Volume 3, Issue 3, March 2014 ISSN 2319 - 4847 Effects of Cd, F … · Volume 3, Issue 3, March...

International Journal of Application or Innovation in Engineering & Management (IJAIEM) Web Site: www.ijaiem.org Email: [email protected]

Volume 3, Issue 3, March 2014 ISSN 2319 - 4847

Volume 3, Issue 3, March 2014

ABSTRACT Effect of Cd, Fe and F dopants on the structural, morphological and optical properties of ZnO thin film deposited on glass substrate at a substrate temperature equal to 450 oC by spray pyrolysis technique have been investigated. X-ray diffraction spectra indicate that the films have polycrystalline nature with a hexagonal wurtzite structure, all the doped ZnO thin films have (002) reflection as the preferred orientation along with the other (100), (101), (102), (110) and (103) reflections, except ZnO: Cd film that appears additional reflection (112) in the XRD pattern. The values of crystallite size are found to be (31, 21.8, 34.1nm) for Cd, Fe and F doped ZnO thin films, respectively. The properties of the thin films changes with the dopants and this cause light loss on the films. Thus, the changes in the transmission take place. The inclusion of dopant into film expands width of localized states as: EU(Cd)>EU(Fe)>EU(F). AFM study reveals that the dopants in the ZnO film had a significant influence on the morphology. 1. INTRODUCTION Zinc oxide (ZnO) is a compound semiconductor of the II-VI family, which has a direct bandgap of (3.37 eV) and large exiton binding energy (60 meV) at room temperature in the ultraviolet (UV) range [1], [2]. Transparent conducting oxide films, such as ZnO thin films, have found extensive applications in optoelectronic devices [3] for example, solar cells [4], liquid crystal displays, heat mirrors and multiplayer photothermal conversion system [5]. ZnO thin films have been deposited by various techniques, such as thermal evaporation [6], [7], sputtering [8], [9], chemical vapor deposition [10], [11], sol-gel [12], [13], pulsed laser deposition [14], [15] and electrochemical deposition [16]-[18]. In addition to these techniques, spray pyrolysis has received a little bit of extra attention because of its simplicity and cost-effectiveness as it does not require sophisticated vacuum apparatus. In this technique, ZnO thin films can be prepared from spray solution of zinc nitrate [19] or zinc chloride [20] or zinc acetate [21]-[24]. Zinc acetate was used in the present study. In this paper, the structural, morphological and optical properties of cadmium, fluorine and iron doped zinc oxide thin films deposited onto glass substrate by spray pyrolysis technique have been investigated. Such doped ZnO thin films are useful in technology of different optoelectronic devices.

2. EXPERIMENTAL Doped ZnO thin films have been deposited on glass substrate at a substrate temperature equal to 450 0C by a home-made chemical spray pyrolysis technique. The experimental setup and other details have been reported elsewhere [25]. A homogeneous solution with 0.1M was prepared by dissolving zinc acetate Zn(CH3COO)2.2H2O in distilled water at room temperature. One drop of HCl was added to (50 ml) solution to prevent the formation of zinc hydroxide Zn(OH)2 and to increase the solubility of the compounds. Similarly aqueous solutions of cadmium acetate dehydrate Cd(CH3COO)2.2H2O, ammonium fluoride NH4F and iron chloride anhydrate FeCl3 were used to obtained Cd, F and Fe doped films. The glass substrates were cleaned with ethanol and acetone for (15 min) by using ultrasonic cleaner and then rinsed in distilled water, and subsequently dried before deposition. The flow of the solution was 6 – 7 ml/min and the oxygen was used as a carrier gas. The substrate to nozzle distance was fixed at (40 cm), and the process involves spraying cycles of (8 s), followed by periods with no spray of (90 s). The structure of the samples was analyzed by X-ray diffraction (XRD) using SHIMADZU X-Ray diffractometer system (XRD - 6000). The optical measurements of the doped ZnO thin films are calculated from the transmittance and absorbance spectra at normal incidence in the wavelength range (300 – 900), using UV-VIS spectrophotometer type (SHIMADZU) (UV-1600/1700 series). The surface morphology of the doped ZnO thin films has been examined using atomic force microscopy (AFM, scanning probe microscope).

3. RESULTS AND DISCUSSION 3.1 STRUCTURAL PROPERTIES The crystal structure and orientation of the doped ZnO thin films were investigated by X-ray diffraction (XRD) patterns. X-ray diffraction spectra of the doped ZnO thin films taken at room temperature are shown in figure 1. These spectra indicate that the films have polycrystalline nature with a hexagonal wurtzite structure, and the peaks with Miller indices given belong to the ZnO (JCPDS card file no. 36-1451) [26]. As shown in figure 1 all the doped ZnO thin films have (002) reflection as the preferred orientation along with the other (100), (101), (102), (110) and (103) reflections, except ZnO: Cd film that appears additional reflection (112) in the XRD pattern. This (002) preferred orientation is due to the

Effects of Cd, F and Fe dopants on the physical properties of ZnO thin films synthesized by

spray pyrolysis

Salam Amir Yousif

Department of Physics, College of Education, the University of Mustansiriyah, Baghdad, Iraq




minimal surface energy which the hexagonal structure, c-plane to the ZnO crystallites, corresponds to the densest packed plane. This result is in a good agreement with literature data [27]-[31].The observed d-values are presented in Table 1, and compared with the standard ones from the (JCPDS) [26]. The matching of the observed and standard d-values confirms that the deposited films are zinc oxide with hexagonal structure.

Figure 1 XRD pattern of doped ZnO thin films

Table 1: XRD parameters of doped ZnO thin films.

Sample ZnO: Cd ZnO: Fe ZnO: F JCPDS

(hkl)

(100) 2θ(deg.) 31.6813 31.6713 31.6713 31.77

d(Ǻ) 2.82200 2.82286 2.82286 2.816 Int. 30 42 45 57

(002) 2θ(deg.) 34.3793 34.5141 34.3268 34.422

d(Ǻ) 2.60646 2.59658 2.61032 2.602 Int. 100 100 100 44

(101)

2θ(deg.) 36.1433 36.2668 36.1299 36.253 d(Ǻ) 2.48319 2.47501 2.48408 2.476 Int. 87 52 86 100

The unit cell parameter (a, c) and the ratio of (c/a) belong to the (002) plane as a preferred orientation of doped ZnO thin films deposited on glass substrate have been calculated using equation (1) [32]. The calculated values of lattice parameter (a, c) and (c/a) are listed in Table 2, found to be in a good agreement with the standard (JCPDS) values [26].

(1)

Table 2: Lattice constants of doped ZnO thin films c/a c (Å) a (Å) Sample

1.59976 5.21292 3.25856 ZnO: Cd 1.59321 5.19316 3.25955 ZnO: Fe 1.60164 5.22064 3.25955 ZnO: F 1.60212 5.20661 3.24982 (JCPDS)

The crystallite size was calculated using a well-known Scherre's formula [33].

(2) Where D is the crystallite size, λ is the wavelength of X-ray used (=1.5406Å), β the broadening of diffraction line measured at half its maximum intensity in radians and θ is the angle of diffraction. The values of crystallite size are found to be (31, 21.8, 34.1nm) for Cd, Fe and F doped ZnO thin films, respectively and listed in Table 3. The crystallite size of the thin films is changed dependence on the different dopant elements, which may be due to the stress arisen from the difference in the ion radii of zinc, cadmium, iron and fluorine

.




The preferred growth orientation was determined using a texture coefficient . This factor is calculated using the following relation [34]:

(3) Where I(hkl) is the measured relative intensity of a plane (hkl), I0(hkl) is the standard intensity of the plane (hkl) taken from the JCPDS data, N is the reflection number and n is the number of diffraction peaks. A sample with randomly oriented crystallite presents TC(hkl)=1, while the larger this value, the larger abundance of crystallites oriented at the (hkl) direction. The texture coefficient calculated for the three main diffraction peaks (100), (002) and (101) are listed in Table 3. It is seen that the highest TC was in (002) plane for all the doped ZnO films. The texture coefficient of (002) plane of the doped ZnO films were between 1.738 and 1.931. The decreasing texture coefficient causes the decreasing in the crystallite size of the films. Therefore the crystallite size of the films decreases with decreasing the texture coefficient, which shows the deterioration in the crystallinity of the films. The micro strain depends directly on the lattice constant (c) and its value related to the shift from the ASTM standard value. The values of micro strain of doped ZnO thin films are listed in Table 3. This strain can be calculated from the following formula [35]:

(4)

The dislocation density )( , defined as the length of dislocation lines per unit volume, has been estimated using the following equation [36], [37].

(5) To derive the film stress (σfilm) parallel to the film surface, the following formula is used, which is valid for a hexagonal lattice [38]:

(6) For the elastic constants Cij, the following values for single crystal ZnO are used: C11 = 208.8 GPa, C33 = 213.8 GPa, C12 = 119.7 GPa, C13 = 104.2 GPa Where: Cfilm and Cbulk are the lattice constants (C) of ZnO film and bulk (or powder).

Table 3: Structural properties of doped ZnO thin films

Sample Crystallite Size (nm)

Texture Coefficient

Micro Strain ε %

Dislocation Density (δ)

Flim stress (GPa) (101) (100) (002)

ZnO: Cd 31 0.711 0.430 1.858 0.121 1.0405 -0.282 ZnO: Fe 21.8 0.441 0.626 1.931 0.258 2.1041 0.601 ZnO: F 34.1 0.657 0.603 1.738 0.269 0.8599 -0.627

3.2 OPTICAL PROPERTIES For most applications, such as solar cells, high transmission in the visible range is important. The transmission of the films changes due to Cd, Fe and F dopants. The properties of the thin films changes with the dopants and this cause light loss on the films. Thus, the changes in the transmission take place.

Figure 2 Transmittance spectra of the doped ZnO thin films




Figure 3 Absorbance spectra of the doped ZnO thin films

The fundamental absorption edge of the films corresponds to electron transition from valence band to conduction band and this edge can be used to calculate the optical band gap of the films. In the direct transition, the absorption coefficient can be expressed by [39]

(7) Where: A is a constant, hυ is the photon energy and Eg is the optical bandgap. Figure 4 shows plots of (αhυ)2 vs. (hυ). The optical band gaps of the films were calculated from these plots and are given in Table 4.

Figure 4 plots of (αhυ)2 vs. (hυ) of the doped ZnO thin films

In the exponential edge region, where , Urbach rule is expressed as [40]

(8) Where: is a constant, is the Urbach energy, which characterizes the slope of the exponential edge. Equation (8) describes the optical transition between occupied states in the valence band tail to unoccupied states of the conduction band edge. Figure 5 shows Urbach plots of the doped films. The value of was obtained from the inverse of the slope of

versus and is given in Table 4. The dopants change the width of localized states in the optical band. values change inversely with optical band gap. The inclusion of dopant into film expands width of localized states as: .




Figure 5 The Urbach plots of the doped ZnO thin films

Table 4: Optical properties of the doped ZnO thin films

EU meV Eg eV Transmittance at 900 nm Dopant 240 3.12 70 Cd 224 3.16 74 Fe 162 3.24 77 F

3.3 MORPHOLOGY The surface morphology of Cd, Fe and F doped ZnO thin films was analyzed using atomic force microscope (AFM), which indicated that the dopants in the ZnO film had a significant influence on the morphology. Figure 6 shows the typical three-dimensional AFM image of Cd, Fe and F doped ZnO films deposited at 450 0C. The pictures show spherical grains that were uniformly distribution on the substrate. The grain size of the films evaluated from AFM pictures are listed in Table 5.The value of grain size was less than (100 nm) which confirms the presence of nanostructures. The difference in the surface roughness of the doped ZnO film belongs to the difference in the ionic radii of the dopants that acts as interstitial or substitution defects in the ZnO lattice. AFM study reveals that the roughness of the film is dependent on the type of the dopant.

Table 5: RMS roughness, Grain size and peak-peak height of film

Sample RMS roughness (nm) Grain size (nm) Peak-Peak height (nm) ZnO: Cd 1.77 98 34.6 ZnO: Fe 0.80 89 4.3 ZnO: F 1.70 77 7.13




Figure 6 AFM images of doped ZnO thin film (1) ZnO: Cd, (2) ZnO: Fe, (3) ZnO: F

4. CONCLUSION The effect of Cd, Fe and F dopants on the structural, morphological and optical properties of ZnO thin films have been investigated by X-ray diffraction technique and optical characterization method. X-ray diffraction spectra indicate that the films have polycrystalline nature with a hexagonal wurtzite structure. The values of crystallite size are found to be 31, 21.8, 34.1nm for Cd, Fe and F doped ZnO thin films, respectively. The optical properties of the ZnO thin films changes with the dopants and this cause light loss on the films. Thus, the changes in the transmission take place. The inclusion of dopant into film expands width of localized states as: EU(Cd)˃EU(Fe)˃EU(F). AFM study reveals that the roughness of the film is dependent on the type of the dopant.

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Volume 3, Issue 3, March 2014 ISSN 2319 - 4847 Effects of Cd, F … · Volume 3, Issue 3, March...

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