Synthesis and Structural study of Rare Earth activated ZnO Thin film Pawan Kumar Department of Physics, University Institute of Sciences, Chandigarh University, Gharuan (Mohali), Punjab (India) e-mail-pawan.uis@cumail.in Abstract The rare earth materials have attracted world-wide attention due to their unique optical properties. In this study, we have prepared pure ZnO and Rare Earth doped ZnO Thin film prepared by simple sol-gel spin coating method. The doping of rare eath in ZnO is a challenging task because of large ionic radius mismatch between Zn ions and rare earth ions. We have studied the change in structural properties of ZnO due to doping of Rare- Earth material (Eu and Dy).XRD analysis confirm that the hexagonal wurtzite structure is present in all samples without any impurity phase. Intensity of XRD peak of doped ZnO is less than pure ZnO thin film which indicates small lattice distortion take place due to incorporation of Rare-Earth material. The (002) peak of XRD pattern is shifted towards lower diffraction angle for doped ZnO thin film. We have observed that crystallite size of Dy and Eu doped ZnO is less than pure ZnO thin film. Keywords: Zinc Oxide, Eu doped ZnO, Dy doped ZnO, Thin film Introduction The study on zinc oxide shows that the doping in ZnO thin films can improve the optical, electrical and magnetic properties of pure ZnO. Recently, there are many reports on the properties of transition metal and rare earth doped ZnO thin films. ZnO doped with a different of transition metals and rare earth has been found a promising materials for creating ferromagnetic semiconductors and semiconductor with enhanced luminescence[1-2]. Among all rare earth, Eu and Dy are most popular element because of their intense emission. The enhanced optical as well as improved magnetic properties have been reported by several authors [3-4]. Eu and Dy have theire characteristic emission due to f-f transitions. Zinc oxide is found in two main form- Hexagonal wurtzite and Cubic zinc blende. Wurtzite is most stable structure at room temperature. The Zinc blende structure is stable when it is deposited on the glass substrate. Zn and Oxygen is in tetragonal form in all the case. The common value of lattice constant of ZnO is a=3.25a 0 & c=5.2a 0 and the ratio of c/a =1.633. The variation from standard c/a ratio indicates the distortion is crystal structure of ZnO. The ZnO have covalent bonds with corresponding radii of 0.74nm for Zn 2+ & 0.140 nm for O 2-. This property supports the wurtzite structure of ZnO and creates strong piezoelectricity of ZnO. This occurs due to polar bonds. ZnO has band gap of 370
3.44 ev at low temperature and 3.37 ev at room temperature. The stability of ZnO structure at room temperature and pressure make it more applicable for several applications like light emitting diodes and photo detectors. The free-exciton binding energy of ZnO is greater than GaN, which make it better as compared to GaN. Large exciton binding energy of ZnO indicates that it ca have efficient excitonic emission at room temperature. These properties make ZnO a very useful material for optical devices based on excitonic effects. In this work we have tried to maintain the crystalline structure of ZnO with small amount of rare earth doping. The cast effective techniques has been used to prepare the sample without any impurity phase formation. Experimental Details The materials used for synthesis of pure and doped ZnO: Zincacetate dihydrate (CH 3COO) 2Zn.2H 2O, Dysprosium(III) acetate tetrahydrate Dy(OOCCH) 34H 2O, Europium (III) acetate hydrate Eu(OOCCH 3) 3xH 2O, Monoethanolamine (MEA) and 2-Methoxethanol. The sample has been preapared with given starting materials with the help of sol-gel spin coating techniques. The materials in appropriate amount have been taken to make a gel of 0.3M. We have prepared the sample for Eu4% doped ZnO and Dy4% doped ZnO thin film. This gel is spin coated on glass substrate by spin coater to get the thin fil structure. The process of coating is repeated to achieve the desire thickness. The crystal structure,size of the particle and lattice parameter were investiged by using Rigaku X- ray diffractometer (HRXRD) High Resolution X-ray diffraction with monochromated Cukα target (λ= 1.54) at room temperature in the scan range 20-90 degree with scan rate of 0.02 Result and Discussion Structural study Fig.1 shows the HR-XRD pattern of ZnO, Zn 0.96Dy 0.4O and Zn 0.96Eu 0.4O annealed at 550 0 C. The observed XRD pattern confirms that the pure and Eu,Dy doped ZnO thin film exhibit single phase with the hexagonal wurtzite type structure of ZnO.(PDF# 792205) 371
Fig 1: HR-XRD spectra of all samples. All the observed peaks are indexes according to (hkl) values of pure ZnO HCP structure. No additional peaks has found, this reveals the formation of pure ZnO in hexagonal wurtzite phase. The prominent peaks of (100),(002)&(101) of pure and doped samples are shown in fig 2. The doping of Dy and Eu ions into ZnO causes a slight shift to the lower diffraction angle side for (002) peaks. The intensity in doped ZnO are less than that of pure ZnO thin film, which indicates small deformation in HCP structure and confirms the doping of Dy and Eu ions. The lattice parameter of the samples are calculated with the help of unitcell software & are tabulated in Table 1. In both the cases lattice parameter c increases from 5.21007A 0 in ZnO to 5.21597A 0 in Zn 0.96Eu 0.4O and 5.76809A 0 in Zn 0.96Dy 0.4O because the radius of Eu 3+ (0.95A 0 ) and Dy 3+ (0.91A 0 ) which is much higher than that of Zn 2+ (.74A 0 ).This is expected due to lattice distortion and incorporation of RE 3+ (Rare earth) ions having higher ionic radius than that of Zn 2+. 372
Table-1: Structural parameters of thin films. Composition a c c/a Cell APF Volume ZnO 3.25486 5.21007 1.6007 47.8013 75.5% Zn 0.96Dy 0.4O 3.43060 5.76809 1.6814 58.7900 71.8% Zn 0.96Eu 0.4O 3.25231 5.21597 1.6038 47.7804 75.3% For H.C.P. structure a=b c and c/a =1.63. In our observation c/a ratio of undoped ZnO is 1.6007 and c/a ratio of Eu and Dy doped ZnO a little change at third and fourth decimal point.i.e. for Eu: ZnO (c/a=1.6038) and for Dy:ZnO (c/a = 1.6814). This change in lattice parameter value with doping may be due to ionic radius mismatched between Zn(0.74A 0 ) & Dy 3+ (0.91A 0 ) & Eu 3+ (0.95A 0 )[63] The HR-XRD analysis reveals the fact that all samples have similar structure i.e. hexagonal wurtzite structure. The lattice parameter variation in different samples confirms the substitution of Eu and Dy ions in Zn site of ZnO. The APF (Atomic Packing Fraction) for HCP structure is calculated by the formula [5] APF=2πa/3 3 c The APF of nanoparticles is 75% and for bulk ZnO is 74% in our observations APF of pure ZnO is 75.5% which is close to nanoparticle APF of ZnO and for Zn 0.96Dy 0.4O APF = 71.8% which indicates that atoms are not closely packed and there are some voids. But in Zn 0.96Eu 0.4O the APF is 75.4% which indicates that the atoms are closely packed. The particle size (D) can be estimated by using Debey Scherrer formula [6] D=0.9λ/βcosθ Where D is crysttalite size, λ is the wavelength(1.54 Å) of incident X-Ray. The crystallite sizes(nm) shown in Table 2. The decrease crystallite size in Zn 0.96Dy 0.4O and Zn 0.96Eu 0.4O sample indicates the loss in crystallinity in the sample and also attributes to formation of Dy-O-Zn & Eu- O-Zn on the surface of doped sample. Table-2: Crystallite size (nm) for different (hkl) (hkl) ZnO Zn 0.96Dy 0.4O Zn 0.96Eu 0.4O (1 0 0) 39.47 22.13 21.90 (0 0 2) 39.74 20.17 20.74 (1 0 1) 36.82 36.38 17.84 373
Conclusion The Eu and Dy doped ZnO thin film is successfully prepared by sol-gel spin coating techniques. The synthesized samples do not have any impurity phase that can be detected by the instrument. The small change in lattice parameters and crystallite size indicate the substitution of Rare earth ions on the crystallite site of Zn+2 ions. The replacement of Zn ions with higher ionic radius rare earth ions creates a lattice distortion but crytal structure is still maintained in all prepared samples. The results indicate that sol-gel spin coating is an effective techniques for preparation of thin film. References 1. L. Zhu and W. Zeng, Room-temperature gas sensing of ZnO-based gas sensor: A review, Sensors Actuators, A Phys., vol. 267, 2017, pp. 242 261. 2. F. Jiang, Z. Peng, Y. Zang, and X. Fu, Progress on rare-earth doped ZnO-based varistor materials, J. Adv. Ceram., vol. 2, no. 3, 2013, pp. 201 212. 3. P. Kumar and P. C. Pandey, Investigations on absorption, photoluminescence and magnetic properties of ZnO: Co nanoparticles, J. Sol-Gel Sci. Technol., vol. 80, no. 2, Nov. 2016, pp. 342 352. 4. P. Kumar, B. K. Singh, B. N. Pal, and P. C. Pandey, Correlation between structural, optical and magnetic properties of Mn-doped ZnO, Appl. Phys. A, vol. 122, no. 8, Aug. 2016, p. 740. 5. P. Kumar, A.K. Yadav, D. Bhattacharyya, S.N. Jha and Praveen C. Pandey, Lithium ion Assisted Luminescence and Ferromagnetism in Europium Doped Zinc Oxide. Material chemistry and Physics 214, 2018, 306-319. 6. P. Kumar, A.K.Yadav, A. G. Joshi, D. Bhattacharyya, S.N. Jha and Praveen C. Pandey Influence of Li co-doping on structural property of sol gel derived Terbium doped Zinc oxide nanoparticles Mater. Charecterization 142, 2018 593 601. 374