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

Authors: Abdelaziz Tchenka; Abdelali Agdad; Mohamed Cheikh Samba Vall; Salma Kaotar Hnawi; Abdelfattah Narjis; Lahcen Nkhaili; Elalami Ibnouelghazi; Elmaati Ech-Chamikh
Journal: Advances in Materials Science and Engineering
 (2021)

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## Abstract

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% In2O3 and 10% SnO2. 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 110–80 W, 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.


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## Body

## 1. Introduction

Tin-doped indium oxide (ITO) thin films have attracted the attention of many researchers owing to their good conductivity and high optical transmittance (∼95%) in the visible region [1]. Besides, ITO is an n-type semiconductor [2] with an energy band gap of 3.5 eV. Consequently, it exhibits high concentration of free electrons. These properties make it useful in several applications, including protective coatings [3], solar cells [4], gas sensors [5], reflecting windows [6], transparent electrodes for flat panel displays [7], organic light emitting diodes [8], plasma display panels [9], lenses [10], and car windows [11]. Sputtering ITO properties depend on various deposition condition parameters, such as initial pressure, RF power [12], argon pressure [13], deposition time [14], angle of incidence of the deposition on the substrate [15], target-to-substrate distance, substrate type [16], film thickness [17], temperature of the substrate [7], and oxygen flow rate [1].Research works on ITO films focused on several methods of elaboration such as dip coating [18], thermal evaporation [19], spray pyrolysis [20], electron beam evaporation [21], electrochemical deposition [22], chemical vapor deposition (CVD) [23], evaporation in reactive atmosphere or vacuum [24], pulsed laser deposition (PLD) [25], direct current (DC) magnetron sputtering [26], and radio-frequency (RF) sputtering [18, 26, 27]. The RF sputtering is one of the most widely used deposition techniques in many industries to prepare transparent and conductive thin films due to its advantages of good film adhesion, reproducibility, high deposition rate, good homogeneity, and suitability for large areas [28].In this study, we investigated the effect of RF sputtering power and deposition time on optical and electrical properties of ITO thin films. The 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. The 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.


## 2. 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 Figure1. 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 substrate-target distance was fixed at 7 cm. The 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. The 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.Figure 1
Schematic diagram of the RF sputtering.Table 1
Conditions of RF sputtering.
ParameterValuePower20 W, 50 W, 80 W, 110 WDeposition times20 min, 40 min, 60 minResidual pressure3.10−6 mbarTargetITO with 90% In2O3 and 10% SnO2The diameter of target10 cmTemperatureRoom temperatureSubstrateGlass substrateTarget-to-substrate distance10 cmArgon pressure10−2 mbarFigure 2
Images of ITO prepared by RF sputtering with various powers and different deposition times.The 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 = 1.5418 Å). SEM morphology images were used to observe the surfaces of ITO films on the substrate. The 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). The TESCAN VEGA3 scanning electron microscope was used to analyze the surface morphology.


## 3. Results and Discussion

XRD spectra of the ITO films prepared with various RF powers and different deposition times are shown in Figure3. 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. The 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. The 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. The peak is first observed followed by the (222) orientation, and then the (400), (440), and (622) orientations are observed. The observed results are similar to those reported by Ghorannevis et al. [30].Figure 3
XRD patterns for ITO films deposited with various powers and different deposition times: (a) 20 min; (b) 40 min; (c) 60 min.
(a)(b)(c)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:(1)D=kλβcosθ,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:(2)δ=1D2,(3)ε=βcosθ4.Figure 4
(a) Crystallite size; (b) microstrain; and (c) dislocation density of the ITO films deposited with various powers and deposition times.
(a)(b)(c)The 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. This indicates that the length of the lines per volume unit of the crystal is constant. In addition, the structural disorder and defects in ITO lattice also decrease for the deposition time of 20 min.


### 3.1. X-Ray Reflectometry

The XRR patterns of the synthesized ITO thin films at variousRF 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. The films thickness d can be calculated by equation (4) where αc is the critical angle, m is the interference order, p1 is the order of the first fringe observed on the reflectometry spectrum, and λ is the wavelength of Cu-Kα radiation [31]:(4)m−12=2dλαm2−αc2−p1.Figure 5
XRR spectra for the ITO films deposited with various powers and different deposition times: (a) 20 min; (b) 40 min; (c) 60 min.
(a)(b)(c)The critical angle increases, by increasing the RF power, between 0.32 and 0.35. The densityρ is related to the electronic density of the material by the following equation [32]:(5)n=1−δ+iβ,θc=2δ,θc=reλ2π.NZ+Δf,θc=reλ2π.NAMZ+Δfρ,(6)Z+ΔfM=∑iciZi+Δfi∑iciMi,ρ=θc2πreλ2NA.MZ+Δf.In equation (6), re is the Bohr radius 2.29×10−1Å, NZ+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. The variations of films density, for various RF powers and 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 6
Density of the ITO films deposited with various powers and different deposition times.Figure7 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. This result is in agreement with the XRD analysis.Figure 7
SEM images for ITO films deposited with various powers and different deposition times.The energy dispersion spectroscopy (EDX) technique was used to determine the chemical composition of our sample, which is shown in Figure8. This confirms the presence of In, Sn, and O in the prepared ITO thin films. The 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.Figure 8
Energy dispersive X-ray (EDX) spectra of ITO films deposited with 60 min and 110 W.Table 2
Composition of ITO films deposited with 60 min and 110 W.
ElementWeight (%)Atomic (%)O K16.5958.36SiK0.551.10InL77.8538.17SnL5.012.38


### 3.2. Optical Properties

The optical transmittance spectra of ITO films deposited with various powers and different deposition times in the 200−2000 nm wavelengths range are reported in Figure9. Interference fringes are observed in the transmittance spectra for the powers greater than 20 W. The 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. This 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].‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬Figure 9
Transmittance of the ITO films deposited with various powers and different deposition times: (a) 20 min; (b) 40 min; (c) 60 min.
(a)(b)(c)Figure 10
(a) Evolution of thickness and (b) transmittance of the ITO films with various powers and different deposition times.
(a)(b)The optical absorption coefficient and band gap are determined using the Lambert–Beer equation and the Tauc equation:(7)α=ln1/Td,(8)αhυn=Ahυ−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 αhυ2vshυ to the energy axis.The evolution of the band gap as a function of the RF power for different deposition times is shown in Figure11(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 [38–40]. 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 [41–43]:(9)α=α0exphυEu,(10)lnα=hυEu+lnα0.Figure 11
(a) Band gap and (b) Energy Urbach of the films deposited with various RF powers and different deposition times.
(a)(b)In equation (10), hυ is the photon energy, α0 is the constant, and Eu is the band tail (Urbach energy). The band tail depends on temperature, induced disorder, strong ionic bonds, static disorder, and average photon energies as shown in Figure 11(b). The Eu value is calculated from the slope of the straight line by plotting lnα versus hυ. It should be noted that using the power of 50 W and 20 min results in a high band tail. This result is consistent with the results of XRD.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 indexn. The expression of the latter is given by the following relations [44, 45]:(11)n=2−TT+41−TT2−k2,n=n∞+bλ2.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. The 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. The refractive index in the infrared range has a value, which varies between 1.75 and 3. This result is consistent with that reported by other researchers [17, 46].Figure 12
Refractive index of the ITO films deposited with various powers and different deposition times: (a) 20 min; (b) 40 min; (c) 60 min.
(a)(b)(c)In order to attain the detailed optical information, the extinction coefficientk of the ITO films can be obtained from the following relationship:(12)k=αλ4π,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. This result is consistent with previous studies [47, 48].Figure 13
Extinction coefficient of the ITO films deposited with various powers and different deposition times: (a) 20 min; (b) 40 min; (c) 60 min.
(a)(b)(c)


### 3.3. Electrical Resistivity

The electrical resistivity of the films deposited with various RF powers and different deposition times is measured using the four-point probe, and the expression used is as follows:(13)ρ=2πdlnS1+S2S2+S3/S1S3VI,where ρ is the resistivity, V is the voltage measured by the voltmeter, I is the current intensity measured by the ammeter, and S1, S2, and S3 are the distances between the probes.The evolution of the sheet resistance and the electrical resistivity as a function of powers and different deposition times is shown in Figure14. It can be clearly seen that the sheet resistance decreases with increase in the RF power and/or the deposition time. The minimal resistivity is of ≈10−4Ω.cm, and it corresponds to the sample prepared without annealing. In addition, the resistivity rapidly decreases to the value 10−3Ω.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.Figure 14
(a) Sheet resistance and (b) resistivity of the films deposited with variousRF powers and different deposition times.
(a)(b)The ITO is widely used in solar cells. To determine the efficiency of ITO in usage as a front contact, the figure_of_merit (FOM) parameter was determined by the Haacke as follows [50]:(14)ϕ=TaRsh,where Rsh the sheet resistance of the films ITO and Ta 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. The best values of FOM are obtained when the power is 110 W.Table3 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. The 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. This 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 Eu values. The 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.Table 3
Carrier concentration, Hall mobility, and figure_of_merit for the ITO films deposited with various RF powers and different deposition times.
Deposition timesRF power (W)Carrier concentration (×1019 cm−3)Hall mobility (cm2/V.s)Figure_of_merit (FOM)φ (× 10−4 sq.Ω−1)20 min20——0.0001500.037.80.8800.101.774.21100.97924.940 min20——0.001500.056.12.363800.5614.825.79211013.5813.7101.46360 min20——0.0001500.183.46.4809.368.8371.111019.418.4111.0


## 3.1. X-Ray Reflectometry

The XRR patterns of the synthesized ITO thin films at variousRF 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. The films thickness d can be calculated by equation (4) where αc is the critical angle, m is the interference order, p1 is the order of the first fringe observed on the reflectometry spectrum, and λ is the wavelength of Cu-Kα radiation [31]:(4)m−12=2dλαm2−αc2−p1.Figure 5
XRR spectra for the ITO films deposited with various powers and different deposition times: (a) 20 min; (b) 40 min; (c) 60 min.
(a)(b)(c)The critical angle increases, by increasing the RF power, between 0.32 and 0.35. The densityρ is related to the electronic density of the material by the following equation [32]:(5)n=1−δ+iβ,θc=2δ,θc=reλ2π.NZ+Δf,θc=reλ2π.NAMZ+Δfρ,(6)Z+ΔfM=∑iciZi+Δfi∑iciMi,ρ=θc2πreλ2NA.MZ+Δf.In equation (6), re is the Bohr radius 2.29×10−1Å, NZ+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. The variations of films density, for various RF powers and 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 6
Density of the ITO films deposited with various powers and different deposition times.Figure7 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. This result is in agreement with the XRD analysis.Figure 7
SEM images for ITO films deposited with various powers and different deposition times.The energy dispersion spectroscopy (EDX) technique was used to determine the chemical composition of our sample, which is shown in Figure8. This confirms the presence of In, Sn, and O in the prepared ITO thin films. The 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.Figure 8
Energy dispersive X-ray (EDX) spectra of ITO films deposited with 60 min and 110 W.Table 2
Composition of ITO films deposited with 60 min and 110 W.
ElementWeight (%)Atomic (%)O K16.5958.36SiK0.551.10InL77.8538.17SnL5.012.38


## 3.2. Optical Properties

The optical transmittance spectra of ITO films deposited with various powers and different deposition times in the 200−2000 nm wavelengths range are reported in Figure9. Interference fringes are observed in the transmittance spectra for the powers greater than 20 W. The 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. This 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].‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬Figure 9
Transmittance of the ITO films deposited with various powers and different deposition times: (a) 20 min; (b) 40 min; (c) 60 min.
(a)(b)(c)Figure 10
(a) Evolution of thickness and (b) transmittance of the ITO films with various powers and different deposition times.
(a)(b)The optical absorption coefficient and band gap are determined using the Lambert–Beer equation and the Tauc equation:(7)α=ln1/Td,(8)αhυn=Ahυ−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 αhυ2vshυ to the energy axis.The evolution of the band gap as a function of the RF power for different deposition times is shown in Figure11(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 [38–40]. 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 [41–43]:(9)α=α0exphυEu,(10)lnα=hυEu+lnα0.Figure 11
(a) Band gap and (b) Energy Urbach of the films deposited with various RF powers and different deposition times.
(a)(b)In equation (10), hυ is the photon energy, α0 is the constant, and Eu is the band tail (Urbach energy). The band tail depends on temperature, induced disorder, strong ionic bonds, static disorder, and average photon energies as shown in Figure 11(b). The Eu value is calculated from the slope of the straight line by plotting lnα versus hυ. It should be noted that using the power of 50 W and 20 min results in a high band tail. This result is consistent with the results of XRD.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 indexn. The expression of the latter is given by the following relations [44, 45]:(11)n=2−TT+41−TT2−k2,n=n∞+bλ2.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. The 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. The refractive index in the infrared range has a value, which varies between 1.75 and 3. This result is consistent with that reported by other researchers [17, 46].Figure 12
Refractive index of the ITO films deposited with various powers and different deposition times: (a) 20 min; (b) 40 min; (c) 60 min.
(a)(b)(c)In order to attain the detailed optical information, the extinction coefficientk of the ITO films can be obtained from the following relationship:(12)k=αλ4π,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. This result is consistent with previous studies [47, 48].Figure 13
Extinction coefficient of the ITO films deposited with various powers and different deposition times: (a) 20 min; (b) 40 min; (c) 60 min.
(a)(b)(c)


## 3.3. Electrical Resistivity

The electrical resistivity of the films deposited with various RF powers and different deposition times is measured using the four-point probe, and the expression used is as follows:(13)ρ=2πdlnS1+S2S2+S3/S1S3VI,where ρ is the resistivity, V is the voltage measured by the voltmeter, I is the current intensity measured by the ammeter, and S1, S2, and S3 are the distances between the probes.The evolution of the sheet resistance and the electrical resistivity as a function of powers and different deposition times is shown in Figure14. It can be clearly seen that the sheet resistance decreases with increase in the RF power and/or the deposition time. The minimal resistivity is of ≈10−4Ω.cm, and it corresponds to the sample prepared without annealing. In addition, the resistivity rapidly decreases to the value 10−3Ω.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.Figure 14
(a) Sheet resistance and (b) resistivity of the films deposited with variousRF powers and different deposition times.
(a)(b)The ITO is widely used in solar cells. To determine the efficiency of ITO in usage as a front contact, the figure_of_merit (FOM) parameter was determined by the Haacke as follows [50]:(14)ϕ=TaRsh,where Rsh the sheet resistance of the films ITO and Ta 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. The best values of FOM are obtained when the power is 110 W.Table3 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. The 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. This 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 Eu values. The 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.Table 3
Carrier concentration, Hall mobility, and figure_of_merit for the ITO films deposited with various RF powers and different deposition times.
Deposition timesRF power (W)Carrier concentration (×1019 cm−3)Hall mobility (cm2/V.s)Figure_of_merit (FOM)φ (× 10−4 sq.Ω−1)20 min20——0.0001500.037.80.8800.101.774.21100.97924.940 min20——0.001500.056.12.363800.5614.825.79211013.5813.7101.46360 min20——0.0001500.183.46.4809.368.8371.111019.418.4111.0


## 4. 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). The 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. The 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. This study revealed that ITO films have excellent optical and electrical properties for photovoltaic cells and optoelectronic applications.



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*Source: 5556305-2021-08-30.xml*