Structural, Optical and Electrical Properties of Pure and Co- Doped ZnO Nano Fiber Thin Films Prepared by Spray Pyrolysis

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1 Applied Science Reports E-ISSN: / P-ISSN: DI: /PSCP.ASR App. Sci. Report. 9 (1), PSCI Publications Structural, ptical and Electrical Properties of Pure and Co- Doped Zn Nano Fiber Thin Films Prepared by Spray Pyrolysis Mahjabin Taskin, Jiban Podder Department of Physics, Bangladesh University of Engineering and Technology, Dhaka-1000, Bangladesh Corresponding author: Mahjabin Taskin Paper Information Received: 7 April, 2014 Accepted: 19 May, 2014 Published: 20 January, 2015 Citation Taksin M, Podder J Structural, ptical and Electrical Properties of Pure and Co-Doped Zn Nano Fiber Thin Films Prepared by Spray Pyrolysis. Applied Science Reports, 9 (1), 1-6. Retrieved from (DI: /PSCP.SA ) A B S T R A C T Cobalt doped zinc oxide thin films have been deposited onto cleaned glass substrate using spray pyrolysis method. In this paper, we investigate systematically the surface morphology, structural, optical and electrical properties of the as-deposited Zn 1-xCo x films with x ranging from 0 to The Scanning electron microscopy micrograph shows nanofiber structure around the nucleation centre of as-deposited thin films. X-ray diffraction studies showed the polycrystalline nature of the films with hexagonal wurtzite structure. For as-deposited Zn 1-xCo x films, the band gap reduced from 3.2 to 2.7 ev with increasing Co concentration indicting that Co 2+ ion replaces Zn 2+ ions in the Zn lattice. The optical transmittance of the films is found to decrease from 80 % to 70 % with the addition of Co doping. The electrical conductivity of Zn 1-xCo x films is found to increases with the incorporation of Co in the solution PSCI Publisher All rights reserved. Keywords: Co-doped Zn; nano fiber; optical band gap; spray pyrolysis; XRD. Introduction II-VI wide band gap semiconductors have given much attention as an important material that we come across in our day-to-day lives. Transparent conducting Zn is one of the most promising materials in many commercial devices due to its unique properties, such as low resistivity and high transmittance in the visible solar region (Minami 2000). Zn is also a good candidate for many device applications, such as transparent electrode, flat panel display, organic light emitting diode, solar cells, field emission device, sensors, ultrasonic oscillators, transducers, photoprotective coating, surface acoustic wave device and laser diodes (Wang, 2007; Breivik et al, 2007; Xu et al, 2005; Ruske et al, 2007). Recently, theoretical works have predicted that Zn based semiconductors can present ferromagnetic behavior at room temperature when doped with transition metals such as cobalt (Ueda et al, 2001). This makes these systems very attractive for both fundamental and technological point of view. 3d transition metals like Fe, Co, and Ni, have partially filled d states. These partially filled d states contain unpaired electrons and responsible for exhibiting magnetic behavior. The addition of 3d transition metal ions can change the Fermi energy state by raising the valance band maximum and lowering the conduction band minimum, thus reducing the band gap. Further, we expect that the doping of Co metal ion into the Zn matrix will change the optoelectronic properties as well as magnetic properties. Many techniques have been used to synthesize Zn films, including pulsed laser deposition, dc reactive magnetron sputtering, atomic layer deposition, sol gel spin coating, radio frequency magnetron sputtering, metal organic chemical vapor deposition and chemical spray pyrolysis (Lin et al, 2009; Srinivasan et al, 2007; Huang et al, 2008; Kong et al, 2007). In this work we report the growth of Co doped Zn thin films on glass substrate by spray pyrolysis and the influence of Co doping on the structural, optical and electrical properties of Zn thin films.

2 Experimental details Zn 1-x Co x (where x= 0, 0.05, 0.15, 0.25 and 0.35) thin films were deposited onto glass substrates by spray pyrolysis technique. Zinc acetate (Zn(CH 3 C) 2.2H 2 ) and cobalt acetate Co(CH 3 C) 2 4H 2 were used as precursors for Zn and Co, respectively. Initially, Zn(CH 3 C) 2.2H 2 was dissolved in water and stirred well for homogeneous solutions. For Co doping, we dissolved stoichiometric amount of Co(CH 3 C) 2. 4H 2 in water, separately. Then we added the droplets of this Cocontaining solution to the starting solution. Homogeneous solution of this mixture was attained by continuously stirring it for 2 hours. The total concentration of the solution was kept at 0.1 M. Undoped and Co-doped Zn thin films were deposited at 573 K on cleaned glass substrate. The spray nozzle and the substrate are kept at a distance of 25 cm from each other. Compressed air was used as the carrier gas with the airflow rate maintained at 0.5 bar. The spray rate of solution was maintained at 0.2 ml min -1 throughout the experiment. The possible chemical reaction that takes place on the heated substrate to produce Zn 1-x Co x may be as follows: Zn(CH 3 C) 2.2H C + ZnCo / Zn 1-x Co x + C 2 + CH 4 + Steam Co(CH 3 C) 2.4H 2 Decomposes The relative stoichiometries of the films were verified using energy dispersive X-ray (EDX) spectroscopy and were found to be within 10% of the expected values according to the precursor mole ratios. Scanning electron microscopy (SEM) micrographs were obtained to study the structural morphology. The crystal structure properties of the films were investigated using X-ray diffraction (XRD) eith a Co Kα (λ = 1.79 Å) source. ptical transmittance measurements were performed to measure the band gap and temperature dependent electrical resistivity was studied from 300 to 440 K via the Vander Pauw method. Results and Discussion Surface Morphology Surface morphology of Zn 1-x Co x thin films was observed using SEM under K magnification. Fig. 1 (a-d) showed SEM micrographs. SEM reveals that sprayed particles (cluster of atoms) are adsorbed onto the glass substrate. Nanofibers are observed in pure Zn thin film. The average diameter of the fibers is around 600 nm as shown in fig 1 (a). Similar fiber feature was also reported by other researchers (Islam and Podder, 2009; Llicana et al, 2006). Figure1. SEM micrograph of the as-deposited Zn 1-x Co x films for (a) x=0, (b) x=0.05, (c) x=0.15, (d) x=0.25. Due to interstitial holes of Zn are filled with cobalt, nanofibers are observed more clearly and the average diameter of the fiber increases around 750 nm in 5% doping concentration as shown is fig 1 (b). However above 5% doping, Zn are 2

3 Intensity (arb. unit) (102) (110) (101) (002) (100) substituted by cobalt nanofibers are observed but the diameter of the fiber decreases around 620 nm in 15% doping fig 1 (c). Finally for 25% doping fibers become narrower around 300 nm. Compositional Studies The quantitative analysis of the as-deposited Zn films carried out by EDX. Two strong peaks corresponding to Zn and were found in the spectrum, which confirms the high purity of the Zn thin film. At high operating voltage the electron beam penetrates the film and reaches the glass surface, which results the Si peak. And all peaks for Zn, Co and are obtained for 15% doping concentration. From the EDX pattern it is observed that the height of the peak for Zn decreases and Co increases with the increase of doping concentration. The percent of Zn, Co and present in different values of Co doping are shown in the table 1. From the above data it is evident that for all the films the amount of Zn, Co and are present at stoichiometric ratio. Table1. Atomic% of different Co compositions of Zn 1-x Co x thin films. Sample Element, Atomic % Zn Si Co Pure Zn % Co Zn Structural Studies The XRD has been taken on as-deposited pure and Co doped Zn thin film. Using the data of diffractogram, the d hkl (inter-planar distance) values and their corresponding <hkl> values have been identified as (100), (002), (101), (102) and (110) for pure Zn films which indicate the hexagonal wurtzite structure of Zn. The calculated lattice parameters (a, c) values, deviation of 2θ and grain size of these samples are tabulated in table 2. The X-ray diffractogram of Zn samples with different concentration of Co are shown in Fig 3. Grain size of the prepared Zn thin film was determined from the stronger peaks of (002) from each XRD patterns using Scherrer formula, D g 0.94 cos where D g is the average grain size, λ is the wavelength of the radiation used as the primary beam of CoK α (λ= Å), θ is the angle of incidence in degree (Bragg angle) and Δ is the full width at half maximum (FWHM) of the peak in radian, which was determined experimentally after correction of instrumental broadening (in the present case it is 0.05 ). Thin films, where two strong peaks (100) and (002) are shown in expanded form to understand the variation of FWHM and peak shift of Bragg peaks with temperature. Lattice parameters a and c were determined from the 2θ value and change with increasing of Co doping. It was observed that Δ2θº value is different for Co doped Zn films than pure Zn thin films. It is observed that the peaks in the doped films shift from their standard positions in the presence of the dopant. The shift in the lattice parameter is mainly due to the dopant occupying interstitial positions in the lattice. Zn 0.65 Co 0.35 Zn 0.75 Co 0.25 Zn 0.85 Co 0.15 Zn 0.95 Co Zn degree) Figure2. X-ray diffraction patterns of Zn 1-x Co x thin films 3

4 Transmittance (arb. unit) No peaks corresponding to either Co metal or any of its oxides are observed in any of the diffractograms, which indicates that there is no additional phase present in Zn 1-x Co x films, at least within the limit of (35%) of Co. When the Co concentration increases the intensity of the (002) peak decreases with increase in FWHM. This is due to the lattice disorder and strain induced by Co 2+ substitution in the host lattice (Subramanian et al, 2010). Table 2. X-ray diffraction data, lattice parameters, grain size for pure and Co doped Zn thin films. Average grain Composition For (100) reflection For (002) reflection size from (002) reflection c/a Ratio 2θ d hkl(å) a (Å) 2θ d hkl(å) c(å) nm Zn Zn 0.95Co Zn 0.85Co Zn 0.75Co Zn 0.65Co ptical Properties ptical Transmittance and Band gap To study the optical properties of Zn 1-x Co x thin films, the transmission spectra of the thin films were recorded in a wavelength range of nm, using a UV-VIS spectrophotometer. The transmission spectra of these films are shown in Fig. 4 (a). It is seen that the values of transmittance is high in the visible and IR region and it is minimum at wavelength ~ 300 nm. Films prepared at 300ºC exhibit a transmission of > 80% in the visible and near IR region; again it is found > 70% in the visible and near IR region for Co doped film, which is found to be smaller than that of undoped film. Below 450 nm there is a sharp fall in the percentage of transmittance, %T of the films, which is due to the strong absorbance of the films in this region. It is found that transmittance decreases with increasing dopant concentration, which is due to the decrease in the crystalline nature of the doped films as revealed by XRD. The transmittance of the films is also influenced by a number of effects, which include surface roughness and optical inhomogenity in the direction normal to the film surface. Additional absorption peaks appear in the case of Co substituted Zn films. Similar result was obtained by other researcher for the absorption peak (Xue- Chao et al, 2007). Figure 4 (b).shows the plots of (αhν) 2 vs. photon energy (hν) for direct transition. The optical band gap (Eg) is determined from the plots of (αhν) 2 vs. photon energy (hν) for direct transition for Zn 1-x Co x thin films. The direct band gap energy of the films have been obtained from the intercept on the energy axis after extrapolation of the straight line section of (αhν) 2 vs. hν curve. The band gap of the films varied between 3.2 ev to 2.7 ev. It is observed that small amount of Co present in the films greatly affects the optical band gap of Zn. The band gap was observed to decrease with an increase of Co in the concentration. A decrease in band gap energy with increase of doping concentration is also reported by K. J. Kim et al and K. Samanta et al (Kim and Park, 2002; Samanta et al, 2005). We think that this shift of the band gap with the Co incorporation interpreted as mainly due to the sp-d exchange interactions between the band electrons and localized d electrons of the Co 2+ ions substituting Zn ions. The s-p and p-d exchange interactions give rise to a negative and a positive correction to conduction and valance band edges, leading to narrowing of the band gap (Ilican at al,2007). The nature of this variation in the band gap energy may be useful to design a suitable window material for fabrication solar cells ( h ) 2 x10 14 in (m -1 ev) x=0.35 x=0.25 x=0.15 x=0.05 x= x=0.35 x=0.25 x=0.15 x=0.05 x= Wavelength (nm) Photon energy, h in (ev) Figure3. (a) Variation of transmittance as a function of wavelength for Zn 1-x Co x films, (b) Plots of (αhν) 2 vs. photon energy (hν) for Zn 1- xco x thin films 4

5 Resisitivity, x10 2 (ohm m) Table 3. ptical band gap and refractive index of Zn thin films of different thicknesses. Composition Band gap (E g ev) Refractive index (n) at 70 nm Zn Zn 0.95Co Zn 0.85Co Zn 0.75Co Zn 0.65Co Electrical Properties Resistivity The variation of electrical resistivity with temperature for as deposited Zn 1-x Co x thin films are shown in Figure 7. From the figure it is seen that resistivity decreases with the increase of temperature. This type of variation indicates the semiconducting behavior of the films. The resistivity decreases with the increasing of cobalt concentration. 3 2 x=0.35 x=0.25 x=0.15 x=0.05 x= Temperature, T (K) Figure4. Variation of electrical resistivity with temperature for as deposited Zn 1-x Co x films of different concentrations. Conclusions In this investigation, the structural and optical properties of Co-doped Zn thin films synthesized by spray pyrolysis with different dopant concentrations are investigated. Spray pyrolysis grown Co-doped Zn films showed good crystallinity suggesting the substation of Zn ion by Co ion as evident from XRD and UV-VIS spectrum. XRD results indicated that the Co ions replaced the Zn ions without changing the wurtzite structure, whose lattice parameters are also similar to that of Zn. Nanofibers are observed in both pure and Co doped Zn thin films from SEM. The optical transmissions of the films are found to decrease from 80 % to 70 % with the addition of Co doping. The films show direct band gap decreasing with increasing doping concentration in the range ev. The conductivity increases with the increasing of cobalt concentration. ur experimental results suggested that Cobalt absorbs UV so for high transmittance and low band gap we can use it in UV sensor. By doping Cobalt the film becomes highly conductive so we can use it in surface acoustic wave devices and transparent conducting electrodes. References Breivik TH, Diplas S, Ulyashin AG, Gunnaes AE, laisen BR, Wright DN, Hott A, lsen A Nano-Structural Properties of Zn Films for Si Based Heterojunction Solar Cells, Thin Solid Films 515(24): Huang B, Li J, Wu Y, Guo D, Wu S ptical Constants of Transparent Zn Films by RF Magnetron Sputtering, Mater Lett 62(8-9): Ilican S, Caglar M, Caglar Y The Effect of Deposition Parameters on the Physical Properties of Cd xzn 1-xS Films Deposited by Spray Pyrolysis Method ptoelectron. Adv. Mater. 9(5): Islam MR, Podder J ptical Properties of Zn Nano Fiber Thin Films Grown by Spray Pyrolysis of Zinc Acetate Precursor, Cryst. Res. Techn 44(3): Kim KJ, Park YR Spectroscopic Ellipsometry Study of ptical Transitions in Zn 1 xco x Alloys, Appl. Phys. Lett. 81(19): Kong BH, Mohanta SK, Kim DC, Cho HK ptical and Structural Properties of Zn Thin Films Grown on Various Substrates by Metalorganic Chemical Vapor Deposition, Condens Matt (15): Lin JM, Zhang YZ, Ye ZZ, Gu XQ, Pan XH, Yang YF, Lu JG, He HP, Zhao BH Nb-doped Zn Transparent Conducting Films Fabricated by Pulsed Laser Deposition, Appl. Surf. Sci. 255(13-14):

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