Zinc oxide (molecular formula : ZnO) is a multifunctional material, with its unique physical and chemical properties such as high chemical stability, high electrochemical coupling coefficient, high photo stability and broad range of radiation absorption[1,2]. It is recognised as a potential II–VI photonic semiconductor materials due to its wide band gap (∼3.3 eV) and high exciton binding energy (∼60 meV)[3]. It possesses considerable potential for applications in optoelectronic devices such as UV lasers, LEDs, as electrode in solar cells, gas and bio sensors etc. The last few years have witnessed tremendous efforts on understanding the physical and optical properties of ZnO with particular attention on fabrication and device applications[4]. Many synthesis routes like sol-gel, hydrothermal, co-precipitation, wet chemical method etc has been used to obtain high quality nano/microstructure ZnO material[5]. It is also well established that ZnO optoelectronic properties strongly vary depending on its defect structure based on synthesis techniques.
Crystal Structure: ZnO generally crystalizes in two forms: Hexagonal Wurtzite and cubic zinc blende. According to the first principle periodic Hartree-Fock linear combination of atomic orbitals theory, the hexagonal Zinc wurtzite is found to be the most thermodynamically stable form[6]. It belongs to the space group of P63mc [6,7] which has two lattice parameters; a= 3.25 Å, c= 5.20 Å and is characterized by two interconnecting sub lattices of Zn2+ and O2- where each anion is surrounded by four cations at the corners of a tetrahedron with a typical sp3 covalent bonding. The number of alternating planes of tetrahedrally coordinated O2- and Zn2+ ions which are pilled alternately along c-axis (Figure 1.1) describe the structure of ZnO. The zinc and oxide center in the wurtzite ZnO is tetrahedral and this tetrahedral symmetry plays an important role for polarity of ZnO. Piezoelectricity and pyroelectricity are the direct consequences of polar symmetry of ZnO along hexagonal axis. ZnO is generally found to be n-type structure. This n-type is due to the structural point defect (vacancies and interstitials) and extended defects (threading/planar dislocations). The presence of oxygen vacancies in ZnO lattice gives it n-type conductivity.
Physical property: Pure ZnO is white in colour and turns yellow on heating. Its molecular weight is 81.37. ZnO has relative density of 5.607. Under high pressure, the melting point of ZnO is 1900ºC and its heat capacity is 9.62cal/deg/mole at 25ºC. It is insoluble in water but soluble in acid.
Opto-electronic property: ZnO has a large exciton binding energy around 60 meV at room temperature due to excitonic recombination[8]. This large exciton binding energy makes efficient excitonic emission in ZnO which suggests ZnO a promising material for optical devices at room temperature and higher. The process of optical absorption and emission are very much influenced transition related to dopants or defects which are usually responsible for creating mid gap discrete electronic state[8]. Many reports show that the photoluminescence of ZnO shows green emission and the intensity of green emission increases with decrease particle size and reduced nanowire diameter which gives quantum size effect. The reduction in particle size increases the binding energy and which in turn enhances the opto-electronic property of ZnO nanomaterial.
Optical properties: As reported in different literature, the optical band gap of ZnO is 3.44eV at low temperature and 3.3eV at room temperature[9] which corresponds to energy of 375.75Å photons. So, zinc oxide is transparent to visible light but strongly absorbs ultra violet light below 375.75 Å. Due to this reason, ZnO is used in varieties of optoelectronic applications like Light Emitting Diode (LED’s), Solar Cells, photo detectors etc. [6,10–12]. The band gap of ZnO depends upon the carrier concentration; Band gap tends to decrease as there is an increase in carrier concentration. Photoluminescence of ZnO represents a relatively sharp absorption peak at 380nm (due to band to band transitions) and a wider yellow-green emission band (due to presence of oxygen vacancies and other related defects).
Electrical Properties: ZnO has a wide bandgap of 3.3 eV at room temperature. This wide band gap has many advantages like higher breakdown voltage, ability to sustain large electrical fields, lower electronic noise, high temperature and high-power operation. These properties make ZnO nanomaterial fit for wide varieties of electrical applications. Electron mobility of ZnO strongly dependent on temperature and possess a maximum of ~2000 cm2/ (V·s) at 80 K.