Study of Structural and Optoelectronic Properties of ZnO Codoped with Ca and Mg

1 P. G. Department of Physics, Patna Science College, Patna University, Patna-800005, Bihar, India 2 Aryabhatta Centre for Nanoscience and Nanotechnology, Aryabhatta Knowledge University, Patna, Bihar, India 3Material Research Centre, Brno University of Technology, Brno, Czech Republic 4 Physics Department, R. R. M. Campus, Janakpur District, Dhanusha, Nepal 5 Nanotechnology Application Centre, Allahabad University, Allahabad, Uttar Pradesh, India


Introduction
Zinc Oxide has attracted a lot of research interest due to its enormous potential for application in a variety of optoelectronic and electronic devices. The main advantages of ZnO for optoelectronic applications are its large exciton binding energy [60 mev], wide band gap energy of 3.2 ev at room temperature, and the existence of well developed bulk and epitaxial growth processes. ZnO can be prepared by an easy and cheap chemical method. It is nonpoisonous; so it can be used widely. ZnO thin films are used as transparent electrodes in photovoltaic cell in place of expensive Indium Tin Oxide [1]. ZnO nanowires have also been investigated as gas sensors [2,3]. ZnO is suitable for UV detection by using its photoconduction properties [3]. ZnO normally forms in the hexagonal (wurtzite) crystal structure with = 3.25 A and = 5.12 A. The Zn atoms are tetrahedrally coordinated with four O atoms where the d-electrons of Zn hybridize with the p-electrons of O. Layers occupied by Zinc atoms alternate with layers occupied by Oxygen atoms. Presence of free electrons in undoped ZnO has been attributed to Zn interstitials and Oxygen vacancies [4]. The intrinsic defect levels that lead to n-type doping lie approximately 0.01 to 0.05 ev below the conduction band. The photoluminescence study of ZnO reflect the intrinsic direct band gap, a strongly bound exciton state, and the gap states due to point defects [4]. Visible emissions in violet blue, green, and red orange range in case of ZnO are due to transitions between selfactivated centers formed by doubly ionized Zinc Vacancy and an ionized interstitial Zn+, Oxygen vacancies, and donor acceptor pair recombination involving an impurity acceptor [4].
For the fabrication of optoelectronic devices, knowledge about the properties of impurities like donors and acceptors is Indian Journal of Materials Science  of essential interest. The binding mechanism can be described as a consequence of the lattice deformation due to atomic size difference between impurity and host atom.

Experimental
The chemical route is simple and economical for preparing high quality nanomaterial like Zinc Oxide. Zinc Oxide nanoparticle can be prepared by treating Zinc Sulphate or Zinc Nitrate with Sodium Hydroxide in aqueous solution and then heating the white precipitate (Zinc Hydroxide) at a temperature greater than 100 C. All chemicals used were of high purity taken from Merc India Ltd. To prepare pure Zinc Oxide, nanomaterial Zinc Nitrate and Sodium Hydroxide were taken in stoichiometric ratio in aqueous solution and stirred for 12 hours. The white precipitate was washed with deionized water 8 times so that only Zinc Hydroxide precipitate remained. It was then dried at 100 ∘ c for 2 hours. Dried samples were annealed at 600 C for half an hour. Next for doping with Ca and Mg, their nitrates were mixed with Zinc Nitrate in the ratio such that the number of atoms of Zn and those of (Ca and Mg) were in the ratio The XRD patterns of these samples were obtained by Rigaku Miniflex 2 X-ray Diffractometer with Cu K X-radiation of wavelength 1.5406Ångstrom. Photoluminescence spectra of all samples were studied with excitation wavelengths of 254 nm by the help of Fluorescence spectrometer (Perkin Elmer LS 55). The photoconduction studies were done by pressing ZnO nanopowders on self-designed interdigital electrode and covering it with glass cavity and illuminating it with visible light from general 100 W bulb kept at two heights such that the illuminance at the sample is 40 Lx, 332 Lx, and 1640 Lx, respectively ( Figure 1). The effective area of crosssection (A) and effective length between two electrodes (L) for the calculation of resistivity were taken as (2.4 × 0.15 × 7 + 3.6 × 0.5 = 4.32 cm 2 ) and 0.15 cm, respectively, by measuring the dimension of electrode.

Results and Discussions
The   SEM micrograph shows that ZnO powder was with small grain of size in nanorange. The SEM study of samples was done one year after its preparation so the size of grain was increased comparing to that found by XRD machine which was smaller than 50 nm at the time of preparation. The structure of grain was like long stones. The picture is given in Figure 3.
The atomic percentage of all elements as studied by the help of EDS of Zn 0.9 Ca 0 Mg 0.1 O [ZnO-2.1] is as shown in Table 1.
EDS study confirm the presence of Mg in ZnO. The amount of Ca is zero matching with the intended doping amount. The amount of doped elements Ca and Mg is found to be less than the actual amount intended to dope in ZnO. This may be due to inhomogeneity of sample.
The spectrum obtained by EDS study of Zn 0.9 Ca 0 Mg 0.1 O [ZnO-2.1] is given in Figure 4.
PL measurement of doped Zinc Oxide was done at excitation wavelength 254 nm. Figure 5  in case of the sample ZnO-2.5 in which amount of Ca and Mg is in ratio (80 : 20). The observed resistance and resistivity of all samples are given in The least resistivity is still higher than that of doped ZnO found by other researchers. This is due to the less compactness of powder in comparison to that of film. The current (in A) verses potential difference (in V) graph at different light intensities are shown in Figure 6.
The data of particle size, interplanar distance, average resistance, resistivity, and initial rate of decay of current can be tabulated as shown in Table 2.

Conclusion
ZnO was doped successfully by a very simple chemical method. XRD pattern shows its high crystallinity. New phases were found in few samples, ZnO-2.2 and ZnO-2.4. The SEM