Spin Coated Cadmium Doped Zinc Oxide Thin Films: Preparation, Structural, Optical and Electrical Characterization

Transcription

1 Indian Journal of Science and Technology, Vol 935), DOI: /ijst/2016/v9i35/98613, September 2016 ISSN Print) : ISSN Online) : Spin Coated Cadmium Doped Zinc Oxide Thin Films: Preparation, Structural, Optical and Electrical Characterization J. Charlesbabu 1*, K. Gopalakrishnan 2 and M. Elango 3 1 Department of Electronics, SNR Sons College, Coimbatore , Tamil Nadu, India; babucharles32@gmail.com 2 Department of Electronics, Bharth University, Chennai , Tamil Nadu, India;gopikrishna2804@gmail.com 3 Department of Physics, PSGCAS,Coimbatore , Tamil Nadu, India; elangopsg05@gmail.com Abstract Pure and cadmium Cd 2+ ) doped zinc oxide ZnO) were prepared by programmable microcontroller assisted spin coating unit using organometallic precursor. The pure and thin films were deposited on well glass substrate at 1500 rpm under alkaline condition. 400ºC sintered films were characterized for their structural, morphological, optical and electrical properties using X-ray diffraction method, scanning electron microscopy, UV-visible absorption method and DC measurements respectively. Hexagonal wurtzite phase with multiple faceted growths was witnessed for pristine film. Without alteration in the phase but dopant induced small variation in the position of prominent peaks were shown by Cd thinfilms. The dopant induced island growth and increasing roughness were clearly unveiled via SEM analysis. Variation in the structure and shape of the optical absorption peak reveal the varying crystallite size in accordance with dopant concentration. The electrical measurements showed that the doping process enhance the conducting property of the films. On comparison 6% Cd films exhibit good conductivity than other films. Keywords: Cd Doped ZnO Thin Films, Electrical Analysis, Optical Properties, Spin Coating, Structural Analysis 1. Introduction The oxides are attractive not only for scientific interest but also for potential application in gas sensing, electronic, and photonic devices. One of the most important wide band gap oxide semiconductors that draw worldwide attention of researchers is zinc oxide ZnO) 1 This ZnO thin film with hexagonal Wurtzite) structure possesses unique properties such as high electro chemical stability, resistivity control, absence of toxicity, important piezo electric properties, and high quantum field. These better properties are utilized wide high optical gain and low loss 2 solar cell 3 light emitting diodes 4, photo detectors 5, Piezoelectric cantilever 6, gas sensor 7 and dye sensitized solar cells 8, and recently ZnO layer has been used to improve the performance of TiO 2 in the dye degradation 9. In addition, ZnO has relatively high physical and chemical stability, and hence it has much high temperature application, such as buffer for III-V nitrides 10. Therefore, it is not surprising that it has been under intensive investigation and many methods have been employed for preparation of such significant material in thin film form 11. Already pure and thin films have been prepared by number of methods which includes thermal evaporation, Spray pyrolysis, anodic oxidation, electro deposition, sputtering, spin coating and chemical bath deposition. Among several techniques spin coating is a well-known simplest and cheapest aqueous technique for depositing large area thin films. The form of ZnO unique properties low temperature deposition is cost effective, uniform particle distribution, high purity and The form of ZnO thin film was formed by spin coating on a substrate at various rpm for various times. The preparation of films was done by changing the absorption rate and reaction rate. Motivated by these facts in the present investigation Cd films were prepared by spin coating * Author for correspondence

2 Spin Coated Cadmium Doped Zinc Oxide Thin Films: Preparation, Structural, Optical and Electrical Characterization technique and the deposited films were characterized by various characterization techniques such as XRD, SEM, UV and electrical conductivity and the obtained results were discussed in detail. 2. Experimental Details The solution to be coated is prepared from standard precursors for 50ml. various molar solutions has been prepared and deposited over cleaned glass substrate. Form various prepared it is observed that solution prepared for 0.6 molarity have most significant coating. The solution is prepared from zinc acetate dyhydrate and monoethanaloamine. The molar ratio of zinc acetate dyhydrate monoethanaloamine is kept as 1:1. Initially desired amount of zinc acetate dehydrate is dissolved in 50ml of ethanol and continuously stirred in a magnetic stirrer at room temperature for 1 hour. In mean time the solution will turn milky white under constant stirring. When the salt is completely dissolved in the solution and the solution is milky white equal amount of monoethanaloamine is added to the solution drop by drop to the solution. Now the solution turns to transparent clear crystal solution under continuous stirring at 333K for 2 hours. The solution is left undisturbed for ageing for 1 day. The solution turns pale golden yellowish in colour reflects the presence of Zinc in the solution. This solution is used to coat pure ZnO films over glass substrate. Similarly solution is doped with cadmium and the coating process is being performed to study for various concentrations. Initially with the desired amount of zinc acetate dyhydrate 2 wt % of cadmium acetate is added and the solution is prepared and allowed for ageing. Now then solution is prepared for various dopant concentrations. Here solution for 2 wt %, 4 wt %, 6 wt %) are prepared and coated over the substrate and various parameters are studied. 3. Results and Discussion 3.1 Structural Properties of ZnO Film X-ray diffraction patterns of Cd thin films were shown in the Figure 1 a), b), c) d). The XRD spectrum of un and cd naro structures grown on glass substrate samples coated at 673K are shown. They all exhibits sharp diffraction peaks characteristics of the ZnO wurtzite hexagonal phase. In the XRD spectrum observe the broad & multiple no of peaks which resembles the film possess the multiple face growth Figure 1 a). The 002) plan peak is more broaden and high intensity shows that the film was highly oriented to that face and thickness as the grain size are small. We have also observed some additional peaks 100), 102), 101), 110) has less intensity compare to the predominant peaks. In which the number of crystallites values are also minimum. From XRD spectrum we have also calculated the strain, dislocation density, grain size values and they resemble the results derived from the other characterization results 12. The crystallite size of ZnO thin film was calculated any Scherer s formula. The several of crystalline grain size D), hkl plane and 2θ values are listed in Table 1. The lattice constants values are calculated and they are found to be a = A, c = A for pure and they are well agreed with previously reported value and it conforms the flow film is in Wurtzite hexagonal) closed pack structure). Table 1. Structural parameters of pure and Cd thin films Sample 2θ deg) θ deg) Strain β ) Grain size D) Miller plane Dislocation Density δ ) Pure ZnO nm wt % Cd 4 wt % Cd 6 wt % Cd nm 11.4nm 11.1nm 8.1nm 8.2nm 8.2nm 2.7nm 2.9nm Vol 9 35) September Indian Journal of Science and Technology

3 Charlesbabu, K. Gopalakrishnan and M. Elango more or less uniformly over the surface. Although no cracks could be detected, some holes indicating porosity is present. The pores in the surface are related with the decomposition reaction of precursor. Figure 1. a-d). XRD spectrum of pure and Cd doped ZnO thin films. 3.2 Morphological Analysis of Pure and Doped ZnO Films The recorded SEM image of ZnO thin film sample annealed at 673 K with 50 spinning cycle is depicted in Figure 2 a), b), c),d).sem scanning electron microscope) is becoming more useful for the direct observation of surfaces because they offer better resolution and depth of the field than optical microscopes. A beam of very small diameter order of 10 nm) is produced by the electron gun and electron lenses. The scanning coils deflect this beam and sweep it over the specimen surface. A cathode ray display tube is modulated by the signal which arises from the interaction of the beam with the surface element which is being probed. The yield of the collected electrons liberated from the specimen depends on the nature of the specimen surface and on its inclination with respect to the probing beam. Consequently one can obtain pictures with highly perspective appearance. From the micrograph one can see that the spherical grains along with some spongy clusters and as the temperature increases, the spongy appearance vanished and the relatively compact spherical grains can be seen. Further increase in temperature might causes increase in crystallite size since more annealing temperature evaporates complex compounds and hence grain size is relatively increased it is good agreement with earlier report. The polycrystalline and porous nature is revealed and the micrometer crystallites distributed Figure 2. a-d). SEM images of pure and Cd doped ZnO thin films. 3.3 Optical Properties of Pure and Cd Doped ZnO Film The optical transmittance spectra of ZnO thin films obtained in the various concentrations and revealed in Figure 3 a), b), c), d). In order to study the optical activity of the thin film samples prepared for various concentration UV visible spectrums is taken for both absorbance and transmittance. Optical properties of the pure & doped ZnO thin film were analyzed using depressing mode using ethanol as solvent at room temperature. Above graphs demonstrate the optical absorption spectra of ZnO thin film and Zn 1-x Cd x O 2wt%, 4wt%, 6wt%) thin film coated over the glass substrate. The absorption spectra of ZnO thin film at nm is get shifted to longer wave length due to doping with Cd2 wt %, 4 wt %, 6 wt %) and their absorption wave length is nm, nm and nm respectively. The dopant cadmium, induce appreciable visible light response which create red shift in the absorption wave length. The red shift of the absorption curve results on the reductively of band gap energy and also the recombination rate in visible spectrum 13. The band gap of pure ZnO film is calculated as 3.44 ev and that of Cd film at 2 wt %, 4 wt %, 6 wt % were 3.43eV, 3.429eV, 3.423eV respectively. Vol 9 35) September Indian Journal of Science and Technology 3

4 Spin Coated Cadmium Doped Zinc Oxide Thin Films: Preparation, Structural, Optical and Electrical Characterization a c Figure 3. a-d). Optical absorption spectrum of pure and Cd thin films. 3.4 Electrical Conductivity DC electrical conductivity for the pure and Cd doped ZnO thin films were carried out using the Keithley 6517B electrometer. The observations were made while cooling the sample. Figure 4 shows the current voltage response for the films. From the graph it is identified that the current was linearly increasing with the applied voltage. Moreover the current increased with respect to the increase in Cd doping. DC conductivity was calculated using the given formula where, d is the thickness of the sample, R is the measured resistance and A is the area of the face in contact with the electrode.figure4shows that electrical conductivity linearly increased with the increase in Cd doping also it confirm the semiconductor nature of the films. The increase in Cd doping creates defects in the films. The defect concentration furthermore increases with dopant concentration, thus increasing the conductivity. Figure 4. films. d I-V spectrum ofpure and Cd thin 4. Conclusion Thickness of the film formed on the glass substrate depends spinning cycles and annealing temperature. The sample with minimum thickness its form to be 0.7mm & the maximum thickness is 1.4mm. The XRD studies reveal that the ZnO then film prepared show hexagonal wurtzite structure. Also observed that crystallization increases with the sp cycles &annealing temperature. The SEM Micrographs shows that the spherical along with the spongy clusters at low annealing temperature the sample with 20 spring and 673 k its minimum 50/nm 673k maximum absorbance. The average band gap of ZnO film coated over a glass substrate is found to be 3.4eV and varies with decrease with dopant concentration. The decrease in band gap of ZnO film may be attributed to the improvement in crystalline quality along with the reduction in the grain size. The XRD results shows predominant peak corresponding to Zinc oxide in the plane 002) and reveals various parameters such as grain size, strain, stacking fault and dislocation density. The Lattice constant a =3.25A and C=5.20A measured agrees with the standard value. The SEM image reveals the polycrystalline structure of pure ZnO film. The spinning cycles and annealing temp are the main factors which changes the resistivity of the samples. The sample with the minimum spinning cycles and minimum annealing temperature has the maximum resistivity. 5. References 1. Ozgur U, Hofstetter D, Morkoc H. ZnO devices and applications: a review of current status and future prospects.proceedings of the IEEE. 2010; 987): Shinde VR, Lokhande CD, Mane RS, Han CH. Hydrophobic and textured ZnO films deposited by chemical bath deposition: annealing effect. Application Surface Science. 2005; 2451): Ennaoui M, Weber R, Scheer HJ, Lewerenz L. Solution energy master. Sol.Cells.1998; 541 4): Pathan HM, Lokhande CD. Deposition of metal chalcogenide thin films by Successive Ionic Layer Adsorption and Reaction SILAR) method. Bulletin of Materials Science. 2004; 272): Mahalingam T, John VS, Raja M, Su YK, Sebastian PJ.Electrodeposition and characterization of transparent ZnO thin films.solar Energy Materials and Solar Cells. 2005; 882): Shinde VR, Lokhande CD, Mane RSS, Gujar TP, Han SH. 4 Vol 9 35) September Indian Journal of Science and Technology

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