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Effect of Indium Doping on Optical Parameter Properties of Sol–Gel-Derived ZnO Thin Films

  • Adem Kocyigit , Mehmet Okan Erdal and Murat Yıldırım EMAIL logo
Published/Copyright: June 27, 2019
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Zeitschrift für Naturforschung A
From the journal Zeitschrift für Naturforschung A Volume 74 Issue 10

Abstract

Zinc oxide (ZnO) has gained great interest for two decades, and its structural, optical, and electrical properties have been investigated by scientists for technological applications. The optical properties of ZnO provide an opportunity for its application in solar cells, lasers, and light-emitting diodes. We prepared ZnO thin films with various In doping levels by using a spin coating technique, and characterised their morphological and detailed optical properties for optical applications. According to the morphological properties obtained by atomic force microscopy, the film surfaces are homogeneous and the In doping level affects the surface morphology of the films. The optical properties of the films were investigated using an ultraviolet–visible spectrometer, and some optical parameters such as band gap, refractive index, extinction coefficient, single oscillator parameters, real and imaginary functions of the dielectric coefficient, and optical conductivity were calculated and discussed in detail. The various In doping levels affected the optical properties, and the In-doped ZnO thin films can find applications in industry.

Keywords: Doping Concentration; In-Doped ZnO; Optical Properties; Thin Film

1 Introduction

The oxide form of zinc, known as zinc oxide (ZnO), has drawn great interest and is a very promising material because it can easily be controlled in terms of structural properties, band gap, optical transmission, and electrical resistivity [1], [2], [3], [4]. In addition, ZnO is an abundant, non-toxic, and chemically stable material, and it is usually wurtzite crystalline in nature and has n-type conductivity [5], [6], [7]. The physical properties of ZnO can be controlled by doping some materials into the ZnO structure such as Al, Co, Ni, Mn, and In [2], [8], [9], [10]. ZnO can have various applications, such as in varistors, solar cells, lasers, and gas sensors because of its promising properties [11], [12], [13].

Thin films have good and adjustable optical properties, and they are very important for various applications [14], [15]. The optical properties of thin films can be understood by investigating some optical constant, band gap energy, transmission, reflectance, dielectric, and dispersion parameters [16]. ZnO is used as a thin-film material for many applications. The thin films of ZnO can be prepared using various thin-film deposition techniques such as spray pyrolysis [17], spin coating [18], atomic layer deposition [19], magnetron sputtering [20], pulsed laser deposition [21], etc. Among them, the spin coating technique is an easier and cheaper way to obtain ZnO thin films [22].

There are many studies about In-doped ZnO thin films in the literature, and indium has shown potential as a doping material to enhance the physical properties of ZnO films; however, there is no study about detailed optical properties including dispersion parameters depending on the In concentration obtained by the sol–gel technique [23], [24], [25], [26], [27], [28], [29]. It has been pointed out that extrinsic donors due to doping atoms are more stable than intrinsic donors due to native defects [30]. Indium has a close ionic radius parameter to that of Zn2+, which means these elements can easily penetrate into the ZnO crystal lattice or substitute the Zn2+ position in crystal [31]. At the same time, the decrease of transmittance may be due to the increase in the scattering of photons by crystal defects created by doping probably due to the increase in metal to oxygen ratio, (Zn+In)/O. However, there is a lack of information about how indium affects the optical and dispersion parameters of ZnO. In this study, various In-doped ZnO thin films were obtained on a glass substrate by using the spin coating technique, and their morphological and optical properties were investigated for potential applications such as in optoelectronic devices and solar cells.

Figure 1: XRD for undoped and In-doped films.
Figure 1:

XRD for undoped and In-doped films.

2 Experimental Details

All chemicals, zinc acetate dehydrate, indium nitrate hydrate, isopropanol, methoxyethanol, Ethanolamine, acetone and propanol were purchased from Merck KGaA (Darmstadt, Germany). They were used directly. For preparing various In-doped ZnO thin films on a glass substrate, we dissolved zinc acetate dehydrate (Zn(CH3COO)2⋅2H2O) in isopropanol as precursor. For 0.1 %, 0.5 %, and 1 % In doping levels, indium nitrate hydrate (In(NO3)3⋅H2O) was dispersed in methoxyethanol in three different vessels. Ethanolamine was added to both ZnO and In solutions as the stabiliser. The substrates were cleaned in an ultrasonic cleaner with acetone and propanol for 25 min, and then they were placed in an ultraviolet (UV)–ozone cleaner (Daihan Scientifc Co. Ltd., Seoul, Korea) for 30 min. After the cleaning, the substrates were dried under nitrogen. These procedures have been explained in our previously published studies [32], [33]. Undoped and In-doped ZnO thin films were deposited on a glass substrate by using the spin coating technique with a spin speed of 3000 rpm, as three layers through a Fytronix SC-500 spin coater (Fytronix Limited, Elazig, Turkey). Finally, the In-doped ZnO thin films were annealed at 400 °C for 30 min. The films were characterised by atomic force microscopy (AFM) and UV–visible (Vis) spectrometry. Morphological images were obtained using a PARK system XE 100E model AFM (Park Systems, Suwon, Korea). The optical measurements were performed at room temperature with a Shimadzu UV–Vis–NIR 3600 spectrophotometer (Shimadzu Corp., Kyoto, Japan).

3 Results and Discussion

3.1 Morphological Properties of In-Doped ZnO Thin Films

X-ray diffraction (XRD) measurement is used to determine the crystal features of ZnO as well as In-doped ZnO. The XRD (Bruker AXS GmbH, Karlsruhe, Germany) diffractograms of undoped and In-doped ZnO thin films are given in Figure 1 to compare them with each other. Peaks are observed at (100), (002), (101), (110), with different intensities, and (102) indicates that growth is for doped films. From Figure 1, it is observed that diffraction patterns exhibit high intensity of peaks, predominantly (100), (002), and (101). This can be evaluated as an evidence for lattice distortion and its related effects, dependent on In doping content. Compared with undoped ZnO, it appears that the intensity of peaks is decreased and the full width at half maximum (FWHM) value is increased in In doping content, showing that the crystalline quality is deteriorated by the implementation of doping. However, the intensity peaks improve again and FWHM is decreased by further doping content. A similar fluctuation has been observed by other researchers for In- and F-doped ZnO thin films applied with different growing techniques [30], [34]. The possible reason for this discrepancy may be the difference in the surface reaction as well as the amorphous nature of the glass substrates.

Figure 2: AFM images (40 μm × 40 μm) for (a) undoped and (b) 0.1 %, (c) 0.5, and (d) 1.0 % In-doped thin films.
Figure 2:

AFM images (40 μm × 40 μm) for (a) undoped and (b) 0.1 %, (c) 0.5, and (d) 1.0 % In-doped thin films.

Figure 3: Transmittance (a, b) and reflectance (c) spectra of various In-doped thin films from 200 to 1200 nm wavelengths.
Figure 3:

Transmittance (a, b) and reflectance (c) spectra of various In-doped thin films from 200 to 1200 nm wavelengths.

The two-dimensional surface morphological images of undoped and 0.1 %, 0.5 %, and 1.0 % In-doped ZnO thin films are shown in Figure 2a–d, respectively, for wide scanning areas (40 μm × 40 μm). The images clearly suggest that In doping affects the surface morphology of the ZnO thin films. The surface roughness values of the films for undoped and 0.1 %, 0.5, and 1.0 % In-doped films are 172, 229, 40, and 167 nm, respectively. The roughness values were also affected by the In doping concentration. While the surface roughness is maximum for the 0.1 % In doping level, it reaches its minimum value with 0.5 % In doping.

3.2 Optical Properties of Various In Doping Levels in ZnO Thin Films

The optical properties of the films have been measured using UV–Vis spectrometry. By utilizing these measurements, various optical parameters have been calculated. The obtained parameters are discussed from the In doping level point of view in detail. Figure 3a–c display the transmission and reflection graphs of the various In-doped ZnO thin films in a wide range of wavelengths from 200 to 1200 nm, respectively. The transmittance values of the films are about 90 % from the Vis range to the infrared range; however, they decrease drastically in the UV region at about 350 nm, which is referred to as the absorption edge. The increasing In doping level caused the transmission values for the Vis and infrared regions to decrease slightly. The reflectance values of the films increased with the In doping level from 3 % to 12 % for the UV region; however, they did not change very much in the Vis and infrared regions and remained between 3 % and 7 %. As a result, increasing In doping concentration in the ZnO structure increased the reflectance and decreased the transmission of the ZnO thin film. This result highlighted that In doping elements contribute to the ZnO structure successfully [10], [35], [36].

The absorption coefficient of the material is used when determining the band structure of the materials and the forbidden energy range. Some of the light falling on the semiconductor material is absorbed, whereas some of it passes through the material and some of it is reflected. The α value is estimated through the T and R spectra by use of the following relation [37]:

(1)α=1dln((1−R)2T),

where d represents the thickness of the materials, and T and R are transmittance and reflectance, respectively. The absorption coefficient of In-doped ZnO thin films were calculated as around 1–2 × 104 cm−1 in the Vis region. While the absorption value is high, it is often called the Tauc regime (α ≥ 104 cm−1), and thus α is written as follows [38]:

(2)αhυ=B(hυ−Eg)m,

where B is a constant, hυ is the photon energy, Eg illustrates the band gap energy of the material, and m is a constant and indicates the transition characteristics of the material as direct (m = 1/2) or indirect (m = 2). By plotting (αhv)2 vs. (hv), the band gap energy values are determined. The band gap plots of various In-doped ZnO thin films are indicated in Figure 4. The band gap energy values of the films decreased slightly from 3.29 to 3.26 eV via increasing the In doping level. The decrease in the band gap values can be attributed to increasing carrier concentration [39].

Figure 4: Plots of (αhv)2 vs. hv for various In-doped ZnO thin films.
Figure 4:

Plots of (αhv)2 vs. hv for various In-doped ZnO thin films.

Figure 5: Variation of ln α with photon energy.
Figure 5:

Variation of ln α with photon energy.

Figure 6: Variation of (a) extinction coefficient and (b) refractive index of the various In-doped thin films with the wavelength.
Figure 6:

Variation of (a) extinction coefficient and (b) refractive index of the various In-doped thin films with the wavelength.

Urbach’s energy, Eu, is an effect of the structural disorder and temperature of the material. The temperature dependence of this energy can be explained as the interaction of electron/excitons with optical phonons. The relation between Eu and α is shown in the equation below depending on incident light energy hv [40], [41]:

(3)α(hv)=α0exp(hvEu),

where α0 is a characteristic parameter of the employed material. Equation (3) helps determine Eu from the slope of ln α vs. hv graphs. Figure 5 exhibits the ln α vs. hv plots of the various In-doped ZnO thin films. The Eu of the thin films was determined to be 367, 386, 409, and 390 meV for undoped and 0.1 %, 0.5, and 1.0 % In-doped films, respectively. According to this result, the Eu values were affected by increasing In doping level in the ZnO structure and confirmed the doping of the In atoms to the ZnO structure. In addition, increasing Eu values with increasing In doping concentration in the ZnO structure indicated increasing disorders [42].

The extinction coefficient (k), which is obtained from the absorption coefficient, and the imaginary complex refractive index (n) are accounted for via the equations below [28], [29], [30], [31]. Here, the profile variation of the refractive index is obtained by using R and k values, as follows [43], [44], [45]:

(4)k=αλ4π,
(5)n=(1+R1−R)+4R(1−R)2−k2.

The k and n values depending on the wavelength of the various In-doped ZnO thin films are indicated in Figure 6. The k values reached the maximum values in the UV region, and then remained constant from the Vis to the infrared region; however, the changing In doping concentration did not affect the k values. The n values increased and exhibited peaks at around 400 nm, and then decreased slightly in the Vis region. The n values increased suddenly at about 1000-nm wavelength and then remained constant. The In doping level in the ZnO structure usually causes an increase in the complex refractive index. This can be attributed to the bigger atomic radius of In atoms than that of Zn atoms in the ZnO structure after substitution [36]. Furthermore, the n values changed between 1.2 and 2.0 for various In doping levels and a wide range of wavelengths. These refractive index values match those reported in other studies [46], [47].

3.3 Dispersion Analysis of the Refractive Index

The variation of the refractive index due to photon energy is referred to a relational dispersion expression that defines the single-oscillator model. In other words, the expression depending on the dispersion parameters of the refractive index can be defined by the single term Wemple DiDomenico oscillator. Thus, the dispersion of the refractive index is expressed via the following formula and exhibits a straight line [32], [48], [49], [50]:

(6)(n2−1)−1=E0Ed−1E0Ed(hv)2,

where E0 is the single oscillator energy and Ed refers to the dispersion energy. The values of E0 and Ed are determined from the intercept of the extrapolation to the (n2−1)−1 axis and the slope, respectively. The (n2−1)−1 vs. (hv)2 graphs of the various In-doped ZnO thin films are indicated in Figure 7. The graphs clearly display straight lines and help determine the E0 and Ed values for various In-doped ZnO thin films. The obtained E0 and Ed values are listed in Table 1. The E0 and Ed values usually increased and reached their maximum values for the 1 % In dopant.

Figure 7: Plots of (n2 − 1)−1 vs. (hv)2 of the various In-doped ZnO thin films.
Figure 7:

Plots of (n2 − 1)−1 vs. (hv)2 of the various In-doped ZnO thin films.

The static refractive index n0 can be determined by using the expression n0=1+Ed/E0. The values of the n0 are also given in Table 1. Their values usually increased with increasing In doping level. This result can be attributed to the bigger atomic radius of In atoms in the ZnO structure [30].

Table 1:

Single oscillator parameters of thin films for various In doping levels in ZnO thin film.

In doping levelE0 (eV)Ed (eV)n0M−1M−3 (eV)2f (eV)2
0 %6.625.361.350.890.02035.48
0.1 %6.668.121.490.810.01854.07
0.5 %6.295.931.390.970.02437.29
1.0 %6.808.561.500.810.01758.20

Furthermore, moments of imaginary part complex dielectric constants M−1 and M−3 can be defined as the E0 and Ed single oscillator parameters as shown by the next equations [51]:

(7)E02=M−1M−3,
(8)Ed2=M−13M−3.

The M−1 and M−3 moment values of the various In-doped ZnO thin films are listed in Table 1. There are fluctuations in the values of M−1 and M−3 with increasing In doping level; however, the maximum values of M−1 and M−3 are shown by the 0.5 % In-doped ZnO thin film. In addition, the oscillator strength (f) is expressed according to Wemple and DiDomenico via the following formula [49], [50]:

(9)f=E0Ed.

The oscillator strength (f) values of the various In-doped ZnO thin films were calculated and tabulated in Table 1. The f value of the films increased from 35.48 to 54.07 (eV)2 with increasing In doping level up to 0.1 %, and then decreased to 37.29 (eV)2 for 0.5 % In doping, and reached its maximum value of 58.20 (eV)2 for the 1.0 % In doping concentration. A similar fluctuation has been observed by other researchers for doped ZnO thin films. The possible reason for this discrepancy may be the difference in the surface reaction as well as in the amorphous nature of the glass substrates.

Figure 8: Variation of (n2 − 1)−1 with λ−2 of CZTSeS thin film.
Figure 8:

Variation of (n2 − 1)−1 with λ−2 of CZTSeS thin film.

The refractive index dispersion can be found by using the Sellmeier dispersion equation for long wavelength regions. The Sellmeier dispersion equation is given by the following formula [49], [52]:

(10)n02−1n2−1=1−(λ0λ)2,

where n0 represents the static refractive index and λ0 is the average interband oscillator wavelength. If (10) is rearranged for n2 − 1, the next equation is obtained:

(11)n2−1=S0λ021−λ02/λ2,

where S0 is the average oscillator strength and given by the following equation [49], [50], [53]:

(12)S0=(n02−1)/λ02.

When (n2−1)−1 vs. λ−2 is plotted, a straight line is obtained, and n0 and λ0 are determined by the slope and y-intercept of this plot, respectively. The (n2−1)−1 vs. λ−2 graphs of the various In-doped ZnO thin films are exhibited in Figure 8, and n0, λ0, and S0 values were obtained and listed in Table 2. The n0 values in Table 2 have been matched with the n0 values listed in Table 1, obtained using the single oscillator and dispersion energy. This harmony at n0 values confirms the accuracy of the Sellmeier dispersion formula.

Table 2:

Single oscillator parameters of thin films for various In doping levels in ZnO structures.

In doping levelλ0 (nm)S0 (nm−2)n0ε∞N/m∗ (1047g−1 cm−3)
0 %2851.06 × 10−51.362.142.11
0.1 %2781.66 × 10−51.512.807.40
0.5 %3318.77 × 10−61.402.372.68
1.0 %3051.34 × 10−51.492.715.77
Figure 9: (a) Real (ε1) and (b) imaginary (ε2) parts of dielectric functions depending on photon energy.
Figure 9:

(a) Real (ε1) and (b) imaginary (ε2) parts of dielectric functions depending on photon energy.

The real (ε1) and imaginary (ε2) parts of dielectric functions can be calculated using the n and k values. ε1 and ε2 are given by the following relation [43]:

(13)ε1=n2−k2;ε2=2nk.

Figure 9a and b display the photon-energy-dependent profile of the ε1 and ε2 functions, respectively. According to Figure 8, while the ε1 values usually increase with increasing In doping level and exhibit peaks at around 3 eV, the ε2 values remain constant from 1.5 to 3.2 eV, and then increase suddenly and are affected by the In doping level after 3.4 eV. The main peaks at the ε2 values can be attributed to the fundamental absorption of the various In-doped ZnO thin films [54].

Figure 10: Variation of the real part of the dielectric coefficient (ε1) with λ2.
Figure 10:

Variation of the real part of the dielectric coefficient (ε1) with λ2.

In addition, ε1 is expressed, dependent on free carriers of the materials, as the following equation [43]:

(14)ε1=ε∞−(Ne24π2c2ε0m∗)λ2,

where ε∞ represents the dielectric constant for high frequencies. The variables e, c, and m∗ are the charge of the electron, speed of light, and effective mass, respectively. While ε0 shows the vacuum permittivity, N indicates the carrier concentration. The ε1 vs. λ2 plot exhibits a straight line for higher wavelengths. The y-intercept of this plot helps determine the ε∞ value. Figure 10 indicates the ε1 vs. λ2 plots of the various In-doped ZnO thin films. The determined values of ε∞ are given in Table 2. The ε∞ values are affected by changing the In doping concentration in the ZnO structure because of changing carrier concentration of the films.

The optical conductivity (σopt) is addressed using the following formula related to the absorption coefficient [55]:

(15)σopt=αnc4π.

The σopt vs. hv graph of the various In-doped ZnO thin films are shown in Figure 11. The σopt values stay independent of both In doping and photon energy up to 3 eV; however, after this energy value, the σopt values increase with increasing photon energy and In doping level.

Figure 11: Variation of optical conductivity with the photon energy of thin films.
Figure 11:

Variation of optical conductivity with the photon energy of thin films.

4 Conclusions

We obtained undoped and 0.1 %, 0.5 %, and 1.0 % In-doped ZnO thin films on a glass substrate by using the spin coating technique and characterised the films with AFM and UV–Vis spectrometry. According to AFM images, the film surfaces are homogenous and smooth and the ZnO surface morphology is affected by the In doping concentration. UV–Vis spectrometry was performed to obtain detailed optical characteristics of the various In-doped ZnO thin films. The transmittance, reflectance, and band gap values were changed by changing the In doping level, because while the transmittance and band values decreased with increasing In doping level, the reflectance values increased. The obtained Eu indicated that the In doping concentration caused an increase in the disorders of the structure. While the k parameters were not influenced by the In doping level, the n values increased with increasing In doping concentration in the ZnO structure. Some important optical parameters such as single oscillator energy, dispersion energy, dielectric constant, and functions were calculated and discussed in detail according to In doping concentration in the ZnO structure. The In-doped ZnO films can be considered and improved for optical applications.

Acknowledgement

The authors would like to thank the Selcuk University BAP office for the grant for the research project (no. 16401044).

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Received: 2019-03-04
Accepted: 2019-05-29
Published Online: 2019-06-27
Published in Print: 2019-09-25

©2019 Walter de Gruyter GmbH, Berlin/Boston

Articles in the same Issue

  1. Frontmatter
  2. Solid State Physics & Materials Science
  3. Electronic and Optical Properties of Cubic Perovskites CsPbCl3−yIy (y = 0, 1, 2, 3)
  4. Effect of Indium Doping on Optical Parameter Properties of Sol–Gel-Derived ZnO Thin Films
  5. Electrical Conductivity of Zirconium Tetrachloride Solutions in Molten Sodium, Potassium and Cesium Chlorides
  6. Hydrogen Sulfide Gas Sensor Based on Copper/Graphene Oxide Composite Film-Coated Tapered Single-Mode Fibre Interferometer
  7. Dynamical Systems & Nonlinear Phenomena
  8. Detecting the Flow Pattern Transition in the Gas-Liquid Two-Phase Flow Using Multivariate Multi-Scale Entropy Analysis
  9. Rapid Communication
  10. Simplistic Synthesis and Enhanced Photocatalytic Performance of Spherical ZnO Nanoparticles Prepared from Arabinose Solution
  11. The Deflection of Rotating Composite Tapered Beams with an Elastically Restrained Root in Hygrothermal Environment
  12. Effect of Dust Ion Collision on Dust Ion Acoustic Solitary Waves for Nonextensive Plasmas in the Framework of Damped Korteweg–de Vries–Burgers Equation
  13. Hydrodynamics
  14. Lie Symmetries and Similarity Solutions for Rotating Shallow Water
  15. A Note on the Significance of Quartic Autocatalysis Chemical Reaction on the Motion of Air Conveying Dust Particles
https://doi.org/10.1515/zna-2019-0070
Keywords for this article
Doping Concentration; In-Doped ZnO; Optical Properties; Thin Film

Articles in the same Issue

  1. Frontmatter
  2. Solid State Physics & Materials Science
  3. Electronic and Optical Properties of Cubic Perovskites CsPbCl3−yIy (y = 0, 1, 2, 3)
  4. Effect of Indium Doping on Optical Parameter Properties of Sol–Gel-Derived ZnO Thin Films
  5. Electrical Conductivity of Zirconium Tetrachloride Solutions in Molten Sodium, Potassium and Cesium Chlorides
  6. Hydrogen Sulfide Gas Sensor Based on Copper/Graphene Oxide Composite Film-Coated Tapered Single-Mode Fibre Interferometer
  7. Dynamical Systems & Nonlinear Phenomena
  8. Detecting the Flow Pattern Transition in the Gas-Liquid Two-Phase Flow Using Multivariate Multi-Scale Entropy Analysis
  9. Rapid Communication
  10. Simplistic Synthesis and Enhanced Photocatalytic Performance of Spherical ZnO Nanoparticles Prepared from Arabinose Solution
  11. The Deflection of Rotating Composite Tapered Beams with an Elastically Restrained Root in Hygrothermal Environment
  12. Effect of Dust Ion Collision on Dust Ion Acoustic Solitary Waves for Nonextensive Plasmas in the Framework of Damped Korteweg–de Vries–Burgers Equation
  13. Hydrodynamics
  14. Lie Symmetries and Similarity Solutions for Rotating Shallow Water
  15. A Note on the Significance of Quartic Autocatalysis Chemical Reaction on the Motion of Air Conveying Dust Particles
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