thermodynamically propitious places to donate free electrons and then contribute to increase the carrier
concentration. In our case, the increase of n for the sample deposited for 20 min could be explained by the increase in the crystallinity of the ITO films with deposition time observed in Fig. 2. However, at larger deposition times, 25 and 30 min, the values of n drop to smaller values. On the other hand, the behavior of carrier mobility of ITO films shows a drop from the film deposited for 10 min to the one deposited for 20 min. At larger deposition times the carrier mobility of the films does not experience strong variations. It is well known that carrier mobility of ITO films is strongly influenced by their degree of crystallinity. The grain boundary scattering of free carriers is sometimes an important factor which reduces the carrier mobility of the films [10,26,37]. Another important scattering mechanism of free carriers which controls their mobility in ITO films is scattering by neutral and ionized impurity centers. It has been reported that the increase of carrier concentration in ITO films reduces their carrier mobility, which suggests that scattering by ionized impurity centers based on oxygen vacancies is the main scattering mechanism in those films [4,6,19,38]. In our case the carrier mobility is lower in the thicker films with larger grain size and carrier concentration, indicating that the mobility is not dominated by grain boundaries but by scattering by ionized impurity centers. The values of electrical resistivity (~ 10− 3 Ωcm) for ITO films on PEN substrates obtained in this work are similar to those reported in reference [14] for the same type of substrate. In general, the values of electrical resistivity of ITO films on plastic substrates are larger than the values observed for films on glass substrates [14,39]. One of the reasons which could explain this result is that surface roughness in plastic substrates is larger than in glass ones and it is transferred to the ITO films causing electron scattering. For example, the higher resistivity measured in ITO films deposited on 125 μm thick PEN substrates compared to that measured on films deposited on 200 μm thick ones was attributed to the higher surface roughness determined in the thinner substrate from AFM measurements [14]. In Table 2 are shown the values of the thickness, d, electrical resistivity, ρ, sheet resistance, Rs, average optical transmission in the visible region, Tvis, and the figure of merit, ϕ, defined as ϕ = T10 vis/Rs [37,39–41] for the ITO films as a function of deposition time. The optical transmission spectra of the ITO-PEN substrate system were corrected employing the optical spectrum of the bare PEN substrate to determine the values of Tvis of the ITO films. The ITO film deposited for 20 min had the lower electrical resistivity and the higher figure of merit. The value of ϕ for this ITO film is in the same order of magnitude as the reported for films deposited on glass [37,40,41] and PET [39] substrates. Since the figure of merit is a quantity which takes in accounts both the optical transparence and electrical resistivity to evaluate the quality of ITO films, it can be concluded that the films deposited for 20 min have the best properties to be applied as a transparent conductive oxide. 4. Conclusions
Fig. 5. Dependence of electrical resistivity (ρ), Hall carrier mobility (μH) and carrier concentration (N) with deposition time of ITO films deposited on PEN substrates at 100 W.
In this work we have reported the properties of ITO films deposited by rf magnetron sputtering on unheated PEN substrates. We studied the influence of the rf power and deposition time on the structural, optical and electrical properties of the ITO films. We found that the films deposited at low rf power or shorter deposition times had the (222) preferred crystalline orientation, which was modified to the (400) one at higher rf power or larger deposition times. The sputtering process at 100 W was quite appropriated for the deposition on PEN substrates because they are not damaged or softened by the exposition to the plasma during the deposition process. Highly crystalline ITO films, with (222) and (400) crystalline orientation, with low lattice crystalline distortion, were deposited at this rf power with electrical resistivity down to the order of 10− 4 Ωcm and sheet resistance between 14 and 53 Ω/sq. The optical transmission of these films is around 80% in the visible spectra region.
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Acknowledgements We acknowledge the technical assistance of P. García-Jiménez, J. E. Urbina-Alvárez, M. A. Hernández-Landaverde, and R. Flores-Farías. This work was partially supported by CONACyT (Project No. 34514-U). References [1] S. Kyu Park, J. In Han, W. Keun Kim, M. Gi Kwak, Thin Solid Films 397 (2001) 49. [2] K. Soo Ong, J. Hu, R. Shrestha, F. Zhu, S. Jin Chua, Thin Solid Films 477 (2005) 32. [3] S. Richter, M. Ploetner, W.J. Fischer, M. Schneider, P.T. Nguyen, W. Plieth, N. Kiriy, H.-J.P. Adler, Thin Solid Films 477 (2005) 140. [4] P.K. Song, H. Akao, M. Kamei, Y. Shigesato, I. Yasui, Jpn. J. Appl. Phys. 38 (1999) 5524. [5] C. Guillén, J. Herrero, Vacuum 80 (2006) 615. [6] P. Canhola, N. Martins, L. Raniero, S. Pereira, E. Fortunato, I. Ferreira, R. Martins, Thin Solid Films 487 (2005) 271. [7] A.M. Gheidari, E.A. Soleimani, M. Mansorhoseini, S. Mohajerzadeh, N. Madani, W. Shams-Kolahi, Mater. Res. Bull. 40 (2005) 1303. [8] S. Takayama, T. Sugawara, A. Tanaka, T. Himuro, J. Vac. Sci. Technol. A 21 (2003) 1351. [9] J.H. Shin, S.H. Shin, J.I. Park, H.H. Kim, J. Appl. Phys. 89 (2001) 5199. [10] F.L. Wong, M.K. Fung, S.W. Tong, C.S. Lee, S.T. Lee, Thin Solid Films 466 (2004) 225. [11] J.W. Bae, H.J. Kim, J.S. Kim, Y.H. Lee, N.E. Lee, G.Y. Yeom, Y.W. Ko, Surf. Coat. Technol. 131 (2000) 196. [12] H. Kim, J.S. Horwitz, G.P. Kushto, Z.H. Kafafi, D.B. Chrisey, Appl. Phys. Lett. 79 (2001) 284. [13] P.F. Carcía, R.S. McLean, M.H. Reilly, Z.G. Li, L.J. Pillione, R.F. Messier, J. Vac. Sci. Technol. A 21 (2003) 745. [14] H. Han, D. Adams, J.W. Mayer, T.L. Alford, J. Appl. Phys. 98 (2005) 083705. [15] M. Fonrodona, J. Escarre, F. Villar, D. Soler, J.M. Asensi, J. Bertomeu, J. Andreu, Solar Energy Mater. Solar Cells 89 (2005) 37. [16] Y. Zoo, D. Adams, J.W. Mayer, T.L. Alford, Thin Solid Films 513 (2006) 170. [17] J.M. Lee, C.P. Judge, S.W. Wright, Solid-State Electron. 44 (2000) 1431.
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[18] Powder Diffraction File, Joint Committee on Powder Diffraction Standards, Grantin-Aid Report, Fargo, North Dakota, 1990, Card 060416. [19] C.V.R. Vassant Kumar, A. Mansingh, J. Appl. Phys. 65 (1989) 1270. [20] Z. Qiao, D. Mergel, Thin Solid Films 484 (2005) 146. [21] E. Terzini, P. Thilakan, C. Minarini, Mat. Sci. Eng. B 77 (2000) 110. [22] L.B. Freund, S. Surech, Thin Film Materials, Cambridge University Press, Cambridge, 2003. [23] H.C. Lee, O. Ok Park, Vacuum 80 (2006) 880. [24] D.M. Mattox, Handbook of Physical Vapor Deposition (PVD) Processing, William Andrew Publishing, Norwich, NY, 1998. [25] A.R. West, Solid State Chemistry and its Applications, John Wiley and Sons, Chichester, 1984. [26] H. Kim, J.S. Horwitz, G. Kushto, A. Pique, Z.H. Kafafi, C.M. Gilmore, D.B. Chrisey, J. Appl. Phys. 88 (2000) 6021. [27] N. Nadaud, N. Lequeux, M. Nanot, J. Jove, T. Roisnel, J. Solid State. Chem. 135 (1988) 140. [28] L.J. Meng, M.P. dos Santos, Thin Solid Films 322 (1998) 56. [29] D. Mergel, W. Stass, G. Ehl, D. Barthel, J. Appl. Phys. 88 (2000) 2437. [30] R.N. Joshi, V.P. Singh, J.C. McClure, Thin Solid Films 257 (1995) 32. [31] L.J. Meng, M.P. dos Santos, Thin Solid Films 303 (1997) 151. [32] S.K. Park, J.I. Han, W.K. Kim, M.G. Kwak, Thin Solid Films 397 (2001) 49. [33] R. Dash, K. Adhikary, S. Ray, Appl. Surf. Sci. 253 (2007) 6068. [34] H.H. Yu, S.J. Hwang, M.C. Tseng, C.C. Tseng, Opt. Commun. 259 (2006) 187. [35] I. Hamberg, C.G. Granqvist, Phys. Rev. B 30 (1984) 3240. [36] Z. Qiao, R. Latz, D. Mergel, Thin Solid Films 466 (2004) 250. [37] M.G. Zebaze Kana, E. Centurioni, D. Iencinella, C. Summonte, Thin Solid Films 500 (2006) 203. [38] R.B. Hadj Tahar, T. Ban, Y. Ohya, Y. Takahashi, J. Appl. Phys. 83 (1998) 2631. [39] A.K. Kulkarni, T. Lim, M. Khan, K.H. Schulz, J. Vac. Sci. Technol. A 16 (1998) 1636. [40] H. Kim, C.M. Gilmore, A. Piqué, J.S. Horwitz, H. Mattousi, H. Murata, Z.H. Kafafi, D.B. Chrisey, J. Appl. Phys. 86 (1999) 6451. [41] E. Aperathitis, M. Bender, V. Cimalla, G. Ecke, M. Mondreanu, J. Appl. Phys. 94 (2003) 1258.
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / t s f
Indium tin oxide films deposited on polyethylene naphthalate substrates by radio frequency magnetron sputtering M.G. Sandoval-Paz, R. Ramírez-Bon ⁎ Centro de Investigación y Estudios Avanzados del IPN-Unidad Querétaro, Apdo. postal 1-798, Querétaro, Qro., 76001, México
a r t i c l e
i n f o
Article history: Received 20 June 2007 Received in revised form 25 August 2008 Accepted 7 October 2008 Available online 17 October 2008 Keywords: Indium tin oxide Structural properties Optical properties Electrical properties and measurements Sputtering X-ray diffraction
a b s t r a c t Indium tin oxide (ITO) thin films were deposited on unheated polyethylene naphthalate substrates by radiofrequency (rf) magnetron sputtering from an In2O3 (90 wt.%) containing SnO2 (10 wt.%) target. We report the structural, electrical and optical properties of the ITO films as a function of rf power and deposition time. Low rf power values, in the range of 100–130 W, were employed in the deposition process to avoid damage to the plastic substrates by heating caused by the plasma. The films were analyzed by X-ray diffraction and optical transmission measurements. A Hall measurement system was used to measure the carrier concentration and electrical resistivity of the films by the Van der Pauw method. The X-ray diffraction measurements analysis showed that the ITO films are polycrystalline with the bixbite cubic crystalline phase. It is observed a change in the preferential crystalline orientation of the films from the (222) to the (400) crystalline orientation with increasing rf power or deposition time in the sputtering process. The optical transmission of the films was around 80% with electrical resistivity and sheet resistance down to 4.9 × 10- 4 Ωcm and 14 Ω/sq, respectively. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Indium tin oxide (ITO) thin films have been used as transparent conductive electrodes in many types of optoelectronic devices, because their exceptional properties of high optical transmittance in the visible range and low electrical resistivity. For most of those current applications, ITO films are deposited on rigid substrates, such as glass. However, the deposition of semiconductor thin films on plastic substrates is a research area of increasing interest because the advantages of light weight, flexibility and low cost which represent these substrates. For those reasons in the last few years many research works have been focused to the semiconductor deposition on plastic substrates for the development of flexible devices [1–23]. The low thermal resistance of plastics is one of the major drawbacks to obtain good quality semiconductor thin films, because the deposition process must be restricted to low substrate temperatures. Several papers in literature report about the properties of ITO films deposited on glass substrates at low temperature [4–8]. The relative high electrical resistivity of those as-deposited ITO films was improved after annealing at temperatures above 200 °C. The deposition of ITO films at room temperature on plastic substrates such as acrylic (AC), polycarbonate (PC), polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) has also been reported in several papers. In the paper by H. Shin, et al. [9] ITO films were deposited on PC, AC and PET substrates. The lowest value of resistivity of 9.3 × 10− 4 Ωcm was attained for the films deposited on PC substrates. On the other hand, values of electrical ⁎ Corresponding author. Tel.: +52 (442) 211 99 06; fax: +52 (422) 211 99 39. E-mail address: [email protected] (R. Ramírez-Bon). 0040-6090/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.10.016
resistivity as low as 5.0 × 10− 4 Ωcm [10], 6.6 × 10− 4 Ωcm [11], 7 × 10− 4 Ωcm [12] and 3 × 10− 4 Ωcm [13], have also been reported for ITO films on PET substrates. The techniques that have been utilized for the deposition of good quality ITO films at low temperature include oxygen assisted ion beam evaporation [11], pulsed laser deposition [12] and direct current (dc) or radio frequency (rf) magnetron sputtering [4–10,13]. PEN is a semicrystalline transparent polymeric material quite appropriated to be used in flexible electronic devices because it is easy to process and has good optical and mechanical properties. Its good optical transparence makes it a potential material to be used as substrate or superstrate in solar cells heterostructures. The glass transition temperature of this polymeric material is around 125 °C, however, its working temperature is up to 155 °C. It is resistant to many diluted acid and solvents. Several types of transparent conductive oxides, metallic and semiconductor films have been deposited on PEN substrates such as ITO [13,14], ZnO:Al [15], Ag [16] and CdSe [17]. In references [13] and [14] were reported the conditions for the deposition by rf sputtering of ITO films on PEN substrates with optical transmission higher than 80% and electrical resistivity between 1 × 10− 3 and 3 × 10− 4 Ωcm. Although the abundance of scientific literature about ITO and that it is currently a commercial available material, there are still some important issues which are worthy of investigation. In fact, that is why the interest on this material has remained for years. Since the importance of ITO as transparent conductive oxide in conventional optoelectronic devices is being translated to flexible devices, the deposition of ITO on plastic substrates is a current interesting research area. In this paper we report on the structural, optical and electrical
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chemical composition of the films was determined from energy dispersive X-ray analysis using a Philips XL 30 ESEM microscope. A Hall measurement system operating on a magnetic field of 0.5 T, perpendicular to the sample surface, was used to measure the carrier concentration, Hall mobility and electrical resistivity by the Van der Pauw method. 3. Results and discussion 3.1. Structural properties
Fig. 1. XRD patterns of ITO films deposited on PEN substrates at different rf powers during 10 min.
properties of the ITO films deposited by rf magnetron sputtering on PEN substrates at room temperature. We studied the influence of the rf power and deposition time on the properties of the films. We provide a set of favorable conditions such as non-heating substrate (low deposition temperature), pure argon plasma which yield to the sputtering deposition of good quality polycrystalline ITO films on PEN which is a plastic substrate of current interest in optoelectronic flexible devices applications. From our results it is possible to determine the most appropriated conditions to deposit (222) or (400) oriented ITO films. 2. Experimental details The ITO films were deposited by the rf magnetron sputtering technique on PEN substrates at room temperature. A sinterized ITO target of 5.08 cm diameter containing In2O3 90 wt.% and SnO2 10 wt.% was utilized. The distance between target and substrate was 5 cm. Before deposition, the deposition chamber was evacuated to a base pressure of 1.3 × 10− 4 Pa. The sputtering gas was only argon, flowing to the chamber at a rate of 50 cc/min. Two sets of films were deposited at different rf powers and deposition times, respectively. For the first set of films the rf power was varied from 100 to 130 W and the deposition time was 10 min for all the films. The rf power was set at these lower values to avoid substrate heating by the plasma during film deposition, however the film deposited at 130 W showed some damage by this effect. The second set of films was deposited at 100 W for deposition times from 10 to 30 min. The thickness of the films was determined from profilometry measurements using a contact probe Dektar II profilometer. The crystalline structure was investigated by X-ray diffraction (XRD) using a Rigaku D-Max/2100 diffractometer equipped with a thin film attachment in order to measure in the glancing angle mode equivalent to the asymmetric Bragg-Brentano geometry. The experimental conditions for the XRD measurements were: step size = 0.04 degrees, scan speed = 5 degrees/min, and scan time = 10 min. The optical transmission spectra of the films were measured with a Film Tek™ 3000 spectrometer in the wavelength range from 240 to 840 nm. The
In Fig. 1 are shown the XRD patterns of the ITO films deposited on PEN substrates at different rf powers, during 10 min of deposition time. Two broad and intense diffraction bands centered at about 15 and 26° are observed in all the patterns evidencing the semicrystalline structure of the PEN substrates. The three most intense diffraction peaks at about 30, 35 and 51° in these patterns coincide with those produced by the (222), (400) and (440) crystalline planes of the bixbite cubic structure of indium oxide [18]. In order to estimate the structural parameters of the ITO films deposited on PEN substrates, the (222) and (400) diffraction peaks in the XRD patterns of all the films were fit to Gaussian functions to determine the intensity (height), Bragg angle (center) and full width at half maximum (FWHM) , as well as the area of the peaks. In Table 1 are shown the values of the ratio of the intensity of (222) diffraction peak to that of the (400) one, (I222/I400), calculated like the quotient of the heights of the corresponding peaks. The ratio of the areas of such diffraction peaks, A222/A400, are also shown in Table 1. The observed decrease in the values of both intensity and area ratios indicates a change in the preferred crystalline orientation of the films produced by the increase of rf power. The films deposited at 100 W show a preferred crystalline orientation along the [222] or equivalent [111] direction, which gradually changes to the [400] or equivalent [100] direction for the film deposited with rf power of 130 W. This change of the preferred crystalline orientation in rf sputtered ITO films has also been observed and explained in previous papers [19-21]. The grain orientation in specific crystallographic directions is related with minimization of free surface energy density of the film and the energy density of the interface between the film and substrate [22]. The mobility of adatoms at the substrate surface depends on their energy when they reach the surface and on the substrate temperature. Thus, the adatoms can move on the substrate surface to equilibrium positions according to energy minimization principles and promote preferred crystalline orientation growth. The (222) crystalline orientation is often observed in sputtered ITO films and it has been related with a crystalline growth by the incidence on the substrate of sputtered thermalized atoms, that is, atoms which reduce their energy after successive collisions to the thermal energy kT [19]. In contrast, the (400) crystalline orientation is related with crystalline growth by the incidence on the substrate of sputtered atoms with higher energy. There are at least two observations to support these results. The first
Table 1 Structural parameters of ITO films deposited on PEN substrates as a function of rf power and deposition time t (min)
P (W)
d222 (Å)
d400 (Å)
Δd/do (%) (222)
(400)
Δa/ao (%)
L (nm)
I(222)/ I(400
A(222)/ A(400)
10 10 10 10 15 25 30
100 110 120 130 100 100 100
2.946 2.945 2.943 2.942 2.943 2.942 2.941
2.545 2.545 2.545 2.544 2.545 2.543 2.543
0.86 0.82 0.75 0.72 0.76 0.73 0.69
0.63 0.63 0.63 0.59 0.61 0.53 0.53
0.70 0.67 0.65 0.59 0.64 0.58 0.58
19.3 20.0 21.4 21.5 20.2 23.1 22.8
4.35 1.68 1.42 0.64 1.03 0.20 0.17
3.95 1.68 1.25 0.57 1.20 0.22 0.17
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one deals with the fact that evaporated ITO films present typically the (222) crystalline orientation, meanwhile many sputtered ITO films have the (400) crystalline orientation [20,23]. The energy of the sputtered atoms is about 4–10 eV higher than that of evaporated atoms [23,24], thus for the same target-substrate or source-substrate distance, the energy of the atoms reaching the substrate will be higher for the case of sputtered atoms. On the other hand, there are reports comparing the properties of dc and rf sputtered ITO films, where the (400) and (222) crystalline orientations observed in dc and rf sputtered films, respectively, are explained as a consequence of the higher energy of the dc sputtered particles reaching the growing film [20]. For the case of the sputtering deposition process, the thermalization of the energetic sputtered atoms from the target occurs in the region nearest to the target to a maximum thermalization distance where the atoms decrease their energy by successive collisions to kT. The thermalization distance depends on the mass and initial energy of the sputtered atoms from the target. The initial energy of the sputtered atoms depends on the growth parameters such as the rf power, in such a way that adjusting the growth parameters it is possible to modify the thermalization distance to values larger than the target-substrate distance. In this case, the substrate lies inside the thermalization region and then the atoms arrive to the substrate surface before being thermalized. As stated above, it has been found, for the case of ITO sputtered films, that the (400) crystalline orientation is promoted when the sputtered particles arrive to the substrate surface with higher energy. In our case the target-substrate distance was 5 cm and based in the previous discussion, the evolution of the XRD patterns of the ITO films with the rf power could be explained as follows. The (222) crystalline orientation of the films deposited at rf power of 100 W indicates that the sputtered particles arriving the growing film are thermalized and therefore the thermalization distance for these growth conditions is shorter than 5 cm. The gradual modification in the (222) crystalline orientation to the (400) one is due to the gradual increase of the thermalization distance with the increase of the rf power. The (400) crystalline orientation of the ITO films deposited at 130 W of rf power indicates that for these growth conditions the thermalization distance is larger than 5 cm. Additional analysis of the XRD measurements was done to obtain the interplanar distances between the (222) and (400) crystalline planes, d222 and d400, the percentage of variation of these distances, Δd/do, the percentage of variation of the lattice constant, Δa/ao, and the grain size, L, of the ITO films on PEN substrates as a function of the rf power, P. The results are shown in Table 1. The interplanar distances d222 and d400 were calculated using the Bragg formula. The grain size of the films was calculated from the FWHM of the peaks by means of the Debye-Scherrer formula [25]. The values of the lattice constant of ITO films were taken as the average of the values obtained from d222 and d400 and the corresponding Miller indexes. The lattice constants and interplanar distances were compared to those of bixbite cubic structure of indium oxide, which lattice constant is 10.118 Å [18] and it can be seen that the crystalline lattice of the ITO films is expanded in both, [222] and [400] crystalline directions. The expansion of the crystalline lattice of In2O3:Sn in comparison to that of undoped In2O3 can be explained in terms of the stoichiometry of the films. The oxygen deficiency, the substitution of Sn ions into In ions sites and/or the inclusion of Sn ions in interstitial sites produce an expansion of the crystalline lattice in ITO films [20,21,26]. It has been found that the lattice constant of ITO ceramics expands up to 10.125 Å when the content of Sn in the In2O3 matrix is 6 at.% [27], corresponding to a percentage of variation respect to the lattice constant of the undoped matrix of 0.069%. Residual strain effects due to the difference in the thermal expansion coefficient between the film and substrate can also yield to an expansion of the crystalline lattice of the ITO films [28]. In our case, as shown in Table 1, the
expansion along the [222] direction is larger than that for the [400] direction. It can be observed a decrease in the values of d222 distance and consequently a decrease in its percentage of variation with increasing rf power. On the other hand, the d400 distance is not modified by the increase of the rf power in the sputtered ITO films, there is only a slight decrease when the power increase from 120 to 130 W. The lattice constant of ITO films relaxes with rf power as consequence of the decreasing of the d222 distance. Taking into account that the lattice constant expansion for the ITO films due to the content of Sn ions, which is about 2.3 at.%, should be of the order of 0.049% at. [27], the experimental results in Table 1 show that there is an additional compressive stress producing larger expansion in the crystalline lattice of the ITO films. The compressive stress decreases with rf power and the crystalline lattice expansion decreases from 0.70% at 100 W to 0.59% at 130 W. A similar effect produced by the increasing of substrate temperature has been observed in sputtered ITO films deposited on glass substrates [28]. The values of percentage of lattice constant expansion shown in Table 1 are smaller than the corresponding values reported for rf sputtered ITO films on glass substrates, which are between 1 and 2.2% [29]. Even in unheated substrates, values of lattice expansion larger than 2% have been reported for ITO films deposited at low pressure [30,31]. Thus, although the large thermal expansion coefficient of PEN substrates (2 × 10− 5 °C− 1) the crystalline lattice distortion observed in ITO films was not as large as could be expected. This is a desirable characteristic of ITO films because larger lattice distortion can result in generation of structural defects which degrades the electrical properties [32]. In Fig. 2 are shown the XRD patterns of the ITO films deposited during different times at 100 W. As expected, the films get thicker at larger deposition times as can be seen by the increase in the intensity of the diffraction peaks. It is also observed a gradual change in the crystalline orientation of the films, from the (222) orientation for the film deposited for 10 min to the (400) for the film deposited for 30 min. In Table 1 are also shown the values of both intensity I222/I400
Fig. 2. XRD patterns of ITO films deposited on PEN substrates at 100 W during 10, 15, 25 and 30 min.
M.G. Sandoval-Paz, R. Ramírez-Bon / Thin Solid Films 517 (2009) 2596–2601
Fig. 3. Optical transmission spectra of ITO films deposited on PEN substrates at different rf powers during 10 min.
and area A222/A400 ratios for this set of films. As discussed above, the (400) crystalline orientation in ITO films is promoted by high energy adatoms with enough mobility to incorporate in the higher energy configuration of (400) crystalline oriented grains. Since it was concluded that the thermalization distance for 100 W of rf power was shorter than 5 cm, which is also the target-substrate distance for this set of films, the change of crystalline orientation from (222) to (400) observed in Fig. 2 with the increase of film thickness, keeping the other growth parameters unmodified, can not be explained by the energy of the sputtered atoms arriving at the substrate surface. This effect has also been reported in previous papers about ITO films [23,26]. There is not still a full explanation about this effect. It has been argued that there is always a competition between the film growth along the (222) and (400) orientations; at the beginning of the growing process the film would be metastable with predominating (222) orientation and at larger thickness, over the time, the film would be rather stable with preferred (400) orientation [23]. In this explanation, nothing is said about the reason for the change of preferred orientation during the film growth. A plausible, general description of the observed change from the (222) crystalline orientation to the (400) one, which considers the evolution of the configuration of the substrate surface-growing film system during the film deposition, could be as follows: As stated above, the grain orientation in specific crystallographic directions is defined by minimum energy configurations of the film-substrate system. The contributions to the energy of the system are the free surface energy of the film and the surface energy of the interface between the film and substrate. It could be expected that at the beginning of the deposition process the main contribution to the system energy comes from the surface energy, because it is inversely proportional to film thickness. At this deposition stage, the grains grow in the crystalline orientation which minimizes the surface energy. As the thickness of the film increases and the heating due to the plasma rises its temperature, the contribution of the surface energy decreases and that of the film free surface energy increases. When the main contribution to the system energy is that of the film free surface the minimum energy configuration changes and there is a transition, where the grains grow in a different crystalline orientation. Thus, the predominating (222) crystalline orientation, observed in our ITO films deposited at shorter deposition times, and the transition to the (400) orientation at larger deposition times could be assigned to minimum energy configurations of the surface energy and film free surface energy, respectively.
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Fig. 4. Optical transmission spectra of ITO films deposited on PEN substrates at 100 W during 10, 15, 25 and 30 min.
The additional structural information obtained from XRD measurements, displayed in Table 1, shows that the increase of deposition time produces a similar effect than the increase of rf power on the properties of ITO films. The expansion of the crystalline lattice of ITO films is also larger in the [222] crystalline direction than that in the [400] one and both interplanar distances relaxes as the deposition time (thickness) increases. The percentage of variation of the lattice constant also reduces with the time deposition from 0.70 to 0.58% for the films deposited for 10 min and 30 min respectively. The deformation of the crystalline lattice is also due mainly to the effect of strain by a compressive stress. The values of grain size of the ITO films (see Table 1) are similar to those reported in literature for sputtered ITO films [33,34]. It is also observed that the grain size of the ITO films slightly increases with film thickness and rf power as reported in other papers [12,34]. 3.2. Optical properties Optical transmission (T) spectra measurements taken at normal incidence were performed in the UV–Vis range to analyze the optical properties of the ITO films on PEN substrates. In Fig. 3 are shown the T spectra of the films deposited at different rf powers during 10 min. In this graph it was also included the T spectrum of the bare PEN substrate as reference. It can be observed that the optical transmission of PEN is about 83%, with absorption edge around 390 nm. The values of T for the ITO films attain values up to 80% for rf power of 100 and 110 W. The film deposited at 130 W had a whitish appearance due to the damage produced to the substrate by the heating of the plasma at this rf power, that is why the optical transmission for this film drops to values between 50 and 60%. The T spectra of all the ITO films show the absorption edge of the PEN substrate. Fig. 4 shows the T spectra for the
Table 2 Thickness, electrical resistivity, sheet resistance, average optical transmission in the visible region and figure of merit of ITO films deposited on PEN substrates at 100 W as a function of deposition time t (min) 10 15 20 25 30
d (nm) 154 236 347 431 523
ρ (Ωcm) −3
2.8 × 10 3.6 × 10− 3 4.9 × 10− 4 2.3 × 10− 3 1.9 × 10− 3
Rs (Ω/sq)
Tvis(%)
ϕ × 10− 3 (Ω− 1)
183 152.7 14.19 53.2 37.5
93.1 90.9 91.9 91.6 93.5
2.64 2.51 30.54 7.87 13.55
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films deposited for 5, 15 and 30 min at 100 W. It was also included the spectra of the PEN substrate. The values of T for the ITO films are in the range of 75–80% in the visible spectra region. The spectra of the films deposited for 15 and 30 min display the characteristic interference fringes produced by their larger thickness. H. Han et al. [14] have reported the optical transmission in the UV–Vis spectral region of ITO films deposited on PEN substrates. They observed a decreasing of T (at wavelength of 550 nm) from 85% to 64% when the thickness of the films increased from 50 to 400 nm. In our case, the decrease of T in the whole measured wavelength range due to the increase of film thickness is not so drastic, in fact, the value of T(550 nm) is about 80% for the three ITO films as can be seen in Fig. 4, although the thickness of the films increased from 154 to 431 nm as shown in Table 2. Neither the experimental results in Fig. 3 nor those in Fig. 4 show a clear evidence of the influence of the preferred crystalline orientation of the ITO films on their optical transparency. 3.3. Electrical properties Since the films deposited at 100 W had the higher values of T in the visible range, we focused the study of the electrical properties on these ITO films. The dependence of the electrical resistivity, ρ, Hall carrier mobility, μH, and carrier concentration, n, as a function of deposition time for ITO films grown on PEN substrates are shown in Fig. 5. It is observed an opposite behavior with deposition time between electrical resistivity and carrier concentration as expected. The carrier concentration in ITO films is controlled by electrons from oxygen ionized vacancies and by electrons from Sn4+ donors replacing In3+ in the In2O3 crystalline lattice [14,35]. The increasing of carrier concentration with deposition time has been related with the higher crystallinity of the thicker ITO films [12,36]. As the crystalline lattice becomes higher ordered, oxygen vacancies and Sn4+ ions have a higher probability to be located at thermodynamically propitious places to donate free electrons and then contribute to increase the carrier
concentration. In our case, the increase of n for the sample deposited for 20 min could be explained by the increase in the crystallinity of the ITO films with deposition time observed in Fig. 2. However, at larger deposition times, 25 and 30 min, the values of n drop to smaller values. On the other hand, the behavior of carrier mobility of ITO films shows a drop from the film deposited for 10 min to the one deposited for 20 min. At larger deposition times the carrier mobility of the films does not experience strong variations. It is well known that carrier mobility of ITO films is strongly influenced by their degree of crystallinity. The grain boundary scattering of free carriers is sometimes an important factor which reduces the carrier mobility of the films [10,26,37]. Another important scattering mechanism of free carriers which controls their mobility in ITO films is scattering by neutral and ionized impurity centers. It has been reported that the increase of carrier concentration in ITO films reduces their carrier mobility, which suggests that scattering by ionized impurity centers based on oxygen vacancies is the main scattering mechanism in those films [4,6,19,38]. In our case the carrier mobility is lower in the thicker films with larger grain size and carrier concentration, indicating that the mobility is not dominated by grain boundaries but by scattering by ionized impurity centers. The values of electrical resistivity (~ 10− 3 Ωcm) for ITO films on PEN substrates obtained in this work are similar to those reported in reference [14] for the same type of substrate. In general, the values of electrical resistivity of ITO films on plastic substrates are larger than the values observed for films on glass substrates [14,39]. One of the reasons which could explain this result is that surface roughness in plastic substrates is larger than in glass ones and it is transferred to the ITO films causing electron scattering. For example, the higher resistivity measured in ITO films deposited on 125 μm thick PEN substrates compared to that measured on films deposited on 200 μm thick ones was attributed to the higher surface roughness determined in the thinner substrate from AFM measurements [14]. In Table 2 are shown the values of the thickness, d, electrical resistivity, ρ, sheet resistance, Rs, average optical transmission in the visible region, Tvis, and the figure of merit, ϕ, defined as ϕ = T10 vis/Rs [37,39–41] for the ITO films as a function of deposition time. The optical transmission spectra of the ITO-PEN substrate system were corrected employing the optical spectrum of the bare PEN substrate to determine the values of Tvis of the ITO films. The ITO film deposited for 20 min had the lower electrical resistivity and the higher figure of merit. The value of ϕ for this ITO film is in the same order of magnitude as the reported for films deposited on glass [37,40,41] and PET [39] substrates. Since the figure of merit is a quantity which takes in accounts both the optical transparence and electrical resistivity to evaluate the quality of ITO films, it can be concluded that the films deposited for 20 min have the best properties to be applied as a transparent conductive oxide. 4. Conclusions
Fig. 5. Dependence of electrical resistivity (ρ), Hall carrier mobility (μH) and carrier concentration (N) with deposition time of ITO films deposited on PEN substrates at 100 W.
In this work we have reported the properties of ITO films deposited by rf magnetron sputtering on unheated PEN substrates. We studied the influence of the rf power and deposition time on the structural, optical and electrical properties of the ITO films. We found that the films deposited at low rf power or shorter deposition times had the (222) preferred crystalline orientation, which was modified to the (400) one at higher rf power or larger deposition times. The sputtering process at 100 W was quite appropriated for the deposition on PEN substrates because they are not damaged or softened by the exposition to the plasma during the deposition process. Highly crystalline ITO films, with (222) and (400) crystalline orientation, with low lattice crystalline distortion, were deposited at this rf power with electrical resistivity down to the order of 10− 4 Ωcm and sheet resistance between 14 and 53 Ω/sq. The optical transmission of these films is around 80% in the visible spectra region.
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