Effect of RF Sputtering Power and Deposition Time on Optical and Electrical Properties of Indium Tin Oxide Thin Film

Indium tin oxide (ITO) ﬁlms are widely used as transparent conducting electrodes in solar cells, gas sensors, and car windows because of their high electrical conductivity and good optical transparency in the visible region. In this work, ITO thin ﬁlms were prepared by cathodic radio-frequency (RF) sputtering using an ITO target with 90% In 2 O 3 and 10% SnO 2 . The structural properties were studied by X-ray diﬀraction (XRD), scanning electronic microscopy (SEM), and X-ray reﬂectometry (XRR). Electrical measurements were performed by applying the four-point method and studying the Hall Eﬀect. Finally, optical properties were taken by the UV-Vis-NIR spectrophotometry. The eﬀect of the RF power and deposition time on optical and electrical properties was investigated. It is shown that by using a RF power of 110–80W, one can prepare crystalline samples with low resistivity, which is an aimed property for TCO semiconductors. Electrical measurements revealed that the resistivity decreases by increasing the RF power and/or the deposition time.

In this study, we investigated the effect of RF sputtering power and deposition time on optical and electrical properties of ITO thin films. e RF sputtering method was used to ensure good adhesion. In fact, the expansion coefficient mismatch between the ITO films could create microcracks and consequently damage the film. e variation in crystal structure, resistance, and optical properties induced in the ITO samples is discussed in detail. It was found that these ITO films have good optical and electrical properties. In particular, the sheet resistance is lower than that reported in some reports.

Experimental Details
ITO films were deposited at room temperature by the RF sputtering technique in an Alcatel SCM 451. A schematic diagram of the RF sputtering that we used in this study is shown in Figure 1. Initially, the glass substrates were cleaned using acetone and distilled water in ultrasonic bath for 15 min prior to deposition, and the sputtering chamber was evacuated to less than 2 × 10 −6 mbar. Besides, the substratetarget distance was fixed at 7 cm. e target purity is 99.99%, and its diameter is 10 cm. Furthermore, high purity argon gas (99.999%) was introduced with a pressure of 10 −2 mbar as shown in Table 1. Before each deposition, the target was presputtering in argon plasma for 20 min to remove contaminants. No oxygen was added during deposition. e films are deposited at different RF powers (20,50,80, and 110 W) for different deposition times (20, 40, and 60 min) and are shown in Figure 2. e structural characterization was performed using the Rigaku Smart lab SE X-ray diffractometer in the 2θ range 20-80°based on the Cu (Kα) radiation in the Bragg Brentano configuration (λ �1.5418Å). SEM morphology images were used to observe the surfaces of ITO films on the substrate. e optical transmittance was measured in the wavelength range 200-2000 nm, using an UV-Vis-NIR spectrometer (Shimadzu UV-PC spectrophotometer). Electrical resistivity was measured using a lab-manufactured device, to apply the four-point probes method, with the KEITHLEY A Tektronix Company generator/counter (6430 Sub-Femtoamp Remote Source Meter). e TES-CAN VEGA3 scanning electron microscope was used to analyze the surface morphology.

Results and Discussion
XRD spectra of the ITO films prepared with various RF powers and different deposition times are shown in Figure 3. It is observed that the preferred orientation does not depend on the power nor the time as all the crystalline film exhibit (222) preferred orientation only. e spectra corresponding to 20 W reveal that the films are amorphous for all deposition times, or the film thickness is not sufficient. In some previous works, (see, e.g., [7,9,29]), no characteristic diffraction peak of ITO was observed for the films with film with thickness lower than 80 nm. e ITO film was found to go from an amorphous state to a microcrystalline or polycrystalline state with increasing the RF power as shown in Figure 3. e peak is first observed followed by the (222) orientation, and then the (400), (440), and (622) orientations are observed. e observed results are similar to those reported by Ghorannevis et al. [30].
To study the effects of the grain size of ITO films on electrical and optical properties, the average crystallite size was calculated using Scherrer's equation as follows: where β is the full width at half maximum (FWHM) of the different peaks, λ is the X-ray wavelength (Cu − Kα � 1.5418Å), k � 0.94 is the shape factor, and 2θ is the peak position. It is observed that the crystallite size is between 5 nm and 37 nm and is increased by increasing the deposition times as shown in Figure 4. In addition, the crystallite size is better as the sputtering power of 80 W. Moreover, the dislocation density (δ) and the microstrain (ε) of the thin films were calculated from the following equations: e analysis of the evolution of one of them is therefore sufficient. Furthermore, for the deposition times 40 and 60 min, the dislocation is constant by increasing the power.
is indicates that the length of the lines per volume unit of   Advances in Materials Science and Engineering the crystal is constant. In addition, the structural disorder and defects in ITO lattice also decrease for the deposition time of 20 min.

X-Ray Reflectometry.
e XRR patterns of the synthesized ITO thin films at various RF powers and different deposition times are shown in Figure 5. For all samples, the reflectometry drops beyond a critical angle, which will be used to determine the sample density and thickness. In addition, it should be noted that some curves exhibit intensity oscillations due to the phenomenon of X-rays interference.
e films thickness d can be calculated by equation (4) where α c is the critical angle, m is the interference order, p 1 is the order of the first fringe observed on the reflectometry spectrum, and λ is the wavelength of Cu-Kα radiation [31]: e critical angle increases, by increasing the RF power, between 0.32 and 0.35. e density ρ is related to the electronic density of the material by the following equation [32]:  Advances in Materials Science and Engineering In equation (6), r e is the Bohr radius (2.29 × 10 − 1Å ), N(Z + Δf) is the electron density, λ is the wavelength (λ � 1.5413Å) of XRD, Zi is the atomic number, Mi is the atomic weight, Ci is the atomic ratio, and Δfi is the correction factor of the abnormal dispersion of the refractive index. e variations of films density, for various RF powers and  Advances in Materials Science and Engineering 5 different deposition times, are reported in Figure 6. We note that the density generally increases with increasing the RF power. On the other hand, the minimum density of ITO films is 5.6 g cm −3 for (20 W, 60 min), and the maximum density reached 6.7 g·cm −3 for (110 W, 40 min). Figure 7 shows the SEM images for the ITO films deposited with different powers and different deposition times. As can be seen, the images are different indicating that there is an influence of the thin film thickness and the RF power on the grain size and the crystallinity of the films. On the other hand, small circles at the nanoscale are seen. is result is in agreement with the XRD analysis. e energy dispersion spectroscopy (EDX) technique was used to determine the chemical composition of our sample, which is shown in Figure 8.
is confirms the presence of In, Sn, and O in the prepared ITO thin films. e peak of Si is due to the substrate elements. In addition, the percentage of elements (see Table 2) shows that the film corresponds to the ITO compound.

Optical Properties.
e optical transmittance spectra of ITO films deposited with various powers and different deposition times in the 200−2000 nm wavelengths range are reported in Figure 9. Interference fringes are observed in the transmittance spectra for the powers greater than 20 W. e transmittance spectra were used to determine the band gap and the Urbach energy [33]. In addition, the interference fringes were used to calculate the refractive index n, the absorption coefficient k, and the thickness d of the thin films [34,35]. It is clear that the transmittance of ITO films decreases with increasing deposition power and the time.
is is due to the increase in the film thickness with increasing RF power and/or deposition time as shown in Figure 10(a). Furthermore, it should also be noted that the average transmittance decreases with increasing RF power and/or deposition time and varies between 70% and 95% as shown in Figure 10(b). Moreover, ITO thin films have an allowed direct transition wide band gap. Consequently, ITO thin films have a good optical quality in the visible region [36]. e optical absorption coefficient and band gap are determined using the Lambert-Beer equation and the Tauc equation: (αhυ) n � A(hυ − Eg).
In the above equations, T is the transmittance, A is the constant, hυ the photon energy, and n takes one of the values 2, 2/3, 1/2, and 1/3, for direct or for allowed direct, forbidden direct, allowed indirect, and forbidden indirect electronic transitions, respectively [37]. ITO is an n-type semiconductor with a direct band gap material for which n � 2. Band gap can be determined by extrapolating the linear part of the curve (αhυ) 2 vs hυ to the energy axis. e evolution of the band gap as a function of the RF power for different deposition times is shown in Figure 11(a). It is observed that the band gap is in the range 3.5-3.6 eV, which is in good agreement with the experimental values reported by other researchers [38][39][40]. In order to obtain further information about structural properties, the relationship between the absorption coefficient and photon energy is known as the Urbach empirical rule which is given by the following equation [41][42][43]:   Advances in Materials Science and Engineering In equation (10), hυ is the photon energy, α 0 is the constant, and E u is the band tail (Urbach energy). e band tail depends on temperature, induced disorder, strong ionic bonds, static disorder, and average photon energies as shown in Figure 11(b). e E u value is calculated from the slope of the straight line by plotting lnα    One of the parameters most influencing the efficiency of the solar cell and which allows to reduce the optical losses in the solar cell is the refractive index n. e expression of the latter is given by the following relations [44,45]: In equation (11), b is the constant, T is the transmittance, k is extinction coefficient, and n (∞) is the value of the refractive index in the infrared region. Figure 12 shows the refractive index as a function of the wavelength. e refractive index increases as a function of the power. In addition, the refractive index increases by increasing the deposition time in the visible and infrared regions because of the effect of thickness and density. e refractive index in the infrared range has a value, which varies between 1.75 and 3.
is result is consistent with that reported by other researchers [17,46].
In order to attain the detailed optical information, the extinction coefficient k of the ITO films can be obtained from the following relationship: where α is the absorption coefficient, and the variation of the extinction coefficient k of the ITO films deposited with various powers and different deposition times is reported in Figure 13. We notice the thin film of ITO is not absorbent in the visible range and the extinction coefficient is in the order of 10 −1 . In the infrared range, the extinction coefficient increases with increasing the RF power and/or the deposition time because of the variation in thickness. is result is consistent with previous studies [47,48].

Electrical Resistivity.
e electrical resistivity of the films deposited with various RF powers and different deposition  Advances in Materials Science and Engineering times is measured using the four-point probe, and the expression used is as follows: where ρ is the resistivity, V is the voltage measured by the voltmeter, I is the current intensity measured by the ammeter, and S 1 , S 2 , and S 3 are the distances between the probes. e evolution of the sheet resistance and the electrical resistivity as a function of powers and different deposition times is shown in Figure 14. It can be clearly seen that the sheet resistance decreases with increase in the RF power and/ or the deposition time.
e minimal resistivity is of ≈10 − 4 Ω.cm, and it corresponds to the sample prepared without annealing. In addition, the resistivity rapidly decreases to the value 10 − 3 Ω.cm corresponding to the power of 50 W; the power after it has a slight decrease. Moreover, the increase in resistivity is due to the increase in carrier concentration as a function of power [49]. On the other hand, the decrease in the resistivity is because of the decrease in the band gap (the obvious red shift). In addition, this result is consistent with the results of the optical properties. In fact, a large crystallite contains numerous atoms, with numerous overlaps between orbits, which results in the decrease in the optical band gap and hereby a low resistivity. It is obvious that the energy gap decreases when electron density increases.  e ITO is widely used in solar cells. To determine the efficiency of ITO in usage as a front contact, the figur-e_of_merit (FOM) parameter was determined by the Haacke as follows [50]: where R sh the sheet resistance of the films ITO and T a is the average transmittance in the visible range (we take the average transmittance for thin film applications in solar cells). FOM is an important value for estimating the quality of transparent conducting oxide (TCO) films. Table 2 illustrates the figure_of_merit of the ITO as a function of the various RF powers and different deposition times [16]. In this work, we obtain that FOM increases as a function of the various RF powers and different deposition times. e best values of FOM are obtained when the power is 110 W. Table 3 illustrates the carrier concentration and Hall mobility of the ITO as a function of the various RF powers and different deposition times. It is clear that the carrier concentration increases with increasing the RF power and/ or deposition time. e increase in carrier concentration can be explained by increasing of oxygen vacancies as donors and the decreasing of In or Sn bivalents as acceptors [39]. On the other hand, in 20 W films, the low crystallinity of ITO thin films is also the reason for the decrease in Hall mobility.
is result is consistent with the results of XRD. In addition, there are a large number of defects in thin films, which is  agreed by the E u values. e greatest Hall mobility and concentration of charge carriers were obtained when the power is 110 W. We conclude that the optimal parameters for good optical and high electrical properties are 110 W and 60 min.

Conclusion
Structural, morphological, optical, and electrical properties of ITO thin film are investigated, as a function of the RF power and deposition time. It is shown that the RF power must exceed 20 W to have crystalline samples with orientation (222). e crystallite size increases with increasing the deposition time. On the other hand, the average transmission and the sheet resistance are decreased by increasing the RF power and/or the deposition time. e best FOM of ITO films is obtained using a power of 110 W. ITO is not absorbent in the visible range, and the extinction coefficient is of the order of 10 −1 . is study revealed that ITO films have excellent optical and electrical properties for photovoltaic cells and optoelectronic applications.

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that they have no conflicts of interest regarding the publication of this paper.